3.2.2(a)-(b) - Collision theory and rate graphs
Reaction rate tells you how quickly a reaction changes a measurable quantity. In this lesson you will connect concentration and gas pressure to collision frequency, then calculate reaction rates from graph gradients. The key skill is to keep the chemistry idea and the graph method joined together: a steeper graph means a faster rate, and collision frequency explains why concentration changes can alter that rate.
What Reaction Rate Means
A reaction rate is a measure of how fast reactants are used up or products are formed. In experiments, you often cannot watch individual particles reacting, so you monitor a physical quantity that changes as the reaction proceeds.
Reaction Rate
Reaction rate is the change in a measured physical quantity per unit time. The physical quantity might be concentration, gas volume, mass or another measurable value linked to the reaction.
Mean Rate
The unit of rate depends on what has been measured:
| Quantity measured | Example graph | Example rate unit |
|---|---|---|
| concentration | concentration against time | mol dm⁻³ s⁻¹ |
| gas volume | gas volume against time | cm³ s⁻¹ |
| mass | mass against time | g s⁻¹ |
If a product quantity increases, the graph has a positive gradient. If a reactant quantity decreases, the mathematical gradient is negative. Chemists usually report the rate of disappearance as a positive value, using the magnitude of the negative gradient.
Worked example: mean rate from gas volume
A reaction produces 30.0 cm³ of gas in the first 75.0 s.
This is a mean rate over the first 75.0 s. It does not say that the rate was exactly 0.400 cm³ s⁻¹ at every moment.
Collision Frequency
For a reaction to happen, reacting particles must collide. In this lesson, the important idea is collision frequency: how often reactant particles collide per unit time.
If there are more reactant particles in the same volume, the particles are closer together on average. They collide more often, so the rate of reaction increases. For reacting gases, increasing pressure has the same particle-level effect as increasing concentration: the gas particles are forced into a smaller volume, so collisions between reactant particles are more frequent.
Worked reasoning: concentration
Magnesium reacts with hydrochloric acid to produce hydrogen gas. If the acid concentration is increased while the temperature and amount of magnesium are kept the same, there are more acid particles per unit volume near the magnesium surface. Collisions between acid particles and the magnesium happen more frequently, so hydrogen is produced faster.
Worked reasoning: gas pressure
In a gas reaction, increasing pressure by decreasing volume pushes the same number of gas particles closer together. The particles collide more often each second, so the reaction rate increases. This pressure explanation is for reacting gases; changing pressure is not a useful way to alter the rate of a reaction involving only solids or liquids.
At fixed temperature, increasing concentration does not increase the energy of each particle or lower the activation energy. The fraction of collisions energetic enough to react is approximately unchanged, but more collisions occur per second, so more successful collisions occur per second. Pressure here means compressing the reacting gas mixture; adding an inert gas at constant volume does not increase the concentrations of reacting gases.
As a reactant is used up, its concentration falls. Collisions become less frequent and the product-volume curve becomes less steep. A plateau can indicate that a limiting reactant has been used up; it does not by itself prove equilibrium.
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