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Ch.15 - Chemical Equilibrium
Chapter 15, Problem 15.63a

The reaction
2 PH31g2 + As21g2 ∆ 2 AsH31g2 + P21g2
has Kp = 2.9 * 10-5 at 873 K. At the same temperature,
what is Kp for each of the following reactions?
(a) 2 AsH31g2 + P21g2 ∆ 2 PH31g2 + As21g2

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1
Identify the relationship between the given reaction and the new reaction. The new reaction is the reverse of the given reaction.
Recall that the equilibrium constant for the reverse reaction is the inverse of the equilibrium constant for the forward reaction.
Write the expression for the equilibrium constant of the reverse reaction: Kp' = 1/Kp.
Substitute the given value of Kp for the forward reaction into the expression for Kp' to find the equilibrium constant for the reverse reaction.
Calculate Kp' using the value Kp = 2.9 * 10^-5.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Equilibrium Constant (Kp)

The equilibrium constant, Kp, is a numerical value that expresses the ratio of the concentrations of products to reactants at equilibrium for a given reaction, with each concentration raised to the power of its coefficient in the balanced equation. It is specific to a particular temperature and provides insight into the extent of a reaction. A larger Kp indicates a greater concentration of products at equilibrium, while a smaller Kp suggests a higher concentration of reactants.
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Reaction Quotient (Q)

The reaction quotient, Q, is a measure of the relative amounts of products and reactants present in a reaction at any point in time, not just at equilibrium. It is calculated using the same formula as Kp but with the current concentrations. Comparing Q to Kp helps predict the direction in which a reaction will proceed to reach equilibrium: if Q < Kp, the reaction will shift to the right (toward products), and if Q > Kp, it will shift to the left (toward reactants).
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Le Chatelier's Principle

Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the system will adjust itself to counteract the change and restore a new equilibrium. This principle can be applied to predict how changes in concentration, pressure, or temperature will affect the position of equilibrium in a chemical reaction. Understanding this principle is crucial for manipulating reactions and predicting the effects of changes on Kp.
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