Calculate the heat of formation of SiO4(s), given that the heats of formation of SiO2(s), -910.90 KJ/mol; H2O(l), -285.8 KJ/mol; HF (g), -271.1 KJ/mol.
To calculate the heat of formation of SiO4(s), you need to use the following equation:
ΔH = ΣnΔH(products) - ΣmΔH(reactants)
where ΔH is the heat of formation, ΣnΔH(products) is the sum of (coefficients × heats of formation) for the products, and ΣmΔH(reactants) is the sum of (coefficients × heats of formation) for the reactants.
First, let's determine the balanced chemical equation for the formation of SiO4(s). Since we know the heats of formation of SiO2(s), H2O(l), and HF(g), we can use these to construct the equation:
SiO2(s) + 2H2O(l) + 4HF(g) → SiO4(s) + 4HF(g)
Now, we can calculate the heat of formation of SiO4(s) using the given heats of formation:
ΔH = [(1 × ΔH(SiO4)) + (4 × ΔH(HF))] - [(1 × ΔH(SiO2)) + (2 × ΔH(H2O)) + (4 × ΔH(HF))]
Substituting the values:
ΔH = [(1 × ΔH(SiO4)) + (4 × -271.1 KJ/mol)] - [(1 × -910.90 KJ/mol) + (2 × -285.8 KJ/mol) + (4 × -271.1 KJ/mol)]
ΔH = ΔH(SiO4) + (-1084.4 KJ/mol) - (-1763.00 KJ/mol)
Simplifying:
ΔH = ΔH(SiO4) + 678.6 KJ/mol
So, the heat of formation of SiO4(s) is ΔH(SiO4) = -678.6 KJ/mol.