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Question

The Heat required for reaction of one litre of H2 (g) with sufficient O2 (g) at STP is 12.78 kJ. Find the approximate value of standard enthalpy of formation of H2O (l).

A
+186 kJ
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B
286 kJ
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C
480 kJ
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D
None of these
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Solution

The correct option is B 286 kJ
The reaction taking place is:
H2 (g)+12O2(g)H2O(l)

Considering,
1 L H2 (g) at STP=122.4 mol
Heat released due to reaction of 122.4 mol of H2(g)=12.78 kJ
But for formation one mole is reacting, so
Heat released due to reaction of 1 mol of H2(g)
=12.78×22.4=286.27 kJ
standard enthalpy of formation of H2O (l)= 286 kJ


Standard enthalpy of formation (Δf H°)

It is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements in their most stable forms (Standard states 1 bar and usually 298 K).

It is denoted by ΔfH°

Example:

Standard enthalpy of formation of HCl(g)

Chemical equation

12H2(g)+12Cl2(g)HCl(g)
Standard enthalpy of reaction in this case is the standard enthalpy of formation of HCl(g)

ΔfH°(HCl,g)=Hm°(HCl,g)12Hm°(H2,g)12Hm°(Cl2,g)


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