Question

From the following organic compounds given below, choose one compound in each case which relates to the description (i) to (iv):[Ethyne, Ethanol, Acetic acid, Ethene, Methane]
(i)An unsaturated hydrocarbon used for welding purposes.
(a)Ethyne
(b)Ethanol
(c)Acid acid
(d)Ethene
(e)Methane

Answer 1

Hydrocarbons are used as excellent fuel materials for welding of metals. Unsaturated hydrocarbons, the double or triple bond is an energy source.

Complete step by step solution : Ethyne is the unsaturated hydrocarbon used for welding purposes. An acetylene compound consists of 2 carbon atoms which is triply bonded and each of them has one hydrogen ($HC\equiv CH$).This triple bond is useful because it stores substantial amounts of energy and this can be released as heat energy during combustion.
For a fuel to be suitable for welding, the fuel gas must have:
-High flame temperature
-The rate of flame propagation is high
-Adequate heat content
-Minimum chemical reaction of the flame with base and filler metals.
Acetylene meets almost all the requirements. Thus, the answer to the question is option (a).

Additional Information: Due to this triple bond, oxy-acetylene flame, the hottest of all gas flames, is responsible for its 2 other properties
(i)Gaseous acetylene when subjected to severe shock , some of the triple bond break, releasing energy which is enough to call all the other molecules in the enclosed volume to decompose and form carbon and hydrogen with an explosive force. The explosion consists of a mixture of oxygen and acetylene and air.
(ii)Next property is that the flammability range of the mixture mentioned above is broader than any other fuel gas/air range. Acetylene/air mixture can be ignited easily when the amount of acetylene ranges from 2.5% to 80%. Mixtures of air and methane are also flammable but here should be around 5-15% of methane in the air.

Note : Double and triple bonds in unsaturated hydrocarbons can store energy. Acetylene has a triple bond, so more energy is stored