The standard enthalpy of formation of gaseous `H_(2)O` at 298 K is `-241.82` kJ/mol. Calculate `DeltaH^(@)` at 373 K given the following values of the molar heat capacities at constant pressure :
`H_(2)O(g)=33.58 " JK"^(-1)" mol"^(-1), " "H_(2)(g)=29.84 " JK"^(-1)" mol"^(-1), " "O_(2)(g)=29.37 " JK"^(-1)" mol"^(-1)`
Assume that the heat capacities are independent of temperature :

Options

A. `-242.6 kJ//mol`
B. `-485.2 kJ//mol`
C. `-121.3 kJ//mol`
D. `-286.4 kJ//mol`

Answer

A

Explanation

`Delta_(r )C_(p)^(@)=-10.945" for"`
`H_(2)(g)+(1)/(2)O_(2)(g)rarrH_(2)O(g)`
According to Kirchhoff's law
`Delta_(r )H_(T_(2))^(@)-Delta_(r )H_(T_(1))^(@)=Delta_(r )C_(p)^(@)[T_(2)-T_(1)]`
`Delta_(r )H_(T_(2))^(@)-(241.82)=[-10.94 J (373-298)//100]`
`implies Delta_(r )H_(T_(2))=-242.6 " kJ"//"mol"`