AQA GCSE C3 Notes

Acids and basesProton Donors & AcceptorsWhen an acid is forms H­­+ ions- a hydrogen atom, that has lost an electron or just a protonThe protons become surrounded by water molecules to keep them in solution-hydratedHydrated hydrogen ions are written as H+ ­­(ag)An alkali is a base which dissolves in water and produces OH- ions – Hydroxide ionsAcids act as a source of protons – proton donorsHydroxide ions combine with protons to form water-OH- (aq) + H+ (aq) → H2O (l)Alkalis are called proton acceptorsStrength of Acids and AlkalisStrength of acid depends on extent to which it ionises in water-strong acid/alkali=100% ionised in waterA weak acid/alkali are only partly ionised in waterCan find the strength of acids by testing for pHA strong acid will be completely ionised so concentration hydrogen ions =1 mol/dm³Weak acids are only partly ionised so concentration of hydrogen ions is less than 1 mol/dm³TitrationsAdding an acidic solution to an alkaline solution will produce a neutralisation reactionAcid + alkali = base + salt We can measure exact volumes of acids and alkalis needed to react with each other using titrationsThe point at which the acid and the alkali have completely reacted is called the end pointEnd point can be shown using an indicator-phenolphthalein/methyl orangeHow to carry out titration:1 Measure known volume of the alkali solution into conical flask using pipette2 Add indicator solution to the alkali3 Put the acidic solution into a burette-record the reading on the burette (i.e. starting volume)4 Open tap to release the acid solution. The solution from the burette is released one drop at a time, alongside swirling of the flask to ensure the solutions are mixed5 Repeat Step 4 until the indicator changes colour to let you know the acid and the alkali have completely mixed6 Record the amount of acid you entered by reading the measurement on the burette Be sure to repeat the entire process two or three times at least to ensure accuracy.Titration calculationsUnits of concentration are mol/dm3 · For example, imagine we were making a sodium hydroxide solution in water by dissolving exactly 40g of sodium hydroxide to make 1dm³ of solution: · We know that the mass of one mole of NaOH is the sum of the atomic masses of sodium, oxygen and hydrogen: 23 + 16 + 1 = 40g · Because 40g is in the solution, we know that there is exactly one mole of NaOH in the solution · And we know that the solution is 1dm³, so the concentration is 1 mol/dm³Energy CalculationsEnergy and reactionsBreaking bonds is an endothermic processMaking bonds is exothermic process-releases energy To measure the amount of energy released in a reaction a calorimeter is usedEnergy Level DiagramsExothermic reaction, we say that the change in energy is negative.Temperature of the surroundings decreases because the change in energy is positive.Endothermic reaction, more energy is needed to break to bonds The products are at a lower energy level than the reactants-energy has been releasedThe amount of energy needed to start a reaction is called the activation energy Adding a catalyst will reduce the activation energyCalculating Energy Changes4.2 joules of energy raises 1g of water by 1°CThe units involved are kJ/g/°C (kilojoules per gram per degree)A + B → CQuestion: 60cm³ of a solution containing 0.1 moles of A is mixed with 40cm³ of a solution containing 0.1 moles of B. Prior to mixing, their temperature was 19,6°C. After mixing, the maximum temperature reached was 26.1°C1. First, calculate the temperature change: 26.1°C – 19.6°C = 6.5°C 2. Since 60cm³ of A added to 40cm³ of B makes 100cm³ overall, we are looking at 100g (assuming the density of the solution is the same as water density). And we know that 4.2J raises 1g by 1°C3. So energy change = 100g x 6.5°C x 4.2J/g/°C = 2,730J = 2.73kJ 4. BUT – don’t forget the solutions are only 0.1 molar – so we have to multiply our value by 10 to find out a 1.0M solution 2.73kJ x 10 = 27.3kJ 5. So the final energy change was -27.3kJ Bond EnergiesThe energy required to break a bond between two atoms is called bond energyTo calculate energy change we need to know: a) The amount of energy needed to break the bonds between atomsb) The amount of energy released in the formation of new chemical bonds.Water and SolubilityThe water cycleThe sun supplies lakes, rivers and oceans with energy-allows water to evaporate.The water vapour forms clouds as the water vapour condenses The water droplets fall as rainThis is the water cycleSolubilityAmount of solute which can be dissolved in an amount of solvent-called the solubility of the substanceMeasured in grams (of solute) per 100g (of solvent)Solubility of a solid solute increases as temp. IncreasesSaturated solution-solution with maximum amount of solute dissolved