In the Classroom JChemEd.chem.wisc.edu • Vol. 75 No. 9 September 1998 • Journal of Chemical Education 1175 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. The Problem The exothermic reaction s(g) d(g) was allowed to come to equilibrium, as represented in the box below: equilibrium system 1. Some d 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 in- creased. Which box (A–E) best represents the new position of equilibrium? Explain your answer. “Conceptual Questions” on LeChâtelier’s Principle Benjamin P. Huddle Chemistry Department, Roanoke College, Salem, VA 24153 Resources for Student Assessment edited by John Alexander University of Cincinnati Cincinnati, OH 45221 3. The pressure of the system at equilibrium was in- creased. Which box (A–E) best represents the new position of equilibrium? Explain your answer. Acceptable Solutions 1. B is the only correct box. The original equilibrium system consisted of 6 d’s and 4 s’s. If some d’s are added to this system, some of the d’s will react (“equilibrium will shift to the left”) and produce some s’s, until the ratio of d’s to s’s is once again the equilibrium ratio. Both boxes B and C have this ratio, but box B has additional d’s and s’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 d’s to s’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 d’s and s’s should still be present.

"Conceptual Questions" on LeChatelier's Principle

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Page 1: "Conceptual Questions" on LeChatelier's Principle

In the Classroom

JChemEd.chem.wisc.edu • Vol. 75 No. 9 September 1998 • Journal of Chemical Education 1175

The questions below are designed to assess the student’sability to conceptualize chemical equilibrium and to predictthe effect of changes made to a system at equilibrium, usingLeChâtelier’s principle, without doing any equilibrium con-stant calculations.

The Problem

The exothermic reaction s(g) d(g) was allowed tocome to equilibrium, as represented in the box below:

equilibrium system

1. Some d was added to the system at equilibrium.Which box (A–E) best represents the new position of equi-librium? Explain your answer.

2. The temperature of the system at equilibrium was in-creased. Which box (A–E) best represents the new positionof equilibrium? Explain your answer.

“Conceptual Questions” on LeChâtelier’s Principle

Benjamin P. HuddleChemistry Department, Roanoke College, Salem, VA 24153

Resources for Student Assessmentedited by

John AlexanderUniversity of CincinnatiCincinnati, OH 45221

3. The pressure of the system at equilibrium was in-creased. Which box (A–E) best represents the new positionof equilibrium? Explain your answer.

Acceptable Solutions

1. B is the only correct box. The original equilibriumsystem consisted of 6 d’s and 4 s’s. If some d’s are added tothis system, some of the d’s will react (“equilibrium will shiftto the left”) and produce some s’s, until the ratio of d’s to s’sis once again the equilibrium ratio. Both boxes B and C havethis ratio, but box B has additional d’s and s’s, as required bythe problem.

2. E is the only correct box. The effect of increasing tem-perature on an exothermic reaction is to decrease the value ofthe equilibrium constant (“equilibrium will shift to the left”),decreasing the amount of product and increasing the amountof reactant. In box E the ratio of d’s to s’s has decreased from6:4 (10 total) to 5:5 (10 total). In all other boxes the ratio isequal to or greater than the original ratio.

3. C is the only correct box. Pressure should have noeffect on the position of equilibrium, since there are the samenumber of gas molecules on both sides of the reaction. Theoriginal number of d’s and s’s should still be present.