DM 3 - Redox Flashcards
What is a half-cell?
A half-cell contains the chemical species present in a redox half equation
What is a metal/metal ion half-cell?
- A half-cell where a metal rod is dipped in a solution containing an ion of the same metal
- An equilibrium will be set up at the boundary where the metal is in contact with its ions
What is an ion/ion half-cell?
- An ion/ion half-cell contains a solution of ions of the same element but of different oxidation states
- The electron is usually graphite or platinum
What is a standard electrode potential?
The EMF/voltmeter reading when a half-cell of interest is connected to a hydrogen half-cell via a salt bridge, with all solutions having a concentration of 1 moldm-3, under standard conditions (101 or 100 kPa and 298K)
What is a hydrogen half-cell?
- A half-cell containing hydrogen gas and a solution containing hydrogen ions
- An inert platinum electrode is used to allow electrons into and out of the half-cell
- 2H+(aq) + 3e- <=> H2(g)
How do you use standard electrode potentials to calculate the standard cell potential, E0cell?
E0cell = E0(positive electrode) - E0(negative electrode)
What is an electrochemical cell?
- Two different half-cells are connected by a salt bridge, with their electrodes connected to a voltmeter (measures EMF/cell potential)
- This allows the flow of electrons
- It generates electrical energy from chemical redox reactions
What is a salt bridge?
- A salt bridge allows the transfer of ions
- It is typically a concentrated solution of an electrolyte i.e. KNO3(aq) that doesn’t react with either half cell solution
Why may a graphite or platinum electrode be used?
They are very unreactive i.e. will not react with the half-cell solutions and will not affect the voltmeter readings. Usually used in ion/ion half-cells
How do you predict which half-cell is being oxidised and which one is being reduced?
- The more negative the E0 value, the greater the tendency for the system to be oxidised
- The more positive the E0 value, the greater the tendency for the system to be reduced
- Hence the half-cell with the more positive E0 is oxidised and the half-cell with the more negative E0 is reduced
How do you predict the feasibility of a redox reaction?
- The reaction is feasible if the oxidising agent has a lower standard cell potential than the reducing agent
- The greater the difference in E0 value, the more likely the reaction to occur
Why may a reaction not occur?
- Non-standard conditions (298 K, 101 kPa, 1 moldm-3 solutions)
- Ambient energy of the system is lower than the activation energy
What is rusting?
- Rust in an iron oxide, (usually red) formed from the redox reaction of iron and oxygen in the presence of water/moisture
- Iron is oxidised, oxygen (dissolved in water) is reduced. The products of this redox reaction then react to form a hydrated iron oxide
How do you prevent rusting?
- To protect iron from rust, the surface needs to be separated from air and water, i.e. by keeping the metal in oil
- Coatings can be applied to iron
- Alloys (e.g. stainless steel) can be made to prevent rusting