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

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2
Q

a combined system containing two half cells is called..?

A

an electrochemical cell

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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
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4
Q

how can the e.m.f be measured?

A

using a voltmeter

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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

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6
Q

in what direction do electrons flow in electrochemical cells?

A

from the left to the right

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7
Q

in an electrochemical cell, which side is the negative side?

A

LHS

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8
Q

what is the salt bridge used for in an electrochemical cell?

A

to complete the circuit (without allowing solutions to mix)

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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
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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
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11
Q

dont use the names anode and cathode

A
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12
Q

for half equations showing reduction, what side of the equations are the electrons??

A

left (LHS)

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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

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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
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15
Q

what does the electrode potential depend on?

A
  • the conditions used:
    • temperature
    • pressure
    • concentration of reactants
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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
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17
Q

electrode potentials measured under standard conditions are known as what?

A
  • standard electrode potentials
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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

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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

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20
Q

what are all electrode potentials measured relative to?

A

the Standard Hydrogen Electrode (SHE)

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21
Q

how to calculate the emf?

A

EMF = E(RHS) - E(LHS)

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22
Q

the more negative a system is, the better ____ agent it is?

A

reducing

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23
Q

what do E° numbers vary with?

A
  • temperature
  • pressure
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24
Q
  • all half equations for electrochemical cells are written as reduction processes
A
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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