Example 3.0: A student investigated how the concentration of hydrochloric acid affected the rate of reaction between hydrochloric acid [tex](HCl)[/tex] and magnesium ribbon to produce magnesium chloride [tex]\left(MgCl_2\right)[/tex] and hydrogen [tex]\left(H_2\right)[/tex].

3.1 Complete and balance the equation for the reaction:
[2 marks]

[tex]\[ \text{Mg (s)} + \text{HCl (aq)} \rightarrow \text{MgCl}_2 \text{(aq)} + \text{H}_2 \text{(g)} \][/tex]



Answer :

Sure, let's balance the chemical equation step-by-step.

### Step-by-Step Solution

1. Identify the Chemical Species Involved:
- We start with magnesium (Mg), which will be in a solid state (s).
- Hydrochloric acid (HCl) is in aqueous state (aq).
- The products of this reaction are magnesium chloride (MgCl₂), which is aqueous (aq), and hydrogen gas (H₂), which is in gaseous state (g).

2. Write the Unbalanced Chemical Equation:
- Mg (s) + HCl (aq) → MgCl₂ (aq) + H₂ (g)

3. Balance Magnesium (Mg) Atoms:
- For magnesium, we have 1 atom on both the reactant side and the product side.
- This means magnesium is balanced.

4. Balance Chlorine (Cl) Atoms:
- On the product side, we have MgCl₂, which contains 2 chlorine atoms.
- Therefore, on the reactant side, we need 2 HCl to provide the necessary 2 chlorine atoms.

Adjusted equation:
- Mg (s) + 2 HCl (aq) → MgCl₂ (aq) + H₂ (g)

5. Balance Hydrogen (H) Atoms:
- With the adjustment to 2 HCl, we now have 2 hydrogen atoms on the reactant side.
- On the product side, H₂ contains 2 hydrogen atoms.
- Thus, the hydrogen atoms are balanced.

6. Verify the Balanced Equation:
- We have 1 Mg atom on each side.
- We have 2 Cl atoms on each side.
- We have 2 H atoms on each side.
- All atoms are balanced.

### Balanced Equation

The balanced chemical equation for the reaction between magnesium and hydrochloric acid is:

[tex]\[ \text{Mg} \, (s) + 2 \, \text{HCl} \, (aq) \rightarrow \text{MgCl}_2 \, (aq) + \text{H}_2 \, (g) \][/tex]

This balanced equation ensures that the law of conservation of mass is satisfied, where the number of atoms of each element is the same on both sides of the equation.