Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: paul.fr on 15/08/2007 07:56:34
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If i am right, this reaction produces no heat. Why is that, don't all chemical reactions produce heat?
If so, what kind of reaction are we looking at?
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NOt all reactions produce heat and this is one that doesn't. In this case the driving force for the reaction is the fact that the gas is given off. Not only does this make the reaction more likely because the reaction can't easily go backwards after the CO2 has left, but the entropy change for the reaction is favourable too.
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Is this endo- and exo-thermic reactions?
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Whether or not a reaction occurs is governed by the Gibbs Free Energy. This has two parts, (enthalpic and entropic) as shown below:
Gibbs = enthalpic part + entropic part
Enthalpy is the heat basis of the reaction. If a reaction gives off heat, that's favorable, if it absorbs heat, that's unfavorable.
Entropy is the energy associated with disorder. If a reaction yields more disorder, that's favorable, if a reaction causes more order, that's unfavorable.
If the two parts are both favorable, the reaction occurs.
If the two parts are both unfavorable, the reaction doesn't occur.
Most times, one part is favorable while the other is unfavorable. That's where temperature comes in. Increasing temperature increases the effect of the entropic part, while leaving the enthalpic part almost unchanged.
So, at low temperature, enthalpy wins, while at high temperature, entropy wins.
This explains phase changes, for instance.
Consider liquid and gaseous water. Liquid water is more favorable enthalpically, because there are more water-water interactions (intermolecular forces of attraction). However, Gaseous water is more disordered. Therefore, at low temperature, enthalpy wins and water exists as a liquid. At high temperature, entropy wins and water exists as a gas. At some temperature, the enthalpic and entropic effects are equal, and you get an equilibrium situation. This is the boiling point.
Dick