To write half-equations for electrolysis reactions, it is essential to identify the ions involved and ensure that both the charges and atoms are balanced in each half-equation.

Half-equations are used to illustrate the processes of electron loss and gain during electrolysis. This process involves the breakdown of an ionic compound, either in a molten state or dissolved in a solution, by passing an electric current through it. In this environment, the ions are free to move and carry the electric current.

To formulate a half-equation, start by identifying the ions that are undergoing oxidation (losing electrons) and reduction (gaining electrons). The ions that migrate towards the cathode (the negative electrode) are positively charged and will undergo reduction by gaining electrons. Conversely, the ions that move towards the anode (the positive electrode) are negatively charged and will undergo oxidation by losing electrons.

Once you have identified the relevant ions, the next step is to balance the charges and atoms in each half-equation. For instance, if a sodium ion ($\text{Na}^+$) gains an electron to form a sodium atom ($\text{Na}$), the corresponding half-equation would be:

This equation demonstrates that the sodium ion has gained an electron during the reaction.

Similarly, if a chloride ion ($\text{Cl}^-$) loses an electron to become a chlorine atom ($\text{Cl}$), the half-equation would be:

This indicates that the chloride ion has lost an electron during the reaction.

It is important to note that the total charge on the left side of the equation must equal the total charge on the right side. If this balance is not achieved, you may need to multiply the entire equation by an appropriate factor. For example, in the case of two chloride ions forming a chlorine molecule ($\text{Cl}_2$), the balanced half-equation would be:

Writing half-equations for electrolysis reactions is a fundamental skill in GCSE Chemistry, as it enhances your understanding of the processes occurring at each electrode during the reaction.