Hi Chemistry students,
Are you struggling to determine the oxidation states of elements in chemical reactions and thus unable to decide if a particular chemical reaction is a redox reaction or not?
It is actually quite easy if you observe several rules when it comes to assigning oxidation states.
The oxidation state is defined as the charge of an atom of an element would have if it existed as an ion in a compound. Note that this is true even for covalent molecule.
When a substance loses or gains electrons, its oxidation state changes. Determining the oxidation states of the reacting atoms is an excellent way to determine whether oxidation or reduction has taken place.
5 Rules to Assign Oxidation States:
To work out the oxidation state, we must apply the following rules.
#1 The oxidation state of an element is always zero.
e.g. Copper atom in Cu has an oxidation state of 0.
e.g. Each iodine atom in I2 has an oxidation state of 0. Each oxygen atom in O2 has an oxidation state of 0.
#2 The oxidation state of a simple ion is the same as the charge on the ion.
e.g. Sodium atom in Na+ ion has an oxidation state of +1.
e.g. Oxygen atom in O2- ion has an oxidation state of -2.
#3 The oxidation states of the atoms in compound add up to zero.
e.g. In MgO, magnesium atom has an oxidation state of +2 while oxygen atom has an oxidation state of -2.
e.g. In CCl4, carbon atom has an oxidation state of +4 while each chlorine atom has an oxidation state of -1.
#4 The sum of the oxidation states of all the atoms in a polyatomic ion is the same as the charge on the ion.
e.g. In OH– ion, oxygen atom has an oxidation state of -2 while hydrogen atom has an oxidation state of +1. The charge on OH– ion is 1-.
#5 Some elements have a fixed oxidation state in their compounds
- Group I elements (e.g. Na, K) in compounds (e.g. NaCl, K2O) have fixed oxidation state of +1.
- Group II elements (e.g. Mg, Ca) in compounds (e.g. MgO, CaSO4) have fixed oxidation state of +2.
- Hydrogen atom in all compounds (e.g. H2O, CH4) always has an oxidation state of +1 except in metal hydrides (e.g. NaH) where it has oxidation state of -1.
- Oxygen atom in all compounds (e.g. H2O, MgO) always has an oxidation state of -2 except in peroxides (e.g. H2O2, Na2O2) where it has oxidation state of -1.
- Fluorine atom in all compounds (e.g. HF, NaF) always has an oxidation state of -1. Rest of the Group VII Halogens has variable oxidation states in compounds.
- Transition metals tend to have variable oxidation states in compounds (e.g. Each iron atom has oxidation state of +2 in FeO whereas each iron atom has oxidation state of +3 in Fe2O3).
With the ability to assign oxidation states to atoms in substances, you can then decide if a chemical reaction is a redox reactions by the following steps:
- Determine the reactants and the products of the reaction
- Determine the oxidation states of all the substances in the reaction
- Compare the oxidation states to check which substance has been oxidised and which substance has been reduced
- Oxidation can be defined as an increase in oxidation state.
- Reduction can be defined as a decrease in oxidation state.
I hope you find the content easy for your understanding and that this article will be part of your Chemistry Notes to master Redox Reactions.
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