Redox II Flashcards
What are electrochemical cells?
Electrochemical cells can be made by placing two different metals in salt solutions containing these metal ions, which are connected by a wire. A salt bridge is used to complete the circuit. Oxidation occurs at the anode (negative electrode) and reduction occurs at the cathode (positive electrode). The more reactive metal is the anode and the less reactive metal is the cathode.
What is the cell potential?
The cell potential (Ecell, EMF) is the voltage (potential difference) between two half-cells in an electrochemical cell.
What is a salt bridge?
A salt bridge is used to complete the circuit in an electrochemical cell and consists of a piece of filter paper or rope soaked in saturated KNO3 (aq) or a U-shaped tube filled with inert solution. It maintains electrical neutrality within the cell by providing ions to either side depending on where charge has built up. The solution on the salt bridge should not react with either of the solutions in the half cells, and you cannot use a wire as this would create more ion potentials, which would interfere with Ecell readings.
How do you set up a half cell where there are no solids involved in the half equation?
The gas can be bubbled over a platinum electrode sitting in a solution of its aqueous ions.
What is electrode potential?
Electrode potential is a measure of how easily the substance in the half cell is oxidised. In an electrochemical cell, the half-reaction with a more positive E value runs forwards, and the half-reaction with a more negative E value runs backwards. A more negative electrode potential means that a substance is more easily oxidised.
What is a standard electrode potential?
The standard electrode potential of a half-cell is the voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode.
What are standard conditions for electrochemical cells?
Salt solutions (or proton concentration for acids) must have a concentration of 1 mol dm-3. The temperature must be 298K, and the pressure must be 100 kPa.
Describe a standard hydrogen electrode.
A platinum electrode is connected to an external circuit with a platinum wire, and is surrounded by hydrogen gas in a glass tube. A 1 mol dm-3 solution of a monoprotic acid can be used as a source of H+ ions. A salt bridge should be used to connect the two half-cells and the hydrogen electrode is always shown on the left hand side, regardless of the Ecell value of the other half cell is. The Ecell of the hydrogen electrode is 0.00V, and acts as a baseline against which other Ecell values can be compared.
How do you work out Ecell values from standard electrode potentials?
Ecell = Ereduction - Eoxidation
If the half cells are drawn the correct way around (the more positive half cell should be on the right, unless there are hydrogen electrodes involved), this is Ecell = Eright - Eleft
What is the conventional representation of electrochemical cells?
- the half cell with the more negative electrode potential goes on the left
- the oxidised forms go in the centre of the cell diagram and the reduced forms go on the outside
- double vertical lines in the middle show the salt bridge, and single vertical lines separate species in different physical states
- commas separate species which are in the same half cell and the same physical state
- standard hydrogen electrode must always go on the left
- if inert electrodes (such as platinum) are used, they must go on the outside of the diagram
How can you use electrode potentials to work out the reactivity of a species?
The more reactive a metal is, the more easily it loses electrons. More reactive metals have more negative standard electrode potentials. The more reactive a non-metal is, the more easily it gains electrons, so more reactive non-metals have more positive standard electrode potentials.
How can Ecell be used to determine if a reaction is feasible?
If Ecell is positive (or zero), the reaction is feasible, but if Ecell is negative it is not feasible.
Why may a reaction with a positive Ecell not occur?
If standard conditions are not used, the concentration or temperature change can change the electrode potentials, which may mean the reaction is not feasible using those conditions. The rate of the reaction may be so slow that it appears not to happen, or a high activation energy may prevent the reaction from occurring.
How is cell potential related to entropy and the equilibrium constant?
E is directly proportional to deltaStotal and E is directly proportional to lnK
How is Ecell related to Gibbs energy?
DeltaG = -nFEcell (where n is the number of moles of electrons transferred and F is the Faraday constant (96500 coulombs per mole).
What are storage cells?
Storage cells are rechargeable batteries. There are many types of storage cell, all of which run the current one way to discharge the battery and the other way to recharge the battery.
What are the half equations for lead-acid batteries (used in vehicles)?
PbO2 (s)+ HSO4- (aq)+ 3H+(aq) +2e- —> PbSO4 (s)+ 2H2O (l) E = +1.69V
PbSO4 (s) + H+ (aq) + 2e- —> Pb (s) + HSO4- (aq) E= -0.36V
During discharge, the top equation will run right and the bottom equation will run left (top equation occurs at the cathode, bottom equation occurs at the anode). During charging, both equations run the other way and the anode and cathode switch over. Both equations are equilibria, so equilibrium arrows should be used.
In an electrochemical cell, what is the anode and what is the cathode?
The anode is the electrode at which oxidation occurs and the cathode is the electrode at which reduction occurs.
What are the equations for the Nickel-Cadmium battery?
2NiO(OH) + 2H2O + 2e- —> 2Ni(OH)2 + 2OH- E= +0.52V
Cd(OH)2 + 2e- —> Cd + 2OH- E= -0.88V
During discharge, the top equation runs right and the bottom runs left (so the top represents the reaction at the cathode and the bottom equation represents the reaction at the anode). Both equations are equilibria, so the arrows should be equilibrium arrows.
What are fuel cells?
Fuel cells provide an electric current but do not need to be charged. They will produce a current as long as they are being provided with reactants.
What is the overall reaction which occurs in a hydrogen fuel cell?
H2 + 1/2O2 —> H2O
Why is the electrochemical cell potential (Ecell) the same for acid and alkaline hydrogen fuel cells?
The overall reaction equation is the same for both, so the electrochemical cell potential is the same.
How does an alkaline hydrogen fuel cell work?
Hydrogen and oxygen are stored on opposite sides of the fuel cell and when a current is required, they are released to come into contact with two separate platinum electrodes. Electrons are exchanged on the electrodes and hydroxide ions can move through the KOH (aq) electrolyte to the hydrogen electrode to react with the hydrogen (first they move through anion exchange membranes). This reaction produces electrons, which move through an external circuit to power a device, then return to the fuel cell to react with oxygen.
What are the half equations for an alkaline hydrogen fuel cell?
At the negative electrode (anode):
H2 (g) + 2OH- (aq) —> 2H2O (l) + 2e-
At the positive electrode (cathode):
1/2O2 (g) + H2O (l) + 4e- —> 4OH- (aq)
How does an acid fuel cell work?
At the anode, the platinum catalysts catalyses the splitting of hydrogen into protons and electrons. The H+ travel through the polymer electrolyte membrane to react with oxygen at the cathode, forming water. The electrons can’t travel through the membrane so are forced around an external circuit to power something else, before returning to the cathode to react with oxygen.
What are the half equations for an acid fuel cell?
At the negative electrode (anode):
H2 (g) —> 2H+ (aq) + 2e-
At the positive electrode (cathode):
1/2O2 (g) +2H+ (aq) + 2e- —> H2O (l)
What are the positives and negatives of hydrogen-oxygen fuel cells?
They produce water as the only waste product and are very efficient compared to combustion engines. They are lightweight and do not require fossil fuels like petrol. However, hydrogen fuel is not widely available and in many causes is produced from natural gas, so does come from non-renewable sources. Also, hydrogen is highly flammable and it is difficult to store gases in fuel cells (they must be stored under very high pressure, which is dangerous, or adsorbed onto a suitable material, which may be expensive and not effective).
What other types of fuel cells are there other than hydrogen-oxygen fuel cells?
Some fuel cells use alcohols like methanol or ethanol as sources of hydrogen for a hydrogen fuel cell. Some newer fuel cells can run on methanol or ethanol directly (CH3OH + H2O —> CO2 + 6e- + 6H+ / 6H+ + 6e- + 3/2 O2 —> 3H2O)
Why are redox titrations often self-indicating?
Redox titrations sometimes use transition metal ions which have multiple stable oxidation states. Usually, each of these ions have different colours, so when the ions change oxidation state (by being oxidised or reduced by oxidising or reducing agents), they will change colour, signalling the end point.
What is the method for redox titrations?
Redox titrations can be used to calculate unknown concentrations of oxidising/reducing agents. An oxidising or reducing agent of known concentration can be placed in the burette or in the conical flask and reacted with the other type of agent in a redox reaction. A colour change will occur showing the end point.
What is the colour change for a permanganate titration?
MnO4- + 8H+ + 5e- —> Mn2+ + 4H2O
Purple to pale pink
In order for this to work, you must titrate against a colourless reducing agent. If the permanganate is in the burette, once all of the reducing agent has been used up, no more MnO4- will be converted to Mn2+, so the tiny excess of MnO4- will give the persistent pale pink colour.
How do thiosulfate/iodine titrations work?
Thiosulfate ions act as a reducing agent, which reduce iodine to iodide ions. Starch is used as the indicator. Thiosulfate ions are added from a burette to iodine solution until the brown iodine solution turns pale yellow. The starch indicator is added to produce a blue colour. The thiosulfate is added dropwise to react with the iodine and decolourise the solution.
2S2O3 2- —> S4O6 2- + 2e- I2 + 2e- —> 2I-
Why is it important in thiosulfate-iodine titrations that the starch indicator is not added too early?
When iodine and iodide ions are in solution together, they form triiodide ions, which fit into the coiled structure of starch. If you add the starch too early, it will form complexes with the iodine and prevents all of it from reacting.
How do you do redox titration calculations?
Combine the redox half equations into one equation and work out the moles of the reagent you know the concentration and volume of. Use the mole ratio from the balanced equation and work out the number of moles of the other reagent. Divide by the volume of the other reagent to work out its concentration.
