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12.11Chapter 12: Chemical Reactions 3 and Rates

Temperature and reaction rate — Alhussan Chemistry - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 11 of 13 2 quizzes

📚 Alhussan Chemistry - Stage 9 Hub / Topical Revision
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Lesson summary

12.11 Temperature and reaction rate — Particle Kinetic Energy and Collisions — Alhussan International Academy — Chemistry (Stages 7–9) notes: Describe and explain how temperature affects reaction rate using collision theory.Faster at higher temperatures — reactions go faster at h…

  • Rule of thumb
  • More frequent collisions
  • More successful collisions
  • What does not change
  • Hypothesis
  • Prediction

Part 1 of 6Effect of temperature

Effect of temperature
Faster at higher temperatures
  • Faster at higher temperatures reactions go faster at higher temperatures.
  • Potatoes cook in 15 minutes in boiling water at 100 °C but more quickly in boiling oil at 200 °C.
  • Food spoils more slowly in a fridge and cooks faster in a hot oven.
Rule of thumb
  • Rule of thumb a 10 °C rise roughly doubles the rate for many reactions, although this is not exact.
Pattern
  • Pattern as temperature increases, the rate of reaction increases.

Part 2 of 6Explaining with particles

Explaining with particles
Kinetic energy
  • Kinetic energy heating gives particles more kinetic energy, so they move faster.
More frequent collisions
  • More frequent collisions faster particles collide more often.
More successful collisions
  • More successful collisions a greater proportion of collisions have energy greater than or equal to the activation energy,
    • so more collisions are effective.
  • This second effect is the more important one, because even a small rise in temperature greatly increases the fraction of particles with enough energy.
What does not change
  • What does not change particles do not get bigger and the activation energy does not change.

Part 3 of 6Farai's investigation

Hypothesis
  • Hypothesis reactions happen when particles collide, and at higher temperatures particles move faster and collide more frequently,
  • so as temperature increases the reaction rate increases.
Reaction
  • Reaction sodium thiosulfate + hydrochloric acid → sodium chloride + water + sulfur dioxide + sulfur.
  • Sulfur is insoluble, so it forms as a precipitate of tiny solid pieces that make the mixture cloudy.
Method
  • Method pour sodium thiosulfate solution into a flask placed over a cross, add hydrochloric acid, start a timer and stop it when the cross can no longer be seen.
  • At 20 °C the cross disappeared after about 244 seconds.
Variables
  • Variables the independent variable is the temperature of the acid, the dependent variable is the time to cover the cross,
    • and the volumes and concentrations are controlled.
Repeats
  • Repeats repeat each temperature three times to reduce error and obtain reliable results.
Prediction
  • Prediction at higher temperatures the reaction will be quicker, so the cross will disappear after a shorter time.

Part 4 of 6Results and analysis

Temperatures
  • Farai repeated the experiment at 20, 30, 40, 50 and 60 °C and recorded the mean time at each.
Anomalous result
  • Anomalous result the first result at 50 °C was very different from the others, probably because the acid had cooled before use,
    • so he left it out of the mean.
Graph
  • Graph both variables are continuous, so he drew a line graph.
Conclusion
  • Conclusion the higher the temperature, the faster the reaction, so his prediction and hypothesis were supported.
Rate from time
  • Rate from time rate = 1 ÷ time, and a graph of rate against temperature curves upward.
Errors
  • Errors judging when the cross disappears is subjective, so using the same observer or a light sensor improves it.

Part 5 of 6Comparing factors that affect rate

FactorCollision frequencyEnergy of collisions
ConcentrationHigherSame
Surface areaHigherSame
TemperatureHigherHigher
CatalystSameLower activation energy needed
Rate graph
  • Rate graph the curve for a higher temperature starts steeper and reaches the plateau sooner,
  • but the final volume is the same if the amounts of reactants are the same.
Not the same
  • Not the same reaction rate is not the same as the amount of product.
Exothermic reactions
  • Exothermic reactions heating them does not stop them working.

Part 6 of 6Applications

Pressure cookers
  • Pressure cookers they cook food faster by raising the temperature.
Refrigerators
  • Refrigerators they slow bacteria.
Industry
  • Industry it chooses a temperature that gives a fast rate without wasting energy or risking safety.
Safety
  • Safety handle hot acids and flasks with care.

Key terms

TermMeaning
Kinetic energythe energy of movement.
Activation energythe minimum energy needed for particles to react.
Ratehow fast a reaction proceeds.
Frequencyhow often something happens.
Precipitatean insoluble solid formed in a reaction.
Anomalous resulta result that does not fit the pattern.

Avoid these mistakes

1
Saying only collision frequency changes — the proportion of effective collisions also increases.
2
Saying activation energy decreases with temperature — it stays the same.
3
Saying the particles get larger — they only move faster.
4
Forgetting that the rule of thumb is only approximate — a 10 °C rise does not exactly double the rate.

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