Question

Phosphoric acid, which is commonly used as rust inhibitor, food additive and etching agent for dental and orthopedic use, can be synthesized using a two-step thermal process. in the first step, phosphorus and oxygen react to form diphosphorus pentoxide: (l) (g) (g)

Answer 1

The first step in the synthesis of phosphoric acid is the reaction of phosphorus and oxygen to form diphosphorus pentoxide.

$P_{4}(l) + 5 O_{2}(g)--->2 P_{2}O_{5} (g)$

The second step in the synthesis of phosphoric acid is the reaction of diphosphorus with water to form phosphoric acid.

$P_{2}O_{5}(g) + 3H_{2}O (l) --> 2H_{3}PO_{4}(aq)$

The net chemical equation representing the formation of phosphoric acid from phosphorus, water and oxygen will be,

$Step -1 : P_{4}(l) + 5O_{2}(g) ---> 2P_{2}O_{5}(g)$

$Step 2: P_{2}O_{5}(g) + 3 H_{2}O(l)-->2 H_{3}PO_{4}(aq)$

Equation in step 2 must be multiplied by 2 in order to cancel diphosphorus pentoxide.

$P_{4}(l) + 5O_{2}(g)---> 2P_{2}O_{5}$

$2 [P_{2}O_{5} (g) + 3 H_{2}O(l) --> 2 H_{3}PO_{4} (aq) ]$

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Net: $P_{4}(l) + 5 O_{2}(g) + 6 H_{2}O(l) ---> 4H_{3}PO_{4}(aq)$