Caroline's weblog

Measuring Enthalpy Changes

A friend of yours is working for a local fast food restaurant, and their boss tells them to throw some cleaning supplies away.  They notice one bottle has some muriatic acid (HCl) in it, and the other has some lye (NaOH) in it.  When the substances were mixed, they spilled them and it burnt them bad.  Now they are trying to sue their employer to get some of the medical costs covered.  The employer says that the substances could not have burned them because they were both at room temperature when they were mixed, after they were mixed, and they had neutral pH after they were mixed.  Your friend asks you to help them win the case because they know that you have been taking this chemistry class, and they don't have a lot of money to hire a good lawyer.  What do you tell your friend?  Do you have the knowledge to help him or not?

Some chemicals should never be mixed together as this lab will teach you.  This is because some chemical reactions either release or absorb massive amounts of heat energy.  Enthalpy is how chemists measure this heat energy at constant pressure.  Chemical reactions can either be exothermic (release heat energy) or endothermic (absorb heat energy).

Here is a cool movie about a chemical reaction that has a lot of enthalpy - the thermite reaction.  If you have never seen this done, you should watch this.

Is this chemical reaction exothermic or endothermic?  I will give you a hint - exothermic reactions release heat energy so the surroundings (what you feel) will increase in temperature.  Enthalpy (or delta H) is negative.  Endothermic reactions absorb heat energy so the surroundings (what you feel) will decrease in temperature.  Enthalpy is positive.  (Sorry about having to type Delta out - blogs have no character key!)

This is another movie about reactions that proceed that absorb a lot of heat energy.  It is about entropy of a reaction.  Entropy is  related to the number of ways that the energy of a system can be distributed or disorder.  If the disorder of a chemical system increases (increase in the number of moles, or it produces a gas), the entropy is increasing (+).  And if the disorder of a chemical system decreases (decrease in the number of moles, or it consumes a gas), the entropy is decreasing (-). 

Both of these terms - enthalpy and entropy - can be combined into one term, Gibbs Free Energy.  Delta G can be used to predict whether ANY chemical reaction will proceed as written!

If you want some practice working these type of problems visit this pdf that I made for a class a while ago!

Posted at 04:17PM Jul 24, 2006 by CHRISTIAN, CAROLINE in General  |  Comments[2]