Chemical Kinetics Flashcards - Rate Laws, Integrated Rate Laws, and Catalysis

Flashcards cover rate concepts, rate laws, integrated rate laws, half-life, collision theory, Arrhenius, energy diagrams, catalysts, enzymes, mechanisms, and experimental determination of rates.

What is the rate of a reaction?

The rate is the quantity that changes per unit time; it is written as a negative change for reactants and a positive change for products.

How are rate expressions written for a general reaction aA + bB → cC + dD?

Rate = −(1/a) d[A]/dt = −(1/b) d[B]/dt = (1/c) d[C]/dt = (1/d) d[D]/dt.

What is the rate law of a reaction and how is it written?

Rate law expresses the rate as rate = k[A]^m[B]^n, where m and n are experimental orders and k is the rate constant.

Are reaction orders equal to stoichiometric coefficients?

No. Orders are determined experimentally and need not match the coefficients in the balanced equation.

What is the Arrhenius equation and what does it describe?

k = A e^(−Ea/RT); in linear form, ln k = ln A − Ea/(RT). It describes how the rate constant depends on temperature.

What is activation energy (Ea)?

Ea is the minimum energy that colliding molecules must have to react; it is the energy barrier that must be overcome.

What is collision theory?

A reaction requires collisions with sufficient energy and proper orientation; the collision frequency factor influences the rate.

What does the 'A' factor in the Arrhenius equation represent?

A is the collision frequency factor, reflecting how often reactant molecules collide in the correct orientation.

What is a catalyst and how does it affect the rate?

A catalyst provides an alternative pathway with a lower activation energy, increasing k and speeding up the reaction without being consumed.

What is the difference between homogeneous and heterogeneous catalysts?

Homogeneous catalysts are in the same phase as reactants; heterogeneous catalysts are in a different phase (e.g., solid catalyst with gases/liquids).

What is an enzyme in kinetics?

Enzymes are biological catalysts that speed up reactions; they bind substrates at an active site to orient them for reaction.

What is a rate-determining step?

The slowest elementary step in a multi-step mechanism, which limits the overall reaction rate.

What is the Method of Initial Rates?

A technique to determine reaction orders by comparing initial rates from experiments with different initial concentrations.

How does doubling a reactant concentration affect the rate for zero-, first-, and second-order reactions (assuming other concentrations constant)?

Zero order: no change; First order: rate doubles; Second order: rate quadruples.

What is the integrated rate law for a first-order reaction?

ln[A]t = −kt + ln[A]0; half-life t1/2 = ln 2 / k.

What is the integrated rate law for a zero-order reaction?

[A]t = [A]0 − kt; the plot of [A] vs t is a straight line with slope −k.

What is the integrated rate law for a second-order reaction with a single reactant?

1/[A]t = kt + 1/[A]0; the plot of 1/[A]t vs t is a straight line with slope k.

How is half-life related to reaction order?

t1/2 depends on order: first order t1/2 = ln2/k; second order t1/2 = 1/(k[A]0); zero order t1/2 = [A]0/(2k).

What are the common units for the rate constant k for 0th, 1st, and 2nd order reactions?

0th: M·s^−1; 1st: s^−1; 2nd: M^−1·s^−1.

What is the difference between a rate expression and a rate law?

Rate expression relates rates of appearance/disappearance via stoichiometry (e.g., −d[A]/dt, d[C]/dt). Rate law is the empirical expression rate = k[A]^m[B]^n with specific orders.

For the reaction NO + O3 with rate law rate = k[NO][O3], how would you solve for k given rate and concentrations?

k = rate / ([NO][O3]).

How does temperature influence the rate constant k according to the Arrhenius equation?

As temperature increases, k increases; the magnitude of the increase depends on Ea.

What is the purpose of an energy diagram in kinetics?

To visualize energy changes during a reaction, identify activation energy (Ea), transition states, and whether the process is exothermic or endothermic.

What is a transition state and activation energy barrier?

The transition state is the highest-energy arrangement along the reaction pathway; Ea is the energy difference between reactants and the transition state.

What is the difference between homogeneous and heterogeneous catalysts in terms of phase?

Homogeneous catalysts are in the same phase as reactants; heterogeneous catalysts are in a different phase (often solid with gases/liquids).

What is the lock-and-key model in enzyme kinetics?

Substrate binds to the enzyme's active site in a precise geometry, forming an enzyme-substrate complex that leads to product formation.

What is an elementary reaction and how does it relate to rate laws?

An elementary reaction is a single step in a mechanism; rates laws are derived from the rate-determining (slowest) elementary step.

What is the rate-dating method for multi-step mechanisms?

Identify the slowest elementary step (rate-determining step) to determine the overall rate law.

Why are rate laws determined experimentally?

Because reaction orders depend on the detailed mechanism and are not given by stoichiometry alone.

What is the graphical criterion for a first-order integrated rate law?

A plot of ln[A]t versus t yields a straight line with slope −k.

What is the graphical criterion for a second-order integrated rate law?

A plot of 1/[A]t versus t yields a straight line with slope k.

What is the practical use of the method of initial rates in determining k and orders?

It uses different starting concentrations to extract m, n, and k from rate data.

What does a catalyst do to the forward and reverse reactions?

A catalyst lowers the activation energy for the forward and reverse pathways, speeding both directions without changing equilibrium composition.

What is the difference between rate of reaction and rates of formation/consumption for individual species?

The rate of reaction is a single value; rates of formation/consumption are the time derivatives of each species’ concentration, related by stoichiometry.

How does a rate constant scale when you double the concentration of one reactant in a rate law with multiple reactants?

If rate = k[A]^m[B]^n, doubling [A] multiplies rate by 2^m; doubling [B] multiplies rate by 2^n (assuming other concentrations constant).