A Level Physical Chemistry AQA questions and answers

Topic 1 - ANS~ Atomic Structure Topic 2 - ANS~ Amount of Substance State what is meant by the term molecular formula. - ANS~ the actual no. atoms of each element in a compound Topic 3 - ANS~ Bonding Explain why CF4 has a bond angle of 109.5°. - ANS~ -there are 4 bonding pairs and no lone pairs -repel equally and therefore spread apart as far as possible Topic 4 - ANS~ Energetics State and explain change in the volume of water in calorimetry that would cause a reduction in heat loss. - ANS~ -higher volume -smaller increase in temperature above room temperature / increased contact with calorimetry / smaller heat loss by evaporation Define the term standard enthalpy of combustion. - ANS~ The enthalpy change when one mole of a substance is burnt completely in oxygen under standard conditions with all reactants and products in their standard states. State why experimental values for enthalpy change is different to mean bond enthalpy. - ANS~ -heat loss / not all heat transferred to apparatus -reactants and products may not be in their standard states -MBE values are taken from a range of compounds -incomplete combustion -water may end up in a gaseous state -MBE refers to gaseous species whereas enthalpy change of combustion is liquid species Give the meaning of the term mean bond enthalpy. - ANS~ Enthalpy for dissociating covalent bonds, averaged for that bond type for a range of values. Describe the steps you would take to determine an accurate minimum temperature that is not influenced from heat from the surroundings. - ANS~ -start clock when compound is added -record every minute for 5 minutes -plot temperature vs time graph -extrapolate time back to the time of mixing to determine the temperature Topic 5 - ANS~ Kinetics Suggest why a catalyst used in a process may become less efficient if the reactants contain impurities. - ANS~ Impurities block the active site. Topic 6 - ANS~ Chemical Equilibria, Le Châtelier's principle and Kc Topic 7 - ANS~ Oxidation, Reduction and Redox Equations Topic 8 - ANS~ Periodicity Topic 9 - ANS~ Group 2, the Alkaline Earth Metals Topic 10 - ANS~ Group 7, the Halogens Topic 11 - ANS~ Introduction to Organic Chemistry State the meaning of the term structural isomers. - ANS~ -same molecular formula -different structural formula Topic 12 - ANS~ Alkanes State the term saturated and hydrocarbon as applied to saturated hydrocarbons. - ANS~ -saturated= no carbon-carbon double bonds -hydrocarbon= compound made only of carbon and hydrogen State two general features of a homologous series of compounds. - ANS~ -same functional group -similar chemical properties -same general formula -each compound differs by CH2 -gradual change in physical properties Explain what is meant by the term fraction as applied to fractional distillation. - ANS~ a group of molecules with similar boiling points / similar chain length Explain why the sulfur compounds in crude oil should be removed from the fractions before they are used for central heating fuel. - ANS~ -react to form sulfur dioxide -which causes acid rain Explain how oxides of nitrogen are formed during the combustion of alkanes. - ANS~ -nitrogen and oxygen in the air -react at high temperatures to form NO, NO2 etc. Explain why the melting point of dodecane is higher than the melting point of the straight-chain alkane produced by cracking dodecane. - ANS~ -larger molecule with more electrons -more and stronger van der Waals' forces Name the catalyst used in catalytic cracking - ANS~ Aluminosilicate / Zeolite State the type of cracking that produces a high proportion of ethene and propene. Give 2 conditions for this process. - ANS~ -thermal cracking -high temperature and high pressure State one hazard associated with carbon particulates produced in combustion. - ANS~ smog / smoke / global dimming / exacerbates asthma / breathing problems etc. Identify a catalyst used in a catalytic converter - ANS~ rhodium / palladium / platinum / iridium Explain why the catalyst in a catalytic converter is coated on a honeycomb. - ANS~ -increases surface area -increases rate of reaction Identify a compound that is used to react with sulfur dioxide in the products of combustion before they enter the atmosphere. Give one reason why this compound reacts with sulfur dioxide. - ANS~ -calcium oxide / calcium carbonate -neutralises SO2 / acid-base reaction Give one reason why 2,3,3,3-tetrafluoropropene is a more environmentally acceptable refrigerant than chlorodifluoromethane. - ANS~ It contains no C-Cl bonds. Name the type of mechanism for the reaction of chlorine with methane - ANS~ Topic 13 - ANS~ Halogenoalkanes Topic 14 - ANS~ Alkenes Topic 15 - ANS~ Alcohols Write an equation for fermentation. Give 2 conditions to produce a good yield of ethanol. Name a process to produce a much more concentrated solution of ethanol a from a dilute aqueous solution. State the meaning of carbon-neutral. - ANS~ -C6H12O6 —> 2C5H10OH + 2CO2 -conditions: anaerobic, yeast enzyme, 25<T<42 -fractional distillation for increased ethanol concentration -carbon neutral= no net CO2 emission into the atmosphere Topic 16 - ANS~ Organic Analysis Topic 26 - ANS~ Carboxylic Acids and Derivatives Outline the method that the student should use for this recrystallisation. - ANS~ -dissolve solvent in the minimum volume of a hot water solvent -allow to cool / allow crystals to form -filter off the pure product under reduced pressure In recrystallisation, why is the minimum volume of hot water used? - ANS~ so that crystals form on cooling In recrystallisation, why is the flask cooled to room temperature before the crystals are filtered off? - ANS~ solubility is higher if warm / yield is lower if warm In recrystallisation, why are the crystals compressed in the funnel? - ANS~ so air passes through the sample rather than around it In recrystallisation, why is a little cold water poured through the crystals? - ANS~ to wash away soluble impurities Outline how you would carry out a simple laboratory process to show that the recrystallised product is a pure sample of N-phenylethanamide. - ANS~ -measure its melting point -using melting point apparatus -sharp melting point indicates pure product Suggest the most likely impurity in recrystallisation and an improvement to the method to limit the amount of the impurity found in the crystals. - ANS~ -water -press crystals between filter papers / allow more time for drying Explain why ethanoic anhydride might be preferable to use when making N-phenylethanamide, although it has a slower rate of reaction. - ANS~ -it has a violent reaction with water -produces toxic HCl gas Topic 27 - ANS~ Aromatic Chemistry State, in terms of bonding, why benzene is more stable than cyclohexa-1,3,5-triene. - ANS~ delocalisation of electrons State the bond angle between carbons in cyclohexane and benzene. - ANS~ -cyclohexane= 109.5 -benzene= 120 Suggest why the reaction of methylbenzene with ethanoyl chloride is quicker than the reaction of benzene with ethanoyl chloride. - ANS~ -methyl group has a positive inductive effect / increases electron density -so makes it a better nucleophiles / electrophiles are more attracted In the reduction of butanone, by consdering the mechanism, explain why the product formed is optically inactive. - ANS~ -the C=O bond is planar -nucleophile is equally likely to attack from either side -racemic mixture is formed