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Chapter 14: Problem 18

(a) What are the units usually used to express the rates of reactions occurring in solution? (b) As the temperature increases, does the reaction rate increase or decrease? (c) As a reaction proceeds, does the instantaneous reaction rate increase or decrease?

Short Answer

Expert verified
(a) Reaction rates in solution are usually expressed in units of moles per liter per unit time (\(\text{M} \cdot \text{s}^{-1}\) or \(\text{M} \cdot \text{min}^{-1}\)). (b) As the temperature increases, the reaction rate typically increases due to greater kinetic energy and more frequent, energetic collisions between particles. (c) As a reaction proceeds, the instantaneous reaction rate generally decreases because of decreasing reactant concentrations and a lower rate of collisions between reacting particles.

Step by step solution

01

(a) Units for reaction rates

For reactions occurring in solution, reaction rates are generally expressed in units of moles per liter per unit time (\(\text{M} \cdot \text{s}^{-1}\) or \(\text{M} \cdot \text{min}^{-1}\)), where M represents molarity or moles per liter.
02

(b) Effect of temperature on reaction rate

As the temperature of a reaction increases, the reaction rate typically increases. This is because as temperature increases, the kinetic energy of the reacting particles increases, leading to more frequent and energetic collisions between particles. According to the Arrhenius equation, the rate constant (k) increases with temperature, resulting in a higher reaction rate.
03

(c) Change in instantaneous reaction rate as a reaction proceeds

As a reaction proceeds, the instantaneous reaction rate usually decreases. This is because over time, the concentrations of the reactants decrease, leading to a lower rate of collisions between reacting particles. Consequently, the rate at which the reaction progresses slows down as it proceeds. Note that this general statement holds true for most, but not all, reactions; the specific behavior can vary depending on the reaction mechanism.

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Most popular questions from this chapter

The following mechanism has been proposed for the gasphase reaction of \(\mathrm{H}_{2}\) with ICl: $$\begin{array}{c}{\mathrm{H}_{2}(g)+\mathrm{ICl}(g) \longrightarrow \mathrm{HI}(g)+\mathrm{HCl}(g)} \\ {\mathrm{HI}(g)+\mathrm{ICl}(g) \longrightarrow \mathrm{I}_{2}(g)+\mathrm{HCl}(g)}\end{array}$$ \(\begin{array}{l}{\text { (a) Write the balanced equation for the overall reaction. }} \\ {\text { (b) Identify any intermediates in the mechanism. (c) If }}\end{array}\) the first step is slow and the second one is fast, which rate law do you expect to be observed for the overall reaction?

For each of the following gas-phase reactions, write the rate expression in terms of the appearance of each product and disappearance of each reactant: \(\begin{array}{l}{\text { (a) } 2 \mathrm{H}_{2} \mathrm{O}(g) \longrightarrow 2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g)} \\ {\text { (b) } 2 \mathrm{SO}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{SO}_{3}(g)} \\\ {\text { (c) } 2 \mathrm{NO}(g)+2 \mathrm{H}_{2}(g) \longrightarrow \mathrm{N}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(g)} \\ {\text { (d) } \mathrm{N}_{2}(g)+2 \mathrm{H}_{2}(g) \longrightarrow \mathrm{N}_{2} \mathrm{H}_{4}(g)}\end{array}\)

(a) The reaction \(\mathrm{H}_{2} \mathrm{O}_{2}(a q) \longrightarrow \mathrm{H}_{2} \mathrm{O}(l)+\frac{1}{2} \mathrm{O}_{2}(g)\) is first order. At 300 \(\mathrm{K}\) the rate constant equals \(7.0 \times 10^{-4} \mathrm{s}^{-1}\) . Calculate the half-life at this temperature. (b) If the activation energy for this reaction is \(75 \mathrm{kJ} / \mathrm{mol},\) at what temperature would the reaction rate be doubled?

Ozone in the upper atmosphere can be destroyed by the following two-step mechanism: $$ \begin{array}{c}{\mathrm{Cl}(g)+\mathrm{O}_{3}(g) \longrightarrow \mathrm{ClO}(g)+\mathrm{O}_{2}(g)} \\ {\mathrm{ClO}(g)+\mathrm{O}(g) \longrightarrow \mathrm{Cl}(g)+\mathrm{O}_{2}(g)}\end{array}$$ (a) What is the overall equation for this process? (b) What is the catalyst in the reaction? (c) What is the intermediate in the reaction?

Indicate whether each statement is true or false. \(\begin{array}{l}{\text { (a) If you measure the rate constant for a reaction at different}} \\ {\text { temperatures, you can calculate the overall }} \\ {\text { enthalpy change for the reaction. }} \\ {\text { (b) Exothermic reactions are faster than endothermic }} \\ {\text { reactions. }} \\ {\text { (c) If you double the temperature for a reaction, you cut }} \\ {\text { the activation energy in half. }}\end{array}\)
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