Question
Bromine, \(Br_2 (l)\), and methanoic acid, HCOOH(aq), react in the presence of sulfuric acid.
\(Br_2 (l) + HCOOH(aq) → 2HBr(aq) + CO_2 (g)\)
(a) Suggest an experimental method that could be used to determine the rate of reaction.
(b) The sulfuric acid is a catalyst in this reaction. Explain how a catalyst increases the reaction rate.
(c) Methanoic acid can react with ethanol to produce an ester.
(i) Draw the full structural formula of the organic product and state its name.
Structural formula:
Name:
(ii) Predict the number of signals, and their splitting patterns, in the \(^1H\) NMR spectrum of this organic product.
Number of signals:
Splitting patterns:
(iii) State one reason why tetramethylsilane, TMS, is often chosen as an internal reference standard for the calibration of \(^1H\) NMR spectroscopy.
(d) (i) Write the equation for the complete combustion of ethanol.
(ii) Determine the enthalpy change for the combustion of ethanol, in kJ \(mol^{−1}\),using section 11 of the data booklet.
Answer/Explanation
Answer:
(a) «measure change in»
mass
OR
pressure
OR
volume of gas/\(CO_2\) produced
OR
«intensity of» colour
OR
«electrical» conductivity
OR
pH
with time
(b) provides an alternative reaction pathway AND lower activation energy/\(E_a\)
larger fraction/number of molecules with E ≥ \(E_a\)/enough energy «for a successful collision»
(c) (i) Structural formula: 
Name:
ethyl methanoate
(ii) Number of signals:
3
Splitting patterns:
singlet/1
AND
quartet/4
AND
triplet/3
(iii) Any two of the following:
chemical shift/signal outside range of common chemical shift/signal
strong signal/12/all H atoms in same environment
singlet/no splitting of signal
volatile/easily separated/easily removed
inert/stable
soluble in most organic solvents
(d) (i) \(CH_3CH_2OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(g)\)
(ii) «bond breaking»
1 C-C + 5 C-H + 1 C-O + 1 O-H + 3 O=O / 346 + 5(414) + 358 + 463 +3(498) /
4731 «kJ»
«bond forming»
4 C=O + 6 O-H / 4(804) + 6(463) / 5994 «kJ»
DH «= 4731– 5994» = −1263 «kJ \(mol^{−1}\) »
Question
The periodic table is a useful tool in explaining trends of chemical behaviour.
(a) (i) Annotate and label the ground state orbital diagram of boron, using arrows to represent electrons.
(ii) Sketch the shapes of the occupied orbitals identified in part (a)(i).
(b) Explain the decrease in first ionization energy from Li to Cs, group 1.
(c) (i) State the electron domain geometry of the ammonia molecule.
(ii) Deduce the Lewis (electron dot) structure of ammonia and sketch its 3D molecular shape.
(iii) Explain, with reference to the forces between molecules, why ammonia has a higher boiling point than phosphine \((PH_3)\).
(d) (i) Ammonia is manufactured by the Haber process.
\(N_2(g)+3H_2(g)\rightleftharpoons 2NH_3 (g)\) \(\Delta H^{\theta}_t = -92.0\) KJ \(mol^{-1}\)
Outline what is meant by dynamic equilibrium.
(ii) Deduce the \(K_c\) expression for the reaction in part (d)(i).
(iii) Determine the entropy change. \( \Delta S^{\theta}\) for the forward reaction to four significant figures, using the data given.
(iv) Calculate the temperature, in K, below which this reaction becomes spontaneous.
Use section 1 of the data booklet. (If you were unable to obtain an answer for part (d)(iii) use -210.0J \(K^{-1} mol^{-1}\), but this is not the correct value.)
(v) The value of \(K_c\) for this reaction is \(6.84 × 10^{-5}\) at 500°C. Suggest, with a reason, how lowering the temperature affects the value of \(K_c\).
(vi) Calculate the standard Gibbs free energy change, \(ΔG^{\theta}\), in kJ \(mol^{-1}\), for this reaction. Use sections 1 and 2 of the data booklet.
(e) (i) The Haber process requires a catalyst. State how a catalyst functions.
(ii) Sketch a Maxwell–Boltzmann distribution curve showing the activation energies with and without a catalyst.
(iii) Suggest how the progress of the reaction could be monitored.
Answer/Explanation
Answer:
(a) (i) 
arrows AND identifies 2s AND 2p sub orbitals
(ii) 
(b) valence electron further from nucleus/«atomic» radius larger «down the group»
«electron» more shielded/ less attractive force/easier to remove
(c) (i) tetrahedral
(ii) 
(iii)
ammonia has intermolecular/IMF hydrogen bonds «phosphine does not»
phosphine «and ammonia» dipole-dipole/London dispersion forces/instantaneous
dipole attractions/Van der Waals forces
hydrogen bonds stronger
(d) (i) «in a closed system» the rate of the forward reaction equals the rate of the
reverse reaction.
(ii) \([NH_3]^2/([N_2][H_2]^3)\)
(iii) \(\Delta S^{\theta}=\Delta S^{\theta}_(products)-\Delta S^{\theta}_{reactants}\)
OR
\((2 \times 192.8\) << J \(mol^{-1} K^{-1}\)>> ) – (3 \(\times \)130.7 << J \(mol^{-1} K^{-1}\) >> + 191.6 << J \(mol^{-1} K^{-1}\) >>)
(d) (iv) 
(v) «reaction» exothermic AND Kc increases «as equilibrium moves right»
(vi) 
(e) (i) alternate pathway AND lowers activation energy/\(E_a\)
(ii) 
correct shape curve starting at the origin, without touching the x axis at high
energy.
(\(E_a\)) catalysed <(\(E_a\)) uncatalysed on x axis.
(e) (iii) change in AND
volume
OR
pressure
OR
temperature
OR
concentration of \(H_2\)/\(N_2\) /reactants/\(NH_3\) /product