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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.
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(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

Consider the reaction \(\mathrm{A}+\mathrm{B} \longrightarrow \mathrm{C}+\mathrm{D} .\) Is each of the following statements true or false? (a) The rate law for the reaction must be Rate \(=k[\mathrm{A}][\mathrm{B}] .\) (b) If the reaction is an elementary reaction, the rate law is second order. (c) If the reaction is an elementary reaction, the rate law of the reverse reaction is first order. (d) The activation energy for the reverse reaction must be greater than that for the forward reaction.

Many primary amines, RNH \(_{2},\) where \(R\) is a carbon-containing fragment such as \(C H_{3}, C H_{3} C H_{2},\) and so on, undergo reactions where the transition state is tetrahedral. (a) Draw a hybrid orbital picture to visualize the bonding at the nitrogen in a primary amine (just use a \(C\) atom for \(^{4} \mathrm{R}^{\prime \prime}\) . (b) What kind of reactant with a primary amine can produce a tetrahedral intermediate?

Hydrogen sulfide \(\left(\mathrm{H}_{2} \mathrm{S}\right)\) is a common and troublesome pollutant in industrial wastewaters. One way to remove \(\mathrm{H}_{2} \mathrm{S}\) is to treat the water with chlorine, in which case the following reaction occurs: $$ \mathrm{H}_{2} \mathrm{S}(a q)+\mathrm{Cl}_{2}(a q) \longrightarrow \mathrm{S}(s)+2 \mathrm{H}^{+}(a q)+2 \mathrm{Cl}^{-}(a q)$$ The rate of this reaction is first order in each reactant. The rate constant for the disappearance of \(\mathrm{H}_{2} \mathrm{S}\) at \(28^{\circ} \mathrm{C}\) is \(3.5 \times 10^{-2} \mathrm{M}^{-1} \mathrm{s}^{-1}\) . If at a given time the concentration of \(\mathrm{H}_{2} \mathrm{S}\) is \(2.0 \times 10^{-4} \mathrm{M}\) and that of \(\mathrm{Cl}_{2}\) is \(0.025 \mathrm{M},\) what is the rate of formation of \(\mathrm{Cl}^{-} ?\)

You have studied the gas-phase oxidation of HBr by \(\mathrm{O}_{2}\) : $$4 \mathrm{HBr}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(g)+2 \mathrm{Br}_{2}(g)$$ You find the reaction to be first order with respect to HBr and first order with respect to \(\mathrm{O}_{2}\) . You propose the following mechanism: $$ \begin{array}{c}{\operatorname{HBr}(g)+\mathrm{O}_{2}(g) \longrightarrow \operatorname{HOOBr}(g)} \\ {\operatorname{HOOBr}(g)+\operatorname{HBr}(g) \longrightarrow 2 \mathrm{HOBr}(g)} \\\ {\operatorname{HOBr}(g)+\operatorname{HBr}(g) \longrightarrow \mathrm{H}_{2} \mathrm{O}(g)+\mathrm{Br}_{2}(g)}\end{array}$$ (a) Confirm that the elementary reactions add to give the overall reaction. (b) Based on the experimentally determined rate law, which step is rate determining? (c) What are the intermediates in this mechanism? (d) If you are unable to detect HOBr or HOOBr among the products, does this disprove your mechanism?

(a) Can an intermediate appear as a reactant in the first step of a reaction mechanism? (b) On a reaction energy profile diagram, is an intermediate represented as a peak or a valley? (c) If a molecule like \(C l_{2}\) falls apart in an elementary reaction, what is the molecularity of the reaction?
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