Electrochemistry 2 Flashcards
In the ECS, best oxidising agents ?
t fluorine gas (F2
) has the maximum
tendency to get reduced to fluoride ions (F–
) and therefore fluorine
gas is the strongest oxidising agent and fluoride ion is the weakest
reducing agent.
reduction potential of F2= +2.87 V
As the reduction potential becomes for +ve, the tendency to get reduced increases and hence it gains electrons easily. The oxidising tendency increases, of the species on leftt hand side while the reducing tendency decreases of the species on the right hand side.
In the ECS, the best reducing agents
Lithium has the lowest electrode potential indicating
that lithium ion is the weakest oxidising agent while lithium metal is
the most powerful reducing agent in an aqueous solution.
As the value becomes more negative, the tendency to gain electrons ie get reduced decreases. Hence more negative electrode potential makes better reducing agents
The reducing tendency increases of the species in the right hand side, while the oxidising tendency decreases of the species in the left hand side
electode potential of Li+ is -3.05V
application of ecs
- calculate standard cell potential of cell
- ## predict spontaneity of a reaction
nernst equation
It is the equation wich relates the electrode potential values and concentration.
It can be seen that E(cell) depends on the concentration of both Cu2+
and Zn2+ ions. It increases with increase in the concentration of Cu2+
ions and decrease in the concentration of Zn2+ ions.
what happens when eqbm is attained
If the circuit in Daniell cell (Fig. 2.1) is closed then we note that the reaction
Zn(s) + Cu2+(aq) ® Zn2+(aq) + Cu(s) (2.1)
takes place and as time passes, the concentration of Zn2+ keeps
on increasing while the concentration of Cu2+ keeps on decreasing.
At the same time voltage of the cell as read on the voltmeter keeps
on decreasing. After some time, we shall note that there is no change
in the concentration of Cu2+ and Zn2+ ions and at the same time,
voltmeter gives zero reading. This indicates that equilibrium has been
attained.
what is the gibbs free energy of a cell reacn
Electrical work done in one second is equal to electrical potential
multiplied by total charge passed. If we want to obtain maximum work
from a galvanic cell then charge has to be passed reversibly. The
reversible work done by a galvanic cell is equal to decrease in its Gibbs
energy and therefore, if the emf of the cell is E and nF is the amount
of charge passed and DrG is the Gibbs energy of the reaction, then
DrG = – nFE(cell)
It may be remembered that E(cell) is an intensive parameter but DrG
is an extensive thermodynamic property and the value depends on n.
what is resistance?
The electrical
resistance is represented by the symbol ‘R’ and it is measured in ohm (W)
. It can be
measured with the help of a Wheatstone bridge with which you are
familiar from your study of physics. The electrical resistance of any object
is directly proportional to its length, l, and inversely proportional to its
area of cross section, A. That is,
The constant of proportionality, r (Greek, rho), is called resistivity
(specific resistance). Its SI units are ohm metre (W m) and quite often
its submultiple, ohm centimetre (W cm) is also used. IUPAC recommends
the use of the term resistivity over specific resistance
what is conductance or conductivity
The inverse of resistance, R, is called conductance, G, and we have
the relation:
The SI unit of conductance is siemens, represented by the symbol
‘S’ and is equal to ohm–1 (also known as mho) or W
–1. The inverse of
resistivity, called conductivity (specific conductance) is represented by
the symbol, k (Greek, kappa).
The SI units of conductivity are
S m–1 but quite often, k is expressed in S cm–1. Conductivity of a
material in S m–1 is its conductance when it is 1 m long and its area
of cross section is 1 m2
. It may be noted that 1 S cm–1 = 100 S m–1
.
how are materials ckassified based on their conductivity
Materials are classified into conductors, insulators and
semiconductors depending on the magnitude of their conductivity. Metals
and their alloys have very large conductivity and are known as conductors.
Certain non-metals like carbon-black, graphite and some organic
polymers* are also electronically conducting. Substances like glass,
ceramics, etc., having very low conductivity are known as insulators.
describe smiconductors
Substances like silicon, doped silicon and gallium arsenide having
conductivity between conductors and insulators are called
semiconductors and are important electronic materials. Certain materials
called superconductors by definition have zero resistivity or infinite
conductivity. Earlier, only metals and their alloys at very low temperatures
(0 to 15 K) were known to behave as superconductors, but nowadays a
number of ceramic materials and mixed oxides are also known to show
superconductivity at temperatures as high as 150 K
what is electrical conducance? what does it depend upon? does the composition of metal change as electricity flows through it
Electrical conductance through metals is called metallic or electronic
conductance and is due to the movement of electrons. The electronic
conductance depends on
(i) the nature and structure of the metal
(ii) the number of valence electrons per atom
(iii) temperature (it decreases with increase of temperature).
As the electrons enter at one end and go out through the other end,
the composition of the metallic conductor remains unchanged. The
mechanism of conductance through semiconductors is more complex.
why does conductance of metal decrease with temperature
when the temperature increases, the kinetic energy of the atoms increases and hence they vibrate more faster. this obstructs the smoooth flow of electrons and hence retards the flow of current.
what is electrolytic conductance? does the composition of the electrolyte change?
When electrolytes are dissolved in water, they furnish
their own ions in the solution hence its conductivity also increases. The
conductance of electricity by ions present in the solutions is called
electrolytic or ionic conductance.
Passage of direct current through ionic solution over a prolonged
period can lead to change in its composition due to electrochemical
reactions. The ions get discharged and hence deposited/liberated.
what does ionic conductance depends upon
The conductivity of electrolytic (ionic)
solutions depends on:
(i) the nature of the electrolyte added
(ii) size of the ions produced and their solvation
(iii) the nature of the solvent and its viscosity
(iv) concentration of the electrolyte
(v) temperature (it increases with the increase of temperature).
problems in measuring ionic conductivity
Firstly, passing direct current
(DC) changes the composition of the solution. Secondly, a solution cannot
be connected to the bridge like a metallic wire or other solid conductor.
The first difficulty is resolved by using an alternating current (AC) source
of power. The second problem is solved by using a specially designed
vessel called conductivity cell.
