6.2 Collision theory and factors affecting rate
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State the two conditions needed for particles to react, according to collision theory.
Mark scheme
- particles must collide (1)
- with at least the activation energy / sufficient energy (1)
Model answer
The particles must collide, and they must collide with at least the activation energy.
Explain why increasing the concentration of an acid increases the rate of its reaction with magnesium.
Mark scheme
- more acid particles in the same volume (1)
- so more frequent collisions (1)
Model answer
There are more acid particles in the same volume, so collisions with the magnesium are more frequent.
Explain why increasing the temperature increases the rate of a reaction.
Mark scheme
- particles move faster / have more (kinetic) energy (1)
- more frequent collisions (1)
- more collisions have energy at least equal to the activation energy / more successful collisions (1)
Model answer
At a higher temperature the particles move faster, so they collide more frequently. They also have more energy, so a greater proportion of collisions have at least the activation energy.
Powdered marble reacts faster with acid than marble chips of the same mass. Explain why.
Mark scheme
- powder has a larger surface area (to volume ratio) (1)
- more particles exposed so more frequent collisions (1)
Model answer
Powder has a much larger surface area to volume ratio, so more particles are exposed to the acid and collisions are more frequent.
Which change does not increase the frequency of collisions?
Choose one answer.
Correct Not quite: the answer is Using larger lumps of solid
Mark scheme
D
Model answer
D: using larger lumps of solid lowers the surface area, so collisions are less frequent.
Explain why increasing the pressure of a gas increases the rate of reaction.
Mark scheme
- same number of particles in a smaller volume / more particles per unit volume (1)
- more frequent collisions (1)
Model answer
The same number of gas particles is squeezed into a smaller space, so they collide more frequently.
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