Q. What is electrolysis?
The decomposition of a compound using an electric current.
Q. What is an electrolyte?
The molten ionic compound or aqueous solution of ions that is decomposed during electrolysis.
Q. Describe what happens at the electrodes during electrolysis.
Negative ions are attracted to the anode, where they are then oxidised. Positive ions are attracted to the cathode, where they are then reduced.
Q. What forms at each electrode when a molten electrolyte containing 2 simple ions undergoes electrolysis?
A molten electrolyte containing 2 simple ions will contain metal and non-metal ions. A metal will form at the cathode, and a non-metal will form at the anode.
Q. What forms at each electrode when an aqueous electrolyte undergoes electrolysis?
H+ and OH- ions are present from the water, as well as the metal and non-metal ions from the ionic compound. Generally, if a halogen is present, it will form at the anode. If not, oxygen is produced at the anode. At the cathode, atoms of substance with a more positive E will form (either the metal or hydrogen). Metals from lead to zinc in the electrochemical series depend on concentration, with the more concentrated ions becoming atoms.
Q. What happens during discharge?
A process in electrolysis in which ions are converted to molecules at the electrodes.
Q. What does discharge depend upon?
- Concentrations of the ions
- Position in the redox series
Q. How does concentration affect discharge at the cathode when aqueous solutions undergo electrolysis?
From around lead to zinc in the electrochemical series:
- If the solution is very concentrated, then the metal will form.
- If the solution is very dilute, then hydrogen will form.
- If the concentration of metal and hydrogen ions is similar, both may form.
Q. How does concentration affect discharge at the anode when aqueous solutions undergo electrolysis?
If a halogen is present, it will form at the anode. If not, oxygen is produced.
Q. How does discharge depend upon the relative electrode potentials of ions?
- Cation is more easily reduced when E is positive.
- An anion is more easily oxidised when E is negative.
- If the cation has a greater E than hydrogen, the cation is discharged. If not, hydrogen is discharged.
Q. What must a half-cell contain?
An element in 2 different oxidation states.
Q. Describe a metal ion half-cell.
A solid metal rod is dipped into a solution containing ions of the metal. An equilibrium will be set up on the surface of the metal.
Q. Describe an ion half-cell.
An ion half cell contains a solution of ions of the same element but different oxidation states. The electrode is usually graphite or platinum.
Q. Define standard electrode potential.
The electromotive force of a half cell compared with a hydrogen half cell under standard conditions.
Q. What is a hydrogen half-cell?
A half cell containing hydrogen gas and a solution of hydrogen ions. An inert platinum electrode provides a surface for the equilibrium.
Q. How can you predict which half-cell is being oxidised and which is being reduced?
Systems with a more negative E tend to be oxidised, while those with a more positive E tend to be reduced.
Q. Describe the flow of electrons in a simple electrochemical cell.
Electrons flow from the negative electrode to the positive electrode, due to redox reactions that take place at these electrodes.
Q. How can you predict the feasibility of a redox reaction in an electrochemical cell?
The reaction is feasible if the oxidising agent has a lower standard cell potential than the reducing agent. The greater the difference in E value, the more likely the reaction is to occur.
Q. How can standard electrode potentials be used to deduce the relative reactivity of the halogens?
As we move down Group 17, the standard electrode potentials decrease. This indicates that halogens further down the group are more likely to be oxidised, leading to a decrease in their oxidising ability. Consequently, reactivity also decreases down the group.
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