Making electricity

• A cell is an arrangement where chemical energy is converted into electrical energy.

• In a cell a pair of different metals are connected with an electrolyte between them.

• The different metals have different voltages.• The metal higher in the electrochemical series( p7

data book) always donate electrons to the metal lower.

• This will continue as long as the electrolyte is present.

Batteries

• A battery is a collection of cells.

• A battery will convert chemical energy into electrical energy until the chemicals are used up.

• There are many types of battery.

Examples of BatteriesBattery Use Advantage Disadvantage

Zn/C Torch, clock

Cheap Leaks

Pb/H+ Cars Rechargeable

Needs topped up

Ni/Cd Torch Rechargeable

Care recharging/Cd poisonous.

Lithium Pacemaker Non toxic Li reactive

Ag2O Watches Long lasting

Expensive

The Electrochemical Series

• Puts metals in order of the ease with which they loose electrons.

• The metals at the top give electrons most readily.

• We can predict a metals position using simple cells and metal pairs.

• We use one metal as a standard – i.e. one of the pair is always the same metal.

Voltage

• The further apart 2 metals are in the electrochemical series – the greater the voltage they produce when arranged in a cell.

• Example – Cu is the standard

Metal Voltage

Mg 2.7

Zn 1.1

Cu 0.0

Ag - 0.5

Displacement Reactions

• When a metal higher up in the electrochemical series is placed in a solution of the ions of a metal lower in the series.

• Example

• Zinc + Copper sulphate —> Zinc sulphate +

Copper

The Zinc takes the place of the Copper ions in the solution, Cu solid appears.

Oxidation and Reduction

• OILRIG

• Oxidation is loss of electrons

• Reduction is gain of electrons

• Oxidation and reduction always happen together.

• REDOX is what we call the whole thing – oxidation and reduction.

Ion Electron Equations

• Oxidation

• The electrons are always on the RHS of the arrow

• Example

• Cu atoms being oxidised to Cu ions

• Cu(s) —> Cu 2+ (aq) + 2e

• Iodide ions loosing electron to form Iodine molecule

• 2I- (aq) —> I2 ( s) + 2e

Reduction

• The electrons will always be at the LHS of the arrow.

• Example• Mg ions gain 2 electrons to form Mg atoms.• Mg 2+(aq) + 2e —> Mg(s)• Cl2 molecule gaining 2 electrons to form

ions• Cl(g) + 2e —> 2 Cl- (aq)

REDOX

• Example• Magnesium reacting with Hydrochloric Acid to form

Magnesium chloride and Hydrogen gas.• Oxidation• Mg (s)—> Mg 2+ (aq) + 2e• Reduction• 2H+ ( aq) + 2e —> H2 (g)• Redox• Mg ( s) + 2H+ (aq) + 2e —> Mg 2+ (aq) + 2e +

H2 (g)