C6 revision cards GCSE

Electrolysis:

- electrolysis occurs when a current is applied to 2 electrodes immersed in an electrolyte. This current causes ions in the electrolyte to move to an electrode which causes a flow of charge, these ions either lose or gain electrons to become the element.

- the ANODE is the POSITIVE electrode and the CATHODE is the NEGATIVE electrode. ANions are negative ions and CATions are positive ions.

- both NaOH(aq) and H2SO4(aq) form hydrogen at the cathode (2H+  +   2e-   --->   H2) and oxygen at the anode (4OH-   --->   O2   +   2H2O). H+ ions from water are more easily reduced that Na+ ions so hydrogen is formed when NaOH is electrolysed. Complex ions are not electrolysed.

- when copper sulphate solution is electrolysed using carbon electrodes, copper is formed at the cathode and oxygen is formed at the anode, over time, the colour of blue solution will disappear.

- the amount of substance produced during an electrolysis is proportional to the current and to the time of electrolysis. Molten ionic liquids can also be electrolysed whereas solid ionic compounds cant be. This is because when solid, the ions are not free to move around.

Fuel cells:

- the reaction between hydrogen and oxygen is exothermic = half equation at H2 electrode (oxidation) 2H2 --> 4H+  + 4e- // half equation at O2 electrode (reduction) O2 + 4H+  +4e- -->2H2O Overall Equation: 2H2 + O2 --> 2H2O

- advantages of using a fuel cell in a spacecraft: provides water to drink, lightweight, compact and has no moving parts.

- Advantages of using a fuel cell: more efficient (no energy lost through heat), only produces water, does not produce CO2 or other pollutants, endless supply of hydrogen from water.

- Disadvantages of using a fuel cell: they contain poisonous catalysts that have to be diposed of at the end of the cells lifetime, catalysts are very expensive, hydrogen and oxygen are normally obtained using electroylsis of water which requirs electricity generated at fossil fuel power stations.

Redox Reactions:

- O I L R I G

- rusting requires iron, water and oxygen, rusting is a redox reaction, iron is oxidised and oxygen is reduced // iron + oxyegn + water --> hydrated iron(III)oxide

- galvanised iron is iron coated with zinc; the zinc stops the oxygen and water reaching the iron. This zinc also acts as a sacrificial metal because it is oxidised instead of the iron as it is more reactive. Magnesium is also a sacrificial metal.

- iron can also be covered with tin. Tin only stops oxygen and water reaching the iron. If the tin layer is scratched, the iron is preferently oxidised and rusts even faster.

- paint, plastic and oil are examples of other ways rusting can be prevented.

- In a displacement reaction, the more reactive displaces the less reactive metal from its salt e.g. zinc displaces the copper from copper sulphate.

Fermentation:

- the molecular formula for ethanol is C2H5OH

- the word equation for fermentation is // glucose --> carbon dioxide + ethanol // C6H12O6 --> 2CO2 + 2C2H5OH

- ethanol made by fermentation is a renewable fuel, but ethanol made by hydration is a non-renewable fuel.

- ethene and steam are passed over a heated phosphoric acid catalyst.

ethane + water --> ethanol // C2H4 +H2O --> C2H5OH

The Ozone Layer:

- CFC molecules are broken down by UV in the stratosphere to form reactive chlorine atoms (radicals) The chlorine atoms react with the ozone layer and destroy it. The chlorine atoms are regenerated and then react with more of the ozone, therefore starting a chain reaction.

- One chlorine atom can destroy thousands of ozone molecules

- the ozone layer is there to absord harmful high-energy UV light, which is protecting us, as UV light can cause many health issues, such as premature aging and skin cancer.

- ozone depletion allows more of this UV light to reach the surface of the earth

- CFCs have now been replaced with alkanes or HFCs that do not destroy the ozone layer.

- CFCs can be found in spray cans and refridgerators.

Water Hardness:

- there are 2 types of water hardness: temporary (caused by calcium hydroencarbonate) and permanent ( caused by calcium sulphate)

- the calcium ions form an insoluble precipitate with soap called scum. More soap is needed to remove grease.

- calcium carbonate in rocks reacts with dissolved carbon dioxide and water to form calcium hydrogencarbonate.

- word equation for the above reaction is: calcium carbonate + water + carbon dioxide --> calcium hydrogencarbonate

- boiling removes temporary hardness by forming insoluble calcium carbonate // Ca(HCO3)2 --> CaCO3 + H2O + CO2

Esters:

- all fats and oils are esters.

- Fats are saturated; they do not have any double bonds in the long chains, and they do not react with bromine. Oils are unsaturated; they have double bonds in the long chains; bromine adds to the double bond to form a colourless dibromo compound.

- Unsaturated oils are healthier but sometimes it is easier to use a solid fat. Margarine is made by adding hydrogen to some of the double bonds in an oil. This makes a solid fat which is healthier than a completely saturated fat.

- oil and water form an immiiscible emulsion.

- fats and oils can be broken into a soap and glycerol. This is called saponification.

- oil in water emulsion (milk in water) has the water separated from the tiny droplets of oil, and the water in oil emulsion (water in butter) has the oil separated from the tiny droplets of water.

Detergents:

- detergents have hydrophilic head and hydrophobic tail. These form intermolecular bonds with water molecules and oil molecules respectively.

- dry cleaning is a cleaning process that uses a solvent instead of water.

- biological detergents have enzymes that are biological catalysts. They help break down some large insoluble molecules into smaller soluble molecules which can be easily removed.