Practice Exam Guidance: Chemical Kinetics & Equilibrium (C117)

Q1. Which equation best represents the reaction given the rates of production and consumption?

Background

Topic: Reaction Stoichiometry & Rate Relationships

This question tests your understanding of how the rates of appearance and disappearance of reactants and products relate to the stoichiometry of a chemical equation.

Key Terms and Formulas:

• Rate of reaction: The change in concentration of a reactant or product per unit time.

• Stoichiometry: The coefficients in a balanced equation show the relative rates of consumption and production.

• General relationship: $\frac{1}{a}\frac{d[A]}{dt} = \frac{1}{b}\frac{d[B]}{dt} = \frac{1}{c}\frac{d[C]}{dt}$ for $aA + bB \rightarrow cC$

Step-by-Step Guidance

1. Assign the given rates to the appropriate species: A (produced at 4.0 M/s), B (consumed at 2.0 M/s), C (produced at 1.0 M/s).

2. Write the general rate relationship using stoichiometric coefficients: $\frac{1}{a}\frac{d[A]}{dt} = -\frac{1}{b}\frac{d[B]}{dt} = \frac{1}{c}\frac{d[C]}{dt}$.

3. Set up ratios using the given rates and compare them to the possible coefficients in the answer choices.

4. Check which equation matches the observed rates by plugging in the coefficients and seeing if the ratios are consistent with the data.

Try solving on your own before revealing the answer!

Q2. What is the rate of disappearance of C2H6O given the rate of appearance of Cr2(SO4)3?

Background

Topic: Stoichiometry and Reaction Rates

This question tests your ability to relate the rates of disappearance and appearance of reactants and products using the balanced chemical equation.

Key Terms and Formulas:

• Balanced equation: $2 \text{K}_2\text{Cr}_2\text{O}_7 + 8 \text{H}_2\text{SO}_4 + 3 \text{C}_2\text{H}_6\text{O} \rightarrow 2 \text{Cr}_2(\text{SO}_4)_3 + 2 \text{K}_2\text{SO}_4 + 11 \text{H}_2\text{O}$

• Rate relationships: $\frac{1}{3}\left|\frac{d[\text{C}_2\text{H}_6\text{O}]}{dt}\right| = \frac{1}{2}\frac{d[\text{Cr}_2(\text{SO}_4)_3]}{dt}$

Step-by-Step Guidance

1. Identify the coefficients for C2H6O and Cr2(SO4)3 in the balanced equation (3 and 2, respectively).

2. Write the rate relationship: $\frac{1}{3}\left|\frac{d[\text{C}_2\text{H}_6\text{O}]}{dt}\right| = \frac{1}{2}\frac{d[\text{Cr}_2(\text{SO}_4)_3]}{dt}$.

3. Plug in the given rate for Cr2(SO4)3 (1.24 M/min) and solve for the rate of disappearance of C2H6O.

4. Set up the equation to isolate $\left|\frac{d[\text{C}_2\text{H}_6\text{O}]}{dt}\right|$ but do not calculate the final value yet.

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Q3. Which reaction does not require experimental data to determine the rate law?

Background

Topic: Rate Laws and Reaction Mechanisms

This question tests your understanding of when a rate law can be written directly from the reaction equation (elementary step) versus when it must be determined experimentally.

Key Terms and Concepts:

• Elementary reaction: A reaction that occurs in a single step; its rate law can be written directly from its molecularity.

• Overall reaction: The sum of elementary steps; its rate law is not always obvious and usually requires experimental data.

Step-by-Step Guidance

1. Identify which reactions are elementary steps (usually given as a single step with no intermediates).

2. Recall that only elementary reactions have rate laws that match their stoichiometry.

3. For each reaction, consider if it is likely to be elementary or if it is a complex/overall reaction.

4. Eliminate choices that require experimental determination of the rate law.

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Q4. Which statement is false about the decomposition of N2O5 with the given rate law?

Background

Topic: Rate Laws and Reaction Order

This question tests your understanding of how the rate law relates to reaction order, the effect of concentration changes, and the units of the rate constant.

Key Terms and Formulas:

• Second order reaction: Rate law is $\text{Rate} = k[N_2O_5]^2$

• Units for k (second order): $M^{-1} s^{-1}$

• Effect of doubling concentration: For second order, rate increases by a factor of 4.

Step-by-Step Guidance

1. Review each statement and compare it to what is true for a second order reaction.

2. Recall that the rate depends on $[N_2O_5]^2$ and not on the concentration of products.

3. Check the units for the rate constant for a second order reaction.

4. Consider how the rate changes if the concentration of N2O5 is doubled.

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Q5. What happens to the rate of a zeroth-order reaction if the concentration of A is tripled?

Background

Topic: Zeroth-Order Kinetics

This question tests your understanding of how the rate of a zeroth-order reaction depends on the concentration of the reactant.

Key Terms and Formulas:

• Zeroth-order rate law: $\text{Rate} = k$

• Effect of concentration: The rate is independent of the concentration of A.

Step-by-Step Guidance

1. Recall the definition of a zeroth-order reaction: the rate does not depend on the concentration of the reactant.

2. Consider what happens to the rate if [A] is increased, decreased, or otherwise changed.

3. Compare this to the behavior of first- and second-order reactions.

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