1.

Which of the reaction defines DeltaH_(f)^(@)

Answer»

`C_(("diamond"))+O_(2(g))rarrCO_(2(g))`
`(1)/(2)H_(2(g))+(1)/(2)F_(2(g))rarrHF_((g))`
`N_(2(g))+3H_(2(g))rarr2NH_(3(g))`
`CO_((g))+(1)/(2)O_(2(g))rarrCO_(2(g))`

Solution :`DeltaH_(f)^(@)` is the enthalpy CHANGE when 1 mole of the substance is formed from its elements in the standard state. In (a) carbon is present in diamond however its standard state is GRAPHITE. Again, in (d) CO (g) is involvedso it can't be the right option. Further inn (c ) 2 MOLES of `NH_(3)` are generated. HENCE the correct option is (b).


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