Question

12(b). Give a chemical test to distinguish between the following pairs of compounds : (iii) Ethene gas and ethane gas.

Answer 1

Step 1:Laboratoy procedure: Bromine water test:

Take Ethane ${\mathrm{C}}_2{\mathrm{H}}_6$ gas and Ethene ${\mathrm{C}}_2{\mathrm{H}}_4$ gas in two separate test tubes. Add Carbon tetrachloride ${\mathrm{CCl}}_4$ to both the gases. Then pass Bromine gas ${\mathrm{Br}}_2$ and observe the color change. In the case of Ethene, the brown color of the gas disappears while the gas remains as such in the case of Ethane. This happens due to unsaturation present in Ethene.

Step 2: Chemical equation involved:

In the case of Ethene:

$$\begin{gathered} {\mathrm{C}}_2{\mathrm{H}}_4+{\mathrm{Br}}_2\uparrow \to {\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Br}}_4 \\ \mathrm{Ethene}\mathrm{Bromine}\mathrm{Ethylbromide} \\ \mathrm{gas} \end{gathered}$$

In the case of Ethane:

$$\begin{gathered} {\mathrm{C}}_2{\mathrm{H}}_6+{\mathrm{Br}}_2\uparrow \to \mathrm{No}\mathrm{reaction} \\ \mathrm{Ethane}\mathrm{Bromine} \\ \mathrm{gas} \end{gathered}$$
Final answer: Bromine water test is given by Ethene and not Ethane.