Using Hess law, calculate ΔH for the equation: O3 (g) +2NO2 (g)→ N2O5 (g) + O2 (g)

Given the following equations:

N2O5 (g) rightwards arrowNO3 (g) + NO2 (g) ΔH=+ 44.3 kJ

NO3 (g) + O2 (g) rightwards arrowO3 (g) + NO2 (g) ΔH=-20.1kJ

1 answer

To use Hess's law, we need to manipulate the given equations so that they add up to the desired equation:

N2O5 (g) rightwards arrowNO3 (g) + NO2 (g) (multiply by 1)

2NO3 (g) + O2 (g) rightwards arrow2O3 (g) + 2NO2 (g) (multiply by 2 and flip the direction)

O3 (g) + 2NO2 (g) rightwards arrowN2O5 (g) + O2 (g) (add the two equations)

Now we can add the enthalpies of the individual reactions to get the total enthalpy change for the desired reaction:

ΔH = ΔH1 + ΔH2

ΔH1 = +44.3 kJ (from the first equation)
ΔH2 = -20.1 kJ (from the second equation, multiplied by 2 and flipped)

ΔH = +44.3 kJ - 2(-20.1 kJ) = +84.5 kJ

Therefore, the enthalpy change for the reaction O3 (g) + 2NO2 (g) → N2O5 (g) + O2 (g) is +84.5 kJ.