3.1 redox and standard electrode potential Flashcards
1
Q
what does the electrode potential mean?
(dont need definition just understand it)
A
how easily a metal looses its electrons into a solution of its own ions
2
Q
a combined system containing two half cells is called..?
A
an electrochemical cell
3
Q
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
A
4
Q
how can the e.m.f be measured?
A
using a voltmeter
5
Q
what is an advantage of using a voltmeter to measure the e.m.f?
A
voltmeters have a high resistance so that they do not divert much current from the main circuit
6
Q
in what direction do electrons flow in electrochemical cells?
A
from the left to the right
7
Q
in an electrochemical cell, which side is the negative side?
A
LHS
8
Q
what is the salt bridge used for in an electrochemical cell?
A
to complete the circuit (without allowing solutions to mix)
9
Q
the salt bridge is often a piece of filter paper saturated with a solution of an inert (unreactive) electrolyte such as KNO3 (aq)
A
10
Q
- in electrochemical cells, the better reducing agents are on the (RHS/LHS)?
- the better oxidising agents are on the (RHS/LHS)??
A
- LHS
- RHS
11
Q
dont use the names anode and cathode
A
12
Q
for half equations showing reduction, what side of the equations are the electrons??
A
left (LHS)
13
Q
if the half-reaction doesnt contain a metal in its elemental state, what must be used?
A
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
14
Q
if a gas is involved in electrochemical cells, what must happen?
A
- it must be bubbled through the solution in such a way that it is in contact with the electrode
15
Q
what does the electrode potential depend on?
A
- the conditions used:
• temperature
• pressure
• concentration of reactants
16
Q
what are the standard conditions for electrochemical cells?
A
- 298K
- a pressure of 1atm
- all species in solution having a concentration of 1 moldm^-3
17
Q
electrode potentials measured under standard conditions are known as what?
A
- standard electrode potentials
- E°
18
Q
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)?
A
because its inert + you dont want it to react
19
Q
the emf of electrochemical cells is easy to measure, but the individual electrode potentials themselves cant actually be measured
A
it is only possible to measure the potential difference between two electrodes
20
Q
what are all electrode potentials measured relative to?
A
the Standard Hydrogen Electrode (SHE)
21
Q
how to calculate the emf?
A
EMF = E(RHS) - E(LHS)
22
Q
the more negative a system is, the better ____ agent it is?
A
reducing
23
Q
what do E° numbers vary with?
A
- temperature
- pressure
24
Q
- all half equations for electrochemical cells are written as reduction processes
A
25
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
26
if the EMF is (positive/negative), we can say that the reaction is feasible/spontaneous and hence will occur??
positive
27
the more positive the EMF, the more ____ the reaction?
feasible
28
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
29
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
30
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)
31
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)
32
what can be used to predict how electrode potentials are affected when non-standard conditions are used?
le chatelier’s principle
33
- 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
34
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
35
- 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
36
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
37
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
38
an increase in temperature favours (endothermic/exothermic) reactions?
endothermic
39
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
40
what is the most widely used fuel cell?
hydrogen-oxygen fuel cell
41
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
42
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
43
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
44
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)
45
OIL
RIG
46
what is a redox reaction?
- a reaction in which oxidation and reduction occur on different species simultaneously
47
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
48
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+
49
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
50
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
51
what moves across the salt bridge?
ions
52
for what range of cell potential values is a process feasible?
- cell potential must be greater than 0
53
why might theoretical cell potential values be different to values obtained experimentally?
- conditions may be non-standard
54
- 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
55
in an electrochemical cell, is the more negative half cell oxidised or reduced?
oxidised
56
what is the only product of a hydrogen-oxygen fuel cell?
water
57
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
58
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
59
- 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
60
- 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
61
a more positive E° value means it is a stronger ____ agent?
oxidising
62
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
63
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
64
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)
65
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
