Rates & Equilibrium — Chapter review · Physical Science: Chemistry, Grade 11

Distinguish reaction speed from equilibrium composition and calculate direction of adjustment.

Review the key ideas

Reaction Rates & Kinetics

Rates depend on a justified pathway and empirical kinetic relationships under specified conditions. Concentration, temperature, surface and catalyst effects are conditional; balanced coefficients do not generally determine rate-law exponents.

Chemical Equilibrium

Dynamic equilibrium has equal opposing rates, not necessarily equal concentrations. A disturbance changes Q or K depending on what changes; pressure predictions require volume/amount constraints, and catalysts do not change K at fixed conditions.

Equilibrium Constants & Reaction Quotients

Compare Q with K to predict adjustment; equilibrium means equal opposing rates, not equal amounts.

Problem 1

A reaction produces 75 mL of gas in 25 s. What is the average rate of gas production?
Answer and explanation

3.0 mL/s

Average rate = change in amount ÷ change in time = 75 mL ÷ 25 s = 3.0 mL/s.

Problem 2

Using collision theory, give one reason why heating the reactants speeds up a reaction.
Answer and explanation

Heating makes particles collide more often, and — more importantly — a larger fraction of collisions carry at least the activation energy, so more collisions are effective.

Higher temperature gives particles more kinetic energy. They collide more frequently, and a much larger share of those collisions now have enough energy to clear the activation-energy barrier. Both effects increase the number of effective collisions per second, so the rate rises.

Problem 3

During a reaction, a reactant's concentration drops from 0.90 M to 0.30 M in 40. s. What is the average rate of reaction in M/s?
Answer and explanation

0.015 M/s

The concentration changed by 0.90 − 0.30 = 0.60 M over 40 s. Average rate = 0.60 M ÷ 40 s = 0.015 M/s.

Problem 4

A reaction is at equilibrium and you add more reactant. Which way does the equilibrium shift?
Answer and explanation

Toward the products

By Le Châtelier's principle, the system responds to the extra reactant by shifting in the direction that consumes it — toward the products.

Problem 5

For H₂(g) + I₂(g) ⇌ 2HI(g), an equilibrium mixture contains [H₂] = 0.10 M, [I₂] = 0.40 M, and [HI] = 0.40 M. Calculate Keq.
Answer and explanation

Keq = 4

Keq = [HI]² ÷ ([H₂][I₂]) = (0.40)² ÷ (0.10 × 0.40) = 0.16 ÷ 0.040 = 4.0. Because Keq is greater than 1, products are favored.

Problem 6

The Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), is exothermic. State two condition changes that would increase the equilibrium yield of ammonia, and briefly justify each.
Answer and explanation

Increase the pressure (the product side has fewer gas moles, 2 vs 4, so higher pressure favors NH₃) and lower the temperature (the forward reaction is exothermic, so cooling favors it). Continuously removing NH₃ would also help.

By Le Châtelier's principle: (1) Raising the pressure shifts the equilibrium toward the side with fewer moles of gas; the left has 1 + 3 = 4 moles and the right has 2, so higher pressure favors ammonia. (2) Because the forward reaction is exothermic, heat behaves like a product, so lowering the temperature shifts the equilibrium forward, making more NH₃. (Removing ammonia as it forms would also pull the equilibrium to the right.)

Problem 7

What changes most directly when a catalyst provides a lower barrier?
Answer and explanation

The kinetic pathway and rate change; overall reaction ΔH and equilibrium K stay the same at fixed conditions.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 8

Chapter review: A selected rate law is r=k[A]²[B]. Doubling A and halving B at the same temperature changes rate by what factor? If r uses M/s, give k units and one condition needed for the prediction.
Answer and explanation

The factor is 2²×0.5=2. For total order three, k units are M⁻²s⁻¹. Assume the same law, temperature, medium and relevant pathway remain valid; the change does not establish a new mechanism.

The factor is 2²×0.5=2. For total order three, k units are M⁻²s⁻¹. Assume the same law, temperature, medium and relevant pathway remain valid; the change does not establish a new mechanism.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 9

How does compressing N₂+3H₂⇌2NH₃ at fixed T affect equilibrium?
Answer and explanation

Reduced volume raises partial pressures; the adjustment favors the side with fewer gas moles, ammonia, under the model.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 10

Chapter review: A⇌B has K=[B]/[A]=4 under a selected ideal concentration convention. Initially at equilibrium [A]=0.20 M and [B]=0.80 M. Add A to 0.70 M at unchanged volume/T. Find immediate Q and predict net direction without claiming original concentrations return.
Answer and explanation

Q=0.80/0.70≈1.14<4, so net forward conversion is favored. Under the closed 1:1 model total concentration becomes 1.50 M and final values are A=0.30 M, B=1.20 M; the ratio returns, not the original amounts.

Q=0.80/0.70≈1.14<4, so net forward conversion is favored. Under the closed 1:1 model total concentration becomes 1.50 M and final values are A=0.30 M, B=1.20 M; the ratio returns, not the original amounts.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 11

A reaction has K=5. Find K for its reverse.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

0.2

1/5=.2.

Problem 12

Q=10 while K=2. Predict adjustment.
Answer and explanation

Net reverse change lowers the product-to-reactant expression toward K.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 13

Chapter review: For 2A⇌A₂ under the normalized ideal convention, K=2.00, [A]=0.400 M and [A₂]=0.160 M. Compute Q, predict net direction, and state the maximum A₂ concentration if only an initial 0.800 M A inventory were available at fixed volume.
Answer and explanation

Q=0.160/(0.400)²=1.00<2.00, so net forward is favored. From an initial 0.800 M A-only inventory, conservation limits A₂ to at most 0.400 M. The equilibrium value need not reach that maximum, and no rate is given.

Q=0.160/(0.400)²=1.00<2.00, so net forward is favored. From an initial 0.800 M A-only inventory, conservation limits A₂ to at most 0.400 M. The equilibrium value need not reach that maximum, and no rate is given.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.
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