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General Chemistry Mock Exam Study Guidance

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Q1. Consider the energy diagram given to the right. Which of the following statements is FALSE?

Background

Topic: Reaction Mechanisms & Energy Diagrams

This question tests your understanding of reaction coordinate diagrams, including the identification of transition states, activation energy, and the characteristics of multi-step reactions.

Key Terms and Concepts:

  • Transition State: The highest energy point along the reaction path.

  • Activation Energy (Ea): The energy barrier that must be overcome for a reaction step to proceed.

  • Endothermic/Exothermic Step: Endothermic steps absorb energy; exothermic steps release energy.

  • Rate-Determining Step: The slowest step in a reaction mechanism, which controls the overall rate.

Step-by-Step Guidance

  1. Examine the energy diagram and count the number of peaks (transition states) and valleys (intermediates).

  2. Identify which step has the highest activation energy by comparing the energy difference between reactants/intermediates and the transition states.

  3. Determine which step is the slowest (rate-determining) by finding the step with the highest activation energy.

  4. Assess whether each step is endothermic or exothermic by comparing the energy of reactants and products for each step.

Try solving on your own before revealing the answer!

Q2. Below are elementary steps and the overall equation of a chemical reaction. Determine the rate law of the overall reaction.

Background

Topic: Reaction Mechanisms & Rate Laws

This question tests your ability to derive the rate law for a reaction based on its mechanism, identifying the slow (rate-determining) step and expressing the rate law in terms of reactant concentrations.

Key Terms and Formulas:

  • Rate Law: An equation that relates the reaction rate to the concentrations of reactants.

  • Elementary Step: A single step in a reaction mechanism; its molecularity determines the rate law for that step.

  • Rate-Determining Step: The slowest step, which controls the overall rate law.

Step-by-Step Guidance

  1. Identify the slow (rate-determining) step in the mechanism.

  2. Write the rate law for the slow step using the concentrations of the species involved in that step.

  3. If the slow step involves an intermediate, use the fast equilibrium step(s) to express the intermediate in terms of reactants.

  4. Combine the expressions to write the overall rate law in terms of the initial reactants only.

Try solving on your own before revealing the answer!

Q3. Consider the reaction 4 CH3I(aq) + NH3 (aq) ⇌ (CH3)4NI(aq) + 3 HI(aq), where the initial concentration of CH3I is 1.50 M and NH3 is 2.50 M. If instantaneous rate of change of HI at 5.0 minutes is 3.0 M/min, then what is the rate of change in [CH3I] at 5.0 minutes?

Background

Topic: Reaction Stoichiometry & Rates

This question tests your ability to relate the rates of change of reactants and products using stoichiometric coefficients from the balanced equation.

Key Terms and Formulas:

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

  • Stoichiometry: The relationship between the coefficients in a balanced equation and the rates of consumption/formation.

Step-by-Step Guidance

  1. Write the rate expressions for both HI and CH3I using their stoichiometric coefficients.

  2. Set up the relationship between the rate of disappearance of CH3I and the rate of appearance of HI using the coefficients from the balanced equation.

  3. Substitute the given rate of HI into your expression to solve for the rate of CH3I.

Try solving on your own before revealing the answer!

Q4. The reaction 2A(aq) + 2B(aq) → 4C(g) + 5D(aq) has a reaction rate that is first order to A and second order to reactant B. What is the overall reaction order?

Background

Topic: Reaction Order

This question tests your understanding of how to determine the overall order of a reaction from the individual orders with respect to each reactant.

Key Terms and Formulas:

  • Order with respect to a reactant: The exponent of the concentration term for that reactant in the rate law.

  • Overall Reaction Order: The sum of the orders with respect to all reactants.

Step-by-Step Guidance

  1. Write the rate law based on the given orders:

  2. Add the exponents (orders) for all reactants to find the overall reaction order.

Try solving on your own before revealing the answer!

Q5. Which of the following statements are true about the rate of the reaction?

Background

Topic: Factors Affecting Reaction Rate

This question tests your understanding of how activation energy and temperature affect the rate of a chemical reaction.

Key Terms and Concepts:

  • Activation Energy (Ea): The minimum energy required for a reaction to occur.

  • Temperature: Increasing temperature generally increases reaction rate.

Step-by-Step Guidance

  1. Recall how changes in activation energy affect reaction rate (higher Ea = slower rate, lower Ea = faster rate).

  2. Recall how temperature changes affect reaction rate (higher temperature = faster rate).

  3. Evaluate each statement (I–IV) based on these principles and identify which are true.

Try solving on your own before revealing the answer!

Periodic table, constants, and equations reference sheet

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