Chemical Properties Of Chemical Compound

CHEMICAL PROPERTIES OF CHEMICAL COMPOUNDS

Most of the fuels we use (coal, wood, CNG, LPG, petrol, kerosene, diesel etc.) are either carbon or its compounds. So, we should know their chemical properties to understand their nature.

COMBUSTION:

Carbon in all its allotropic forms burns in air or oxygen to give carbon dioxide and releases energy in the form of heat and light.

Most carbon compound also release a large amount of heat and light on burning.

Saturated hydrocarbons will generally give a clean flame while unsaturated carbon compounds will give a yellow flame with lots of black smoke. This results in a sooty deposit on the metal plate. However, limited supply of air results even in incomplete combustion of saturated hydrocarbons giving a sooty flame.

Disadvantages of incomplete combustion

The most common fuels contain a high percentage of carbon, so it is obviously very important to burn them completely. The incomplete combustion of fuels has the following disadvantages:

(i) Incomplete combustion in insufficient supply of air, leads to unburnt carbon in the form of soot which pollutes the atmosphere, blackens cooking utensils, and blocks chimneys in factories.

(ii) The incomplete combustion also leads to the formation of an extremely poisonous gas called carbon monoxide.

(iii) A yet another disadvantage is that the incomplete combustion of a fuel produces less heat than that produced by complete combustion.

OXIDATION:

Addition of oxygen to any substance is called oxidation and the substances which are capable of adding oxygen to other substances are called oxidizing agents.

We have discussed above the combustion of carbon compounds. Combustion, in fact, means complete oxidation. In addition to this complete oxidation, there are reactions in which partial oxidation occurs. Partial oxidation of substances is carried out by using certain oxidizing agents. Alkaline potassium permanganate and acidified potassium dichromate are good oxidizing agents. These can easily oxidize alcohols to carboxylic acids. For example,

To demonstrate oxidation of alcohols to carboxylic acids, let us perform the following experiment.

Take about 3 ml of ethanol in a test tube and warm it gently in a water bath. Add a 5% solution of alkaline potassium permanganate drop by drop to this solution. The pink colour of potassium permanganate will disappear due to the oxidation of ethanol to ethanoic acid and a brown precipitate of manganese dioxide will be formed due to the reduction of potassium permanganate by ethanol.

SUBSTITUTION REACTION:

The reaction in which an atom or group of atoms in a molecule is replaced or substituted by different atoms or group of atoms are called substitution reactions. Saturated hydrocarbons are fairly unreactive. For example, chlorine does not react with methane at room temperature. However, in the presence of sunlight the reaction of chlorine and hydrocarbons is fairly fast reaction. It gives a variety of products.

In this reaction H - atom of methane has been replaced by a –CI atom converting CH₄, to CH₃Cl. However, if CI₂ is used in excess, all the hydrogen atoms are replaced by chlorine atom one by one.

ADDITION REACTION:

Reactions which involve addition of two reactants to form a single product are called addition reactions.

Due to the presence of double and triple bonds, unsaturated hydrocarbons are more reactive and hence add hydrogen in presence of a catalyst such as nickel, platinum or palladium to form saturated hydrocarbons. This process is called catalytic hydrogenation.

For example,

Addition reaction of Ethene with hydrogen

This addition reaction is commonly used in the hydrogenation (addition of hydrogen) of vegetable oils (such as soyabean oil, cotton seed oil, groundnut oil etc.) in presence of nickel as catalyst to form fats (vegetable ghee such as Gagan, Rath, Dalda, etc.).

The addition of hydrogen to an unsaturated hydrocarbon to obtain a saturated hydrocarbon is called hydrogenation. It is used to prepare vegetable ghee from vegetable oils.

Vegetable oils generally have long unsaturated carbon chains while animal fats have saturated carbon chains.

Need for hydrogenation:

1. When vegetable oils are exposed to hot and humid weather for a long time, they turn rancid, i.e., they develop unpleasant smell and taste. This process of developing unpleasant smell and taste is called rancidity and occurs due to the formation of carboxylic acids and aldehydes by the action of oxygen and moisture on the long unsaturated carbon chains present in vegetable oils. Hydrogenation reduces (but does not eliminate) the number of such unsaturated carbon chains and hence slows down the development of rancidity.

2. Vegetable oils are good for health. Saturated carbon chains present in saturated fats increase the level of bad cholesterol (LDL, low density lipoprotein) in blood which sticks to the walls of the arteries and thus causes coronary heart disease. Vegetable oils (like Sundrop, Saffola, Fortune etc.) are good for health. On the other hand, animal fats like ‘ghee’ and butter (desi ghee) contain saturated fatty acids which are said to be bad for health. Therefore, oils containing unsaturated fatty acids should be used for cooking.