14 - Redox II Flashcards
What is oxidation?
Oxidation is the loss of electrons.
Oxidation number becomes more positive.
What is reduction?
Reduction is the gain of electrons.
Oxidation number becomes more negative.
Standard electrode potential?
Standard electrode potential is the potential difference / emf measured when a half-cell is connected to a standard hydrogen electrode, under standard conditions.
What are the standard conditions when measuring standard electrode potential?
298K temperature
100kPa pressure of gases
1.00 moldm^-3 concentration of ions.
Why is a reference electrode necessary?
- It is impossible to measure the absolute potential of a half-cell on its own. Only possible to measure potential difference between two electrodes.
- By convention, standard electrode potential of a half-cell is measured by connecting to a standard hydrogen electrode, which is given a potential of 0 volts. Measured under standard conditions.
Features of a standard hydrogen electrode
Hydrogen gas (in) at 100kPa.
Platinum wire.
Platinum foil covered in porous platinum.
Surrounding solution = HCl acid [H+(aq)] = 1moldm^-3
What is the salt bridge for?
- Needed to complete the electrical circuit.
- usually contains concentrated solution of potassium nitrate KNO3 (liquid/gel).
- Salt should be unreactive with the half-cells and their solutions.
Why use a high resistance voltmeter?
- To avoid any flow of current through the circuit.
- In this state, it is possible to measure the maximum possible potential difference.
- The reactions will not be occuring as the very high resistance voltmeter stops the current from flowing.
electromotive force?
emf
The standard electrode potential of a half-cell connected to a standard hydrogen electrode, measured under standard conditions.
E cell
How is a half cell for chlorine set up?
- Very similar to SHE
- Cl2 gas 100kPa
- bubbled over platinium foil covered in porous platinium.
- platinum wire
- immersed in a solution of [Cl-(aq)] = moldm^-3, e.g sodium chloride solution.
How do you a half cell?
substance losing electrons (most easily oxidised) written closest to the electrode (not salt bridge).
How to write Cl2 half cell?
Pt(s) I 2Cl-(aq), Cl2(g) II
notice how Cl- is closest to the platinum electrode, since it is the thing that loses electrons (oxidised).
Chlorine and its E value
Value is postive and moderately high. Means that position of chlorine equilibrium lies further to the right than the hydrogen equilibrium.
Chlorine is more likely to gain electrons than hydrogen ions. (Notice not chlorine ions. Chlorine is the substance that gains electrons to form the ions).
Chlorine is therefore good at removing electrons from other substances and gaining them itself. Therefore it is a good oxidising agent (it is reduced itself).
How to measure redox potential for Fe2+ / Fe3+ system (ions of the same element with different oxidation numbers).
- insert a platinum electrode (platinum foil covered in porous platinum and platinum wire) into a beaker containing solution of Fe2+ and Fe3+ ions, [Fe2+(aq)] = 1.00moldm^-3 and [Fe3+(aq)] = 1.00moldm^-3.
- connect it to a standard hydrogen electrode.
How is the electrochemical series arranged?
- The most negative E values are placed at the top of the series; most positive at the bottom.
- The more negative the E value, the more powerful the reducing agent.
- The more positive the E value, the more powerful the oxidising agent.
Useful rule of thumb in electrochemical cells
- The equilibrium with the more negative (less positive) E value will shift to the left.
- The equilibrium with the more positive (less negative) E value will shift to the right.
How to calculate E cell of an electrochemical cell?
E (more positive) - E (more negative)
E (right) - E (left)
e.g zinc = -0.76V, copper = +0.34 V,
E cell = 0.34 - (-0.76) = +1.10V
Although a reaction may be thermodynamically feasible (when comparing E values), why might the reaction not take place?
- Reaction has a very large activation energy.
- Reaction may not be taking place under standard conditions.
A reaction is deemed not thermodynamically feasible under standard conditions.
How can you make the reaction take place?
- increase the concentration of the two reactants.
- causes shift in equilibrium to desired side. Causes E values to become more positive and more negative.
Therefore, a reaction that is initially deemed not thermodynamically feasible under standard conditions, may become feasible (and hence the reaction occurs) under non-standard conditions.
Gibbs free energy and emf equation
∆G = -nFEcell
What does positive E cell mean?
If E cell is +ve, the reaction as written from left to right in the cell diagram is thermodynamically feasible as ∆S total will be positive.
If you have done E (right) - E (left) and you have E cell which is -ve, it means that reaction from left to right is not thermodynamically feasible. Reaction will occur from right to left instead.
E cell can be used to predict the direction of a cell reaction.
Relationship between equilibrium constant (k) and E cell?
lnK ∝ E cell
Relationship between total entropy change and E cell?
∆S total ∝ E cell
If a question asks you to write the equation of a storage cell being recharged, what do you write?
write the reverse of the discahrge equation.
What is a fuel cell?
A fuel cell uses the energy released from the reaction of a fuel with oxygen to generate a voltage.
Electricity is generated by a continuous external supply of chemicals (fuel).
Why is a PLATINUM electrode used for non-metals and in fuel cells?
- good conductor of electricity.
- inert metal.
What are the reactions taking place in an alkaline hydrogen-oxygen fuel cell (with alkaline (KOH) electrolyte)?
2H2(g) + 4OH-(aq) -> 4H2O(l) + 4e-
O2(g) + 2H2O (l) + 4e- -> 4OH-(aq)
Overall equation:
2H2(g) + O2(g) -> 2H2O (l)
E cell = +1.23V
Be careful. Water is (g) in overall equation, which is released into the atmosphere.
Overall reaction same for both alkaline and acidic hydrogen-oxygen fuel cells.
What are the reactions taking place in an acidic hydrogen-oxygen fuel cell (with acidic (acid) electrolyte)?
2H2(g) -> 4H+(aq) + 4e-
O2(g) + 4H+(aq) + 4e- -> 2H2O(l)
Overall equation:
2H2(g) + O2(g) -> 2H2O(l)
E cell = +1.23V
Overall reaction same for both alkaline and acidic hydrogen-oxygen fuel cells.
What fuels do fuel cells use?
- Hydrogen
- Hydrogen rich fuels (molecules) such as methanol and ethanol.
What determines if you are using a alkaline or acidic hydrogen-oxygen fuel cell?
alkaline = alkaline electrolyte (such as KOH)
acidic = acidic electrolyte (such as an acid).
Why can fuel cells maintain a constant voltage over time?
- Fuel cells can maintain a constant voltage over time.
- Because they are constantly supplied with fresh O2 and H2, which maintains the concentrations of the reactants and keeps them constant.
Why can storage cells not maintain a constant voltage over time?
- In storage cells, voltage drops over time.
- This is because over time, the reactant concentrations fall.
They are not constantly supplied with fresh reactants to keep the reactant concentrations constant.
Are standard conditions used in hydrogen-oxygen fuel cells?
- NO
- Using standard conditions means that the rate of reaction is too slow to produce an acceptable current.
- Higher temperatures are used to increase rate of reaction.
- However, reaction is exothermic, so by applying le chatelier’s principle, emf would fall.
- A higher pressure can be used to counteract this effect.
What are the advantages of fuel cells over conventional fossil fuel powered vehicles?
- Less pollution.
- Less CO2 emissions. Fuel cells using pure H2 emits only water (H2O), and hydrogen-rich fuels (such as methanol and ethanol) produce only small amounts of air pollutants and CO2.
- Greater efficiency.
What are the limitations of hydrogen fuel cells?
- Very expensive.
- Storing and transporting hydrogen can be dangerous.
- Requires regular replacement and disposal.
- High production costs.
- Toxic chemicals are used in their production.
For the hydrogen needed for use in fuel cells, how is it obtained?
- H2 is readily available by the electrolysis of water.
- But this process is expensive.
What needs to be done in order for H2 to be a green fuel?
- The electricty required for the electrolysis of water to produce H2 would need to be produced from renewable resources.
How can hydrogen be stored in fuel cells?
- As a liquid under pressure.
- Adsorbed on the surface of a solid material.
- Adsorbed within a solid material.
What are the advantages of ethanol fuel cells compared to hydrogen fuel cells?
- Ethanol can be formed from renewable sources in a carbon neutral way.
- The raw materials needed to produce ethanol by fermentation are abundant.
- Ethanol is less explosive than hydrogen.
- Ethanol is easier to store than hydrogen.
- New petrol stations would not need to be made as ethanol is a liquid fuel (and not gas).
Methanol can also be used in fuel cells as it is also a hydrogen-rich fuel.
What is the molar ratio for Fe2+ : MnO4-
5:1
What acid is used for manganate titration?
dilute sulfuric acid.
What is the molar ratio for C2O42- : MnO4-
5:2
What is the molar ratio for Fe3+ : MnO4-
5:1
What is the molar ratio for FeC2O4 : MnO4-
5:3
In thiosulfate redox titration, when and why is starch indicator added?
- Added near end point when iodine has fades into a pale yellow.
- Starch indicator is added to clarify/emphasise the end point.
- When all of the iodine has reacted, colour change from blue/black to colourless.
What is the molar ratio for S2O32- : I2
2:1
How to calculate % uncertainty of a burette?
((2 x uncertainty) / measurement made) x 100
MAKE SURE TO MULTIPLY THE UNCERTAINTY BY 2 FOR BURETTE AS WHEN YOU USE A BURETTE, YOU TAKE 2 READINGS.
How to calcualte % uncertainty of a pipette?
(uncertainty / measurement made) x 100
How to decrease apparatus uncertainties?
Make the titre a larger volume. This can be done by:
- Increase volume and concentration of the substance in the conical flask.
- Decrease the concentration of the substance in the burette.
What are the reactions taking place in a methanol fuel cell?
CH3OH + H2O -> CO2 + 6e- + 6H+
6H+ + 6e- + 1.5O2 -> 3H2O
overall equation:
CH3OH + 1.5O2 -> CO2 + 2H2O
What is the purpose of alkali/acid in a alkaline/acidic hydrogen-oxygen fuel cell?
Electrolyte.
To allow the movement of ions between electrodes.
