Tough questions series – 2013 CHEM 13 NEWS exam

The second question we’ll examine (#38) is reproduced below. Most students (87%) entered a response, but less than 13% chose the correct response (E) and almost 60% chose answer D.  Because the success rate was so low, this question was not as effective at discriminating between the “stronger” and “weaker” students.

If the reaction W + X ⇄ Y + Z is exothermic in the forward direction, then what is the effect of an increase in temperature on the forward rate, the reverse rate and the equilibrium constant?

I think that many students recognize that an increase in temperature generally causes an increase in reaction rate and, for a system initially at equilibrium, net reaction in the endothermic direction. The results of this question suggest that many students do not appreciate that an increase in temperature causes increases in the rates of both the forward and reverse reactions (i.e., the rates of converting reactants into products and of converting products into reactants), or have difficulty reconciling the increases in these rates with a shift in the equilibrium position. Thinking back on the many conversations I’ve had with students, I suspect that many students might reason through this problem as follows:

1. Because the reaction is exothermic in the forward direction, an increase in temperature would cause the equilibrium position to shift to the left, thereby producing a smaller value for the equilibrium constant.³
2. The shift to the left actually occurs because the rate of the forward reaction decreases whereas the rate of the reverse reaction increases. This reasoning breaks down in step (2). The shift to the left actually occurs because the rate of the reverse reaction increases by a greater factor than that of the forward reaction. For example, if an increase in temperature causes the forward reaction rate to increase by a factor of 10 and the reverse reaction rate to increase by a factor of 100, there will be net reaction to the left even though both reactions are occurring faster than they were initially.

I encourage teachers, when discussing chemical equilibrium, to emphasize that although an increase in temperature may cause the equilibrium constant K to increase or decrease, depending on whether the reaction is endothermic or exothermic, the rates of the forward and reverse reactions always increase with temperature.⁴

Notes:

1. Although many students know that atomic radii decrease across a period and increase down a group, relatively few are able to provide a solid explanation.
2. The atomic radius of P is about 110 pm and that of Br is about 114 pm.
3. Actually, it is more correct to say that there is a net reaction to the left because, for an exothermic reaction, the equilibrium constant decreases as the temperature increases.
4. There are some reactions for which the rate decreases as temperature increases, so the use of “always” is not totally justified.

Most students (87%) entered a response, but less than 13% chose the correct response (E) and almost 60% chose answer D.  Because the success rate was so low, this question was not as effective at discriminating between the “stronger” and “weaker” students.

I think that many students recognize that an increase in temperature generally causes an increase in reaction rate and, for a system initially at equilibrium, net reaction in the endothermic direction. The results of this question suggest that many students do not appreciate that an increase in temperature causes increases in the rates of both the forward and reverse reactions (i.e., the rates of converting reactants into products and of converting products into reactants), or have difficulty reconciling the increases in these rates with a shift in the equilibrium position. Thinking back on the many conversations I’ve had with students, I suspect that many students might reason through this problem as follows:

1. Because the reaction is exothermic in the forward direction, an increase in temperature would cause the equilibrium position to shift to the left, thereby producing a smaller value for the equilibrium constant.³
2. The shift to the left actually occurs because the rate of the forward reaction decreases whereas the rate of the reverse reaction increases. This reasoning breaks down in step (2). The shift to the left actually occurs because the rate of the reverse reaction increases by a greater factor than that of the forward reaction. For example, if an increase in temperature causes the forward reaction rate to increase by a factor of 10 and the reverse reaction rate to increase by a factor of 100, there will be net reaction to the left even though both reactions are occurring faster than they were initially.

I encourage teachers, when discussing chemical equilibrium, to emphasize that although an increase in temperature may cause the equilibrium constant K to increase or decrease, depending on whether the reaction is endothermic or exothermic, the rates of the forward and reverse reactions always increase with temperature.⁴

Notes:

1. Although many students know that atomic radii decrease across a period and increase down a group, relatively few are able to provide a solid explanation.
2. The atomic radius of P is about 110 pm and that of Br is about 114 pm.
3. Actually, it is more correct to say that there is a net reaction to the left because, for an exothermic reaction, the equilibrium constant decreases as the temperature increases.
4. There are some reactions for which the rate decreases as temperature increases, so the use of “always” is not totally justified.