Chemistry, Third edition

CONSERVATION OF ENERGY 217

Standard enthalpy changes of reaction (H^

—

)

The enthalpy change of a reaction when the reactants and products are in their

standard statesis called the standard enthalpy change of reaction,H^ —(with the

superscript— sign being read as ‘standard’):

H—^ HP—^ HR—^ (13.2)

What is a standard state? A substance is said to be in its standard state when it

is in its pure form at an atmospheric pressure of 1 atm(101 kPa). Temperature is

not part of the definition of a standard state and (for example) the standard state of

iron is pure iron at a pressure of 1 atm whatever its temperature, although it happens

that at 25 °C the iron would be a solid and at 1600 °C the iron would be a liquid.

The idea of a standard state is equally applicable to compounds. For instance, the

standard state of sodium chloride would be pure sodium chloride at a pressure of 1 atm.

Where elements may exist in several allotropic forms, one form is chosen as the

reference state. For example, carbon exists in two common forms, graphite and

diamond, but graphite has been chosen as the reference state for the element carbon.

Similarly dioxygen (O 2 , but not ozone (O 3 )), has been chosen as the reference state

for the element oxygen. Usually, reference states are the most stable forms of an ele-

ment. An exception to this rule is that white phosphorus (and not the more stable

red phosphorus), has been chosen as the reference state for the element phosphorus.

It is most common for standard enthalpy changes to be measured at 25 °C (more

precisely, at 298.15 K), or else for standard enthalpy changes obtained at other

temperatures to be theoretically corrected to 25 °C. Such enthalpy changes are repre-

sented by the notation H—^ (298 K). Unless otherwise stated, we will assume that all

standard enthalpy changes refer to a reaction where the temperature at the begin-

BOX 13.1

Cooling packs

Instant ice packsare used to cool food and also to numb sports injuries on the field of play.

The ice packs do not require refrigeration and can be activated simply by striking them with a

fist.

The ice packs consist of a thin plastic bag containing water, which is contained in a larger

outer bag containing ammonium nitrate. When the pack is thumped, the bag containing the

water breaks and the ammonium nitrate dissolves in the water. Ammonium nitrate is used

because when it dissolves, heat is taken in

NH 4 NO 3 (s) NH 4 (aq)NO 3 (aq) H

26 kJ mol^1

A mixture containing 50% (by mass) of water and ammonium nitrate reaches a temperature of

about15 °C.

Enthalpy change

A few reactions, such as the decomposition of silver chlorate(III) upon heating,

2AgClO 2 (s) 2Ag(s)Cl 2 (g)2O 2 (g) H 

0

involve changes in enthalpies which are so near zero that they cannot be measured. Sketch

an enthalpy diagram (following the pattern in Fig. 13.1) for such a reaction.

Exercise 13A

The burning of acetylene

(HCCH) in oxygen produces

so much heat that part of the

flame reaches 3000 °C. This

is high enough to melt most

metals (iron melts at 1535 °C)

and is used to melt metals

together, an operation known

as gas welding.