Question

A mixture of oxygen and ethyne is burnt for welding. Can you tell why the mixture of ethyne and air is not used?

Answer 1

The process of complete and incomplete combustion should be known to solve this question. As complete combustion is required for welding let us solve the reaction –ethyne with air and ethyne with oxygen to check for complete combustion.

Complete step by step solution:
First let us understand what combustion is,
Combustion also known as burning (at high temperature) is an exothermic redox reaction between the reductant and the oxidant, usually atmospheric oxygen ,that produces oxidized products in the form of smoke.
Complete combustion produces a limited number of products when the reactant is burned with oxygen. Carbon dioxide and water is formed primarily when a hydrocarbon burns in oxygen.
Incomplete combustion takes place when there is not enough oxygen for the reactant to react completely to form the products (carbon dioxide and water).
Now let us see how combustion takes place with ethane;
In complete combustion of ethane a general reaction of reactant and oxygen to give the products .i.e. carbon dioxide and hydroxides.
The equation is given by;
$2C{}_2H{}_2 + 7O{}_2 \to 4CO{}_2 + 6H{}_2O$
In incomplete combustion, ethyne forms Carbon dioxide, carbon monoxide and water vapours.
The equation is given by;
$C{}_2H{}_2 + \dfrac{5}{2}O{}_2 \to CO{}_2 + CO + 2H{}_2O$
A sooty flame is produced when ethyne. It happened because of limited supply of oxygen present in air .i.e. incomplete combustion
$ethyne + air \to incomplete$ $combustion \to sooty$ $flame$
In case of complete combustion, ethyne is burnt with oxygen which gives out clean flame having high temperature $(3000^\circ C)$ where complete combustion of ethyne takes place.
$ethyne + O{}_2 \to clean$ $flame \to oxy - acetylene$ $flame$
The overall reaction of complete combustion of ethane can be written as
$2HC \equiv CH + 5O{}_2 \to 4CO{}_2 + 2H{}_2O + Heat$
Therefore, the oxy-acetylene flame is used for welding. Attaining such a high of $(3000^\circ C)$ is not possible with air or without mixing oxygen. Hence the mixture of ethyne and air is not used, whereas the mixture of ethyne and burnt oxygen is used for welding purposes.

Note:
Having knowledge of combustion and its types is very important as it can get confusing. Knowing the difference between complete combustion and incomplete combustion makes it easier to solve such types of questions.