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CHM 112
Section 1

Homework Assignment #8
Due February 15, 2006 by 8:05 a.m.

Practice (not to be turned in): Review Questions 14.8


Consider the reaction

C(s) + O2(g) + S(s) COS(g)

which has H = –138.4 kJ/mol

The reaction is initially at equilibrium. Which way does the reaction shift if:

0) Oxygen is added to the reaction?

1) The pressure is raised by reducing the volume?

2) Graphite is added to the reaction?

3) The temperature is raised by 50 oC?

4) The pressure is doubled by adding argon?

5) Platinum is added as a catalyst?

6) The reaction vessel is cooled by 20 oC?

7) Oxygen is removed from the reaction?

8) The pressure is reduced by increasing the volume of the reaction vessel?

9) Sulfur is added to the reaction?



Answers:

First, balance the reaction (and add heat as a product for this exothermic reaction):

2 C(s) + O2(g) + 2 S(s) 2 COS(g) + heat



0) Oxygen is added to the reaction?

Right. Oxygen, O2, is a reactant so the reaction will shift away from reactants towards products to remove some of the added oxygen.

1) The pressure is raised by reducing the volume?

Left. Reducing the volume will increase the partial pressure of all the gases in the reaction. The reaction will shift towards fewer moles of gases, the reactants, to remove some of the increased pressure.

2) Graphite is added to the reaction?

No shift. Graphite, C, is a solid and does not show up in the mass action expression so does not affect equilibrium composition (as long as some is present).

3) The temperature is raised by 50 oC?

Left. Raising the temperature adds heat so the reaction will shift in the endothermic direction to remove the added heat, i.e. towards reactants.

4) The pressure is doubled by adding argon?

No shift. Adding an inert gas like argon has no effect on the partial pressures of the reaction components.

5) Platinum is added as a catalyst?

No shift. A catalyst will increase the rate of reaction but not the equilibrium composition.

6) The reaction vessel is cooled by 20 oC?

Right. Cooling the reaction removes heat so the reaction will shift to replace the lost heat, i.e. towards the exothermic direction, which are the products.

7) Oxygen is removed from the reaction?

Left. Oxygen, O2, is a reactant so the reaction will shift towards reactants away from products to replace some of the lost oxygen.

8) The pressure is reduced by increasing the volume of the reaction vessel?

Right. Increasing the volume will decrease the partial pressure of all the gases in the reaction. The reaction will shift towards more moles of gases, the products, to recover some of the decreased pressure.

9) Sulfur is added to the reaction?

No shift. Sulfur, S, is a solid and does not show up in the mass action expression so does not affect equilibrium composition (as long as some is present).