Question

The enthalpy of formation of H₂O (l) is – 290 kJ/mol and enthalpy of neutralisation of strong acid with strong alkali is – 56 kJ/equiv. What is the enthalpy of formation of OH^– (aq) ? Given D_formH (H⁺, aq) = 0

• A. – 334 kJ/mol
• B. – 234 kJ/mol
• C. – 346 kJ/mol
• D. – 178 kJ/mol

Answer 1

Answer: (B)

– 234 kJ/mol

Answer 2

H₂ (g) + $\frac{1}{2}$O₂ (g) ¾® H₂O (l) DH₁ = – 290 kJ

H⁺ + OH^– ¾® H₂O DH₂ = – 56 kJ

$\frac{1}{2}$ H₂ ¾® H⁺ (aq)

DH = 0

\ $\frac{1}{2}$ H₂ (g) + $\frac{1}{2}$O₂ (g) ¾® OH^– (aq) D_rH

D_rH = – 290 + 56 = – 234 kJ/mol