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endothermic reactions

Discussion in 'Science' started by aldebaran, Jan 5, 2011.

  1. Here's a poser from one of my students: If a reaction is endothermic, and so heat energy will flow from the surroundings into the system, why does the temperature of the reaction mixture fall? Well?
    thanks!
     
  2. Here's a poser from one of my students: If a reaction is endothermic, and so heat energy will flow from the surroundings into the system, why does the temperature of the reaction mixture fall? Well?
    thanks!
     
  3. blazer

    blazer Star commenter

    Initially the bonds being formed will absorb energy from their surroundings such as the solvent the chemicals are dissolved in or the particles themselves, the vessel they are in etc. Once these have cooled below room temp then energy will flow in from the room or your hand as well. Eventually once the reaction has ceased the contents of the vessel will return to room temp. However the chemicals will have increased the amount of chemical energy stored within the bonds.
     
  4. blazer

    blazer Star commenter

    Or put another way the reaction is turning heat energy into chemical energy thus the amount of heat energy in the system is falling and thus the temperature falls as well.
     
  5. The reaction mixture is the surroundings from which the energy is being taken. Hence it gets cold.
     
  6. Rhysboy

    Rhysboy New commenter

    The products have more chemical energy than the reactants - therefore as mentioned above heat energy is taken from the surroundings, 'changed' to chemical energy and stored in the chemical bonds of the products. Hence it gets cold.
     
  7. thanks for the replies.. but I must admit I still don't get it... so cannot go on to explain to my students. We dissolve ammonium nitrate in water. The thermometer is in the water so recording the temperature...but surely the solution is the final state of the system? The temperature falls, and if heat energy has flowed into it, where has it gone?
     
  8. blazer

    blazer Star commenter

    Think of it as two separate stages. Firstly the chemical reaction removes thermal energy from the chemicals in the tube and turns it into chemical energy. Thus with less thermal energy the temperature of the tube's contents falls. Then because the tube and contents now has a lower temperature than the romm (or your hand if you are holding it) thermal energy flows into the tube through the glass until the temp inside and outside is in equilibrium.

    The tube feels colder because the reaction has removed heat energy from it.
     
  9. blazer

    blazer Star commenter

    If you did the reaction in a thermos flask (lets assume the thermos flask is a perfect insulator) then the temperature would fall as the thermal energy of the chemicals in the flask was transformed into Chemical energy in the chemical bonds. There is still the same amount of energy inside the flask but now there is less Heat and more Chemical. As it is in a thermos flask then the temp will stay lower as heat cannot now flow in the equilibrate.
     
  10. blazer

    blazer Star commenter

    Try the reation between citric acid and sodium hydrogen carbonate.

    Mix equal quantities of dry chemicals in a tube. Add a few drops of water. A reaction occurs that produces a gas (CO2) and the temp falls.

    Get them to consider photosyntheseis. An endothermic reaction that uses light instead of heat. The light hits the leaf where it is captured as chemical energy within the glucose and oxtgen formed.

    Burn some glusose to release the energy!
     
  11. thanks . That seems to make sense now.
     

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