Equilibrium Constant Examples in Chemistry

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

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

Concept Recap

The ratio of product concentrations to reactant concentrations at equilibrium, each raised to its stoichiometric coefficient.

K is the scoreboard at the end of the game β€” it tells you which side won. Large K means products dominate; small K means reactants dominate.

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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: Equilibrium Constant starts by naming reactants and products, then checks conservation with a balanced equation.

Common stuck point: Students often know a formula related to equilibrium constant but skip the recognition step: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation?

Worked Examples

Example 1

medium
For N2O4β‡Œ2NO2N_2O_4 \rightleftharpoons 2NO_2, [N2O4]=0.040[N_2O_4] = 0.040 M and [NO2]=0.040[NO_2] = 0.040 M at equilibrium. Find KcK_c.

Answer

Kc=0.040K_c = 0.040

First step

1
Kc=[NO2]2/[N2O4]K_c = [NO_2]^2 / [N_2O_4].

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Example 2

medium
For Aβ‡ŒBA \rightleftharpoons B, K1=4K_1 = 4. If reaction 2 is Bβ‡ŒAB \rightleftharpoons A, what is K2K_2?

Example 3

hard
For H2+I2β‡Œ2HIH_2 + I_2 \rightleftharpoons 2HI, a 1.01.0 L vessel initially holds 0.500.50 mol H2H_2 and 0.500.50 mol I2I_2. At equilibrium [HI]=0.80[HI] = 0.80 M. Find KcK_c.

Example 4

hard
For Aβ‡ŒBA \rightleftharpoons B, Kc=4K_c = 4. Start with [A]0=1.0[A]_0 = 1.0 M and no B. Find [A][A] and [B][B] at equilibrium.

Example 5

hard
For Aβ‡Œ2BA \rightleftharpoons 2B with Kc=1.0K_c = 1.0 and starting with [A]0=1.0[A]_0 = 1.0 M and no BB, set up and solve for [B][B] at equilibrium.

Example 6

hard
For 2SO2+O2β‡Œ2SO32SO_2 + O_2 \rightleftharpoons 2SO_3, at equilibrium [SO2]=0.20[SO_2] = 0.20, [O2]=0.10[O_2] = 0.10, [SO3]=0.80[SO_3] = 0.80 M. Find KcK_c.

Example 7

challenge
Phosgene decomposes: COCl2β‡ŒCO+Cl2COCl_2 \rightleftharpoons CO + Cl_2, Kc=0.090K_c = 0.090 at TT. A 1.01.0 L flask holds 1.01.0 mol COCl2COCl_2 initially. Find [CO][CO] at equilibrium.

Practice Problems

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

Example 1

easy
Write the equilibrium expression KcK_c for Aβ‡ŒBA \rightleftharpoons B.

Example 2

easy
In the expression for 2Aβ‡ŒB2A \rightleftharpoons B, what power does [A][A] take?

Example 3

easy
A reaction has a very large KK. Do products or reactants dominate at equilibrium?

Example 4

easy
Should pure solids appear in an equilibrium constant expression?

Example 5

easy
Write KcK_c for N2+3H2β‡Œ2NH3N_2 + 3H_2 \rightleftharpoons 2NH_3.

Example 6

easy
Does adding more reactant change the value of KK (at constant temperature)?

Example 7

easy
If K=1K = 1, what does that say about products and reactants at equilibrium?

Example 8

easy
Write KcK_c for the heterogeneous equilibrium CaCO3(s)β‡ŒCaO(s)+CO2(g)CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g).

Example 9

medium
At equilibrium [A]=2.0[A]=2.0 M, [B]=4.0[B]=4.0 M for Aβ‡ŒBA \rightleftharpoons B. Find KcK_c.

Example 10

medium
For H2+I2β‡Œ2HIH_2 + I_2 \rightleftharpoons 2HI, equilibrium gives [H2]=0.10[H_2]=0.10, [I2]=0.20[I_2]=0.20, [HI]=0.40[HI]=0.40 M. Find KcK_c.

Example 11

medium
A reaction has Q=5Q = 5 and K=2K = 2. Which direction does the reaction shift?

Example 12

medium
For 2SO2+O2β‡Œ2SO32SO_2 + O_2 \rightleftharpoons 2SO_3, write KcK_c and state its concentration dependence on [O2][O_2].

Example 13

medium
If Kforward=4K_{forward} = 4, what is KK for the reverse reaction?

Example 14

medium
A reaction Aβ‡Œ2BA \rightleftharpoons 2B has K=9K = 9. If [A]=1.0[A] = 1.0 M at equilibrium, find [B][B].

Example 15

medium
Two reactions are added: reaction 1 has K1=2K_1 = 2, reaction 2 has K2=3K_2 = 3. What is KK for the overall reaction?

Example 16

medium
A reaction has Q=1Q = 1 and K=5K = 5. Which direction does it shift?

Example 17

medium
Write the equilibrium expression KcK_c for CO+2H2β‡ŒCH3OHCO + 2H_2 \rightleftharpoons CH_3OH.

Example 18

challenge
For N2+3H2β‡Œ2NH3N_2 + 3H_2 \rightleftharpoons 2NH_3, equilibrium gives [N2]=1.0[N_2]=1.0, [H2]=2.0[H_2]=2.0, [NH3]=4.0[NH_3]=4.0 M. Compute KcK_c.

Example 19

challenge
A 1.0 L vessel starts with 1.0 mol H2H_2 and 1.0 mol I2I_2. At equilibrium 0.20 mol H2H_2 remains for H2+I2β‡Œ2HIH_2 + I_2 \rightleftharpoons 2HI. Find KcK_c.

Example 20

challenge
Reaction 1: Aβ‡ŒBA \rightleftharpoons B, K1=4K_1 = 4. Reaction 2: Bβ‡ŒCB \rightleftharpoons C, K2=5K_2 = 5. Find KK for Aβ‡ŒCA \rightleftharpoons C and for Cβ‡ŒAC \rightleftharpoons A.

Example 21

easy
What is the symbol for the reaction quotient (not at equilibrium)?

Example 22

easy
Write KcK_c for 3Aβ‡ŒB3A \rightleftharpoons B.

Example 23

easy
A reaction has K=1Γ—10βˆ’8K = 1 \times 10^{-8}. Are products or reactants favored at equilibrium?

Example 24

easy
Write KcK_c for H2(g)+I2(g)β‡Œ2HI(g)H_2(g) + I_2(g) \rightleftharpoons 2HI(g).

Example 25

easy
Does a catalyst change the value of KK?

Example 26

medium
For Aβ‡Œ2BA \rightleftharpoons 2B, [A]=0.50[A] = 0.50 M and [B]=1.0[B] = 1.0 M at equilibrium. Find KcK_c.

Example 27

medium
Reaction has K=10K = 10. If Q=100Q = 100, which direction does it shift?

Example 28

medium
Write KcK_c for the heterogeneous equilibrium 2NaHCO3(s)β‡ŒNa2CO3(s)+H2O(g)+CO2(g)2NaHCO_3(s) \rightleftharpoons Na_2CO_3(s) + H_2O(g) + CO_2(g).

Example 29

medium
Reaction Aβ‡ŒBA \rightleftharpoons B has K=8K = 8 at 300300 K. After increasing TT, KK becomes 44. Is the forward reaction endothermic or exothermic?

Example 30

medium
For 2Aβ‡ŒB2A \rightleftharpoons B, Kc=25K_c = 25 and [A]=0.20[A] = 0.20 M at equilibrium. Find [B][B].

Example 31

medium
Reaction Aβ‡ŒBA \rightleftharpoons B has K=2K = 2. If you instead write 2Aβ‡Œ2B2A \rightleftharpoons 2B, what is KK?

Example 32

medium
At 500500 K, Kc=0.50K_c = 0.50 for Aβ‡ŒBA \rightleftharpoons B. Starting with [A]=1.0[A] = 1.0 M, [B]=0[B] = 0 M, in which direction does the reaction proceed?

Example 33

medium
For A+Bβ‡ŒCA + B \rightleftharpoons C, Kc=50K_c = 50. At equilibrium [A]=0.10[A] = 0.10 M and [C]=0.50[C] = 0.50 M. Find [B][B].

Example 34

medium
For a reaction with Ξ”ngas=0\Delta n_{gas} = 0, how are KpK_p and KcK_c related?

Example 35

hard
For Aβ‡Œ2BA \rightleftharpoons 2B, Kc=0.040K_c = 0.040. Starting with pure AA at 0.100.10 M and pure BB at 0.200.20 M, predict the direction of net change.

Example 36

hard
For N2O4β‡Œ2NO2N_2O_4 \rightleftharpoons 2NO_2 in the gas phase at TT, Kc=0.36K_c = 0.36 and RT=24.5RT = 24.5 LΒ·atm/mol. Find KpK_p.

Example 37

hard
Two reactions: R1R_1: Aβ‡ŒBA \rightleftharpoons B, K1=4K_1 = 4. R2R_2: 2Bβ‡ŒC2B \rightleftharpoons C, K2=9K_2 = 9. Find KK for 2Aβ‡ŒC2A \rightleftharpoons C.

Example 38

challenge
For Aβ‡ŒBA \rightleftharpoons B, K=4.0K = 4.0 at T1=300T_1 = 300 K and K=1.0K = 1.0 at T2=400T_2 = 400 K. State whether the forward reaction is exothermic or endothermic, and justify with Le ChΓ’telier.
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Background Knowledge

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

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