Principles to Production: Chemical Energy Bond Enthalpies

Breaking bonds If we want to break a covalent bond between two atoms, we need to to overcome the attractive force. put energy in put energy in C C © Nueyer Bond breaking is an endothermic process.

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 2H ∆H = 432 kJ 2H H ∆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

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.

This will require energy to be put in. C2H2 (g) + 2H2 (g) C2H2 (g) + 2H2 (g) C2H6 (g) C2H6 (g) In this reaction, we must first break all the bonds inside the reactant molecules. This will require energy to be put in. Next, new bonds must be formed between the atoms in the product molecule. This releases energy. Potential energy Reaction pathway

What is the enthalpy change when hydrogen is added to ethyne, producing ethane? C2H2 (g) + 2H2 (g) C2H6 (g) 2 C H H C H + H To answer this question we can follow these steps. Step One: Draw the full structural formulae of all the molecules from the equation. This will show exactly what bonds are involved.

What is the enthalpy change when hydrogen is added to ethyne, producing ethane? C2H2 (g) + 2H2 (g) C2H6 (g) + C H H C H C C H Bond Breaking C 1 x C 1 x C H 2 x C H 2 x H 2 x Step Two: Make a list of all the bonds being broken in the reactants

What is the enthalpy change when hydrogen is added to ethyne, producing ethane? C2H2 (g) + 2H2 (g) C2H6 (g) + C H Bond Breaking 1 x C C = 835 2 x C H = 2 x 414 = 828 2 x H H = 2 x 432 = 864 Step Three: Fill in the values for the bond enthalpies from Page 9 of the data book.

What is the enthalpy change when hydrogen is added to ethyne, producing ethane? C2H2 (g) + 2H2 (g) C2H6 (g) + C H C H C Bond Breaking Bond Making 1 x C C = 835 C 1 x = 346 2 x C H = 2 x 414 = 828 C H 6 x = 6 x 414 = 2484 2 x H H = 2 x 432 = 864 Step Four: Repeat this process for the Bond Making steps.

What is the enthalpy change when hydrogen is added to ethyne, producing ethane? C2H2 (g) + 2H2 (g) C2H6 (g) + C H Bond Breaking Bond Making 1 x C C = 835 C 1 x = 346 2 x C H = 2 x 414 = 828 C H 6 x = 6 x 414 = 2484 2 x H H = 2 x 432 = 864 Total put in = 2527 kJ Step Five: Calculate the total energy put in breaking bonds and total energy given out making new bonds. Total 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? C2H2 (g) + 2H2 (g) C2H6 (g) + C H Bond Breaking Bond Making 1 x C C = 835 C 1 x = 346 2 x C H = 2 x 414 = 828 C H 6 x = 6 x 414 = 2484 2 x H H = 2 x 432 = 864 Total put in = 2527 kJ Total given out = –2830 kJ ∆H = 2527 + (–2380) Step Six: Calculate the enthalpy change for the reaction. = 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. For example, the enthalpy of formation of methane can be represented by: C (s) + 2H2 (g) CH4 (g) The data book quotes the value for this reaction as ∆Hformation = –75 kJ mol–1

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 C H C (s) Calculate the enthalpy of formation of propane. H + C (s) H H 3C (s) + 4H2 (g) C3H8 (g)

Example 2 Calculate the enthalpy of formation of propane. 3C (s) + 4H2 (g) C3H8 (g) C H C (s) C (s) H H H C (s) + C (s) H H Bond Breaking 3 3 x C(s) C(g) = 3 x 715 = 2145 4 x 4 H H = 4 x 432 = 1728 Total put in = 3873 kJ

Example 2 Calculate the enthalpy of formation of propane. 3C (s) + 4H2 (g) C3H8 (g) C H C (s) H H C (s) + C (s) H H Bond Breaking 3 x C(s) C(g) = 3 x 715 = 2145 4 x H H = 4 x 432 = 1728 Total put in = 3873 kJ

Example 2 Calculate the enthalpy of formation of propane. 3C (s) + 4H2 (g) C3H8 (g) C H C H C H C H C H C (s) H H C (s) + C H C C C H C C H C H C H C (s) H H Bond Breaking Bond Making 3 x C(s) C(g) = 2145 C 2 x 2 = 2 x 346 = 692 4 x H H = 1728 8 C H 8 x = 8 x 414 = 3312 Total put in = 3873 kJ Total given out = –4004 kJ

Example 2 Calculate the enthalpy of formation of propane. 3C (s) + 4H2 (g) C3H8 (g) C H C (s) H H C (s) + C (s) H H Bond Breaking Bond Making 3 x C(s) C(g) = 2145 C 2 x = 2 x 346 = 692 4 x H H = 1728 C H 8 x = 8 x 414 = 3312 Total put in = 3873 kJ Total given out = –4004 kJ ∆H = 3873 + (–4004) = 3873 – 4004 = – 131 kJ mol-1