Question

Draw the electron-dot structure for ethyne. A mixture of ethyne and oxygen is burnt for welding. In your opinion, why cannot we use a mixture of ethyne and air for this purpose?

Answer 1

i) An electron dot structure is a structure in which valence electrons represented as dots are placed around the chemical symbol of atoms. An ethyne is a form of alkyne which will contain a triple bond between C atoms.
ii) Incomplete burning is not a favourable condition for welding.

Complete-step- by- step answer:
Let us first understand what is meant by an electron dot structure. As the name suggests an electron dot structure is a structure in which valence electrons of an atom are represented as dots around the chemical symbol of the atom. This structure can be used to represent the bonding between atoms in a molecule. In this case, we can use the electron dot structure to show the bonding between atoms in an ethyne molecule.

Ethyne is an alkyne consisting of 2 C atoms that form triple bonds with each other, and 2 H atoms that are attached to each of the C atoms. 1 bond can be represented by 2 electrons, in this case one from each atom. 3 of the 4 valence electrons of each C is used to form the triple bond. As there is a triple bond between the C atoms, we can put 6 electrons in between the C atoms. The remaining valence electron of each carbon is used to bond with 1 H atom each. The C-H bond can be represented by showing 2 electrons in between the atoms.

Let us now understand why a mixture of ethyne and oxygen is burnt for welding and why a mixture of ethyne and air is not used for this purpose.
First see why ethyne and oxygen is the suitable combination. When a mixture of ethyne and oxygen is burnt for welding, it will give a clean flame with a high temperature of upto ${{3000}^{o}}C$ . This is because complete combustion of ethyne takes place in the presence of oxygen.

Let us now discuss why the mixture of ethyne and air is not suitable for welding. When ethyne burns in air, the burning will be incomplete and it will produce a sooty flame due to the limited supply of oxygen that is present in the air.

Hence, a mixture of ethyne and oxygen is used for welding instead of a mixture of ethyne and air.

Note: The electron dot structure is not to be confused with the lewis structure. They are different. A lewis structure represents bonds by drawing a line in between the atoms. Whereas an electron dot structure represents bonds by showing electrons as dots in between the atoms.