Hi UltimaOnline,
I have further questions on electrochem:
I. The diagram below shows the sodium-nickel(II) chloride battery,
which is a high power, high capacity cell suitable for electric
traction applications.

The electrolyte used is molten sodium aluminium chloride,
which has a melting point of 157°C. In the reaction, nickel(II)
chloride is reduced to nickel. Which of the following statements is
incorrect?
A: Terminal E is the negative terminal.
Question: How do I know which is the positive and negative
terminal?
II: In the construction of pacemakers for the heart, a tiny
magnesium electrode can be used to create an electrical cell with
the inhaled oxygen. The relevant half‐equations are as
shown:
Mg2+
+ 2e−
⇌ Mg (Equilibrium 1)
½ O2
+ 2H+
+ 2e−
⇌ H2O
(Equilibrium 2)
In the body, a potential of 3.20V is usually obtained. What is the
best explanation for this e.m.f.?
A The small size of the magnesium electrode
B The low concentration of Mg2+
surrounding the magnesium electrode
C The high resistance of the body fluids surrounding the
electrodes
D The physiological pH of between 7 and 8 of the body fluid
surrounding the electrodes
Answer: D
Remarks: Since the Ecell according to the Data Booklet for this
reaction +3.61V, and in the body the usual emf is 3.20V, this would
mean either the [Mg2+]
is greater in reality, or that there or lower [H+]
or [Oxygen] in reality. I can infer that D is correct because of
the low [H+] in a pH environment between 7 to 8, but how do I know
the other choices are wrong/not good explanations?
III:
http://img.photobucket.com/albums/v700/gohby/Chemistry/sludge_zpsimhrdrbj.jpg
Answer: D
Remarks: Comparing the Ered
values, I can gather that Cu2+
will be reduced to Cu at the cathode.
(i) What is meant by “Ag and Fe impurities”? If they refer to
metallic compounds, wouldn’t it be unfavourable for the metallic
ions to be oxidised (since it is at the anode) even
further?
(ii) If “Ag and Fe impurities” refers to the metals itself, how do
I know that Cu2+ and Ag+ will be formed, since the
Eox
(Cu/Cu2+)
and Eox
(Ag/Ag+)
are negative? And how does Ag “fall off” the electrode and form the
sludge?
IV:
http://img.photobucket.com/albums/v700/gohby/Chemistry/doublelec_zpshzwpp2ja.jpg
Answer: A
Remarks:
(i) If it is an open-ended question, will I know if the product at
S would be Fe2+
(instead of Fe)? I think it can be both (so long as the Ecell is
positive) but I can’t be sure.
(ii) At electrode Q, comparing the Ered
potentials between (Cu2+/Cu)
and (H+/H2),
wouldn’t Cu2+
be preferentially reduced compared to the latter - so why would
hydrogen gas be the products formed at Q?
Thank you! :)