Question

Which of the following is the dehydrating agent in the given reaction?
${{C}_{6}}{{H}_{12}}{{O}_{6}}\xrightarrow[conc.]{{{H}_{2}}S{{O}_{4}}}6C+6{{H}_{2}}O$
A. Glucose
B. Concentrated sulphuric acid
C. Carbon
D. Water

Answer 1

Hint: The answer is known by the name “oil of vitriol”. It is a colourless, odourless, and viscous liquid that is soluble in water and is synthesized in reactions that are highly exothermic. It is widely used in the manufacture of chemicals and fertilizers.

Step-by-step answer:
We should know that Sulphuric acid is a powerful drying agent. It extracts the water from the sugar to form carbon and generates so much heat the water boils and expands the whole mess. So, to put that all together, sulphuric acid and sugar are mixed in a beaker, due to a low energy being required for this reaction to occur (activation energy) the heat at room temperature is enough to break the bonds of the two elements, and the reaction starts. The sulphuric acid breaks down the sugar into carbon (C) and hydrogen and oxygen, which re-bond to form water (${{H}_{2}}O$). The energy created by these new bonds forming is more than what is required for the reaction to happen, so energy as heat is given off. This heat turns the newly formed water into vapour, which we see as steam. This steam is what pushes the black carbon up out of the beaker.
So, now we come to the conclusion that when concentrated sulphuric acid reacts with sugar it acts as a dehydrating agent; removing water from the sucrose. It caused the carbohydrate to convert all the way down to elemental carbon. Steam and sulphurous fumes are produced plus a smell of caramel.
So, from the options given above we can now say that concentrated sulphuric acid in option B is used as a dehydrating agent.

Note: We should note that sulphuric acid is one of the most important industrial chemicals. We use sulphuric acid in the production of fertilizers, such as superphosphate of lime and ammonium sulphate. It is widely used in the manufacture of chemicals, such as in making hydrochloric acid, nitric acid, sulphate salts, synthetic detergents, dyes and pigments, explosives, and drugs. It is used in petroleum refining to wash impurities out of gasoline and other refinery products.