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Factors That Influence Equilibrium and Rate of Reaction

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Ammonia reacts with oxygen in the presence of a platinum catalyst to give nitric oxide and water, according to the following gas phase equilibrium:

4NH3(g) + 5O2(g) <--> 4NO(g) + 6H2O(g) delta H = -906 kJ/mol

(a) Write an appropriate equilibrium expression for the reaction. What would the units of the equilibrium constant K be if the concentrations of all the gases in the equilibrium expression were measured in mol/dm^3? Show how you determined your units for K using your equation.

(b) Write a short account of around 250 words in total, with reference to Le Chatelier's principle, in which you explain the following:
(i) The effect on the equilibrium yield of nitric oxide of increasing the pressure.
(ii) The effect on the equilibrium yield of nitric oxide of increasing the temperature.
(iii) The effect on the rate of reaction of increasing the temperature.
Write your account using concise scientific language. For the purposes of this exercise you should assume that your tutor is your target audience.

(c) Describe what a catalyst is and what effect the presence of a platinum catalyst has on the equilibrium yield and on the rate of reaction. (Three or four sentences)

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Solution Preview

(a) Please see attached World file for complete answer with equations.
Equilibrium expression:
To write an equilibrium expression for any chemical equation, multiply the concentration (or partial pressure) of the species on the product side raise to the power of their coefficients and divide by the same treatment with the species on the reactant side.
Units for K:
Looking at the equilibrium expression for this specific reaction (see attached World file), we see that power of 4 associated with the NO(g) on the product side would cancel with the power of 4 associated with the NH3(g) on the reactant side. Also the power of 5 associated with the O2(g) on the reactant side would cancel with the 5 of the 6 powers associated with the H2O(g) on the product side. This leaves only a coefficient of 1 on the product side, so units of K will be just mol/dm^3.

(b) Le Chatelier's Principle basically says that when a system at equilibrium is disturbed, it will react in ...

Solution Summary

The concepts of Le Chatelier's Principle, equilibrium expression, and catalyst are briefly explained. The effects of pressure, temperature and catalyst on the equilibrium yield (a thermodynamic parameter) are discussed using the reaction of ammonia with oxygen as a specific example. The effects of temperature and catalyst on the rate of reaction (a kinetic parameter) are also discussed. The attached Word file contains step-by-step explanations along with relevant equations to illustrate the relationships discussed.