In the Classroom edited by
Resources for Student Assessment
John Alexander University of Cincinnati Cincinnati, OH 45221
“Conceptual Questions” on LeChâtelier’s Principle Benjamin P. Huddle Chemistry Department, Roanoke College, Salem, VA 24153
The questions below are designed to assess the student’s ability to conceptualize chemical equilibrium and to predict the effect of changes made to a system at equilibrium, using LeChâtelier’s principle, without doing any equilibrium constant calculations.
3. The pressure of the system at equilibrium was increased. Which box (A–E) best represents the new position of equilibrium? Explain your answer.
The Problem
1. B is the only correct box. The original equilibrium system consisted of 6 䊉’s and 4 䊊 ’s. If some 䊉’s are added to this system, some of the 䊉’s will react (“equilibrium will shift to the left”) and produce some 䊊’s, until the ratio of 䊉’s to 䊊’s is once again the equilibrium ratio. Both boxes B and C have this ratio, but box B has additional 䊉’s and 䊊’s, as required by the problem. 2. E is the only correct box. The effect of increasing temperature on an exothermic reaction is to decrease the value of the equilibrium constant (“equilibrium will shift to the left”), decreasing the amount of product and increasing the amount of reactant. In box E the ratio of 䊉’s to 䊊’s has decreased from 6:4 (10 total) to 5:5 (10 total). In all other boxes the ratio is equal to or greater than the original ratio. 3. C is the only correct box. Pressure should have no effect on the position of equilibrium, since there are the same number of gas molecules on both sides of the reaction. The original number of 䊉’s and 䊊’s should still be present.
The exothermic reaction 䊊 (g) 䊉(g) was allowed to come to equilibrium, as represented in the box below: equilibrium system
1. Some 䊉 was added to the system at equilibrium. Which box (A–E) best represents the new position of equilibrium? Explain your answer. 2. The temperature of the system at equilibrium was increased. Which box (A–E) best represents the new position of equilibrium? Explain your answer.
Acceptable Solutions
JChemEd.chem.wisc.edu • Vol. 75 No. 9 September 1998 • Journal of Chemical Education
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