ACS CP Thermodynamics & Chemical Kinetics 2 — Questions and Answers
Question 1: For a reaction with rate law r = k[A]²[B], what is the overall reaction order?
- First order
- Second order
- Third order (Correct answer)
- Sixth order
Correct answer: Third order
The overall reaction order is the sum of all exponents in the rate law: 2 + 1 = 3 (third order overall).
Question 2: The Arrhenius equation k = A·e^(-Ea/RT) predicts that increasing temperature will:
- Decrease k while increasing Ea
- Increase k while leaving Ea unchanged (Correct answer)
- Decrease both k and Ea
- Increase both k and Ea
Correct answer: Increase k while leaving Ea unchanged
Increasing T raises k via the exponential factor e^(-Ea/RT), while activation energy Ea is an intrinsic molecular property unaffected by T.
Question 3: For a first-order reaction A → products with rate constant k, the half-life (t₁/₂) is given by:
- t₁/₂ = k/ln(2)
- t₁/₂ = ln(2)/k (Correct answer)
- t₁/₂ = 1/(2k[A]₀)
- t₁/₂ = 2/k[A]₀
Correct answer: t₁/₂ = ln(2)/k
For a first-order reaction, t₁/₂ = ln(2)/k = 0.693/k, which is independent of initial concentration.
Question 4: Which experimental technique determines the order of a reaction with respect to individual reactants?
- Van't Hoff equation analysis
- Method of initial rates (Correct answer)
- Arrhenius plot analysis
- Integrated rate law fitting only
Correct answer: Method of initial rates
The method of initial rates involves varying one reactant concentration while holding others constant, then measuring the initial rate to determine individual orders.
Question 5: A homogeneous catalyst increases reaction rate primarily by:
- Raising the temperature of the reaction mixture
- Shifting the equilibrium position toward products
- Providing an alternative pathway with lower activation energy (Correct answer)
- Increasing the concentrations of reactants
Correct answer: Providing an alternative pathway with lower activation energy
A catalyst provides an alternative mechanism with a lower activation energy barrier, increasing k without being consumed in the overall reaction.
Question 6: For a zero-order reaction, plotting [A] versus time yields:
- An exponential decay curve
- A straight line with slope = -k (Correct answer)
- A straight line with slope = +k
- A curve with increasing slope
Correct answer: A straight line with slope = -k
For a zero-order reaction, [A] = [A]₀ - kt, producing a straight line with slope -k when [A] is plotted against time.
Question 7: In the Arrhenius equation, the pre-exponential factor A (frequency factor) physically represents:
- The activation energy normalized by the gas constant
- The maximum reaction rate at infinitely high temperature
- The collision frequency weighted by the steric factor for correct orientation (Correct answer)
- The equilibrium constant at standard conditions
Correct answer: The collision frequency weighted by the steric factor for correct orientation
The pre-exponential factor A accounts for the collision frequency and the probability that colliding molecules have the correct geometric orientation to react.
ACS CP Thermodynamics & Chemical Kinetics 2
For a reaction with rate law r = k[A]²[B], what is the overall reaction order?