Physical Chemistry : Le Chatelier's Principle

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #1 : Le Chatelier's Principle

Ammonia is created using the Haber-Bosch process:

A reaction vessel is used to combine the nitrogen and hydrogen gas. The reaction proceeds until the vessel is at equilibrium.

What would you predict to happen if the pressure of the vessel increases?

Possible Answers:

More nitrogen gas would be created

Changing the pressure will not alter the equilibrium

The system will shift to the left

More ammonia would be created

Correct answer:

More ammonia would be created

Explanation:

According to Le Chatelier's principle, a change in pressure will cause a shift in order to counteract the pressure change. When the pressure of a vessel is increased, the side of the reaction with fewer gas molecules will be preferred in order to minimize contact between gas molecules. For this reaction, there are four gas molecules on the left side and two gas molecules on the right side. As a result, the products side will be preferred and more ammonia will be created.

Example Question #2 : Le Chatelier's Principle

Ammonia is created using the Haber-Bosch process:

A reaction vessel is used to combine the nitrogen and hydrogen gas until the vessel is at equilibrium.

What will happen to the system if ammonia is removed from the vessel?

Possible Answers:

The amount of hydrogen gas will increase

The equilibirum will shift to the right

The equilibrium will not shift

The equilibrium will shift to the left

Correct answer:

The equilibirum will shift to the right

Explanation:

When a system is at equilibrium, it is possible to predict how a system will respond to sudden changes using Le Chatelier's principle. In this scenario, ammonia has been removed from the system. This will cause the reaction to produce more ammonia, and proceed to the right in order to reestablish equilibrium.

Example Question #3 : Le Chatelier's Principle

 

Based on the given information, under what conditions should this reaction be carried out to promote the most formation of product? 

Possible Answers:

Low pressure, high temperature, in the dark

High pressure, high temperature, in a vacuum

Low temperature, low pressure, in a semi-lit area

The reaction will proceed as written regardless of conditions because it is spontaneous

High pressure, low temperature, under a lamp

Correct answer:

High pressure, low temperature, under a lamp

Explanation:

High pressure will favor the side of the reaction with fewer moles of gas, which in this case is the product side. 

The reaction is exothermic, meaning that it generates heat. By removing heat from the reaction, we are constantly stressing the system towards the product side.

Finally, the reaction should occur under a lamp. This is due to the hv symbol above the double arrows, which signifies that light promotes the reaction. Note: had the hv symbol been below the double arrows, the reaction should have occurred in the dark.  

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