Practice Questions Section 2.2
The Equilibrium Constant

1.	Write balanced chemical equations for each of the following. Pay close attention to the physical states! 

		Also - you must include the charge when writing ions, otherwise your answer is incorrect.

		Do not balance these equations using fractions for coefficients.

	sulfur dioxide gas combines with oxygen gas to produce sulfur trioxide gas

	carbon monoxide gas burns in gaseous oxygen to produce carbon dioxide gas 

	hydrogen chloride gas is produced from hydrogen gas and chlorine gas.

	nitrogen gas and oxygen gas combine to produce gaseous dinitrogen oxide. 

	solid hydrogen cyanide dissolves to produce hydrogen ions and cyanide ions in solution. 

	solid silver chloride dissolves to produce silver ions and chloride ions in solution. 

	calcium ions and phosphate ions come out of solution to produce solid calcium phosphate.

2.	For each of the above reactions, write the equilibrium expression, Keq, for the reaction. Remember not to include solids or liquids in the equilibrium constant expression.

3.	The equilibrium equation for the formation of ammonia is

		N2 (g) + 3 H2 (g) ↔ 2 NH3 (g)

	At 200°C the concentrations of nitrogen, hydrogen, and ammonia at equilibrium are measured and found to be:

		 [N2] = 2.12	[H2] = 1.75	[NH3] = 84.3

	Calculate Keq at this temperature.

4.	For each of the following equilibrium systems, identify whether the reactants or products are favored at equilibrium, or whether they are equally favored.

COCl2 (g) ↔ CO(g) + Cl2 (g)	Keq = 8.2× 10-2 at 627°C	
C(s) + 2 H2 (g) ↔ CH4 (g)	Keq = 8.1× 108 at 25°	
PCl5 (g) ↔ PCl3 (g) + Cl2 (g)	Keq = 2.24 at 227°C	
H2 (g) + Cl2 (g) ↔ 2 HCl(g)	Keq = 1.8 × 1033 at 25°C	
C(s) + H2O (g) ↔ CO(g) + H2 (g)	Keq = 1.96 at 1000°C	
Mg(OH)2 (s) ↔ Mg2+(aq) + 2 OH-(aq)	Keq = 1.2× 10-11 at 25°C	

5.	For the reaction: carbon monoxide burns in oxygen to produce carbon dioxide

		You are given the following equilibrium conditions:

		[O2] = 1.30 × 10-3		[CO2] = 2.50 × 10-4	Keq = 3.60 × 10-3

	Calculate [CO]

Practice Questions Section 2.2
The Equilibrium Constant	Answers

1.	Write balanced chemical equations for each of the following. Pay close attention to the physical states! 

		Also - you must include the charge when writing ions, otherwise your answer is incorrect.

		Do not balance these equations using fractions for coefficients.

	sulfur dioxide gas combines with oxygen gas to produce sulfur trioxide gas

			2 SO2 (g) + O2 (g) ↔ 2 SO3 (g)

	carbon monoxide gas burns in gaseous oxygen to produce carbon dioxide gas 

			2 CO (g) + O2 (g) ↔ 2 CO2 (g)

	hydrogen chloride gas is produced from hydrogen gas and chlorine gas.

			H2 (g) + Cl2 (g) ↔ 2 HCl (g)

	nitrogen gas and oxygen gas combine to produce gaseous dinitrogen oxide. 

			2 N2 (g) + O2 (g) ↔ 2 N2O (g)

	solid hydrogen cyanide dissolves to produce hydrogen ions and cyanide ions in solution. 

			HCN(s) ↔ H+(aq) + CN-(aq)

	solid silver chloride dissolves to produce silver ions and chloride ions in solution. 

			AgCl(s) ↔ Ag+(aq) + Cl-(aq)

	calcium ions and phosphate ions come out of solution to produce solid calcium phosphate.

			3 Ca2+(aq) + 2 PO43-(aq) ↔ Ca3(PO4)2 (s)

2.	For each of the above reactions, write the equilibrium expression, Keq, for the reaction. Remember not to include solids or liquids in the equilibrium constant expression.

2 SO2 (g) + O2 (g) ↔ 2 SO3 (g)		
2 CO (g) + O2 (g) ↔ 2 CO2 (g)		
H2 (g) + Cl2 (g) ↔ 2 HCl (g)		
2 N2 (g) + O2 (g) ↔ 2 N2O (g)		
HCN(s) ↔ H+(aq) + CN-(aq)		
AgCl(s) ↔ Ag+(aq) + Cl-(aq)		
3 Ca2+(aq) + 2 PO43-(aq) ↔ Ca3(PO4)2 (s)		

3.	The equilibrium equation for the formation of ammonia is

		N2 (g) + 3 H2 (g) ↔ 2 NH3 (g)

	At 200°C the concentrations of nitrogen, hydrogen, and ammonia at equilibrium are measured and found to be:

		 [N2] = 2.12	[H2] = 1.75	[NH3] = 84.3

	Calculate Keq at this temperature.

	Solution:

			answer

4.	For each of the following equilibrium systems, identify whether the reactants or products are favored at equilibrium, or whether they are equally favored.

		Answer	
COCl2 (g) ↔ CO(g) + Cl2 (g)	Keq = 8.2× 10-2 at 627°C	reactants; COCl2 (g)	
C(s) + 2 H2 (g) ↔ CH4 (g)	Keq = 8.1× 108 at 25°	products; CH4 (g)	
PCl5 (g) ↔ PCl3 (g) + Cl2 (g)	Keq = 2.24 at 227°C	equal	
H2 (g) + Cl2 (g) ↔ 2 HCl(g)	Keq = 1.8 × 1033 at 25°C	products; HCl(g)	
C(s) + H2O (g) ↔ CO(g) + H2 (g)	Keq = 1.96 at 1000°C	equal	
Mg(OH)2 (s) ↔ Mg2+(aq) + 2 OH-(aq)	Keq = 1.2× 10-11 at 25°C	reactants; Mg(OH)2 (s)	

5.	For the reaction: carbon monoxide burns in oxygen to produce carbon dioxide

		You are given the following equilibrium conditions:

		[O2] = 1.30 × 10-3		[CO2] = 2.50 × 10-4	Keq = 3.60 × 10-3

	Calculate [CO]

	Solution:

		Begin by writing a balanced equation for the reaction:

				2 CO(g) + O2 (g) ↔ 2 CO2 (g) 

		Next, set up the equilibrium constant expression and solve for the unknown, [CO]. You will find it easier to let [CO] =  while you rearrange the equation: