Julia Tran
Preliminary Year Assessment – 4
Open Ended Investigation
Research Assignment Topic: Energy
1.
“The energy for life comes from the sun.”
Outline the role of the photosynthesis process to explain what is meant by this statement.
(3 marks)
chlorophyll
6CO2(g) + 6H2O(l) + energy C6 H12 O6 (aq) + 6O2(g)
2830 kJ
glucose
Photosynthesis is the process in which plants use the sun’s light energy to convert carbon dioxide
and water into carbohydrates such as glucose, sucrose, starch and cellulose.
Carbohydrates are high energy compounds because when they react with oxygen, they produce
large amounts of energy. Plants make their own food by using the glucose they produces as an
energy source for growth, reproduction, repairing damaged tissues... Animals cannot carry out
photosynthesis, so they eat the plants to obtain food, hence, energy. For that reason the energy for
life comes from the sun.
2. Explain what is meant by thermal pollution and discuss the implications for aquatic life. (3 marks)
Thermal pollution is a discharge into a river or lake of a lot of hot water which is enough to
significantly (2°C to 5°C) increase the temperature of water. This abnormal increase in temperature
is potentially harmful to aquatic organisms in these ways:
 Stress to organisms due to less dissolved oxygen.
 Increase metabolism which would increase oxygen demand, and this would lead to
decreasing oxygen supply.
 Fish eggs don’t hatch.
 Migration and spawning would be set off at the wrong time due to false temperature cues.
 Sudden change in temperature would kill off fish eggs even if the temperature is in the
egg’s survival range.
3. Identify the sources of pollution which accompany the combustion of organic compounds and
explain how these can be avoided. (6 marks)
Carbon dioxide is produced by power plants and motor vehicles by complete combustion when
there is plenty of oxygen supply. It is a major pollution of enhanced greenhouse effect. This can be
avoided by using filters such as catalytic converters to remove it.
C(s) + O2(g)  CO2(g)
2C8H18(l) + 25O2(g)  16CO2 + 18H2O(l)
Sulfur dioxide is produced by the combustion of fossil fuels which contributes to form acid rain
which leads to:
 Increasing acidity of lake and/or river.
 Erosion of marble and limestone buildings.
 Accelerating the rate of corrosion.
This can be avoided by lowering combustion temperatures to prevent the formation of sulfur
dioxides.
S(s) + O2(g)  SO2(g)
2SO2(g) + O2(g)  2SO3(g)
SO3(g) + H2O(l)  H2SO4(aq)
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Julia Tran
Preliminary Year Assessment – 4
Open Ended Investigation
Research Assignment Topic: Energy
Nitrogen oxide is produced in the combustion chamber of petrol or from diesel engines and
combustion of coal in power plants. The combustion of nitrogen in fuels occurs at high
temperatures. It causes respiratory problems and contributes to form photochemical smog and
acid rain like sulfur dioxide. This can be avoided by lowering the combustion temperature to
prevent the formation of nitrogen oxides.
N2(g) + O2(g)  2NO(g)
No(g) + O2(g)  NO2(g)
2NO(g) + O2(g)  2NO2(g)
Carbon monoxide or soot is produced by incomplete combustion when there is insufficient
oxygen supply. Carbon monoxide is toxic when it combines with haemoglobin in red blood cells in
preference to oxygen; this reduces the ability of blood to transport oxygen - suffocation. Soot is
carcinogenic. This is can be avoided by ensuring complete combustion.
2C8H18(l) + 17O2(g)  16CO(g) + 18H2O(l)
OR
C8H18(l) + 5O2(g)  CO(g) +7C(s) + 9H2O(g)
Tetraethyl lead (Pb(C2H5)4) is added to petrol to produce leaded petrol. The leaded emitted to the
environment is a toxic metal which can enter the food chain.
4. Explain combustion in terms of slow, spontaneous and explosive reactions and explain the
conditions under which these occur. (4 marks)
Combustion is a self-sustaining chemical reaction in which a substance reacts with oxygen and
release energy, usually in the form of heat and light. Therefore, it is an exothermic reaction.
Slow combustion occurs at low temperatures and non-rapid rates. It occurs when we use big
lumps of fuel and restrict the supply of air. For example: respiration; fuel is burnt at low ignition
temperature (body temperature) and produces carbon dioxide, water and energy.
Spontaneous combustion occurs when a substance doesn’t require the application of external
energy in order to combust. It occurs when small particles of coal or gaseous fuel are sprayed into
excess air. For example: the exposure of white phosphorus forms an oxide because the heat from
this reaction is enough to ignite the rest of the phosphorus.
Explosive combustion is an uncontrolled chemical reaction that occurs at a very fast rate in which
heat is released. Explosions occur when there are high concentrations of gases or finely divided
solid particles of materials (as the surface area is increased) in excess heated air. For example:
cylinders of petrol and diesel engines in vehicles.
5. Describe the role of catalysts in chemical reactions, using a named industrial catalyst as an
example. (2 marks)
A catalyst is a substance which increases the rate of
chemical reaction by providing an alternative pathway with
lower activation energy without itself being consumed or
permanently changed.
Palladium or nickel catalyst is used in the margarine
industry in the process of hydrogenation of unsaturated fats
to saturated fats. Hydrogen molecules and unsaturated
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Julia Tran
Preliminary Year Assessment – 4
Open Ended Investigation
Research Assignment Topic: Energy
hydrocarbon molecules are absorbed onto the surface of the catalyst and the bonds between the
hydrogen atoms break as new metal-hydrocarbon molecules form. The absorbed atoms migrate to
the metal surface and react; the materials convert from an absorbed state to form products.
6. Explain the relationship of ignition temperature and activation energy. (2 marks)
Ignition temperature is the minimum temperature required for the gas or mixture to
spontaneously combust.
Activation energy is the minimum amount of energy reactant molecules must possess in order to
form products.
The greater the activation energy, the higher is the ignition temperature.
7. Milk kept at room temperature can sour in as little as a day, but can be kept in a refrigerator. Use
energy profile and molecular kinetic energy distribution diagrams to explain these observations.
(4 marks)
The temperature is the measure of the average
kinetic energy of the reacting particles. The increase in
temperature also changes the distribution of molecular
kinetic energies.
Not all reactant particles have the same kinetic
energy at the same temperature. At a higher
temperature, there are greater proportions of
molecules that have kinetic energy more than
activation energy.
Therefore, increasing temperature will result in
greater number of reactant molecules with kinetic
energy greater than the required activation energy, which results in more successful collisions.
The souring of milk is a chemical reaction, so the rate of chemical reaction increases with heat.
As the heat increases, the average kinetic energy also increases, meaning that particles collide
more often and thus react more often.
8. (6 marks)
a) Explain why the flash point for pentane (- 49°C) is much lower than its ignition temperature
(260°C).
Flash point is the minimum temperature for a liquid fuel to produce sufficient vapour for a
spontaneous combustion to occur in the presence of a small flame.
Flash point is the temperature where a product develops vapours, not the temperature where
the product will burst into flames or support combustion. A product will not ignite until it
reaches its ignition temperature, consequently, flash point is always much lower than the
ignition temperature.
b) Account for the explosive risks in small motor boats, wheat storage silos and underground coal
mines.
The flashpoint of a product determines the safety precautions needed for handling and storing the
fuel.
An explosive reaction occurs when fuel molecules (molecules that can burn, including: wheat
and coal particles and petrol vapour) are well mixed with the air and there is no limit on the flow
of fuel to the combustion area in a small boat, mine or silo. When there is a fine mixture with the
right concentrations and a spark or ignition appears, this spark begins a reaction that accelerates
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Julia Tran
Preliminary Year Assessment – 4
Open Ended Investigation
Research Assignment Topic: Energy
so quick that the heat release and gas pressure creates an explosion. Therefore, in small and or
confined spaces e.g. the silo, there is a possibility of explosive risks because the wheat particles
cannot move out and so remain mixing with the air if there is no ventilation. This goes same for
petrol vapour in small motor boats and coal particles in underground coal mines.
c) Explain how white phosphorus, which has much lower ignition temperature than butane, can
have a lower risk of explosive combustion.
As explained in parts (4 & 8b), an explosive combustion would need a mixture of fuel and air and
the addition of a spark or ignition. Assuming white phosphorus to be solid, it would not mix with air
as successfully as butane as a gas, thus, an explosive reaction or a high-risk explosion would not
take place, in spite of the ignition temperature of white phosphorus and butane.
Bibliography
 Macmillan Chemistry Pathways 1 – Geoffrey Thickett
 Excel Preliminary Chemistry – C.M.Roebuck
 www.cem.msu.edu
 greenlighton.files.wordpress.com
 www.chemguide.co.uk
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