Chemical Equilibrium

Chemical Equilibrium

​Reversible Reactions:
A chemical reaction in which the products can react to re-form the reactants

Chemical Equilibrium:
When the rate of the forward reaction equals the rate of the reverse reaction and the concentration of products and reactants remains unchanged

2HgO(s) ⮀ 2Hg(l) + O2(g)
Arrows going both directions ( ⮀ ) indicates equilibrium in a chemical equation

Approaching Equilibrium

Reactant concentrations start high and decrease as the reaction proceeds. The forward rate, which depends on collisions of the reactants, also decreases. 

 

Product concentrations start at zero and increase as the reaction proceeds. The reverse rate, which depends on collisions of the products, also increases. 

 

Eventually the forward rate is equal to the reverse rate and the concentrations are constant. This is equilibrium. 

Multiple Choice

Which of the following would NOT affect the rate of a reaction?

• Temperature

• Pressure

• Catalyst

• Concentration

• Electronegativity

When a system at

equilibrium is placed under stress, the system will undergo a change in such a way as to relieve that stress.

Video

Le Chatelier's Principle

Multiple Choice

2SO₂(g)+O₂(g) ⇌ 2SO₃(g) + Heat

Increasing the temperature will...

• shift equilibrium right

• shift equilibrium left

• increase pressure

• have no change

Multiple Choice

2SO_2(g) + O_2(g) ⇌ 2SO_3(g) + Heat

Removing O_2(g) will

• shift equilibrium right

• shift equilibrium left

• increase pressure

• have no change

Multiple Choice

2SO₂(g)+O₂(g) ⇌ 2SO₃(g) + Heat

Adding SO₃(g) will

• shift equilibrium right

• shift equilibrium left

• increase K

• have no change

Multiple Choice

2SO₂(g)+1O₂(g) ⇌ 2SO₃(g) + Heat

Increasing the volume of the container will...

• shift equilibrium right

• shift equilibrium left

• slow rate of reaction

• have no change

Multiple Choice

2SO₂(g)+1O₂(g) ⇌ 2SO₃(g) + Heat

Increasing the pressure on the system will...

• shift equilibrium right

• shift equilibrium left

• slow rate of reaction

• have no change

Multiple Select

2SO₂(g) + O₂(g) ⇌ 2SO₃(g) + Heat

Using a catalyst

• shift equilibrium right

• shift equilibrium left

• increase the rate of reaction

• have no change

Multiple Choice

CoCI₄^2- +6H₂O →Co(H₂O)₆²⁺ + 4CI^-.

What would happen when H₂O is added?

• Position of equilibrium will shift to left (and become more blue)

• Position of equilibrium will shift to right (and become more pink)

• K_eq will increase as H₂O is added

• Position of equilibrium will shift to left to reduce the added H₂O

Multiple Choice

1N₂ + 3H₂ →2NH₃

When the pressure on the system is increased, the equilibrium position shifts to the right. Why?

• To increase the amount of products

• To reduce the pressure, as the right side has fewer molecules of gas

• K_eq will increase when it is shifted to the right

• To increase the pressure, as the right side has more molecules of gas

Multiple Choice

CoCI₄^2- +6H₂O →Co(H₂O)₆²⁺ + 4CI^-

What will happen when CI^- ions are added?

• Position of equilibrium will shift to left and become more pink

• Color of system will turn to all pink

• Concentration of reactants and products remain unchanged

• Position of equilibrium will shift to left to reduce the added CI^- ions

Multiple Choice

2CrO₄^2- + 2H⁺ → Cr₂O₇^2- + H₂O

What will happen when H⁺ ions are added to the system?

• Position of equilibrium will shift to left and become more yellow

• Color of system will turn all yellow

• Color of system will turn all orange

• Equilibrium will shift to right and become more orange

​The Equilibrium Constant

The equilibrium constant, represented as Kc​ is a number that shows how far a chemical reaction goes before it stops changing and reaches a state of balance or equilibrium.

• If Kc​ is much greater than 1: At equilibrium, there are a lot more products than reactants. The reaction strongly favors forward.

• If Kc​ is much less than 1: This means that at equilibrium, there are a lot more reactants than products. The reaction favours reverse.

• If Kc​ is around 1: This means that at equilibrium, the amounts of reactants and products are somewhat similar. Neither side is heavily favored.

Multiple Choice

The square brackets in a K_c expression represent ?

• concentration

• concentration in mol L^-1

• concentration in g L^-1

• the same as round brackets

Multiple Choice

Kc = ?

• products / reactants

• reactants /products

Multiple Choice

The Kc expression for the reaction 2NH₃ ⇌ N₂ + 3H₂

Multiple Choice

What is the equation/reaction for this Kc expression?

• O₃ ⇌ 3O₂

• 2O₃ ⇌ 3O₂

• 2O₃ ⇌ O₂

• 3O₂ ⇌ 2O₃

Multiple Choice

What is the reaction/equation for this K_c expression_

• 2N₂ + O₂ ⇌ 2NO

• N₂ + O₂ ⇌ NO

• NO ⇌ 2NO

• N₂ + O₂ ⇌ 2NO

Multiple Choice

This symbol indicates that a reaction is ____________

• reversible

• irreversible

Multiple Choice

When the rate of the forward reaction is equal to the rate of backward reaction, the system is said to be in

• Chemical Equilibrium

• Chemical Balance

• Chemical Constant

• Chemical Reaction

Multiple Choice

At what time does the reaction reach equilibrium?

• t₁

• t₂

• t₃

• t₄

Multiple Choice

W is a chemical reaction where the reactants form products and the products form reactants.

• reversible reaction

• nonreversible reaction

• decomposition reaction

• standard reaction

Multiple Choice

Identify the labels (a) and (b) on the graph.

• a is the amount (concentration) of reactants and b is the concentration of products

• a is the amount (concentration) of products and b is the concentration of reactants

• a is the volume of reactants and b is the volume of products

• a is the mass of reactants and b is the mass of products