Cells (OCR GCSE Chemistry A (Gateway)): Revision Note

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

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

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