To investigate the factors affecting the rate of a chemical reaction, researchers conducte… — ACT

To investigate the factors affecting the rate of a chemical reaction, researchers conducted two experiments measuring the rate of decomposition of nitrogen dioxide ( $NO_2$ ) into nitrogen monoxide ( $NO$ ) and oxygen ( $O_2$ ):

2NO_2(g) \rightarrow 2NO(g) + O_2(g)

Experiment 1
Researchers introduced $1.0\text{ mol}$ of $NO_2$ gas into four separate rigid 10-liter containers at different temperatures. No other gases were initially present. The reaction rate was measured at the start of the reaction (initial rate). The results are shown in Table 1.

Table 1

Container	Temperature ( $^\circ\text{C}$ )	Initial Rate ( $\text{mol}/(\text{L}\cdot\text{s})$ )
1	25	$1.2 \times 10^{-5}$
2	100	$4.8 \times 10^{-5}$
3	200	$1.9 \times 10^{-4}$
4	300	$7.6 \times 10^{-4}$

Experiment 2
Using Container 1 ( $25^\circ\text{C}$ ), researchers repeated the reaction but added different amounts of helium ( $He$ ), an inert gas that does not participate in the reaction, to test whether the total pressure of the container affects the reaction rate. The initial concentration of $NO_2$ was kept at $0.10\text{ mol/L}$ ( $1.0\text{ mol}$ in the 10-liter container) in all trials. The results are shown in Table 2.

Table 2

Trial	Amount of $He$ added (mol)	Total Initial Pressure (atm)	Initial Rate ( $\text{mol}/(\text{L}\cdot\text{s})$ )
5	0.5	3.6	$1.2 \times 10^{-5}$
6	1.0	4.8	$1.2 \times 10^{-5}$
7	2.0	7.2	$1.2 \times 10^{-5}$

Based on the results of Experiments 1 and 2, which of the following setups serves as the control group to determine the effect of adding helium gas on the initial reaction rate in Experiment 2?

Container 1, because it represents the reaction under the same conditions as Experiment 2 but with no helium gas added.Answer
B
Container 4, because it represents the reaction at the highest temperature, which maximizes the initial reaction rate.
C
Trial 5, because it has the lowest amount of helium added among the trials in Experiment 2.
D
Trial 7, because it has the highest initial pressure, establishing the upper limit for the pressure variable.

Answer

Container 1, because it represents the reaction under the same conditions as Experiment 2 but with no helium gas added.

The correct answer is the option specifying Container 1. In Experiment 2, researchers are testing the effect of adding helium gas on the initial reaction rate at a constant temperature of

25^\circ\text{C}

and initial

NO_2

concentration of

0.10\text{ mol/L}

. To determine if helium has any effect, they must compare these trials to a baseline setup where no helium was added. Container 1 in Experiment 1 provides this baseline, as it was conducted at

25^\circ\text{C}

with the same initial

NO_2

concentration but without any helium gas.

Step-by-Step Solution

Identify the independent variable in Experiment 2.

The independent variable is the amount of helium gas (

He

) added to the reaction mixture, which in turn changes the total initial pressure.

To determine the control group, we must first establish what variable is being manipulated in the experiment.

Determine the baseline condition for the independent variable.

The baseline condition is the reaction rate when zero helium (

0.0\text{ mol}

) is added, while keeping all other controlled variables constant.

A control group or baseline condition represents the state of the system before the independent variable is manipulated.

Locate the experimental setup that matches the baseline condition.

In Table 2, all trials (5, 6, and 7) have helium added. Looking back at Experiment 1, Container 1 represents the reaction at

25^\circ\text{C}

with

1.0\text{ mol}

NO_2

in a 10-L container (concentration of

0.10\text{ mol/L}

) and no other gases (helium =

0.0\text{ mol}

Comparing the reaction rates in Experiment 2 to Container 1 allows researchers to isolate and measure the specific effect of adding helium on the reaction rate.

Key Concept

Determining Control Groups and Baseline Conditions

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic