Question

What happens when (write chemical equation in each case):
(1) Ethanol is burnt in air
(2) Ethanol is heated with excess sulphuric acid at 443K
(3) A piece of sodium metal is dropped in ethanol?

Answer 1

Hint: Ethanol is an alcohol and it undergoes combustion reaction when burnt in air. If we heat acid with ethanol, it will dehydrate ethanol and give an alkene. When we add a piece of sodium metal in ethanol, alkoxide is formed.

Complete step by step answer:
We know that ethanol is a primary alcohol with the formula ${{C}_{2}}{{H}_{5}}OH$. We know that for alcohols, the functional group is –OH. In ethanol, the functional group is attached to an ethane alkyl group where one hydrogen atom is replaced by the –OH functional group and thus gives rise to the name ethanol. Now we will discuss the reactions asked in question one by one.
- In the first statement we have ethanol that is burnt in air.
Ethanol is a flammable volatile liquid and thus it burns efficiently and completely in air and shows a blue flame. When it burns in air i.e. in presence of oxygen, it produces carbon dioxide gas and water. Combustion of ethanol is exothermic thus it also produces heat. We can write the reaction as-
     \[{{C}_{2}}{{H}_{5}}OH+3{{O}_{2}}\to 2C{{O}_{2}}+3{{H}_{2}}O+\Delta \left( heat \right)\]

- Next we have that ethanol is treated with excess sulphuric acid at a very high temperature.
We know that sulphuric acid is an oxidising agent but it also is a dehydrating agent. When we heat ethanol at a very high temperature with excess of concentrated sulphuric acid, we get ethene. Sulphuric acid acts as a dehydrating agent and rips off a water molecule from ethanol. We can write the reaction as-
     \[{{C}_{2}}{{H}_{5}}OH+{{H}_{2}}S{{O}_{4}}(conc.\text{ and excess})={{C}_{2}}{{H}_{4}}+{{H}_{2}}S{{O}_{4}}(dil.)\]

Sulphuric acid just dehydrates ethanol and thus we get it as it is but diluted in the product.

- And lastly we have that a piece of sodium metal is dropped in ethanol.
When we drop a small piece of sodium metal in ethanol, we get sodium ethoxide and hydrogen gas is evolved in the form of bubbles. We can write the reaction as-
     \[{{C}_{2}}{{H}_{5}}OH+2Na\to 2{{C}_{2}}{{H}_{5}}ONa+{{H}_{2}}(g)\]

Note: We should not be confused between ethanol and ethanal. Ethanol is an alcohol where ethanal is an aldehyde.
As we can see from the above reactions that after combustion of ethanol the only products formed are carbon dioxide and water. This property of ethanol has been used to fuel buses as it favoured for environment conscious transportation.