Endothermic and Exothermic Reactions w00t w00t

The word of the day is:

ululate

\ UL-yuh-layt; YOOL- \  , intransitive verb;

1. To howl, as a dog or a wolf; to wail; as, ululating jackals.

Used in a sentence: The simplicity of comparing endothermic and exothermic equations caused the groundhog to ululate.

Say what?

Here is some review from previous years:
-All chemical reactions involve changes in energy or heat
-Exothermic reactions GIVE OFF energy
-Endothermic reactions TAKE IN energy

Today, we will be looking at ENERGY DIAGRAMS in depth. These diagrams show the changes the reactants go through as they become products - how much energy they start out with, how much energy they have at their peak, and how much energy they are left with.

With this graph, which looks something like this, we will be able to see whether the reaction is endothermic or exothermic.

 -The x-axis of the graph represents the time passed, the y-axis of the graph represents the amount of energy in kJ

-The flat part of the graph (just before the "hill) labeled "Substrate" represents the energy of the reactants. This is how much energy the compounds START with.

-The flat part of the graph (after the "hill") labeled "Product" represents the energy of the products. This is how much energy the compounds END with.

Comparing just these two features will allow you to determine whether it is a endothermic or exothermic reaction, but in order to add this to the chemical equation, we must investigate further.

-The energy of activated complex, labeled transition state on the graph above, is the potential energy of the transition state between the reactants and the products. In other words, it's the highest possible amount of energy the compounds can reach in the reaction process

-The activation energy is the energy that is added to to the reactants to get them to progress. This is found by subtracting the energy of the activated complex by the energy of the reactants.

-Delta H or the change in enthalpy (heat contained in the system) during the reaction. This is found by subtracting the energy of the reactants from the energy of the products.
     -A negative H will signify an exothermic reaction - you end up with less energy than you started with
     -A positive H will signify an endothermic reaction - you end up with more energy than you started with

Once you find the H, you can use it in equations like this:

Fe + Cl2 = FeCl3 + 432kJ

This means that an exothermic reaction occurred as the product needs 432kJ of energy to equal the reactants (it lost this energy).

In an endothermic reaction, the added energy appears on the reactant side.

You will be asked to draw graphs when given certain information like activation energy, delta H, the amount of energy in the products and the reactants, or you may be given the activated complex instead of activation energy.