Cells (AQA GCSE Chemistry): Revision Notes
Cells
What are cells?
A cell is a device that contains chemicals which react together to make electricity. When you connect two or more cells together, you create a battery. Batteries have a higher voltage than single cells.
Cells are really useful because they let us store energy and use it when we need it. There are two main types of cells you need to know about.
The key difference: a cell is a single unit, while a battery is multiple cells connected together to increase the total voltage output.
Chemical cells
How chemical cells work
Chemical cells are made using two different metals connected to an electrolyte (a liquid that conducts electricity).
The key parts are:
- Two different metals (called electrodes) - like copper and magnesium
- Electrolyte - like ammonium chloride solution
- The metals must be different to create a voltage
When the metals react with the electrolyte, electrons flow from one metal to the other. This creates an electric current.
How the current flows:
The chemical reaction causes electrons to build up on one electrode (making it negative) while the other electrode loses electrons (making it positive). When you connect a wire between them, electrons flow through the wire from negative to positive, creating the electric current we can use.
Which metals give the best voltage?
The further apart the metals are in the reactivity series, the higher the voltage you get. For example:
- Magnesium and copper = high voltage (big gap in reactivity series)
- Zinc and copper = lower voltage (smaller gap in reactivity series)
Understanding the Reactivity Series:
The reactivity series ranks metals by how easily they lose electrons. The bigger the difference between two metals in this series, the greater the voltage produced. This is why we can't just use any two metals - we need to choose metals that are far apart in reactivity.
Advantages of chemical cells
- Portable - you can carry them anywhere
- Rechargeable types available - some can be recharged by applying electricity backwards
Disadvantages of chemical cells
- Run down eventually - when chemicals are used up, they stop working
- Hard to dispose of safely - contain chemicals that can harm the environment
Fuel cells
How fuel cells work
Fuel cells work differently from chemical cells. Instead of using up chemicals inside them, they are continuously supplied with fresh fuel from outside.
The most common type uses:
- Hydrogen as fuel (goes in at one side)
- Oxygen from air (goes in at the other side)
- These react to produce electricity and water
The chemical reaction is:
This means hydrogen + oxygen makes water (and electricity).
Worked Example: Hydrogen Fuel Cell Reaction
Step 1: Hydrogen gas enters one side of the fuel cell
- 2 molecules of hydrogen ()
Step 2: Oxygen from air enters the other side
- 1 molecule of oxygen ()
Step 3: They react to produce:
- 2 molecules of water ()
- Electrical energy
The beauty of this reaction is that the only waste product is clean water!
Advantages of fuel cells
- Constant voltage - doesn't drop as fuel runs out
- Don't run down - work as long as you supply fuel
- Only produce water - very clean, no harmful waste
- Use renewable fuel - hydrogen can be made from water using renewable electricity
Environmental Benefits:
Fuel cells are considered one of the cleanest energy technologies because their only emission is water vapour. The hydrogen fuel can also be produced using renewable energy sources like solar or wind power, making the entire process carbon-neutral.
Disadvantages of fuel cells
- Not portable - need fuel tanks and supply systems
- Fuel storage problems - hydrogen needs special heavy tanks
Comparing the two types
| Feature | Chemical cells | Fuel cells |
|---|---|---|
| Portability | Portable | Not portable |
| Fuel supply | Built-in chemicals | Continuous fuel supply needed |
| Voltage | Drops over time | Stays constant |
| Waste products | Various chemicals | Only water |
| Recharging | Some types can recharge | Not needed - just add fuel |
Real-world examples
Petrol cars vs hydrogen fuel cell cars:
- Petrol: Can travel far on one tank, but petrol is not renewable
- Hydrogen: Uses renewable fuel, but needs heavy fuel storage tanks
Rechargeable batteries:
- Good: No carbon dioxide when generating electricity
- Bad: Takes time to recharge when they run down
Why This Matters:
As we move towards cleaner energy solutions, understanding these different technologies helps us make better choices. Chemical cells are great for portable devices, while fuel cells show promise for larger applications like powering vehicles and buildings cleanly.
Key Points to Remember:
- Cells contain chemicals that react to make electricity
- Chemical cells use two different metals in an electrolyte - the bigger the gap in the reactivity series, the higher the voltage
- Fuel cells need a continuous supply of fuel (like hydrogen) and oxygen to keep working
- Chemical cells are portable but run down, fuel cells give constant power but aren't portable
- Fuel cells only produce water as waste, making them very environmentally friendly