Rates & Equilibrium — Chapter Practice, Grade 11

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

Problem 1

For a reaction whose rate increases with this reactant’s concentration, which change usually speeds it up while other conditions stay fixed?
  1. removing the catalyst
  2. cooling the reactants
  3. using large chunks instead of a fine powder
  4. increasing the concentration of a reactant
Answer and explanation

increasing the concentration of a reactant

Increasing that reactant’s concentration can increase rate under the stated conditions. Concentration effects depend on the actual reaction and rate law; this is not a universal rule for every reaction.

Problem 2

According to collision theory, an effective collision requires…
  1. enough energy and the correct orientation
  2. that a catalyst be present
  3. that the reaction be exothermic
  4. only that the particles touch
Answer and explanation

enough energy and the correct orientation

A collision only produces a reaction if the particles collide with at least the activation energy and are properly oriented.

Problem 3

A reaction releases 48 mL of gas in 12 s. What is the average rate of gas production in mL/s?
Answer and explanation

4 mL/s

Rate = 48 mL ÷ 12 s = 4.0 mL/s.

Problem 4

Explain, using collision theory, why powdered zinc reacts with acid much faster than a single lump of zinc of the same mass.
Answer and explanation

Powder has far more exposed surface area than a single lump, so many more zinc atoms are in contact with the acid at once. That allows far more effective collisions per second between zinc and acid particles, which speeds up the reaction.

Problem 5

In a reaction, 0.36 mol of a reactant is consumed in 45 s. What is the average rate of consumption in mol/s?
Answer and explanation

0.008 mol/s

Rate = 0.36 mol ÷ 45 s = 0.008 mol/s.

Problem 6

Doubling B at fixed A doubles rate. Find order in B. Show the reasoning, not just the conclusion.
Hints

Reaction coefficients do not generally supply rate-law exponents.

Answer and explanation

First order, since 2^n=2 gives n=1.

Problem 7

Why does a catalyst not increase equilibrium yield solely by lowering activation barriers?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

It accelerates forward and reverse processes without changing the equilibrium constant at fixed temperature.

Problem 8

Temperature changes the population of particles above a barrierTemperature changes the population of particles above a barrierProbability density (inverse energy unit)00.10.20.30.40.5024681012Translational energy (selected units)Lower T modelHigher T model
Original normalized ideal translational-energy distributions with kBT values of 1 and 2 selected energy units. The dashed threshold is 4 units; the finite plot truncates the tails.
Interpret the supplied visual. Does the lower peak of the higher-temperature distribution mean fewer total particles?
Answer and explanation

No. These are normalized distributions. The population is spread over a wider energy range; compare areas and the threshold tail rather than peak height alone.

At thermal equilibrium a gas has a distribution of translational energies, not one common energy per molecule. Raising temperature shifts and broadens the ideal distribution so a greater fraction can exceed a selected activation threshold. This helps explain many temperature effects on rates, though orientation, mechanism and transport also matter. A catalyst changes the pathway/barrier rather than simply raising every molecule’s energy or changing equilibrium. The shaded-free curves here encode a probability density; an area, not peak height alone, represents a fraction.

No. These are normalized distributions. The population is spread over a wider energy range; compare areas and the threshold tail rather than peak height alone.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 9

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 10

At dynamic equilibrium…
  1. the reaction has completely stopped
  2. the forward and reverse reactions occur at equal rates
  3. reactants and products are always present in equal amounts
  4. no more collisions occur
Answer and explanation

the forward and reverse reactions occur at equal rates

Equilibrium is dynamic: both reactions still run, but at equal rates, so concentrations stay constant.

Problem 11

Adding more reactant to a system at equilibrium causes the equilibrium to shift…
  1. toward the reactants
  2. toward the products
  3. so the reaction stops
  4. in no direction at all
Answer and explanation

toward the products

By Le Châtelier's principle, the system shifts toward the products to consume the added reactant.

Problem 12

At equilibrium, the forward reaction has a rate of 0.020 mol/(L·s). What is the rate of the reverse reaction at that same moment?
Answer and explanation

0.02 mol/(L·s)

At equilibrium the forward and reverse rates are equal by definition, so the reverse rate is also 0.020 mol/(L·s).

Problem 13

A student says, 'At equilibrium, the reaction has stopped.' Explain why this statement is incorrect.
Answer and explanation

The forward and reverse reactions are both still happening — they have not stopped. They simply run at equal rates, so the concentrations stay constant. Equilibrium is dynamic, not static.

Problem 14

For the exothermic reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) + heat, increasing the temperature will…
  1. cause the reaction to stop entirely
  2. shift the equilibrium right, making more SO₃
  3. shift the equilibrium left, making less SO₃
  4. have no effect on the equilibrium
Answer and explanation

shift the equilibrium left, making less SO₃

Heat behaves like a product in an exothermic reaction, so adding heat shifts the equilibrium in the reverse (endothermic) direction — left, reducing the amount of SO₃.

Problem 15

What does adding a catalyst do to an established equilibrium composition in the ideal model? Show the reasoning, not just the conclusion.
Hints

Equilibrium concentrations change, while K remains fixed at that temperature.

Answer and explanation

It does not shift equilibrium; it changes rates and how quickly equilibrium is reached.

Problem 16

Why can changing temperature change K while adding reactant at fixed temperature does not?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

K depends on thermodynamics and temperature; concentration changes alter Q and the response, not fixed-temperature K.

Problem 17

Not every intervention changes the equilibrium constantNot every intervention changes the equilibrium constantChange at same reaction conditionsK effect / responseConcentration disturbance at fixed TQ changes; K does notCatalyst at fixed TRates change; K does notTemperature changeK can changeInert gas at fixed V, ideal mixtureReactant partial pressuresunchangedPressure effects require the actual volume/amount constraint.
Original condition-comparison table; ideal mixture assumptions and unchanged reaction identity are explicit.
Interpret the supplied visual. Why is “raising total pressure always shifts equilibrium toward fewer gas molecules” incomplete?
Answer and explanation

The method matters. Adding inert gas at fixed volume does not change ideal reacting-species partial pressures; compression is a different disturbance.

For a specified reaction and standard-state convention, K depends on temperature rather than how much material was initially added. A catalyst changes approach rates but not the equilibrium constant for the same conditions. Temperature changes can alter K; their direction requires the appropriate thermodynamic account. Adding an inert gas to an ideal mixture at fixed volume can raise total pressure without changing reacting-species partial pressures. Distinguish total pressure from the quantities entering Q before predicting a composition shift.

The method matters. Adding inert gas at fixed volume does not change ideal reacting-species partial pressures; compression is a different disturbance.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 18

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 19

For A⇌B, [A]=.20 M and [B]=.60 M at equilibrium. Find K.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

3

K=.60/.20.

Problem 20

Why omit a pure solid from a simple K expression?
Answer and explanation

Its activity is approximately constant while that pure phase is present.

Response criteria

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

Problem 21

For A⇌B, equilibrium [A]=.40 M and [B]=.80 M. Find K.
Hints

List the given quantities, select a relationship, and check units before substituting.

Answer and explanation

2

.80/.40=2.

Problem 22

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 23

For A⇌B with K=3 and Q=6, predict direction. Show the reasoning, not just the conclusion.
Hints

Compare Q and K for the same expression and temperature.

Answer and explanation

Reverse toward A, reducing Q toward K.

Problem 24

Why are pure solids omitted from standard equilibrium expressions?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Their activities are treated as unity when the pure phase is present; changing its amount alone does not change that activity.

Problem 25

A quadratic solution must also satisfy material constraintsA quadratic solution must also satisfy material constraints2A ⇌ A₂[A] (M)[A₂] (M)Initial1.000Change−2x+xEquilibrium1.00−2xxK=2: x/(1−2x)² = 2 → x≈0.3048 MChoose the physical root: 0 ≤ x ≤ 0.500 M.
Original calculated hypothetical dimerization using ideal normalized concentrations and no volume change; A and A₂ conserve the same atomic building units.
Interpret the supplied visual. Why must x be no greater than 0.500 M for the selected initial 1.00 M A inventory?
Answer and explanation

Each A₂ requires two A units, so producing more than 0.500 M A₂ would require a negative remaining A amount.

For dimerization, producing x concentration units of A₂ consumes 2x of A. The ICE table therefore differs from a one-to-one isomerization. Substitution into the selected K equation can yield more than one algebraic root; reject any root that makes a species concentration negative or exceeds available material. The displayed K=2 case gives x≈0.3048 M and [A]≈0.3904 M. Substitute the accepted values back into both the equilibrium ratio and atom-equivalent conservation to check the result.

Each A₂ requires two A units, so producing more than 0.500 M A₂ would require a negative remaining A amount.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 26

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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