Energy in food
Energy is measured in joules.
One joule is the energy needed to lift a weight of one newton (N) one metre.
A small apple has a weight of about 1N. How many apples would you need to lift one metre to use up all the energy in this chocolate bar?
Energy (kJ) 1075 Energy (kcal) 255 Protein 3.7g
Carbohydrate 27.9g Fat 14.6g
Fibre 0.3g
Energy in food
Energy in food is usually measured in calories. This comes from calor, the Latin word for heat.
1 calorie = 4.2 joules
1 kilocalorie (kcal) = 1000 calories
How many joules of energy does this cheeseburger contain?
Energy (kJ) ?
Energy (kcal) 295 Protein 16g
Carbohydrate 31g Fat 12g
Fibre 2g
Calorimetry involves burning a known mass of food and
measuring the energy released.
Calorimetry
The energy content of food is measured using a technique called calorimetry.
Knowing the temperature change and the specific heat capacity of water, it is possible to calculate the energy released by the burning food.
This is done by transferring the energy to a measured volume of water and working out the
Making and breaking chemical bonds
Most chemicals will break up (decompose) if they are
heated strongly enough. This means that energy is needed to break chemical bonds – an endothermic process.
energy absorbed
Because bond-breaking is endothermic, bond-making must therefore be exothermic. This means that energy is
released when chemical bonds are made.
All reactions need a certain amount of energy to get started. This is called the activation energy (Ea).
Activation energy is needed to start breaking the bonds of the reactants. In most chemical reactions, some existing bonds need to be broken (an endothermic process) before new bonds can be made (an exothermic process).
What is activation energy?
In some reactions, the bonds are easily broken and a low activation energy is needed; for example, the
reaction between sodium hydroxide and water starts at room temperature.
In other reactions, the bonds are strong and not easily broken.
These reactions have a high activation energy.
Do different reactions need different E
a?
An example is the
Bond energies
The energy changes in a reaction can be calculated from the bond energies of the reactants and the products.
The amount of energy needed to break or make a bond is called the bond energy.
Different chemical bonds have different bond energies. For example:
432 240 428
H–H
Cl–Cl
H–Cl
bond bond
energy (kJ)
Bond energies are given per mole.
en er g y (k J)
What are catalysts?
Catalysts are substances that change the rate of a reaction without being used up in the reaction.
Catalysts never produce more product – they just produce the same amount more quickly.
Different catalysts work in different ways, but most lower the reaction’s
activation energy (Ea).
Catalysts in industry
Some industrial processes, like making paper, use biological catalysts called enzymes.
Why are catalysts so important for industry?
Products can be made more quickly, saving time and money.
Catalysts reduce the need for high temperatures, saving fuel and reducing pollution.
