Topic 4.3 - Carbon cycling

Question 1

Carbon dioxide in solution

Answer 1

Carbon dioxide can either remain as CO2 (g) or dissolve to form carbonic acid.
Aquatic plants can either use CO2 or for photosynthesis.
H2CO3 can dissociate to form hydrogen (H+) and hydrogen carbonate (HCO3-) ions which explains how carbon dioxide may reduce the pH of water.

Question 2

Removing carbon dioxide in the atmosphere

Answer 2

Carbon dioxide is fixed out of the atmosphere by the protein RuBisCo and used during photosynthesis which reduces the carbon dioxide concentration in the atmosphere.

Question 3

Carbon dioxide cycle - removal of CO2 and production

Answer 3

Carbon dioxide diffuses into autotrophs and is then converted into carbon compounds (stomata in underside of leaves/ any part of aquatic plant) and dissolves into water. Carbon dioxide is released by aerobic respiration and dissolves out of water.

Question 4

Methanogenesis

Answer 4

Produced from organic matter in anaerobic conditions by methanogenic archaeans and released into the atmosphere.
Can occur in many locations:
• Mud along shore and bed of lakes
• Waterlogged environments (ie swamps)
• Guts of termites, cattle and sheep
• Landfill sites where organic material is buried

Question 5

Oxidation of methane

Answer 5

Methane is oxidised to carbon dioxide and water in the atmosphere so naturally only persists for around 12 years.
This explains why atmospheric concentrations are not high despite large production of methane.

Question 6

Peat formation

Answer 6

Peat forms when organic matter is not fully decomposed because of anaerobic conditions in waterlogged soils.
Saprotrophs cannot thrive in waterlogged, anaerobic conditions so dead organic matter is not fully decomposed.
In these conditions acidic conditions also tend to develop which further inhibits methanogens and saprotrophs.

Question 7

Fossilized organic matter

Answer 7

Partially decomposed organic matter from past geological eras convert into oil and gas in porous rocks or into coal.

Question 8

Carbon stability

Answer 8

Very chemically stable, can remain unchanged in rocks for hundreds of millions of years.

Question 9

Coal formation

Answer 9

Coal -> formed when deposits of peat are buried under other sediments. The peat is compressed and heated, forming coal.

Question 10

Crude oil and natural gas formation

Answer 10

Formed in the mud at the bottom of seas and lakes. As more mud or other sediments are deposited the partially decomposed matter is compressed and heated. Chemical changes start to occur, producing complex mixtures of liquid carbon compounds or gases.
Largest part of natural gas -> methane.
Deposits are found where there are porous rocks that prevent escape, such as shales.

Question 11

Combustion

Answer 11

Carbon dioxide is produced by the combustion of biomass and fossilised organic matter.
Combustion are the oxidisation reactions that occur when something is burned.

Question 12

Animals causing limestone to be produced

Answer 12

Reef-building corals produce their exoskeleton by secreting calcium carbonate and molluscs’ shells composed of CaCO3.

Question 13

Limestone formation

Answer 13

In shallow tropical seas CaCO3 is deposited which forms limestone rock and hard parts of animals are often visible as fossils.
10% of sedimentary rock on Earth is limestone, 12% of mass of CaCO3 is carbon so huge amounts of carbon are locked up in limestone rock on Earth.