Question

Prove that ${P_2}{O_5}\;$ is a powerful dehydrating agent.

Answer 1

To answer this question, you should recall the concept of drying agents. Drying agents can be defined as any hygroscopic substance that induces or sustains a state of dryness in its vicinity. Strong bases such as soda-lime, sodium hydroxide, potassium hydroxide, and lithium hydroxide can remove carbon dioxide by chemically reacting with it. ${P_2}{O_5}\;$ is the anhydride of phosphoric acid.

Complete Step by step solution:
We know that common drying agents are anhydrous inorganic salts that acquire waters of hydration when exposed to moist air or a wet solution. Upon hydrolysis of phosphorus pentoxide, the reaction which takes place is exothermic. It can extract water from many inorganic compounds which include sulphuric acid, nitric acid and several organic compounds. The reactions can be represented as:
${H_2}S{O_4}\xrightarrow{{{P_2}{O_5}\;}}S{O_3} + {H_2}O$ and $HN{O_3}\xrightarrow{{{P_2}{O_5}\;}}{N_2}{O_5} + {H_2}O$

Additional information: Important acidic nature of oxoacids of phosphorus: You are already aware of the term acids. These acids show varied physical and chemical properties of acids. Some simple ones are – they need a pH below 7, they turn blue paper red, they need a sour taste and that they react with alkalis to make salts. There is little difference of acidic strength among ${{\mathbf{H}}_{\mathbf{3}}}{\mathbf{P}}{{\mathbf{O}}_{\mathbf{4}}},{{\mathbf{H}}_{\mathbf{3}}}{\mathbf{P}}{{\mathbf{O}}_{{\mathbf{3}}}}\;{\mathbf{and}}\;{{\mathbf{H}}_{\mathbf{3}}}{\mathbf{P}}{{\mathbf{O}}_{\mathbf{2}}}$ because the hydrogens in these acids aren't all bonded to oxygen, and phosphorus isn't a highly electronegative element i.e. its oxides are less basic and electronegativities of ${\text{P}}$ and ${\text{H}}$ are almost the same. Hence, they all have almost similar acidic strength.

Note: Even though the structure of the compound contains four phosphorus and ten oxygen chemists always refer to this material as ${P_2}{O_5}$( four and ten can be divided by two so we go for the smaller numbers). This is the reason it is termed as diphosphorus pentoxide, and generally the prefix: di dropped as it is a known fact that oxygen is divalent. Make sure you remember the difference between the drying agent and dehydrating agent. A drying agent refers to a chemical substance which is used to remove water from an organic compound that is in solution while a dehydrating agent refers to a chemical that dries or removes water from a material.