Specification Point 5.25C:
Recall that a chemical cell produces a voltage until one of the reactants is used up.
- A chemical cell is a source of electrical energy.
- The simplest design consists of two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external circuit.
- A common example is zinc and copper.
- Zinc is the more reactive metal and forms ions more easily, readily releasing electrons.
- The electrons give the more reactive electrode a negative charge and they then flow around the circuit to the copper electrode which is now more positive.
- The difference in the ability of the electrodes to release electrons causes a voltage to be produced.
- The greater the difference in the metals reactivity then the greater the voltage.
- Chemical cells include the familiar batteries used in everyday appliances and cars.
- There are different designs of cells depending on the use.
- Chemical cells produce a voltage until one of the reactants is used up.
Chemical cell made with copper and magnesium. These metals are further apart on the reactivity series than copper and zinc and would hence produce a greater voltage.
Hydrogen-Oxygen Fuel Cell
Specification Point 5.26C:
Recall that in a hydrogen–oxygen fuel cell hydrogen and oxygen are used to produce a voltage and water is the only product.
- A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode.
- These cells are becoming more common in the automotive industry to replace petrol or diesel engines.
- In the hydrogen-oxygen fuel cell, H2 and O2 are pumped through two porous electrodes where the half reactions occur.
- The overall reaction produces only water:
2H2 + O2 → 2H2O
- The electrons move around the external circuit from the cathode to the anode.
- This movement of electrons produces a potential difference which is used to drive an electric motor.
Diagram showing the movement of hydrogen, oxygen and electrons in a Hydrogen-Oxygen fuel cell
Evaluating Fuel Cells
Specification Point 5.27C:
Evaluate the strengths and weaknesses of fuel cells for given uses.
- They do not produce any pollution.
- They produce more energy per kilogram than either petrol or diesel.
- No power is lost in transmission as there is much less moving parts than in an internal combustion engine.
- Continuous process and will keep producing energy as long as fuel is supplied.
- Materials used in producing fuel cells are expensive.
- High pressure tanks are needed to store the oxygen and hydrogen in sufficient amounts.
- Fuel cells are affected by low temperatures, becoming less efficient.
- Hydrogen is expensive to produce and store.
Edexcel GCSE Chemistry Notes
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Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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