Chemical Equilibrium Examples: 69 Problems with Answers

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Chemical Equilibrium.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

A dynamic state in a reversible reaction where the forward and reverse reactions proceed at equal rates, so the macroscopic concentrations of reactants and products stay constant even though molecules keep converting in both directions.

The reaction is still happening both ways, but the amounts stay constant.

Read the full concept explanation →

How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Chemical Equilibrium starts by naming the reversible reaction, the stress, and which side is favored.

Common stuck point: Students often know a formula related to chemical equilibrium but skip the recognition step: Am I reasoning about a reversible reaction where forward and reverse processes continue and a stress shifts the composition? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I reasoning about a reversible reaction where forward and reverse processes continue and a stress shifts the composition?

Worked Examples

Example 1

easy

Explain what it means for a reaction to be at chemical equilibrium.

Answer

{Rate}_{forward} = {Rate}_{reverse}

First step

At equilibrium, the forward reaction rate equals the reverse reaction rate.

Full solution

2
Concentrations of reactants and products remain constant (but not necessarily equal).
3
Equilibrium is dynamic — both forward and reverse reactions continue to occur, but there is no net change.

Chemical equilibrium does not mean the reaction has stopped. It means the rates of the forward and reverse reactions are balanced, so concentrations remain steady.

Example 2

medium

For the reaction

\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3

, at equilibrium

[\text{N}_2] = 0.50\,\text{M}

[\text{H}_2] = 1.50\,\text{M}

, and

[\text{NH}_3] = 0.60\,\text{M}

. Calculate

K_{eq}

N₂ + 3H₂ ⇌ 2NH₃

Example 3

medium

Build an ICE table for

\text{N}_2\text{O}_4(g) \rightleftharpoons 2\text{NO}_2(g)

starting with

[\text{N}_2\text{O}_4]_0 = 0.100\,\text{M}

and no

\text{NO}_2

. At equilibrium,

[\text{NO}_2] = 0.060\,\text{M}

. Compute

K_c

N₂O₄ ⇌ 2NO₂ (ICE table)

Example 4

medium

For

\text{H}_2(g) + \text{F}_2(g) \rightleftharpoons 2\text{HF}(g)

K_c = 1.0 \times 10^2

. Starting with

[\text{H}_2]_0=[\text{F}_2]_0=1.00\,\text{M}

, find

[\text{HF}]

at equilibrium.

Example 5

medium

Acetic acid dissociates as

\text{HC}_2\text{H}_3\text{O}_2 \rightleftharpoons \text{H}^+ + \text{C}_2\text{H}_3\text{O}_2^-

with

K_a = 1.8 \times 10^{-5}

. Find

[\text{H}^+]

in a

0.10\,\text{M}

solution.

Example 6

hard

For

\text{COCl}_2(g) \rightleftharpoons \text{CO}(g) + \text{Cl}_2(g)

K_c = 2.2 \times 10^{-10}

T

. A

1.00\,\text{L}

vessel is loaded with

0.500\,\text{mol}

\text{COCl}_2

. Find

[\text{CO}]

at equilibrium.

Example 7

hard

For

2\text{HI}(g) \rightleftharpoons \text{H}_2(g) + \text{I}_2(g)

K_c = 0.0156

T

. A

1.00\,\text{L}

flask is loaded with

0.50\,\text{mol HI}

. Find

[\text{H}_2]

at equilibrium.

Example 8

hard

K_p

for

\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3

T = 500\,\text{K}

1.5 \times 10^{-5}\,\text{atm}^{-2}

. Convert to

K_c

. Use

R = 0.0821\,\text{L atm/(mol K)}

Example 9

hard

2.00\,\text{L}

vessel contains

0.040\,\text{mol}

\text{SO}_2

0.020\,\text{mol}

\text{O}_2

, and

0.080\,\text{mol}

\text{SO}_3

at equilibrium for

2\text{SO}_2 + \text{O}_2 \rightleftharpoons 2\text{SO}_3

. Compute

K_c

2SO₂ + O₂ ⇌ 2SO₃

Example 10

challenge

For

\text{H}_2(g) + \text{CO}_2(g) \rightleftharpoons \text{H}_2\text{O}(g) + \text{CO}(g)

K_c = 4.40

T

. A

1.00\,\text{L}

vessel starts with

1.00\,\text{mol}

each of

\text{H}_2

and

\text{CO}_2

. Find equilibrium

[\text{CO}]

Example 11

challenge

A buffer is made by dissolving

0.20\,\text{mol}

acetic acid (

K_a = 1.8 \times 10^{-5}

) and

0.20\,\text{mol}

sodium acetate in

1.00\,\text{L}

water. Find the pH using the equilibrium expression.

Example 12

medium

For

\text{H}_2 + \text{I}_2 \rightleftharpoons 2\text{HI}

, at equilibrium

[\text{H}_2] = 0.20\,\text{M}

[\text{I}_2] = 0.20\,\text{M}

[\text{HI}] = 1.00\,\text{M}

. Find

K

Example 13

medium

Given

\text{A} \rightleftharpoons \text{B}

with

K = 3.0

. At equilibrium

[\text{A}] = 0.20\,\text{M}

. Find

[\text{B}]

Example 14

medium

For

\text{N}_2\text{O}_4 \rightleftharpoons 2\text{NO}_2

K = 0.211

at equilibrium

[\text{N}_2\text{O}_4] = 0.500\,\text{M}

. Find

[\text{NO}_2]

Example 15

hard

For

\text{A} \rightleftharpoons 2\text{B}

, start with

[\text{A}]_0 = 1.00\,\text{M}

, no B. At equilibrium

[\text{A}] = 0.40\,\text{M}

. Find

K

Example 16

hard

Reaction

\text{X} \rightleftharpoons \text{Y}

has

K = 16

25^\circ\text{C}

and

K = 4

80^\circ\text{C}

. Is the forward reaction exothermic or endothermic?

Example 17

challenge

For

\text{H}_2 + \text{I}_2 \rightleftharpoons 2\text{HI}

K = 50

. If

[\text{H}_2]_0 = [\text{I}_2]_0 = 1.00\,\text{M}

, no HI initially, find

[\text{HI}]

at equilibrium.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium

For

\text{N}_2\text{O}_4 \rightleftharpoons 2\text{NO}_2

, if additional

\text{N}_2\text{O}_4

is added, which direction does the equilibrium shift? Apply Le Chatelier's principle.

N₂O₄ ⇌ 2NO₂ — Le Chatelier shift direction

Example 2

medium

At equilibrium in a reversible reaction, have the reactions stopped? Explain what is equal at equilibrium.

Example 3

easy

At chemical equilibrium, how do the forward and reverse reaction rates compare?

Example 4

easy

At equilibrium, are reactant and product concentrations necessarily equal?

Example 5

easy

Which symbol indicates a reaction is reversible (at equilibrium)?

Example 6

easy

Is a reaction at equilibrium static (stopped) or dynamic (ongoing)?

Example 7

easy

For

\text{A}\rightleftharpoons\text{B}

, write the equilibrium constant expression

K

Example 8

easy

K\gg1

for a reaction at equilibrium, are products or reactants favored?

Example 9

easy

For

\text{N}_2+3\text{H}_2\rightleftharpoons2\text{NH}_3

, write the equilibrium expression

K

N₂ + 3H₂ ⇌ 2NH₃

Example 10

easy

Does changing temperature change the value of the equilibrium constant

K

Example 11

medium

For

\text{H}_2+\text{I}_2\rightleftharpoons2\text{HI}

, at equilibrium

[\text{H}_2]=0.10

[\text{I}_2]=0.10

[\text{HI}]=0.80\,\text{M}

. Find

K

H₂ + I₂ ⇌ 2HI

Example 12

medium

A reversible reaction reaches equilibrium. Explain why concentrations stop changing even though reactions continue.

Example 13

medium

K=4

for

\text{A}\rightleftharpoons\text{B}

. At equilibrium

[\text{A}]=0.20\,\text{M}

. Find

[\text{B}]

Example 14

medium

For an exothermic reaction at equilibrium, raising temperature decreases

K

. Does the equilibrium shift toward reactants or products?

Example 15

medium

Why does adding a catalyst NOT change the equilibrium position of a reversible reaction?

Example 16

medium

For

2\text{SO}_2+\text{O}_2\rightleftharpoons2\text{SO}_3

, at equilibrium

[\text{SO}_2]=0.20

[\text{O}_2]=0.50

[\text{SO}_3]=0.40\,\text{M}

. Find

K

2SO₂ + O₂ ⇌ 2SO₃

Example 17

medium

A sealed flask of

\text{N}_2\text{O}_4\rightleftharpoons2\text{NO}_2

shows constant brown color. Is the reaction stopped? Explain.

Example 18

medium

For

\text{A}\rightleftharpoons\text{B}

with

K=0.01

, are reactants or products favored, and by roughly how much?

Example 19

medium

For

\text{A}+\text{B}\rightleftharpoons\text{C}

, at equilibrium

[\text{A}]=0.50

[\text{B}]=0.40

[\text{C}]=0.20\,\text{M}

. Find

K

Example 20

challenge

For

\text{A}\rightleftharpoons2\text{B}

, start with

[\text{A}]_0=1.00\,\text{M}

, no B. At equilibrium

[\text{A}]=0.60\,\text{M}

. Find

K

Example 21

challenge

Reaction

\text{X}\rightleftharpoons\text{Y}

has

K=9

300\,\text{K}

and

K=4

400\,\text{K}

. Is the forward reaction exothermic or endothermic? Explain.

Example 22

challenge

\text{A}\rightleftharpoons\text{B}

with

K=2

, the reaction quotient is computed as

Q=0.5

at some instant. Which direction will the reaction proceed to reach equilibrium?

Example 23

easy

Write the equilibrium expression

K_c

for

2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g)

Example 24

easy

K_c \gg 1

for a reaction at equilibrium, are reactants or products favored?

Example 25

easy

For

\text{CaCO}_3(s) \rightleftharpoons \text{CaO}(s) + \text{CO}_2(g)

, write

K_c

Example 26

easy

For

\text{H}_2(g) + \text{I}_2(g) \rightleftharpoons 2\text{HI}(g)

, at equilibrium

[\text{H}_2] = 0.10\,\text{M}

[\text{I}_2] = 0.10\,\text{M}

, and

[\text{HI}] = 0.80\,\text{M}

. Compute

K_c

Example 27

easy

Which direction does

\text{CO}(g) + \text{H}_2\text{O}(g) \rightleftharpoons \text{CO}_2(g) + \text{H}_2(g)

shift if

\text{H}_2

is removed?

Example 28

medium

For

\text{PCl}_5(g) \rightleftharpoons \text{PCl}_3(g) + \text{Cl}_2(g)

K_c = 0.040

T

. If

[\text{PCl}_5]=0.10\,\text{M}

[\text{PCl}_3]=0.20\,\text{M}

[\text{Cl}_2]=0.10\,\text{M}

, compute

Q

and predict the shift direction.

Example 29

medium

The reaction

\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)

is exothermic. How does

K_c

change as temperature increases?

Example 30

medium

For

2\text{NO}_2(g) \rightleftharpoons \text{N}_2\text{O}_4(g)

, what is the effect on the equilibrium position of decreasing the container volume (increasing pressure)?

Example 31

medium

Adding a catalyst to a system at equilibrium has what effect on

K_c

and on the equilibrium position?

Example 32

medium

For

\text{N}_2\text{O}_4(g) \rightleftharpoons 2\text{NO}_2(g)

K_p = 0.15\,\text{atm}

T

. If

P_{\text{N}_2\text{O}_4} = 0.40\,\text{atm}

at equilibrium, what is

P_{\text{NO}_2}

Example 33

medium

For

\text{AgCl}(s) \rightleftharpoons \text{Ag}^+(aq) + \text{Cl}^-(aq)

K_{sp} = 1.8 \times 10^{-10}

. Compute the molar solubility of AgCl in pure water.

Example 34

hard

Reaction 1:

\text{A} \rightleftharpoons \text{B}

has

K_1 = 2

. Reaction 2:

\text{B} \rightleftharpoons \text{C}

has

K_2 = 3

. Find

K

for

\text{A} \rightleftharpoons \text{C}

Example 35

hard

K_c = 4.0

for

\text{A} + \text{B} \rightleftharpoons \text{C} + \text{D}

, what is

K_c

for the reverse reaction

\text{C} + \text{D} \rightleftharpoons \text{A} + \text{B}

Example 36

hard

K_c

for

\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3

0.50

T

. Compute

K_c

for

\tfrac{1}{2}\text{N}_2 + \tfrac{3}{2}\text{H}_2 \rightleftharpoons \text{NH}_3

Example 37

hard

For an endothermic reaction at equilibrium, predict the effect on

K

of (a) raising

T

, (b) adding an inert gas at constant volume.

Example 38

easy

At chemical equilibrium, what is true about the forward and reverse reaction rates?

Example 39

easy

Which arrow symbol indicates a reversible reaction at equilibrium?

Example 40

easy

K \gg 1

for

\text{A} \rightleftharpoons \text{B}

, which side dominates at equilibrium?

Example 41

easy

K \ll 1

for a reversible reaction, which side dominates?

Example 42

easy

For

\text{A}+\text{B} \rightleftharpoons \text{C}

, write the equilibrium expression

K

Example 43

medium

For

\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3

, if you add more

\text{N}_2

, which way does the equilibrium shift?

Example 44

medium

For an exothermic equilibrium, increasing temperature shifts it which way?

Example 45

medium

For

2\text{SO}_2 + \text{O}_2 \rightleftharpoons 2\text{SO}_3

, if pressure is increased (constant T), which way does the equilibrium shift?

Example 46

medium

For

\text{A} \rightleftharpoons \text{B}

, at some instant

Q = 5

and

K = 2

. In which direction does the reaction proceed?

Example 47

medium

For a reversible reaction at equilibrium, if you remove a product, what happens?

Example 48

hard

For an endothermic reaction at equilibrium, what happens to

K

when temperature is increased?

Example 49

hard

If the equation

\text{A} \rightleftharpoons \text{B}

has

K_1 = 4

, what is

K_2

for the reverse reaction

\text{B} \rightleftharpoons \text{A}

Example 50

hard

For

2\text{A} \rightleftharpoons \text{B}

with

K_1

, what is K for

4\text{A} \rightleftharpoons 2\text{B}

Example 51

hard

For

\text{CaCO}_3(s) \rightleftharpoons \text{CaO}(s) + \text{CO}_2(g)

, write the equilibrium expression

K

Example 52

challenge

Explain why two simultaneous equilibria sharing a common species (e.g., a common ion) interact through Le Chatelier's principle.

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Related Concepts

Chemical Reaction Reaction Rate Le Chatelier's Principle Equilibrium Constant

Background Knowledge

These ideas may be useful before you work through the harder examples.

chemical reactionreaction rate