1.) 2× + 4 (×-1)=2+4× 2.) 25- × =15-(3× +10) 3.) 4× =2× +2× +5 (× -×)

1.) x=3
2.)x=-10
3.)x=0

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