Unit 2, Module 3, Energy

Bond EnthalpiesWednesday 19 April 2023

Objectives - Enthalpy

2.3.1 Enthalpy Changes

Candidates should be able to:

• Bond Enthalpies• (h) explain exothermic and endothermic reactions

in terms of enthalpy changes associated with the breaking and making of chemical bonds;

• (i) define and use the term average bond enthalpy (ΔH positive; bond breaking of one mole of bonds);

• (j) calculate an enthalpy change of reaction from average bond enthalpies;

Homework

• Energy exam questions

• Some of the questions will be from Monday’s lesson

Bond Enthalpy• Energy is stored in chemical bonds.• It takes energy to break a chemical bond• Energy is released when we form a

chemical bond.

Definition (on worksheet)Bond enthalpy is the enthalpy change that takes place when breaking by homolytic fission one mole of a given bond in the molecules of a gaseous species.

Bond Enthalpy• It takes energy to break bonds so

breaking bonds is ___________• Energy is given out when making

bonds so making bonds is __________

• e.g. H-H(g) 2H(g) ΔH = +436 kJ.mol-1

exothermic

endothermic

DefinitionBond enthalpy is the enthalpy change that takes place when breaking by homolytic fission one mole of a given bond in the molecules of a gaseous species.

• Look at the diagrams.• What do you notice?

Average Bond Enthalpy• H-H and H-Cl occur only in H2 and HCl

molecules respectively.

• However, most bonds can occur in different molecules. E.g. The C-H bond can occur in a huge variety of different molecules.

• The bond strength (enthalpy) will vary depending on what else is attached, and where the bond is in the molecule.

• For this reason we usually report average bond enthalpy. This is the average of enthalpies for that bond in a number of chemical species.

Definition (on worksheet)

Average bond enthalpy is the average enthalpy change that takes place when breaking by homolytic fission one mole of a given type of bond in the molecules of a gaseous species.

What happens in chemical reactions?

• In a chemical reaction we must first break bonds and then make some different ones.

• This is an endothermic process followed by an exothermic process.

• So how can we describe a reaction as exothermic or endothermic?

What happens in chemical reactions?

• So how can we describe a reaction as exothermic or endothermic?

• It depends which bonds are stronger.• If the bonds that are formed are

stronger then the reaction is _______• If the bonds that are broken are

stronger then the reaction is _______

exothermic

endothermic

Using bond enthalpies• We can use bond enthalpies to work

out the overall energy change for a reaction.

• The overall energy change is the total amount of energy used to break bonds minus the energy released by the bonds formed.

ΔH = Σ(enthalpy of bond broken) - Σ (enthalpy of bond made)

ExampleCH4(g) + 2O2(g) CO2(g) + 2H2O (g)

What bonds do we have in the reactants?How many of each?

(draw out the molecules to be sure)

These bonds all get broken.

What bonds do we have in the products?How many of each?

These bond all get formed.

Example

CH4(g) + 2O2(g) CO2(g) + 2H2O (g)

Average bond enthalpies:• C-H +413 kJ.mol-1

• O=O +497 kJ.mol-1

• C=O +805 kJ.mol-1

• O-H +463 kJ.mol-1

Example

CH4(g) + 2O2(g) CO2(g) + 2H2O (g)

We break:4 x C-H2 x O=O

We form:2 x C=O4 x H-O

What is the total enthalpy change?

Practice• Complete “using bond energy data

practice” on the worksheet.

• This is broken down into the different stages

• Follow the instructions• Don’t forget to complete the extension

More practice questions• Answer the rest of the questions on the

worksheet• Questions 1 and 2 a, b, c are from page

197 in the OCR book

• Follow the same steps from the worked example and the previous question– Draw the molecules out.– Include units– Include +/- sign

Objectives - Enthalpy

2.3.1 Enthalpy Changes

Candidates should be able to:

• Bond Enthalpies• (h) explain exothermic and endothermic reactions

in terms of enthalpy changes associated with the breaking and making of chemical bonds;

• (i) define and use the term average bond enthalpy (ΔH positive; bond breaking of one mole of bonds);

• (j) calculate an enthalpy change of reaction from average bond enthalpies;