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Can someone explain why this dissolution is exothermic rather than endothermic?

  1. Jul 24, 2012 #1
    A pair of students found the temperature of 100 g of water to be 27.0°C. They then dissolved 6.32 g of KOH in the water. When the salt had dissolved, the temperature of the water was 42.5°C.

    If the temperature of the water is raised, doesn't that mean it retained heat, thus showing that the dissolution was endothermic? I'm having trouble understanding why it is exothermic. Can someone please explain?
     
  2. jcsd
  3. Jul 25, 2012 #2

    cepheid

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    Exothermic means that energy is released by the chemical reaction. In other words, energy is an output of the reaction, along with the reaction products. Endothermic means that energy is an *input* to the reaction. In other words, the reaction won't occur unless if you give energy to the reactants. So, if the dissolution of potassium hydroxide had been endothermic, then the KOH would have had to steal energy from the surroundings (the water, in this case) in order to dissolve. Therefore, heat would have been removed from the water and the water would have become cooler. Instead, since the dissolution is exothermic, energy is released into the surroundings. So heat is transferred from the dissolving KOH into the water, which gets warmer as a result.
     
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