Topic 16 - Chemical Cells and Fuel Cells Flashcards
5.25C - What are chemical cells and why do they go ‘flat’?
Everyday batteries in mobile phones and torches are chemical cells.
In a simple chemical cell ( Daniell cell ), there are two different metals dipped into a solution of one of their salts and a ‘salt bridge’ to allow dissolved ions to pass from one solution to another.
In these cells, a voltage is produced as there is a difference in the reactivity of the metals, the further apart in the reactivity series the two metals are the higher the voltage.
A current flows when the cells are connected in a circuit.
This is an exothermic reaction, although most of the energy is transferred by electricity.
When one of the reactants are used up, the reaction stops and a voltage is no longer produced. This is a ‘flat’ battery.
5.26C - What happens in a hydrogen-oxygen fuel cell?
Supplied with hydrogen fuel and oxygen from the outside and water as the product.
Fuel cells produce a voltage ( from the electrons that pass through the electric motor ) as long as the reactants are supplied, therefore they don’t go ‘flat’.
At the left electrode, hydrogen atoms lose electrons to form H+ ions. These electrons flow through the external circuit, from the negative Pt electrode ( Catalyses formation of electrons from hydrogen fuel ) to the positive Pt electrode ( Catalyses reaction of H+ ions with electrons ) through the membrane which lets H+ ions pass through.
Cathode - 2H+ + 2e = H2
Anode - 4OH- = O2 + 2H2O + 4e
5.27C - What are the strengths and weaknesses of fuel cells?
They are much quieter and needs less maintenance than a petrol engine, although hydrogen still needs to be stored in a tank.
Hydrogen-oxygen fuel cells don’t produce greenhouse gases like CO2, although most hydrogen is made by reacting steam with fossil fuels which release CO2 as a by-product.
