3:07 (Triple only) use bond energies to calculate the enthalpy change during a chemical reaction

3:07 (Triple only) use bond energies to calculate the enthalpy change during a chemical reaction

Each type of chemical bond has a particular bond energy. The bond energy can vary slightly depending what compound the bond is in, therefore average bond energies are used to calculate the change in heat (enthalpy change, ΔH) of a reaction.

Example: dehydration of ethanol

Note: bond energy tables will always be given in the exam, e.g:

BondAverage bond energy in kJ/mol
H-C412
C-C348
O-H463
C-O360
C=C612

So the enthalpy change in this example can be calculated as follows:

Breaking bondsMaking bonds
BondsEnergy (kJ/mol)BondsEnergy (kJ/mol)
H-C x 5(412 x 5) = 2060C-H x 4(412 x 4) = 1648
C-C348C=C612
C-O360O-H x 2(463 x 2) = 926
O-H463
Energy needed to break all the bonds3231Energy released to make all the new bonds3186

Enthalpy change, ΔH = Energy needed to break all the bonds - Energy released to make all the new bonds

ΔH = 3231 – 3186 = +45 kJ/mol (ΔH is positive so the reaction is endothermic)

Select a set of flashcards to study:

     Terminology

     Skills and equipment

     Remove Flashcards

Section 1: Principles of chemistry

      a) States of matter

      b) Atoms

      c) Atomic structure

     d) Relative formula masses and molar volumes of gases

     e) Chemical formulae and chemical equations

     f) Ionic compounds

     g) Covalent substances

     h) Metallic crystals

     i) Electrolysis

 Section 2: Chemistry of the elements

     a) The Periodic Table

     b) Group 1 elements: lithium, sodium and potassium

     c) Group 7 elements: chlorine, bromine and iodine

     d) Oxygen and oxides

     e) Hydrogen and water

     f) Reactivity series

     g) Tests for ions and gases

Section 3: Organic chemistry

     a) Introduction

     b) Alkanes

     c) Alkenes

     d) Ethanol

Section 4: Physical chemistry

     a) Acids, alkalis and salts

     b) Energetics

     c) Rates of reaction

     d) Equilibria

Section 5: Chemistry in industry

     a) Extraction and uses of metals

     b) Crude oil

     c) Synthetic polymers

     d) The industrial manufacture of chemicals