Question

A mixture of oxygen and ethyne is burnt for welding. Why is a mixture of ethyne and air not used?
A. Ethyne cannot burnt in air
B. Incomplete combustion takes place.
C. Ethyne burn in air with explosion
D. None of these

Answer 1

Hint- In order to solve this question, firstly we must know the concept of complete combustion and incomplete combustion. Then we will do both the reactions of ethane with oxygen and ethane with air to check where the complete combustion occurs to get the required result.

Complete answer:
Firstly, we must know complete combustion of ethane and incomplete combustion of ethane to proceed towards the answer.
Complete combustion of ethane- A combustion reaction has a general reaction of fuel and oxygen to form carbon dioxide and hydro-oxide.
Balanced equation-
$2{C_2}{H_2} + 7{O_2} \to 4C{O_2} + 6{H_2}O$
Incomplete combustion of ethane- Ethyne undergoes incomplete combustion to form carbon dioxide, carbon monoxide and water vapour.
The balanced equation of this incomplete combustion is as follows-
${C_2}{H_4} + \dfrac{5}{2}{O_2} \to C{O_2} + CO + 2{H_2}O$
Now, when ethyne (acetylene) is burnt in air, it produces a sooty flame. This is due to incomplete combustion caused by the limited supply oxygen present in the air that is not enough to melt metals for metals.
$ethyne + air \to incomplete{\text{ combustion}} \to {\text{sooty flame}}$
However, if ethyne is burnt with oxygen, it gives a clean flame with high temperature i.e. 3000 degrees celsius due to complete combustion of ethyne.
$ethyne + {O_2} \to clean{\text{ flame}} \to {\text{oxy - acetylene flame}}$
The reaction of complete combustion of ethane can be written as-
$2HC \equiv CH + 5{O_2} \to 4C{O_2} + 2{H_2}O + Heat$

Hence, this oxy-acetylene flame is used for welding and it is not possible to attain such a high temperature with air or without mixing oxygen. Due to this reason, a mixture of ethyne and air is not used for welding whereas mixture ethyne burnt in oxygen is used for welding purposes.

Note- Whenever we face such a type of problem, one can get confused with complete combustion and incomplete combustion so we must know the difference between complete combustion and incomplete combustion to get the required result.