How greenhouse gases cause global warming

1. Carbon dioxide and methane are greenhouse gases.
2. Thermal energy from the Sun travels through the atmosphere and reaches the Earth.
3. Some of this energy is reflected straight back into space by clouds, ice and the Earth's surface.
4. The rest is absorbed by the Earth's surface, which warms up.
5. The warm surface then emits thermal energy back out towards space as infrared radiation.
6. Greenhouse gases in the atmosphere absorb some of this thermal energy.
7. The greenhouse gas molecules then emit the thermal energy again in all directions, so some of it travels back towards the Earth's surface.
8. This reduces thermal energy loss to space, so the Earth's surface and the lower atmosphere stay warmer than they otherwise would.
9. A small amount of this warming is natural, and without it the Earth would be too cold for most life.
10. Higher levels of carbon dioxide and methane absorb and re-emit more of the thermal energy, so even less is lost to space.
11. The average temperature of the Earth therefore rises, which is increased global warming.

Oxides of nitrogen in car engines

1. The air drawn into a car engine is about $78\%$ nitrogen and $21\%$ oxygen.
2. Nitrogen is normally very unreactive, because a strong triple bond holds the two atoms together in each ${\mathrm{N}}_2$ molecule.
3. Inside the engine, the burning fuel produces a very high temperature.
4. At this high temperature there is enough energy for nitrogen and oxygen from the air to react together, forming nitrogen monoxide: ${\mathrm{N}}_2+{\mathrm{O}}_2\to 2NO$.
5. Nitrogen monoxide then reacts with more oxygen to form nitrogen dioxide: $2NO+{\mathrm{O}}_2\to 2N{\mathrm{O}}_2$.
6. The nitrogen in these oxides comes from the air, not from the fuel, because petrol is a mixture of hydrocarbons and contains no nitrogen.

Removing oxides of nitrogen with catalytic converters

1. A catalytic converter contains a ceramic honeycomb coated with a thin layer of transition metal catalysts, such as platinum and rhodium.
2. The honeycomb gives the catalyst a very large surface area, so the exhaust gases make frequent contact with it and react quickly.
3. The converter works best once it is hot, so it removes pollutants most effectively after the engine has warmed up.
4. On the surface of the catalyst, harmful gases react to form less harmful gases.
5. Nitrogen monoxide reacts with carbon monoxide to form nitrogen and carbon dioxide: $2CO+2NO\to 2C{\mathrm{O}}_2+{\mathrm{N}}_2$.
6. This single reaction removes two pollutants at once: carbon monoxide, which is toxic, and nitrogen monoxide, which causes acid rain.
7. In this reaction nitrogen monoxide is reduced, because it loses oxygen, and carbon monoxide is oxidised, because it gains oxygen.
8. Nitrogen dioxide is removed in the same way: $2N{\mathrm{O}}_2+4CO\to {\mathrm{N}}_2+4C{\mathrm{O}}_2$.
9. The products, nitrogen and carbon dioxide, are not toxic, although the carbon dioxide still adds to global warming.

The symbol equation for photosynthesis

1. The symbol equation for photosynthesis is $6C{\mathrm{O}}_2+6{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2$.
2. Glucose has the formula ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$, so six molecules of carbon dioxide are needed to provide its six carbon atoms.
3. Six molecules of water provide the twelve hydrogen atoms in each glucose molecule.
4. The oxygen atoms that are not used to make glucose are released as six molecules of oxygen gas.
5. For every six molecules of carbon dioxide taken in, six molecules of oxygen are given out.