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Ch.15 - Chemical Equilibrium
Chapter 15, Problem 131

Baking soda (sodium bicarbonate) decomposes when it is heated:2 NaHCO31s2 ∆ Na2CO31s2 + CO21g2 + H2O1g2ΔH° = + 136 kJConsider an equilibrium mixture of reactants and products in a closed container. How does the number of moles of CO2 change when the mixture is disturbed by the following:(b) Adding water vapor

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Identify the reaction: The decomposition of sodium bicarbonate is an endothermic reaction, as indicated by the positive ΔH° value (+136 kJ).
Understand the effect of adding water vapor: According to Le Chatelier's Principle, if a system at equilibrium is disturbed, it will adjust to minimize the disturbance.
Analyze the equilibrium shift: Adding water vapor increases the concentration of one of the products, H2O(g).
Predict the direction of the shift: The system will shift towards the reactants to counteract the increase in water vapor, thereby reducing the number of moles of CO2.
Conclude the effect on CO2: As the equilibrium shifts towards the reactants, the number of moles of CO2 will decrease.

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Key Concepts

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

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 that change and restore a new equilibrium. This principle is crucial for predicting how the addition of substances, such as water vapor, will affect the concentrations of reactants and products in a chemical reaction.
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Equilibrium Constant (K)

The equilibrium constant (K) is a numerical value that expresses the ratio of the concentrations of products to reactants at equilibrium for a given reaction at a specific temperature. Changes in concentration, pressure, or temperature can shift the position of equilibrium, affecting the amounts of CO2 and other species present in the system.
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Effect of Water Vapor on Gas Equilibrium

In the context of gas equilibria, adding water vapor increases the total pressure of the system. According to Dalton's Law of Partial Pressures, this can shift the equilibrium position depending on the number of moles of gaseous products and reactants. In this case, the addition of water vapor may favor the production of CO2 if it leads to a decrease in the overall pressure of the gaseous components.
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