electrochemistry - chemical energy Flashcards
(16 cards)
how are chemical cells able to do work?
the transformation/reaction in an electrochemical cell has a certain energy change/ΔG associated with it, like all reactions
- this ΔG corresponds to the Ecell
moving a charge Q through the potential difference, ΔV, results in a change in energy, this energy change = electrical work
ΔG = -nFEcell
where n = no. moles
F = faradays number = 96500
Ecell = cell potential
when is work done by the system vs on the system?
when reactions go in a favourable direction, ΔG<0, the reaction can be harnesses to do electrical work, so Ecell >0
if ΔG>0 due to QΔV = Ecell, Ecell<0 due to electrons, so work is done on the system
give 4 reasons as to why it is useful to work with E°, and standard conditions specifically?
- E°/standard reduction potentials can be found in tables/databases
- for non electrochemical reactions, ΔG° can be found from ΔH° or ΔS°
- maximum work that can be done by harnessing favourable reactions is if all products and reactants are in standard state, which is unlikely
- can get information about equilibrium constant k via ΔG°=-RTlnK
what is the effect of changes in the system on ΔG and Ecell?
considering le chateliers principle, a change in system that moves position of equilibrium from LHS –> RHS, in a favourable direction, will cause ΔG to decrease and Ecell to increase
- this means we are not at standard ocnditions anymore!
an example change that would have this effect is increasing [reactant] or decreasing [product], both push the favourable reaction forward
give the nerst equation + its function
Ecell = E°cell - (RT/nF)*lnQ
where Q = reaction quotient
n = moles
F = faradays number
this equation links electrochemistry and thermodynamics
what are ΔG and Ecell when the system is at equilibrium?
considering thermodynamics + the nerst equation:
at equilibirum Q = K therefore ΔG = Ecell = 0
there is no reaction or current
how can the effect of concentration in the nerst equation be exploited?
- can be used to measure pH relative to H=/H- half cell using nerst equation, or using a H2 or other cell to measure the pH of that cell
- can be used to store energy - consider biological membranes + the use of protons/ions/electrochemical gradients to store energy or set up p.d.
what reversible process is used by rechargeable batteries?
electrolysis is the process by which batteries and used/recharged
how can the favourable reaction in a battery be reversed the other way?
a voltage needs to be applied that is at least = Ecell, to drive the unfavourable reaction
- this can be known as Eapplied
if Eapplied = Ecell, no reaction will happen
to significantly run the reverse reaction, Eapplied»_space; Ecell
Eapplied - Ecell = overpotential
why doesn’t the rechargeable battery mechanism work on galvanic cells?
in galvanic cells the anode + cathode facilitate opposite process
in an electrochemical cell oxidation is always at the anode, and reduction is always at the cathode
what are the 2 types of rechargeable batteries?
lead acid batteries - these are older, used in car batteries
newer type = nickel metal hydroxide batteries
how are newer type batteries improved from older batteries?
as the older lead batteries are used, the voltage changes and remaining charge can be determined from voltage, however this is not the case for the newer batteries
this is because of the nerst equation, all reactants + products in new batteries are solid, and so =1 in the nerst equation, therefore there is no voltage change, whereas lead batteries have aq components so voltage is everchanging
give 1 use of industrial electrolysis
often used to produce chemicals
give 2 disadvantages of using industrial electrolysis to produce chemicals
- any side reactions that occur are uncontrollable and this can harm yield of desired product, only voltage can be controlled
- also any side product with a less -ve reduction potential will be reduced at the cathode
what is the equation that relates charge, current and time?
Q = I*t
how would electrochemical calculations change for real world calculations?
cannot assume ideality, so {} is used instead of []
