Pupil sheet · Bond energies

Bond energies — final check

Name ______________________ Date ____________

Work alone, with the lesson closed. Answer every question; there are no hints.

1. calculate / determine

Supplement. Ammonia can be burned as a fuel in ships: 4NH₃ + 3O₂ → 2N₂ + 6H₂O. Breaking all the bonds in the reactants takes in 6186 kJ/mol. Making all the bonds in the products gives out 7446 kJ/mol. (a) Calculate ΔH. (b) On the reaction pathway diagram, drawn to scale, the top of the hump is 1600 kJ/mol above the products' line. Determine the activation energy, Ea.

(a) ΔH: ______________ unit ________

(b) Ea: ______________ unit ________

2. calculate / deduce / explain

Supplement. On a hot catalyst, methanol vapour breaks down into carbon monoxide and hydrogen: CH₃OH → CO + 2H₂. Breaking all the bonds in methanol takes in 2057 kJ/mol. Making all the bonds in carbon monoxide and hydrogen gives out 1949 kJ/mol. (a) Calculate ΔH. (b) Deduce whether the reaction is exothermic or endothermic, and explain your answer in terms of bond breaking and bond making.

(a) ΔH: ______________ unit ________

(b) Exothermic or endothermic, explained in terms of bond breaking and bond making.

3. calculate

Supplement. Hydrazine, N₂H₄, is a rocket fuel: N₂H₄ + O₂ → N₂ + 2H₂O. Use the displayed formulae and the bond energies in the table to calculate (a) the energy taken in breaking bonds; (b) the energy given out making bonds; (c) ΔH.

Displayed formulae: hydrazine has an N–N single bond and four N–H bonds. Oxygen is O=O, nitrogen is N≡N with a triple bond and water is H–O–H. The equation is N₂H₄ + O₂ → N₂ + 2H₂O; one water molecule is drawn.

Bond	Bond energy / kJ/mol
N–N	158
N–H	391
O=O	498
N≡N	945
O–H	463

(a) Energy in: ______________ unit ________

(b) Energy out: ______________ unit ________

4. calculate / deduce

Supplement. Ethene reacts with chlorine to make 1,2-dichloroethane, used to make PVC: C₂H₄ + Cl₂ → C₂H₄Cl₂. (a) Use the displayed formulae and the bond energies in the table to calculate ΔH. (b) Deduce whether the reaction is exothermic or endothermic. Use your answer to (a).

Displayed formulae: ethene has a C=C double bond and four C–H bonds. Chlorine is Cl–Cl. The product, 1,2-dichloroethane, has a C–C single bond, four C–H bonds and two C–Cl bonds, one on each carbon atom.

Bond	Bond energy / kJ/mol
C=C	612
C–H	412
Cl–Cl	242
C–C	348
C–Cl	338

(a) ΔH: ______________ unit ________

(b) Exothermic or endothermic, from your answer to (a).

5. calculate

Supplement. Hydrogen peroxide, H–O–O–H, breaks down into water and oxygen: 2H₂O₂ → 2H₂O + O₂. ΔH = −206 kJ/mol. Use the bond energies in the table to calculate the bond energy of the O–O single bond.

Bond	Bond energy / kJ/mol
O–H	463
O=O	498

O–O bond energy: ______________ unit ________