Hess's law and enthalpies of reactions (bond dissociation, combustion, formation etc.)
- C(s) + 2H_2(g) → CH_4(g); Δ H = -74.8 kJ mol^-1Which of the following diagrams gives an accurate representation of the above reaction? [R → reactants; P → products] Answer: 1. Energy diagram (1) Key idea: Δ H=-74.8 kJ/mol means an exothermic reaction: products lie 74.8 kJ below the reactants. Diagram (1)... 1
- The standard heat of formation, in kcal/mol of Ba^2+ is :[Given : standard heat of formation of SO_4^2- ion (aq) = -216 kcal/mol, standard heat of crystallisation of BaSO_4(s) = -4.5 kcal/mol, standard heat of formation of BaSO_4(s) = -349 kcal/mol] Answer: 1. -128.5 Key idea: mathrmBa²⁺(aq)+SO₄²⁻(aq)→ BaSO₄(s) has Δ H=-4.5 kcal (crystallisation). Δ H_f(BaSO₄)=Δ H_f(mathrmBa²⁺)+Δ H_f(mat... 1
- The bond dissociation energies of X_2, Y_2 and XY are in the ratio of 1 : 0.5 : 1. ΔH for the formation of XY is -200 kJ mol^-1. The bond dissociation energy of X_2 will be Answer: 2. 800 kJ mol ⁻¹ Let the bond energies be X-X = a , Y-Y = 0.5a , X-Y = a . For 1/2X₂+1/2Y₂→XY : Δ H=1/2 a+1/2(0.5a)-a=-0.25a . -0.25... 1