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
