5.6 Redox and Electrode Flashcards
What is reduction ?
Gain of electrons or decrease in oxidation number.
What is oxidation ?
Loss of electrons or increase in oxidation number.
What do oxidising agents do ?
Take electrons from species being oxidised. Contains species species that is reduced.
What do reducing agents do ?
Adds electrons to the species being reduced. Contains the species being oxidised.
What is reduced and oxidised in manganate titrations ?
MnO4- ions are reduced and the other chemical must be a reducing agent that is oxidised.
What is the end point in manganate titrations ?
Colourless to pink.
What is oxidised and reduced during iodine and thiosulphate titrations ?
Thiosulphate ions are oxidised and iodine is reduced.
What is the ionic equation for iodine thiosulphate titrations ?
2S2O3 2- +I2 = 2I- + S4O6 2-
What is the end point for iodine thiosulphate titrations ?
Starch is used as indicator and the colour goes from blue/black to colourless.
What is a voltaic cell ?
Type of electrochemical cell that converts chemical energy into electrical energy. Takes place in batteries.
What is a half cell ?
Contains the chemical species present in a redox half-equation. A voltaic cell can be made by connecting 2 half-cells together. In a cell, the 2 halves must be kept separate so electrons do not flow in an uncontrolled way.
What is the simplest half-cell ?
Metal rod dipped into a solution of its aqueous metal ion. Represented by a vertical line for the phased boundary between the aqueous solution and the metal.
What happens when the half cells contain same element in different oxidation states ?
Eg Fe 3+ and Fe 2+
In this type of cell there is no metal to transport electrons either into to out of the half-cell, so an inert metal electrode made out of platinum is used.
How do you know which electrode has a greater tendency to gain or lose electrons ?
In a cell with 2 metal/ metal ion half cells connected the more reactive metal releases electrons and is more readily oxidised.
What is defined as the negative electrode ?
In an operating cell, this is the electrode with more reactive metal loses electrons and is oxidised.
What is defined as a positive electrode ?
In an operating cell, this is the electrode with the less reactive metal gaining electrons and is reduced.
What is the standard electrode potential ?
Is the EMF of a half cell compared with a standard hydrogen half cell, measured under standard contains of 100 kPa and 298K with a solution concentration of 1 mol dm-3.
How do you measure standard electrode potential ?
Half cell is connected to a standard hydrogen electrode. The 2 electrodes are connected by a wire to allow a controlled flow of electrons. 2 solutions are connected with a salt bridge, which allows the ions to flow.
What does a more negative standard electrode potential indicate ?
The more negative the E value, the greater tendency to lose electrons and undergo oxidation and the less tendency to gain electrons and undergo reduction. Metals tend to have more negative E values.
What does a more positive standard electrode potential indicate ?
The more positive the E value, the greater the tendency to gain electrons and undergo reduction and the less tendency to lose electrons and undergo oxidation. Non-metals tends to have positive E values.
How to directly calculate standard cell potential from standard electrode potentials ?
E cell = E (positive electrode) - E (negative electrode)
How to predict feasibility from E values ?
Stronger oxidising agents at the bottom and stronger reducing agents at the top with equations going from most to least negative. The reaction is always anticlockwise.
How does concentration affect the equilibrium of electrode cell reactions ?
If concentration is increased, the equilibrium will swing in the opposite direction to oppose the change. If the concentration is reduced, the equilibrium swings in the same direction making more of what has been reduced.
What are primary cells ?
They are non-rechargable and are designed to be used once. Electrical energy is produced by oxidation and reduction at the electrodes, but the reactions cannot be reversed. Eventually chemicals will be used up, voltage will fall and the battery will go flat.
What are secondary cells ?
They are rechargeable and the cell reaction producing electrical energy can be reversed during recharging. Chemicals are then regenerated and can be used again.
What are fuel cells ?
Fuel cells use energy from the reaction of a fuel with oxygen to create a voltage. Fuel cells can operate continuously provided that fuel and oxygen are supplied into the cell. They do not have to be recharged. Hydrogen is the most common fuel cell.
