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10.3.2 Reducing pollution and photosynthesis

Reducing climate change

Definition

Renewable energy resource

An energy resource that is replaced naturally and will not run out, such as wind or solar energy.

  1. Climate change is caused by increased global warming from higher levels of carbon dioxide and methane, so each strategy reduces the amount of one of these gases in the atmosphere.
  2. Planting trees
    1. Trees take in carbon dioxide from the air for photosynthesis, so planting more trees removes carbon dioxide from the atmosphere.
    2. Cutting down and burning forests has the opposite effect, because it releases carbon dioxide and removes the trees that would have absorbed it.
  3. Reduction in livestock farming
    1. Cattle and sheep release methane as a waste gas from digestion.
    2. Keeping fewer animals reduces the amount of methane released into the atmosphere.
  4. Decreasing use of fossil fuels
    1. Burning coal, oil and natural gas releases carbon dioxide.
    2. Burning less of them, for example by insulating homes and using public transport, reduces the amount of carbon dioxide released.
  5. Increasing use of hydrogen
    1. Hydrogen burns to form only water, 2H2+O2→2H2O\mathrm{2H_{2} + O_{2} \rightarrow 2H_{2}O}2H2​+O2​→2H2​O, so using it as a fuel releases no carbon dioxide.
    2. Hydrogen can also power vehicles through a fuel cell, which produces only water.
  6. Increasing use of renewable energy
    1. Wind turbines and solar cells generate electricity without burning any fuel, so they release no carbon dioxide while they produce electricity.
    2. Every unit of electricity from a renewable resource replaces electricity that would otherwise come from burning fossil fuels.

Reducing acid rain

Definition

Catalytic converter

A device in the exhaust system of a vehicle that uses a catalyst to convert harmful gases into less harmful gases before they are released into the air.

Definition

Flue gas desulfurisation

The removal of sulfur dioxide from the waste gases of a power station by reacting it with calcium oxide.

  1. Acid rain is caused by sulfur dioxide and oxides of nitrogen, so each strategy reduces the amount of one of these gases released into the air.
  2. Catalytic converters in vehicles
    1. A catalytic converter is fitted in the exhaust system of a vehicle, between the engine and the end of the exhaust pipe.
    2. It converts oxides of nitrogen into harmless nitrogen before the exhaust gases leave the vehicle, so less of the gases that cause acid rain reach the air.
  3. Using low-sulfur fuels
    1. Sulfur compounds are removed from petrol and diesel at the refinery before the fuels are sold.
    2. A fuel that contains less sulfur forms less sulfur dioxide when it burns.
  4. Flue gas desulfurisation with calcium oxide
    1. Power stations that burn coal release their waste gases, called flue gases, through tall chimneys.
    2. Before the flue gases leave the chimney, they pass through calcium oxide.
    3. Sulfur dioxide is an acidic oxide and calcium oxide is a basic oxide, so they react together and the sulfur dioxide is removed from the gases.
    4. The reaction forms solid calcium sulfite, CaO+SO2→CaSO3\mathrm{CaO + SO_{2} \rightarrow CaSO_{3}}CaO+SO2​→CaSO3​.
Example

How flue gas desulfurisation prevents acid rain

  • Coal contains sulfur compounds, so burning it in a power station forms sulfur dioxide.
S+O2→SO2 \mathrm{S + O_{2} \rightarrow SO_{2}} S+O2​→SO2​
  • Without treatment, this sulfur dioxide would escape from the chimney, dissolve in the water in clouds and fall as acid rain.
  • In the chimney, the sulfur dioxide meets calcium oxide and reacts with it.
CaO+SO2→CaSO3 \mathrm{CaO + SO_{2} \rightarrow CaSO_{3}} CaO+SO2​→CaSO3​
  • The sulfur dioxide is trapped in a solid, so much less of it leaves the chimney and much less acid rain forms.

Photosynthesis

Definition

Photosynthesis

The reaction between carbon dioxide and water to produce glucose and oxygen, in the presence of chlorophyll and using energy from light.

Definition

Chlorophyll

The green substance in plants that absorbs energy from light for photosynthesis.

  1. Photosynthesis is the reaction that green plants use to make glucose.
  2. It is the reaction between carbon dioxide and water to produce glucose and oxygen.
  3. The plant takes in carbon dioxide from the air through its leaves and water from the soil through its roots.
  4. The reaction takes place in the presence of chlorophyll, the green substance in leaves.
  5. Chlorophyll absorbs energy from light, and this energy is used to make the reaction happen.
  6. The word equation for photosynthesis is carbon dioxide+water→glucose+oxygen\text{carbon dioxide} + \text{water} \rightarrow \text{glucose} + \text{oxygen}carbon dioxide+water→glucose+oxygen.
  7. Light and chlorophyll are needed for the reaction, but they are not reactants, so they do not appear on either side of the equation.
  8. Photosynthesis removes carbon dioxide from the atmosphere and releases oxygen into it, which is why planting trees helps to reduce climate change.
Exam technique

Explaining a strategy to reduce pollution

  • An explanation needs three links: the strategy, the gas it reduces, and the problem that gas causes.
  • For example, planting trees removes carbon dioxide by photosynthesis, which reduces global warming and so reduces climate change.
  • Match each strategy to the right problem: trees, fewer livestock, fewer fossil fuels, hydrogen and renewable energy reduce climate change, while catalytic converters, low-sulfur fuels and flue gas desulfurisation reduce acid rain.
  • Name calcium oxide as the substance used in flue gas desulfurisation.
  • Write glucose in the word equation for photosynthesis, not sugar.
Common Mistake
  • Do not say that catalytic converters reduce global warming, because they remove oxides of nitrogen and do not remove carbon dioxide.
  • Do not say that planting trees reduces acid rain, because trees remove carbon dioxide, which is not a cause of acid rain.
  • Do not write light or chlorophyll as reactants in the equation for photosynthesis.
  • Do not say that low-sulfur fuels cause no pollution, because they still form carbon dioxide when they burn.
Self review
  • Explain how planting trees helps to reduce climate change.
  • Why does a reduction in livestock farming help to reduce climate change?
  • Name the compound that reacts with sulfur dioxide in flue gas desulfurisation.
  • State the word equation for photosynthesis.
  • Apart from carbon dioxide and water, what two things are needed for photosynthesis?
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10.3.2 Reducing pollution and photosynthesis Revision Guide

  1. IGCSE
  2. /Chemistry
  3. /10.3.2 Reducing pollution and photosynthesis

Revision notes for CIE IGCSE Chemistry 10.3.2 Reducing pollution and photosynthesis: explanations and worked examples.

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