Shanghai gained importance due to its prime location at:

Shanghai gained importance due to its prime location at the mouth of the Yangtze River.
This location brought Shanghai many advantages and allowed it to prosper and thrive and become and important part of the Asian part of the world, as well as the rest of it. In the past, and even now, it was very important for large cities to be situated near rivers and lakes and other types of waters so as to ensure the advancement of that city.

So  if  the sum of the enthalpies of the reactants is greater than the products, the  reaction  will be  exothermic.  if  the products side has a larger enthalpy, the  reaction  is  endothermic. you may wonder why  endothermic reactions, which soak up energy or enthalpy from the environment, even happen