15.3.4 The Effects of Temperature and Catalysts on Equilibrium

Question 1

The Effects of Temperature and Catalysts on Equilibrium

Answer 1

• Equilibrium constants are temperature dependent.
• Catalysts increase the rate at which equilibrium is established by lowering the activation energy (E a ), but do not change K eq .

Question 2

note

Answer 2

• Le Châtelier’s principle states that if a system is in
  equilibrium and a stress is applied to that system, the system responds in such a way as to minimize the effects of the stress. Le Châtelier’s principle can be used to explain why equilibrium constants are temperature dependent.
• Heat can be thought of as a reactant for endothermic
  reactions. Raising the temperature of an endothermic reaction will cause the reaction to shift to favor the products. The equilibrium constant increases as temperature increases.
• Heat can be thought of as a product for exothermic reactions. Raising the temperature of an exothermic reaction will cause the reaction to shift to favor the reactants. The equilibrium constant decreases as temperature increases.
• Cyclopentadiene (Cp) dimerizes in an exothermic Diels-Alder reaction.
• What happens if the temperature is raised?
• At high temperature, Q > K c , and the reaction shifts to the left.
• Adding a catalyst will increase the rate at which products are formed, but will this cause a reaction to shift toward products?
• Adding a catalyst decreases the activation energy (E a ) by providing an alternate pathway for the reaction to proceed to form products. This decrease in activation energy causes the forward rate to increase. However, the catalyst lowers the energy barrier between products and reactants. The rate of products decomposing into reactants increases as well.
• Since both rates are impacted, there is no overall change to K eq .

Question 3

Which statement about an endothermic reaction is not true?

Answer 3

The value for Keq is constant and is not affected by temperature.

Question 4

Look at the reaction pathways diagram for two reactions. The reactions are identical except that one reaction uses a catalyst. Which statement about this graph is not correct?

Answer 4

The reaction with the catalyst will yield a higher concentration of products.

Question 5

Look at the following reaction involving cylopentadiene:
2Cp <==> Cp2 + heat
At a lower temperature, the Kc for the reaction is 1.5*10^4. Suppose that you start with 0.38 M Cp2 and 0.0058 M Cp. Which of the following identities for this reaction at a higher temperature could be correct if nothing else was changed in the system?

Answer 5

KC = 5.2 × 10^3

Question 6

Look at the diagram that compares the values of Keq and (rising) temperature for endothermic and exothermic reactions. Which statement about this diagram is not correct?

Answer 6

As temperature decreases, the reactants are more favored in an exothermic reaction.

Question 7

Which statement about endothermic or exothermic reactions is not correct?

Answer 7

In an endothermic reaction, adding more reactant will cause more heat to be released

Question 8

Which statement about exothermic or endothermic reactions is not correct?

Answer 8

Equilibrium shifts toward the products in an exothermic reaction in which the temperature is raised.

Question 9

Which statement is characteristic of an exothermic reaction?

Answer 9

The reactants are favored at high temperatures.

Question 10

Consider a snake pit as an analogy to illustrate the difference between a reaction at low temperature and a reaction at high temperature, which statement about this analogy is not correct?

Answer 10

At the higher temperature, the snakes tend to move into the cooler pit.

Question 11

Which statement about the plots for Keq vs. temperature is not correct?

Answer 11

At Points 4 and 5, the products are strongly favored.