CHEMICAL PROPERTIES OF CARBON
The combustion of carbon:
Carbon reacts with oxygen to form two oxides, carbon dioxide, CO2, and carbon monoxide CO. The proportions of these two oxides formed during combustion depend on the conditions. At about 500 ºC, carbon dioxide is produced almost exclusively, provided that oxygen is in excess:
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C(s) + O2(g) CO2(g)
At higher temperatures, or when the supply of oxygen is restricted, carbon monoxide is the main product.
2C(s) +O2 (g) 2CO (g)
Reaction with acids
Carbon reacts with concentrated sulphuric acid and concentrated nitric acid. Nitric acid is a powerful oxidizing agent. Both acids oxidize carbon to carbon dioxide gas.
Carbon + sulphuric acid carbon dioxide + sulphur dioxide +water
C(s) + 2H2SO4(conc) CO2(g) + SO2(g) + 2H2O(l)
Carbon + nitric acid carbon dioxide + Nitrogen (IV) oxide + water
C(s) + 4HNO3 (conc) CO2(g) + 4NO2(g) + 2H2O(l)
Reducing action of carbon
When carbon is mixed with iron (III) oxide and heated strongly, pure iron metal is produced.
Carbon monoxide + iron (III) oxide carbon dioxide + iron.
3CO(g) + Fe2O3(s) 3CO2(g) + 2Fe(l)
Carbon (IV) Oxide:
Carbon dioxide is easily prepared by the action of dilute hydrochloric acids on metal carbonates (normally calcium carbonate or marble).
Vigorous effervescence occurs as bubbles of carbon dioxide are liberated.
CaCO3(s) + 2HCl (aq) CaCl2+ CO2 (g) + H2O (l)
Since carbon dioxide is 1.53 times as heavy as air, it usually collected by upward displacement of air. In this case, it is collected by downward displacement of water.
Properties of carbon (IV) oxide
Carbon dioxide is a colourless gas with a faint pungent smell.
It does not burn or support combustion, except in extreme cases, and is not poisonous (it is the gas in fizzy cool drinks). It can however cause death by suffocation, when it is present in sufficient concentrations. Carbon dioxide is a linear molecule. The gas condenses to a liquid at 0 ºC under a pressure of 35 atm.
At normal pressure, carbon dioxide condenses directly to a solid at -78.5 ºC. This solid, known as dry ice, is widely used as cooling agent. Solid carbon dioxide does not melt under conditions of normal atmospheric pressure, but passes directly into the gas phase, a process known as sublimation.
- Reaction with water
When carbon dioxide is dissolved in water, carbonic acid, H2CO3, is produced in small quantities:
CO2 (g) + H2O (l) H2CO3 (aq) Carbonic acid is a weak diprotic acid which gives rise to salts known as carbonates, which contain the carbonate anion CO32-.
- Reaction with limewater
Test for carbon dioxide
Limewater is a clear colourless solution of calcium hydroxide (slaked lime). Calcium carbonate is precipitated when carbon dioxide is passed through a clear solution of calcium hydroxide in water. The lime water turns milky serving as a test for carbon dioxide liberation. The above reaction of carbon dioxide with potassium hydroxide is used to purify air. When air is passed through a solution of potassium hydroxide, it absorbs carbon dioxide.
Hence, air, which is first passed through aqueous potassium hydroxide, and then passed through limewater, does not turn the latter milky, as all the carbon dioxide is absorbed by the potassium hydroxide
Carbon dioxide is slightly acidic. It turns blue litmus paper red.
During rainy season, blue litmus paper kept open in the laboratory slowly turns red, due to the presence of carbon dioxide in air.
Carbon dioxide is neither combustible, not a supporter of combustion.
A burning splinter or a burning candle gets put off, but metals like potassium, sodium, magnesium etc. continue to burn in carbon dioxide.
Burning magnesium in carbon dioxide
Ignite a ribbon of magnesium, and introduce it in the jar of carbon dioxide. The magnesium ribbon continues to burn in carbon dioxide. Deposits of carbon can be seen on the inner sides of the jar. At the temperature of ignition, these metals are able to reduce the carbon dioxide to carbon by taking away the oxygen i.e., tiny black solid particles of carbon get deposited inside the jar.
Action on heated carbon
When carbon dioxide is passed over red-hot carbon in the form of coke charcoal, the carbon dioxide loses one of its two atoms of oxygen.
As a result, carbon dioxide gets reduced and becomes carbon monoxide. At the same time the hot carbon also gets converted to carbon monoxide.
Uses of Carbon Dioxide
- In nature
- a) Carbon dioxide is used in photosynthesis by green plants to produce carbohydrates.
- b) To induce natural breathing.
- To extinguish fires
Soda-acid fire extinguishers produce carbon dioxide to put out fires.
- As a refrigerant
Solid carbon dioxide called “Dry ice” can provide temperatures as low as -109.3o F.
It is superior to ordinary ice, for the following reasons:
- i) It provides much lower temperature than ice.
- ii) It lasts longer.
- iii) It freezes faster.
- iv) It does not wet the food being chilled, as it sublimes directly into gaseous state.
2. Manufacture of fertilizer
Carbon dioxide is used extensively in the manufacture of urea, an important in nitrogenous fertilizer.
3. In the baking industry
Baking powder is used in all the food preparations. The addition of baking powder during baking produces carbon dioxide which makes the dough “rise”.
The small pores in a loaf of bread are the spaces in which carbon dioxide was formed . Yeast is also used in baking instead of baking powder. It produces carbon dioxide by anaerobic respiration.
Baking powder contains starch, sodium hydrogen carbonate and an acid forming ingredient, such as tartaric acid of calcium hydrogen phosphate [Ca(H2PO4)2] or alum [Na2SO4.Al2(SO4)3.24 H2O].
4. In medicine
A mixture of 97% oxygen and 3% carbon dioxide, called carbogen is used to revive persons affected by carbon monoxide poisoning, pneumonia, asphyxiation etc.
5. Manufacture of aerated drinks
Carbon dioxide is extensively used in aerated drinks. Increasing the pressure increases the solubility of the gas. The fizz in the drink is due to carbon dioxide being liberated when the pressure is reduced.
6. Food storage
It is use to fill silos (storage bins) and containers, that are used for storing food grains.
After the silos are packed with the food grains, carbon dioxide is pumped into the silos from the top.
The gas being heavier then air, slowly sinks down, pushing out the air in between the grains.
This prevents the growth of bacteria, fungus, etc. on the grains.
7. In the entertainment industry
Dry ice is used to create artificial clouds as special effects, for stage shows, films, discotheques etc. Dry ice is dropped into water; absorbing heat from the water, the solid carbon dioxide sublimes, carrying with it a lot of water vapor. These two together form thick white ‘clouds’.
But unlike real clouds, which are formed above due to the low vapor density of water vapor, the artificial cloud fills the floor of the stage, as the vapor density of carbon dioxide is 22. So while the lower part of the body of the artist is covered by ‘cloud’, the upper half is visible.