How Fast? The Rate of Chemical Change · Paper 1 — Free Chemistry HL Practice Questions

When a few drops of potassium iodide solution are added to a solution of hydrogen peroxide, the rate of decomposition according to $\text{2H}_2\text{O}_2\text{(aq)} \rightarrow \text{2H}_2\text{O(l)} + \text{O}_2\text{(g)}$ increases dramatically. Which statement best explains this observation in terms of collision theory?

For the reaction $\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightarrow 2\text{NH}_3\text{(g)}$, the activation energy for the forward reaction is $+335 \ \text{kJ mol}^{-1}$ and $\Delta H = -92 \ \text{kJ mol}^{-1}$. What is the activation energy for the reverse reaction?

The reaction between marble chips (calcium carbonate) and excess hydrochloric acid is first order with respect to HCl. A student monitors the volume of $\text{CO}_2$ produced over time and determines the initial rate from the gradient of the curve at $t = 0$. The experiment is repeated with the concentration of HCl doubled, all other conditions remaining constant. What is the expected ratio of the new initial rate to the original initial rate?

In an experiment, the time for a sulfur precipitate to obscure a cross is measured for the reaction between sodium thiosulfate solution and dilute hydrochloric acid at two temperatures. At $20\ ^\circ\text{C}$ the time is $120\ \text{s}$, and at $30\ ^\circ\text{C}$ the time is $60\ \text{s}$. Using the Arrhenius equation with $R = 8.31\ \text{J K}^{-1}\text{mol}^{-1}$, what is the approximate activation energy for this reaction?