Chemistry (14 - 16)

Chemical Reactions & Energy Changes

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From very early times we have burnt fuels to keep warm and to cook our food.

Everyday we use energy in our homes, schools and work places.

‘a substance which burns in air to produce heat’

A fuel can be defined as...

Three of the most important fuels are...

What is a Fuel?

At home we burn fuels to provide warmth and to cook our food. We also use fuels to generate electricity, power motor vehicles and create new materials in a whole range of different industries.

coal...

oil...

and natural gas.

All fuels contain the elements carbon and hydrogen. When a fuel is burnt, the carbon and hydrogen combine with oxygen (in the air) to produce carbon dioxide, water and energy (mostly heat).

Fuels

CH4(g) + 2O2(g) ⇒ CO2(g) + 2H2O(g) + energy

fuel + oxygen ⇒ carbon dioxide + water + energy

The burning of fuels in air is called combustion.

As heat energy is given out, this is an exothermic reaction.

Living things all carry out a process called respiration, whereby food (equivalent to the fuel in combustion), is used to produce energy. Oxygen is usually present and carbon dioxide and water are also produced...

Every day, the world requires energy equivalent to approximately 200 million barrels of oil.

The World’s Energy Needs

The World’s energy is provided by a range of fuels and other energy sources, as shown by the following pie chart.

The World’s Energy Needs

Source: United Nations Development Programme, 2000

The World’s energy is provided by a range of fuels and other energy sources, as shown by the following pie chart.

The World’s Energy Needs

Every year, the United Kingdom requires energy equivalent to that provided by over 300 million tonnes of coal.

The World’s Energy Needs

Fuels For Living Things

C6H12O6 + 6O2 ⇒ 6CO2 + 6H2O + energy

food + oxygen ⇒ carbon dioxide + water + heat

When humans inhale, air enters the lungs. From the lungs the oxygen diffuses into the blood and passes around the body. All of the body’s cells are thereby supplied with oxygen and this enables them to carry out the chemical reaction known as respiration. In respiration the actual fuel is sugar - even fats and oils have to be converted into sugars before energy can be derived from them:

The foods we eat contain carbon and hydrogen in a similar way to coal, oil and natural gas - foods are the fuel for respiration:

sugar + oxygen ⇒ carbon dioxide + water + energy

Measuring the Energy in Food and Fuels

Energy is measured in units called joules (J) or kilojoules (kJ).

Dietary information, about energy, on food packaging is given in units called calories (cal) or kilocalories (kcal).

One calorie is equal to 4.2 joules.

The energy content of foods and fuels can be worked out using a device called a calorimeter.

§  20 cm3 (20g) of water is placed into a boiling tube and clamped.

§  The mass of the pasta is recorded. §  The initial temperature of the

water (ºC) is recorded. §  The pasta is held in a bunsen flame

until it burns and then supported beneath the boiling tube.

§  The water is stirred, and when the pasta has finished burning, the final water temperature is recorded.

A simple method for calculating the amount of chemical potential energy in a food or fuel sample is to burn a small amount and to heat a known mass of water.

stirrer

thermometer

boiling tube

20 cm3 water

pasta plasticine

cotton wool plug

Measuring Energy Content Using a Calorimeter

Pasta is a good food to use in the laboratory, e.g.

Energy(J) = water mass (g) x temp rise (ºC) x 4.2

0.2g of pasta is used to heat 20 cm3 water.

25ºC

Energy(J) = 20 g x 25ºC x 4.2

Energy(J) = 2,100 J Energy(kJ) = 2.1 kJ

Starting temp = 20ºC Final temp = 45ºC Temperature Rise =

Energy value of pasta = 2.1kJ (from 0.2g pasta) = 10.5kJ/g = 1050kJ/100g)

(N.B. The kJ value is normally expressed/100g)

Measuring Energy Content Using a Calorimeter

Results:

Observe the features of the experiment and suggest sources of error and improvements to the method.

Measuring Energy Content Using a Calorimeter

§  Heat loss around the edge of the test tube §  Heat used in warming up the glass of the tube §  The pasta does not burn completely (incomplete combustion) §  Heat losses occurring from the water through the glass and

cotton wool plug

Sources of Error

Most of these errors can be eliminated by measuring heat energy with a proper calorimeter (bomb calorimeter).

The energy content of food is determined by placing a known dry mass of food in an atmosphere of pure oxygen

within the calorimeter. The material is ignited, and the heat produced is passed entirely to a known mass of

water, whose rise in temperature is recorded.

The Bomb Calorimeter

stirrer (to mix water and keep the temperature uniform)

lid

measured volume of water

inner glass chamber (combustion takes place here)

heating coil

asbestos base

pure oxygen

waste gas outlet

weighed food/fuel sample in crucible

oxygen atmosphere for complete combustion

thermometer

coiled copper tube (transfers heat to water)

A more accurate calorimeter is shown below:

The following results were calculated by using a calorimeter...

These results show that crisps release the largest amount of energy - this energy is mostly provided by the fats in the food.

Food Energy kJ/100g Crisps 2600 Pasta 1470

Crackers 2116

Bread sticks 1280

Cereal 1400

Measuring Energy Content Using a Calorimeter

Eating too many fatty foods...

…can lead to obesity.

Energy Intake Too High?