### Feb. 16 Endothermic & Exothermic Reactions!!

This is an ENDOTHERMIC REACTION

This is an EXOTHERMIC REACTION

What are endothermic and exothermic reactions?
You can classify them by their prefixes.  Endo means "within heating".  It describes a process or reaction in which the system absorbs energy from the surroundings.  An example of an endothermic reaction would be cracking an instant cold pack.  Once you crack it, the pack loses heat and becomes cold instantly.  Exo means "outside heating".  It describes a process or reaction that releases energy from the system.  An example of an exothermic reaction would be an explosion.  It releases energy causing the explosion.  Most of the time heat is absorbed or released in there reactions.

Energy Diagrams

Both endothermic and exothermic reactions can be shown on energy diagrams.  A way to tell if a diagram is endothermic or exothermic reaction is to look at the start and end of the graph.  If the end part of the graph is more than the start, it is an endothermic reaction meaning it absorbed energy.  If the end part of the graph is less than the start, it is an exothermic reaction meaning it released energy.

The one shown above is an endothermic reaction because the reactant's energy is less than the product's energy meaning it absorbed energy.
Terms:
(a) is the "Activation Energy"
(c) is ΔH or change in enthalpy.  It is the change in energy during the reaction.  To find this, you can use this equation: Hprod - Hreact.

The one shown above is an exothermic reaction because the reactant's energy is more than the product's energy meaning it released energy.
Terms:
(a) is the "Activation Energy"
(b) is ΔH or change in enthalpy.  It is the change in energy during the reaction.  To find this, you can use this equation: Hprod - Hreact.

Energy In the Equations

The energy that is absorbed or released can be placed into an equation:
Exothermic reaction in an equation.  If the ΔH is negative, it means that reactants lost heat and change into products. :
NaOH + HCl-> NaCl+H20 AH = -57.3 kj

Endothermic reaction in an equation.  If the ΔH is positive, it mean that the heat is absorbed and is classified as endothermic reactions:
2C + H2-> C2H2 AH = + 225.9 kj

Here is a site that has a few questions on exothermic and endothermic reactions: