 # Balancing Simple Half-Equations For Ions Tutorial

## Key Concepts

• The terms "half-equation" or "half-reaction" can refer to either an oxidation reaction equation or to a reduction reaction equation.
• The electrons lost by an atom or ion during an oxidation reaction can be treated as a product in the reaction.

For atom M undergoing oxidation:

 reactant → products M → M+ + e-

Electrons, e-, are a product of the reaction.

• The electrons gained by an atom or ion during a reduction reaction can be treated as a reactant in the reaction.

For atom X undergoing reduction:

 reactants → product X + e- → X-

Electrons, e-, are a reactant.

• When the oxidation or reduction reaction equation is balanced:
(i) The number of atoms of each element on the left hand side of the equation must equal the number of atoms of each element on the right hand side of the equation.

(ii) The sum of the charges on the left hand side of the equation must equal the sum of the charges on the right hand side of the equation.

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## Writing Oxidation and Reduction Equations

Writing a balanced half-equation for an oxidation reaction or a reduction reaction is a 3 step process:

1. Write a skeletal equation for the oxidation or reduction equation based on the information provided.
2. Balance the number of atoms of each element present
(refer to balancing chemical equations)
.
3. Balance the charge by adding electrons (e- or e) to the side deficient in negative charge.

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## Worked Example: Write a Balanced Half-Equation for an Oxidation Reaction

Question: Write a balanced half equation for the oxidation of Fe2+ to Fe3+.

Solution:

(Based on the StoPGoPS approach to problem solving.)

1. What is the question asking you to do?

Write a balanced half-equation for the oxidation of Fe2+

2. What data (information) have you been given in the question?

Extract the data from the question:

(i) Fe2+ is the reactant to be oxidised.

(ii) Fe3+ is the product of the reaction.

3. What is the relationship between what you know and what you need to find out?
Step 1: Write a skeletal equation for the equation.

 reactant → product Fe2+ → Fe3+

Step 2: Balance the number of atoms of each element present.

 reactant → product skeletal equation: Fe2+ → Fe3+ 1 = 1 balanced

Step 3: Balance the charge by adding electrons (e-) to the side deficient in negative charge.

 skeletal equation: Fe2+ → Fe3+ Find the charge on each side of the equation: 2+ 3+ Which side is deficient in negative charge? less positive more positive (that is, deficient in negative charge) Add electrons, e-, to the more positive side Fe2+ → Fe3+ + e- Add together the charges on each side of the equation: 2+ 3+ + 1- 2+ 3+ + 1- = 2+ Compare left hand side total charge and right hand side total charge 2+ = 2+ Charges are balanced, so the equation is balanced ! Fe2+ → Fe3+ + e-

4. Write the balanced half-equation:

Fe2+ → Fe3+ + e-

First, check that you have written an equation for the oxidation of Fe2+
Oxidation means the Fe2+ will lose electrons, so electrons will be a product of the reaction.
An electron, e-, is written on the right hand side (product side) of our equation so, yes, we have written an equation for the oxidation reaction.

Next, check that the charges are balanced:
+ 2 on left hand side
+3 -1 = +2 on right hand side
charges are balanced.

Then, check that number of iron "atoms" are balanced:
1 Fe on left hand side
1 Fe on right hand side
Iron "atoms" are balanced.

So we are reasonably confident that our answer is plausible.

6. State your solution to the problem "write a balanced half-equation for the oxidation of Fe2+":

Fe2+ → Fe3+ + e-

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## Worked Example: Write a Balanced Half-Equation for a Reduction Reaction

Question: Write a balanced half equation for the reduction of I2 to I-

Solution:

(Based on the StoPGoPS approach to problem solving.)

1. What is the question asking you to do?

Write a balanced half-equation for the reduction of I2

2. What data (information) have you been given in the question?

Extract the data from the question:

(i) I2 is the reactant to the reduced.

(ii) I- is the product of the reaction.

3. What is the relationship between what you know and what you need to find out?
Step 1: Write a skeletal equation for the equation.

 reactant → product I2 → I-

Step 2: Balance the number of atoms of each element present.

 reactant → product skeletal equation: I2 → I- 2 ≠ 1 NOT balanced I2 → 2I- 2 = 2 Balanced

Balanced skeletal equation is: I2 → 2I-

Step 3: Balance the charge by adding electrons (e-) to the side deficient in negative charge.

 skeletal equation: I2 → 2I- Find the charge on each side of the equation: 0 2 × 1- = 2- Which side is deficient in negative charge? more positive less positive (that is, deficient in negative charge) Add electrons, e-, to the more positive side e- + I2 → 2I- Add together the charges on each side of the equation: 1- + 0 2- 1- 2- Compare left hand side total charge and right hand side total charge 1- ≠ 2- Charges are NOT balanced, so multiply number of electrons by 2 2e- + I2 → 2I- Check to see if the charges are now balanced: 2 × 1- = 2- + 0 2- 2- = 2- Charges are balanced, so equation is balanced: 2e- + I2 → 2I-

4. Write the balanced half-equation:

I2 + 2e- → 2I-

First, check that you have written an equation for the reduction of I2
In a reduction reaction a species gains electrons, electrons must be a reactant.
Electrons are present on the left hand side (reactant side) of our equation, so yes we have written an equation for a reduction reaction.

Next, chaeck that the charges are balanced:
Charge on left hand side = 0 + (2 × 1-) = 0 + 2- = 2-
Charge on right hand side = 2 × 1- = 2-
Charged is balanced.

Then check that the number of "atoms" of I is balanced:
No. I atoms on left hand side = 2
No. I atoms on right hand side = 2
Iodine "atoms" are balanced.

We are now reasonably confident that our balanced equation is plausible.

6. State your solution to the problem "write a balanced half-equation for the reduction of I2":

I2 + 2e- → 2I-

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