Chemical equation with heat of formation

You can simply write it inside the equation and maybe add some horizontal space between with \quad or \qquad:

\documentclass{article}
\usepackage[version=4]{mhchem} % Package for chemical equation typesetting
\usepackage{amssymb}
\usepackage{siunitx}
\begin{document}

\begin{equation}
  \ce{N2(g) + 3H2(g) -> 2NH3(g)} \qquad \Delta H_{\mathrm{f}}^\circ = \SI{-92.5}{kJ}
\end{equation}  

\end{document}

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An alternative using chemmacros:

\documentclass{article}

\usepackage{chemmacros}
\chemsetup{
  formula = chemformula , % or mhchem
  modules = thermodynamics
}

\begin{document}

\begin{equation}
  \ch{N2\gas{} + 3 H2\gas{} -> 2 NH3\gas} \qquad \enthalpy(f){-92.5}
\end{equation}  

\end{document}

For a non-molar heat of formation it would let you easily define a command \formation:

\documentclass{article}

\usepackage{chemmacros}
\chemsetup{
  formula = chemformula , % or mhchem
  modules = thermodynamics
}

\NewChemState\formation{
  symbol = H ,
  subscript =f ,
  unit = \kilo\joule
}

\begin{document}

\begin{equation}
  \ch{N2\gas{} + 3 H2\gas{} -> 2 NH3\gas} \qquad \formation{-92.5}
\end{equation}  

\end{document}

There are a number of options for customization (e.g. regarding the position of the subscript).

You can use the align environment from »amsmath«. You only need to use \cee for the chemical part of the equation.

\documentclass[11pt]{article}
\usepackage[T1]{fontenc}
\usepackage[version=3]{mhchem} % Package for chemical equation typesetting
\usepackage{amsmath}
\usepackage{siunitx}

\begin{document}
  \begin{align}
    \cee{N2(g) + 3H2(g) &-> 2NH3(g)} & \Delta H_{\text{f}}^\circ = \SI{-92.5}{\kilo\joule}
  \end{align}
\end{document}

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