Chemistry (endothermic and exothermic)

Question

A chemical reaction absorbs energy when occurring. Is this reaction Exothermic or Endothermic? If the enthalpy of the reactants was 300kJ and the enthalpy of the products was 450kJ, list one possible value for the enthalpy value of the transition state (the activation energy). 

Answer 1

Any time you are told that a chemical reaction absorbs energy, it is endothermic. Think of it as entering in, the energy is "entering in" hence "endo" it absorbs the energy in to complete the reaction. Exothermic reactions release energy, think of "exit", "exo".

When you have a positive enthalpy it is endothermic, think + in the positive, like for example a chemical reaction being represented and said to have taken place with 500kJ. This reaction is endothermic. A negative reaction will be "exo" exothermic. for example they give you a complete reaction and tell you the energy represented by this is -700kJ. This is an exothermic reaction.

Most reactions are exothermics. They release energy when going from the reactant to the product side.

Reactions that are endothermic usually require an activation energy, because they don't usually proceed spontaneously.

One of the major laws of thermodynamics is that energy cannot be created or destroyed, but only transfered.

If your product 300kJ and your reactant side is 450kJ, 150kJ is the difference of what it would take to keep everything balanced. So 150kJ would be the activation energy. Energy cannot just dissapear and be unaccounted for, and all of the partial or half reaction energies must sum up to or reflect in the final enthalpy equation, this is part of Hess's law.

If you had some actual problems I would be able to further illustrate this for you.

Answer 2

If the reaction absorbs energy, then according to convention, the reaction is ENDOTHERMIC.

If the enthalpy of reactants is 300 kJ and that of the products is 450 kJ, what could be the value of the transition state. Well, it must be greater than 300 kJ and it must be greater than 450 kJ, since it is endothermic. I wish I could draw an energy diagram but I cannot. But picture this....

You start with energy of reactants at 300 kJ. The Ea is greater so you climb the hill to get over the Ea. Then you come down the hill, but since the reaction is endothermic, the energy of the products will be greater than the energy of the reactants, but less than the Ea. So one possible value of Ea would be 450 kJ (actual value, but deducting the starting value of 300, it would be 150 kJ)