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Energy in food

Energy is measured in joules.

One joule is the energy needed to lift a weight of one newton through one metre.

A small apple has a weight of about 1 N. How many apples would you need to lift through one metre to use up all the energy in this chocolate bar?

Energy (kJ) 1075Energy (kcal) 255Protein 3.7 gCarbohydrate 27.9 gFat 14.6 gFibre 0.3 gSodium 0.04 g

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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) 295Protein 16 gCarbohydrate 31 gFat 12 gFibre 2 gSodium 1.5 g

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Higher or lower?

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Age 2-10 Age 11-15Age 2-15

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

10

20

30

Levels of obesity in many countries around the world are on the increase. It is a modern problem, with a complex mixture of causes, including increasing car use, less active lifestyles and the popularity of convenience foods.

Obesity is linked to increased rates of heart disease, diabetes and some forms of cancer.

Food and obesity

A particular concern is the increasing prevalence of obesity among children.

%

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The obesity debate

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Treatments for obesity

The recommended treatment for obesity is a reduction in calorie intake and increased exercise. Medical treatments have been developed that work alongside these measures.

The treatments act in different ways:

Blocking body enzymes which digest fat. The undigested fat is not absorbed by the body.

Affecting chemicals in the brain which control appetite.

Gastric banding is a surgical procedure in which a band is fitted around the upper part of the stomach. It reduces the amount of food that can be held in the stomach.

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The obesity debate

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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 temperature change.

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The energy content of food is found using this equation:

Specific heat capacity is a measure of how much energy it takes to heat up a given mass of a substance.

It takes 4.18 J of energy to heat up 1 cm3 of water by 1 °C.

Using calorimetry

Q = energy given out by burning food (J)

m = mass of water heated by burning food (g)

c = specific heat capacity of water = 4.18 Jg-1°C-1

T = change in temperature of the water (°C)

Q = mcT

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Performing a calorimetry experiment

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The 1.5 g crisp that was burned gave out 33 kJ of energy.

Finding the energy content of food

Energy per gram = 33 / 1.5

= 22 kJ/g

How likely do you think it would be to obtain this answer experimentally?

How much energy was given out per gram of crisp?

The mass of crisp that burned was 1.5 g, so to find the amount of energy per gram, divide the energy given out by the mass that burned:

One bag of crisps has a mass of 25 g.

How much energy does one bag of crisps contain?

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Calorimetry calculations

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Comparing the actual amounts of energy in food with the values found in calorimetry experiments, the experimental values are often a lot lower than the actual values.

Errors in calorimetry

Calorimetry experiments are not very accurate. A lot of the energy released by the burning food is not transferred into the water. It is simply lost into the atmosphere.

How could the experiment be made more accurate?

Why is this?

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To reduce inaccuracies in calorimetry, a bomb calorimeter is used.

The bomb calorimeter

The heat energy is trapped inside a sealed, insulated container to reduce heat loss to the surroundings.

The water is stirred continuously to make sure the heat energy is dispersed evenly throughout.

thermometermotorized stirrer

ignition coil

water bath

insulated jacket

system (combustible material and compressed oxygen)

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A fuel is a substance that is burned to release useful energy.

Is burning a fuel endothermic or exothermic?

Fuels

The amount of energy in a fuel can be found in the same way as the amount of energy in food.

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Energy content of fuels

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Alcohols like ethanol and propanol can be used as fuels.

EthanolC2H5OH

PropanolC3H7OH

C O

H

H

C

H

H

H

H

C O

H

H

C

H

H

C

H

H

H

H

Alcohols as fuels

Many biofuels are alcohols collected from fermented plant matter, such as sugar cane grown specially for the purpose. Biofuels are most commonly used to power cars, heat homes and to run cooking stoves.

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Alcohols as fuels

Alcohols have a variety of uses as fuels:

biofuels: ethanol and butanol

methylated spirits burners: methanol

cooking: ethanol

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Alcohols as fuels

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The reaction which takes place in a hydrogen fuel cell is:

2H2 + O2 2H2O

The only waste product is water vapour.

Hydrogen fuel cells produce electricity through the reaction of hydrogen with oxygen.

Hydrogen fuel cell

Hydrogen fuel cells do not produce other pollutants like carbon dioxide, sulfur dioxide or carbon particles.However, water vapour is itself a greenhouse gas and could contribute to global warming.

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How does a hydrogen fuel cell work?

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Calculating the energy

432

498

463

H–H

O=O

O–H

Bond Bond Energy (kJ)

Bond energies can be used to calculate the amount of energy given out by the reaction in a hydrogen fuel cell. Energy is taken in to break bonds, and released when bonds are created.

energy for bond-breaking= 2H–H + O=O= 2 × 432 + 498= 1362 kJ

= 4H–O= 4 × 463= 1852 kJ

energy from bond-making

= energy in – energy out= 1362 kJ – 1852 kJ = –490 kJ

total energy change

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Energy level diagram

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At the moment the hydrogen for fuel cells is extracted from crude oil.

It may be possible to use solar power to extract hydrogen from water, but this method is still under development.

What are the advantages and disadvantages of each method of making hydrogen?

Where does the hydrogen come from?

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Hydrogen has a low density, making vehicles light and efficient.

Hydrogen gives out more energy per gram than conventional fuels.

Advantages of fuel cells

The big advantage of hydrogen fuel cells is that the only emission they produce is water vapour.

What are the other advantages?

Production of hydrogen from fossil fuels produces carbon dioxide – a greenhouse gas.

Hydrogen is highly flammable, making it risky to store.

Hydrogen is not cheap!

What are some of the disadvantages of fuel cell technology?

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Fuel cells activity

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Glossary

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Anagrams

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Multiple-choice quiz