Rate of
Reactions

IP Chemistry (6092), Sec 3

By the end of this topic, you can answer

1

How can you measure how fast a reaction goes?

2

Why does a reaction slow down and then stop?

3

Why do concentration, pressure, particle size and temperature change the rate?

4

How would you plan an experiment to test one of these factors?

5

How does a catalyst speed up a reaction without being used up?

6

What are enzymes, and why do high temperatures stop them working?

Starter

PredictMilk left on the table turns sour in a day. In the fridge, it stays fresh for about a week.

What does the fridge change about the reactions that turn milk sour?

NoteThe lower temperature slows the reactions down. This topic explains why temperature, and several other conditions, change how fast a reaction goes.
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)
AnalogyA rate works like the speed of a car. Speed is the distance travelled per hour. A reaction rate is the amount of product made, or reactant used, per second or per minute.
1

Measuring the Rate

1.1Volume of gas

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

1.1Loss in mass

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

1

Measuring the Rate

1.2Reading a rate graph

  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.
1

Measuring the Rate

1.3Following the loss in mass

  • A graph of the mass of the reaction mixture against time slopes down, as the gas escapes.
  • It is steepest at the start, when the reaction is fastest, and levels off when the reaction stops.
  • A graph of the mass of a product slopes up instead.
1

Measuring the Rate

1.3The disappearing cross

  • 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)
1

Measuring the Rate

Quick checkMarble chips react with excess dilute hydrochloric acid. Why does the graph of volume of gas against time level off?

AThe catalyst has been used up.
BThe marble chips have all reacted.
CThe acid has evaporated.
DThe gas has stopped dissolving.
NoteB: the acid is in excess, so the marble chips are the limiting reactant. When they are used up, no more gas forms.
1

Measuring the Rate

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 tipName the apparatus, 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.
1

Measuring the Rate

1.5Comparing rate graphs

  • Each curve is the same mass of marble chips with excess acid of a different concentration.
  • The curve with the steepest initial gradient shows the fastest rate.
  • The final volume depends only on the amount of the limiting reactant, so all three end at the same volume.
  • Halving the amount of the limiting reactant halves the final volume.
1

Measuring the Rate

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?

ProductProducts form as the reaction goes on, so the mass of a product increases: the gradient is positive.

FlaskThe 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.
  • The rate increases when the frequency of effective collisions increases.
AnalogyA ball thrown softly at a window just bounces off. Only a throw with enough energy breaks the glass. The activation energy is that minimum energy.
2

What Affects the Rate

2.2Concentration and pressure

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 ↑

A higher pressure pushes gas particles closer together. Pressure has almost no effect on solids and liquids.

2

What Affects the Rate

2.3Particle size

Particle size of the solid ↓

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

What Affects the Rate

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

Step 1Powdered marble has a larger total surface area of contact with the acid.

Step 2This increases the frequency of effective collisions, so the rate is faster.

2

What Affects the Rate

2.4Temperature

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.
2

What Affects the Rate

2.4Watching the collisions

  • Twice the concentration: more collisions per second, so more product in the same time. The fraction of collisions that react stays about the same.
  • Hotter: faster particles collide more often, and a larger fraction of the collisions have enough energy to react.
2

What Affects the Rate

Quick checkWhich change does not increase the rate of reaction between marble chips and dilute hydrochloric acid?

Ausing powdered marble
Bwarming the acid
Cusing a larger volume of the same acid
Dusing more concentrated acid
NoteC: more acid of the same concentration has the same number of particles per unit volume, so the frequency of effective collisions is unchanged.
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

Catalysts

3.2A lower activation energy

Catalyst present

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

The energies of the reactants and the products are unchanged.

3

Catalysts

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

Step 1Reaction B is faster.

Step 2Its activation energy is lower, so more particles have energy greater than or equal to it.

Step 3The frequency of effective collisions is higher, so the rate is faster.

3

Catalysts

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
  • 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.

Exit ticket

1Name two quantities that can be measured to follow the rate of a reaction that gives off a gas.

The volume of gas given off and the mass of the reaction mixture.[1]

2What is an effective collision?

A collision with energy greater than or equal to the activation energy, which leads to a reaction.[1]

3Define a catalyst.

A substance that increases the rate of a reaction without being chemically changed at the end of the reaction.[1]

4Explain why a higher temperature increases the rate of a reaction.

Particles have more kinetic energy and collide more often, and more of them have energy greater than or equal to the activation energy, so the frequency of effective collisions increases.[2]