Answer:
The ballance half reactions are:
Mg²⁺ + 2e⁻ → Mg
6OH⁻ + Si → SiO₃²⁻ + 4e⁻ + 3 H₂O
Coefficients for H2O and OH– are 3 for H₂O (in products side) and 6 for OH⁻ (in reactants side)
Explanation:
Si (s) + Mg(OH)₂ (s) → Mg (s) + SiO₃²⁻ (aq)
Let's see the oxidations number.
As any element in ground state, we know that oxidation state is 0, so Si in reactants and Mg in products, have 0.
Mg in reactants, acts with +2, so the oxidation number has decreased.
This is the reduction, so it has gained electrons.
Si in reactants acts with 0 so in products we find it with +4. The oxidation number increased it, so this is oxidation. The element has lost electrons.
Let's take a look to half reactions:
Mg²⁺ + 2e⁻ → Mg
Si → SiO₃²⁻ + 4e⁻
In basic medium, we have to add water, as the same amount of oxygen we have, IN THE SAME SIDE. We have 3 oxygens in products, so we add 3 H₂O and in the opposite site we can add OH⁻, to balance the hydrogen. The half reaciton will be:
6OH⁻ + Si → SiO₃²⁻ + 4e⁻ + 3 H₂O
If we want to ballance the main reaction we have to multiply (x2) the half reaction of oxidation. So the electrons can be ballanced.
2Mg²⁺ + 4e⁻ → 2Mg
Now, that they are ballanced we can sum the half reactions:
2Mg²⁺ + 4e⁻ → 2Mg
6OH⁻ + Si → SiO₃²⁻ + 4e⁻ + 3 H₂O
2Mg²⁺ + 4e⁻ + 6OH⁻ + Si → 2Mg + SiO₃²⁻ + 4e⁻ + 3 H₂O
