A. explain the difference between an exothermic chemical reaction and an endothermic chemical reaction.
b. how is the enthalpy of reaction related to the enthalpies of formation for the products and reactants?

An exothermic reaction creates heat, whereas a endothermic reaction absorbs heat.  

Explanation:

in cartography, a contour line (often just called a "contour") joins points of equal elevation (height) above a given level, such as mean sea level. a contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness or gentleness of slopes.

explanation: