Destroyer GC #57

[NonTradHopeful]

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Which statement is correct?

a) Temperature is average kinetic energy.
b) 30 ml of 70 degree C water has a greater avg kinetic energy than 50 ml of 40 degrees C water.
c) When solid NH4NO3 is dissolved in water at 25-degrees C, the solution temperature decreases. The reaction is endothermic.
d) A and B Only
e) All are correct.


I put the answer as D, but they said the answer is E. How is C correct? If a solution's temperature decreases, that means it has released heat to the environment, right? If so, it should be considered exothermic. However, the solutions say that the solution actually absorbed heat. Is this right? Could somebody explain this to me logically?

Thanks!

 

[Demps]

If you put D, means you obviously know them as correct statement, so I will skip choice A and B.

I understand why you are getting confused with choice C, but it is the other way around of how you are thinking of it. When the system's temperature decreased, heat was absorbed = endothermic... If the temperature increased, it is because heat is released into the system and that is exothermic.

Hope this helped

 

[SN1]

The explanation does sound very counter-intuitive. When you say temperature is decreased, then it makes sense to think that you are releasing heat and vice-versa. However, that is NOT the case.

The way I think about answer choice C is that when NH4NO3 is dissolved in standard temperature of water, I know that it will dissociate completely to NH4+ and NO3-(solubility rules). The bond that connect these two are broken therefore you can assume that the reaction is endothermic.

In GENERAL:
Endothermic= bond breaking, heat on reactant side
Exothermic = bond making, heat on product side

 

[NOORALLAH]

I agree with Demps....

 

[jwnichols21]

If you put D, means you obviously know them as correct statement, so I will skip choice A and B.

I understand why you are getting confused with choice C, but it is the other way around of how you are thinking of it. When the system's temperature decreased, heat was absorbed = endothermic... If the temperature increased, it is because heat is released into the system and that is exothermic.

Hope this helped

I still don't get it..if you are absorbing heat wouldn't that make your temperature rise!!

 

[AlbinoPolarBear]

I still don't get it..if you are absorbing heat wouldn't that make your temperature rise!!

In an endothermic reaction, the molecules absorb the heat from the environment, lowering the temperature.

In an exothermic reaction, the molecules releases heat into the environment, raising the temperature.

 

[Demps]

I still don't get it..if you are absorbing heat wouldn't that make your temperature rise!!

The term is relative to the system.

If the heat was used to form a bond for example, that means the heat left the system, hence making the system cooler (Endothermic).

If the heat was released as a result of bond breaking, now that released heat makes the temperature rise for the system (Exothermic).

 

[jwnichols21]

The term is relative to the system.

If the heat was used to form a bond for example, that means the heat left the system, hence making the system cooler (Endothermic).

If the heat was released as a result of bond breaking, now that released heat makes the temperature rise for the system (Exothermic).

Now you've thoroughly confused me..

You add heat to break bonds (endo) and release heat when you form bonds (exo)..isn't your post saying the opposite?

 

[Demps]

does PolarBear's reply confuse you too? He pretty much said the same thing.

 

[jwnichols21]

does PolarBear's reply confuse you too? He pretty much said the same thing.

Yup.

 

[SN1]

In an endothermic reaction, the molecules absorb the heat from the environment, lowering the temperature.

In an exothermic reaction, the molecules releases heat into the environment, raising the temperature.

.

 

[jwnichols21]

I get that it's relative to the system..but how does the compound dissolving in the solution equate to temperature leaving the system and entering the "system?" Because now there's more energy in the system thus temperature was transferred there?

 

[BTown DDS]

The question is talking about the solution. So When a solid was added, it dissolved into a liquid. Solid=> liquid requires an input of energy (endothermic). The energy was taken from the water so the temperature decreased.

 

[jwnichols21]

The question is talking about the solution. So When a solid was added, it dissolved into a liquid. Solid=> liquid requires an input of energy (endothermic). The energy was taken from the water so the temperature decreased.

Thank you..this was the explanation I was looking for! Appreciate it

 

[jevan]

The question is talking about the solution. So When a solid was added, it dissolved into a liquid. Solid=> liquid requires an input of energy (endothermic). The energy was taken from the water so the temperature decreased.

this is true, however according to this explanation alone, every time we dissolve salt in water it should be an endothermic process bcs of the heat "stolen" from the water in order to break the ionic bonds of the salt...

but there are many salts that dissolve in water and cause a net increase in solution temp- those are exothermic processes...

this is becuase after the heat has been absorbed from the water molecules and successfully broken apart the ionic lattice, the ions immediately associate with the water molecules in the solution...these interactions cause a release of energy back into the water (now its a solution actually). bcs these ion- dipole interactions are at lower energy than ions being in one corner of the beaker, and the water hiding away in another(anti-entropic)...and when we start at higher energy (lonely ions and lonely water molecules) and end at lower energy(water/ions mingling), the energy must go somewhere..so its released as heat into the solution.

so now we have two competing subprocesses in the dissolving process- the absorption of energy to break the salt into its ions, and the release of energy from the stabilizing interactions of the ions with water.
the relative amount of energy absorbed and released in these 2 subprocesses determines whether it is a net exothermic or endothermic process...

hope this wasnt too confusing!