Question

Which compound has the lowest melting point?
(A) Ethane
(B) Ethene
(C) Ethyne
(D) All have approximately equal melting points.

Answer 1

The melting point of a compound or a substance is the temperature at which the substance or the compound changes its state from solid to liquid. At the melting point, both solid and liquid phases exist in equilibrium. The melting point of hydrocarbons depends on so many factors such as the number of carbon atoms and intermolecular forces of attraction.

Complete step by step answer
In the question, we are asked to figure out the compound which has the lowest melting point. In the options, we have given the hydrocarbons ethane, ethene, and ethyne. As we know that the given options are the hydrocarbons and hydrocarbons are the compounds that are formed with the help of carbon and hydrogen only. The given hydrocarbons are ethane, ethene, and ethyne. Ethane is an alkane with two carbon atoms and six hydrogen atoms. It follows the general formula of alkanes which is ${C_n}{H_{2n + 2}}$ . For ethane, the value is $n = 2$ and hence it is represented as $C{H_3} - C{H_3}$ or ${C_2}{H_6}$ . Similarly, ethene is represented using the general formula of alkene ${C_n}{H_{2n}}$ , for $n = 2$ the chemical formula will be ${C_2}{H_4}$ and the structure will be $C{H_2} = C{H_2}$ . Similarly, ethyne is represented using the general formula of an alkyne ${C_n}{H_{2n - 2}}$ , for $n = 2$ the chemical formula will be ${C_2}{H_2}$ and the structure will be $C{H_2} \equiv C{H_2}$ .
So, from the structures of the hydrocarbons, we can observe that ethane has a single bond, ethene has a double bond, and ethyne has a triple bond. So, more temperature will be required to break the triple bond and double bond as compared to a single bond which is in ethane. Hence, ethane is the compound that has the lowest melting point.
Therefore, the correct option is (A).

Note:
The melting point of hydrocarbons also depends on the number of carbon atoms. If the number of carbon atoms in the compound is unequal then the compound having a greater number of carbon atoms will have a higher melting or boiling point.