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Thread: exothermic and endothermic reactions

  1. #1 exothermic and endothermic reactions 
    Forum Freshman almirza's Avatar
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    hi friends
    In a reaction first the reactances will be in molecules form , for instance, after that the reaction will reach to the activation energy where the atoms are at high energy and when the bonds form energy is released if the energy released is less than the energy level at the beginning of the reaction which means there is some energy absored and if it is under the first energy level , which means there is some energy released more than what was taken.
    I understood these reactions like that.
    When we use a thermometer , it should increase until the activation level then it should decrease since the thermometer measures the thermal energy of the particles but this does not happen.

    In both cases there is a release of energy to the surroundings and we assume always that all the energy is used in the reaction and the energy level diagrams show the energy of the reactance not the suuroundings .

    How in exothemic reaction the temperature of the suuroundings rises and in endothermic falls , by following what i mentioned above???


    ALMIRZA
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    Forum Senior silkworm's Avatar
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    I'm sorry, I'm having trouble following what you wrote above.

    Here's the deal though. Endothermic reactions make the temperature of their immediate environments colder because they require energy in order to proceed. They are not spontaneous. We, or something else, must input energy into the reaction in order for it to proceed.

    Exothermic reactions occur spontaneously. They do not require energy to proceed. They warm up their immediate environments because, being spontaneous, the bonds they're making are of lower energy than the bonds they are making, and the difference is lost as heat.

    Study Gibbs Free Energy.


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  4. #3  
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    Quote Originally Posted by silkworm
    I'm sorry, I'm having trouble following what you wrote above.

    Here's the deal though. Endothermic reactions make the temperature of their immediate environments colder because they require energy in order to proceed. They are not spontaneous. We, or something else, must input energy into the reaction in order for it to proceed.
    Endothermic reactions can be spontaneous if they result in an increase in entropy. If the associated change in entropy times the temperature is greater than the endothermic energy change, the reaction will occur spontaneously. You can google for the "gibbs equation" for more info.
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  5. #4  
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    every reacting partice, atom, molecule, formula unit have energy in the potencial form. in the process of reaction they use some quantity of energy from the environment to reach their activation place and change to form the products, releasing another quantity of energy to the environment. the products also have some potencial energy related to them. now to see weather a chemical reaction is endo or exothermic you have to see the change in enthalpy.

    this is thermochemistry which studies energy and chemical change. to explain these chemical reactions they use enthalpy because it relates the work, the pressure ,and volume done by a system in this equation: H=E+PV, but to describe exo and endothermic reactions it is used the change in enthalpy, the same equation but with a triangle (delta) in front of the H,E, and V symbolizing change. the pressure stays constant

    so the exothermic reation releases energy. its deltaH<0
    an endothermic reaction absorbs energy. its deltaH>0


    in an exothermic rxn, for example, you have hydrogen gas and oxygen gas as reactants. their potential energy is high. to complete their reation they use some energy from the environment so they can join themselves to form water. in this process water can not reatain all the energy from the reactants and then turns the potencial energy into heat which difusses into the suroundings. now the product, water, is left with a low potencial energy. in the overall rxn, the intake energy is less than the energy released.
    Code:
    ΔH<0
    in an endothermic rxn the products have a higher potencial energy than the reactants, which means that when they reacted they took more energy from the environment than the energy they released. overall energy is absorbed.
    Code:
    ΔH>0
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