electrochemistry - redox

Question 1

what is electrochemistry?

Answer 1

the chemistry/study of charged things, such as ions or electrons

Question 2

give 4 assumptions of electrochemistry

Answer 2

• all substances are liquid or solid
• all gases are atmospheric pressure
• no temperature changes
• all solutions are ideal

Question 3

how is charge, Q measured?

Answer 3

coulombs, C

Question 4

voltage, V definition

Answer 4

energy per unit charge, or electric potential difference - measured in volts, V
thus work (aka energy) = ΔE = QΔV

Question 5

current, I definition

Answer 5

rate of flow of charge, measured in amps, A

Question 6

what is the purpose of electrochemistry?

Answer 6

to use electricity to drive unfavourable processes and harness favourable reactions into electricity, to do electrical work

Question 7

how does the electrolysis of water produce energy? + give a benefit of this as a source of energy

Answer 7

H2 + 1/2O2 <–> H2O
the forward reaction is favourable, releasing an electrical energy output as the fuel cell is used
the reverse reaction recharges the system to produce electricity again, like a battery

this specific reaction is good for the environment as no CO2 is emitted

Question 8

give one challenge of using electrolysis/formation of water as an energy source

Answer 8

requires hydrogen gas, which is difficult to store

Question 9

give one advantage and one disadvantage of typical fuels used today

Answer 9

typical fuels include methanol, ethanol, etc
these are easily stored but produce CO2
- recently CO2 has been successfully converted back into methanol, so net 0 CO2 is possible soon

Question 10

electrochemical cell definition

Answer 10

a device/system capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions - this specifically only works with redox reactions

Question 11

how do redox reactions allow the generation of electricity?

Answer 11

they can be harnessed in the form of electrochemical cells to generate a flow of electrons

Question 12

what are the components of an electrochemical cell?

Answer 12

2 half cells, one containing an anode and one containing a cathode, which are connected to a voltmeter, and the cells are connected via a salt bridge, completing the circuit

Question 13

what are the 3 types of half cell?

Answer 13

metal ion electron - contains a metal electrode and matching metal ion solution
redox electrode - the electrode is made of an inert metal e.g. Pt that transfers electrons between solvated ions
gas electrode - reaction between gas and solution via an inert electrode e.g. Pt (such as the hydrogen half cell, used as a reference electrode)

Question 14

in which direction are shorthand electrochemical cells usually written?

Answer 14

usually written in the direction that the reaction flows, normally electrons will flow from LHS -> RHS therefore anode half cell written on LHS and cathode half cell written on RHS

Question 15

what does | mean on a cell diagram?

Answer 15

phase boundaries, between the electrode + solution + any gas

Question 16

what does || mean on a cell diagram?

Answer 16

salt bridge, separating 2 half cells

Question 17

standard reduction potential definition

Answer 17

a measure of the reducing power of elements/compounds relative to a hydrogen half cell under standard conditions
= E°

Question 18

in what direction are half equations for cells written + why?

Answer 18

half equations for E° are always written in the direction of reduction, as they model standard reduction potential

Question 19

how is direction of reaction found with E°?

Answer 19

more +ve E° means reduction is more favourable, more -ve E° means reduction is unfavourable
when pairing together 2 half cells, standard reduction potentials dictate that the more +ve half cell will be reduced at the cathode, and the more -ve half cell will be oxidised at the anode
therefore E°cell = E°cathode - E°anode

Question 20

what is the difference between E° and E?

Answer 20

E = general cell/electrode potentil
E° = potential under standard conditions for both products and reactants

Question 21

what conditions must E°cell be measured at?

Answer 21

standard temp, pressure and concentration, and also under 0 current and with a high resistance voltmeter as other wise electrodes can react/change

Question 22

how can E°cell be used to determine favourability?

Answer 22

if E°cell < 0 reaction is favourable

Question 23

resistance, R definition + equation

Answer 23

a measure of how difficult it is to get current to flow through something - measured in ohms, Ω
R = V/I = p.d. / current

Question 24

how are V and E related for electrochemical cells?

Answer 24

V = potential difference, same as E = cell potential
they are the same thing, interchangeable

Question 25

how are resistance and voltage related?

Answer 25

for 2 resistors in series, the total potential difference is divided between them proportionally to the resistance of each Vx=IRx and Vy=IRy

Question 26

what is internal resistance?

Answer 26

all electrochemical cells have a small unavoidable internal resistance = Rinternal, from electrodes, solvated ions, etc therefore Ecell = Rinternal + Rexternal

Question 27

give 2 ways to ensure Ecell is measured as accurately as possible

Answer 27

ensure Rexternal >> Rinternal so Emeasured is as close as possible to Ecell or minimise current to minimise Einternal as Eint = IRint

Question 28

give 1 disadvantage of using H2(g) as a reference electrode?

Answer 28

H2(g) is difficult + dangerous to store, can be inconvenient

Question 29

give 1 alternative reference electrode + an advantage of it

Answer 29

AgCl(s) +e- --> Ag(s) + Cl-(aq) this is much easier to work with as there are no gas moles, oxidised Ag(s) doesn't dissolve so doesn't need a separate half cell, also is a solid at room temperature so easy to work with under standard conditions, always gives a very even E°cell value