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प्रश्न
Define standard enthalpy of formation.
उत्तर
- The standard enthalpy of formation of a compound is the enthalpy change that accompanies a reaction in which one mole of pure compound in its standard state is formed from its elements in their standard states.
- Consider the reaction, \[\ce{H2_{(g)} + 1/2 O2_{(g)} -> H2O_{(l)}}\], ΔrH° = −286 kJ
When one mole of liquid water is generated from H2 and O2 gases in their standard states, the enthalpy changes correspond to the standard enthalpy of water creation.
ΔfH° of water is −286 kJ mol−1 - Similarly, the formation of one mole of CH4 from carbon and hydrogen in their standard states is depicted as:
\[\ce{C_{(graphite)} + 2H2_{(g)} -> CH4_{(g)}}\]
ΔrH° = −74.8 kJ or
ΔfH (CH4)
= −74.8 kJ mol−1
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\[\begin{array}{cc}
\phantom{}\ce{H}\phantom{...}\ce{H}\phantom{...................}\ce{H}\phantom{...}\ce{H}\phantom{....}\\
\phantom{.}|\phantom{....}|\phantom{....................}|\phantom{....}|\phantom{.....}\\
\ce{C = C + H - H -> H - C - C - H}\\
\phantom{.}|\phantom{....}|\phantom{....................}|\phantom{....}|\phantom{.....}\\
\phantom{}\ce{H}\phantom{...}\ce{H}\phantom{...................}\ce{H}\phantom{...}\ce{H}\phantom{....}
\end{array}\]
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