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Chapter 14: Problem 81

(a) What is a catalyst? (b) What is the difference between a homogeneous and a heterogeneous catalyst? (c) Do catalysts affect the overall enthalpy change for a reaction, the activation energy, or both?

Short Answer

Expert verified
(a) A catalyst is a substance that increases the rate of a chemical reaction without being consumed, by lowering the activation energy required for the reaction to occur. (b) A homogeneous catalyst is in the same phase as the reactants, while a heterogeneous catalyst is in a different phase. The key difference is their phase relative to the reactants. (c) Catalysts do not affect the overall enthalpy change for a reaction but do affect the activation energy, providing an alternative reaction pathway with a lower activation energy, thus increasing the reaction rate.

Step by step solution

01

(a) Definition of a Catalyst

A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. It does this by lowering the activation energy, which is the energy required for the reaction to occur.
02

(b) Homogeneous vs. Heterogeneous Catalysts

A homogeneous catalyst is a catalyst that is present in the same phase (solid, liquid, or gas) as the reactants in a chemical reaction. In contrast, a heterogeneous catalyst is a catalyst in a different phase than the reactants. For example, a solid catalyst in a reaction involving liquid reactants is a heterogeneous catalyst. The key difference between the two types of catalysts is their phase relative to the reactants.
03

(c) Enthalpy Change and Activation Energy

Catalysts do not affect the overall enthalpy change for a reaction, which is the difference in energy between the reactants and the products. The enthalpy change is a property of the reaction itself and is not impacted by the presence or absence of a catalyst. However, catalysts do affect the activation energy required for a reaction to take place. By providing an alternative reaction pathway with a lower activation energy, a catalyst allows the reaction to proceed more quickly, increasing the reaction rate.

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Most popular questions from this chapter

Indicate whether each statement is true or false. If it is false, rewrite it so that it is true. (a) If you compare two reactions with similar collision factors, the one with the larger activation energy will be faster. (b) A reaction that has a small rate constant must have a small frequency factor. (c) Increasing the reaction temperature increases the fraction of successful collisions between reactants.

The iodide ion reacts with hypochlorite ion (the active ingredient in chlorine bleaches) in the following way: \(\mathrm{OCl}^{-}+\mathrm{I}^{-} \longrightarrow \mathrm{OI}^{-}+\mathrm{Cl}^{-}\). This rapid reaction gives the following rate data: $$ \begin{array}{lll} \hline\left[\mathrm{OCl}^{-}\right](M) & {\left[I^{-}\right](M)} & \text { Initial Rate }(M / s) \\ \hline 1.5 \times 10^{-3} & 1.5 \times 10^{-3} & 1.36 \times 10^{-4} \\ 3.0 \times 10^{-3} & 1.5 \times 10^{-3} & 2.72 \times 10^{-4} \\ 1.5 \times 10^{-3} & 3.0 \times 10^{-3} & 2.72 \times 10^{-4} \\ \hline \end{array} $$ (a) Write the rate law for this reaction. (b) Calculate the rate constant with proper units. (c) Calculate the rate when \(\left[\mathrm{OCl}^{-}\right]=2.0 \times 10^{-3} \mathrm{M}\) and \(\left[\mathrm{I}^{-}\right]=5.0 \times 10^{-4} \mathrm{M}\)

For the generic reaction \(\mathrm{A} \longrightarrow \mathrm{B}\) that is zero order in \(\mathrm{A}\), what would you graph in order to obtain the rate constant?

Enzymes are often described as following the two-step mechanism: $$ \begin{array}{l} \mathrm{E}+\mathrm{S} \rightleftharpoons \mathrm{ES} \quad(\text { fast }) \\ \mathrm{ES} \longrightarrow \mathrm{E}+\mathrm{P} \quad(\text { slow }) \end{array} $$ where \(\mathrm{E}=\) enzyme, \(\mathrm{S}=\) substrate, \(\mathrm{ES}=\) enzyme- substrate complex, and \(\mathrm{P}=\) product. (a) If an enzyme follows this mechanism, what rate law is expected for the reaction? (b) Molecules that can bind to the active site of an enzyme but are not converted into product are called enzyme inhibitors. Write an additional elementary step to add into the preceding mechanism to account for the reaction of \(\mathrm{E}\) with \(\mathrm{I}\), an inhibitor.

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 \(\mathrm{Cl}^{-} ?\)
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