Stupid thermodynamics question

[blebl34]

Ok i know that an exothermic reaction is -H (releases heat to surroundings), and endothermic is +H (takes in heat from surroundings)....

But for an exothermic reaction, lets say the Haber process (i think its exothermic), does the actual reaction mixture heat up or cool down? If its releasing heat does its internal temp cool, even though it'll feel hot to the touch?

any insight?
Thanks

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[liverotcod]

Ok i know that an exothermic reaction is -H (releases heat to surroundings), and endothermic is +H (takes in heat from surroundings)....

But for an exothermic reaction, lets say the Haber process (i think its exothermic), does the actual reaction mixture heat up or cool down? If its releasing heat does its internal temp cool, even though it'll feel hot to the touch?

any insight?
Thanks

Since the reaction produces heat, it needs to go somewhere. In a typical lab situation, most of the heat is absorbed by the reaction mixture, while some of it is given off of the container (test tube, flask, whatever). So, unless you are chilling the container in an ice bath, it's going to heat up for an exothermic reaction. Think hot pack. Likewise, endothermic reactions (so long as they're exergonic) will cool the reaction mixture and the container. Think cold pack.

 

[MDtoBe777]

I may be wrong but I've never read anything about exergonic reactions being endothermic? Usually exergonic goes with exothermic. Any ideas?

 

[liverotcod]

I may be wrong but I've never read anything about exergonic reactions being endothermic? Usually exergonic goes with exothermic. Any ideas?

Separate ideas. Exergonic = spontaneous. Exothermic = produces heat.

Most exothermic reactions are exergonic, by the equation:
Change in free energy = change in heat - (temperature * change in entropy)
A negative free energy change implies a spontaneous reaction. A zero free energy change implies a reaction at equilibrium. A positive free energy change implies a non-spontaneous reaction (which simply won't happen without being tied to some exergonic reaction).

Because change in entropy is usually a much smaller numeric value than change in heat, endothermic reactions are usually exergonic.

One good way to remember all this is to look at state-change reactions. Thawing of water ice, for example, is exergonic above 0 degrees, and endergonic below 0 degrees, and in equilibrium at 0 degrees. This is because the heat of fusion is exactly balanced by the temperature times the change in entropy at zero degrees.

Edit: I remember all this because for a long time I couldn't see how instant ice packs work. They work because they produce tons of moles of ions in solution when you use them --> large entropy increase --> offsets extremely endothermic reaction --> spontaneous.