3.1 redox and standard electrode potential Flashcards
what does the electrode potential mean?
(dont need definition just understand it)
how easily a metal looses its electrons into a solution of its own ions
a combined system containing two half cells is called..?
an electrochemical cell
if a half cell is connected to another half cell with a different metal, there will be a potential difference or electromotive force (emf) between the two electrode, causing a current to flow between them
how can the e.m.f be measured?
using a voltmeter
what is an advantage of using a voltmeter to measure the e.m.f?
voltmeters have a high resistance so that they do not divert much current from the main circuit
in what direction do electrons flow in electrochemical cells?
from the left to the right
in an electrochemical cell, which side is the negative side?
LHS
what is the salt bridge used for in an electrochemical cell?
to complete the circuit (without allowing solutions to mix)
the salt bridge is often a piece of filter paper saturated with a solution of an inert (unreactive) electrolyte such as KNO3 (aq)
- in electrochemical cells, the better reducing agents are on the (RHS/LHS)?
- the better oxidising agents are on the (RHS/LHS)??
- LHS
- RHS
dont use the names anode and cathode
for half equations showing reduction, what side of the equations are the electrons??
left (LHS)
if the half-reaction doesnt contain a metal in its elemental state, what must be used?
an inert platinum electrode
- this is required in order to connect the redox couple to the external circuit.
THE Pt ELECTRODE DOESNT GET INVOLVED IN THE REACTION
if a gas is involved in electrochemical cells, what must happen?
- it must be bubbled through the solution in such a way that it is in contact with the electrode
what does the electrode potential depend on?
- the conditions used:
• temperature
• pressure
• concentration of reactants
what are the standard conditions for electrochemical cells?
- 298K
- a pressure of 1atm
- all species in solution having a concentration of 1 moldm^-3
electrode potentials measured under standard conditions are known as what?
- standard electrode potentials
- E°
why do you use an electrode made of platinum when you want to measure the electrode potential of something thats not a metal (e.g liquid)?
because its inert + you dont want it to react
the emf of electrochemical cells is easy to measure, but the individual electrode potentials themselves cant actually be measured
it is only possible to measure the potential difference between two electrodes
what are all electrode potentials measured relative to?
the Standard Hydrogen Electrode (SHE)
how to calculate the emf?
EMF = E(RHS) - E(LHS)
the more negative a system is, the better ____ agent it is?
reducing
what do E° numbers vary with?
- temperature
- pressure
- all half equations for electrochemical cells are written as reduction processes
physically drawing an electrochemical cell each time is time consuming, so this process can be summarised as a cell diagram: what are some of the rules to follow:
- two vertical double lines represents the salt bridge in the middle of the electrochemical cell (you may see this as a single vertical dotted line)
- on the LHS of the salt bridge, the order of placing species is as follows:
• electrode —> reduced species —> oxidised species - on the RHS of the salt bridge, the order of placing species is as follows:
• oxidised species —> reduced species —> electrode - include state symbols when writing these out
- a change of state between one species and the next is represented by a solid line
- a non-change of state is indicated by a comma
- be aware that sometimes your electrode will be the reduced species
if the EMF is (positive/negative), we can say that the reaction is feasible/spontaneous and hence will occur??
positive
the more positive the EMF, the more ____ the reaction?
feasible
can cell potentials be used to predict how fast a reaction can happen?
no
- they CAN be used effectively to predict whether or not a given reaction will take place
- but they CANT give any indication as to how fast a reaction will proceed
if something has a positive Ecell but no apparent reaction occurs it’s because the reactants are kinetically stable, has a high activation energy so is very slow at room temperature
if reactions are expected to occur but dont seem to, what could be the reasons?
- too dilute (conditions not standard)
- reactions too slow (reactants are kinetically stable)
if a reaction is not expected to take place but does, why might this happen?
- because the conditions are non-standard (ie solutions are concentrated)
what can be used to predict how electrode potentials are affected when non-standard conditions are used?
le chatelier’s principle
- if the oxidizing agent has a concentration greater than 1moldm^-3, it is more likely to favour reduction and the electrode potential will be more positive than the standard electrode potential
- if it has a concentration of less than 1mol^-3, it is more likely to favour oxidation and the electrode potential will be more negative than the standard electrode potential
- for reducing agents, the reverse is true
e.g Fe2+ (aq) + 2e- ⇌ Fe (s)
standard electrode potential = -0.44V
if [Fe2+]= 0.1moldm^-3 the electrode potential = -0.50V
- the concentration is lower than standard so reduction is less likely to take place, and hence the electrode potential is more negative than expected
- if the temperature is higher than 298K, then the system will move in the endothermic direction and the electrode potential will change accordingly
- if the pressure is greater than 1atm, then the system will move to decrease the pressure and the electric potential will change accordingly
- a change which favours the reduction direction will make the electrode potential more ____
- a change which favours the oxidation direction will make the electrode potential more ____
- positive
- negative
e.g Fe2+ + e- ⇌ Fe+
E° = 0.50v
- what happens to E° when decreasing the concentration of Fe2+?
- equilibrium shifts to LHS
- oxidation is favoured (bc e- lost)
- ∴ better/more effective reducing agent
- E° value becomes more negative
e.g Cu2+ ⇌ Cu3+ + e-
E° = +0.34V
∆H = +
what happens to the E° if the temperature increases?
- ∆H = + ∴ endo
- equilibrium shifts to RHS
- oxidation favoured
- ∴ more effective reducing agent
- E° value becomes more negative
an increase in temperature favours (endothermic/exothermic) reactions?
endothermic
what is a fuel cell?
- a cell in which a chemical reaction between a fuel and oxygen is used to create a voltage
- the fuel and oxygen flow into the cell continuously and the products flow out of the cell
- therefore the cell does not need to be recharged
what is the most widely used fuel cell?
hydrogen-oxygen fuel cell
what does a fuel cell consist of?
- two half-cells connected by a semi-permeable membrane
- an aqueous solution of sodium hydroxide is used as the electrolyte
hydrogen-oxygen fuel cell:
- oxygen is pumped into one of the half-cell:
O2 (g) + H2O (l) + 4e- —> 4OH- (aq) E°=+0.40V - hydrogen is pumped into the other half-cell:
H2O (l) + 2e- —> H2 (g) + 2OH- (aq) E°=-0.83V - the oxygen half cell is more positive and therefore undergoes reduction
- the hydrogen half cell is more negative and undergoes oxidation:
O2 + H2O + 4e- —> 4OH- reduction
H2 + 2OH —> H2O + 2e- oxidation
O2 (g) + 2H2 (g) —> 2H2O (l) overall cell reaction, emf=1.23V
- hydroxide ions are generated in the oxygen half cell and travel through the membrane into the hydrogen half cell, where they are used up
- water is the product of the reaction and is allowed to off
what are some advantages of using hydrogen fuel cells?
- the hydrogen-oxygen fuel cell produces water as the only product (∴ doesn’t produce any greenhouse or polluting gases associated with combustion energies. the process of generating hydrogen for use in fuel cells produces a small quantity of CO2 but much less than would be generated by a combustion engine
- fuel cells are more efficient than combustion energies
- process is continuous as long as the fuel is supplied
what are some limitations of using hydrogen fuel cells?
- hydrogen is a flammable gas with a low boiling point - it is ∴ both difficult and dangerous to store and transport. It can be stored as a liquid under pressure or as a solid adsorbed to the surface of a solid, but both of these techniques are expensive
- as a result obtaining hydrogen as a fuel is difficult - and this means that people will not buy hydrogen powered vehicles
- fuel cells use toxic chemicals in their manufacture
- fuel cells have a limited lifetime
- efficiency is affected by temperature
(-hydrogen is expensive and hard to store
- high pressure tanks are required to store oxygen and fuels like hydrogen
- materials used to make them are expensive)
OIL
RIG
what is a redox reaction?
- a reaction in which oxidation and reduction occur on different species simultaneously
what is the standard electrode potential for hydrogen assumed to be ?
how is this an advantage to it being the standard electrode potential?
- 0 volts
- so it is used to allow for easy comparison between thr electrode potential of different elements
what is the experimental setup to calculate the standard electrode potential for zinc?
LEFT:
- hydrogen standard cell always placed on left
- Pt electrode
- H2 enters
- solution of HNO3 - dissociation into H+
- e.g NaCl salt bridge
RIGHT:
- zinc electrode
- Zn(NO3)2 solution - dissociation into Zn2+
why must metal electrodes be cleaned with sandpaper before creating an electrochemical cell?
- to remove any metal oxide that has formed on the surface and improve electrical conductivity
why must an inert salt bridge be used in the salt bridge?
- the salt must be inert so that it doesn’t react with the solutions and alter the ion concentrations
- if a reactive salt was used, the cell potential would change
what moves across the salt bridge?
ions
for what range of cell potential values is a process feasible?
- cell potential must be greater than 0
why might theoretical cell potential values be different to values obtained experimentally?
- conditions may be non-standard
- a cell is made up of the following half cells:
Ag+ (aq) + e- ⇌ Ag (s) E°=+0.80V
Cu2+ (aq) + 2e- ⇌ Cu (s) E°=+0.34V - write the overall cell equation and calculate the standard cell potential
2Ag + (aq) + Cu(s) —> Ag(s) + Cu2+ (aq)
E°cell= +0.80 - (+0.34) = 0.46V
in an electrochemical cell, is the more negative half cell oxidised or reduced?
oxidised
what is the only product of a hydrogen-oxygen fuel cell?
water
how does a hydrogen-oxygen fuel cell work?
- hydrogen and oxygen are pumped through porous electrodes. the electrolyte is often an acid such as phosphoric acid
- hydrogen and oxygen react, producing energy and water
what are the 2 half equations taking place in a hydrogen-oxygen fuel cell?
2H2 + 4OH- ⇌ 4H2O + 4e-
O2 + 2H2O + 4e- ⇌ 4OH-
write an equation for the overall reaction that takes place in a hydrogen-oxygen fuel cell
2H2 + O2 —> 2H2O
the gas oxygen, O2, is converted into ozone, O3, in the upper atmosphere. the equation for this process is:
3O2 —> 2O3
use oxidation states to explain why this is not a redox reaction [2]
- both O2 and O3 have oxidation states of zero
- no change in oxidation state
- an important technological development in recent years has been the hydrogen fuel cell
- this uses electrochemical methods to get energy from hydrogen
- write the half-equations for the processes occurring at the electrodes and an equation for the overall reaction [3]
- (at anode) H2 —> 2H+ + 2e-
- (at cathode) O2 + 4H+ + 4e- —> 2H2O
- (overall) 2H2 + O2 —> 2H2O
- the following equations show the standard electrode potential for the Cl2/Cl-and I2/I- systems
- Cl2 + 2e- ⇌ 2Cl- E°= +1.36V
- I2 + 2e- ⇌ 2I- E°= +0.54V
- use these values to explain why only hydrogen iodide (represented as I- in the equation) is able to further react with concentrated sulphuric acid in this way [2]
- the values show that chlorine is the best oxidising agent, as it has the most positive E° value and therefore iodide is the better reducing agent
- and is strong enough to reduce the sulfuic acid
a more positive E° value means it is a stronger ____ agent?
oxidising
define the term standard electrode potential [3]
- emf/potential difference/voltage (of electrochemical cell)
- between a standard hydrogen electrode and a half cell
- 1 atm, 1moldm^-3, 298k
consider the half reaction:
Fe2+ + 2e- ⇌ Fe
define the term standard electrode potential with reference to this electrode [3]
- emf of cell / potential difference of cell containing Fe2+ AND Fe
- AND standard hydrogen electrode
- 1 moldm^-3, 1 atm, 298k
explain how the salt bridge provides an electrical connection between the two solutions [1]
- completes the circuit
- without allowing solutions to mix
(it allows ions to move through it)
what is the role of the e.g platinum electrode in a cell? [1]
its an (inert) electrode that is used to carry the charge/current/electron flow
