(a)(i) State Faraday's first law of electrolysis. (ii) Distinguish between a strong electrolyte and a weak electrolyte
(b) State one chemical property of ethyne.
(c)( i) What is meant by the tern unsaturated hydrocarbon? (ii) Complete the following reaction equation: \(\mathrm{CH_3 + CH_3OH \rightarrow}\) (iii) Name the major product formed in the cation stated in 1(c)(ii).
(d) State one way by which the rate of esterification could be increased.
(e) Consider the reaction represented by the following equation: \(\mathrm{Zn + H_2SO_4 \rightarrow ZnOS_4 + H_2}\). If 3.75g of Zn dust was added to excess \(\mathrm{H_2SO_4}\). Calculate the number of molecules of hydrogen gas produced. [ \(\mathrm{Zn = 65.0,\ Na = 6.02 \times 10^{23}}\) ].
(f) State one effect of global warming.
A. \(\mathrm{Pb(NO_3) + H_2S \rightarrow PbS + 2HNO_3}\);
B. \(\mathrm{H_2 + C_2H_4 \rightarrow C_2H_6}\).
C. \(\mathrm{Zn(OH)_2 + 2OH \rightarrow [Zn(OH)_4]^{2}}\).
(i) Which of the equations represent(s) redox process? (ii) State the change in Oxidation number of the species that are oxidized or reduced. (h)(i) State two of the main concepts of Bohr's model of the atom. (ii) State the limitations of Bohr's model. (i) List three factors that could influence the equilibrium position of a reversible reaction. (j) Calcium trioxocarbonate(iv) powder is added to separate equimolar solutions of hydrochloric acid and ethanoic acid. State one: (i) similarity in the observation in both reactions: (ii) difference in the observation in both reactions.
(a)(i) Faraday's first law of electrolysis
The mass of a substance liberated or deposited at an electrode during electrolysis is directly proportional to the quantity of electricity (charge) that passes through the electrolyte.
(a)(ii) Strong versus weak electrolyte
A strong electrolyte is almost completely ionized in aqueous solution, whereas a weak electrolyte is only partially ionized in aqueous solution.
(b) One chemical property of ethyne
Ethyne is unsaturated and undergoes addition reactions, e.g. it decolourizes bromine water; it also burns in air. \(C_2H_2 + 2Br_2 \to C_2H_2Br_4\).
(c)(i) Unsaturated hydrocarbon
A hydrocarbon that contains at least one carbon-carbon double or triple bond.
(c)(ii) and (iii) Esterification
Reading the reaction as an ethanoic acid and methanol esterification:
\[CH_3COOH + CH_3OH \overset{H_2SO_4}{\rightleftharpoons} CH_3COOCH_3 + H_2O\]
The major product is the ester methyl ethanoate (\(CH_3COOCH_3\)).
(d) Increasing the rate of esterification
Use a small amount of concentrated tetraoxosulphate(VI) acid as catalyst (also warming the mixture increases the rate).
(e) Number of hydrogen molecules from 3.75 g Zn
\[Zn + H_2SO_4 \to ZnSO_4 + H_2\]
Moles of Zn \(= 3.75/65 = 0.0577\,mol\). From the equation 1 mol Zn gives 1 mol \(H_2\), so moles of \(H_2 = 0.0577\,mol\).
\[\text{Number of molecules} = 0.0577 \times 6.02 \times 10^{23} = 3.47 \times 10^{22}\]
(f) One effect of global warming
Melting of polar ice caps leading to a rise in sea level (and flooding of low-lying areas). Other acceptable: climate change, drought, desertification.
(g) Redox analysis
- (i) Equation B (\(H_2 + C_2H_4 \to C_2H_6\)) is the redox process. (In A and C no oxidation number changes.)
- (ii) In B, hydrogen changes from oxidation number 0 (in \(H_2\)) to \(+1\) (in \(C_2H_6\)) so it is oxidized; carbon changes from \(-2\) (in \(C_2H_4\)) to \(-3\) (in \(C_2H_6\)) so it is reduced.
(h)(i) Two concepts of Bohr's model
- Electrons revolve round the nucleus only in certain fixed circular paths (orbits/shells) of definite (quantized) energy.
- An electron does not radiate energy while in a given orbit; it absorbs or emits energy only when it jumps from one energy level to another.
(h)(ii) Limitations of Bohr's model
It successfully explains only the hydrogen (one-electron) atom and fails for atoms with more than one electron; it cannot explain the fine structure/splitting of spectral lines and contradicts the Heisenberg uncertainty principle (an electron cannot have a fixed orbit and definite momentum simultaneously).
(i) Three factors influencing equilibrium position
Concentration of reactants/products, temperature, and pressure (for reactions involving gases). A catalyst does not shift the position of equilibrium.
(j) Calcium trioxocarbonate(IV) with HCl and with ethanoic acid
- (i) Similarity: in both, effervescence occurs and a colourless gas (carbon(IV) oxide) is evolved which turns limewater milky.
- (ii) Difference: the reaction with hydrochloric acid (a strong acid) is faster and more vigorous, while the reaction with ethanoic acid (a weak acid) is slower and less vigorous.
