①C(s)+H2O(g)=CO(g)+H2(g) H1=akJ mol−1
②CO2(g)+H2(g)=CO(g)+H2O(g) H2=bkJ mol−1
③CO2(g)+3H2(g)=CH3OH(g)+H2O(g) H3=ckJ mol−1
④2CH3OH(g)=CH3OCH3(g)+H2O(g) H4=dkJ mol−1