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)
