Redox in Terms of Electrons
Redox reactions describe the transfer of electrons between chemical species. Understanding whether a substance loses or gains electrons is crucial for analysing many chemical processes, from batteries to biological systems.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Oxidation is the Loss of electrons (OIL).
- Reduction is the Gain of electrons (RIG).
- When a species is oxidised, its charge becomes more positive or less negative.
- When a species is reduced, its charge becomes more negative or less positive.
- Oxidation and reduction always happen simultaneously in a complete reaction, known as a redox reaction.
- The species that causes oxidation is the oxidising agent (and gets reduced itself). The species that causes reduction is the reducing agent (and gets oxidised itself).
Formulae
X → X^(n+) + n e^- This represents an oxidation half-equation, showing a species X losing 'n' electrons to become a positive ion.
Y + n e^- → Y^(n-) This represents a reduction half-equation, showing a species Y gaining 'n' electrons to become a negative ion.
Definitions
- Oxidation
- The process of a chemical species losing one or more electrons.
- Reduction
- The process of a chemical species gaining one or more electrons.
- Redox Reaction
- A chemical reaction in which electron transfer occurs, meaning both oxidation and reduction take place.
Worked example
In the reaction between solid sodium and fluorine gas, sodium fluoride is formed: 2 Na(s) + F2(g) → 2 NaF(s). Which statement correctly describes the process?
- 1
First, identify the initial and final states of each element.
Sodium (Na) starts as a neutral element.
Fluorine (F2) also starts as a neutral element.
- 2
Next, determine the states in the product, sodium fluoride (NaF).
NaF is an ionic compound composed of Na+ ions and F- ions.
- 3
Analyse the change for sodium.
It goes from neutral Na to a Na+ ion.
To form a positive ion, it must have lost one electron.
Loss of electrons is oxidation.
- 4
Analyse the change for fluorine.
It goes from neutral F2 to F- ions.
To form a negative ion, each fluorine atom must have gained one electron.
Gain of electrons is reduction.
- 5
Therefore, sodium is oxidised and fluorine is reduced.
Answer: Sodium (Na) is oxidised and fluorine (F2) is reduced.
Common mistakes
- ×Sign errors are common. Remember that gaining negative electrons makes a species' charge *decrease* (become more negative), while losing negative electrons makes its charge *increase* (become more positive).
- ×Forgetting that diatomic elements like F2, Cl2, O2 etc. are neutral (charge 0) in their elemental state.
- ×Mixing up the process with the agent. The reducing agent is the substance that gets oxidised; the oxidising agent is the substance that gets reduced.
No-calculator tips
- ✓Memorise the mnemonic OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
- ✓Track the charge: If an atom's charge goes UP (e.g., 0 to +2), it's oxidised. If its charge goes DOWN (e.g., 0 to -1), it's reduced.
- ✓For reactions involving simple ionic compounds, mentally split the product into its ions (e.g., MgCl2 into Mg2+ and two Cl^-) to easily see the final charges.