Electrochemistry 4 Flashcards
what is a cell
Any battery (actually it may have one or more than one cell connected
in series) or cell that we use as a source of electrical energy is basically
a galvanic cell where the chemical energy of the redox reaction is
converted into electrical energy. However, for a battery to be of practical
use it should be reasonably light, compact and its voltage should not
vary appreciably during its use.
what are primary cells
In the primary batteries, the reaction occurs only once and after use
over a period of time battery becomes dead and cannot be reused
again.
describe leclanche cell
i) electrodes
ii) electrolyte
iii) electrode reactions
The most familiar example of this type is the dry
cell (known as Leclanche cell after its discoverer) which is used commonly in our transistors and clocks.
The cell consists of a zinc container that also acts as anode and the cathode is a carbon (graphite) rod surrounded by powdered manganese dioxide and carbon (Fig.2.8). The
space between the electrodes is filled by a moist paste of ammonium chloride (NH4Cl) and zinc chloride (ZnCl2).
The electrode reactions are complex, but they can be written
approximately as follows
Anode: Zn(s) → Zn2+ + 2e–
Cathode: MnO2+ NH4++ e– → MnO(OH) + NH3
In the reaction at cathode, manganese is reduced
from the + 4 oxidation state to the +3 state.
Ammonia produced in the reaction forms a complex with Zn2+ to give[Zn (NH3)4]2+. The cell has a potential of nearly 1.5 V.
describe mercury cell
i) uses
ii) electrodes/ electrolyte
iii) electrode reactions
iv) potential
Mercury cell, (Fig. 2.9) suitable for low current devices like hearing aids, watches, etc. consists of zinc – mercury amalgam as anode and a paste of HgO and carbon as the cathode. The electrolyte is a paste of KOH and ZnO. The electrode reactions for the cell are given below:
Anode: Zn(Hg) + 2OH– → ZnO(s) + H2O + 2e–
Cathode: HgO + H2O + 2e– → Hg(l) + 2OH–
The overall reaction is represented by
Zn(Hg) + HgO(s) → ZnO(s) + Hg(l)
The cell potential is approximately
1.35 V and remains constant during its
life as the overall reaction does not
involve any ion in solution whose
concentration can change during its life
time.
what is a secondary cell
A secondary cell after use can be recharged by passing current
through it in the opposite direction so that it can be used again. A
good secondary cell can undergo a large number of discharging
and charging cycles.
describe the lead storage battery
i) use
ii) electrodes and electrolyte
iii) reactions
iv) reversed
The most important secondary cell is the lead
storage battery (Fig. 2.10) commonly used in automobiles and
invertors.
It consists of a lead anode (lead grid filled with spongy lead) and a grid of lead packed with
lead dioxide (PbO2 ) as cathode. A 38% solution of sulphuric acid is used as an electrolyte.
The cell reactions when the battery is in use are given below:
Anode: Pb(s) + SO42–(aq) → PbSO4 (s) + 2e–
Cathode: PbO2(s) + SO42–(aq) + 4H+(aq) + 2e– → PbSO4(s) + 2H2O (l)
i.e., overall cell reaction consisting of cathode and anode reactions is:
Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l)
On charging the battery the reaction is reversed and PbSO4 (s) on anode and cathode is converted into Pb and PbO2
, respectively.
describe nickel cadmium battery
Another important secondary
cell is the nickel-cadmium cell
(Fig. 2.11) which has longer life
than the lead storage cell but
more expensive to manufacture.
We shall not go into details of
working of the cell and the
electrode reactions during
charging and discharging.
The overall reaction during
discharge is:
Cd (s) + 2Ni(OH)3 (s) → CdO (s) + 2Ni(OH)2 (s) + H2O (l )
A Ni-Cd cell is in the form of a jelly roll arrangement, where the positive and negative sheets are seperated by a seperator ( a layer soaked in sodium/potassium hydroxide)
what are some methods of early diagnosis of disease
