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AS, Chemistry, KS5

Principles of green chemistry

The Bhopal disaster

Union Carbide Disaster In Bhopal India | Flashback | NBC News
Bhopal disaster | Britannica

Methyl isocyanate (MIC) is a highly toxic, volatile, and flammable organic compound (CH3NCO) used primarily as an intermediate in the production of carbamate pesticides, polyurethane foam, and plastics. It is notoriously hazardous, causing severe respiratory, eye, and skin irritation

What is green chemistry?

Using renewable resources

Using renewable resources is the first key point in green chemistry.

Ethanol, C2H5OH, is often used as a solvent for chemical reactions. It is also a raw material in the synthesis of useful compounds such as pharmaceuticals and pesticides, and can be used as a fuel.

Ethanol can be made by the reaction of ethene (derived from crude oil) and water. Alternatively, it can be made by fermentation of biomass.

Exam Question #1

29%

Ethanol, C2H5OH, is manufactured on a large scale for a wide range of uses such as alcoholic drinks, as an industrial solvent and as a raw material for the synthesis of many organic compounds.

Ethanol, C2H5OH, is manufactured on a large scale by two methods:

  • Fermentation, using yeast, of sugars, such as glucose, C6H12O6.

C6H12O6(aq) → 2C2H5OH(aq) + 2CO2(g)

The ethanol is then distilled off.

  • Hydration of ethene, C2H4, with steam in the presence of an acid catalyst.

C2H4(g) + H2O(g) → C2H5OH(g)

Compare the sustainability of these methods of manufacturing ethanol in terms of:

  • availability of starting materials and energy requirements;
  • atom economy.

In your answer, you should make clear how the atom economy of the processes links with chemical theory.

(7 marks)

 Compare 

Learners look for the similarities and differences of two (or more) things. Should not require the drawing of a conclusion. Answer must relate to both (or all) things mentioned in the question. The answer must include at least one similarity and one difference. 

Atom economy

A measure of the efficiency of a reaction in terms of the mass of the desired product as a percentage of the total mass of the reactants.

Any SEVEN from:

  • availability
  • sugar is renewable because it can be grown (1)
  • ethane is finite because it is obtained by processing of crude oil (1)
  • energy
  • fermentation: energy is required for distillation/ hydration: energy is required to generate steam (1)
  • atom economy and waste products:
  • atom economy for fermentation < atom economy hydration (1)
  • In fermentation, CO2 is produced in addition to ethanol/ethanol is not the only product (1)
  • In hydration, ethanol is the only product/hydration is an addition
    reaction (1)
  • Atom economy of fermentation could be increased by finding a use CO2 (1)

Atom economy linked to a chemical equation to show that hydration has 100% atom economy/fermentation has 51% atom economy (1)

7 max

Alternatives to hazardous chemicals

Many reactions in the chemical industry use a solvent to dissolve the raw materials

Some solvents can be flammable and toxic, so safer alternatives are being researched.

An example of a safer solvent is the use of liquid carbon dioxide in dry cleaning. This is an alternative to the solvent 1,1,2,2-tetrachloroethane, which is toxic and harmful.

Ball-and-stick model of the 1,1,2,2-tetrachloroethane molecule

The Chemistry of Solvents

© SnapRevise

Atom economy

Use catalysts when possible

Common industrial catalysts include iron, platinum, vanadium(V) oxide and nickel. Enzymes can also be used as catalysts in industrial processes.

Catalysts act by providing an alternative route for a reaction with a lower activation energy. At any given temperature, more particles are likely to have enough energy to react when they collide.

Only a small amount of a catalyst is needed in a reaction, and it can be recycled and reused multiple times.

Enzyme or metal?

Exam Question #2

46%

Chemical companies are using catalysts to develop processes that are more sustainable. These processes reduce costs and are less harmful to the environment.

Suggest two ways in which the use of catalysts helps chemical companies to make their processes more sustainable.

(4 marks)

Suggest

Always used with another command word, e.g. Suggest an explanation. Suggest tells you that you need to apply your knowledge to a new situation, and in this case to give a possible explanation.

Sustainable

An activity that meets the needs of society today and can be carried on for an extended period without depleting resources or damaging the environment so that future generations can also meet their needs.

Any FOUR from:

  • catalyst not used up in reaction
  • reactions take place at lower temperatures
  • with lower energy demand OR lower activation energy OR use less fuel
  • so less carbon dioxide emitted into atmosphere OR so fossil fuels last longer
  • different reactions can be used
  • with better atom economy OR less waste 
  • less hazardous chemicals
  • catalysts or enzymes can generate specific products
  • ALLOW catalysts can work at room temperature OR enzymes work at room temperature
  • IGNORE cheaper

Exam Question #3

75%

Explain, using an enthalpy profile diagram and a Boltzmann distribution, how the presence of a catalyst increases the rate of reaction.

A QWC (Quality of Written Communication) question

A long-answer exam question, common in GCSE and A-Level science, that assesses both subject knowledge and communication skills. Usually worth 6 marks, it requires a clear, logical, and well-structured argument, often using specialized terminology with correct spelling, punctuation, and grammar.

(7 marks)

Explain

Learners’ explanations require a justification/exemplification of a point. The answer must contain some element of reasoning/justification – this can include mathematical explanations.

Catalyst

A substance that speeds up a chemical reaction without itself being consumed by the reaction. It works by providing an alternative route by which the reaction can take place, which has lower activation energy.

  • one activation energy labelled on enthalpy profile diagram
  • ALLOW double headed arrows on the activation energy label
  • ALLOW vertical line with no arrows
  • DO NOT ALLOW arrow just pointing downwards
  • IGNORE any enthalpy change label drawn
  • one activation energy labelled on enthalpy profile diagram
  • marks can be awarded via, reaction profile, in words or from Boltzmann
  • catalyst has a different reaction pathway
    OR different reaction mechanism
    OR two curves drawn on profile

A QWC (Quality of Written Communication) question

correct diagram of reaction profile for endothermic or exothermic reaction with products and reactants at different heights -y axis labelled as energy or enthalpy (1 mark)

  • Drawing of Boltzmann distribution AND axes labelled (number of) molecules and energy
  • Boltzmann distribution – must start at origin and must not end up at O on y-axis i.e. must not touch x-axis.
  • DO NOT ALLOW Boltzmann mark if two distributions are drawn one for non-catalysed and one for catalysed
  • ALLOW particles instead of molecules
  • IGNORE atoms instead of particles
  • More molecules with energy above activation energy with a catalyst OR More molecules that overcome the activation energy
  • DO NOT ALLOW more molecules have sufficient energy to react
  • More effective collisions OR more successful collisions

Energy is used in the chemical industry for heating, separating and drying.

Heat is often produced by burning fossil fuels. However, some reactions can now be carried out using microwaves to generate the heat needed.

There is also the potential for using solar power for heating and for water purification.

What are the advantages and disadvantages of using microwaves or solar power for heating?

The advantage of solar panels is that they use a renewable source of energy. The disadvantages are that enough space is needed to install them and the Sun doesn’t always shine. Because they are so large, it is only worthwhile to install them if they produce enough power for large scale use.

Reducing water use

  • Turn off the tap while brushing your teeth to save up to 6–9 litres per minute.
  • Choose showers over baths, as a standard bath uses about 80 litres, while a typical shower uses much less.
  • Only boil what you need in the kettle to save water and energy.
  • Use a washing-up bowl instead of a running tap when cleaning dishes or rinsing vegetables.
  • Use a watering can rather than a hose or sprinkler, as a hose can waste up to 1,000 litres per hour.
  • Apply mulch (like bark or compost) to flower beds to help the soil retain moisture.

Reducing pollution

  • Choose active travel: Walk or cycle for short trips to eliminate vehicle emissions and improve personal fitness.
  • Ditch single-use plastics: Swap disposable cutlery, straws, and bags for reusable alternatives like stainless steel bottles and cloth totes.
  • Upgrade appliances: Look for the ENERGY STAR label when buying new equipment to ensure higher efficiency and lower power consumption.
  • Plant native greenery: Trees and native shrubs filter pollutants, absorb CO2, and provide habitat for local wildlife.

Reduce and recycle waste

  • A reduction in landfill and industrial waste by reuse or recycling.
  • Reduced energy usage by reuse or recycling.
  • Less pollution through reuse or recycling.
  • Increased employment by reuse or recycling.
  • Sustainable usage of natural resources by reuse and recycling.

Which is greener?

Greenhouse gases

Global warming: true or false?

What are greenhouse gases?

Global warming

Global warming potential

Not all greenhouse gases contribute to global warming to the same degree.

The global warming potential (GWP) is a measure of how much a given gas can contribute to global warming over a given time period. It is measured relative to carbon dioxide, which is given a global warming potential of 1.

The GWP of a gas is calculated using the following factors:

  • molar mass of the gas
  • lifetime of the gas in the atmosphere
  • ability of the gas to absorb infra-red radiation.

Greenhouse gases compared

The contribution of a greenhouse gas to global warming depends on its global warming potential (GWP) and the amount of that gas present in the atmosphere.

Greenhouse
gas

Average % of 
atmosphere

After
20 years

After
100 years

water vapour*

1–4

–

–

carbon dioxide

0.038

1

1

methane

0.0002

72

25

nitrogen
monoxide

0.00003

289

298

*Water vapour is estimated to be the most powerful of these greenhouse gases. Its concentration in the atmosphere varies depending on the temperature so no GDP is calculated for it.

The local effects of climate change

Exam Question #4

83%

Research scientists working in the Antarctic are taking measurements for the global warming potential. They have measured the concentration of carbon dioxide inthe ice. This study has allowed the scientists to estimate the atmospheric concentration of carbon dioxide over many thousands of years.

The graphs below show these atmospheric concentrations and the corresponding average surface temperature.

Do the graphs provide reliable evidence that an increase in atmospheric carbon dioxide concentration will result in global warming?

Explain your answer.

(2 marks)

Explain

Learners’ explanations require a justification/exemplification of a point. The answer must contain some element of reasoning/justification – this can include mathematical explanations.

Global warming

An increase in the average temperature of the Earth.

Any two from the following:

  • There are times when CO2 has a high concentration and the temperature is also high 
  • OR
  • There are times when CO2 has a low concentration and the temperature is low
  • ALLOW a (positive) correlation between temperature and carbon dioxide concentration
  • DO NOT ALLOW just ‘a correlation’
  • IGNORE ‘graphs are the same shape’
  • IGNORE ‘graphs are similar’
  • It is impossible to measure with certainty the average temperature years ago 
    There are other gases that may cause a greenhouse effect
  • OR
  • There are other factors that may cause a greenhouse effect 
    There are very few anomalous results

Causes of climate change

Causes of climate change – class activity

How scientists can help

It is now widely accepted that human activity is causing an increase in the concentration of carbon dioxide in the atmosphere, and that this is leading to global warming.

What is the role of scientists in combating global warming?

  • Analysing the available evidence to find out the current extent of the problem and predict its future extent.
  • Investigating the viability of controversial solutions, such as carbon capture and storage (CCS).

What do you think? – class activity

Carbon footprints

What are carbon footprints?

Carbon footprint is a measure of the impact of a person’s activities on the environment, in terms of how much CO2 is produced by each activity.

Data for 2023 | Wikipedia

Country

Average carbon footprint radius
(tonnes of CO2 per person per year)

USA

13 – 15

Australia

14 – 21

UK

9 – 11

China

7 – 9

India

1.2 – 2

Carbon footprints

Reducing your carbon footprint

What is carbon neutral?

Carbon neutral describes any activity that produces no overall emissions of CO2.

There are two ways of working towards carbon neutrality:

  • reducing carbon emissions
  • carbon offsetting.

Carbon offsetting is the practice of balancing activities that emit CO2 with activities that absorb it; for example, planting trees that take in CO2 as they grow.

Carbon offsetting is controversial because the act of planting trees in large numbers uses energy and emits CO2 even if the trees themselves absorb it.

Coldplay’s “Music of the Spheres” world tour was designed to be their most sustainable yet by focusing on three core pillars:

  • Reduce (cutting emissions by 50%)
  • Reinvent (developing new green tech)
  • Restore (funding nature-based solutions)
Coldplay appeared on The One Show in early 2025

Are biofuels carbon neutral?

Advantages of biofuels

Is hydrogen carbon neutral?

When hydrogen is burned as a fuel, the only product is water.

Fuel cells that use this method of energy production can be incorporated into engines, as shown on the right.

The hydrogen fuel is generated by the electrolysis of water.

Do you think hydrogen is a carbon neutral fuel?

Hydrogen is generated for fuel by the electrolysis of water. This uses electricity, which must be generated somehow, and most often by the burning of fossil fuels. Although hydrogen is, therefore, cleaner than fossil fuels, it is not carbon neutral. Students should be able to reach this decision after some discussion.

CFCs and the ozone layer

What are CFCs?

Chlorofluorocarbons (CFCs), are a family of organic compounds.

dichlorodifluoromethane
1,1,1–trichloro–2,2,2–trifluoroethane

In the 1930s, CFCs were developed for use as refrigerants, to cool the air inside refrigerators.

They were then found to be useful in many other products, including aerosol sprays and foam fire extinguishers.

What is the ozone layer?

  • The ozone layer is a high concentration of ozone (O3) molecules in a layer of the stratosphere about 15–30km above the Earth’s surface.
  • Ozone absorbs a high proportion of the Sun’s ultraviolet radiation. There is strong evidence that this ultraviolet radiation is harmful.

CFCs and ozone

At ground level, CFCs are very stable. This is what allows them to exist in the atmosphere for long enough to reach the stratosphere. Once there, they absorb UV radiation from the Sun, which breaks their covalent bonds to form highly reactive free radicals.

CCl2F2   →   •CClF2   +   Cl•

These react very quickly with ozone molecules in a free radical chain reaction:

Cl• + O3 → ClO• + O2

ClO• + O3 → Cl• + 2O2

Exam Question #5

100%

Oxides of nitrogen, NOx, are atmospheric pollutants.

Nitrogen monoxide reacts with oxygen to form NO2.

  • Write an equation for the formation of NO2 from nitrogen monoxide and oxygen.

(1 mark)

Aeroplane engines produce nitrogen monoxide.

  • Describe, with the aid of equations, how nitrogen monoxide catalyses ozone depletion in the stratosphere.

(3 marks)

Describe

Learners give an account of something. Statements in the response need to be developed as they are often linked but do not need to include a justification or reason.

Ozone layer

A high concentration of ozone (O3) molecules in a layer of the stratosphere about 15–30 km above the Earth’s surface.

Write an equation for the formation of NO2 from nitrogen monoxide and oxygen.

  • 2N0 + 02 → 2N02
  • ALLOW any correct multiple including fractions
  • IGNORE state symbols

Describe, with the aid of equations, how nitrogen monoxide catalyses ozone depletion in the stratosphere.

  • N0 is not consumed OR overall reaction is 03 + 0 → 202
  • ALLOW 203 → 302
  • OR It is a chain reaction
    OR N0 is reformed
    OR mechanism of ozone depletion is changed
    OR N0 made can react with more ozone
  • N0 + 03 → N02 + 02           
  • IGNORE dots
  • N02 + 0 → N0 + 02
  • ALLOW N02 + 03 → N0 + 202

4 max

CFCs and ozone: true or false

Summary activities

Glossary

What’s the keyword?

Multiple-choice quiz

justeen.clements@jclemuk.co.uk

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