Writing Oxidation and Reduction Half-Equations
What Are Half-Equations, and Why Do We Use Them?
Definition
Half-equation in a redox reaction
A half-equation focuses on one part of a redox reaction: either the oxidation or reduction process.
By breaking a redox reaction into these two components, we can better track the movement of electrons.Example
Let’s take the reaction between zinc and copper(II) sulfate as an example:
Full Reaction:
$$
\text{Zn}(s) + \text{Cu}^{2+}(aq) \to \text{Zn}^{2+}(aq) + \text{Cu}(s)
$$
This reaction involves two simultaneous processes:
- Zinc metal ($\text{Zn}$) is oxidized to form zinc ions ($\text{Zn}^{2+}$).
- Copper ions ($\text{Cu}^{2+}$) are reduced to form copper metal ($\text{Cu}$).
Writing Oxidation and Reduction Half-Equations
Step-by-Step Guide:
- Identify the species being oxidized and reduced:
- Oxidation: The species losing electrons.
- Reduction: The species gaining electrons.
- Write the unbalanced equation for each process:
- For oxidation, place the electrons ($e^-$) on the right-hand side of the equation.
- For reduction, place the electrons ($e^-$) on the left-hand side.
- Balance the atoms:
- Ensure the number of atoms of each element is the same on both sides of the equation.
- Balance the charges:
- Add electrons to one side of the equation to balance the overall charge.
- Combine the half-equations:
- Ensure the number of electrons lost in oxidation equals the number of electrons gained in reduction.
- Add the two half-equations together, canceling out the electrons.
Zinc and Copper(II) Sulfate Reaction
Step 1: Identify the species being oxidized and reduced
- Zinc($\text{Zn}$) is oxidized: it loses electrons to form $\text{Zn}^{2+}$.
- Copper ions($\text{Cu}^{2+}$) are reduced: they gain electrons to form $\text{Cu}$.
Step 2: Write the unbalanced half-equations
- Oxidation: $\text{Zn} \to \text{Zn}^{2+}$
- Reduction: $\text{Cu}^{2+} \to \text{Cu}$
Step 3: Balance the atoms
- Both equations already have balanced atoms.
Step 4: Balance the charges
- Oxidation: $\text{Zn} \to \text{Zn}^{2+} + 2e^-$
- Reduction: $\text{Cu}^{2+} + 2e^- \to \text{Cu}$
Step 5: Combine the half-equations
- Add the two half-equations together:
$$
\text{Zn} + \text{Cu}^{2+} \to \text{Zn}^{2+} + \text{Cu}
$$ - The electrons cancel out, and the overall reaction is balanced.
Balancing Half-Equations in Acidic or Neutral Solutions
When redox reactions occur in aqueous solutions, water ($\text{H}_2\text{O}$), hydrogen ions ($\text{H}^+$), or hydroxide ions ($\text{OH}^-$) may need to be included to balance oxygen and hydrogen atoms in the half-equations.
Steps for Acidic Solutions:
- Balance all atoms except hydrogen and oxygen.
