Rate of Reaction (AQA GCSE Chemistry Combined Science): Revision Notes
Catalysts
What are catalysts?
Catalysts are special substances that speed up chemical reactions. The most important thing to remember is that catalysts are not used up during the reaction. This means you can use the same catalyst again and again.
Think of a catalyst like a helpful friend who makes a job easier for you, but doesn't do the work themselves.
Understanding that catalysts remain unchanged is crucial - this property allows them to be used repeatedly in industrial processes, making them economically valuable and environmentally efficient.
How do catalysts work?
Catalysts work by giving reactions an easier pathway to follow. Here's what happens:
- Without a catalyst: Particles need lots of energy to react (high activation energy)
- With a catalyst: Particles need less energy to react (lower activation energy)
When there's less energy needed, more particles can successfully react. This means the reaction happens faster.
The key concept here is activation energy - catalysts don't change the overall energy change of a reaction, they just make it easier for the reaction to get started by providing a route that requires less initial energy.
Energy diagrams
You can see this on an energy diagram:
- The blue line shows the normal reaction pathway
- The red line shows the easier pathway when a catalyst is present
- The catalyst pathway has a lower peak - this is the reduced activation energy
Key properties of catalysts
Catalysts have several important features you need to know:
They provide an alternative reaction pathway
This is the fundamental way catalysts work. By offering a different route for the reaction, they reduce the activation energy barrier that particles must overcome.
- This pathway needs less activation energy
- More collisions become successful
- The reaction rate increases
Catalyst Specificity
Different reactions need different catalysts because each catalyst is designed to work with specific types of molecules and reaction mechanisms. This is similar to how different keys work for different locks.
They are not consumed
This is perhaps the most important property of catalysts:
- Catalysts don't appear in the chemical equation
- The same amount of catalyst exists at the start and end
- This is why they can be used over and over again
Because catalysts are not consumed, they can theoretically work indefinitely. In practice, catalysts may become less effective over time due to contamination or physical degradation, but they are not chemically changed by the reaction itself.
Biological catalysts
In living things, catalysts are called enzymes. These work in the same way as other catalysts but are found in biological systems. Enzymes are highly specific and operate under the mild conditions found in living organisms.
Testing for catalysts
You can prove something is a catalyst by checking if it remains unchanged after the reaction.
Testing Method: Mass Comparison
Step 1: Weigh the suspected catalyst before the reaction
Step 2: Allow the reaction to proceed completely
Step 3: Weigh the substance after the reaction
Step 4: Compare the masses
Result: If the mass is the same before and after, it's a catalyst. If the mass has decreased, it was a reactant that got used up.
Simple example
Hydrogen peroxide breaks down to make water and oxygen. This reaction demonstrates catalyst behaviour perfectly.
Worked Example: Hydrogen Peroxide Decomposition
The reaction:
Without catalyst: This reaction is very slow at room temperature
With catalyst: Adding manganese oxide (MnO₂) makes the reaction speed up dramatically
Key observation: The manganese oxide doesn't appear in the chemical equation because it's not used up - it's a catalyst that can be recovered unchanged after the reaction.
Key Points to Remember:
- Catalysts speed up reactions but are not used up
- They work by lowering the activation energy needed
- They provide an alternative, easier reaction pathway
- Different reactions need different catalysts (specificity)
- You can test for catalysts by checking their mass before and after the reaction
- In biological systems, catalysts are called enzymes