Pupil sheet · Bond energies

Bond energies — return questions

Name ______________________ Date ____________

Use one to two weeks after the lessons. Keep the notes and the lesson closed.

1. deduce

Supplement. For the reaction X₂ + Y₂ → 2XY, breaking the bonds in the reactants takes in 600 kJ/mol, and making the bonds in the product gives out 720 kJ/mol. The activation energy is 170 kJ/mol. Which row describes the reaction pathway diagram correctly?

Row	Products' level	Top of the hump	ΔH arrow
A	120 kJ/mol below the reactants	600 kJ/mol above the reactants	down, from the reactants' level to the products' level
B	120 kJ/mol below the reactants	170 kJ/mol above the reactants	down, from the reactants' level to the products' level
C	120 kJ/mol above the reactants	170 kJ/mol above the reactants	up, from the reactants' level to the products' level
D	120 kJ/mol below the reactants	170 kJ/mol above the reactants	down, from the top of the hump to the products' level

Circle one answer.

2. identify

Supplement. When limestone is heated strongly it breaks down: CaCO₃ → CaO + CO₂. The reaction is endothermic. The figure is its reaction pathway diagram. Match each lettered feature, W, X, Y and Z, to what it shows.

Write the number of the matching option.

The separate atoms
Enthalpy change, ΔH
Products
Reactants
Activation energy, Ea

W: ______
X: ______
Y: ______
Z: ______

3. calculate / deduce / explain

Supplement. Chloroethane breaks down when it is heated strongly: C₂H₅Cl → C₂H₄ + HCl. Chloroethane has one C–C bond, five C–H bonds and one C–Cl bond; ethene has a C=C double bond and four C–H bonds. (a) Use the bond energies in the table to calculate ΔH. (b) Deduce ΔH for the reverse reaction, C₂H₄ + HCl → C₂H₅Cl. (c) Explain your answer to (b) in terms of bond breaking and bond making.

Bond	Bond energy / kJ/mol
C–C	348
C–H	412
C–Cl	338
C=C	612
H–Cl	431

(a) ΔH: ______________ unit ________

(b) ΔH for the reverse reaction: ______________ unit ________