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Bond Enthalpies
Principles to Production: Chemical Energy
Breaking bonds
If we want to break a covalent bond between two atoms, we need to to overcome the attractive force.
Bond breaking is an endothermic process.
C C
put energy input energy in
© Nueyer
Making bonds
The opposite is true if we want to make new bonds.
Energy is released when new chemical bonds are formed.
Bond making is an exothermic process.
Breaking or making the same chemical bond will require the same energy to be put in or released.
H H 2H ∆H = 432 kJ
H H2H ∆H = –432 kJ
Bond enthalpy values
The values for bond enthalpies are found on page 9 of the data book.
For some bonds, the mean bond enthalpy is quoted. This is to give an average value to work from since the precise enthalpy value for a bond may be different in different molecules.
For example, the energy needed to break a bond in ethane (C2H6) will be different to the energy needed to break a bond in decane (C10H22)
C C
C C
Gaseous state
The bond enthalpies quoted in the data book are the energies required to break 1 mole of a particular bond between a pair of atoms in the gaseous state.
We can use these bond enthalpies to approximately calculate the enthalpy change for a given reaction.
Example 1
What is the enthalpy change when hydrogen is added to ethyne to produce ethane?
C2H2 (g) + 2H2 (g) C2H6 (g)
To answer this we must look at what types of bonds must be broken in the reactants and formed in the products.
In this reaction, we must first break all the bonds inside the
reactant molecules.
This will require energy to be put in.
C2H2 (g) + 2H2 (g) C2H6 (g)
Reaction pathway
Pot
entia
l ene
rgy
C2H2 (g) + 2H2 (g) C2H6 (g)
Next, new bonds must be formed between the atoms in
the product molecule.
This releases energy.
What is the enthalpy change when hydrogen is added to ethyne, producing ethane?
C2H2 (g) + 2H2 (g) C2H6 (g)
Step One: Draw the full structural formulae of all the molecules from the equation. This will show exactly what bonds are involved.
+CH C HH H
H HCH C H
H
H
H
H
To answer this question we can follow these steps.
2
What is the enthalpy change when hydrogen is added to ethyne, producing ethane?
C2H2 (g) + 2H2 (g) C2H6 (g)
+CH C HH H
H HCH C H
H
H
H
H
Step Two:
Make a list of all the bonds being broken in the reactants
CH C H
Bond Breaking
C C1 x
C H2 xH H2 x
C C
C C1 x
C H2 x
H H
H H
What is the enthalpy change when hydrogen is added to ethyne, producing ethane?
C2H2 (g) + 2H2 (g) C2H6 (g)
+CH C HH H
H HCH C H
H
H
H
H
Step Three:
Fill in the values for the bond enthalpies from Page 9 of the data book.
Bond Breaking
C C1 x
C H2 xH H2 x
= 835
= 2 x 414= 2 x 432
= 828= 864
What is the enthalpy change when hydrogen is added to ethyne, producing ethane?
C2H2 (g) + 2H2 (g) C2H6 (g)
+CH C HH H
H HCH C H
H
H
H
H
Step Four:
Repeat this process for the Bond Making steps.
Bond Breaking
C C1 x
C H2 xH H2 x
= 835
= 2 x 414= 2 x 432
= 828= 864
Bond Making
C C1 x
C H6 x
= 346
= 6 x 414 = 2484
C CCH C H
H
H
H
H
What is the enthalpy change when hydrogen is added to ethyne, producing ethane?
C2H2 (g) + 2H2 (g) C2H6 (g)
+CH C HH H
H HCH C H
H
H
H
H
Step Five:
Calculate the total energy put in breaking bonds and total energy given out making new bonds.
Bond Breaking
C C1 x
C H2 xH H2 x
= 835
= 2 x 414= 2 x 432
= 828= 864
Bond Making
C C1 x
C H6 x
= 346
= 6 x 414 = 2484
Total put in = 2527 kJ Total given out = –2830 kJRemember: Bond breaking is an endothermic process
Remember: Bond making is an exothermic process
What is the enthalpy change when hydrogen is added to ethyne, producing ethane?
C2H2 (g) + 2H2 (g) C2H6 (g)
+CH C HH H
H HCH C H
H
H
H
H
Step Six:
Calculate the enthalpy change for the reaction.
Bond Breaking
C C1 x
C H2 xH H2 x
= 835
= 2 x 414= 2 x 432
= 828= 864
Bond Making
C C1 x
C H6 x
= 346
= 6 x 414 = 2484
Total put in = 2527 kJ Total given out = –2830 kJ∆H = 2527 + (–2380)
= 2527 – 2380= – 303 kJ mol-1
Enthalpy of formation
Bond enthalpies are also useful when calculating the enthalpy change in making 1 mole of a compound from its elements in their standard states.
C (s) + 2H2 (g) CH4 (g)
The data book quotes the value for this reaction as ∆Hformation = –75 kJ mol–1
For example, the enthalpy of formation of methane can be represented by:
Enthalpy of formation
To calculate the enthalpy of formation for methane using bond enthalpies, we would need to change 1 mole of solid carbon atoms into 1 mole of gaseous carbon atoms.
This change, C (s) C (g), is known as the enthalpy of sublimation of carbon and has a value of 715 kJ mol–1 (see page 9 of the data book).
Example 2
Calculate the enthalpy of formation of propane.
3C (s) + 4H2 (g) C3H8 (g)
C (s)H H
H HCH C
H
H
H
H
C H
H
H
H H
H HC (s)
C (s)+
Example 2Calculate the enthalpy of formation of propane.
3C (s) + 4H2 (g) C3H8 (g)
C (s)
Bond Breaking
3 x
H H4 x = 4 x 432
= 3 x 715
= 1728
Total put in = 3873 kJ
H H
H HCH C
H
H
H
H
C H
H
H
H H
H HC (s)
C (s)
C(s) C(g)
+
= 2145
C (s)
C (s)
C (s)
3
4
H H
H H
H H
H H
Example 2Calculate the enthalpy of formation of propane.
3C (s) + 4H2 (g) C3H8 (g)
C (s)
Bond Breaking
3 x
H H4 x = 4 x 432
= 3 x 715
= 1728
Total put in = 3873 kJ
H H
H HCH C
H
H
H
H
C H
H
H
H H
H HC (s)
C (s)
C(s) C(g)
+
= 2145
Example 2Calculate the enthalpy of formation of propane.
3C (s) + 4H2 (g) C3H8 (g)
C (s)
Bond Making
C C2 x
C H8 x = 8 x 414 = 3312
Total given out = –4004 kJ
H H
H HCH C
H
H
H
H
C H
H
H
H H
H HC (s)
C (s)
= 2 x 346 = 692
+
Bond Breaking
3 x
H H4 x = 1728
Total put in = 3873 kJ
C(s) C(g) = 2145 2
C C CC CC C
8
CH C
H
H
H
H
C H
H
H
CH C
H
C HC
H
C
H
C
H
C
H
C
H
Example 2Calculate the enthalpy of formation of propane.
3C (s) + 4H2 (g) C3H8 (g)
C (s)
Bond Making
C C2 x
C H8 x = 8 x 414 = 3312
Total given out = –4004 kJ
H H
H HCH C
H
H
H
H
C H
H
H
H H
H HC (s)
C (s)
= 2 x 346 = 692
+
Bond Breaking
3 x
H H4 x = 1728
Total put in = 3873 kJ
C(s) C(g) = 2145
∆H = 3873 + (–4004)= 3873 – 4004= – 131 kJ mol-1
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