http://www.ck12.org Chapter 19. Equilibrium
Lesson Summary
- A system at equilibrium can be disrupted by a change in concentration of one of the substances or by a change
in temperature or pressure. Le Châtelier’s principle states that such a system will respond by attempting to
counteract the stress. Either the forward or reverse reaction will temporarily be favored until equilibrium is
reestablished. - A catalyst increases the rate of both the forward and reverse reactions equally and does not change the
equilibrium position. - Reactions in which a product is continually removed from the system do not attain equilibrium and, instead,
proceed to completion. - A change in concentration or pressure does not change the value of the equilibrium constant for a reaction.
Only a temperature change alters the equilibrium constant.
Lesson Review Questions
Reviewing Concepts
- What are three stresses that can upset the equilibrium of a chemical system?
- Which stress or stresses change the value of the equilibrium constant?
- What conditions can drive a reaction to completion?
- What must be true of the reaction in order for pressure to have an effect on the equilibrium position?
- Does the use of a catalyst influence the position of an equilibrium? Explain.
Problems
- Given the following equilibrium equation: N 2 (g)+2O 2 (g)+ 66 .2 kJ⇀↽2NO 2 (g). Predict the direction of
equilibrium that will be favored (forward, reverse, or neither) for each of the following changes.
a. N 2 is added.
b. O 2 is removed.
c. The temperature is increased.
d. The pressure is increased.
e. A catalyst is used.
f. NO 2 is removed.
g. The temperature is decreased.
h. The system volume is increased. - For the system in question 6, how would the concentration of NO 2 at equilibrium be affected by each change?
- For the system in question 6, how would the value of Keqbe affected by each change?
- Given the following reaction for the formation of sulfur trioxide from sulfur dioxide and oxygen: 2SO 2 (g)+O 2 (g)⇀↽2SO 3 (g)+198 kJ.
What conditions of temperature and pressure would maximize the concentration of SO 3 at equilibrium?
Further Reading / Supplemental Links
- LeChâtelierPrinciple, http://www.kentchemistry.com/links/LeChatelier.htm
- LeChâtelier’sPrinciple, http://www.chemguide.co.uk/physical/equilibria/lechatelier.html