Kinetics

Question 1

Free Energy Change of the Reaction (ΔGrxn) and Activation Energy (Eₐ)

Answer 1

• Difference between the free energy of the products and the free energy of the reactants is ΔG(rxn).
• -ΔG(rxn) = Exergonic (energy released) and Spontaneous.
• +ΔG(rxn) = Endergonic (energy absorbed) and Nonspontaneous.
• Activation Energy is energy required to reach Transition State.
• Difference in free energy between the transition state and the reactants is the activation energy of the forward reaction.
• Difference in free energy between the transition state and the products is the activation energy of the reverse reaction.

Question 2

Mechanisms

Answer 2

• Intermediate may be found in reaction mechanisms.
• Rate-Limiting Step is slowest step.
• Rate Constant (k) increases as Temperature (T) increases and as Activation Energy (Eₐ) decreases, according to Arrhenius Equation.

Question 3

Factors Affecting Reaction Rate

Answer 3

• Reaction rate increases as Reactant Concentrations increase for all but zero-order reactions, because the number of effective collisions per unit time, and thus the Frequency Factor (A), increases.
• Reaction rate increases as Temperature increases, because the average kinetic energy, and thus the proportion of reactants gaining enough energy to surpass Eₐ, increases.
• Reaction rate may depend on aqueous or nonaqueous environment and the physical state (solid, liquid, gas) of medium.
• Reaction rate increases via action of Catalysts, which reduce Eₐ for reaction to proceed.

Question 4

Reaction Rates

Answer 4

• General Rxn: aA + bB -> cC + dD.
• rate = -Δ[A]/(aΔt) = -Δ[B]/(bΔt) = +Δ[C]/(cΔt) = +Δ[D]/(dΔt), in units of M/s.
• Rate Law: r = k[A]ˣ[B]ʸ.
• Order of Reaction = x + y.
• Rate laws only involve reactants, never products.
• K(eq) for reversible reaction is rate constant for forward reaction divided by rate constant for reverse reaction (Keq = k/k₋₁).

Question 5

Zero-Order Reaction

Answer 5

• r = k[A]⁰[B]⁰ = k, where k has units M/s.
• In zero-order reactions, k can only be changed by increasing temperature or adding catalysts.
• On Conc vs Time graph, k = -slope.

Question 6

First-Order Reaction

Answer 6

• r = k[A]¹, where k has units s⁻¹.
• Radioactive decay is example of first-order reaction. r = -Δ[A]/(aΔt) = k[A].
• On ln[A] vs Time graph, k = -slope.

Question 7

Second-Order Reaction

Answer 7

• r = k[A]¹[B]¹ or r = k[A]², where k has units M⁻¹s⁻¹.
• Second-order reaction often suggests physical collision between reactants.
• On 1/[A] vs Time graph, k = slope.

Question 8

Broken-Order Reactions and Mixed-Order Reactions

Answer 8

• Broken-Order Reactions have non-integer orders (fractions).
• Mixed-Order Reactions have orders that vary over course of reaction; reaction appears first-order at the beginning and second-order at the end.