A student performed an experiment to study the reaction between magnesium metal and hydroc… — ACT

A student performed an experiment to study the reaction between magnesium metal and hydrochloric acid ( $HCl$ ). The student hypothesized that as the concentration of $HCl$ increases, the reaction time will decrease at a constant rate. The student measured the time required for a $1.0\text{ g}$ pellet of magnesium to completely dissolve in $50\text{ mL}$ of $HCl$ at $25^\circ\text{C}$ using different concentrations of the acid. The results are shown in the table below.

$HCl$ Concentration (M)	Reaction Time (s)
0.5	120
1.0	60
1.5	40
2.0	30

Do the results of the experiment support the student's hypothesis?

No; although the reaction time decreased as the concentration of $HCl$ increased, the rate of decrease was not constant.Answer
B
Yes; as the concentration of $HCl$ increased, the reaction time decreased by a constant $60\text{ s}$ for every $0.5\text{ M}$ increase.
C
Yes; the reaction time decreased by exactly $20\text{ s}$ for each $0.5\text{ M}$ increase in concentration.
D
No; as the concentration of $HCl$ increased, the reaction time increased rather than decreased.

Answer

No; although the reaction time decreased as the concentration of

HCl

increased, the rate of decrease was not constant.

The correct answer is correct because the student's hypothesis predicted that the reaction time would decrease at a constant rate. While the reaction time did decrease as the concentration of

HCl

increased, the rate of decrease was not constant: the time decreased by

60\text{ s}

(from

120\text{ s}

60\text{ s}

) when concentration increased from

0.5\text{ M}

1.0\text{ M}

, but only decreased by

20\text{ s}

(from

60\text{ s}

40\text{ s}

) when concentration increased from

1.0\text{ M}

1.5\text{ M}

. Because the rate of decrease varies, the hypothesis of a constant rate of decrease is not supported.

Step-by-Step Solution

Identify the student's hypothesis regarding the relationship between

HCl

concentration and reaction time.

The hypothesis states that as concentration increases, reaction time will decrease at a constant rate.

This establishes the criteria required to support the hypothesis.

Analyze the trend in the data table to determine if reaction time decreases as concentration increases.

As concentration increases from

0.5\text{ M}

2.0\text{ M}

, reaction time decreases from

120\text{ s}

30\text{ s}

This checks the first part of the hypothesis (decreasing trend).

Calculate the change in reaction time for equal increments of concentration to determine if the rate of decrease is constant.

For each

0.5\text{ M}

increase, the reaction time changes by

-60\text{ s}

, then

-20\text{ s}

, and finally

-10\text{ s}

. The rate of decrease is not constant.

This tests the second part of the hypothesis (constant rate) to draw the final conclusion.

Key Concept

Evaluating a hypothesis by comparing its predictions (specifically, a constant rate of change) against quantitative experimental results.

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