exothermic reactions and KE

[Mae16]

My understanding is that an exothermic reaction converts potential energy stored in bonds to kinetic energy. This raises the temperature of the system (which ultimately releases heat into the surroundings).

So I'm confused by this Examkrackers question in gen chem (pg. 74, # 85): When two volatile solvents are mixed, the vapor pressure drops below the vapor pressure of either solvent in its pure form. what can be predicted about the solution of these solvents?

the answer is: the heat of solution is exothermic.

But I would think that if this were an exothermic rxn, temp of the solution would have increased, and KE energy of the molecules in the solution would have increased, thus INCREASING the vapor pressure of the mixture.

what am I missing here? I guess the bottom line question is: how do exothermic reactions result in stronger bonds (while they increase the temp of the solution simultaneously)?

thanks.

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[kobe200LATE]

I think you're attacking this question from the wrong angle

When the vapor pressure decreases, this means that stronger intermolecular forces are formed between the two solutions. Stronger intermolecular forces means stronger bonds are formed than before, which makes it an exothermic reaction because heat is released whenever you form stronger bonds. Therefore, because you are forming stronger bonds, the reaction releases energy and is thus exothermic.

 

[Mae16]

thanks. It makes sense that once you get the stronger bonds, vapor pressure would decrease.

but I'm not following how an exothermic reaction can raise the temperature of the solution (which must mean KE is increased), and, at the same time, build stronger bonds. I would think if KE is going up, the bonds are getting weaker, not stronger.

can anyone explain?

thanks!

 

[kobe200LATE]

The increase in temperature and therefore slight weakening of the bonds is only secondary compared to the increase in intermolecular forces that led to the decrease in vapor pressure

Because vapor pressure decreased, then IMF's increased, releasing heat. This heat then weakens some of the IMF bonds, but only slightly so overall there is still and increase in the strength of the bonds.

The increase in temperature is only secondary.

 

[loveoforganic]

Yeah, you've got to consider what's causing what. You mix the two solvents together, the molecules of the two solvents form stronger bonds to one another (more stable bonds), energy is released, temperature rises.

Think about it in another way - You react sodium and chlorine gas to make salt, NaCl. This is a highly exothermic reaction, yet the salt doesn't turn back into chlorine and sodium.

 

[Mae16]

very helpful, thanks!

 

[ssiding]

ok so when you combine the two solvents they end up forming stronger with eachother thus decrease pressure below each ones individuals pressure. The answer states it must be an exothermic rxn, these reaction release heat from the system into the surrounding thus the systems heat ends up decreasing. If you decrease the heat energy(heat), then shouldn't a decrease in pressure follow, because at with a decrease in heat/temp gases will be less likely to be in the gas phase.

I just always thought that if you have an exothermic rx then the system is losing heat to the surround. Please let me know if I am complete wrong.
Thanks

 

[helix]

ok so when you combine the two solvents they end up forming stronger with eachother thus decrease pressure below each ones individuals pressure. The answer states it must be an exothermic rxn, these reaction release heat from the system into the surrounding thus the systems heat ends up decreasing. If you decrease the heat energy(heat), then shouldn't a decrease in pressure follow, because at with a decrease in heat/temp gases will be less likely to be in the gas phase.

I just always thought that if you have an exothermic rx then the system is losing heat to the surround. Please let me know if I am complete wrong.
Thanks

This is exactly what I'm thinking and making a mistake in picking answers. If exothermic means the system is releasing heat to the surrounding then shouldn't the temperature of the solution itself drop?
Please explain.

 

[loveoforganic]

The solution is the surroundings. The system that's releasing energy is the reactants/products

 

[apex954]

ok so when you combine the two solvents they end up forming stronger with eachother thus decrease pressure below each ones individuals pressure. The answer states it must be an exothermic rxn, these reaction release heat from the system into the surrounding thus the systems heat ends up decreasing. If you decrease the heat energy(heat), then shouldn't a decrease in pressure follow, because at with a decrease in heat/temp gases will be less likely to be in the gas phase.

I just always thought that if you have an exothermic rx then the system is losing heat to the surround. Please let me know if I am complete wrong.
Thanks

You're not completely wrong but you're taking it from a wrong approach...the way "Kobe200LATE" and "loveoforganic" users explained it above is the right approach. You have to remember that its not like ur system here is completely isolated from the surrounding, so the energy released into the surrounding can easily go right back into the system weakening those stable bonds, but just like those two users explained, the overall effect of that is slight in comparison to the increase in IMF/bond strength, resulting in a decrease in vapor pressure.

 

[helix]

I'm clear now...thank you for clearing my doubts!!