Rate: temperature (AQA GCSE Chemistry): Revision Notes
Rate: temperature
Collision theory and temperature
Collision theory helps us understand why temperature changes the rate of reactions. For particles to react, they must collide with each other. But not all collisions lead to reactions.
Two things must happen for a successful reaction:
- Particles must collide with enough energy
- Particles must hit each other in the right way
How temperature affects particle movement
When you heat up reactants, the particles gain more energy. This energy makes them move faster around the container.
Higher temperature means:
- Particles move faster
- More collisions happen per second
- Collisions are more energetic
Activation energy
Every reaction needs a minimum amount of energy to get started. This is called activation energy.
Think of activation energy like a hill that particles must climb over to react. Only particles with enough energy can get over this hill.
The reaction profile shows:
- Reactants start with some energy
- They need extra energy (activation energy) to react
- Products form with different energy levels
Why higher temperature increases reaction rate
At higher temperatures, more particles have enough energy to overcome the activation energy barrier.
This happens because more particles are moving fast enough, a higher proportion of collisions are successful, and the frequency of successful collisions increases.
Analogy: Jumping Over a Fence
Think of people trying to jump over a fence. If everyone runs faster (higher temperature), more people will successfully jump over (more successful reactions). The fence height represents the activation energy - it stays the same, but more people can now clear it.
Practical example - zinc and hydrochloric acid
When zinc reacts with dilute hydrochloric acid, hydrogen gas is produced:
Worked Example: Testing Temperature Effects
To test how temperature affects this reaction:
- Add measured volume of acid to a conical flask
- Add known mass of zinc to the acid
- Measure the volume of hydrogen gas produced over time
- Repeat at different temperatures
- Higher temperatures will produce gas faster
Measuring temperature effects
You can measure the effects of temperature changes using specific equipment and following important safety procedures.
Equipment needed:
- Gas syringe (to measure hydrogen gas)
- Conical flask (to hold reactants)
- Measuring cylinder (for acid volume)
- Balance (to weigh zinc)
Safety points:
- Wear safety goggles (acid is corrosive)
- Keep away from naked flames (hydrogen is flammable)
Other examples of temperature effects
Marble chips with hydrochloric acid:
- Crushing chips into smaller pieces increases surface area
- Diluting the acid decreases concentration
- Heating the acid increases temperature and rate
All these changes affect how fast the reaction happens.
Key Points to Remember:
- Higher temperature always increases reaction rate
- Particles move faster and collide more often at higher temperatures
- More collisions have enough energy to overcome activation energy
- You can measure temperature effects by timing gas production or measuring temperature changes
- Safety equipment is essentialwhen working with acids and flammable gases