Question

Use the standard enthalpies of formation for the reactants and products to solve for the ΔHrxn for the following reaction

Ca(s)+ 2H20(l) → Ca(OH)2(s)+H2(g)

∆Hrxn=

The reaction is =

Answer 1

Answer:

- 386.6 kJ/mol.

Explanation:

• To solve this problem, we can use Hess's Law:  

ΔHrxn = ΣΔHf(products) - ΣΔHf(reactants)

• For this reaction: Ca(s)+ 2H₂O(l) → Ca(OH)₂(s) + H₂(g),

∴ ΔHrxn = ΣΔHf(products) - ΣΔHf(reactants) = [ΔHf(Ca(OH)₂(s)) + ΔHf(H₂)(g)] - [ΔHf(Ca)(s) + 2*ΔHf(H₂O)(l)].

• knowing that:

ΔHf(Ca(OH)₂(s)) = - 985.2 kJ/mol, ΔHf(H₂)(g) = 0.0 kJ/mol, ΔHf(Ca)(s) = 0.0 kJ/mol, ΔHf(H₂O)(l) = - 285.8 kJ/mol.

∴ ΔHrxn = ΣΔHf(products) - ΣΔHf(reactants) = [ΔHf(Ca(OH)₂(s)) + ΔHf(H₂)(g)] - [ΔHf(Ca)(s) + 2*ΔHf(H₂O)(l)] = [(- 985.2 kJ/mol) + (0.0 kJ/mol)] - [(0.0 kJ/mol) + 2*(- 285.8 kJ/mol)] = - 386.6 kJ/mol.