CHEM0037 Electron Transfer Exam Question
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CHEM0037
Candidates should attempt ALL questions. Each question is marked out of 25 and the numbers in square brackets in the right-hand margin indicate the provisional allocation of marks to the subsections of a question.
1. Rechargeable lead-acid batteries have half-cell reactions (1) and (2).
PbO2 + H2 SO4 + 2H+ + 2e– → PbSO4 + 2H2O E0 = 1.698 V (2)
(a) Determine the overall cell reaction, standard cell potential and Gibbs energy per
mol of Pb for this battery. [3]
(b) Apply the Nernst equation to half-cell reaction (2) to explain why the voltage of
the battery declines during use. Assuming a constant current is drawn from the
battery, describe the time dependence of this voltage decline. [6]
(c) A cyclic voltammogram (CV) was recorded for reaction (1) as shown below. The CV starts at –0.2 V and sweeps first towards more negative potentials.
(i) Assign the reactions responsible for peaks A and B and propose the reaction that results in increased current C. [3]
(ii) Sketch and label a typical CV expected for reversible reduction of a
dissolved redox species under diffusion control and explain the features and shape of the CV. Hence explain why peaks A and B look different
from those obtained for diffusing species. [7]
(d) Explain the following observations:
(i) Over-charging this battery can lead to water loss and hydrogen and oxygen gas production. [3]
(ii) Use of a sulfuric acid electrolyte in gel form could result in sluggish
charge and discharge rates compared to a liquid electrolyte. [3]
2. Answer ALL parts.
(a) Two cylindrical polypropylene particles of radii R1 and R2 and length l = 500 nm
are placed in water at T = 20 °C. The total pair potential V(s) for the two particles
approaching each other perpendicularly is V (s ) = Ze−Ks − .
where s is the separation distance, κ−1 is the Debye length, Z is the interaction constant and A is the Hamaker constant. Assume κ−1 = 0.81 nm, Z = 4.26 × 10−20 J nm−1 and A = 0.3 × 10−20 J for polypropylene in water.
(i) Obtain an expression for the total pair-potential when R = R1 = R2 . [1] (ii) Sketch a graph showing the variation of V(s) with s. At what values of s is
the interaction between the particles attractive, and at what values is it repulsive? Explain at what values of s the particles are stable in solution
when R = 10 nm. [6]
(iii) How would the stability of the particles differ if the hydrophobic
interaction VLW (s) =−4πRl e−ds were added to V(s)? Assume that
γ = 3 × 10−5 J m−2 and d−1 = 1 nm. [3]
(b) The spreading coefficient S of a droplet of radius R on a solid surface is given by
S =g(cosqc - 1), where g is the surface tension of the droplet at the liquid-air interface and θc is its contact angle on the substrate. If S varies as a function of
position x, the droplet can move under a force F = πR2 .
(i) Calculate θc for water ( g= 72.8 mN m− 1) when S = −9.75 mN m− 1 . [ 1] (ii) A voltage V applied to the substrate modulates the contact angle as
cosqc(V) = cosqc + V2 , where k is constant. Discuss how the wetting
properties of the surface change with the applied voltage. [2]
(iii) Show that, when V = V0 x , the force exerted on the droplet is given by
F = pkV02R2 with V0 = 1 V m−0.5 . Justify all steps in the derivation. [5]
(iv) Calculate F when R is equal to the capillary length of the water droplet
and k = 2.2 × 10−7 F m−1 . Would you expect the droplet to move towards
lower or higher wettabilities? Justify your answer. [4]
(v) Beyond a change in the value of S, what else would you expect to drive droplet motion on a horizontal surface? Justify your answer. [3]
3. Answer ALL parts.
(a) Ni has a latent heat of vaporization ΔvapH = 378 kJ mol– 1 and adopts an FCC
crystal structure with a lattice parameter b = 3.52 Å . Estimate the surface energy
for the bulk terminated Ni(101) surface. [7]
(b) The decomposition of HI on a Ni catalyst proceeds as follows:
HI(ads.) — 1 H2 (gas) + 1 I2 (gas)
2 2
Assuming a Langmuir-Hinshelwood mechanism, the decomposition rate of HI THI is given by
kbHI PHI
THI = keHI =
where k is the rate constant for the reaction and bHI is the Langmuir b-factor. The table below shows the rate of I2 formation r on 10 g of a Ni(101) catalyst at different values of PHI and a temperature of 25 °C.
r / dm3 s– 1 PHI / Pa
8.5 × 10–6 0.05 0.13 101325
(i) By considering the expression for THI at limiting cases of low and high
PHI use the values in the table to estimate bHI and k. [3]
(ii) The turnover frequency Tf for the Ni(101) catalyst is defined as the
average number of HI molecules decomposed at an individual adsorption site per second. Given that Tf = 5.3 s– 1 for the Ni(101) catalyst at PHI = 101325 Pa, calculate the surface area of the catalyst. Give your
answer in units of m2 g– 1 . [5]
(c) Solid strontium has a density of 2.63 g cm–3 and molar mass of 87.62 g mol– 1 . Use the free electron model to estimate values for the following:
(i) the Fermi energy (in eV), [4] (ii) the Fermi wave vector (in m– 1), [2] (iii) the Fermi wavelength (in nm), [2] (iv) the Fermi velocity (in m s– 1). [2]
2023-04-18