Question

Which of the following reactions is not a combination reaction?

• A. reaction of iodine with white phosphorus
• B. reaction of iron with sulfur
• C. addition of water to lime
• D. addition of concentrated sulphuric acid to sugar.

Answer 1

The correct option is D

addition of concentrated sulphuric acid to sugar.

Correct answer. explanation

$$\begin{gathered} {\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+{\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}\left(\mathrm{aq}\right)\to 12\mathrm{C}\left(\mathrm{aq}\right)+11{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right) \\ (\mathrm{Conc}.\mathrm{sulphuric}(\mathrm{sugar})(\mathrm{carbon}\left(\mathrm{water}\right) \\ \mathrm{acid})\mathrm{black}) \end{gathered}$$

It is a dehydration reaction because in this reaction concentrated sulphuric acid is reacting with sugar and forming a product of carbon black and dehydrating some molecules of the water.

Incorrect answer explanation

$$\begin{gathered} {\mathrm{P}}_4\left(\mathrm{g}\right)+{\mathrm{I}}_2\left(\mathrm{g}\right)\stackrel{{\mathrm{CS}}_2}{\to }{\mathrm{P}}_2{\mathrm{I}}_4\left(\mathrm{g}\right) \\ (\mathrm{White}\mathrm{phosphorus})\left(\mathrm{Iodine}\right)(\mathrm{phosphorus}\mathrm{iodide}) \end{gathered}$$

In this reaction, iodine is reacting with white phosphorus and gives phosphorus iodide $\left({\mathrm{P}}_2{\mathrm{I}}_4\right)$. Therefore, it is a combination reaction.

The reaction of iron with sulphur.

$$\begin{gathered} \mathrm{Fe}\left(\mathrm{s}\right)+\mathrm{S}\left(\mathrm{s}\right)\to \mathrm{FeS}\left(\mathrm{s}\right) \\ \left(\mathrm{Iron}\right)\left(\mathrm{sulphur}\right)(\mathrm{Iron}\mathrm{sulphide}) \end{gathered}$$

It is a combination reaction because in this reaction iron and sulphur are combined to form iron sulphide.

When water is added to lime.

$$\begin{gathered} {\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+\mathrm{CaO}\left(\mathrm{s}\right)\to \mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right) \\ \left(\mathrm{water}\right)(\mathrm{calcium}(\mathrm{calcium} \\ \mathrm{oxide})\mathrm{hydroxide}) \end{gathered}$$

Here, in this reaction water is combined with calcium oxide and forms a product of calcium hydroxide. So. it is also a combination reaction.

Hence, the addition of concentrated sulphuric acid to sugar is not a combination reaction.