Question

Consider the following statements:
(a) Rusting is an exothermic process.
(b) Dissolution of ammonium chloride in water is an exothermic process.
(c) Addition of chlorine is also an oxidation process
(d) The formula of calcium phosphate is $\text{ C}{{\text{a}}_{\text{3}}}{{\left( \text{P}{{\text{O}}_{\text{4}}} \right)}_{\text{2}}}\text{ }$
Which of the above statements are correct.
A) (a), (b) and (c)
B) (a), (b) and (d)
C) (b), (c) and (d)
D) (a), (c) and (d)

Answer 1

the endothermic reaction is the reaction in which the reactant absorbs the energy from the surrounding and forms a product. This lowers the surrounding temperature. The exothermic reaction liberates energy. This process proceeds via the formation of a bond. When salt is introduced in a solution, the ions are surrounded by the water molecules. This reduces the energy of dissolution.

Complete step by step answer:
a) Rust is a chemical reaction between iron and oxygen. It is a slow reaction of oxidation of iron. This produces the iron oxide. This formation of new bonds releases energy in the form of heat. The reaction which is associated with the liberation of heat is called the exothermic reaction. Thus rusting of iron is an exothermic process. The reaction of formation of rust is as shown below,
$\text{ Fe + }{{\text{H}}_{\text{2}}}\text{O + }{{\text{O}}_{\text{2}}}\text{ (air) }\to \text{ F}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}\text{.x}{{\text{H}}_{\text{2}}}\text{O }$
b) When ammonium chloride $\text{ N}{{\text{H}}_{\text{4}}}\text{Cl }$is dissolved in the water, the water molecules separated the ammonium chloride from the solution as a result of dipole-dipole interaction. This process is also known as hydration. Thus $\text{ N}{{\text{H}}_{\text{4}}}\text{Cl }$is dissolved and yields as a $\text{ NH}_{4}^{+}\text{ }$ and $\text{ C}{{\text{l}}^{-}}\text{ }$ .the reaction are depicted as follows,
$\text{ N}{{\text{H}}_{\text{4}}}\text{Cl }(s)\text{ +}{{\text{H}}_{\text{2}}}\text{O(}l)\text{ + Heat }\to \text{ NH}_{4}^{+}\text{ }(aq)\text{ + C}{{\text{l}}^{-}}\text{ }(aq)\text{ }$
The dissolution of ammonium chloride thus requires extra energy .hence, $\text{ N}{{\text{H}}_{\text{4}}}\text{Cl }$withdraws the energy from surrounding, thus the dissolution of $\text{ N}{{\text{H}}_{\text{4}}}\text{Cl }$is an endothermic reaction.
c) Oxidation is a chemical process in which there is a removal of an electron, removal of hydrogen, or loss of electropositive radicals. We know that the increase in the positive charge is also known as oxidation .so, the chlorine is bonded to the carbon atom it increases the partial positive charge on the carbon atom as therefore addition of chlorine is known as the oxidation reaction.
d) Calcium phosphate is a salt of phosphoric acid. The phosphate $\text{ PO}_{4}^{3-}\text{ }$ forms a compound with the calcium ions. Calcium loses its two electrons and exists in the divalent state. Therefore, calcium has a valency of $\text{ +2 }$ .the phosphate has a valency equal to $\text{ }-3\text{ }$ . Thus the phosphate forms a compound with calcium ions to form calcium phosphate compounds. The formula for calcium phosphate is $\text{ C}{{\text{a}}_{\text{3}}}{{\left( \text{P}{{\text{O}}_{\text{4}}} \right)}_{\text{2}}}\text{ }$. Therefore, statement (a), (c) and (d) are correct but (B) is incorrect.

Hence, (D) is the correct option.

Note: Note that, ions dissolved in the solution get surrounded by the water molecule. Thus the hydration process is accompanied by the liberation of energy. Thus hydrated or solvated ions require extra energy to free from the water molecule cage. Thus, the energy is taken from the surroundings to break the cage of a water molecule.