CIE A Level Chemistry (9701) Solved Past Paper May/June 2020 P21
1 ai) Subatomic particles of an atom are protons, neutrons & electrons. Isotopes are atoms of same element with same number of protons but different number of neutrons.
ii) Relative atomic mass of elements, those who have got isotopes, can be calculated by taking their isotopic masses since each isotope has got a different atomic mass and their relative abundances into account.
bii) Gallium has 1 pair of electrons left to complete its valency of outermost shell in GaCl3. This gets completed when Cl atom attached to Gallium from other GaCl3 molecule share its pair of electrons with it. Likewise, Cl from the first GaCl3 molecule shares its lone pair of electrons with the Gallium of second GaCl3 molecule. This share of electrons forms dative bond.
2 aiii) Whenever oxidation and reduction take place in the same chemical reaction, that reaction becomes redox reaction. Oxidation is increase in oxidation number while reduction is decrease in oxidation number.
b) If in these kind of Qs, you are able to find moles of CuSO4 and moles of H2O, good to go. You can find their ratio and hence the value of y.
3b) If Q asks you to use enthalpies of formation to find the enthalpy of a reaction, following should be the cycle you’d be using:
If Q asks you to use enthalpies of combustion to find the enthalpy of a reaction, following should be the cycle you’d be using:
All enthalpy values usually given are for 1 mole of product forming so you should multiply that to 2 or 3 depending upon number of moles of that product forming.
4a) Brønsted-Lowry base says that a base is a proton acceptor and Brønsted-Lowry cd theory says that an acid is a proton donor.
5 ai) Cl is a Chlorine radical which can react with ethane through free-radical substitution reaction.
ii) Since ? bonds are electron dense meaning they tend to attract positively charged species. Out of 5 species given in the Q, only 2 of them can break heterolytically giving a partial positive charge to one specie that would be attracted towards ? bond and they are HCl & H2O.
ci) Nucleophilic addition reaction in organic chemistry means a nucleophile like H-CN first breaks heterolytically with H having a partial positive charge and CN having a partial negative charge and a lone of electrons. This :CN will be attracted towards a partially positive charged specie (usually C+ of carbonyl group) after a ? bond breaks and form 2 new ? bonds.
ii) For substitution reactions, its usually a halogenoalkane however when it says that reaction is of SN1 mechanism, it means no ordinary halogenoalkane but tertiary halogenoalkane. Halogen attached to a tertiary carbon chain is the most stable structure possible which leads to forming an intermediate product due to stability and positive inductive effect of electron donating alkyl groups.
di) But-1-ene has double bond between 1st and 2nd Carbon atom and when it reacts with water, there are chances that -OH group may be attached to 1st or 2nd Carbon atom.
ii) More stable structure is the one which has -OH attached to 2nd Carbon atom because that forms secondary carbocation which is more stable than primary carbocation which would be made if -OH attaches to 1st Carbon atom. This stability is defined by number of alkyl groups attached to that Carbon atom to which -OH group is also attached, the more the number of alkyl groups attached to that Carbon atom to which -OH group is also attached, the higher the positive inductive effect.
6 bii) Oxidation is gain of Oxygen.
fii) LiAlH4 is a reducing agent and reduction of carboxylic acid gives primary alcohol.