Question

Explain why?

(a) Digestion of food is a decomposition reaction.

(b) All decomposition reactions are endothermic reactions.

(c) A popping sound is produced when a burning candle is brought near mouth of a test tube used in electrolysis of water.

Answer 1

Decomposition reactions

Reactions in which two or products are formed from a single reactant are called decomposition reactions and it includes breaking of certain bonds in a compound. Decomposition reactions are classified as thermal decomposition, photodecomposition and electrolytic decomposition based on the source of energy of decomposition. The general form of a decomposition reaction is:

$AB\to A+B$

(a) During digestion, the complex compounds in our food like carbohydrates, fats and proteins are decomposed into simpler compounds that can easily be absorbed into the body fluids. Carbohydrates are decomposed to simple sugars like glucose, galactose, fructose etc. and proteins are decomposed into amino acids. Fats form fatty acid and glycerol. These decomposition reactions are catalyzed by enzymes and the acid in stomach. Therefore, digestion is an example for decomposition reaction.

(b) In decomposition reactions, larger molecules are broken down into simpler compounds or elements. Therefore, the decomposition reactions involve breaking of bonds which require energy. This energy will be equal to the bond dissociation energy of the corresponding bond and the compounds absorbs energy in the form of heat, light or electricity. Since bonds cannot be broken without absorption of energy by the compound, decomposition reactions are always endothermic.

(c) Electrolysis of Water (H₂O) is an example for decomposition reaction in which electricity is provided to convert Water into Hydrogen (H₂) and Oxygen (O₂). The chemical equation for the reaction is:

$H_{2}O\left(l\right)\to H_2\left(g\right)+O_2\left(g\right)$

(Water) (Hydrogen) (Oxygen)

The combustion of Hydrogen is an explosive reaction. Therefore, it burns in air with a pop sound. When a candle is brought near mouth of test tube used in electrolysis of water, the Hydrogen produced from electrolysis reacts with the oxygen in air and pop sound is produced.