OCR A-level Chemistry - Paper 3 Questions and Answers

OCR A-level Chemistry - Paper 3 Questions and Answers Why does HF have a higher boiling point than CH4? -Boiling point increases as electronegativity increases -HF has hydrogen bonding, CH4 has London forces How do you prepare a standard solution? -Weigh out correct mass of solid -Dissolve in distilled water -Transfer to a 250cm^3 volumetric flask -Wash beaker with distilled water -Add washings to the volumetric flask -Make up to 250cm^3 with distilled water -Invert to ensure mixing Why can we sometimes assume a reaction in zero order with respect to X? Because it is in excess How to test order of X from concentration time graph? -Calculate half-life -If the next half-life is within 10s of the first, it is first order Why is the reaction of Ethane with UV light a bad way to get a high yield of Chloroethane? Because further substitution can occur Why are benzene bonds all the same length? They're all π bonds with one delocalised electron Why is the rate of hydrolysis of 1-bromobutane faster than 1-chlorobutane? C-Br is weaker than C-Cl Reactions on Fe2+ Mn2+ NH4+ with aqueous NaOH Fe(OH)2 pale green precipitate Mn(OH)2 pale brown precipitate NH3 + H2O red litmus paper turns blue from NH3 gas How to test for CO32- React with nitric acid Forms CO2 + H2O Fizzing How to test for SO42- React with barium nitrate Forms BaSO4 White precipitate How to test for Cl- React with aqueous silver nitrate Forms AgCl White precipitate In a reaction how can the concentration of bromine be monitored? Measure the reduction of the orange/brown colour of bromine If X is in excess, why does it mean its zero order? Because the concentration of X is constant How to calculate the number of isomers 2^number of chiral centres What does the ability to undergo electrophilic substitution suggest? There is a benzene ring What does the ability to undergo electrophilic addition mean? It is an alkene, has C=C in molecule How to choose which equation is correct from a minimum volume Minimum volume means that there must be a maximum amount of moles Why does skipping boiling in a titration reduce the titre? -Boiling speeds up the reaction Lattice enthalpy The energy required to change one mole of ionic solid into its gaseous ions Enthalpy change of solution The enthalpy change when one mole of ionic substance dissolves in water to make a solution of infinite concentration Enthalpy change of hydration The enthalpy change when one mole of gaseous ions are dissolved in water to give one mole of aqueous ions