AP Chemistry Equilibrium Lesson

​Chapter 6

Equilibrium Constant (Kc)

• ​Solid and liquid substances are not represented in a K_c expression, since their concentrations do not vary in the course of reaction.

• For example, C(s) + CO₂(g) 2CO(g)

  The equilibrium expression of the reaction is;

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

For the graph, choose the CORRECT statement.

• The rate of reaction slowly increase

• The systems never reach equilibrium

• At equilibrium, more NO₂ is present than N₂O₄.

• At start of reaction, only NO₂ was present.

Multiple Choice

Which of the two graphs reaches equilibrium?

• The left one.

• The right one.

• Both of them.

• Neither.

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

Multiple Choice

At what time does the reaction reach equilibrium?

• t₁

• t₂

• t₃

• t₄

Multiple Choice

During equilibrium, the rates of the forward and the reverse reaction are ________

• equal

• unequal

• unequal for exothermic reactions

• unequal for endothermic reactions

Multiple Choice

For the reaction, choose the CORRECT equilibrium constant, K_c.

Multiple Choice

When writing the expression for an equilibrium constant, which type of substance is included?

• pure solids

• pure liquids

• gases and aqueous species

• only gases

Multiple Choice

What is the equilibrium-constant expression for 
CO₂(g) + H₂(g) ↔ CO(g) + H₂O(l)

• K_c= [CO][H₂O] / [CO₂][H₂]  

• K_c= [CO₂][H₂] / [CO]

• K_c= [CO₂][H₂] / [CO][H₂O]

• K_c= [CO] / [CO₂][H₂]  

Multiple Choice

What is the Kc expression for the reaction below: SOCl₂(g)  +  H₂O(g)   ⇌    SO₂(g)  +  2HCl(g)

• $$\frac{[SO_2][2HCl]}{[SOCl_2][H_2O]}$$  

• $$\frac{[SO_2][HCl]^2}{[SOCl_2][H_2O]}$$  

• $$\frac{[SOCl_2][H_2O]}{[SO_2][2HCl]}$$  

• $$\frac{[SOCl_2][H_2O]}{\left[[SO_2\right][2HCl]}$$  

​Example: Write an expression for Kc:

N₂(g) + 3H₂(g) 2NH₃(g)

The reaction starts with 2 mol of NH₃ in 1L container. When 25 percent of NH₃ decomposes into N₂ and H₂, the reaction establishes an equilibrium.

What is the value of Kc?

​Example: The colourless gas dinitrogen tetroxide decomposes to the brown coloured air pollutant nitrogen dioxide and exists in equilibrium.

A 0.125 mol sample of dinitrogen tetroxide is introduced into a 1.00 L container and allowed to decompose at a given temperature. When equilibrium is reached, the concentration of the dinitrogen tetroxide is 0.0750 mol/L.

What is the value of Kc for this reaction?

​Examples: 1. Phosphorus pentachloride decomposes to phosphorus trichloride and chlorine gas. A 1.10 mol/L sample of PCl₅ was placed into a vessel. At equilibrium it was determined that the concentration was 0.33 mol/L. Calculate the Kc for this reaction? PCl₅(g) ⇌ PCl₃(g) + Cl₂(g)

2. Gaseous dinitrogen tetroxide is placed in a flask and allowed to decompose to nitrogen dioxide and reach equilibrium at 100°C. At 100°C, the value of K_eq is 0.212. If the concentration of dinitrogen tetroxide at equilibrium is 0.155 mol/L, what is the concentration of nitrogen dioxide at equilibrium?

N₂O₄(g) ⇌ 2NO2(g)