Topic 17 · Rate of Reactions

Rate of Reactions

1

Measuring the Rate

1.1What the rate is

DefinitionThe rate of reaction is how fast a reactant is used up or a product is formed, measured as the change in amount per unit time.
  • Follow a property that changes as the reaction goes on: the volume of gas given off, or the mass of the reaction mixture.
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
Volume of gasgas syringedilute hydrochloric acidmarble chips

Read the volume in the gas syringe at regular time intervals.

Loss in mass197.00 gcotton woolacid + marble chipsbalance

Carbon dioxide escapes, so the mass on the balance falls. The cotton wool stops acid spraying out.

1.2Reading a rate graph

timevolume of gastotal volume123
  1. The gradient is the rate. It is steepest at the start, as the reactants are most concentrated.
  2. The gradient falls as the reactants are used up, so the reaction slows down.
  3. The graph levels off when the limiting reactant is used up and the reaction stops.
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1

Measuring the Rate (continued)

1.3Other ways to follow a reaction

timemass / gfinal mass seen from abovestart: cross seen60153045time tseen from abovesulfur hides the cross
  • A graph of the mass of the reaction mixture against time slopes down, as the mass of reactants falls. A graph of the mass of a product slopes up.
  • When a precipitate forms, time how long it takes for a cross under the flask to disappear. A shorter time means a faster rate:
rate ∝ 1time taken
Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + SO2(g) + S(s)
  • The yellow precipitate of sulfur slowly hides the cross.

1.4Planning an investigation

  1. Change one variable only, e.g. the concentration of the acid.
  2. Keep every other variable the same: the volume of acid, the mass and size of the solid, and the temperature.
  3. Measure the volume of gas (or the mass) at regular time intervals with a stopwatch, until it stops changing.
  4. Plot the results for each experiment on the same axes and compare the initial gradients.
Exam tipIn a plan, name the apparatus (gas syringe or balance, and stopwatch), say what is kept the same, and say how the result shows the rate: a steeper initial gradient, or a shorter time, means a faster rate.
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1

Measuring the Rate (continued)

1.5Comparing rate graphs

timevolume of CO₂2.00 mol/dm³1.00 mol/dm³0.50 mol/dm³
  • The curve with the steepest initial gradient shows the fastest rate.
  • The final volume depends only on the amount of the limiting reactant. Here the marble chips are limiting, so all three end at the same volume.
  • Halving the amount of the limiting reactant halves the final volume.

Worked exampleThe gradient of a mass against time graph is positive when the mass of a product is measured, but negative when the mass of the reaction flask and its contents is measured. Why?

Products form as the reaction goes on, so the mass of a product increases: the gradient is positive.The gas given off escapes from the flask, so the mass of the flask and its contents decreases: the gradient is negative.
2

What Affects the Rate

2.1Effective collisions

Particles react only when they collide with energy greater than or equal to the activation energy, Ea. Such a collision is an effective collision.
less than the activation energy: they bounce apartproductat least the activation energy: they react
  • The rate increases when the frequency of effective collisions increases.
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2

What Affects the Rate (continued)

2.2Concentration and pressure

  • A higher concentration means more reacting particles per unit volume. They collide more often, so the frequency of effective collisions increases.
  • For gases, a higher pressure pushes the particles closer together, with the same effect. Pressure has almost no effect on solids and liquids.

Concentration (or pressure, for a gas) ↑

  1. Number of reacting particles per unit volume ↑
  2. → frequency of collisions ↑, and hence frequency of effective collisions ↑ → rate ↑

2.3Particle size

  • Smaller pieces have a larger total surface area for the same mass, so more particles are exposed to collide.

Particle size of the solid ↓

  1. Larger total surface area of contact
  2. → frequency of effective collisions ↑ → rate ↑

Worked exampleExplain, in terms of collisions, why powdered marble reacts faster with dilute hydrochloric acid than the same mass of marble lumps.

Powdered marble has a larger total surface area of contact with the acid.This increases the frequency of effective collisions, so the rate is faster.

2.4Temperature

  • Particles gain kinetic energy and move faster, so they collide more often.
  • More particles have energy greater than or equal to the activation energy, so a larger fraction of collisions are effective.
  • Both increase the frequency of effective collisions, so the rate increases.

Temperature ↑

  1. Particles gain kinetic energy and move faster → frequency of collisions ↑
  2. → more particles have energy ≥ Ea, so a larger fraction of collisions are effective
  3. → frequency of effective collisions ↑ → rate ↑
Exam tipExplain a rate in three links: what changes about the particles, so they collide more frequently (or with more energy), so the frequency of effective collisions increases. Say "frequency", not just "more collisions".
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3

Catalysts

3.1What a catalyst does

DefinitionA catalyst increases the rate of a reaction without being chemically changed at the end of the reaction.
  • It is not used up, so a small amount works for a long time. Its physical appearance may change.
  • It does not change the yield of a reaction.
  • It is selective: each catalyst speeds up only certain reactions.

3.2A lower activation energy

Eₐ withoutcatalystEₐ withcatalystreactantsproducts
  • A catalyst provides an alternative pathway with a lower activation energy.
  • More particles now have energy greater than or equal to the activation energy, so the frequency of effective collisions increases.
  • The energies of the reactants and the products are unchanged.

Catalyst present

  1. Provides an alternative pathway with a lower activation energy
  2. → more particles have energy ≥ Ea
  3. → frequency of effective collisions ↑ → rate ↑

Worked exampleReaction A has a higher activation energy than reaction B. Under the same conditions, which reaction is faster? Explain your answer.

Reaction B is faster.Its activation energy is lower, so more particles have energy greater than or equal to it.The frequency of effective collisions is higher, so the rate is faster.
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3

Catalysts (continued)

3.3Catalysts in industry and in living things

CatalystUsed in
ironthe Haber process, to make ammonia
aluminium oxide or silicon dioxidethe cracking of hydrocarbons
nickelmaking margarine from vegetable oils
platinum, palladium and rhodiumcatalytic converters in cars
  • Industrial catalysts are usually transition metals or their compounds. They let reactions run at lower temperatures and pressures, which saves energy.
  • Enzymes are biological catalysts. They are proteins, and each works best at a particular temperature (often body temperature, 35 to 40 °C) and pH. High temperatures denature them.
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