    Endothermic and Exothermic Reactions

Chemical bonds store energy.

In a given reaction, if the new bonds store less energy than the old, the excess energy is given off to the environment and we have an exothermic reaction.

If in a different reaction the new bonds store more energy than the old, the reaction is endothermic and energy is absorbed, we have an endothermic reaction.

To calculate the energy change the formula is: Hrxn =  Hproducts - Hreactants

When  Hrxn is negative, the molecules have lost energy. We in the outside have gained it, that's an exothermic reaction. When  Hrxn is positive, the molecules have gained energy. The energy goes into the new bonds formed. It's an "indothermic" reaction :).

Things to remember: the energy given off depends on the amount. Think: a big fire with a lot of fuel gives more heat than a small fire with a couple of twigs.

That means in calculations, we need to multiply the Hf's (enthalpies of formation) by the number of moles present in the balanced equation. Look up the Hf's on the handout. Keep in mind most elements in their most common state are defined as Hf=0

The most common problem is not adding up all of the products or all of the reactants and or making a sign error.

Example problem:

 Substance (phase) Enthalpy of formation (Hf) O2(g) 0 kcal/mole S (s) 0 kcal/mole SO2 (g) - 70.75 kcal/mole

S (s) + O2(g)--> SO2 (g)

Hrxn =  Hproducts - Hreactants

Hrxn = ( - 70.75 kcal/mole * 1 mole) - (0 + 0 ) = -70.75 kcal.

Negative Hrxn so exothermic. This is burning.