Question

In the reaction, H₂ (g) + Cl₂ (g) → 2HCl (g) ΔH = -184 kJ, if 2 moles of H₂ react with 2 moles of Cl₂, then ΔU is equal to
(a)-184 kJ
(b) -368 kJ
(c) Zero
(d) +368 kJ

Answer 1

(d) +368 kJ is the correct answer.  

Work this problem, think of the reaction in terms of simple steps:

Step 1 The reactant molecules, H2 and Cl2, break down into their atoms

H2(g) → 2 H(g)

Cl2(g) → 2 Cl(g)

Step 2 these atoms combine to form HCl molecules

2 H (g) + 2 Cl (g) → 2 HCl (g)

The H-H and Cl-Cl bonds are broken. In both cases, one mole of bonds is broken.