AP Intermediate 2nd Year Chemical Kinetics — practice questions — page 3

A catalyst is a substance that increases the rate of a chemical reaction by providing an alternative reaction mechanism with a lower activation energy ($E_a$). \nKey characteristics of a catalyst include: 1. **Kinetic Effect**: It lowers the energy barrier ($E_a$) for both the forward and reverse reactions, allowing the system to reach equilibrium faster. 2. **Conservation**: Although it may participate in intermediate steps, it is regenerated by the end of the reaction and is not consumed. 3. **Thermodynamic Neutrality**: It does not change the enthalpy ($\Delta H$), the Gibbs free energy ($\Delta G$), or the equilibrium constant ($K_{eq}$) of the reaction. It only affects the kinetics (speed). 4. **Product Identity**: The identity and amount of products at equilibrium remain unchanged. \nTherefore, the correct choice is that a catalyst lowers $E_a$ without being consumed.

1. First-order half-life: $t_{1/2} = 0.693/k$. 2. Plug in $k = 6.93\times 10^{-3}\,\text{s}^{-1}$. 3. $t_{1/2} = 0.693 / (6.93\times 10^{-3}) = 100\,\text{s}$. 4. Notice the clean cancellation: $0.693 / 6.93 = 0.1$, then $0.1 / 10^{-3} = 100$. 5. Distractor analysis: option B uses $k = 0.0139$ (twice as fast). Option A uses $k = 0.0693$. Option D uses $k = 3.47\times 10^{-3}$ (half as fast). _Source: syllabus Chemistry Part 1, Ch 3, §3.3.2 (Example calculations of first-order half-life), p. 13–14._

They agree only for an elementary reaction, which is the whole point of keeping them apart. Source: syllabus Chemistry Part I, Ch 3 Chemical Kinetics.