Air quality and climate Revision Notes

Air is a homogeneous mixture of gases that surrounds the Earth. When referring to clean, dry air, it means air that is free from water vapour and pollutants. The major components of clean, dry air are nitrogen, oxygen, noble gases, and carbon dioxide.

By volume, air consists of approximately 78% nitrogen (N₂), 21% oxygen (O₂), and about 1% other gases including noble gases and carbon dioxide (CO₂). These percentages can vary slightly depending on location and environmental conditions, but these values are standard for dry air at sea level.

• Nitrogen: ~78% by volume
• Oxygen: ~21% by volume
• Noble gases (argon, neon, helium, krypton, xenon): ~0.9% by volume
• Carbon dioxide: ~0.04% by volume

Nitrogen gas (N₂) is the most abundant component of air, making up about 78% by volume. It is a diatomic molecule with a strong triple covalent bond between the two nitrogen atoms, making it chemically inert under normal conditions.

Nitrogen is colourless, odourless, and tasteless. It does not support combustion or respiration but is essential in diluting oxygen to prevent rapid burning.

Because of its inertness, nitrogen is often used in industrial processes to provide an unreactive atmosphere.

• Molecular formula: N₂
• Colourless, odourless, tasteless gas
• Chemically inert due to strong triple bond
• Does not support combustion or respiration

Oxygen gas (O₂) constitutes about 21% of clean, dry air by volume. It is a diatomic molecule essential for respiration in most living organisms and supports combustion.

Oxygen is colourless, odourless, and tasteless. It readily reacts with many substances, including metals and non-metals, often forming oxides.

In chemical equations, oxygen is represented as O₂ (g). For example, combustion of methane in oxygen is: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

• Molecular formula: O₂
• Colourless, odourless, tasteless gas
• Supports combustion and respiration
• Highly reactive, forms oxides

Noble gases are present in trace amounts, approximately 0.9% of air by volume. Argon is the most abundant noble gas (~0.93%), followed by neon, helium, krypton, and xenon in much smaller quantities.

Noble gases are monatomic, colourless, odourless, and chemically inert due to their full outer electron shells.

Carbon dioxide (CO₂) is present at about 0.04% by volume. It is a colourless, odourless gas produced naturally by respiration, decomposition, and volcanic activity, and increased by human activities such as burning fossil fuels. CO₂ is important in photosynthesis and as a greenhouse gas.

• Argon: ~0.93% by volume, inert monatomic gas
• Other noble gases: neon, helium, krypton, xenon in trace amounts
• Carbon dioxide: ~0.04% by volume, colourless, odourless gas
• CO₂ is involved in photosynthesis and contributes to the greenhouse effect

Air is a mixture, not a compound, because its components retain their individual chemical properties and are not chemically bonded together.

The gases in air can be separated by physical methods such as fractional distillation of liquefied air, which exploits differences in boiling points.

Because air is a mixture, the proportions of gases can vary slightly, especially water vapour content, which is excluded in the definition of clean, dry air.

• Components retain individual properties
• No fixed chemical formula for air
• Can be separated physically (e.g., fractional distillation)
• Water vapour content varies and is excluded in dry air

Carbon dioxide (CO₂) is a colourless gas produced when carbon-containing fuels burn completely in oxygen. Common fuels include coal, petrol, diesel, natural gas, and wood.

Complete combustion occurs when there is sufficient oxygen to convert all carbon atoms into carbon dioxide.

The general word equation for complete combustion of a hydrocarbon fuel is: hydrocarbon + oxygen → carbon dioxide + water vapour. For example, methane combustion: CH₄ + 2O₂ → CO₂ + 2H₂O (balanced symbol equation).

• Carbon dioxide is a greenhouse gas contributing to global warming.
• Produced by burning fossil fuels in power stations, vehicles, and heating.
• Complete combustion requires ample oxygen supply.

Incomplete combustion occurs when there is insufficient oxygen, causing carbon-based fuels to burn partially.

This produces carbon monoxide (CO), a colourless, odourless, and highly toxic gas, and particulates, which are tiny solid particles mainly of carbon (soot).

Carbon monoxide forms because carbon atoms do not fully oxidise to CO₂; instead, some form CO due to limited oxygen availability.

• Incomplete combustion example: 2C + O₂ → 2CO (carbon to carbon monoxide).
• Particulates consist mainly of carbon particles released as soot.
• Incomplete combustion is common in poorly maintained engines and inefficient stoves.
• Carbon monoxide binds to haemoglobin in blood, reducing oxygen transport, making it dangerous.

Methane (CH₄) is a colourless, odourless gas produced naturally by anaerobic decomposition of organic matter in wetlands, landfills, and digestive processes of ruminant animals (e.g., cows, sheep).

In the absence of oxygen, microorganisms break down plant and animal material, releasing methane as a by-product.

Methane is a potent greenhouse gas with a global warming potential much higher than carbon dioxide over a 100-year period.

• Methane is produced biologically, not by combustion.
• Sources include wetlands, rice paddies, landfills, and livestock digestion.
• Methane contributes significantly to climate change despite lower atmospheric concentration than CO₂.

Nitrogen oxides (NOₓ) are a group of gases including nitric oxide (NO) and nitrogen dioxide (NO₂).

They form in vehicle engines due to high temperatures causing nitrogen and oxygen in air to react: N₂ + O₂ → 2NO (at high temperature).

Nitric oxide can further react with oxygen to form nitrogen dioxide: 2NO + O₂ → 2NO₂, a brown toxic gas contributing to smog and acid rain.

• Produced mainly by internal combustion engines in vehicles.
• High temperature in engines breaks the strong N≡N bond in nitrogen gas.
• Nitrogen oxides cause respiratory problems and environmental damage.
• Catalytic converters in vehicles reduce NOₓ emissions by converting them to nitrogen and oxygen.

Sulfur dioxide (SO₂) is a colourless gas with a sharp, choking smell, produced when fossil fuels containing sulfur impurities are burned.

Coal and oil often contain sulfur compounds; during combustion, sulfur reacts with oxygen: S + O₂ → SO₂.

Sulfur dioxide dissolves in atmospheric moisture to form sulfurous acid, contributing to acid rain, which damages ecosystems and buildings.

• Main source is burning coal and oil with sulfur impurities.
• SO₂ is a major cause of acid rain.
• Flue gas desulfurisation can reduce SO₂ emissions in power stations.
• Sulfur dioxide is toxic and irritates the respiratory system.

Carbon dioxide (CO₂) and methane (CH₄) are significant greenhouse gases that contribute to global warming and climate change. They trap thermal energy in the Earth's atmosphere by absorbing infrared radiation emitted from the Earth's surface, reducing the amount of heat lost to space.

Carbon dioxide is produced mainly by burning carbon-based fuels such as coal, oil, and natural gas. Methane is released from natural sources like decaying vegetation and animal digestion, as well as human activities such as livestock farming and landfill decomposition.

The increased concentration of these gases enhances the greenhouse effect, leading to a rise in global temperatures, melting polar ice, rising sea levels, and altered weather patterns. This contributes to climate change with widespread environmental and socio-economic impacts.

• CO₂ and CH₄ absorb infrared radiation, trapping heat in the atmosphere.
• Increased levels lead to global warming and climate change.
• Sources include fossil fuel combustion (CO₂) and biological decay or digestion (CH₄).

Carbon monoxide (CO) is a colourless, odourless, and highly toxic gas formed by incomplete combustion of carbon-containing fuels when oxygen supply is limited.

CO binds strongly to haemoglobin in red blood cells, forming carboxyhaemoglobin, which reduces the blood's oxygen-carrying capacity. This can cause symptoms ranging from headaches and dizziness to unconsciousness and death at high concentrations.

Exposure to carbon monoxide is particularly dangerous in enclosed or poorly ventilated spaces, such as garages or homes with faulty heating appliances.

• CO is produced by incomplete combustion of fuels.
• It binds to haemoglobin, preventing oxygen transport.
• Exposure causes poisoning and can be fatal.

Particulates are tiny solid particles or liquid droplets suspended in air, produced by incomplete combustion of fuels, industrial processes, and natural sources like dust and pollen.

Inhalation of particulates can irritate the respiratory system, causing problems such as asthma, bronchitis, and other lung diseases. Fine particulates can penetrate deep into the lungs and enter the bloodstream.

Long-term exposure to certain particulates, especially those containing carcinogenic substances like polycyclic aromatic hydrocarbons, increases the risk of lung cancer.

• Particulates arise from incomplete combustion and industrial emissions.
• They cause respiratory irritation and diseases.
• Some particulates increase cancer risk due to carcinogenic components.

Nitrogen oxides (NOₓ), mainly nitric oxide (NO) and nitrogen dioxide (NO₂), are formed in high-temperature combustion processes such as vehicle engines.

NO reacts with oxygen to form NO₂, a brown toxic gas contributing to photochemical smog formation. NO₂ can react with water in the atmosphere to form nitric acid (HNO₃), leading to acid rain.

Acid rain damages aquatic ecosystems, soils, and vegetation, and corrodes buildings and monuments. NOₓ gases also irritate the respiratory system, worsening conditions like asthma and bronchitis.

• NO and NO₂ form during high-temperature combustion.
• NO₂ contributes to photochemical smog and acid rain formation.
• Nitric acid from NOₓ causes environmental damage and respiratory issues.
• Balanced equation for NO oxidation: 2NO (g) + O₂ (g) → 2NO₂ (g)

Sulfur dioxide (SO₂) is produced by burning fossil fuels containing sulfur impurities, such as coal and oil.

In the atmosphere, SO₂ dissolves in water droplets forming sulfurous acid (H₂SO₃), which can be further oxidised to sulfuric acid (H₂SO₄). These acids fall as acid rain.

Acid rain lowers the pH of soils and water bodies, harming plants, aquatic life, and infrastructure. SO₂ also irritates the respiratory system, causing coughing and aggravating lung diseases.

• SO₂ originates from burning sulfur-containing fossil fuels.
• It forms sulfurous and sulfuric acids in the atmosphere.
• Acid rain damages ecosystems and man-made structures.
• SO₂ is a respiratory irritant.
• Relevant equation: SO₂ (g) + H₂O (l) → H₂SO₃ (aq)

Climate change is driven primarily by increased concentrations of greenhouse gases such as carbon dioxide (CO₂) and methane (CH₄) in the atmosphere, which trap thermal energy and raise global temperatures. Several methods can reduce these gases:

Planting trees (afforestation) increases the uptake of CO₂ through photosynthesis. Trees absorb CO₂ and convert it into glucose and oxygen, thus lowering atmospheric CO₂ levels. The general photosynthesis equation is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (chlorophyll and light energy required)

• Trees act as carbon sinks by storing carbon in biomass.
• Increased tree cover helps balance carbon emissions from human activities.

Methane is a potent greenhouse gas produced during the digestion of food by ruminant animals such as cows and sheep. Reducing livestock farming lowers methane emissions, thus mitigating climate change.

Methane (CH₄) has a higher global warming potential than CO₂ over a 20-year period, so even small reductions in methane emissions have significant effects.

• Methane is released mainly via enteric fermentation in livestock.
• Reducing livestock numbers or improving farming practices can decrease methane output.

Burning fossil fuels releases CO₂ and sulfur dioxide (SO₂), contributing to climate change and acid rain. Reducing fossil fuel consumption lowers these emissions.

Renewable energy sources such as wind and solar power generate electricity without emitting greenhouse gases.

Hydrogen can be used as a clean fuel; when combusted, it produces only water vapour: 2H₂ + O₂ → 2H₂O (balanced equation). This reduces CO₂ emissions if hydrogen is produced sustainably.

• Renewables are sustainable and reduce reliance on fossil fuels.
• Hydrogen fuel cells produce electricity with water as the only by-product.

Acid rain forms when sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) dissolve in rainwater forming sulfuric and nitric acids. Catalytic converters fitted to vehicle exhaust systems reduce emissions of harmful gases.

Catalytic converters use catalysts such as platinum, palladium, and rhodium to convert carbon monoxide (CO), nitrogen oxides (NO and NO₂), and unburnt hydrocarbons into less harmful gases.

Key reactions include: 2CO + 2NO → N₂ + 2CO₂ (carbon monoxide reacts with nitrogen monoxide to form nitrogen and carbon dioxide).

• Catalytic converters reduce NOₓ and CO emissions.
• They help decrease acid rain and improve air quality.

Burning fossil fuels containing sulfur produces sulfur dioxide (SO₂), a major cause of acid rain. Using fuels with low sulfur content reduces SO₂ emissions.

Flue gas desulfurization (FGD) is a chemical process used in power stations to remove SO₂ from exhaust gases before release into the atmosphere.

In FGD, calcium oxide (CaO) reacts with sulfur dioxide and oxygen to form calcium sulfate (CaSO₄), a solid that can be removed: 2SO₂ + O₂ + 2CaO → 2CaSO₄ (balanced equation).

• Low-sulfur fuels emit less SO₂ when burned.
• FGD chemically removes SO₂, preventing acid rain formation.

Photosynthesis is the process by which green plants, algae, and some bacteria synthesise glucose from carbon dioxide and water. This process requires light energy, which is absorbed by the green pigment chlorophyll found in chloroplasts within plant cells. Photosynthesis is essential because it produces glucose, a source of chemical energy for plants and other organisms, and oxygen, which is released into the atmosphere.

The overall word equation for photosynthesis is: carbon dioxide + water → glucose + oxygen.

• Occurs in green plants, algae, and some bacteria.
• Takes place in chloroplasts containing chlorophyll.
• Requires light energy, usually from the sun.
• Produces glucose (C₆H₁₂O₆) and oxygen (O₂).

The raw materials for photosynthesis are carbon dioxide (CO₂) from the air and water (H₂O) absorbed by roots. These reactants undergo a series of complex reactions powered by light energy to form glucose and oxygen.

The balanced symbol equation for photosynthesis is: 6CO₂ (g) + 6H₂O (l) → C₆H₁₂O₆ (s) + 6O₂ (g).

This equation shows that six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen.

• Carbon dioxide enters leaves through stomata.
• Water is absorbed by roots and transported to leaves.
• Glucose is a carbohydrate used for energy and growth.
• Oxygen is released as a by-product into the atmosphere.

Chlorophyll is a green pigment located in chloroplasts that absorbs light energy, mainly from the blue and red regions of the visible spectrum. This absorbed light energy provides the energy required to drive the endothermic reactions of photosynthesis.

Without chlorophyll, plants cannot capture light energy efficiently, and photosynthesis cannot proceed. Light energy is converted into chemical energy stored in the glucose molecules.

The process involves two main stages: the light-dependent reactions, where light energy is converted to chemical energy (ATP and NADPH), and the light-independent reactions (Calvin cycle), where carbon dioxide is fixed into glucose.

• Chlorophyll absorbs light energy to initiate photosynthesis.
• Light energy is converted into chemical energy.
• Light-dependent reactions produce ATP and NADPH.
• Light-independent reactions synthesise glucose from CO₂.

Photosynthesis occurs in the chloroplasts of leaf cells, primarily in the palisade mesophyll layer where chlorophyll concentration is highest. Adequate light intensity, carbon dioxide concentration, and water availability are necessary for optimal photosynthesis.

Temperature also affects the rate of photosynthesis, with enzymes involved working best within a certain range (usually 20–35°C). Insufficient light or carbon dioxide limits the rate, while excessive heat can denature enzymes.

Oxygen produced diffuses out through stomata, small pores on the leaf surface.

• Occurs in chloroplasts of leaf cells.
• Requires sufficient light intensity and carbon dioxide.
• Optimal temperature range is 20–35°C.
• Water must be available for the reaction.
• Oxygen is released through stomata.

Balance the symbol equation for photosynthesis given the unbalanced equation: CO₂ + H₂O → C₆H₁₂O₆ + O₂.

Step 1: Identify the number of atoms in glucose (C₆H₁₂O₆): 6 carbon, 12 hydrogen, 6 oxygen atoms.

Step 2: Balance carbon atoms by placing 6 before CO₂: 6CO₂ + H₂O → C₆H₁₂O₆ + O₂. Carbon atoms balanced (6 on each side). Step 3: Balance hydrogen atoms by placing 6 before H₂O: 6CO₂ + 6H₂O → C₆H₁₂O₆ + O₂. Hydrogen atoms balanced (12 on each side). Step 4: Balance oxygen atoms. Left side: 6×2 (from CO₂) + 6×1 (from H₂O) = 18 oxygen atoms. Right side: 6 (in glucose) + x×2 (in O₂). Set 6 + 2x = 18 → 2x = 12 → x = 6. Place 6 before O₂: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Equation balanced.

• Balanced equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
• Carbon, hydrogen, and oxygen atoms balanced.
• Shows stoichiometric relationship of reactants and products.

Photosynthesis is a chemical process that occurs in green plants, algae, and some bacteria. It enables these organisms to produce glucose, a carbohydrate that serves as an energy source, from simple inorganic substances.

The process requires carbon dioxide and water as raw materials and uses energy from sunlight, which is absorbed by the green pigment chlorophyll found in chloroplasts of plant cells.

Photosynthesis also produces oxygen as a by-product, which is released into the atmosphere and is essential for aerobic respiration in most living organisms.

• Occurs in chloroplasts of plant cells.
• Requires chlorophyll to absorb sunlight.
• Converts carbon dioxide and water into glucose and oxygen.

The overall chemical change in photosynthesis can be summarised by a word equation that shows the reactants and products involved.

This equation is: Carbon dioxide + Water → Glucose + Oxygen

This means that plants take in carbon dioxide from the air and water from the soil, and using energy from sunlight, produce glucose and release oxygen.

• Reactants: carbon dioxide and water.
• Products: glucose and oxygen.
• Energy source: sunlight absorbed by chlorophyll.

Photosynthesis takes place in the chloroplasts, where chlorophyll absorbs light energy, which drives the chemical reactions.

Carbon dioxide enters the leaves through small pores called stomata, while water is absorbed by roots and transported to leaves.

The process requires light energy, so photosynthesis occurs mainly during daylight hours under suitable light intensity and temperature conditions.

• Chlorophyll captures light energy.
• Carbon dioxide enters through stomata.
• Water transported from roots to leaves.
• Light and suitable temperature needed for the reaction.

The balanced symbol equation represents the stoichiometric relationship between reactants and products in photosynthesis.

It is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (with chlorophyll and light energy)

This shows that six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen.

• 6 carbon dioxide + 6 water → 1 glucose + 6 oxygen
• Chlorophyll and light energy are necessary for the reaction.
• Balanced to conserve atoms of each element.

Photosynthesis is vital for life on Earth as it provides the organic compounds needed for plant growth and development.

Glucose produced can be used immediately for energy or converted into starch for storage.

Oxygen released during photosynthesis replenishes the atmosphere and supports respiration in animals and humans.

• Produces glucose as food for plants.
• Releases oxygen essential for respiration.
• Supports the food chain and ecosystem balance.

Greenhouse gases are atmospheric gases that can absorb and emit infrared radiation (thermal energy). The main greenhouse gases include carbon dioxide (CO₂), methane (CH₄), water vapour (H₂O), nitrous oxide (N₂O), and ozone (O₃). This note focuses on carbon dioxide and methane, which are significant contributors to the greenhouse effect and global warming.

Carbon dioxide is produced mainly by the combustion of fossil fuels and respiration, while methane is released from natural sources such as wetlands and from human activities like livestock digestion and landfill decomposition. Both gases are present in the atmosphere in trace amounts but have a strong effect on Earth's energy balance.

• Carbon dioxide (CO₂) and methane (CH₄) are key greenhouse gases.
• They exist in small concentrations but have significant thermal effects.
• Sources include combustion, agriculture, and natural processes.

Greenhouse gases absorb thermal energy emitted by the Earth's surface. When sunlight reaches Earth, the surface absorbs shortwave radiation and warms up. The Earth then emits this energy as longwave infrared radiation.

Carbon dioxide and methane molecules have vibrational modes that can absorb this infrared radiation. Upon absorption, the molecules gain vibrational energy and later re-emit infrared radiation in all directions, including back towards the Earth's surface.

This absorption and re-emission process effectively traps heat within the atmosphere, a phenomenon known as the greenhouse effect. It differs from reflection of visible light by clouds or ice, as greenhouse gases specifically interact with infrared radiation.

• Earth emits longwave infrared radiation after absorbing sunlight.
• CO₂ and CH₄ absorb infrared radiation due to molecular vibrations.
• They re-emit infrared radiation, partly back to Earth, trapping heat.

Normally, Earth loses thermal energy to space by emitting infrared radiation. However, greenhouse gases absorb some of this radiation and re-emit it in all directions, including back towards the surface.

This reduces the net thermal energy loss from Earth to space, causing more heat to remain in the atmosphere and near the surface. The result is an increase in the average global temperature, known as global warming.

The balance between incoming solar radiation and outgoing infrared radiation determines Earth's climate. Increasing concentrations of greenhouse gases disrupt this balance by decreasing outgoing radiation.

• Greenhouse gases absorb outgoing infrared radiation.
• Re-emission back to Earth reduces net heat loss to space.
• This causes Earth's average temperature to rise (global warming).

Carbon dioxide is the most abundant anthropogenic greenhouse gas and has a long atmospheric lifetime, making it a major driver of global warming.

Methane is more effective at absorbing infrared radiation per molecule than carbon dioxide but is present in smaller amounts and has a shorter atmospheric lifetime (about 12 years).

Despite its shorter lifetime, methane's global warming potential over 100 years is approximately 28–36 times that of carbon dioxide, making it a potent greenhouse gas in the short term.

• CO₂ is the primary anthropogenic greenhouse gas with a long lifetime.
• CH₄ absorbs more infrared radiation per molecule but is less abundant.
• Methane's global warming potential is much higher than CO₂ over 100 years.

The enhanced greenhouse effect caused by increased concentrations of carbon dioxide and methane leads to global warming, which affects climate patterns worldwide.

Human activities such as burning fossil fuels, deforestation, and agriculture increase emissions of these gases.

Understanding the role of greenhouse gases helps in developing strategies to mitigate climate change by reducing emissions and enhancing carbon sinks.

• Increased greenhouse gases cause global warming and climate change.
• Human activities contribute significantly to CO₂ and CH₄ emissions.
• Mitigation requires reducing emissions and enhancing natural absorption.

Nitrogen oxides (NOx) are a group of gases composed mainly of nitric oxide (NO) and nitrogen dioxide (NO2). They form in car engines during the combustion process when nitrogen and oxygen from the air react at the high temperatures inside the engine's combustion chamber.

Air contains about 78% nitrogen (N2) and 21% oxygen (O2). Under normal conditions, nitrogen is relatively inert. However, the extreme temperatures (above 1500 °C) in the engine cause nitrogen and oxygen molecules to react, forming nitrogen oxides.

The main reaction for the formation of nitric oxide is: N2 (g) + O2 (g) → 2NO (g) (at high temperature) This reaction is endothermic and favoured by high temperature and pressure conditions in the engine. Some of the nitric oxide formed can further react with oxygen to form nitrogen dioxide: 2NO (g) + O2 (g) → 2NO2 (g) Nitrogen oxides are harmful pollutants contributing to acid rain, photochemical smog, and respiratory problems.

• Nitrogen oxides form when nitrogen and oxygen react at high temperatures in car engines.
• Main nitrogen oxides are nitric oxide (NO) and nitrogen dioxide (NO2).
• High temperature (above 1500 °C) in combustion chambers enables these reactions.

Catalytic converters are devices fitted in the exhaust systems of vehicles to reduce harmful emissions, including nitrogen oxides (NOx), carbon monoxide (CO), and unburnt hydrocarbons.

The catalytic converter contains a ceramic honeycomb coated with catalysts such as platinum, palladium, and rhodium. These catalysts speed up chemical reactions without being consumed.

Inside the catalytic converter, nitrogen oxides are reduced back to nitrogen and oxygen gases, which are harmless. Simultaneously, carbon monoxide is oxidised to carbon dioxide, which is less toxic.

• Catalytic converters use catalysts (platinum, palladium, rhodium) to speed up reactions.
• They convert nitrogen oxides into nitrogen and oxygen.
• They convert carbon monoxide into carbon dioxide.
• These reactions reduce air pollution from vehicle exhausts.

The key reactions occurring in catalytic converters include the reduction of nitrogen oxides and the oxidation of carbon monoxide. These can be summarised by balanced symbol equations:

1. Reduction of nitric oxide by carbon monoxide: 2NO (g) + 2CO (g) → N2 (g) + 2CO2 (g)

2. Reduction of nitrogen dioxide by carbon monoxide: 2NO2 (g) + 2CO (g) → N2 (g) + 2CO2 (g) + O2 (g) (Note: This reaction may vary; often NO2 is first reduced to NO, then NO is reduced to N2.)

• Carbon monoxide acts as a reducing agent, converting NOx to nitrogen gas.
• Carbon monoxide is oxidised to carbon dioxide.
• Balanced equations show conservation of atoms and charge.
• Catalysts lower activation energy, allowing these reactions at lower temperatures.

Nitrogen oxides contribute to environmental problems such as acid rain, which damages ecosystems and buildings, and photochemical smog, which affects human health by causing respiratory issues.

Carbon monoxide is a colourless, odourless, and highly toxic gas that binds to haemoglobin in blood, reducing oxygen transport.

By converting nitrogen oxides to nitrogen and oxygen, and carbon monoxide to carbon dioxide, catalytic converters significantly reduce the harmful impact of vehicle emissions on air quality and human health.

• NOx gases cause acid rain and smog, harming health and environment.
• CO is toxic and reduces oxygen delivery in the body.
• Catalytic converters improve air quality by removing these pollutants.

Write the balanced symbol equation for the reaction between nitric oxide and carbon monoxide in a catalytic converter.

Step 1: Write the unbalanced equation: NO + CO → N2 + CO2

Step 2: Balance nitrogen atoms: There are 2 nitrogen atoms in N2, so multiply NO by 2: 2NO + CO → N2 + CO2 Step 3: Balance carbon atoms: There is 1 carbon atom on each side, so CO and CO2 are balanced as written. Step 4: Balance oxygen atoms: Left side has 2 oxygen atoms (from 2NO) + 1 oxygen atom (from CO) = 3 oxygen atoms. Right side has 2 oxygen atoms (from CO2). Oxygen is unbalanced. Adjust CO to 2 molecules: 2NO + 2CO → N2 + 2CO2 Now oxygen atoms: Left side 2 (from 2NO) + 2 (from 2CO) = 4 oxygen atoms; right side 4 (from 2CO2). Balanced equation: 2NO + 2CO → N2 + 2CO2

• Balance nitrogen atoms first.
• Balance carbon atoms next.
• Adjust coefficients to balance oxygen atoms last.
• Final balanced equation: 2NO + 2CO → N2 + 2CO2.

Photosynthesis is a vital biochemical process carried out by green plants, algae, and certain bacteria. It enables these organisms to synthesise glucose, a carbohydrate that serves as an energy source, from simple inorganic molecules: carbon dioxide and water. This process also releases oxygen as a by-product, which is essential for aerobic respiration in most living organisms.

Photosynthesis occurs in specialised cell structures called chloroplasts, which contain the green pigment chlorophyll. Chlorophyll absorbs light energy, primarily from the sun, which drives the chemical reactions of photosynthesis.

• Occurs in chloroplasts of plant cells and algae.
• Requires sunlight, carbon dioxide (CO₂), and water (H₂O).
• Produces glucose (C₆H₁₂O₆) and oxygen (O₂).

The overall chemical reaction of photosynthesis can be summarised by the balanced symbol equation:

6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂. This equation shows that six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen.

This equation is balanced, meaning the number of atoms of each element is the same on both sides, ensuring the law of conservation of mass is obeyed.

• Reactants: 6 molecules of carbon dioxide and 6 molecules of water.
• Products: 1 molecule of glucose and 6 molecules of oxygen.
• Energy from sunlight is required but not shown in the symbol equation.

Chlorophyll is the pigment responsible for capturing light energy from the sun. This energy is necessary to drive the endothermic reactions that convert carbon dioxide and water into glucose and oxygen.

Light energy excites electrons in chlorophyll, initiating a series of reactions known as the light-dependent reactions. These reactions produce energy-rich molecules (ATP and NADPH) used in the light-independent reactions (Calvin cycle) to synthesise glucose from carbon dioxide.

• Chlorophyll absorbs sunlight, mainly blue and red wavelengths.
• Light energy converts ADP and NADP⁺ into ATP and NADPH.
• ATP and NADPH provide energy and reducing power for glucose synthesis.

Photosynthesis is fundamental to life on Earth as it provides the primary source of organic matter and oxygen. Glucose produced is used by plants for energy and as a building block for other organic compounds such as cellulose and starch.

Oxygen released during photosynthesis replenishes the atmosphere and supports aerobic respiration in animals and other organisms.

• Glucose is an energy source and building block for plants.
• Oxygen supports respiration in most living organisms.
• Photosynthesis maintains atmospheric oxygen and carbon dioxide balance.