Chemical Equilibrium

Question 1

Conditions for a dynamic equilibrium (4)

Answer 1

1. Reversible rxn
2. Rate of forward and reversed rxn is equal
3. Opposing rxns occur never stop
4. Constant concentrations of reactants and products

Question 2

What is equilibrium law (mass action expression)?

Answer 2

Kc = [C]^c *[D]^d / [A]^a * [B]^b

For Kp, partial pressures are used instead of molar concentration [X]

Question 3

If Q<K: ____ rxn is favored
If Q=K: ____ rxn is favored
If Q>K: ____ rxn is favored

Answer 3

If Q<K: forward rxn is favored, increase products, decrease reactants

If Q=K: neither forward nor reversed rxn is favored, equilibrium = initial

If Q>K: reversed rxn is favored, decrease products, increase reactants

Question 4

Interpret K values:
High K value
Intermediate K value
Low K value

Answer 4

High K value = almost all reactants -> products
Intermediate K value = some reactants -> products
Low K value = almost none of reactants -> products

Question 5

According to Le Chatelier’s Principle, increasing pressure of a system at equilibrium will shift the equilibrium towards ____ and decreasing pressure will shift towards ____

Answer 5

high pressure -> side with lower mol of gas
low pressure -> side with higher mol of gas

Question 6

According to Le Chatelier’s Principle, addition of inert gas will ____. Why?

Answer 6

shift the equilibrium towards side with higher moles of gas

total volume will increase and will lower partial pressures

Question 7

According to Le Chatelier’s Principle, adding catalyst will ____

Answer 7

No effect to the equilibrium. Catalyst only speeds up both the forward and reversed reaction by lowering the activation energy

Question 8

How to approach this problem?

Methanol is manufactured industrially by the reaction of carbon monoxide and hydrogen. The Kc for the reaction is 10.5 at 220°C. Calculate the value of Kp at this temperature.

Answer 8

1. Balanced equation
   CO (g) + 2H2 (g) <-> CH3OH (g)
2. Convert °C to K
3. use the formula: Kp = Kc (RT)^Δng

IMPORTANT: Δng = mol gas products - mol of gas reactants. In this problem, Δng = 1-3 = -2

Question 9

How to approach this problem?

The equilibrium process below has been studied at 230°C. In one experiment, the concentrations of the reacting species at equilibrium are found to be [NO] = 0.0542 M, [O2] = 0.127 M, and [NO2] = 15.5 M. Calculate Kc of the reaction at this temperature.

2NO (g) + O2 (g) <-> 2NO2 (g)

Answer 9

1. Check the balanced equation
2. Use equilibrium law expression

IMPORTANT: always products/reactant; mole coefficients are exponents

Question 10

How to approach this problem?

A mixture of 0.500 mol H, and 0.500 mol I, was placed in a 1.00-L stainless steel flask at 430°C. The Kc for the reaction H2 + I2 = 2HI is 54.3 at this temperature. Calculate the equilibrium concentrations of all three species.

Answer 10

1. Check the balanced equation
2. Use ICE table
3. Construct equilibrium expression
4. Solve for unknown

IMPORTANT: sometimes, molarity (M) is given and not in moles. convert it first to moles before using ICE table