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Author Topic: The changing solubility of sugar.  (Read 3146 times)

paul.fr

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The changing solubility of sugar.
« on: 25/08/2007 17:15:24 »
in a sugar solution, the solubility increases when you apply heat to it. But why?


 

Offline lightarrow

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The changing solubility of sugar.
« Reply #1 on: 25/08/2007 19:32:38 »
in a sugar solution, the solubility increases when you apply heat to it. But why?
For two reasons:
1.that reaction is endothermic, so you accelerate it giving heat, and solubility increases, increasing the temperature;
2.every reaction's speed increases with temperature, because particles move faster.
 

Offline Bored chemist

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The changing solubility of sugar.
« Reply #2 on: 25/08/2007 20:08:25 »
"1.that reaction is endothermic, so you accelerate it giving heat, and solubility increases, increasing the temperature;"
ammonium nitrate disolves endothermically in water but the solubillity still rises with temperature so that can't be the reason.
"2.every reaction's speed increases with temperature, because particles move faster."
All reactions, including thr reprecipitation of the sugar from the solution so that's not really  a reason.

The reason is to do with entropy. The solution has less order than the nice organised crystals. Disolving is favoured by this increase in entropy but the problem is that it needs energy to do it. Heating the solution provide that energy and therfore lets more sugar dissolve.
 

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The changing solubility of sugar.
« Reply #3 on: 26/08/2007 12:22:14 »
"1.that reaction is endothermic, so you accelerate it giving heat, and solubility increases, increasing the temperature;"
ammonium nitrate disolves endothermically in water but the solubillity still rises with temperature so that can't be the reason.
It seemed to me to have written the same.
Quote
"2.every reaction's speed increases with temperature, because particles move faster."
All reactions, including thr reprecipitation of the sugar from the solution so that's not really  a reason.
What you say is true at chemical equilibrium: the 2 speeds are equal, but before that equilibrium is reached, dissolution's speed is greater than reprecipitation's speed, and this difference increases with temperature.
Quote
The reason is to do with entropy. The solution has less order than the nice organised crystals. Disolving is favoured by this increase in entropy but the problem is that it needs energy to do it. Heating the solution provide that energy and therfore lets more sugar dissolve.
That is to say: the reaction is endothermic, so it is accelerated if you heat it, the same I wrote. Of course I could have written ΔG = ΔH - TΔS and explained that with a positive ΔH and ΔS, the reaction starts to be spontaneous (ΔG < 0) only when T > ΔS/ΔH and that the cinetic constant k is related to activation energy Ea by the equation
k = Ae-Ea/RT, but it would have been quite long to explain and not easy to understand.
 

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The changing solubility of sugar.
« Reply #3 on: 26/08/2007 12:22:14 »

 

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