Grade by Grade
E. Chemical Systems and Equilibrium · Chemical Equilibrium

Chemical Equilibrium: Unit Review

E1.1 E1.2 E2.1 E2.2 E2.3 E2.4 E3.1 E3.2 E3.3 E3.4 A1.1 A1.2 A1.4 A1.5 A1.6 A1.8 A1.9 A1.10 A1.11 A1.12 A1.13 A2.1 A2.2About 40 minutes
Learning goals
  • I can explain dynamic equilibrium and write and use K, Kp and Q.
  • I can solve equilibrium problems with ICE tables, including the small-K approximation.
  • I can predict shifts with Le Châtelier's principle and apply equilibrium ideas to industry, living things and the environment.

Words to know

dynamic equilibrium
Forward and reverse rates are equal in a closed system, so concentrations stay constant.
K
The equilibrium constant: products over reactants, coefficients as exponents, at equilibrium; changes only with temperature.
Q
The reaction quotient: same form as K, any concentrations; compare with K to find the direction of shift.
Le Châtelier's principle
A system at equilibrium shifts to partly counteract a change.

Let's review!

In this unit you learned what dynamic equilibrium is, wrote equilibrium constant expressions (K and Kp) and combined them, and used ICE tables to find K and equilibrium concentrations, including the small-K approximation. You compared Q with K to predict the direction of change, applied Le Châtelier's principle to changes in concentration, pressure, temperature and catalysts, and saw how chemists investigate shifts and measure K. In the extension you linked K to entropy and Gibbs free energy. Finally, you applied it all to the Haber and contact processes and to equilibria in the body and the environment.

Next comes Acid-Base and Solubility Equilibria, which uses every tool in this unit for Ka, Kb, Kw, Ksp, pH and buffers. This review has questions from every lesson. Try each one, then press Show answer.

How to use this review

Do two or three sections at a time with a scientific calculator. For each calculation, write the balanced equation, the K expression and an ICE table in mol/L before touching numbers. For each Le Châtelier question, name the stress, the shift and whether K changes. When you miss a question, write down why (wrong exponent? used moles? forgot to check 5 %? forgot that only temperature changes K?) and go back to that lesson. Finish with the 10-question quiz.

Dynamic equilibriumrates equal (1)K expressionK, Kp, combining (2, 3)ICE tablesfind K or x (4, 5, 6)Q versus Kdirection of shift (7)Le Châtelierstresses and shifts (8, 9)Investigationsshifts, measuring K (10, 11)Why K? ΔG° = −RT ln K(12, extension)Applications: industry (13), living things and environment (14)
How the ideas in this unit connect. Numbers in brackets are lesson numbers.
Just for fun

Riddle: What did one chemical equilibrium say to the other at the end of the unit?

"I'm not stopping. I'm just balanced."

Reversible Reactions and Dynamic Equilibrium

1

Give the four conditions that describe dynamic equilibrium.

2

0.80 mol/L each of H₂ and I₂ react until [HI] = 1.20 mol/L. Find [H₂] at equilibrium and the percent reaction.

3

Is a saturated salt solution with undissolved salt in a sealed jar a static or dynamic system? Explain.

The Equilibrium Law and Writing K Expressions

1

Write K for 4 HCl(g) + O₂(g) ⇌ 2 Cl₂(g) + 2 H₂O(g).

2

Write K for Ni(s) + 4 CO(g) ⇌ Ni(CO)₄(g).

3

For 2 NO₂ ⇌ N₂O₄, [NO₂] = 0.040 and [N₂O₄] = 0.0080 mol/L at equilibrium. Find K.

Kp, Combining Equilibrium Constants and the K Family

1

For 2 NO₂(g) ⇌ N₂O₄(g), suppose Kc = 5.0 at 298 K. Find Kp in kPa.

2

K = 0.25 for A ⇌ B. Find K for 2 B ⇌ 2 A.

3

Write Kw and the Ksp expression for BaSO₄(s) ⇌ Ba²⁺(aq) + SO₄²⁻(aq).

ICE Tables I: Calculating K from Experimental Data

1

2.00 mol of PCl₅ is placed in a 2.00 L flask. At equilibrium, 0.40 mol of Cl₂ is present. Find K for PCl₅ ⇌ PCl₃ + Cl₂.

2

For 2 SO₂ + O₂ ⇌ 2 SO₃, a flask starts with [SO₂] = 0.60 and [O₂] = 0.40 mol/L. At equilibrium [SO₃] = 0.40 mol/L. Find K.

3

Which row of an ICE table follows the coefficients?

ICE Tables II: Finding Equilibrium Concentrations from K

1

For H₂ + I₂ ⇌ 2 HI, K = 36.0. Start: [H₂] = [I₂] = 0.400 mol/L. Find [HI].

2

For A ⇌ B + C, K = 0.20, [A]ᵢ = 0.50 mol/L. Find [B].

3

How do you decide which root of a quadratic to keep?

ICE Tables III: The Small-K Approximation

1

For A ⇌ 2 B, K = 4.0 × 10⁻⁸, [A]ᵢ = 0.10 mol/L. Find [B] and check the 5 % rule.

2

x = 0.009 and [A]ᵢ = 0.12 mol/L. Is the approximation valid?

3

Why can't you drop the x in (2x)²?

The Reaction Quotient Q: Which Way Will It Shift?

1

For N₂ + 3 H₂ ⇌ 2 NH₃, K = 0.50. A mixture has [N₂] = 1.0, [H₂] = 1.0 and [NH₃] = 1.0 mol/L. Which way does it shift?

2

For CO + H₂O ⇌ CO₂ + H₂, K = 4.00. Start: all four at 0.200 mol/L. Find [CO] at equilibrium.

3

What does Q = K mean?

Le Châtelier's Principle I: Changing Concentrations

1

For 2 CrO₄²⁻ + 2 H⁺ ⇌ Cr₂O₇²⁻ + H₂O, predict the colour change when a base is added.

2

After extra N₂ is added to an N₂/H₂/NH₃ equilibrium, how do [H₂] and [NH₃] change?

3

Does adding more solid sulfur shift S(s) + O₂(g) ⇌ SO₂(g)? Why?

Le Châtelier's Principle II: Pressure, Volume, Temperature and Catalysts

1

For 2 H₂S(g) ⇌ 2 H₂(g) + S₂(g), ΔH > 0, predict the shift for (a) decreasing the volume; (b) raising T; (c) adding a catalyst.

2

Which stress changes K?

3

Argon is added to a rigid container at equilibrium. What happens?

Investigation: Testing Le Châtelier's Principle in the Lab

1

Cobalt(II) chloride solution turns blue when heated. Is the pink-to-blue reaction endothermic or exothermic?

2

Why is silver nitrate waste collected rather than poured down the drain?

3

Write an "If ... then ... because ..." prediction for adding Cl⁻ to the cobalt equilibrium.

Investigation: Determining an Equilibrium Constant

1

2.00 mL of 2.00 × 10⁻³ mol/L KSCN is mixed with 5.00 mL of Fe(NO₃)₃ solution and 3.00 mL of water. Find [SCN⁻]ᵢ.

2

With a calibration slope of 4.80 × 10³ L/mol, what [FeSCN²⁺] gives A = 0.288?

3

Five trials give K = 147 to 152. What does this show?

Extension: Entropy, Gibbs Free Energy and K

1

Predict the sign of ΔS for 2 H₂O(g) → 2 H₂(g) + O₂(g).

2

A reaction has ΔH = −40 kJ and ΔS = −100 J/K. Below what temperature is it favourable?

3

A reaction has ΔG° = −11.4 kJ at 298 K. Find K.

Industrial Equilibria: The Haber Process and Sulfuric Acid

1

List the four main features of the industrial conditions for the Haber process and the reason for each.

2

Why is excess air used in the contact process?

3

Name one environmental impact of ammonia production and one way to reduce it.

Equilibrium in Living Things, the Environment and Technology

1

Why does breathing pure oxygen help a person with CO poisoning?

2

Explain in two steps why ocean acidification threatens shellfish.

3

How does fluoride protect tooth enamel?

What are you given?equilibrium amounts→ substitute into Kinitial amounts + oneequilibrium value→ ICE table → KK + initial amounts→ Q for direction →ICE in x → solveperfect square? square rootsmall K? approximate + 5 % checkotherwise: quadratic formulaAlways: mol/L, coefficients in the C row,equilibrium values in K, check your answer
A decision guide for equilibrium calculations.
Check your understanding

Quiz

1.

At dynamic equilibrium:

2.

The K expression for 2 A(g) + B(s) ⇌ C(g) is:

3.

If K = 2.0 for X ⇌ Y, then K for 2 Y ⇌ 2 X is:

4.

For A ⇌ B, [A]ᵢ = 1.0 mol/L and [B]ₑ = 0.20 mol/L. K is:

5.

When Q > K, the system:

6.

Which change increases K for an exothermic reaction?

7.

For N₂(g) + O₂(g) ⇌ 2 NO(g), decreasing the volume:

8.

The small-K approximation is valid when:

9.

A catalyst added to the Haber process:

10.

A reaction has ΔG° < 0 at 25 °C. Its K at 25 °C is:

For parents and teachers

How to use this review: each practice set matches one lesson. If your teen misses questions, send them back to that lesson:

  • Equal amounts versus equal rates, closed systems: lesson 1 (Reversible Reactions and Dynamic Equilibrium).
  • Wrong exponents, including solids or liquids in K: lesson 2 (The Equilibrium Law).
  • Kp, Δn, reversing or combining equations: lesson 3.
  • ICE-table setup, moles versus mol/L, finding K: lesson 4.
  • Solving for x, choosing the right root: lesson 5.
  • Approximation errors or a missing 5 % check: lesson 6.
  • Direction of shift, signs in the C row: lesson 7 (Q).
  • Concentration stresses and concentration-time graphs: lesson 8.
  • Pressure, temperature, inert gas or catalyst confusion; thinking K changes with concentration: lesson 9.
  • Planning, safety, observations, error analysis: lessons 10 and 11.
  • ΔG, ΔS and ln K (extension): lesson 12.
  • Industrial conditions and "compromise" reasoning: lesson 13.
  • Hemoglobin, CO, oceans, teeth and kidney stones: lesson 14.

A strong finish: your teen can solve a full "Q, then ICE, then check" problem and explain a Le Châtelier shift in words, with the reason (Q or K) each time.