Question

What happens when orthophosphoric acid is heated to $\text{600 }\!\!{}^\circ\!\!\text{ C}$.?
A. Phosphorus is formed
B. Phosphorus acid is formed
C. Hydrogen gas is released
D. Phosphorus pentoxide is formed

Answer 1

Hint: Orthophosphoric acid is another name for Phosphoric acid. It is a weak acid. The chemical formula for Orthophosphoric acid is ${{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}$.

Step by step answer:
Orthophosphoric acid or ${{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}$ when heated to a temperature of $\text{250 }\!\!{}^\circ\!\!\text{ C}$ yields pyrophosphoric acid and water as products of its reaction. This following chemical equation signifies the reaction that takes place when orthophosphoric acid is heated to a temperature of $\text{250 }\!\!{}^\circ\!\!\text{ C}$.
$\text{2}{{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}\xrightarrow{\text{250 }\!\!{}^\circ\!\!\text{ C}}{{\text{H}}_{\text{4}}}{{\text{P}}_{\text{2}}}{{\text{O}}_{\text{7}}}\text{ + }{{\text{H}}_{\text{2}}}\text{O}$
When further heated up to a temperature of $\text{316 }\!\!{}^\circ\!\!\text{ C}$, orthophosphoric acid yields metaphosphoric acid. Metaphosphoric acid being a strong dehydrating agent, moisture is lost. This is denoted from the equation of the reaction that takes place which is given below. The heating of orthophosphoric acid to $\text{316 }\!\!{}^\circ\!\!\text{ C}$ or the heating of pyrophosphoric acid to $\text{316 }\!\!{}^\circ\!\!\text{ C}$ yields metaphosphoric acid and water as products of the reaction.
${{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}\xrightarrow[\Delta ]{\text{316 }\!\!{}^\circ\!\!\text{ C}}\text{HP}{{\text{O}}_{3}}\text{ + }{{\text{H}}_{\text{2}}}\text{O}$
or
\[{{\text{H}}_{\text{4}}}{{\text{P}}_{\text{2}}}{{\text{O}}_{\text{7}}}\xrightarrow[\text{ }\!\!\Delta\!\!\text{ }]{\text{316 }\!\!{}^\circ\!\!\text{ C}}\text{2HP}{{\text{O}}_{\text{3}}}\text{ + }{{\text{H}}_{\text{2}}}\text{O}\]
On further heating, at $\text{600 }\!\!{}^\circ\!\!\text{ C}$ orthophosphoric acid yields Phosphorus pentoxide and water as the products of the reaction. The equation for the reaction of orthophosphoric acid when heated to a $\text{600 }\!\!{}^\circ\!\!\text{ C}$ mark is written below.
$\text{2}{{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}\xrightarrow[\text{ }\!\!\Delta\!\!\text{ }]{\text{600 }\!\!{}^\circ\!\!\text{ C}}{{\text{P}}_{2}}{{\text{O}}_{5}}\text{ + 3}{{\text{H}}_{\text{2}}}\text{O}$

Note: Phosphoric acid is a colourless, odourless liquid which is crystalline in nature. Orthophosphoric acid is a diprotic acid which means that in an aqueous solution, it can donate two hydrogen atoms or protons per molecule. When Orthophosphoric acid is heated at $\text{600 }\!\!{}^\circ\!\!\text{ C}$, it might also yield some amount of ${{\text{P}}_{\text{4}}}{{\text{O}}_{\text{10}}}$ other than water as one of its products. The following equation shows the reaction that takes place when orthophosphoric acid yields ${{\text{P}}_{\text{4}}}{{\text{O}}_{\text{10}}}$ on heating to a temperature of $\text{600 }\!\!{}^\circ\!\!\text{ C}$.
$\text{4}{{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}\xrightarrow[\text{ }\!\!\Delta\!\!\text{ }]{\text{600 }\!\!{}^\circ\!\!\text{ C}}{{\text{P}}_{\text{4}}}{{\text{O}}_{\text{10}}}\text{ + 6}{{\text{H}}_{\text{2}}}\text{O}$