Half Equations

The Mechanism of Electrolysis

Think about two powerful magnets snapping together; electrostatic attraction works in exactly the same way to pull chemical compounds apart. In an electrolytic cell, the power supply creates a potential difference that electrically charges the electrodes.

This creates an electrostatic attraction, which is the force that pulls particles with opposite electrical charges together.
Positively charged ions (cations) are electrostatically attracted to the negatively charged cathode.
Negatively charged ions (anions) are electrostatically attracted to the positively charged anode.

This movement of charged ions through the electrolyte is what carries the electric current. When the ions reach the electrodes, they are discharged, meaning they lose their charge by transferring electrons to become neutral atoms or molecules.

Writing Half Equations

You can track exactly where every electron goes during a chemical reaction using a half equation. This is a balanced symbol equation that describes only the oxidation or reduction part of a reaction, using the symbol $e^-$ to represent an electron.

Reduction occurs at the cathode (RED CAT). Cations gain electrons to become neutral.
Oxidation occurs at the anode (AN OX). Anions lose electrons to become neutral.
A helpful way to remember this is the OIL RIG mnemonic: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

When balancing half equations, you must ensure that both the number of atoms and the total electrical charge are identical on both sides of the arrow. For example, if the left side has a total charge of $2-$ , the right side must also have a total net charge of $2-$ .

Deriving Monatomic and Diatomic Products

Like balancing a seesaw, a half equation must have perfectly matched elements and charges, which requires different steps depending on the state of the final product. Metals form monatomic (single) neutral atoms, while hydrogen, oxygen, and halogens always form diatomic molecules (e.g., $Cl_2$ , $O_2$ ).

Worked Example: Formation of sodium metal

Derive the half equation for the discharge of sodium ions ( $Na^+$ ).

Identify the ion and its movement: Positive sodium ions ( $Na^+$ ) are attracted to the negative cathode.
Write the unbalanced discharge process: $Na^+ \rightarrow Na$
Balance the charge by adding electrons: The $Na^+$ $N a^{+}$ ion must gain one electron to become neutral.
- $Na^+ + e^- \rightarrow Na$

Worked Example: Formation of chlorine gas

Derive the half equation for the discharge of chloride ions ( $Cl^-$ ) to form chlorine gas.

Identify the ion and write the initial discharge process: Negative chloride ions ( $Cl^-$ $C l^{-}$ ) move to the anode to form diatomic chlorine ( $Cl_2$ $C l_{2}$ ).
- $Cl^- \rightarrow Cl_2$
Balance the atoms: We need two chloride ions to form one diatomic molecule.
- $2Cl^- \rightarrow Cl_2$
Balance the charge by adding electrons: The left side has a total charge of $2-$ $2 -$ . The right side must match this by adding two electrons.
- $2Cl^- \rightarrow Cl_2 + 2e^-$

Worked Example: Formation of oxygen gas

Derive the half equation for the formation of oxygen gas from hydroxide ions ( $OH^-$ ).

Identify the reactants and products: Hydroxide ions ( $OH^-$ $O H^{-}$ ) react to form oxygen gas ( $O_2$ $O_{2}$ ) and water ( $H_2O$ $H_{2} O$ ).
- $OH^- \rightarrow O_2 + H_2O$
Balance the oxygen and hydrogen atoms: To make $O_2$ $O_{2}$ and $H_2O$ $H_{2} O$ while balancing both elements, we require four hydroxide ions.
- $4OH^- \rightarrow O_2 + 2H_2O$
Balance the total charge by adding electrons: The left side has a charge of $4-$ $4 -$ . We must add four electrons to the right side to balance it.
- $4OH^- \rightarrow 2H_2O + O_2 + 4e^-$

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Exam Tips

Common Mistake
Students often forget to double the ions and electrons when forming diatomic gases. Never write Cl⁻ → Cl + e⁻; you must write 2Cl⁻ → Cl₂ + 2e⁻.
2
In Edexcel mark schemes, always show electrons being added to the right-hand side for oxidation equations (e.g., X → Y + e⁻) rather than subtracted from the left.
3
Writing and balancing half equations is specifically required for Higher Tier students, so make sure you memorize the 4-step derivation for oxygen gas from hydroxide ions!

Key Terms(9)

Electrostatic attraction: The force of attraction between particles with opposite electrical charges.
Cation: A positively charged ion that is attracted to the cathode during electrolysis.
Anion: A negatively charged ion that is attracted to the anode during electrolysis.
Cathode: The negative electrode where reduction (gain of electrons) takes place.
Anode: The positive electrode where oxidation (loss of electrons) takes place.
Discharge: The process where an ion loses its charge at an electrode to become a neutral atom or molecule.
Half equation: A balanced symbol equation describing only the oxidation or reduction part of a redox reaction, specifically showing the transfer of electrons.
Reduction: The gain of electrons by a chemical species (occurs at the cathode).
Oxidation: The loss of electrons by a chemical species (occurs at the anode).

Previous Subtopic: Predicting Molten Electrolysis ProductsPredicting Molten Electrolysis Products Next Subtopic: Oxidation and Reduction (Electrons)Oxidation and Reduction (Electrons)

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Key Terms(9)

Electrostatic attraction: The force of attraction between particles with opposite electrical charges.
Cation: A positively charged ion that is attracted to the cathode during electrolysis.
Anion: A negatively charged ion that is attracted to the anode during electrolysis.
Cathode: The negative electrode where reduction (gain of electrons) takes place.
Anode: The positive electrode where oxidation (loss of electrons) takes place.
Discharge: The process where an ion loses its charge at an electrode to become a neutral atom or molecule.
Half equation: A balanced symbol equation describing only the oxidation or reduction part of a redox reaction, specifically showing the transfer of electrons.
Reduction: The gain of electrons by a chemical species (occurs at the cathode).
Oxidation: The loss of electrons by a chemical species (occurs at the anode).

Half Equations

The Mechanism of Electrolysis

This creates an electrostatic attraction, which is the force that pulls particles with opposite electrical charges together.
Positively charged ions (cations) are electrostatically attracted to the negatively charged cathode.
Negatively charged ions (anions) are electrostatically attracted to the positively charged anode.

Writing Half Equations

Reduction occurs at the cathode (RED CAT). Cations gain electrons to become neutral.
Oxidation occurs at the anode (AN OX). Anions lose electrons to become neutral.
A helpful way to remember this is the OIL RIG mnemonic: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

Deriving Monatomic and Diatomic Products

Worked Example: Formation of sodium metal

Derive the half equation for the discharge of sodium ions ( $Na^+$ ).

Identify the ion and its movement: Positive sodium ions ( $Na^+$ ) are attracted to the negative cathode.
Write the unbalanced discharge process: $Na^+ \rightarrow Na$
Balance the charge by adding electrons: The $Na^+$ $N a^{+}$ ion must gain one electron to become neutral.
- $Na^+ + e^- \rightarrow Na$

Worked Example: Formation of chlorine gas

Derive the half equation for the discharge of chloride ions ( $Cl^-$ ) to form chlorine gas.

Identify the ion and write the initial discharge process: Negative chloride ions ( $Cl^-$ $C l^{-}$ ) move to the anode to form diatomic chlorine ( $Cl_2$ $C l_{2}$ ).
- $Cl^- \rightarrow Cl_2$
Balance the atoms: We need two chloride ions to form one diatomic molecule.
- $2Cl^- \rightarrow Cl_2$
Balance the charge by adding electrons: The left side has a total charge of $2-$ $2 -$ . The right side must match this by adding two electrons.
- $2Cl^- \rightarrow Cl_2 + 2e^-$

Worked Example: Formation of oxygen gas

Derive the half equation for the formation of oxygen gas from hydroxide ions ( $OH^-$ ).

Identify the reactants and products: Hydroxide ions ( $OH^-$ $O H^{-}$ ) react to form oxygen gas ( $O_2$ $O_{2}$ ) and water ( $H_2O$ $H_{2} O$ ).
- $OH^- \rightarrow O_2 + H_2O$
Balance the oxygen and hydrogen atoms: To make $O_2$ $O_{2}$ and $H_2O$ $H_{2} O$ while balancing both elements, we require four hydroxide ions.
- $4OH^- \rightarrow O_2 + 2H_2O$
Balance the total charge by adding electrons: The left side has a charge of $4-$ $4 -$ . We must add four electrons to the right side to balance it.
- $4OH^- \rightarrow 2H_2O + O_2 + 4e^-$

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Get full access to revision notes, key terms, and exam tips for every subtopic.

Exam Tips

Common Mistake
Students often forget to double the ions and electrons when forming diatomic gases. Never write Cl⁻ → Cl + e⁻; you must write 2Cl⁻ → Cl₂ + 2e⁻.
2
In Edexcel mark schemes, always show electrons being added to the right-hand side for oxidation equations (e.g., X → Y + e⁻) rather than subtracted from the left.
3
Writing and balancing half equations is specifically required for Higher Tier students, so make sure you memorize the 4-step derivation for oxygen gas from hydroxide ions!

Key Terms(9)

Electrostatic attraction: The force of attraction between particles with opposite electrical charges.
Cation: A positively charged ion that is attracted to the cathode during electrolysis.
Anion: A negatively charged ion that is attracted to the anode during electrolysis.
Cathode: The negative electrode where reduction (gain of electrons) takes place.
Anode: The positive electrode where oxidation (loss of electrons) takes place.
Discharge: The process where an ion loses its charge at an electrode to become a neutral atom or molecule.
Half equation: A balanced symbol equation describing only the oxidation or reduction part of a redox reaction, specifically showing the transfer of electrons.
Reduction: The gain of electrons by a chemical species (occurs at the cathode).
Oxidation: The loss of electrons by a chemical species (occurs at the anode).

Previous Subtopic: Predicting Molten Electrolysis ProductsPredicting Molten Electrolysis Products Next Subtopic: Oxidation and Reduction (Electrons)Oxidation and Reduction (Electrons)

Back to Half Equations

Put your knowledge into practice — try past paper questions for Chemistry

Key Terms(9)

Electrostatic attraction: The force of attraction between particles with opposite electrical charges.
Cation: A positively charged ion that is attracted to the cathode during electrolysis.
Anion: A negatively charged ion that is attracted to the anode during electrolysis.
Cathode: The negative electrode where reduction (gain of electrons) takes place.
Anode: The positive electrode where oxidation (loss of electrons) takes place.
Discharge: The process where an ion loses its charge at an electrode to become a neutral atom or molecule.
Half equation: A balanced symbol equation describing only the oxidation or reduction part of a redox reaction, specifically showing the transfer of electrons.
Reduction: The gain of electrons by a chemical species (occurs at the cathode).
Oxidation: The loss of electrons by a chemical species (occurs at the anode).