During chemical reactions the bonds between atoms break and new bonds form. Energy must be absorbed to break a bond, so breaking bonds is endothermic.

Making new bonds is exothermic because energy is released.

a) When green copper carbonate decomposes, the equation is:

CuCO3 CuO CO2

copper carbonate copper oxide carbon dioxide

Is the reaction exothermic or endothermic? Use ideas about bonds to explain why.