Chemical Reactions and Compounds Practice Questions — Armed Services Vocational Aptitude Battery (ASVAB)

Question 1Question →

Consider the chemical equation for the formation of ammonia from its constituent elements: $\text{N}_2 + \underline{\quad}\text{H}_2 \rightarrow 2\text{NH}_3$ . What missing coefficient and reaction type correctly complete the statement?

Fill in the blanks below

In the chemical equation $\text{N}_2 + \text{H}_2 \rightarrow 2\text{NH}_3$, the coefficient required to balance the hydrogen atoms is , and because two distinct reactants combine to yield a single compound, this process is classified as a reaction.

Show answer & explanation

Answer

The missing coefficient is 3 and the reaction is classified as a synthesis (or combination) reaction.

On the product side,

2\text{NH}_3

contains 2 nitrogen atoms and 6 hydrogen atoms. On the reactant side,

\text{N}_2

accounts for the 2 nitrogen atoms, so 3 molecules of

\text{H}_2

(

3 \times 2 = 6

atoms) are required to balance the hydrogen. Furthermore, because two separate elemental substances combine to form a single compound, the reaction represents a synthesis (combination) reaction.

Step-by-Step Solution

1

Count the number of hydrogen atoms in the product side of the equation.

The product side contains

2\text{NH}_3

, which has

2 \times 3 = 6

hydrogen atoms.

According to the law of conservation of mass, the total number of atoms of each element must be equal on both sides of the equation.

2

Determine the coefficient for the reactant

\text{H}_2

needed to provide 6 hydrogen atoms.

Since each diatomic hydrogen molecule (

\text{H}_2

) contains 2 atoms, dividing 6 by 2 yields a coefficient of 3.

Setting the coefficient of

\text{H}_2

to 3 balances the hydrogen atoms (

3 \times 2 = 6

) while nitrogen is already balanced (

2 = 2

).

3

Identify the reaction type based on the reactants and products.

Two elemental reactants (

\text{N}_2

and

\text{H}_2

) combine directly to form a single chemical compound (

\text{NH}_3

).

A chemical reaction in which two or more simple substances combine to form a single complex substance is defined as a synthesis (or combination) reaction.

Key Concept

Balancing chemical equations by conserving atoms and classifying reaction types (synthesis/combination).

Estimated Time:1m 0s

Question 2Question →

A technician observes several material transformations while working in a maintenance shop. Which of the following observations indicates that a chemical reaction has occurred to form a new compound?

Solid solder wire melting into a liquid puddle when touched with a hot iron

An iron bracket developing a reddish-brown layer of rust after exposure to humid air

Rubbing alcohol evaporating completely from a metal surface into vapor

A sheet of brass being hammered and shaped into a curved shield

Show answer & explanation

Answer: An iron bracket developing a reddish-brown layer of rust after exposure to humid air

Answer

An iron bracket developing a reddish-brown layer of rust after exposure to humid air

The formation of rust occurs when elemental iron reacts with oxygen in the presence of water to synthesize iron oxide (

Fe_2O_3

), a completely new chemical compound with distinct chemical and physical properties from the original metallic iron.

Step-by-Step Solution

1

Differentiate between physical transformations and chemical changes

Physical changes alter state, shape, or appearance without changing molecular structure, whereas chemical changes form entirely new substances with distinct chemical bonds and properties.

Understanding the fundamental definition of a chemical reaction is required to evaluate each scenario.

2

Analyze the formation of rust on iron

Iron (

Fe

) combines with oxygen (

O_2

) and moisture from the environment to synthesize iron(III) oxide (

Fe_2O_3

), which is a brand-new compound with different physical and chemical properties.

This transformation demonstrates a true chemical reaction (oxidation/synthesis).

3

Evaluate phase changes and mechanical shaping

Melting solder, evaporating alcohol, and shaping brass preserve the original chemical identities of the materials.

Eliminating physical state changes confirms the only chemical change.

Key Concept

Identification of Chemical Reactions and Compound Formation versus Physical Changes

Estimated Time:45s

Question 3Question →

Fill in the blank with the correct integer coefficient needed to balance the chemical equation.

Fill in the blanks below

In the complete combustion of methane, one molecule of methane reacts with oxygen to form carbon dioxide and water:

\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 +

\text{H}_2\text{O}

Show answer & explanation

Answer

2

According to the law of conservation of mass, the total number of atoms for each element must be equal before and after the reaction. The reactant side contains 1 carbon atom, 4 hydrogen atoms, and 4 oxygen atoms. On the product side,

\text{CO}_2

contains 1 carbon atom and 2 oxygen atoms. Placing a coefficient of 2 in front of

\text{H}_2\text{O}

provides the remaining 4 hydrogen atoms and 2 oxygen atoms, fully balancing the equation.

Step-by-Step Solution

1

Count the number of atoms of each element on the reactant side.

Reactants have 1 carbon atom, 4 hydrogen atoms (from

\text{CH}_4

), and 4 oxygen atoms (from

2\text{O}_2

).

The law of conservation of mass requires the number of atoms of each element to be identical on both sides of a balanced chemical equation.

2

Count the atoms present in the products and determine the coefficient needed for

\text{H}_2\text{O}

.

One molecule of

\text{CO}_2

contains 1 carbon atom and 2 oxygen atoms. To balance the 4 hydrogen atoms and remaining 2 oxygen atoms, 2 molecules of

\text{H}_2\text{O}

are required.

A coefficient of 2 provides

2 \times 2 = 4

hydrogen atoms and

2 \times 1 = 2

oxygen atoms, giving a total of 4 oxygen atoms on the product side.

Key Concept

Balancing chemical equations using conservation of mass

Estimated Time:45s

Question 4Question →

During a laboratory experiment, a student mixes an aqueous solution of sodium carbonate ( $\text{Na}_2\text{CO}_3$ ) with an aqueous solution of calcium chloride ( $\text{CaCl}_2$ ). A solid white precipitate of calcium carbonate ( $\text{CaCO}_3$ ) immediately forms and settles at the bottom, while sodium chloride ( $\text{NaCl}$ ) remains dissolved. Which of the following statements best explains why this process is classified as a chemical reaction rather than a physical change?

The liquid solution underwent a physical phase change where dissolved particles froze into solid crystals due to temperature loss.

The substances formed a simple mechanical mixture in which both original compounds retain their identical chemical bonds.

Chemical bonds were broken and reformed to produce a new substance with different chemical properties from the original reactants.

New atoms of calcium and carbon were generated during the reaction to create the additional mass observed in the solid precipitate.

Show answer & explanation

Answer: Chemical bonds were broken and reformed to produce a new substance with different chemical properties from the original reactants.

Answer

Chemical bonds were broken and reformed to produce a new substance with different chemical properties from the original reactants.

A chemical reaction occurs when bonds within reactant molecules break and new bonds form to create one or more chemically distinct substances. The formation of solid calcium carbonate precipitate from aqueous sodium carbonate and calcium chloride demonstrates the synthesis of a new compound with unique solubility and chemical properties.

Step-by-Step Solution

1

Analyze the starting reactants and resulting products in the reaction.

Reactants are soluble

\text{Na}_2\text{CO}_3

and

\text{CaCl}_2

; products are solid

\text{CaCO}_3

precipitate and dissolved

\text{NaCl}

.

Identifying the initial and final chemical species confirms whether new substances were produced.

2

Distinguish between chemical transformations and physical changes.

The formation of a solid precipitate from two clear liquids via double displacement indicates bonds broke and reformed to yield a new insoluble compound.

A physical change alters form or state without changing chemical identity, whereas this reaction alters chemical identity and molecular structure.

3

Apply the Law of Conservation of Mass to verify atom balance.

The balanced equation is

\text{Na}_2\text{CO}_3\text{(aq)} + \text{CaCl}_2\text{(aq)} \rightarrow \text{CaCO}_3\text{(s)} + 2\text{NaCl}\text{(aq)}

, showing atom conservation rather than atom creation.

All atoms present in the products originate from the reactants; no atoms are generated or destroyed.

Key Concept

Chemical change involves breaking and forming chemical bonds to produce new substances, distinguished from physical phase changes and governed by the Law of Conservation of Mass.

Estimated Time:1m 0s

Question 5Question →

A student performs several classroom experiments to study how substances interact. Which of the following processes represents a chemical change?

Melting an ice cube into liquid water

Crushing a solid piece of chalk into fine powder

Baking bread dough into a loaf of bread

Dissolving granulated sugar in warm water

Show answer & explanation

Answer: Baking bread dough into a loaf of bread

Answer

Baking bread dough into a loaf of bread represents a chemical change.

Baking bread is a chemical change because thermal energy causes chemical reactions among ingredients (such as gas production and protein structural changes), resulting in new substances that cannot be converted back by simple physical means.

Step-by-Step Solution

1

Recall the distinction between chemical and physical changes.

A chemical change results in the formation of one or more new substances through the breaking and forming of chemical bonds, whereas a physical change alters only the form, state, or appearance of matter without changing its chemical identity.

This criterion allows accurate categorization of each observed process.

2

Evaluate each choice against the definition of a chemical change.

Melting ice, crushing chalk, and dissolving sugar all preserve the original chemical composition of the materials. In contrast, baking bread involves heat-driven chemical reactions that form completely new compounds.

Only baking creates new substances with distinct chemical properties.

Key Concept

Differentiating chemical changes (which form new chemical substances) from physical changes (which alter physical state or appearance without changing molecular composition).

Estimated Time:45s

Question 6Question →

In an industrial wastewater neutralization facility, aqueous calcium hydroxide [ $Ca(OH)_2$ ] reacts with aqueous phosphoric acid [ $H_3PO_4$ ] to yield an insoluble calcium phosphate [ $Ca_3(PO_4)_2$ ] precipitate and liquid water [ $H_2O$ ]. When the chemical equation for this reaction is completely balanced using the lowest whole-number coefficients, what is the sum of all stoichiometric coefficients, and which fundamental type of chemical reaction is demonstrated?

Total sum of

11

; synthesis reaction

Total sum of

6

; single replacement reaction

Total sum of

12

; double replacement reaction

Total sum of

12

; physical dissolution change

Show answer & explanation

Answer: Total sum of $12$ ; double replacement reaction

Answer

Total sum of

12

; double replacement reaction

The balanced chemical reaction is

3Ca(OH)_2 + 2H_3PO_4 \rightarrow Ca_3(PO_4)_2 + 6H_2O

. Summing all stoichiometric coefficients yields

3 + 2 + 1 + 6 = 12

. Because the cations and anions of two reacting compounds trade partners to produce two new substances (an insoluble precipitate and water), the process is classified as a double replacement reaction.

Step-by-Step Solution

1

Write the unbalanced skeleton equation for the described chemical reaction.

Ca(OH)_2 + H_3PO_4 \rightarrow Ca_3(PO_4)_2 + H_2O

Establishing the correct chemical formulas of reactants and products is the necessary starting point for stoichiometry.

2

Balance the metal cations, polyatomic phosphate ions, and remaining hydrogen and oxygen atoms.

3Ca(OH)_2 + 2H_3PO_4 \rightarrow 1Ca_3(PO_4)_2 + 6H_2O

Balancing calcium requires

3Ca(OH)_2

, balancing phosphate requires

2H_3PO_4

, which yields

6

hydrogen atoms from hydroxide and

6

from acid, forming

6H_2O

and satisfying the Law of Conservation of Mass.

3

Calculate the sum of all stoichiometric coefficients in the balanced equation.

3 + 2 + 1 + 6 = 12

Every term in the balanced equation has a coefficient, where an unwritten coefficient represents exactly

1

.

4

Classify the reaction mechanism based on atomic and ionic reorganization.

Double replacement (acid-base neutralization and precipitation)

The calcium cations and hydrogen ions exchange their respective polyatomic anion partners (

OH^-

and

PO_4^{3-}

), which defines a double replacement reaction.

Key Concept

Balancing chemical equations and classifying chemical reaction types

Estimated Time:1m 30s

Question 7Question →

In chemistry, chemical reactions are categorized by the pattern in which reactants rearrange to form new substances. Match each chemical reaction equation with its correct classification type.

Click a left item, then click its matching right item

Show answer & explanation

Answer

The reaction

2H_2 + O_2 \rightarrow 2H_2O

matches Synthesis reaction;

2H_2O_2 \rightarrow 2H_2O + O_2

matches Decomposition reaction;

C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O

matches Combustion reaction; and

HCl + NaOH \rightarrow NaCl + H_2O

matches Neutralization reaction.

Each chemical reaction matches its respective classification based on the fundamental pattern of atomic rearrangement: synthesis combines multiple reactants into one product; decomposition splits a single reactant into multiple products; combustion oxidizes a fuel with oxygen to form carbon dioxide and water; and neutralization combines an acid with a base to produce salt and water.

Step-by-Step Solution

1

Analyze the number and structure of reactants and products in

2H_2 + O_2 \rightarrow 2H_2O

.

Two distinct reactant molecules join together to form a single product species (

H_2O

).

When multiple simple reactants unite into a single compound (

A + B \rightarrow AB

), the process is classified as synthesis.

2

Examine the reactant and products in

2H_2O_2 \rightarrow 2H_2O + O_2

.

A single reactant species (

H_2O_2

) splits into two distinct product species (

H_2O

and

O_2

).

When one complex substance breaks apart into simpler components (

AB \rightarrow A + B

), the process is classified as decomposition.

3

Examine the chemical species participating in

C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O

.

A hydrocarbon fuel (

C_3H_8

) reacts rapidly with oxygen gas (

O_2

) to yield

CO_2

and

H_2O

.

Oxidation of a hydrocarbon fuel producing carbon dioxide and water vapor is the hallmark of a combustion reaction.

4

Identify the nature of the reactants in

HCl + NaOH \rightarrow NaCl + H_2O

.

An acid (

HCl

) reacts with a strong base (

NaOH

) to produce an ionic salt (

NaCl

) and liquid water (

H_2O

).

An acid-base double replacement yielding salt and water is specifically classified as a neutralization reaction.

Key Concept

Classification of Chemical Reactions

Estimated Time:45s

Question 8Question →

When solid baking soda (sodium bicarbonate) is mixed with liquid vinegar (acetic acid), vigorous bubbling occurs as carbon dioxide gas is released. Which of the following statements correctly classifies and explains this transformation?

It is a physical change because the vinegar underwent rapid boiling caused by mixing.

It is a chemical change because bonds are broken and rearranged to form new substances with different properties.

It is a physical change because dissolving a solid into a liquid only separates molecules without creating new substances.

It is a physical change because the solid baking soda simply melted into a liquid state upon contact with the vinegar.

Show answer & explanation

Answer: It is a chemical change because bonds are broken and rearranged to form new substances with different properties.

Answer

It is a chemical change because bonds are broken and rearranged to form new substances with different properties.

The reaction between baking soda and vinegar is a chemical change because the original substances react to form entirely new compounds (carbon dioxide gas, water, and dissolved sodium acetate) with properties distinct from the starting materials.

Step-by-Step Solution

1

Analyze the observed interaction and identify products.

Mixing sodium bicarbonate (

\text{NaHCO}_3

) and acetic acid (

\text{HC}_2\text{H}_3\text{O}_2

) generates carbon dioxide gas (

\text{CO}_2

), water (

\text{H}_2\text{O}

), and sodium acetate (

\text{NaC}_2\text{H}_3\text{O}_2

).

The production of a new gas and temperature change serve as clear evidence of a chemical transformation.

2

Classify the type of change based on chemical identity.

The process is classified as a chemical change rather than a physical change.

In a chemical change, atoms are rearranged and chemical bonds are broken and formed, creating entirely new chemical substances with distinct chemical and physical properties.

Key Concept

Chemical vs. Physical Changes

Estimated Time:45s

Question 9Question →

In emergency life-support systems, oxygen is generated through the chemical breakdown of solid sodium chlorate ( $NaClO_3$ ) into solid sodium chloride ( $NaCl$ ) and oxygen gas ( $O_2$ ) according to the balanced chemical equation:

2NaClO_3(s) \rightarrow 2NaCl(s) + 3O_2(g)

Fill in the blanks to identify the reaction type and calculate the total moles of oxygen produced.

Fill in the blanks below

Because a single reactant compound breaks apart to form multiple simpler products, this transformation is classified as a reaction. Furthermore, based on the stoichiometric coefficients in the balanced equation, the complete reaction of

6

moles of

NaClO_3

produces moles of

O_2

gas.

Show answer & explanation

Answer

The chemical process is classified as a decomposition reaction, and 6 moles of sodium chlorate produce 9 moles of oxygen gas.

A chemical reaction in which a single compound decomposes into two or more simpler elements or compounds is a decomposition reaction. According to the balanced equation

2NaClO_3(s) \rightarrow 2NaCl(s) + 3O_2(g)

, every

2

moles of

NaClO_3

consumed produce

3

moles of

O_2

. Therefore, reacting

6

moles of

NaClO_3

yields

6 \times \frac{3}{2} = 9

moles of

O_2

.

Step-by-Step Solution

1

Analyze the reactant and product patterns to determine the reaction classification.

The reaction is classified as a decomposition reaction.

A single complex reactant (

NaClO_3

) breaks down into two distinct, simpler substances (

NaCl

and

O_2

).

2

Identify the stoichiometric molar ratio between

NaClO_3

and

O_2

from the balanced equation.

The molar ratio is

2\text{ moles } NaClO_3 : 3\text{ moles } O_2

.

The coefficients preceding

NaClO_3

and

O_2

in the balanced equation are

2

and

3

, respectively.

3

Calculate the moles of

O_2

produced from

6

moles of

NaClO_3

.

6\text{ mol } NaClO_3 \times \frac{3\text{ mol } O_2}{2\text{ mol } NaClO_3} = 9\text{ moles of } O_2

.

Multiplying the given quantity by the stoichiometric conversion factor yields the theoretical product amount.

Key Concept

Reaction classification criteria and stoichiometric molar ratio calculations from balanced chemical equations

Estimated Time:1m 30s

Question 10Question →

Chemical compounds are identified by specific chemical formulas and possess distinct everyday applications and properties. Match each chemical compound on the left with its corresponding description and practical use on the right.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Calcium carbonate matches the primary mineral compound in limestone, marble, and antacids; Sodium hydroxide matches the strong alkaline base lye used in soap making and drain cleaning; Hydrochloric acid matches the corrosive acid found in gastric juices and muriatic acid; Methane matches the simple hydrocarbon gas that serves as the primary component of natural gas.

Each compound correctly corresponds to its standardized chemical formula, common name, and primary practical usage: Calcium carbonate (

\text{CaCO}_3

) is limestone/antacid; Sodium hydroxide (

\text{NaOH}

) is lye/drain cleaner; Hydrochloric acid (

\text{HCl}

) is stomach acid/muriatic acid; Methane (

\text{CH}_4

) is natural gas fuel.

Step-by-Step Solution

1

Analyze the formula and chemical nature of Calcium carbonate (

\text{CaCO}_3

).

Identify it as an insoluble ionic salt that constitutes limestone and marble, frequently used to neutralize stomach acids.

Connecting chemical formula to geological presence and antacid utility.

2

Analyze the formula and chemical nature of Sodium hydroxide (

\text{NaOH}

).

Identify it as a soluble, highly caustic alkali base commonly called lye.

Distinguishing hydroxide bases and their commercial cleaning roles.

3

Analyze the formula and chemical nature of Hydrochloric acid (

\text{HCl}

).

Identify it as a strong binary acid present in the stomach and sold as muriatic acid.

Matching common commercial acid names with biological roles.

4

Analyze the formula and chemical nature of Methane (

\text{CH}_4

).

Identify it as a simple flammable hydrocarbon that makes up the majority of natural gas.

Classifying organic fuel compounds by molecular formula.

Key Concept

Nomenclature, chemical formulas, and practical applications of common household and industrial compounds

Estimated Time:1m 0s

Question 11Question →

Complete the statement and equation for the reaction between solid magnesium metal and hydrochloric acid. What coefficient balances the acid reactant, and what classification describes this reaction?

Fill in the blanks below

When solid magnesium reacts with aqueous hydrochloric acid, it forms dissolved magnesium chloride and hydrogen gas according to the equation: $\text{Mg} + \text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$. Because an uncombined element takes the place of another element in a compound, this process is classified as a replacement reaction.

Show answer & explanation

Answer

The balanced coefficient for

\text{HCl}

is

2

, and the reaction type is single replacement.

According to the Law of Conservation of Mass, matter is neither created nor destroyed during a chemical reaction. On the product side,

\text{MgCl}_2

contains two chlorine atoms and

\text{H}_2

contains two hydrogen atoms. Therefore, a coefficient of

2

in front of

\text{HCl}

is necessary to supply two hydrogen and two chlorine atoms from the reactants. Furthermore, because elemental magnesium displaces hydrogen from hydrochloric acid to yield magnesium chloride and free hydrogen, the reaction is by definition a single replacement (displacement) reaction.

Step-by-Step Solution

1

Count the atoms of each element on the product side of the equation

The products

\text{MgCl}_2 + \text{H}_2

contain

1

magnesium atom,

2

chlorine atoms, and

2

hydrogen atoms.

The Law of Conservation of Mass requires the number of atoms of each element to be identical on both sides of a chemical equation.

2

Determine the coefficient needed for

\text{HCl}

on the reactant side

Placing a coefficient of

2

in front of

\text{HCl}

gives

2

hydrogen atoms and

2

chlorine atoms, balancing

1\text{Mg} + 2\text{HCl} \rightarrow 1\text{MgCl}_2 + 1\text{H}_2

.

Changing coefficients balances the total atom counts without altering chemical identities.

3

Classify the reaction mechanism based on reactant and product structures

The reaction is classified as a single replacement (or single displacement) reaction.

Free magnesium metal (

\text{Mg}

) displaces hydrogen (

\text{H}

) from hydrochloric acid (

\text{HCl}

) to form a new compound and release elemental hydrogen gas.

Key Concept

Balancing chemical equations by conserving atoms and identifying single replacement reaction patterns.

Estimated Time:45s

Question 12Question →

An automotive technician observes several processes taking place in a workshop. Which of the following processes is an example of a chemical change?

The evaporation of spilled degreaser on the concrete floor

The rusting of an iron vehicle frame exposed to moisture

The melting of solid solder wire using a heated soldering iron

The grinding of a steel brake rotor into fine metal shavings

Show answer & explanation

Answer: The rusting of an iron vehicle frame exposed to moisture

Answer

The rusting of an iron vehicle frame exposed to moisture

The rusting of an iron frame represents a chemical reaction (oxidation) where metallic iron combines with oxygen and water to synthesize a new compound, iron oxide. Because a new chemical substance with unique properties is created, it is a chemical change.

Step-by-Step Solution

1

Differentiate between physical and chemical changes.

A physical change alters form, state, or appearance without changing chemical identity. A chemical change produces one or more new chemical substances through the breaking and forming of bonds.

Establishing the core scientific definitions allows accurate classification of each workshop process.

2

Analyze the process involving the formation of rust on iron.

Iron (

Fe

) reacts with oxygen (

O_2

) and moisture (

H_2O

) to form hydrated iron(III) oxide (

Fe_2O_3 \cdot nH_2O

), which is a brand-new compound.

Since a new chemical compound with distinct properties is formed, the process is definitively a chemical change.

Key Concept

Chemical change involves a transformation where original substances combine or break down into new substances with different chemical properties.

Estimated Time:45s

Question 13Question →

Chemical transformations involve the rearrangement of atoms and bonds to create distinct substances. Match each described chemical reaction on the left with its corresponding balanced chemical equation on the right.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Each reaction description matches its stoichiometrically balanced chemical equation representing the correct reaction archetype (decomposition, single displacement, neutralization, and combustion).

The correct pairings accurately connect standard chemical nomenclature and reaction archetypes (decomposition, single displacement, acid-base neutralization, and hydrocarbon combustion) to their fully balanced IUPAC stoichiometric equations.

Step-by-Step Solution

1

Identify reactants, products, and reaction classification for each narrative prompt.

Decomposition produces simpler compounds from one reactant; displacement involves an elemental metal replacing an aqueous cation; neutralization pairs an acid and base to form salt and water; combustion combines a hydrocarbon/alcohol with oxygen to yield carbon dioxide and water.

Categorizing the reaction type narrows the corresponding structural formula patterns.

2

Verify chemical formulas and conservation of mass (balancing coefficients) for each equation.

Thermal decomposition corresponds to

\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2

; single displacement corresponds to

\text{Cu} + 2\text{AgNO}_3 \rightarrow \text{Cu}(\text{NO}_3)_2 + 2\text{Ag}

; neutralization corresponds to

\text{H}_2\text{SO}_4 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}

; combustion corresponds to

\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}

.

Every atom must balance across the reaction arrow in accordance with the Law of Conservation of Mass.

Key Concept

Classification and Stoichiometric Balancing of Chemical Reactions

Estimated Time:2m 0s

Question 14Question →

In physical science, chemical reactions are categorized by how reactant atoms and compounds interact to form products. Match each described chemical reaction scenario on the left with its correct reaction classification on the right.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Burning propane in oxygen matches Combustion; Splitting water into hydrogen and oxygen matches Decomposition; Combining magnesium and oxygen into magnesium oxide matches Synthesis; Mixing hydrochloric acid and sodium hydroxide matches Neutralization.

Each process demonstrates a core category of chemical change: burning propane with oxygen to produce carbon dioxide and water is combustion; splitting water into hydrogen and oxygen gas via electricity is decomposition; combining magnesium and oxygen into a single compound is synthesis; and mixing hydrochloric acid with sodium hydroxide to create salt and water is neutralization.

Step-by-Step Solution

1

Identify the reaction where a fuel rapidly reacts with oxygen gas (

O_2

) producing energy, carbon dioxide (

CO_2

), and water (

H_2O

).

Burning propane corresponds to a Combustion reaction (

C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O

).

Combustion occurs when a substance combines with oxygen rapidly, releasing heat and light.

2

Identify the process where a single reactant compound breaks down into two or more distinct products.

Splitting water into hydrogen and oxygen corresponds to a Decomposition reaction (

2H_2O \rightarrow 2H_2 + O_2

).

Decomposition represents the breakdown of a single substance into simpler parts.

3

Identify the process where two or more individual reactants join together to create a single compound.

Combining magnesium and oxygen into magnesium oxide corresponds to a Synthesis reaction (

2Mg + O_2 \rightarrow 2MgO

).

Synthesis involves building up a larger or more complex molecule from simpler elements or compounds.

4

Identify the reaction where an acidic solution and a basic solution react to yield a neutral salt and water.

Mixing hydrochloric acid and sodium hydroxide corresponds to a Neutralization reaction (

HCl + NaOH \rightarrow NaCl + H_2O

).

Neutralization is an acid-base double displacement reaction where hydrogen and hydroxide ions combine to form water.

Key Concept

Classifying Fundamental Chemical Reactions

Estimated Time:45s

Question 15Question →

In residential heating, propane gas ( $\text{C}_3\text{H}_8$ ) burns in the presence of oxygen ( $\text{O}_2$ ) to form carbon dioxide ( $\text{CO}_2$ ) and water vapor ( $\text{H}_2\text{O}$ ) according to the unbalanced chemical equation:

\text{C}_3\text{H}_8 + \underline{\hspace{0.8cm}}\,\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}

What coefficient must be placed before $\text{O}_2$ to completely balance this equation using the lowest whole numbers?

10

5

7

3

Show answer & explanation

Answer: 5

Answer

5

The coefficient 5 is correct because the product side contains a total of 10 oxygen atoms (6 from

3\text{CO}_2

and 4 from

4\text{H}_2\text{O}

). Since molecular oxygen is diatomic (

\text{O}_2

), 5 molecules of

\text{O}_2

(

5 \times 2 = 10

atoms) are required to balance the reaction.

Step-by-Step Solution

1

Count the total number of oxygen atoms on the product side of the equation.

The products contain

(3 \times 2) + (4 \times 1) = 6 + 4 = 10

oxygen atoms.

By the Law of Conservation of Mass, the number of oxygen atoms in the reactants must equal the number in the products.

2

Determine the coefficient needed for diatomic oxygen gas (

\text{O}_2

) on the reactant side.

10 \div 2 = 5

Because each

\text{O}_2

molecule contains 2 oxygen atoms, 5 molecules are required to provide the 10 needed atoms.

Key Concept

Balancing chemical equations by conserving atoms across reactants and products

Estimated Time:1m 0s

Question 16Question →

In a submarine life-support system, carbon dioxide ( $CO_2$ ) is scrubbed from the cabin atmosphere by reacting with solid lithium hydroxide ( $LiOH$ ) to produce solid lithium carbonate ( $Li_2CO_3$ ) and water vapor ( $H_2O$ ). Which of the following represents the correctly balanced chemical equation for this reaction?

LiOH + CO_2 \rightarrow Li_2CO_3 + H_2O

2LiOH + CO_2 \rightarrow Li_2CO_3 + H_2O

Li_2OH + CO_2 \rightarrow Li_2CO_3 + H_2O

LiOH + CO_2 \rightarrow LiCO_2 + H_2O

Show answer & explanation

Answer: $2LiOH + CO_2 \rightarrow Li_2CO_3 + H_2O$

Answer

The balanced chemical equation is

2LiOH + CO_2 \rightarrow Li_2CO_3 + H_2O

.

Placing a coefficient of 2 in front of lithium hydroxide yields equal quantities of each element on both sides: 2 lithium atoms, 1 carbon atom, 4 oxygen atoms, and 2 hydrogen atoms, satisfying the law of conservation of mass.

Step-by-Step Solution

1

Write the unbalanced skeleton equation using the correct chemical formulas for all reactants and products.

LiOH + CO_2 \rightarrow Li_2CO_3 + H_2O

Establishing the correct formulas ensures that the identities of the chemical substances are maintained.

2

Count the number of each type of atom on both sides of the equation.

Reactants: 1 Li, 1 C, 3 O, 1 H. Products: 2 Li, 1 C, 4 O, 2 H.

An atom inventory reveals which elements violate the law of conservation of mass.

3

Place a coefficient of 2 in front of

LiOH

on the reactant side to balance the lithium and hydrogen atoms.

2LiOH + CO_2 \rightarrow Li_2CO_3 + H_2O

Using whole-number coefficients balances atoms without altering compound identities.

4

Re-verify the atom count for every element across the entire equation.

Reactants: 2 Li, 1 C, 4 O, 2 H. Products: 2 Li, 1 C, 4 O, 2 H.

Confirming equal atom counts ensures the equation satisfies the law of conservation of mass.

Key Concept

Balancing chemical equations by adding stoichiometric coefficients while keeping chemical formulas unchanged.

Estimated Time:1m 30s

Question 17Question →

Fill in the missing stoichiometric coefficient and chemical formula in the neutralization equation below. What coefficient and compound formula correctly complete the reaction?

Fill in the blanks below

When sulfuric acid (

\text{H}_2\text{SO}_4

) reacts with sodium bicarbonate (

\text{NaHCO}_3

), the complete neutralization reaction is represented by:

\text{H}_2\text{SO}_4 +

\text{NaHCO}_3 \rightarrow

+ 2\text{H}_2\text{O} + 2\text{CO}_2

.

Show answer & explanation

Answer

The missing coefficient is 2 and the missing chemical formula is Na2SO4.

A coefficient of 2 is necessary for sodium bicarbonate (NaHCO3) to provide the 2 carbon atoms and 2 sodium atoms found in the products (2CO2 and Na2SO4). The resulting salt is sodium sulfate, whose empirical formula is Na2SO4 because two +1 sodium cations (Na+) are needed to balance one -2 sulfate polyatomic anion (SO4^2-).

Step-by-Step Solution

1

Balance the carbon and hydrogen atoms between reactants and products to find the coefficient for sodium bicarbonate.

The coefficient for NaHCO3 is 2.

The product side contains 2 carbon atoms (in 2CO2) and a total of 4 hydrogen atoms (in 2H2O), requiring 2 moles of NaHCO3 to combine with 1 mole of H2SO4 on the reactant side.

2

Determine the chemical formula of the salt produced by pairing sodium cations with sulfate anions.

The chemical formula of the salt is Na2SO4.

Sodium forms a singly charged cation (Na+) while the sulfate polyatomic ion carries a -2 charge (SO4^2-), meaning two sodium ions are required to balance one sulfate ion electrically.

3

Verify the conservation of each atomic species across the complete chemical equation.

Reactants: 4 H, 1 S, 10 O, 2 Na, 2 C. Products: 4 H, 1 S, 10 O, 2 Na, 2 C.

The Law of Conservation of Mass requires that the number of atoms of each element on the reactant side must equal the number on the product side.

Key Concept

Stoichiometric equation balancing and compound formula determination in acid-carbonate neutralization reactions.

Estimated Time:1m 30s

Question 18Question →

A researcher conducts two separate experiments in a laboratory.

In Experiment 1, a chunk of solid carbon dioxide is submerged in room-temperature water, producing vigorous bubbling.
In Experiment 2, a piece of solid calcium metal is submerged in room-temperature water, which also produces vigorous bubbling, along with a noticeable increase in temperature and the formation of a cloudy white suspension.

Based on the evidence provided, which of the following best classifies the two experiments and correctly identifies the primary gas generating the bubbles in the experiment that represents a chemical change?

Experiment 1 is a physical change and Experiment 2 is a chemical change; the gas released in Experiment 2 is hydrogen (

H_2

).

Both experiments represent chemical changes; the gas released in Experiment 1 is carbon dioxide (

CO_2

) and the gas released in Experiment 2 is oxygen (

O_2

).

Experiment 1 is a physical change and Experiment 2 is a chemical change; the gas released in Experiment 2 is oxygen (

O_2

).

Experiment 1 is a chemical change and Experiment 2 is a physical change; the gas released in Experiment 1 is carbon dioxide (

CO_2

).

Show answer & explanation

Answer: Experiment 1 is a physical change and Experiment 2 is a chemical change; the gas released in Experiment 2 is hydrogen ( $H_2$ ).

Answer

Experiment 1 is a physical change and Experiment 2 is a chemical change; the gas released in Experiment 2 is hydrogen (

H_2

).

The correct answer accurately identifies that solid carbon dioxide sublimating into carbon dioxide gas is a physical change because the chemical structure does not change. It also accurately recognizes that calcium reacting with water is a chemical change that produces a new solid compound (calcium hydroxide) and hydrogen gas (

H_2

).

Step-by-Step Solution

1

Analyze Experiment 1 for signs of chemical vs. physical change.

Solid carbon dioxide (dry ice) in water undergoes sublimation (solid to gas).

Bubbling occurs as carbon dioxide transitions phases. No new substance is formed, meaning this is a physical change.

2

Analyze Experiment 2 for signs of chemical vs. physical change.

Calcium in water produces heat (exothermic), a precipitate (cloudy suspension), and gas bubbles.

The combination of heat generation and the formation of new distinct substances indicates a chemical change.

3

Determine the products of the chemical reaction in Experiment 2.

The reaction is

Ca + 2H_2O \rightarrow Ca(OH)_2 + H_2

.

Alkaline earth metals react with water in a single replacement reaction to form a metal hydroxide and hydrogen gas, not oxygen gas.

Key Concept

Distinguishing physical phase changes from chemical reactions and predicting products of metal-water reactions.

Estimated Time:1m 30s

Question 19Question →

During an outdoor camping trip, several activities take place around the campsite. Which of the following activities is an example of a chemical change?

Melting ice cubes inside an insulated cooler

Burning dry firewood in a campfire

Boiling river water in a metal pot

Crushing aluminum beverage cans for recycling

Show answer & explanation

Answer: Burning dry firewood in a campfire

Answer

Burning dry firewood in a campfire

Burning dry firewood is a combustion reaction where organic compounds in wood chemically react with oxygen gas to yield new chemical products, including carbon dioxide, water vapor, and ash, accompanied by the release of heat and light.

Step-by-Step Solution

1

Analyze each activity to determine whether new chemical substances are formed or only the physical state/shape changes.

Melting ice, boiling water, and crushing cans alter physical state or form without creating new molecules; burning wood reacts with oxygen to form new substances (ash,

\text{CO}_2

, and

\text{H}_2\text{O}

).

A chemical change is defined by the rearrangement of atoms to form one or more new chemical substances.

2

Select the activity that involves a chemical reaction.

Combustion of firewood is identified as the chemical change.

Combustion is an irreversible chemical reaction that releases energy and produces distinct products.

Key Concept

Chemical change versus physical change

Estimated Time:40s

Question 20Question →

Complete the statement by providing the correct numerical coefficient and reaction classification for the chemical equation shown.

Fill in the blanks below

In the industrial synthesis of ammonia, the balanced chemical equation is $\text{N}_2 + \text{H}_2 \rightarrow 2\text{NH}_3$, and because multiple reactants combine into one compound, it is classified as a reaction.

Show answer & explanation

Answer

The missing coefficient is 3 (or three) and the reaction type is synthesis (or combination).

To balance the equation according to the law of conservation of mass, the 6 hydrogen atoms in

2\text{NH}_3

require a coefficient of 3 for diatomic

\text{H}_2

(

3 \times 2 = 6

). A chemical process where two or more substances combine into a single compound is classified as a synthesis (or combination) reaction.

Step-by-Step Solution

1

Count the total number of nitrogen and hydrogen atoms in the product molecules.

Two ammonia molecules (

2\text{NH}_3

) contain

2 \times 1 = 2

nitrogen atoms and

2 \times 3 = 6

hydrogen atoms.

The law of conservation of mass dictates that the number of atoms of each element on the reactant side must equal the number of atoms on the product side.

2

Determine the coefficient needed for diatomic hydrogen gas (

\text{H}_2

) on the reactant side.

Dividing the required 6 hydrogen atoms by the 2 atoms per

\text{H}_2

molecule yields a coefficient of 3.

Three molecules of

\text{H}_2

supply exactly 6 hydrogen atoms (

3 \times 2 = 6

), completely balancing the equation.

3

Classify the reaction based on the relationship between reactants and products.

Two distinct elemental reactants (

\text{N}_2

and

\text{H}_2

) combine chemically to produce a single product compound (

\text{NH}_3

).

Reactions that follow the general form

A + B \rightarrow AB

are defined as synthesis or combination reactions.

Key Concept

Balancing chemical equations via conservation of mass and identifying synthesis reactions

Estimated Time:1m 0s

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