✅3.1 Redox and Standard Electrode Potential Flashcards
What is an oxidising agent?
A substance that takes electrons from another substance and so it itself is reduced
What is a reducing agent
A substance that gives electrons to another substance and so it itself is oxidised
What is oxidation?
A process where electrons are lost
What is reduction
A process where electrons are gained
Name the parts of an electrochemical cell
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
Why is the wire important in an electrochemical cell
Allows electrons to flow from the half cell where oxidation occurs to the half cell where reduction occurs
When would you use a high resistance voltmeter in an electrochemical cell?
When measuring the potential difference of the cell (voltage)
What is the significance of a salt bridge in an electrochemical cell
Completed the circuit allowing ions to flow without the solutions mixing
What solution is a salt bridge generally soaked in
Potassium nitrate solution
What must each half cell contain
The products and reactants of the half equation e.g
Cu(s) electrode —> Cu2+ solution + 2e-
Why must a half cell contain metal?
To allow electrons to flow in/out of the half cell
What are the 3 types of half cell?
- Metal/Metal ions
- A gas in contact with a solution of non metal ions, with an inert metal electrode
- A solution containing ions of a metal in two different oxidation states, again using an inert metal electrode
What is present in the metal/metal ions half cell?
A piece of metal as the electrode with a solution containing a 1 mol solution of the metal ions
State the colour change present as the copper ions are reduced
Solution may go from blue—> colourless as the copper ions are reduced
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?
Non metals are not conductors and :: we use an inert platinum electrode to allow electrons to follow in/out of the half cell.
Give two examples of a half cell with gas in contact with a solution of non metal ions
Hydrogen or oxygen electrode half cells
Describe the colour change of a half cell with a gas in contact with a solution of non metal ions.
There is no apparent colour change
When is a half cell with a solution containing ions of a metal in two different oxidation states used?
Typically used for transition metals with several oxidation states e.g Fe2+ and Fe3+ or Mn2+/MnO4-
What is the colour change for the reaction:
Fe3+ + e- —> Fe2+
Yellow/orange (Fe3+) —> pale green (Fe2+)
What is the colour change for this reaction:
Mn2+ (Aq)—> MnO4-(aq)
Colourless —> purple
Why is a comma used in this representation of a half cell:
Pt(s)I Mn2+,MnO4-
A comma separates the two manganese containing ions in aqueous solution as they are in the same physical state
What is the EMF of a cell
Difference in redox power of the two half cells
What does a large EMF value represent?
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)
What is used to measure the ability of a half cell to gain or lose electrons?
Standard electrode potential
What is the scale of standard electrode potential based on?
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
What is the hydrogen half cell called?
Standard hydrogen electrode
What does the standard hydrogen electrode consist of?
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.
What is the pressure gas is pumped in standard hydrogen electrode and at what temperature
298K
1atm
What colour is the Pt electrode?
Black
How do you measure the standard electrode potential?
Set up a half cell under standard conditions and connect it to the standard hydrogen electrode
What is the E value for the zinc half cell?
What does this mean?
-0.76 v
This means that the electrode potential for Zn2+|Zn is more negative than the standard hydrogen electrode.
What happens to electrons in a cell with a negative E value?
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)
What is the E value of a copper half cell?
What does this mean?
+0.34V
This means that the standard hydrogen electrode has the more negative potential
What happens to electrons in a cell with a positive E value?
Electrons flow from the standard hydrogen electrode along the wire to the other half cell :: hydrogen electrode becomes the negative electrode
How do the reduction potentials reflect the reactivity series?
The most reactive metals have the most negative E value
The most reactive non metals have the most positive E value
What is the full chromate equation?
Cr2O7(2-) + 14H(+) + 6e- —> 2Cr(3+) + 7H2O
Ratios: 1:14:6:2:7
What is the full permanganate equation?
MnO4(-) + 8H(+) + 5e- —> Mn(2+) +4H2O
Ratio: 1:8:5:1:4
How can you differentiate a strong oxidising agent from a weak one?
The strongest oxidising agents (themeselves being reduced) have the most positive standard electrode potentials
Is the EMF always positive?
Yes for a feasible reaction
Why may a reaction not occur if an oxidising agent and a reducing agent are mixed?
The oxidising agent may not be strong enough to oxidise the reducing agent and :: the reaction is not feasible
What is a feasible reaction
A feasible reaction is one that occurs spontaneously
Describe the anticlockwise rule
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
What is the EMF equation?
EMF= E(reduction) - E(oxidation)
Why are fuel cells being developed?
Method of releasing energy very efficiently from fuels such as hydrogen methane or methanol
Describe what happens inside of a fuel cell
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 💧
Give an example where fuel cells are being used
Used as a store to release electricity or heat e.g in zero emission cars where no CO2 is released
List 3 advantages of fuel cells
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
Name 3 drawbacks of fuel cells
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
Why are platinum electrodes used?
Platinum electrodes are chemically inert and therefore don’t partake in the redox reactions allowing current to flow through the cell
