Chemistry, Third edition

Energy Changes in

Chemical Reactions

Objectives

Defines different types of standard enthalpy change

Explains Hess’s law

Introduces calculations involving enthalpy changes

Shows how enthalpy changes may be measured

Discusses fuels, nutrition and explosives

Conservation of energy

Following the experiments of J. P. Joule (1818–1889), it was concluded that energy is

neither created or destroyed but is merely converted from one form to another. For

example, a dynamo converts mechanical energy to electrical energy. If the dynamo

does 100 J of work, then the sum of the electrical energy produced and the energy

lost as friction also equals 100 J. This preservation (or ‘conservation’) of the total

amount of energy is called the Law of Conservation of Energy.

In a chemical reaction, new substances are made. The Law of Conservation of

Energy tells us that the total energy of the reactants must equal the total energy of the

products and any energy lost to (or gained from) the surroundings:

total energy of reactantstotal energy of productsenergy lost (or gained)

Enthalpy

The energy of a substance under constant atmospheric pressure (such as in an open

test tube or beaker) is called its enthalpy(symbol,H). The enthalpy of elements and

compounds cannot be measured or calculated, but differences in enthalpy (enthalpy

changes), symbolised H, are easily measured in the laboratory. If a chemical or

physical change is carried out at constant pressure, the amount of heat energy

absorbed or produced in the change equals the change in enthalpy that has taken

place.

13.1

Contents

13.1Conservation of

energy 215

13.2Key points about

enthalpy changes 218

13.3Determination of

Hin the

laboratory 222

13.4Special kinds of

standard enthalpy

change 225

13.5Standard enthalpy

of formation 226

13.6Standard enthalpy

of combustion 230

13.7Nutrition 231

13.8Lattice enthalpy 233

13.9Energetics of bond

breaking and bond

making 236

13.9Revision questions 239

13

UNIT