Chemistry, Third edition

Nutrition

Energy value of food

Table 13.3 includes the energy value of common foodstuffs, since these can be

thought of as ‘human fuel’. If 1 g of apple were completely burned in a flame, it

would give out about 2 kJ of heat energy. Energy is also produced when foods are

digested in the body, but there are three important differences between the break-

down of foodstuffs (more properly called food metabolism) and the burning of a fuel

in a flame:

1.In the human body, foodstuffs are consumed slowly in a complicated series of

chemical reactions. About 40% of the enthalpy change of combustion of a food is

available to the body to be used in exercise.

2.Oxygen gas is not used as the oxidizing agent in the metabolism of food. The oxi-

dizing agent used in the cell is called nicotinamide adenine dinucleotide(NAD).

3.Some of the energy released in the metabolism of foods is not given out as heat, but

is used to make a substance called adenosine triphosphate(ATP). ATP may be con-

verted to adenosine diphosphate(ADP) when required, and it is the ATP ADP

reaction that produces the energy upon which we depend for the synthesis of bio-

logically important molecules, for the transport of vital nutrients within the body

and for muscle action (see Appendix 13 in the website).

Measuring energy use during bodily exercise

The metabolism of fats and glucose during prolonged exercise requires oxygen and

produces carbon dioxide and water. The energy consumption of an individual during

exercise can be estimated by measuring the ratio of the volume of CO 2 produced to the

volume of O 2 used up. This ratio is called the respiratory exchange ratio(RER):

RER

volume of CO 2 produced per minute

volume of O 2 consumed per minute

Experimentally, the RER is obtained by analysis of the inhaled and exhaled air of the

person exercising.

13.7

NUTRITION 231

Energy from fuels

(i)Given the thermochemical equation

C 12 H 22 O 11 (s)12O 2 (g) 12CO 2 (g)11H 2 O(l)

H^ —c 5644 kJ mol^1

calculate the energy value of sucrose in kJ g^1.

(ii)From Table 13.3, work out the volume of hydrogen gas which, when burned completely,

produces 100 kJ of heat (assume the molar volume of a gas at room temperature and

pressure is 24 dm^3 ).

Exercise 13I

Nutrition

Diet plans often tabulate

the energy value of food in

kilocalories (symbolized

Cal) with 1 Cal 4.18 kJ. A

20 g bar of chocolate

possessed a calorific value

of 120 Cal. What is the

energy value of the

chocolate, in kJ g^1?

Exercise 13J

For more about

energy and nutrition

see Appendix 13 on

the website.