Question

calculate ΔH for the formation of one mole of dinitrogen pentoxide from its elements in their stable state at 25°C and 1 atm.

Answer 1

$N_2+$ $5\over 2$ $O_2\to N_2O_5$

From the equation;

$H_2O\to H_2+$ $1\over 2$ $O$

$2HNO_3\to N_2O+H_2O$

$N_2+3O_2\to 2HNO_3$

Cancelling the equations;

$H_2O$ $\to$ Equation 1 and 3 cancel

$2HNO_3\to$ Equation 2 and 3 cancel

$1\over 2$ $O_2\to$ Equation 1 and 3 cancel

Cancelling the equations reduces the enthalpy of $O_2$ from $6\over 2$ to $5\over 2$

Adding the enthalpies;

$(+285.8)+(+73.7)+(-348.2)=+11.3kJ$

$\therefore$ $\Delta H_f=11.3kJ$