Chapter 17, Problem 29A

Interpretation Introduction

Interpretation: The effect of decreasing the volume of the system on the reaction equilibrium needs to be explained.

Concept introduction: According to Le Chatelier's principle if any stress is given to the equilibrium, it will shift in a direction to reduce the given stress. If change in concentration, pressure or temperature takes place at equilibrium, it will shift in the direction to undo the change or its effect.

Answer to Problem 29A

When the volume decreases the reaction shifts towards the side of fewer moles of gas.

Explanation of Solution

Consider a hypothetical reaction: $A+2B\rightleftharpoons C+D$

According the ideal gas equation ( $PV=nRT$ ) volume is inversely proportional to the pressure. If volume is decreased pressure will increase.

According to Le Chatelier’s principle, if the pressure is increased, the direction of equilibrium will shift so as to reduce the pressure. Pressure is created when gas molecules collides with the sides of their container. The more the number of molecules in the container, the more will be the pressure. The system can decrease the pressure by reacting in such a way as to generate fewer molecules.

In this reaction example equilibrium will shifts towards forward direction as the product side has fewer number of moles.

Conclusion

When the volume decreases the reaction shifts towards the side of fewer moles of gas.