Classifying reactions
Chemists find it very useful to classify reactions that are similar in some way. Any one reaction may be classified in a variety of ways. One way of classifying reactions is based on whether oxidation numbers of atoms in the products are different to those in the reactants.
In acid-base reactions there are no changes in oxidation number.
In redox reactions two oxidation number changes occur.
One atom in a second species known as the reductant donates electrons.
With few exceptions, reactions having an element as a reactant or product are redox.
In acid-base reactions there are no changes in oxidation number.
Proton transfer reactions are also acid-base reactions:
NH3(aq) + HCl(aq)
NH4Cl(aq)
Precipitation reactions are also acid-base reactions:
Ag+(aq) + Cl–(aq) AgCl(s)
NH3(aq) + HCl(aq)
NH4Cl(aq)In both reactants and products, H is +1, Cl is -1 and N is -3.
Precipitation reactions are also acid-base reactions:
Ag+(aq) + Cl–(aq)
AgCl(s)Proton-transfer and precipitation are other classification terms.
In redox reactions two oxidation number changes occur.
Redox reaction example:
Cu2+(aq) + Fe(s) Cu(s) + Fe2+(aq)
One atom in a species (molecule, atom or ion) known as the oxidant accepts electrons.
The oxidation state of this atom is more negative in a reaction product (Cu2+ above).
One atom in a second species known as the reductant donates electrons.
The oxidation state of this atom is more positive in a reaction product (Fe above).
The oxidant and reductant are also known as the oxidizing agent and the reducing agent.
The oxidant and reductant are also known as the oxidizing agent and the reducing agent.
With few exceptions, reactions having an element as a reactant or product are redox.
This is because atoms in compounds generally have either negative or positive oxidation state, and elements always have an oxidation state of zero.