NEETChemistryThermodynamics (C)
For which of the following reactions is the change in enthalpy ( H ) equal to the change in internal energy ( U ) at a given constant temperature?
Options
- APCl ₅( g ) PCl ₃( g ) + Cl ₂( g )
- B2 CO ( g ) + O ₂( g ) 2 CO ₂( g )
- CH ₂( g ) + Cl ₂( g ) 2 HCl ( g )
- DH ₂( g ) + I ₂( s ) 2 HI ( g )
Correct answer
C. H ₂( g ) + Cl ₂( g ) 2 HCl ( g )
Step-by-step solution
The relationship between change in enthalpy ( H ) and change in internal energy ( U ) is given by: H = U + n _ g RT For H = U , the term n _ g RT must be equal to zero. Since R and T are non-zero, this requires n _ g = 0 . n _ g is the difference between the number of moles of gaseous products and gaseous reactants. Let us evaluate n _ g for each option: Option 1: n _ g = (1 + 1) - 1 = 1 Option 2: n _ g = 2 - (2 + 1) = -1 Option 3: n _ g = 2 - (1 + 1) = 0 Option 4: n _ g = 2 - 1 = 1 (Note: I ₂ is in the solid state