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Homework Help: Endo or Exothermic?

  1. Mar 9, 2008 #1
    I have the equation Mg(OH)2 (s) -> Mg2+ (aq) + 2 OH- (aq)

    I started out with 20 drops of 1.0M MgCl2 in a well & 10 drops of .5M NaOH. Phenolphthalein was also added. When I heated the mixture, it became white, when it cooled it became pink. So, how can I tell if it was endo or exothermic? I want to say it was endothermic because it shifted to the reactant side when heated, but I'm not sure, almost doesn't seem right if you look at the original equation.
  2. jcsd
  3. Mar 10, 2008 #2
    It might be helpful to consider energy like a component in the reaction. Let me explain.

    An exothermic reaction releases energy to its surroundings. Usually we write this as:

    (reactants) <=> (products) delta-H < 0

    However, it's often useful to think of it like this:

    (reactants) <=> (products) + (energy)

    So by Le Chatelier's principle, if you add energy (by heating) to an exothermic reaction, you should shift the equilibrium to the left, i.e. towards the reactant's side. Vice versa for an endothermic reaction, which shifts to the right, i.e., towards the product's side.

    Do you now see the answer to your original question?
  4. Mar 10, 2008 #3
    Yes I do, thank for the explanation.
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