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Chemistry Unit 3 > ✅3.1 Redox and Standard Electrode Potential > Flashcards

✅3.1 Redox and Standard Electrode Potential Flashcards

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1
Q

What is an oxidising agent?

A

A substance that takes electrons from another substance and so it itself is reduced

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2
Q

What is a reducing agent

A

A substance that gives electrons to another substance and so it itself is oxidised

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3
Q

What is oxidation?

A

A process where electrons are lost

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4
Q

What is reduction

A

A process where electrons are gained

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5
Q

Name the parts of an electrochemical cell

A 
1 mol of solution 
Electrode of that metal 
Wire 
High resistance voltmeter 
Salt bridge 
Another metal electrode 
1 mol of another solution 
All measured under standard conditions
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6
Q

Why is the wire important in an electrochemical cell

A

Allows electrons to flow from the half cell where oxidation occurs to the half cell where reduction occurs

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7
Q

When would you use a high resistance voltmeter in an electrochemical cell?

A

When measuring the potential difference of the cell (voltage)

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8
Q

What is the significance of a salt bridge in an electrochemical cell

A

Completed the circuit allowing ions to flow without the solutions mixing

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9
Q

What solution is a salt bridge generally soaked in

A

Potassium nitrate solution

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10
Q

What must each half cell contain

A

The products and reactants of the half equation e.g

Cu(s) electrode —> Cu2+ solution + 2e-

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11
Q

Why must a half cell contain metal?

A

To allow electrons to flow in/out of the half cell

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12
Q

What are the 3 types of half cell?

A
  1. Metal/Metal ions
  2. A gas in contact with a solution of non metal ions, with an inert metal electrode
  3. A solution containing ions of a metal in two different oxidation states, again using an inert metal electrode
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13
Q

What is present in the metal/metal ions half cell?

A

A piece of metal as the electrode with a solution containing a 1 mol solution of the metal ions

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14
Q

State the colour change present as the copper ions are reduced

A

Solution may go from blue—> colourless as the copper ions are reduced

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15
Q

Why must you use an inert platinum electrode in a half cell with a gas in contact with a solution of non metal ions or a half cell containing ions of metal in two different oxidation states?

A

Non metals are not conductors and :: we use an inert platinum electrode to allow electrons to follow in/out of the half cell.

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16
Q

Give two examples of a half cell with gas in contact with a solution of non metal ions

A

Hydrogen or oxygen electrode half cells

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17
Q

Describe the colour change of a half cell with a gas in contact with a solution of non metal ions.

A

There is no apparent colour change

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18
Q

When is a half cell with a solution containing ions of a metal in two different oxidation states used?

A

Typically used for transition metals with several oxidation states e.g Fe2+ and Fe3+ or Mn2+/MnO4-

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19
Q

What is the colour change for the reaction:

Fe3+ + e- —> Fe2+

A

Yellow/orange (Fe3+) —> pale green (Fe2+)

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20
Q

What is the colour change for this reaction:

Mn2+ (Aq)—> MnO4-(aq)

A

Colourless —> purple

21
Q

Why is a comma used in this representation of a half cell:

Pt(s)I Mn2+,MnO4-

A

A comma separates the two manganese containing ions in aqueous solution as they are in the same physical state

22
Q

What is the EMF of a cell

A

Difference in redox power of the two half cells

23
Q

What does a large EMF value represent?

A

Large EMF values occur when a half cell contains a species that is easy to oxidise (e.g magnesium) is connected to a half cell with a species that is easy to reduced (e.g manganate vII)

24
Q

What is used to measure the ability of a half cell to gain or lose electrons?

A

Standard electrode potential

25
Q

What is the scale of standard electrode potential based on?

A

Uses hydrogen as zero, any species that is easier to reduce has a positive E value, any species that is easier to oxidise has a negative E value

26
Q

What is the hydrogen half cell called?

A

Standard hydrogen electrode

27
Q

What does the standard hydrogen electrode consist of?

A

Platinum electrode coated with fine platinum grains called platinum black, dipped into a 1 mol solution of H+ ions, hydrogen gas is then slowly passed over the electrode.

28
Q

What is the pressure gas is pumped in standard hydrogen electrode and at what temperature

A

298K

1atm

29
Q

What colour is the Pt electrode?

A

Black

30
Q

How do you measure the standard electrode potential?

A

Set up a half cell under standard conditions and connect it to the standard hydrogen electrode

31
Q

What is the E value for the zinc half cell?

What does this mean?

A

-0.76 v

This means that the electrode potential for Zn2+|Zn is more negative than the standard hydrogen electrode.

32
Q

What happens to electrons in a cell with a negative E value?

A

Electrons flow along the wire from the negative half cell (e.g zinc) to the standard hydrogen electrode which becomes the positive electrode
(Positive electrode has the more positive E value and electrons flow to the cell with the more positive E value)

33
Q

What is the E value of a copper half cell?

What does this mean?

A

+0.34V

This means that the standard hydrogen electrode has the more negative potential

34
Q

What happens to electrons in a cell with a positive E value?

A

Electrons flow from the standard hydrogen electrode along the wire to the other half cell :: hydrogen electrode becomes the negative electrode

35
Q

How do the reduction potentials reflect the reactivity series?

A

The most reactive metals have the most negative E value

The most reactive non metals have the most positive E value

36
Q

What is the full chromate equation?

A

Cr2O7(2-) + 14H(+) + 6e- —> 2Cr(3+) + 7H2O

Ratios: 1:14:6:2:7

37
Q

What is the full permanganate equation?

A

MnO4(-) + 8H(+) + 5e- —> Mn(2+) +4H2O

Ratio: 1:8:5:1:4

38
Q

How can you differentiate a strong oxidising agent from a weak one?

A

The strongest oxidising agents (themeselves being reduced) have the most positive standard electrode potentials

39
Q

Is the EMF always positive?

A

Yes for a feasible reaction

40
Q

Why may a reaction not occur if an oxidising agent and a reducing agent are mixed?

A

The oxidising agent may not be strong enough to oxidise the reducing agent and :: the reaction is not feasible

41
Q

What is a feasible reaction

A

A feasible reaction is one that occurs spontaneously

42
Q

Describe the anticlockwise rule

A

1) write the two half equations on top of each other, with the most negative E value on top
2) The top reaction is feasible in the reverse of what is presented, the bottom reaction occurs in the foreword direction

43
Q

What is the EMF equation?

A

EMF= E(reduction) - E(oxidation)

44
Q

Why are fuel cells being developed?

A

Method of releasing energy very efficiently from fuels such as hydrogen methane or methanol

45
Q

Describe what happens inside of a fuel cell

A

Fuel passes over platinum metal which acts as a catalyst and electrode for the electrochemical system, removing electrons from hydrogen atoms.
Protons(H+) then diffuse through SPM to the other electrode where they receive electrons and oxygen molecules to form water 💧

46
Q

Give an example where fuel cells are being used

A

Used as a store to release electricity or heat e.g in zero emission cars where no CO2 is released

47
Q

List 3 advantages of fuel cells

A

1) convenient way of storing and releasing energy
2) energy efficiency is much higher than standard fuel systems
3) emissions from fuel cells are less damaging than carbon dioxide from traditional engines

48
Q

Name 3 drawbacks of fuel cells

A

1) hydrogen fuel must be generated elsewhere which is likely to use fossil fuel energy sources which will cause CO2 emissions. There’s an energy loss at this point and :: it’s not 100% efficient
2) gases are difficult to store compared to liquid fuels
3) fuel cells operate at lower temperatures (80 degrees C) so need very efficient catalysts which use expensive metals

49
Q

Why are platinum electrodes used?

A

Platinum electrodes are chemically inert and therefore don’t partake in the redox reactions allowing current to flow through the cell

Chemistry Unit 3 (6 decks)

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