C5 revision cards GCSE

Moles and RFM:

- one mole of atoms or molecules of any substance will have a mass in grams equal to the relative formula mass for that substance

- Number of Moles = mass in g (of element or compound) / Mr (of element or compound)

- a mole of a substance has the same mass in grams as the RFM

- the RFM is the sum of the RAMs

- the unit for RFM is g/mol

- mass is conserved in a chemical reaction as no atoms are created or destroyed

Emperical Formula:

- the emperical formula gives the simplest whole number ratio of each type of atom in a compound

- deducing the emperical formula of a compound, given the mass of each element in a compound, is done by following these steps;

1) divide the mass of each element by its RAM

2)simplify your ratio by dividing by the smallest answer in the last step

3) write an emperical formula using the answers to step 2

- % by mass = (RAM of element x number of atoms of this element in compound) / RFM x 100

Concentrations:

- concentrations have the unit g/dm3 and mol/dm3

- volume is measured in cm3 or dm3 (1000cm3 = 1dm3)

- number of moles = concentration x volume (n=cv/1000)

- the guidline daily amount listd on packaging may list the amount of sodium or the amount of salt. Salt is not the only source of sodiumions and so converting the amount of salt into sodium may not be accurate.

pH:

- a pH curve shows the original pH of the solution, the final pH of the solution and the volume of the acid/alkali added to neutralise the solution.

- the volume of acid/alkali needed to neutalise  is the shown in the pH curve is verticle at pH 7

- colours of indicators in acid and alkali: universal indicator (red/purple) litmus (red/blue) phenolphthalein (colourless/pink)

- single indicators such as litimus and phenolphthalein are more useful when performing titrations as there is a distinct colour change, whereas with universal indicator there is only a gradual colour change.

- the average titre (amount of acid and alkali added) is calculated by repeating the titration until 3 consistent values are obtained and calculating the average volume using only these values.

Volumes of Gas:

- identify the apparatus used to collect of a volume of gas and be able to state which method is better based on the solubility of the gas.

- the amount of product formed is directly proportional to the mass of the limiting reactant. The limiting reactant is the one with the smallest number of moles and is not in excess. When the limiting reactant is all used up, the reaction stops.

- the volume of 1 mole of gas is 24dm3 at room temperature and pressure.

Equilibrium:

- reversible reactions are reactions that proceed in both directions.

- at equilibrium the rate of the forward reaction is equal to the rate of the backward reaction, and the concentration of the reactants and the products do not change

- the position of equilibrium is related to the ratio of the concentration of products to reactants e.g. if the position of equilibrium shifts to the left then the concentration of the reactants increases and the concentration of the products decreases.

- the symbol of a reversible reaction is

- in order to reach equilibrium the reaction must be done in a closed system.

- adding an extra reactant to the reaction shifts the position of equilibrium to the right, and taking away a reactant shifts the position of equilibrium to the left.

More on Equilibrium:

- increasing the temperature shifts the reaction in the endothermic reaction

- increasing the pressure shifts the reaction to the side with fewer molecules

- the raw materials for the Contact process are sulphur, air and water

- sulphur is burnt to make sulphur dioxide (S + O2 --> SO2); Sulphur dioxide reacts with oxygen to form sulphur trioxide (SO2 + O2            SO3) using a V2O5 catalyst, 450*C and atmospheric pressure.

- Increasing temperature shifts the position of equilibrium to the left so a compromise temperature is used. Using catalyst increases the rate but des not change the position of equilibrium, as the position of equilibrium is already on the right at low pressure, an expensive high pressure is not necessary.

Strong and Weak Acids:

- ethanoic acid is a weak acid; Hydrochloric acid, nitric acid and sulphuric acid are all strong acids; Strong acids have a lower pH than weak acids

- all acid ionise to produce H+ ions. Strong acids completely ionise by weak acids only partially ionise so the concentration of H+ ions is lower.

- pH measures the concentration of H+ ions in a solution, the higher the concentration of H+ ions, the lower the pH. A weak acid with the same concentration as a strong acid will have a higher pH

- weak acids react slower than strong acids because they have a lower concentration of H+ ions and therefore the collision frequency will be lower.

- the volume of H2 produced when equal volumes of a strong acid and a weak acid react with magnesium is the same although the rate of reaction will be slower with the weak acid.

- the conductivity of a weak acid solution is lower than the strong acid solution.

Precipiation Reactions:

- a precipitation reaction is where 2 solutions are added together and an insoluble solid is formed

- these reactions are fast as the ions in a solution have an extremely high collision frequency

- lead nitrate can be used to test for halide ions. The precipitates have different colours. White precipitate with Cl- ; cream precipitate with Br- and yellow precipitate with I-

- barium chloride can be used to test for sulphate ions. Barium sulphate is formed which is a white precipitate

- the state symbol for a solid is (s), a liquid (l), a gas (g) and a solution is (aq)