Electrolysis is the flow of charge by moving ions and discharging ions at the electrodes.
- NaOH (aq) - hydrogen at the cathode and oxygen at the anode
- H2SO4 (aq) - hydrogen at the cathode and oxygen at the anode
- Cathode - 2H+ + 2e- H2
- Anode - 4OH- - 4e- O2 + 2H2O
The electrolysis of NaOH makes H2 at the cathode rather than Na because it is easier to decompose the water than the compound that is dissolved in it.
Electrolysis of Copper:
- copper is formed at the cathode and oxygen is formed at the anode
- Cathode - Cu2+ + 2e- Cu
- Anode- 4OH- - 4e- O2 + 2H2O
The amount of substance produced during electrolysis varies with time and current, more substance is made if:
- a larger current flows
- the current flows for a longer time
Quality of electricity (coulombs) = current x time
On mole of substance with a 1+ charge will be deposited by 96500 coulombs.
An ionic substance cannot be electrolysed but the molten liquid can be electrolysed because:
- ionic solid has ions which are in fixed positions and cannot move
- ions in a molten liquid can move.
Balanced symbol equation for each electrode in a fuel cell:
At the cathode (negative electrode): H2(g) – 2e– → 2H+(aq)
At the anode (positive electrode): 4H+(aq) + O2(g) + 4e– → 2H2O(g)
The reaction at the cathode is an oxidation reaction because hydrogen loses electrons, and the reaction at the anode is a reduction reaction because hydrogen ions gain electrons. The overall reaction in the fuel cell is a redox reaction.
Fuel Cells 2
Advantages of using hydrogen-oxygen fuel cell to porvide electrical power in a spacecraft:
- provides water that can be used by astronauts
- no moving parts
Advantages over normal ways of creating electricity:
- produce less pollution
- very efficient
- transfer energy directly
- few stages and simple to construct
- have no moving parts
Fuel Cells 3
The car industry is developing fuel cells because:
- no carbon dioxide emmisions from the car
- fossil fuels such as petrol are non-renewable
- large source of hydrogen is available by decomposing water
Fuel cells still produce pollution because:
- fuel cells often contain poisonous catalysts that have to be disposed of at the end of the lifetime of the fuel cell
- production of the hydrogen fuel and oxygen will involve the use of energy which may come from the use of burning fossil fuels
Redox reactions involve both oxidisation and reduction.
Oxidisation is loss of electrons, reduction is gain of electrons.
- Oxidising agent - a chemical that removes electrons from another substance
- Reducing agent - a chemical that gives electrons to another substance
Fe loses electrons (oxidisation) to become Fe2+.
Rusting is a redox reaction because iron loses electrons and oxygen gains electrons.
iron + oxygen +water hydrated (III) iron oxide.
Galvenising protects iron from rusting:
- galvenised iron is covered with a layer of zinc
- layer of zinc stops water and oxygen from reaching the surface of the iron
- zinc also acts as a sacrificial metal.
Sacrificial protection protects iron from rusting:
- use of a metal such as magnesium or zinc
- sacrificial metal is more reactive than iron
- sacrificial metal will lose electrons in preference to iron.
Disadvantages of using tin plate as a means of protection:
- tin only acts as a barrier stopping water and air reaching the surface of the iron.
- when the tin is scratched the iron will lose electrons in preference to tin and so the iron rusts faster than on its own.
Word equations for displacement reactions between metals and metal salt solutions:
magnesium + zinc chloride zinc + magnesium chloride
Mg + ZnCl2 Zn + MgCl2
- metal ion is reduced by gaining electrons
- metal atom is oxidised by losing electrons
Molecular formula for ethanol =
Displayed formula for ethanol =
The general formula of an alcohol =
glucose carbon dioxide + ethanol
C6H12O6 2CO2 + 2C2H5OH
Ethanol can be made by fermentation:
- glucose solution
- reaction catalysed by enzymes in yeast
- absence of oxygen
- fractional distillation to get ethanol
Conditions used in fermentation:
- temperature too low the yeast is inactive
- temperature too high the enzymes in yeast are denatured
- absence of air to prevent the formation of ethanoic acid
Fermentation 2 and Hydration
Ethanol made by fermentation is a renewable fuel because plants provide the glucose needed, absorbing carbon dioxide from the atmosphere as they grow. When the ethanol is burned, the carbon dioxide returns to the atmosphere.
Ethanol is produced for industrial use by passing ethene and steam over a heated phosphoric acid catalyst.
ethene + water ethanol
Balanced symbol equation:
Fermentation Vs Hydration
Feature of process
Hydration of ethene
warm, normal pressure
high temperature, high pressure
Type of process
continuous (runs all the time)
uses renewable resources (glucose from plants)
uses non-renewable resources (ethene from crude oil)
low purity – needs fractional distillation
high purity – no by-products are made
low – about 15%
high – around 100%
medium – 51%
maximum – 100%
CFCs have been banned in the UK because society has agreed with scientists views that CFCs deplete the ozone layer.
Scientists attitude to CFCs has changed:
- initial enthusiasm for the use of CFCs based upon their inertness
- later discovery of ozone depletion and link to presence of CFCs
- acceptance by scientists and the rest of the world community that use CFCs should be banned.
Ozone Layer 2
CFCs deplete the ozone layer:
- CFC molecules are broken down in the stratosphere by ultraviolet light to give highly reactive chlorine atoms
- chlorine atoms react with ozone molecules
- chlorine atoms are regenerated so can react with more ozone molecules.
Formation of Chlorine atoms from CFCs: CCl3F → •CCl2F + •Cl
The chlorine atoms are called chlorine radicals. They are shown as •Cl in equations.
CFCs are only removed slowly from the stratosphere because they are only brokent down slowly by ultraviolet light.
Ozone Layer 3
When the C-Cl bond breaks, one electron goes to the Cl to make a Cl radical and then the other electron goes to the main part of the molecule.
Only a small number of chlorine atoms will destroy a large number of ozone molecules because the chlorine molecule is regenerated to make more chlorine radicals.
CFCs will continue to deplete the ozone a long time after their use has been banned because of the reason above and also the inertness of the molecules.
The depletion of the ozone layer allows more ultraviolet light to reach the surface of the earth because the ozone layer would normally absorb some of this light but instead it is not absorbed so reaches earth.
CFCs can be replaced with alkanes or HFCs and these will not damage the ozone layer.
Hardness of Water
- calcium carbonate in rocks reacts with dissolved carbon dioxide and water to form soluble calcium hydrogencarbonate
- calcium carbonate + water + carbon dioxide calcium hydrogencarbonate
Permanent Hardness caused by dissolved calcium sulfate.
Boiling removes temporary hardness:
- decomposition of calcium hydrogencarbonate to give insoluble calcium carbonate (limescale), water and carbon dioxide
- soluble calcium ions are changed into insoluble compounds
Balanced symbol equation:
Hardness of Water 2
Ion exchange resin can soften water:
- the resin is made into small balls which are packed into a tube/column. The ion resin starts with sodium ions stuck to it
- as the hard water passes through the resin, the sodium ions come off the resin and go into the water, while calcium ions come out of the water and onto the resin
- sodium ions do not cause water hardness
Washing soda (sodium carbonate) can soften water:
- soluble in water and add a huge number of carbonate ions to the water.
- these react with the calcium ions to form a precipitate
Hardness of Water 3
Adding soap solution can see the hardness of the water.
If the water lathers, the water is soft
If the water doesnt lather with soap, the water is hard.
Natural Fats and Oils
Animal and vegetable fats and oils are esters.
Unsaturated fats are healthier as part of a balanced diet because it reduces the risk of cholesterol. Cholesterol could lead to heart disease due to blockage in blood vessels.
- When a fat is saturated - no double bonds
- when a fat is unsaturated - double bond
Unsaturation in fats and oils can be shown using bromine water:
- with saturated fats, the bromine water stays orange
- with unsaturated fats, the bromine water goes colourless.
- addition reaction takes place at the carbon-carbon double bond
- a colourless dibromo compound is formed
- saturated compounds cannot react with bromine since they do not have a carbon-carbon double bond.
Natural Fats and Oils 2
Margarine is manufactered from vegetable oils: reacting unsaturated vegetable oils with hydrogen, using a nickel catalyst. This makes a solid saturated fat, which can then be blended with other ingredients to make it taste/look like butter.
Immiscible liquids (not not mix) can be made into an emulsion by shaking vegetable oil with water to break down the oil into small droplets that disperse into the water.
- oil in water - mostly water with tiny droplets of oil dispersed in it
- water in oil - mostly oil with droplets of water dispersed in it
Natural Fats and Oils 3
Natural fats and oils can be split up by hot sodium hydroxide solution to produce soap and glycerol. This process is saponification.
Fat + sodium hydroxide soap + glycerol
This is a hydrolysis reaction because it involves breaking down ester groups in the oil molecule using an alkali.
Advantages of low temperature washes:
- enzymes are denatured at high temperatures
- some types of clothes can be damaged by high temperatures
- lower temperatures require less energy so the wash is cheaper
Detergents are molecules that have a hydrophyllic head and a hydrophobic tail.
Detergents remove fats and oils:
- hydrophillic head of the detergent molecule forms strong intermolecular forces with water molecules
- hydrophobic end of detergent forms strong intermolecular forces with molecules of fat and oil.
Dry cleaning is a process used to clean clothes that does not involve water
- solvent that is not water
- stain that will not dissolve in water
Dry cleaning removes stains by:
- there are weak intermolecular forces between molecules of grease
- there are weak intermolecular forces between solvent molecules
- solvent molecules form intermolecular forces with molecules of grease and so solvent molecules can surround molecules of grease.