OCR B Salters A level chemistry Questions and answers

OCR B Salters A level chemistry Questions and answers percentage error Uncertainty / reading x 100 JJ Thomson Plum pudding model (1897) Ernest rutherford Gold foil experiment- discovered nucleus (1909) Niels Bohr 1913- discovered that electrons move around the nucleus in orbits called electron shells. EM emitted/ absorbed when moving between levels- fixed frequency Mass Spectrometer steps 1. Vaporisation 2.Ionisation 3.Acceleration 4.Ion drift 5.Detection relative atomic mass Sum of (%isotopic abundance x relative isotopic mass)/total moles equation Moles = mass/Mr Concentration equation moles/volume How to convert between g/dm^3 and mol/dm^3 Divide by rfm Stoichiometry how many moles of one reactant will react with how many moles of another empirical formula a formula with the lowest whole-number ratio of elements in a compound molecular formula (molecular mass/relative mass) x empirical formula How to calculate empirical formula divide the percentages given by the molecular mass of the element; then divide everything by the lowest number you get percentage composition of an element Total mass of element in compound / total mass of compound * 100 percentage yield actual yield/theoretical yield x 100 methyl orange Red in acid, yellow in alkali phenolphtalein an indicator which is clear in acidic solution and pink in a basic solution ionic compounds properties - High melting points (high electrostatic forces of attraction) - High boiling points - Can dissolve easily in water (polar) - lattice structure - can conduct electricity when molten Giant covalent structure A huge 3D network of covalently bonded atoms High melting points (lots of strong bonds) Hard Won't dissolve in water Good thermal conductors Won't conduct electricity other than graphite (3/4 carbon electrons bonded) metallic bonding the chemical bonding that results from the attraction between metal atoms and the surrounding sea of electrons ductile high mp (affected by size of metal ion and how many electrons per atom) insolube except in liquid metals good conductors Tetrahedral molecule a molecule in which four atoms are bound to a central atom, resulting in a tetrahedral shape; the atoms bonded to the central atom lie at the corners of a tetrahedron with 109.5° angles between them pyramidal molecule Has a central atom that is bonded to 3 other atoms and has an unshared pair of valence electrons (lone pair); ammonia is an example 107 degrees bent molecule Molecule with 4 areas of electron density- 2 bonded pairs and 2 lone pairs 104.5 degrees e.g. water linear molecule 2 areas of electron density 180 degree angles 2D e.g: CO2 trigonal planar molecule 3 areas of electron density 120 degree angles 2D e.g: BF3 Bipyramidal molecule 5 areas of electron density, 120 and 90 degree bond angles, e.g. PCL5 Octahedral molecule 6 areas of electron density 90 degree angles 3D e.g.: SF6 First ionisation enthalpy The enthalpy change when 1 mole of gaseous 1+ ions is formed from 1 mole of gaseous atoms first ionisation enthalpy trend increases across a period (more protons increasing nuclear charge without an increase of shielding) decreases down a group (increased shielding) S block= low (low nuclear charges) P block= high (more protons) Group 2 reactions with water Forms metal hydroxide and hydrogen e.g. Ca + 2H2O -- Ca(OH)2 + H2 Hydroxide is a base Group 2 reactions with oxygen Forms metal oxides Oxide is a base Group 2 oxide + water Metal ion + OH- Then the solution becomes saturated. Metal ion + OH - -- group 2 hydroxide Group 2 hydroxide solubility Increases down the group Group 2 carbonates thermal decomposition MCO3 --- MO + CO2 Group 2 thermal stability trend Increases down the group, larger cation means less distortion (lower polarising power) therefore less soluble as you move down the group Li+ flame test crimson Na+ flame test orange-yellow K+ flame test lilac Ca2+ flame test Brick red Ba2+ flame test green Cu2+ flame test blue-green How to carry out a flame test - Dip nichrome wire loop into concentrated hydrochloric acid - Dip wire into sample of compound - Hold in clear blue part of flame Test for Ag+ Few drops of NaOH Brown precipitate of Ag2O Test for Ca2+ Few drops of NaOH White Ca(OH)2 precipitate Test for Cu2+ Few drops of NaOH Blue Cu(OH)2 precipitate Test for Pb2+ Few drops of NaOH white Pb(OH)2 precipitate Test for Fe2+ Few drops of NaOH Green Fe(OH)2 precipitate Test for Fe3+ Few drops of NaOH Reddish Brown Fe(OH)3 precipitate Test for Zn2+ Few drops of NaOH white Zn(OH)2 precipitate that redissolves in excess Zn(OH)3- Test for Al3+ Few drops of NaOH White Al(OH)3 precipitate redissolves in excess Al(OH)4- Test for sulfate ions SO₄²⁻ Dissolve salt in water in test tube, add dilute hydrochloric acid (removes carbonate ions which would produce precipitate), add drops of barium chloride solution White precipitate (of barium sulphate) formed Test for ammonium ions ~ Ammonia gas is alkaline so use a damp piece of red litmus paper ~ Add a few drops of aqueous sodium hydroxide and warm the mixture ~ Litmus paper turns blue then there is ammonia gas produced Test for hydroxide ions Litmus paper Blue if hydroxide present Test for halides Add dilute nitric acid and then silver nitrate solution Cl- + Ag+ --- white AgCl Br- + Ag+ --- cream AgBr I- + Ag+ --- yellow AgI Test for nitrates -boil with sodium nitrate and aluminium -aluminium recudes nitrate ions to ammonium ions -if nitrate ions are present ammonia gas is produced (NH4+ + OH- --- NH3 +H2O) -ammonia gas turns damp red litmus paper blue absorption spectrum EM radiation passed through gaseous element Electrons only absorb certain frequencies Radiation passing through has certain frequencies missing (corresponds to difference between energy levels) Dark lines on a coloured background Emission spectrum When electrons drop to lower energy levels- certain amounts of energy given out Frequencies in emission spectra same as those missing in absorption Coloured lines on a dark background Difference in energy between two shells frequency x Planck's constant E=hv Hydrogenation of ethene Nickel catalyst 150 degrees, 5 atm finely powdered Platinum catalyst under standard conditions Fluorine appearance at room temperature pale yellow gas Chlorine appearance at room temperature Yellow-Green gas Bromine appearance at room temperature red- brown liquid Iodine appearance at room temperature shiny grey solid Halogen volatility trend Decreases down the group Chlorine colour in water Virtually colourless/ pale green Bromine colour in water yellow-orange Iodine colour in water brown Chlorine colour in cyclohexane pale green Bromine colour in cyclohexane orange/ brown/red Iodine colour in cyclohexane violet Silver chloride colour white Silver bromide colour cream Silver iodide colour yellow Silver chloride in ammonia solution Soluble Silver bromide in ammonia solution Partially soluble Silver iodide in ammonia solution Insoluble If bromide is displaced and bromine formed what is the colour change? Mixture turns orange- hexane added leads to orange/ red layer If iodide displaced and iodine formed what is the colour change? Mixture turns brown- hexane added leads to purple layer Dangers of chlorine Toxic and corrosive Oxidising agent so increases fire risk Irritates respiratory system Transporting chlorine Liquid under pressure in cylinders Uses of chlorine Disinfection of swimming pools + drinking water (sterilising) Manufacture of bleach Manufacture of HCL % atom economy (Mass of desired product/total mass of reactants) x 100 Formation of hydrogen halides Adding concentrated acid to solid ionic halide Phosphoric acid molecular formula H3PO4 Products of adding sulfuric acid to sodium bromide+ change in sulfur oxidation state Na2SO4+ Br2+ SO2+ H2O S=+6 to S=+4 Products of adding sulfuric acid to sodium iodide 4Na2SO4+4I2+H2S+4H2O S=+6 to S=-2 Which hydrogen halides are strong acids? HCl+ HBr+ HI Which hydrogen halides are strong reducing agents? HBr+HI Hydrogen bromide+ sulfuric acid Br2+SO2+2H2O Hydrogen iodide+ sulfuric acid 4I2+ H2S+ 4H2O What is Kc equilibrium constant Equation for Kc [products]^moles/[reactants]^moles How does changing concentration of reagents affect Kc? Will remain constant When Kc1 equilibrium lies right When Kc1 Equilibrium lies left How does increasing pressure alter equilibrium position? Shifts to side with fewer gas molecules to reduce pressure How does increasing concentration alter equilibrium position? Shifts to side with fewer moles How does increasing temperature alter equilibrium position? Shifts in endothermic direction