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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 catalyst is a substance that accelerates a chemical reaction without being consumed by providing an alternative reaction pathway with lower activation energy. Homogeneous catalysts exist in the same phase as the reactants, while heterogeneous catalysts exist in a different phase. Catalysts do not affect the overall enthalpy change (鈭咹) of a reaction, but they do lower the activation energy, allowing the reaction to proceed faster.

Step by step solution

01

(a) Definition of a Catalyst

A catalyst is a substance that speeds up the rate of a chemical reaction without being consumed in the reaction. It does so by providing an alternative reaction pathway or mechanism with a lower activation energy.
02

(b) Homogeneous vs Heterogeneous Catalysts

Homogeneous catalysts are those that are in the same phase (solid, liquid, or gas) as the reactants, and they usually form intermediate species with the reactants during the reaction. Heterogeneous catalysts, on the other hand, are in a different phase than the reactants and typically work by adsorbing the reactants onto their surface, where the reaction takes place.
03

(c) Effect of Catalysts on Enthalpy Change and Activation Energy

Catalysts do not affect the overall enthalpy change for a reaction, as they only provide an alternative pathway for the reaction to proceed. The overall enthalpy change (鈭咹) is a state function, which means it depends only on the initial and final states of the reactants and products, and not on the path taken to reach those states. However, catalysts do affect the activation energy of a reaction, as this is the minimum energy required for reactants to transform into products through a particular pathway. By providing an alternative pathway with a lower activation energy, catalysts allow the reaction to proceed faster, as more reactant molecules have enough energy to overcome the lowered activation energy barrier.

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Most popular questions from this chapter

(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?

(a) Consider the combustion of hydrogen, \(2 \mathrm{H}_{2}(g)+\) \(\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(g) .\) If hydrogen is burning at the rate of 0.48 \(\mathrm{mol} / \mathrm{s}\) , what is the rate of consumption of oxygen? What is the rate of formation of water vapor? (b) The reaction \(2 \mathrm{NO}(g)+\mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{NOCl}(g)\) is carried out in a closed vessel. If the partial pressure of \(\mathrm{NO}\) is decreasing at the rate of 56 torr/min, what is the rate of change of the total pressure of the vessel?

You perform a series of experiments for the reaction \(\mathrm{A} \longrightarrow \mathrm{B}+\mathrm{C}\) 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 \(8 .\)

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: \(\begin{array}{l}{\text { (a) } 2 \mathrm{H}_{2} \mathrm{O}(g) \longrightarrow 2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g)} \\ {\text { (b) } 2 \mathrm{SO}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{SO}_{3}(g)} \\\ {\text { (c) } 2 \mathrm{NO}(g)+2 \mathrm{H}_{2}(g) \longrightarrow \mathrm{N}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(g)} \\ {\text { (d) } \mathrm{N}_{2}(g)+2 \mathrm{H}_{2}(g) \longrightarrow \mathrm{N}_{2} \mathrm{H}_{4}(g)}\end{array}\)

Platinum nanoparticles of diameter \(\sim 2 \mathrm{nm}\) are important catalysts in carbon monoxide oxidation to carbondioxide. Platinum crystallizes in a face-centered cubic arrangement with an edge length of 3.924 A. (a) Estimate how many platinum atoms would fit into a 2.0 -nm sphere; the volume of a sphere is \((4 / 3) \pi r^{3} .\) Recall that \(1 \hat{\mathrm{A}}=1 \times 10^{-10} \mathrm{m}\) and \(1 \mathrm{nm}=1 \times 10^{-9} \mathrm{m} .\) (b) Estimate how many platinum atoms are on the surface of a \(2.0-\mathrm{nm}\) Pt sphere, using the surface area of a sphere \(\left(4 \pi r^{2}\right)\) and assuming that the "footprint" of one Pt atom can be estimated from its atomic diameter of 2.8 A. (c) Using your results from (a) and (b), calculate the percentage of Pt atoms that are on the surface of a 2.0 -nm nanoparticle. (d) Repeat these calculations for a 5.0 -nm platinum nanoparticle. (e) Which size of nanoparticle would you expect to be more catalytically active and why?
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