topic 14: redox II Flashcards
what does an electrochemical cell consist of
- a cell has two half–cells
- the two half cells have to be connected with a salt bridge
- simple half cells will consist of a metal (acts an
electrode) and a solution of a compound containing
that metal (eg Cu and CuSO4) - 1 moldm-3 - these two half cells will produce a small voltage if connected into a circuit. (i.e. become a battery or cell)
- standard conditions
- electrodes connected to a high resistance voltmeter
- LHS - oxidised
- RHS - reduced
why does a voltage form
- when connected together the zinc half-cell has more of a tendency to
oxidise to the Zn2+ ion and release electrons than the copper half-cell
(Zn Zn2+ + 2e-) - more electrons will therefore build up on the zinc electrode than the copper electrode
- a potential difference is created between the two electrodes. the zinc strip is the negative terminal and the copper strip is the
positive terminal
why use a high resistance voltmeter
- the voltmeter needs to be of very
high resistance to stop the current
from flowing in the circuit. in this
state it is possible to measure the
maximum possible potential difference (E). - the reactions will not be occurring
because the very high resistance
voltmeter stops the current fromflowing
what is the salt bridge used for
the salt bridge is used to connect up the circuit. the free moving ions conduct the charge
what is a salt bridge made up of
- a salt bridge is usually made from a piece of filter paper (or material) soaked in a salt solution, usually potassium nitrate
- the salt should be unreactive with the electrodes and electrode solutions
——-> e.g. potassium chloride would not
be suitable for copper systems as chloride ions can form complexes with copper ions
- the salt should be unreactive with the electrodes and electrode solutions
why is a wire not used in place of a salt bridge
because the metal wire would set up its own electrode system with the solutions and wires do not allow the flow of ions
what would happen if the voltmeter is removed and replaced with a bulb
a current flows. the
reactions will then occur separately at each electrode. the voltage will fall to zero as the reactants are used up
how can electrochemical cells be represented
cell diagram
Zn(s) | Zn2+ (aq) | | Cu2+ (aq) | Cu (s)
what does each part of the cell diagram represent
- the solid vertical line represents the boundary between phases e.g. solid (electrode) and solution (electrolyte) * the double line represents the salt bridge between the two half cells
- the voltage produced is indicated
- the more positive half cell is written on the right if possible (but this is not essential)
what electrode is used if the system doesn’t have an electrode and why
- platinum electrode
- provides a conducting surface for electron transfer
- a platinum electrode is used because it is unreactive and can conduct electricity
how do you measure the electrode potential of a cell
- it is not possible to measure the absolute potential of a half electrode on its own. it is
only possible to measure the potential difference between two electrodes - to measure it, it has to be connected to another half-cell of known potential, and the potential difference between the two half-cells measured
- by convention we can assign a relative potential to each electrode by linking it to a reference electrode (hydrogen electrode), which is given a potential of zero volts
what is the potential of all electrodes measured by comparing
comparing their potential to that of the standard hydrogen electrode
what are the components of a standard hydrogen electrode (SHE)
- hydrogen gas at pressure of 100 kPa
- solution containing the hydrogen ion at 1.00 mol dm-3 (solution is usually 1 mol dm-3 HCl)
- temperature at 298K
describe how to draw an electrode set up
- on the right hand side, the electrode getting reduced is drawn in 1 mol dm-3 of a corresponding solution - anode - positive terminal
- on the left hand side, the electrode is getting oxidised in 1mol dm-3 of the corresponding solution - cathode - negative terminal
- salt bridge between each beaker
high resistance voltmeter drawn - the direction of current goes from LHS to RHS
WRITE OUT THE STATNDARD CONDITIONS
what is standard electrode potential
when an electrode system is connected to the hydrogen electrode system, and standard conditions apply the potential difference measured
what is the equation for working out the EMF of a cell
E(rhs) - E(lhs)
what does it mean for the more negative half cell
it is oxidised so it goes the opposite way
what happens as the standard electrode potential increases and becomes more positive
tendency for
species on left to
reduce, and act
as oxidising
agents
what happens as the standard electrode potential gets more negative
tendency for
species on right
to oxidise, and
act as reducing
agents
what is Ecell directly proportional to
total entropy change and lnK
positive Ecell will lead to a positive total entropy change
what is Ecell a measure of
a measure of how far from equilibrium the
cell reaction lies. the more positive the Ecell the more likely the reaction is to occur
what is the effect of concentration on Ecell
as the concentration of the reactants increases so will the Ecell
what is the effect of temperature on Ecell
most cells are exothermic in the spontaneous direction so applying Le Chatelier to a temperature rise to these would result in a decrease in Ecell because the equilibrium reactions would shift backwards
what is a fuel cell
uses the energy from the reaction of
a fuel with oxygen to create a voltage
what is the overall equation of a hydrogen fuel cell
2e- + 2H+ —> H2 E=0V
4e- + 4H+ +O2 —> 2H2O E=1.23V
in acidic conditions these are the
electrode potentials. the Ecell is the same as alkaline conditions as
the overall equation is the same
overall 2H2 + O2 —> 2H2O E=1.23V
what are some advantages of hydrogen fuel cells
- less pollution and less CO2 (pure hydrogen emits only water whilst hydrogen-rich fuels
produce only small amounts of air pollutants and
CO2) - greater efficiency
- hydrogen is readily available by the
electrolysis of water
what are some limitations of hydrogen fuel cells
- expensive
- storing and transporting hydrogen, in terms of safety, feasibility of a pressurised liquid and a limited
life cycle of a solid ‘adsorber’ or ‘absorber’ - limited lifetime (requiring regular replacement and
disposal) and high production costs - use of toxic chemicals in their production
what are the different ways that hydrogen can be stored in fuel cells
- as a liquid under pressure
- adsorbed on the surface of a solid material
- absorbed within a solid material
describe advantages of ethanol/methanol fuel cells
ethanol fuel cells have also been developed. compared to hydrogen fuel cells they have certain advantages including…
- ethanol can be made from renewable sources in a carbon neutral way
- raw materials to produce ethanol by fermentation are
abundant
- ethanol is less explosive and easier to store than hydrogen
- new petrol stations would not be required as ethanol is a liquid fuel
what are the equations that occur in an ethanol/methanol fuel cells
equation that occurs at oxygen electrode
4e- + 4H+ +O2 —> 2H2O E=1.23V
equation that occurs at ethanol electrode
C2H5OH + 3O2 → 2CO2 + 3H2O
overall equation
C2H5OH + 3H2O → 2CO2 + 12H+ + 12e-
manganate redox titration
- the acid is needed to supply the 8H+ ions. some acids are not suitable as they set up alternative redox reactions and hence make the titration readings inaccurate.
only use dilute sulfuric acid for manganate titration.
insufficient volumes of sulfuric acid will mean the solution is not acidic enough and MnO2 will be produced instead of Mn2+
MnO4-(aq) + 4H+(aq)+ 3e- —> MnO2 (s) + 2H2O
the brown MnO2 will mask the colour change and lead to a greater (inaccurate) volume of manganate being
used in the titration - using a weak acid like ethanoic acid would have the same effect as it cannot supply the large amount of hydrogen ions needed (8H+)
- it cannot be conc HCl as the Clions would be oxidised to Cl2 by MnO4- as the Eo MnO4-/Mn2+ > Eo Cl2/Cl-
MnO4-(aq) + 8H+(aq)+ 5e– —>Mn2(aq) + 4H2O(l) E+1.51V
Cl2(aq) +2e– —> 2Cl–(aq) E +1.36V this would lead to a greater volume of manganate being used and poisonous Cl2 being produced - it cannot be nitric acid as it is an oxidising agent. it oxidises Fe2+ to Fe3+ as Eo NO3-/HNO2> Eo Fe3+/Fe2+
NO3- (aq) + 3H+(aq) + 2e– —> HNO2(aq) + H2O(l) Eo +0.94V
Fe3+ (aq)+e– Fe2+ (aq) Eo +0.77 V
this would lead to a smaller volume of manganate being used
in the redox titration between Fe2+ and MnO4- what is the colour change when the manganate is in the burette
colourless to pink
what can cells be
non-rechargeable (irreversible), rechargeable and fuel cells
