In a study of reaction kinetics, a student investigates the reaction between sodium thiosu… — ACT

In a study of reaction kinetics, a student investigates the reaction between sodium thiosulfate ( $Na_2S_2O_3$ ) and hydrochloric acid ( $HCl$ ). The reaction produces solid sulfur, which precipitates and clouds the solution.

Experiment 1
The student placed a beaker containing $50\text{ mL}$ of $0.1\text{ M}$ $Na_2S_2O_3$ over a sheet of paper marked with a black cross. The student then added $10\text{ mL}$ of $1.0\text{ M}$ $HCl$ to the beaker, immediately started a stopwatch, and recorded the time (in seconds) required for the liquid to become so opaque that the black cross was no longer visible. The reaction was conducted at $25^\circ\text{C}$ . The student repeated this procedure at temperatures of $35^\circ\text{C}$ , $45^\circ\text{C}$ , and $55^\circ\text{C}$ , keeping the volumes and concentrations of the reactants constant.

Suppose the student wants to design a follow-up experiment to determine how the concentration of $HCl$ affects the reaction rate, while ensuring that temperature does not act as a confounding variable. Which of the following modifications to the procedure of Experiment 1 would best achieve this goal?

A
Conduct multiple trials using different concentrations of $HCl$ , and increase the reaction temperature by $10^\circ\text{C}$ for each higher concentration tested.
B
Conduct multiple trials at $25^\circ\text{C}$ keeping the $HCl$ concentration at $1.0\text{ M}$ while varying the concentration of the $Na_2S_2O_3$ solution.
Conduct multiple trials at a constant temperature of $25^\circ\text{C}$ using $50\text{ mL}$ of $0.1\text{ M}$ $Na_2S_2O_3$ , while varying the concentration of the $10\text{ mL}$ of $HCl$ added.Answer
D
Conduct multiple trials at a constant temperature of $25^\circ\text{C}$ using $1.0\text{ M}$ $HCl$ and $0.1\text{ M}$ $Na_2S_2O_3$ , but vary the total volume of the reaction mixture.

Answer

Conduct multiple trials at a constant temperature of

25^\circ\text{C}

using

50\text{ mL}

0.1\text{ M}

Na_2S_2O_3

, while varying the concentration of the

10\text{ mL}

HCl

added.

To investigate the effect of hydrochloric acid concentration on the reaction rate, the concentration of hydrochloric acid must be systematically varied as the independent variable. To prevent confounding variables, all other parameters—including the temperature, the volume of both reactants, and the concentration of the other reactant (sodium thiosulfate)—must be kept constant. Conducting trials at a constant temperature of

25^\circ\text{C}

while keeping the volume and concentration of sodium thiosulfate constant and varying only the concentration of the added hydrochloric acid achieves this design.

Step-by-Step Solution

Identify the target independent variable for the follow-up study.

The target independent variable is the concentration of hydrochloric acid (

HCl

The student wants to determine how the concentration of

HCl

affects the reaction rate.

Determine the control variables that must remain constant.

The reaction temperature must remain constant (e.g., at

25^\circ\text{C}

), and the volume and concentration of sodium thiosulfate (

Na_2S_2O_3

), as well as the volume of the

HCl

solution, must remain constant.

Controlling these factors prevents them from acting as confounding variables, ensuring that any variation in reaction time is due solely to the changes in

HCl

concentration.

Evaluate the proposed modifications and select the one that isolates the independent variable.

The only procedure that varies only the concentration of the

HCl

solution while holding the temperature and the properties of the sodium thiosulfate reactant constant is the correct choice.

This isolates the

HCl

concentration as the single variable under test.

Key Concept

Isolating a single independent variable while controlling other variables to study its specific effect in a follow-up experiment.

Estimated Time:1m 30s

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Answer

Step-by-Step Solution

Key Concept

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