9701 · 8.2
Effect of temperature on reaction rates and the concept of activation energy flashcards
Revision flashcards for Cambridge 9701 Effect of temperature on reaction rates and the concept of activation energy (syllabus 8.2). Flip, recall, then mark a real past-paper question.
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What is Activation Energy ($E_a$)?
The minimum amount of kinetic energy that colliding particles must possess for a chemical reaction to occur.
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What are the TWO reasons why increasing temperature increases reaction rate?
1. Particles gain kinetic energy, move faster, and collide more frequently. 2. A significantly larger proportion of particles have kinetic energy greater than or equal to the activation energy ($E \ge E_a$).
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Which factor has the greater impact on rate when temperature is increased?
The increase in the proportion of particles with energy greater than or equal to the activation energy ($E \ge E_a$) has a much larger effect than the increase in collision frequency.
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What is the 'rule of thumb' for the effect of temperature on reaction rate?
For many reactions at around room temperature, the rate of reaction approximately doubles for every 10°C (or 10 K) increase in temperature.
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What does a Boltzmann distribution curve show?
It shows the statistical distribution of molecular kinetic energies within a sample of a gas or liquid at a constant temperature.
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What does the area under a Boltzmann distribution curve represent?
The total number of molecules in the sample. This area remains constant even if the temperature changes.
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How does the shape of a Boltzmann distribution curve change when temperature INCREASES?
The curve becomes lower and broader. The peak (most probable energy) shifts to the right, towards higher kinetic energy.
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How is activation energy ($E_a$) shown on a Boltzmann distribution curve?
As a vertical line on the x-axis (Kinetic Energy). The area under the curve to the right of this line represents the number of molecules with enough energy to react.
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How does a catalyst increase reaction rate?
A catalyst provides an alternative reaction pathway with a lower activation energy. It does not change the energy of the particles themselves.
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How does adding a catalyst affect the Boltzmann distribution curve?
It doesn't. The curve itself is unchanged because the temperature and number of molecules are the same. The catalyst lowers $E_a$, so the activation energy line shifts to the left.
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Why does a Boltzmann distribution curve always start at the origin (0,0)?
Because there are zero molecules that have zero kinetic energy. All particles are in continuous, random motion (unless at absolute zero).
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What is activation energy ($E_a$)?
The minimum kinetic energy that colliding particles must possess for a successful collision to occur, leading to a chemical reaction.
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What do the axes on a Boltzmann distribution curve represent?
The y-axis represents the 'Number of particles' (or proportion of molecules), and the x-axis represents 'Kinetic energy'.
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Describe two changes to the Boltzmann distribution curve when temperature is increased.
1. The peak of the curve becomes lower and shifts to the right (higher most probable energy). 2. The curve becomes broader/more spread out.
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Why does a small increase in temperature cause a large increase in reaction rate?
Because it causes a disproportionately large increase in the number of particles that have kinetic energy equal to or greater than the activation energy ($E_a$), leading to a much higher frequency of successful collisions.
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How does a catalyst increase the rate of reaction in terms of activation energy?
A catalyst provides an alternative reaction pathway with a lower activation energy ($E_a$). This means more particles have sufficient energy to react at the same temperature.