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Chapter 14: Problem 75

(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 by lowering the activation energy, without undergoing any permanent chemical change. (b) Homogeneous catalysts are in the same phase as the reactants, while heterogeneous catalysts are in a different phase. Homogeneous catalysts generally allow for faster reactions but are harder to separate, while heterogeneous catalysts are easier to separate and more suited for large-scale applications. (c) Catalysts only affect the activation energy of a reaction, not the overall enthalpy change, allowing the reaction to proceed faster and at lower temperatures while the overall energy difference between reactants and products remains unchanged.

Step by step solution

01

Definition of a catalyst

A catalyst is a substance that increases the rate of a chemical reaction by lowering the activation energy. Importantly, a catalyst remains chemically unchanged after the reaction and can be reused multiple times.
02

Homogeneous vs. heterogeneous catalysts

The main difference between homogeneous and heterogeneous catalysts lies in their phase relative to the reactants. A homogeneous catalyst is in the same phase (solid, liquid, or gas) as the reactants, while a heterogeneous catalyst is in a different phase. Generally, homogeneous catalysts are more effective in accelerating reactions, as their particles can easily interact with the reactants. However, heterogeneous catalysts are often easier to separate from the reaction mixture and can be more practical for large-scale applications.
03

Catalysts' effect on enthalpy change and activation energy

Catalysts only affect the activation energy of a reaction, not the overall enthalpy change. By lowering the activation energy, catalysts enable the reaction to proceed faster and at lower temperatures. However, the overall enthalpy change for the reaction remains unchanged, as it is determined by the difference in energy between the reactants and products, which is independent of the catalyst's presence.

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Most popular questions from this chapter

(a) The activation energy for the isomerization of methyl isonitrile (Figure 14.7) is \(160 \mathrm{~kJ} / \mathrm{mol}\). Calculate the fraction of methyl isonitrile molecules that has an energy of \(160.0 \mathrm{~kJ}\) or greater at \(500 \mathrm{~K}\). (b) Calculate this fraction for a temperature of \(520 \mathrm{~K}\). What is the ratio of the fraction at \(520 \mathrm{~K}\) to that at \(500 \mathrm{~K} ?\)

You perform a series of experiments for the reaction \(A \longrightarrow B+C\) and find that the rate law has the form rate \(=k[\mathrm{~A}]^{x}\). Determine the value of \(x\) in each of the following cases: (a) There is no rate change when \([\mathrm{A}]_{0}\) is tripled. (b) The rate increases by a factor of 9 when \([\mathrm{A}]_{0}\) is tripled. (c) When \([\mathrm{A}]_{0}\) is doubled, the rate increases by a factor of 8 .

A first-order reaction \(\mathrm{A} \longrightarrow \mathrm{B}\) has the rate constant \(k=3.2 \times 10^{-3} \mathrm{~s}^{-1}\). If the initial concentration of \(\mathrm{A}\) is \(2.5 \times 10^{-2} M\), what is the rate of the reaction at \(t=660 \mathrm{~s}\) ?

The accompanying graph shows plots of \(\ln k\) versus \(1 / T\) for two different reactions. The plots have been extrapolated to the \(y\)-intercepts. Which reaction (red or blue) has (a) the larger value for \(E_{a}\) and (b) the larger value for the frequency factor, \(A\) ? [Section 14.5]

The oxidation of \(\mathrm{SO}_{2}\) to \(\mathrm{SO}_{3}\) is accelerated by \(\mathrm{NO}_{2}\). The reaction proceeds according to: $$ \begin{aligned} &\mathrm{NO}_{2}(g)+\mathrm{SO}_{2}(g) \longrightarrow \mathrm{NO}(g)+\mathrm{SO}_{3}(g) \\ &2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}_{2}(g) \end{aligned} $$ (a) Show that, with appropriate coefficients, the two reactions can be summed to give the overall oxidation of \(\mathrm{SO}_{2}\) by \(\mathrm{O}_{2}\) to give \(\mathrm{SO}_{3}\). (b) \(\mathrm{Do}\) we consider \(\mathrm{NO}_{2}\) a catalyst or an intermediate in this reaction? (c) Is this an example of homogeneous catalysis or heterogeneous catalysis?
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