Chemical Reactions and the Environment
In 2023, Australia emitted 487 million tonnes of CO2-equivalent, a BOM report confirmed average temperatures are now 1.47°C above the 1961–1990 baseline.
Printable Worksheets
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Q1 · Where does the carbon in carbon dioxide from a car exhaust originally come from, and where does it end up?
Q2 · If chemical reactions cause environmental damage, do you think we should try to stop all chemical reactions, or is that impossible? Why?
● Know
- The main steps of the carbon cycle and how combustion moves carbon into the atmosphere
- The difference between biodegradable and non-biodegradable materials
- Examples of alternative fuels and cleaner combustion technologies
● Understand
- Why burning fossil fuels contributes to climate change while recent plant growth does not
- How chemical reactions can be designed to reduce environmental harm
- That Indigenous knowledge includes sophisticated understanding of chemical reactions
● Can do
- Describe the carbon cycle using word equations for key reactions
- Evaluate environmental impacts of different materials and fuels using evidence
- Communicate scientific arguments about sustainability using chemical reaction knowledge
Stand in Melbourne on a hot summer day when a temperature inversion traps car exhaust at ground level, the air smells acrid and your eyes sting, because the same nitrogen oxides that are harmless at high altitude form toxic ozone and smog at street level. The environmental impact of a chemical depends critically on where it is, how concentrated it is, and what else is present. Context matters enormously.
Ozone (O3): In the stratosphere (15-35 km altitude), ozone absorbs harmful UV-B and UV-C radiation, protecting life on Earth. The ozone layer is essential. But at ground level, ozone is a toxic pollutant formed by photochemical reactions involving nitrogen oxides (NOx) and volatile organic compounds in sunlight. Ground-level ozone causes respiratory problems, damages vegetation, and is a greenhouse gas.
Nitrogen oxides (NOx): In the stratosphere, NOx catalyses ozone destruction (a problem). In the troposphere, NOx contributes to smog and acid rain (also a problem). In soil, nitrogen compounds are essential fertilisers (beneficial). The same element plays different roles in different contexts.
Carbon dioxide: Essential for photosynthesis and life itself. But excess atmospheric CO2 drives climate change. The difference is concentration and location.
The Montreal Protocol (1987) banned chlorofluorocarbons (CFCs) because they destroy stratospheric ozone. CFCs were safe, non-toxic, non-flammable chemicals used in refrigerators, air conditioners, and aerosols. Their harm was not toxicity but their chemical stability, which allowed them to reach the stratosphere where UV radiation broke them down, releasing chlorine atoms that catalysed ozone destruction. This is a classic example of unintended consequences: a seemingly safe chemical caused global environmental damage because of where it ended up and what it reacted with.
Australian ozone research: Australian scientists played a key role in discovering the Antarctic ozone hole in 1985. The British Antarctic Survey, including Australian collaborators, measured dramatically depleted ozone over Antarctica each spring. This discovery led to the Montreal Protocol, the most successful international environmental agreement in history. Australia continues to monitor ozone through the Bureau of Meteorology and contributes to international assessments. The ozone layer is recovering slowly and is projected to return to 1980 levels by mid-century.
Natural chemicals are always safe; synthetic chemicals are always dangerous. This is false. Some of the most toxic substances known are natural (botulinum toxin, ricin, aflatoxin). Some synthetic chemicals are completely harmless (sodium chloride produced in a lab is identical to sea salt). Safety depends on dose, exposure route, and chemical properties, not origin. The natural vs synthetic distinction is scientifically meaningless - a molecule is a molecule regardless of its source.
A student claims: "All ozone is bad because it is a pollutant." Identify the evidence that contradicts this claim.
Chemical reactions in the environment have profound consequences for ecosystems and human health.
Acid rain: Sulfur dioxide and nitrogen oxides from burning fossil fuels dissolve in atmospheric water to form sulfuric acid and nitric acid. These acids fall as rain, acidifying lakes, damaging forests, and corroding buildings and statues. Acid rain has been largely controlled in developed countries through emissions regulations and flue-gas desulfurisation, but remains a problem in industrialising regions.
Ocean acidification: About 30% of anthropogenic CO2 dissolves in the oceans, forming carbonic acid: CO2 + H2O -> H2CO3. The pH of surface seawater has dropped from 8.2 to 8.1 since the Industrial Revolution - a 30% increase in acidity. This acidification reduces the availability of carbonate ions, making it harder for shell-forming organisms (corals, oysters, pteropods) to build calcium carbonate shells.
Greenhouse effect: CO2, methane, and water vapour absorb infrared radiation emitted by Earth surface, trapping heat in the atmosphere. Without this greenhouse effect, Earth would be frozen. But enhanced greenhouse effect from fossil fuel combustion is warming the climate.
The Great Barrier Reef is experiencing mass coral bleaching events driven by rising ocean temperatures and acidification. When water temperatures rise, corals expel their symbiotic algae (zooxanthellae), losing their colour and food source. If conditions do not improve, the corals die. Meanwhile, acidification reduces the saturation state of aragonite (the form of calcium carbonate corals use), making calcification more energetically expensive. The combination of thermal stress and acidification threatens the long-term survival of coral reef ecosystems worldwide.
Australian marine science: The Australian Institute of Marine Science (AIMS) operates the National Sea Simulator (SeaSim) in Townsville, a world-class facility for studying ocean warming and acidification effects on marine organisms. Researchers expose corals, fish, and shellfish to predicted future conditions and measure their physiological responses. This research informs reef management strategies and international climate policy. Australia marine scientists are global leaders in understanding how chemical changes in the ocean affect biodiversity.
Ocean acidification means the oceans will become literally acidic (pH < 7). This is false. Ocean acidification refers to a decrease in pH, not necessarily reaching acidic conditions. The oceans are buffered by dissolved carbonate and bicarbonate, maintaining pH above 7. However, even small pH changes have large biological effects because many marine organisms are finely adapted to the current pH range. A drop from pH 8.2 to 8.0 would be catastrophic for calcifying organisms even though the water is still basic.
Chemists and chemical engineers are increasingly focused on sustainability - designing processes that meet human needs while minimising environmental harm.
Green chemistry principles: Prevent waste rather than treating it. Design safer chemicals and syntheses. Use renewable feedstocks. Design for degradation. Use catalysis rather than stoichiometric reagents. Avoid auxiliary substances (solvents, separating agents). Maximise energy efficiency. Use inherently safer chemistry.
Atom economy: Calculate what fraction of reactant atoms end up in the desired product. A reaction with 100% atom economy uses every atom efficiently. Addition reactions have 100% atom economy; substitution and elimination reactions have lower atom economy because they produce by-products.
Life cycle assessment (LCA): Evaluate environmental impacts across the entire life cycle: raw material extraction, manufacturing, distribution, use, and disposal. LCA prevents problem-shifting - improving one stage while worsening another.
Traditional PVC production uses mercury catalysts in the vinyl chloride monomer step, creating toxic waste and occupational hazards. Modern processes use copper catalysts or non-catalytic routes that eliminate mercury entirely. This green chemistry improvement protects workers and the environment. Similarly, the pharmaceutical industry has shifted from stoichiometric oxidation reagents (which generate large amounts of metal waste) to catalytic oxidation using molecular oxygen or hydrogen peroxide, dramatically reducing waste per kilogram of product.
Australian green chemistry: The Centre for Green Chemistry at the University of Melbourne develops sustainable chemical processes, including bio-based plastics from agricultural waste, cleaner methods for pharmaceutical synthesis, and CO2 utilisation technologies. The Australian Research Council funds green chemistry research through dedicated programs. Australian companies like Boron Molecular specialise in flow chemistry, which uses continuous processing with minimal solvent waste. These innovations reduce the environmental footprint of Australian manufacturing.
Green chemistry means no chemicals are used. This is absurd - everything is made of chemicals. Green chemistry means designing chemical processes to be safer, more efficient, and less polluting. It is about better chemistry, not no chemistry. Water is a chemical (H2O). Air is a mixture of chemicals. Our bodies are chemical factories. The goal is to make human chemical activities sustainable, not to eliminate them.
Match each environmental chemistry concept to its definition.
Wrong: "Burning biofuels produces no CO₂ at all." No � burning biofuels does release CO₂. The advantage is that the plants recently absorbed that CO₂ from the atmosphere, so the net increase is lower than fossil fuels.
Right: Burning biofuels does release CO₂, but the carbon was recently absorbed by the plants as they grew. This makes biofuels roughly carbon-neutral over their lifecycle, a much smaller net increase in atmospheric CO₂ than burning fossil fuels.
Wrong: "All plastics are non-biodegradable." No � some newer bioplastics are designed to be biodegradable, but they often need specific conditions. Most everyday plastics are not biodegradable.
Right: Some modern bioplastics are engineered to be biodegradable, but most everyday plastics, including PET bottles and polystyrene, are not. Biodegradable plastics also typically require specific industrial composting conditions, not just landfill.
Wrong: "Catalytic converters make cars pollution-free." No � they reduce some pollutants but do not eliminate CO₂ emissions. Cars still produce CO₂ from fuel combustion.
Right: Catalytic converters reduce pollutants like carbon monoxide, nitrogen oxides and unburnt hydrocarbons, but they do not remove CO₂. Every kilogram of petrol burned still produces about 3 kg of CO₂ regardless of whether a converter is fitted.
Aboriginal and Torres Strait Islander Knowledge of Chemical Reactions
Aboriginal and Torres Strait Islander Peoples have practised sophisticated chemical knowledge for tens of thousands of years, developed through careful observation of Country and deep understanding of materials.
Cool burning demonstrates controlled combustion chemistry. By burning at low intensity when conditions are right, Traditional Custodians manage fuel loads without reaching the temperatures that destroy mature trees. This practice reduces the risk of catastrophic hot fires, a chemistry-informed land management strategy.
Tool making involves chemical reactions. Heating certain rocks changes their properties through thermal decomposition, making them easier to shape into stone tools. Resins and plant saps are heated to create adhesives for hafting spear points.
Food preparation uses chemical reactions to detoxify and preserve. Some plant foods contain toxins that are broken down through leaching, fermentation or heating. These are deliberate chemical processes applied to make food safe and nutritious.
Traditional Knowledge about chemical processes on Country is the Cultural and Intellectual Property of Aboriginal and Torres Strait Islander Peoples and should be acknowledged and respected.
✍ Copy Into Your Books
▾Carbon Cycle Reactions
- Photosynthesis: CO₂ + H₂O → glucose + O₂
- Respiration: glucose + O₂ → CO₂ + H₂O + energy
- Combustion: fuel + O₂ → CO₂ + H₂O + energy
Alternative Fuels
- Ethanol: made from fermented plants, lower net CO₂
- Biodiesel: cleaner burning than fossil diesel
- Hydrogen: burns to produce only water
Biodegradable vs Non-Biodegradable
- Biodegradable: broken down by microorganisms
- Non-biodegradable: persists in environment
- Conditions matter, some need industrial composting
Evaluate the Fuel
Biodegradable or Not?
At the start of this lesson, the hook told you that Australia emits about 500 million tonnes of CO₂ per year, every kilogram once locked inside coal, oil, or gas, and that combustion releases that stored carbon in minutes while photosynthesis took millions of years to capture it.
Now that you understand the carbon cycle and the chemistry of combustion in detail, can you explain why the mismatch between release rate and capture rate is the core of the climate problem? How has this lesson changed the way you think about burning fossil fuels compared to your initial reaction to that hook?
Q1. 1. Explain the difference between biodegradable and non-biodegradable materials. Include one example of each and describe the chemical reactions (or lack of them) that determine their environmental fate. 4 MARKS
Q2. 2. Using word equations, explain how the carbon cycle moves carbon between the atmosphere, plants and animals. Then explain why burning fossil fuels disrupts this cycle. 4 MARKS
Q3. 3. Aboriginal cool burning demonstrates a sophisticated understanding of combustion chemistry. Explain how controlling the fuel, temperature and oxygen supply changes the combustion reaction, and how this helps manage the Australian landscape. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now explain the difference between "recent" carbon and "ancient" carbon?
- How does your new knowledge change how you think about the fuels you use?
Model answers (click to reveal)
Answers
▾MCQ 1
BPhotosynthesis removes CO₂ from the atmosphere by converting it into glucose. Combustion and respiration both add CO₂ to the atmosphere.
MCQ 2
CFossil fuels contain carbon that was removed from the atmosphere millions of years ago and stored underground. Burning them releases this "old" carbon, increasing the total CO₂ in the atmosphere today. Recently grown wood recycles carbon that was in the atmosphere within the last few decades.
MCQ 3
AHydrogen + oxygen → water + energy. This is why hydrogen is considered a clean fuel, it produces no carbon dioxide.
MCQ 4
DMany biodegradable plastics need specific conditions like heat, moisture and microorganisms found in industrial composting facilities. The ocean does not provide these conditions, so the bottle persists.
MCQ 5
BCatalytic converters reduce harmful gases like carbon monoxide and nitrogen oxides, but they do not remove CO₂. Cars still produce CO₂ through fuel combustion, contributing to climate change.
Short Answer 1
Model answer: Biodegradable materials can be broken down by natural chemical reactions (decomposition) carried out by microorganisms. For example, a cotton T-shirt will decompose into simpler substances like carbon dioxide, water and minerals when buried in soil. Non-biodegradable materials resist these natural decomposition reactions. For example, a conventional plastic bottle is made of long-chain polymers that microorganisms cannot easily break apart, so it persists in the environment for hundreds of years, potentially harming wildlife and ecosystems.
Short Answer 2
Model answer: Photosynthesis removes carbon from the atmosphere: carbon dioxide + water → glucose + oxygen. Cellular respiration returns it: glucose + oxygen → carbon dioxide + water + energy. In a natural cycle, the CO₂ released by respiration was recently captured by photosynthesis, so the amount of CO₂ in the atmosphere stays relatively balanced. Burning fossil fuels disrupts this because the carbon in coal, oil and gas was removed from the atmosphere millions of years ago. Combustion releases this ancient carbon rapidly, increasing atmospheric CO₂ faster than photosynthesis can remove it, leading to climate change.
Short Answer 3
Model answer: Cool burning controls the amount of fuel (fine leaves and bark rather than heavy logs), which limits how much heat the combustion reaction can produce. With less fuel, the fire temperature stays lower, and less oxygen is consumed. This means mature trees survive because the fire does not reach the high temperatures needed to kill them. The landscape benefits because fuel loads are reduced, preventing catastrophic hot fires, and the ecosystem maintains its biodiversity. This demonstrates deep understanding of how controlling reactants (fuel and oxygen) controls the reaction.