9701 · 8.1
Rate of reaction flashcards
Revision flashcards for Cambridge 9701 Rate of reaction (syllabus 8.1). Flip, recall, then mark a real past-paper question.
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What is the formal definition of 'rate of reaction'?
The change in concentration of a reactant or product per unit time. The standard units are mol dm⁻³ s⁻¹.
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According to collision theory, what two conditions must be met for a reaction to occur?
1. Particles must collide with energy equal to or greater than the activation energy ($E_a$). 2. Particles must collide with the correct geometric orientation.
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Define 'activation energy' ($E_a$).
The minimum kinetic energy that colliding particles must possess for a successful collision to result in a chemical reaction.
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How does increasing the concentration of an aqueous reactant affect the reaction rate?
It increases the number of reactant particles per unit volume. This leads to more frequent collisions, increasing the rate of reaction.
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For a reaction involving gases, how does increasing the pressure affect the rate?
Increasing pressure forces the gas particles closer together, increasing their concentration. This results in more frequent collisions and therefore a faster reaction rate.
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Why does increasing temperature cause a large increase in reaction rate?
Two reasons: 1. Particles have more kinetic energy, move faster, and collide more frequently. 2. Crucially, a significantly larger proportion of particles now have energy greater than or equal to the activation energy ($E_a$), leading to a much higher frequency of successful collisions.
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How does a catalyst speed up a chemical reaction?
A catalyst provides an alternative reaction pathway with a lower activation energy ($E_a$). This means a greater proportion of collisions are successful, increasing the rate. The catalyst is not consumed.
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What is the effect of increasing the surface area of a solid reactant?
It exposes more particles of the solid to the other reactant(s). This increases the frequency of collisions at the surface, leading to a faster rate of reaction.
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Suggest a method to follow the rate of the reaction: $CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$
Monitor the volume of carbon dioxide gas produced at regular time intervals using a gas syringe, or monitor the decrease in total mass of the flask and contents as CO₂ escapes.
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What does the gradient of a concentration vs. time graph represent?
The gradient at any point on the curve represents the instantaneous rate of reaction at that time. The initial rate is the gradient at t=0.
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Why does the rate of most reactions decrease over time?
As the reaction proceeds, the concentration of reactants decreases. This leads to a lower frequency of collisions between reactant particles, causing the reaction rate to slow down.