Question

How can you find heat of formation of water?

Answer 1

Hint: Heat of formation of compounds is the fraction of heat evolved or absorbed when \[1\] mole of the substance is directly obtained from its constituent elements. Usually the conditions at which the compound is formed are taken to be temperature of \[{{25}^{\circ }}C\] and pressure of \[1\] atm in which case the heat of formation is called as standard heat of formation.

\[\Delta H=\][sum of bond energy of reactants- bond energy of products]

Complete step by step answer:
As we know during any reaction evolution or absorption of heat energy occurs. This is in accordance with law of conservation of energy (1st law of thermodynamics) enthalpy of formation or heat of formation is the fraction of heat evolved or absorbed when \[1\] mole of substance is directly obtained from its constituent elements.
To obtain enthalpy of formation of water
\[{{H}_{2}}\left( g \right)+\dfrac{1}{2}{{O}_{2}}\left( g \right)\to {{H}_{2}}O\left( g \right)\to \Delta H=?\]
for this you should know the bond energies of the reactants and products.
BE-Bond Energy
\[\Delta H=[Be\,of\,{{H}_{2}}+\dfrac{1}{2}\times BE\,of\,{{O}_{2}}]-[2BE\,ofO-H]\]
\[=433+\left( \dfrac{1}{2}\times 492 \right)\times -\left( 2\times 464 \right)\]
\[=433+246-928\]
\[=-249KJ\]
The negative sign indicates that the reaction is exothermic and heat is evolved during reaction.

So, the answer to this question is \[-249KJ\] which is the heat of formation of water.

Note: The enthalpy of formation depends upon the conditions of formation i.e, temperature, pressure and physical state (gas, solid or liquid) or allotropic state of their reactants. By using the thesis law of heat summation, we can calculate the heat absorbed or evolved in any chemical reaction by summing the known heats of formation or combustion for the steps in that reaction.