Download Food and Fuels

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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) 1075
Energy (kcal) 255
Protein 3.7 g
Carbohydrate 27.9 g
Fat 14.6 g
Fibre 0.3 g
Sodium 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) 295
Protein 16 g
Carbohydrate 31 g
Fat 12 g
Fibre 2 g
Sodium 1.5 g
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Higher or lower?
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Food and obesity
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.
A particular concern
is the increasing
prevalence of obesity
among children.
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30
20
%
10
Age 2-10
Age 2-15
Age 11-15
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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
The energy content of food is measured using a technique
called calorimetry.
Calorimetry involves burning a
known mass of food and
measuring the energy released.
This is done by transferring the
energy to a measured volume of
water and working out the
temperature change.
Knowing the temperature change and the specific heat
capacity of water, it is possible to calculate the energy
released by the burning food.
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Using calorimetry
The energy content of food is found using this equation:
Q = mcT
 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)
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.
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Performing a calorimetry experiment
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Finding the energy content of food
The 1.5 g crisp that was burned gave out 33 kJ of energy.
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:
Energy per gram = 33 / 1.5
= 22 kJ/g
One bag of crisps has a mass of 25 g.
How much energy does one bag of
crisps contain?
How likely do you think it would be to
obtain this answer experimentally?
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Calorimetry calculations
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Errors in calorimetry
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.
Why is this?
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?
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The bomb calorimeter
To reduce inaccuracies
in calorimetry, a bomb
calorimeter is used.
thermometer
motorized stirrer
ignition coil
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.
system (combustible
material and
compressed oxygen)
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water bath
insulated jacket
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Fuels
A fuel is a substance that is
burned to release useful energy.
The amount of energy in a
fuel can be found in the
same way as the amount of
energy in food.
Is burning a fuel
endothermic or exothermic?
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Energy content of fuels
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Alcohols as fuels
Alcohols like ethanol and propanol can be used as fuels.
H
H
H
C
C
H
H
Ethanol
C2H5OH
H
O
H
H
H
H
C
C
C
H
H
H
O
H
Propanol
C3H7OH
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
cooking: ethanol
methylated spirits burners: methanol
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Alcohols as fuels
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Hydrogen fuel cell
Hydrogen fuel cells produce electricity through the reaction
of hydrogen with oxygen.
The reaction which takes
place in a hydrogen fuel
cell is:
2H2 + O2  2H2O
The only waste product is
water vapour.
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
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.
Bond
Bond Energy (kJ)
H–H
432
O=O
498
O–H
463
energy for bond-breaking energy from bond-making
= 2H–H + O=O
= 4H–O
= 2 × 432 + 498
= 4 × 463
= 1362 kJ
= 1852 kJ
total energy change = energy in – energy out
= 1362 kJ – 1852 kJ = –490 kJ
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Energy level diagram
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Where does the hydrogen come from?
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?
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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?
 Hydrogen has a low density, making
vehicles light and efficient.
 Hydrogen gives out more energy per gram than
conventional fuels.
What are some of the disadvantages of fuel cell technology?
 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!
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Fuel cells activity
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Glossary
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Anagrams
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Multiple-choice quiz
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