1. this energy diagram is for the thermal decomposition of solid mercury (ii) oxide (also known as mercuric oxide) into liquid mercury and oxygen gas.
• write a balanced equation for the reaction.
• explain what feature is shown by the arrow labeled (a).
• using chemical symbols and dashed lines (this can be done with type), draw what the activated complex or transition state might look like.
• is this reaction exothermic or endothermic? explain.

1) Balanced chemical equation:
         2 HgO(s) → 2 Hg(l) + O₂(g)
2) The feature indicated by line (a) is Activation energy which is the smallest amount of energy required to activate molecules in order to undergo a chemical reaction (i.e. in this case energy required to break the bond between mercury and oxygen atoms.
3) The activated complex of transition state in this case will be: Hg- - -O at which the bond between Hg and O start to be broken by heat
4) The reaction is considered as Endothermic due to the energy of products is higher than energy of reactant 

The answer is letter c