Question

The heat of formation for a compound in the following reaction.
\[{{\text{H}}_2}({\text{g}}) + {\text{C}}{{\text{l}}_2}({\text{g}}) \to 2{\text{HCl}} + 44{\text{ kcal mo}}{{\text{l}}^{ - 1}}\]
A.\[ - 44{\text{ kcal mo}}{{\text{l}}^{ - 1}}\]
B.\[ - 22{\text{ kcal mo}}{{\text{l}}^{ - 1}}\]
C.\[11{\text{ kcal mo}}{{\text{l}}^{ - 1}}\]
D.\[ - 88{\text{ kcal mo}}{{\text{l}}^{ - 1}}\]

Answer 1

Hint: Heat of formation is the energy released or produced when I mole of product is formed. Since the energy released is given to us for formation of 2 moles, so divide it with 2.

Complete step by step answer:
In a chemical reaction reactant reacts to form products.
In some reactions energy is consumed during the process while in some reactions energy releases during the process. This energy change whether energy is released or consumed is known as the enthalpy of reaction.
Enthalpy of formation or heat of formation is defined as the energy change when 1 mole of compound is formed from its constituent elements in the most stable state. Enthalpy of formation is just another case of standard enthalpy of reaction. In enthalpy of formation two or more than two reactants combine to form a product.
We can check whether energy is released or absorbed by looking on which side the energy is written. When the energy is released it is written on the product side but when energy is consumed it is written on the reactant side.
When the energy is released we put a negative sign before it and when energy is consumed we put a positive sign before it.
In the question 2 moles of product are formed but we need to want energy for 1 mole of product. We will divide the energy by 2. Hence the energy released will be \[ - 22{\text{ kcal mo}}{{\text{l}}^{ - 1}}\].

Thus, the correct option is B.

Note:
Energy of formation is defined when reactants are in the most stable state. For example in the given question hydrogen and chlorine are in the gaseous state which is their most stable state, but if suppose hydrogen would be given to us in liquid or solid state then we cannot determine the enthalpy of formation from the given data.