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. CC put 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. HH2HH = 432 kJ HH2HH = –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 (C 2 H 6 ) will be different to the energy needed to break a bond in decane (C 10 H 22 ) CC CC

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? C 2 H 2 (g) + 2H 2 (g)C 2 H 6 (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. C 2 H 2 (g) + 2H 2 (g)C 2 H 6 (g) Reaction pathway Potential energy C 2 H 2 (g) + 2H 2 (g)C 2 H 6 (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? C 2 H 2 (g) + 2H 2 (g)C 2 H 6 (g) Step One: Draw the full structural formulae of all the molecules from the equation. This will show exactly what bonds are involved. + CHCH HH HH CHCH 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? C 2 H 2 (g) + 2H 2 (g)C 2 H 6 (g) + CHCH HH HH CHCH H H H H Step Two: Make a list of all the bonds being broken in the reactants CHCH Bond Breaking CC1 x CH2 x HH CC CC1 x CH2 x HH HH

What is the enthalpy change when hydrogen is added to ethyne, producing ethane? C 2 H 2 (g) + 2H 2 (g)C 2 H 6 (g) + CHCH HH HH CHCH H H H H Step Three: Fill in the values for the bond enthalpies from Page 9 of the data book. Bond Breaking CC1 x CH2 x HH = 835 = 2 x 414 = 2 x 432 = 828 = 864

What is the enthalpy change when hydrogen is added to ethyne, producing ethane? C 2 H 2 (g) + 2H 2 (g)C 2 H 6 (g) + CHCH HH HH CHCH H H H H Step Four: Repeat this process for the Bond Making steps. Bond Breaking CC1 x CH2 x HH = 835 = 2 x 414 = 2 x 432 = 828 = 864 Bond Making CC1 x CH6 x = 346 = 6 x 414= 2484 CC CHCH H H H H

What is the enthalpy change when hydrogen is added to ethyne, producing ethane? C 2 H 2 (g) + 2H 2 (g)C 2 H 6 (g) + CHCH HH HH CHCH H H H H Step Five: Calculate the total energy put in breaking bonds and total energy given out making new bonds. Bond Breaking CC1 x CH2 x HH = 835 = 2 x 414 = 2 x 432 = 828 = 864 Bond Making CC1 x CH6 x = 346 = 6 x 414= 2484 Total put in = 2527 kJTotal given out = –2830 kJ Remember: 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? C 2 H 2 (g) + 2H 2 (g)C 2 H 6 (g) + CHCH HH HH CHCH H H H H Step Six: Calculate the enthalpy change for the reaction. Bond Breaking CC1 x CH2 x HH = 835 = 2 x 414 = 2 x 432 = 828 = 864 Bond Making CC1 x CH6 x = 346 = 6 x 414= 2484 Total put in = 2527 kJTotal 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) + 2H 2 (g)CH 4 (g) The data book quotes the value for this reaction asH formation = –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) + 4H 2 (g)C 3 H 8 (g) C (s) HH HH CHC H H H H CH H H HH HH +

Example 2 Calculate the enthalpy of formation of propane. 3C (s) + 4H 2 (g)C 3 H 8 (g) C (s) Bond Breaking 3 x HH4 x= 4 x 432 = 3 x 715 = 1728 Total put in = 3873 kJ HH HH CHC H H H H CH H H HH HH C (s) C(s) C(g) + = 2145 C (s) 3 4 HH HH HH HH

Example 2 Calculate the enthalpy of formation of propane. 3C (s) + 4H 2 (g)C 3 H 8 (g) C (s) Bond Breaking 3 x HH4 x= 4 x 432 = 3 x 715 = 1728 Total put in = 3873 kJ HH HH CHC H H H H CH H H HH HH C (s) C(s) C(g) + = 2145

Example 2 Calculate the enthalpy of formation of propane. 3C (s) + 4H 2 (g)C 3 H 8 (g) C (s) Bond Making CC2 x CH8 x= 8 x 414= 3312 Total given out = –4004 kJ HH HH CHC H H H H CH H H HH HH C (s) = 2 x 346= Bond Breaking 3 x HH4 x= 1728 Total put in = 3873 kJ C(s) C(g) = CCCCCCC 8 CHC H H H H CH H H CH C H CHC H C H C H C H C H

Example 2 Calculate the enthalpy of formation of propane. 3C (s) + 4H 2 (g)C 3 H 8 (g) C (s) Bond Making CC2 x CH8 x= 8 x 414= 3312 Total given out = –4004 kJ HH HH CHC H H H H CH H H HH HH C (s) = 2 x 346= Bond Breaking 3 x HH4 x= 1728 Total put in = 3873 kJ C(s) C(g) = 2145 H= 3873+(–4004) = 3873–4004 = – 131 kJ mol -1