Use of fullpage style (must be the shortest CTAN manual that nobody reads :-)
you could just add in preamble
\usepackage{fullpage} 
\documentclass{article} \usepackage{fullpage} \usepackage{amsmath} \usepackage{mhchem} \usepackage{amssymb} \begin{document} Overall formation of ethane thingy: \ce{2 C(s) + 3 H2(g) -> C2H6(g)} \\ \begin{alignat}{2} \ce{2C(s) + 2O2(g) &-> 2CO2(g)} &\Delta H &= -786 kJ\\ \ce{3H2(g) + 3/2 O2(g) &-> H2O(l)} &\Delta H &= -858 kJ\\ \ce{2 CO2(g) + 3 H2O(l) &-> C2H6(g) + 7/2 O2 (g)} &\Delta H &= 1560 kJ\\ \nonumber \ce{2 C(s) + 3 H2(g) + 7/2 O2(g) &-> C2H6(g) + 7/2 O2(g)} &\indent \Delta H &= -786-858+1560\\ \nonumber & &&= -84 kJ \\ \nonumber &&\therefore \Delta H_{f(C_2H6)}^0 &= -84kJ \end{alignat} \end{document}