Thread: strongest endothermic reactions

I am doing a little independent research, and I am having a hard time finding an answer to this question. I want to know what some of the reactions are that absorb the most heat possible from their surroundings. I can find lists of exothermic reactions pretty easily, but for some reason it is tough to find the endothermic ones. If possible I would prefer to know the reactants, with the amount of energy absorbed. I would much appreciate any help I could get on finding this information.

I don't have a specific answer but I can give you some pointers on the type of process to look for.

Whether a chemical reaction is thermodynamically spontaneous or not depends upon the change in Gibbs free energy during a reaction. For a reaction to occur spontaneously this must be negative.



where is the change in enthalpy, the change in entropy and T the temperature.

If a reaction is endothermic the term is positive and so for the reaction to be spontaneous this means must be positive and large enough to offset the positive term. For these calculations the T is constant (at the starting temperature of the reaction) so must be positive and large enough to offset . The easiest way to have a reaction with a large is to create a lot of gas from solid or liquid reactants (the entropies of gases are much larger than those of liquids and solids leading to large values). So look for reactions of this type and calculate the and for the reactions. As Gibbs free energy, enthalpy and entropy are all state functions you can do this using Hess's Law. http://en.wikipedia.org/wiki/Hess's_law

Some of the most commonly accessible endothermic reactions are the dissolution of salts in water. Ammonium chloride is a common example. Other things heat up when you dissolve them in water, like sodium hydroxide for example. You can look at a list like this one to see the relative heating and cooling effects of a range of compounds.

http://sites.chem.colostate.edu/dive...ectrolytes.pdf

Yes but endothermic dissolution reactions are not very endothermic. I was trying to point you in the direction you asked for.

EDIT: Apologies to Warron but I thought he was the same poster as Mr. Warren who made the OP.

Easy mistake

I don't think that there are too many spontaneous endothermic reactions. The most commonly quoted one seems to be the reaction of barium hydroxide octahydrate with ammonium thiocyanate, described here.

Endothermic Reaction - UW Dept. of Chemistry

As someone else has commented:

"Most endothermic reactions have to be driven by some external force, much as work has to be done to roll a boulder uphill."

Thank you for the help. Specifically I was trying to think of a way to use an endothermic reaction to reduce the energy output of an exothermic reaction, and crunch some numbers. This was merely an Idea I had, and was thinking of possibilities of controlling extremely power full exothermic reactions by using a power full endothermic reaction. I could not find much written on this topic. I was just curious for my own knowledge.

Originally Posted by Mr. Warren

Thank you for the help. Specifically I was trying to think of a way to use an endothermic reaction to reduce the energy output of an exothermic reaction, and crunch some numbers. This was merely an Idea I had, and was thinking of possibilities of controlling extremely power full exothermic reactions by using a power full endothermic reaction. I could not find much written on this topic. I was just curious for my own knowledge.

In that case, you're not limited by the constraint of exergonic reactions mentioned above. What you need to do is couple the reactions together. But you need to be specific about the reactions.

Originally Posted by Mr. Warren

Thank you for the help. Specifically I was trying to think of a way to use an endothermic reaction to reduce the energy output of an exothermic reaction, and crunch some numbers. This was merely an Idea I had, and was thinking of possibilities of controlling extremely power full exothermic reactions by using a power full endothermic reaction. I could not find much written on this topic. I was just curious for my own knowledge.

I suppose it's cheating a bit, but wouldn't phase changes such as evaporation or melting have these characteristics?