Year 11 Taster day – Chemistry A-Level homework task
Bond Energies
Objective: That bond energies can be used to calculate energy changes in a reaction.
Success Criteria:
All students should be able to: State a definition of bond energy.
Most students should be able to: Use a data table to get a bond energy
Some students should also be able to: Calculate the energy transferred in reactions using
supplied bond energies
During the Chemistry session at Ossett Academy, you carried out a practical to find the
enthalpy change of a reaction, complete the following worksheet.
Test yourself on the key ideas
Aim
To help you to understand the ideas and conventions that underlie bond energy calculations.
1
When you use mains gas for cooking or heating, you are using methane, CH4.
Here is the equation for burning methane:
CH4 + 2O2  CO2 + 2H2O ΔH = 890 kJ
a) Which substances are the reactants?
b) Is this reaction exothermic or endothermic?
c) What is the sign for ΔH?
d) If you burn 1 mole of methane, how many moles of
i)
CO2
ii) H2O
are produced?
2
When completed, the diagram should show molecules being broken up and put
together. Complete both the diagram and the key. You will need to fill in all the
blank key cells.
Key to diagram
Pictogram
Meaning
Pictogram
Meaning
3
Complete and label the energy diagram.
Energy of the
reactants
Before reaction
4
After the reaction
Complete the equation using structural formulae.
H
H
C
H
+
2 O=O
H
5
Complete the following table.
Bond breaking
Bond making
Bond
Number
Bond
energy
ΔH (kJ)
Bond
Number
Bond
energy
ΔH (kJ)
C–H
4
413 kJ
+1652
C=O
2
805 kJ
–1610
O=O
2
498 kJ
+ 0000
O–H
Total
ΔH =
464 kJ
Total
Answer the following exam question, you will have to look up the structure of some of the
compounds in order to work out the enthalpy.
1. (a) Use the bond enthalpy data provided below to work out an enthalpy
change for the reaction in the equation below.
2H2S(g)  3O2(g)  2SO2(g)  2H2O(g)
f)
bond enthalpy/kJ
mol–1
e)
bond
g)
H–S
h)
364
i)
S=O
j)
525
k)
SO
l)
265
m)
H–O
n)
464
o)
O=O
p)
498
Clearly lay out all your working using the grid below.
bonds broken
enthalpy change
total for breaking bonds
overall enthalpy change/kJ mol–1
bonds formed
enthalpy change
total for forming new bonds