Redox reactions are where reduction and oxidation occurs. Reduction is the gain of electrons whereas oxidation is the loss of electrons. Redox occurs in electrochemical cells at the anode and cathode, producing electricity. A voltmeter measures voltage difference of electrodes to calculate cell potential.
Redox reactions
A redox reaction is a reaction where both reduction and oxidation occurs, transferring electrons.
Oxidation is the loss of electrons and the oxidation number of an element will increase when it has been oxidised. Reduction is the gain of electrons and the oxidation number of an element will decrease when it has been reduced. You can determine if an element has been oxidised or reduced by half reactions.
Tip: You can remember the difference between these two terms by the acronym 'OILRIG' which stands for Oxidation Is Loss and Reduction Is Gain.
Example
Na++21Cl2→NaCl
Half reactions:
Na→Na++e−
21Cl2+e−→Cl−
Nahas been oxidised as there is a loss of electrons whereas Cl2 has been reduced as there is a gain of electrons. The oxidation number of Na has increased from 0 to +1 and the oxidation number of Cl2 has decreased from 0 to −1.
Element type
redox
Oxidation number
Example
s-block
Oxidation
Oxidation number = group number
Mg→Mg2++2e−
p-block elements (groups 3−4)
Oxidation
Oxidation number = group number
Al→Al3++3e−
p-block elements (groups 5−8)
Reduction
Oxidation number = group number−8
21O2+2e−→O2−
d-block
Oxidation
Variable oxidation numbers
Cu→Cu++e−
Cu→Cu2++2e−
Cu→Cu3++3e−
Note: These are general rules for predicting oxidation numbers and there are exceptions. Groups 3−4 in the p-block can be referred to as metals and groups 5−7 can be referred to as non-metals.
Electrochemical cells
Electrochemical cells exhibit redox reactions as there are two different metal electrodes (positive and negative) immersed in a solution of ions and connected by an electrical circuit. This allows electrons to flow between the electrodes which produces electricity.
At the anode
This is the negative electrode where oxidation (loss of e−) will take place and electrons will flow from this electrode. The more reactive metal will typically be the anode as it loses electrons so has become oxidised.
At the cathode
This is the positive electrode where reduction (gain of e−) will take place and electrons flow to this electrode. The less reactive metal will typically be the cathode as it loses electrons less easily so will gain electrons and be reduced.
Example
Zinc is labelled as the anode as it is more reactive compared to copper so will undergo oxidation. Copper is labelled as the cathode as it is less reactive than zinc so undergoes reduction. You can determine whether the metals are anodes and cathodes by half reactions that are occurring in the electrochemical cell.
Half reactions:
Anode:
Zn→Zn2++2e−
Cathode:
Cu2++2e−→Cu
Note: Half reactions can be shown with reversible arrows, however as one of the metals will oxidise more readily, they are drawn like this to show if the metal is oxidised or reduced.
Each electrode is immersed in an aqueous salt solution and connected to a wire to allow the flow of electrodes from the anode to the cathode. A salt bridge made out of filter paper soaked in a salt solution such as KCl allows flow of ions to counteract the flow of electrons in the circuit.
A voltmeter is used within the electrical circuit to measure the voltage V between the half cells (electrodes). The calculated difference in voltage between the half cells is referred to as the cell potential or the electromotive force (EMF) and shows the flow of electrons in an electrochemical cell.
Electrolysis in an electrochemical cell
procedure
1.
The reactive anode will oxidise to form positive ions and lose electrons
2.
The electrons will travel around the circuit which generates electricity
3.
When the electrons reach the cathode, positive ions in the solution will gain electrons
4.
As positive ions both build up and are reduced in the solutions, a salt bridge is used so that positive and negative ions can flow to either solution to maintain charge neutrality
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FAQs - Frequently Asked Questions
What is the difference between an anode and cathode?
An anode is the positive electrode where oxidation occurs as electrons are lost. A cathode is the negative electrode where reduction occurs as electrons are gained.
What is an electrochemical cell?
An electrochemical cell produces electricity by electrons flowing from an anode to a cathode through a current and ions flowing in the reverse direction.
What is a redox reaction?
A redox reaction is where both reduction and oxidation occur in a reaction, resulting in the transfer of electrons.
How do you work out if an element has been oxidised or reduced?
To work out if an element has been oxidised, it will have lost electrons and the oxidation number will increase. If an element has been reduced, it will have gain electrons and the oxidation number will decrease.
