Given the following experimental data:
D H rxn Ca (s) + 2 H₂ O ( ) Ca(OH) 2 (s) + H₂ (g) - 415.1 kJ 1 / 2 H₂ (g) + 1 / 2 Cl₂ 2 HCl (g) - 92.31 kJ CaCl₂ (s) + 2 H₂ O ( ) Ca(OH) 2 (s) + 2 HCl (g) 195.3 kJ What is the standard heat of formation ( D H f ) for CaCl₂ (s)?

Question

Given the following experimental data:
D H rxn Ca (s) + 2 H₂ O (  ) Ca(OH) 2 (s) + H₂ (g) - 415.1 kJ 1 / 2 H₂ (g) + 1 / 2 Cl₂ 2 HCl (g) - 92.31 kJ CaCl₂ (s) + 2 H₂ O (  ) Ca(OH) 2 (s) + 2 HCl (g) 195.3 kJ What is the standard heat of formation ( D H f ) for CaCl₂ (s)?

Quizplus · Accepted Answer

To determine the standard heat of formation of CaCl₂ (s), we can use Hess's Law and add the two given reactions together in a way that cancels out the Ca(OH)₂ (s) and HCl (g) on both sides:

Ca(s) + 2H₂O(l) → Ca(OH)₂(s) + H₂(g)    ΔH₁ = -415.1 kJ/mol
1/2H₂(g) + 1/2Cl₂(g) → HCl(g)            ΔH₂ = -92.31 kJ/mol
Ca(OH)₂(s) + 2HCl(g) → CaCl₂(s) + 2H₂O(l)    ΔH₃ = 195.3 kJ/mol

Multiplying the second reaction by 2 and adding it to the third reaction gives:

Ca(s) + Cl₂(g) → CaCl₂(s)      ΔH = -311.3 kJ/mol

This equation represents the standard heat of formation of CaCl₂ (s), since it is the heat released when one mole of CaCl₂ (s) is formed from its constituent elements in their standard states at 298 K and 1 bar.

Therefore, the answer is A) -795.0 kJ/mol.

Given the Following Experimental Data