OCR Gateway GCSE Chemistry

Revision Notes

6.2.10 Cells

Chemical Cells

  • A simple 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 voltmeter by wire, creating a complete 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 sets up a charge difference between the electrodes
  • The electrons then flow around the circuit to the copper electrode which is now the more positive electrode
  • 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 produced
  • The electrolyte used also affects the voltage as different ions react with the electrodes in different ways
  • Cells produce a voltage until one of the reactants is used up

How Reactivity Affects Voltage in Cells 1, downloadable IGCSE & GCSE Chemistry revision notesHow Reactivity Affects Voltage in Cells 2, downloadable IGCSE & GCSE Chemistry revision notes

Simple cell made with Cu and Mg. These metals are further apart on the reactivity series than Cu and Zn and produce a greater voltage

Hydrogen Fuel Cells

  • 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
  • As the fuel enters the cell it becomes oxidised which sets up a potential difference or voltage within the cell
  • Different electrolytes and fuels can be used to set up different types of fuel cells
  • An important cell is the hydrogen-oxygen fuel cell which combines both elements to release energy and water

Hydrogen-oxygen fuel cell, IGCSE & GCSE Chemistry revision notes

Diagram showing the movement of hydrogen, oxygen and electrons in a hydrogen-oxygen fuel cell

  • At the anode, hydrogen molecules lose electrons and become hydrogen ions:

2H2 (g) → 4H+ (aq) + 4e-

  • The electrons flow through the external circuit to the cathode
  • Hydrogen ions migrate through a special membrane separating the anode and cathode
  • At the cathode hydrogen ions gain electrons and react with oxygen to form water:

4H+ (aq) + O2 (g) + 4e-→  H2O (g)

  • The overall reaction is:

2H2 (g) + O2 (g) → 2H2O (l)

Pros & Cons of Hydrogen Fuel Cells

Pros

  • 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 are no moving parts, unlike an internal combustion engine
  • No batteries to dispose of which is better for the environment
  • Continuous process and will keep producing energy as long as fuel is supplied

Cons

  • Materials used in producing fuel cells are expensive
  • High pressure tanks are needed to store the oxygen and hydrogen in sufficient amounts which are dangerous and difficult to handle
  • Fuel cells are affected by low temperatures, becoming less efficient
  • Hydrogen is expensive to produce and store

Exam Tip

You should be able to state 3 advantages and 3 disadvantages of the hydrogen cell.

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