Rate Laws
What is the general form of a rate law for a reaction?
Rate = k[Reactant₁]ⁿ[Reactant₂]ᵐ, where k is the rate constant and n, m are reaction orders.
What is another name for the slowest step in a reaction mechanism?
Rate-determining step
What assumption does the steady-state approximation make about intermediates?
Their concentration remains relatively constant throughout the reaction.
What is activation energy, and how is it shown on a reaction energy profile diagram?
Activation energy (Ea) is the energy required for reactants to reach the transition state. It is the peak height in the diagram.
What is the primary function of a catalyst in a reaction?
Lowers activation energy to speed up the reaction.
Explain why the rate law must be determined experimentally and not from the balanced equation.
Reaction orders do not always match the coefficients in the balanced equation. Rate laws depend on the mechanism of the reaction, which must be determined experimentally.
Which step is the slow step in the following mechanism?
Step 1: NO₂ + NO₂ → NO₃ + NO (slow)
Step 2: NO₃ + CO → NO₂ + CO₂ (fast)
Step 1 (since it is explicitly labeled as slow).
Explain why the steady-state approximation is useful when the slow step is not the first step.
It allows us to express the concentration of intermediates in terms of reactants.
Sketch a reaction energy diagram for a reaction with two steps where the first step is fast and the second step is slow.
First hill is small, second hill is large (higher Ea for slow step).
Compare and contrast homogeneous and heterogeneous catalysts,.
Given the reaction 2A + B → C, the rate law is Rate = k[A]²[B]. What is the reaction order with respect to each reactant and overall?
Order with respect to A = 2, B = 1, and overall order = 3 (2 + 1 = 3).
Why must the slow step determine the overall rate law?
The reaction cannot proceed faster than the slowest step, so the rate law is derived from that step.
Using the mechanism below, derive the overall rate law using the steady-state approximation.
Step 1: A ⇌ B (fast, reversible)
Step 2: B + C → D (slow)
What do valleys between peaks on a reaction profile diagram represent?
Intermediates
How do enzymes function as catalysts?
Bind to substrates at the active site, stabilizing the transition state.
If the rate law for a reaction is Rate = k[X][Y]², what would be a possible two-step mechanism that supports this rate law?
Explain how an energy profile diagram can help determine the rate-determining step.
Explain how an energy profile diagram can help determine the rate-determining step.
How does the steady-state approximation explain the experimental rate law Rate = k[O₃]² / [O₂] for ozone decomposition?
It allows us to express the intermediate [O] in terms of reactants, leading to the given rate law.
How would the addition of a catalyst change the reaction profile diagram?
Lowers Ea, creating a new pathway with a smaller peak.
Why is platinum used in catalytic converters?
It provides a large surface area for reactions and lowers activation energy for NO and CO breakdown.
A reaction mechanism consists of the following steps:
Step 1: A + B ⇌ C (fast, reversible)
Step 2: C + D → E (slow)
Given a three-step mechanism, describe a scenario where the slow step is not the first step. How does this affect the rate law?
If the first step is fast and reversible, an intermediate builds up and must be substituted into the rate law using the steady-state approximation.
Describe a scenario where an intermediate appears in the rate law but must be replaced using steady-state conditions.
If the slow step includes an intermediate (e.g., B), it must be substituted using equilibrium expressions from previous steps.
A reaction is exothermic overall. If the reaction occurs in two steps and the second step is endothermic, explain how this would appear on an energy profile diagram.
The second peak is higher than the first, but the final energy level is lower than the initial reactants (since the reaction is exothermic overall).
Explain why a catalyst appears in the rate law but not in the overall reaction equation.
A catalyst is used in one step and regenerated in a later step, meaning it is not a reactant or product.