During the period each year when we see mars undergoing apparent retrograde motion in our sky, what is really going on in space?

It's the point where we are passing it while we're orbiting the sun and it looks like it's moving backwards

Themes; subfields is the answer

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