I read on my school book about chemistry the reaction:

$$\ce{H2O + NH4Cl}$$

with $T(\ce{H2O}) = \pu{25 °C}.$

The temperature of the final solution is $\pu{20 °C}$. I know that it is an endothermic reaction, and the initial solution absorbs temperature from the environment.

I don't understand why the temperature of the final solution is lower than the initial solution, I think it should not be lower because all the heat that it needs comes from the environment.

  • 2
    $\begingroup$ The environment is whatever is closest to the reactants, and that is principally the solvent the reaction is conducted in $\endgroup$
    – Waylander
    Mar 3 '20 at 14:02
  • $\begingroup$ @Waylander I am sorry, what do you mean? The environment is a solvent? $\endgroup$
    – Edoardo
    Mar 3 '20 at 14:03
  • 1
    $\begingroup$ As I said in comment 1, the environment is whatever is around the reactants. the bulk solvent is closest so that is where the energy (as heat energy) comes from. Hence the reaction mixture gets colder. $\endgroup$
    – Waylander
    Mar 3 '20 at 14:05
  • $\begingroup$ @Waylander thanks, so it means that the reaction absorbs energy (heat) from the H2O and NH4Cl? $\endgroup$
    – Edoardo
    Mar 3 '20 at 14:09
  • $\begingroup$ @Ed V so I'll do for sake of clarity $\endgroup$
    – Alchimista
    Mar 5 '20 at 7:38

An exothermic reaction occurs when the temperature of a system increases due to the evolution of heat. This heat is released into the surroundings, resulting in an overall negative quantity for the heat of reaction (-ΔE ).

An endothermic reaction occurs when the temperature of an isolated system decreases while the surroundings of a non-isolated system gains heat. Endothermic reactions result in an overall positive heat of reaction (+ΔE ).

Exothermic and endothermic reactions cause energy level differences and therefore differences in enthalpy (ΔE ), the sum of all potential and kinetic energies.

ΔH is determined by the system, not the surrounding environment in a reaction.

A system that releases heat to the surroundings, an exothermic reaction, has a negative ΔH by convention, because the enthalpy of the products is lower than the enthalpy of the reactants of the system.

The enthalpies of these reactions are less than zero, and are therefore exothermic reactions.

A system of reactants that absorbs heat from the surroundings in an endothermic reaction has a positive , because the enthalpy of the products is higher than the enthalpy of the reactants of the system. Because the enthalpies of these reactions are greater than zero, they are endothermic reactions.

EDIT ( See this )

I understand your confusion. See, in endothermic reaction the product will be "colder"....this might seem counterintuitive as in endothermic reaction heat is absorbed by the system form the environment. Therefore the heat absorbed must increase the temperature right?

No! See temperature shows the direction of heat flow. If the temperature of the system increased ( assuming that previously the system and environment were in thermal equilibrium ~ same temperature ) this would mean that heat should flow from system to the environment as the system has a higher temperature than environment.

The following example makes everything clear:

$$T_{(env)}= 25°$$

$$A + B _{(25°)} = C _{(20°)} ...... (1)$$

$$A' + B' _{(25°)} = C' _{(30°)} ..... (2)$$

Compare the temperature of system and environment in the above rxn.

Therefore in rxn. 1 heat flows from environment to system ( from 25° to ~20°). This heat energy is used to break bonds and stuff.( Endothermic )

And in rxn. 2 heat flows from system to environment ( from 30° to 25°).( Exothermic )

Also see this practical video: https://youtu.be/NHOHKH3808c

  • 1
    $\begingroup$ Heat is absorbed from outside the system, that is the environment. $\endgroup$ Mar 9 '20 at 4:34
  • 1
    $\begingroup$ I think you should make the point that such analyses depend on the system being isolated, specifically that system is adiabatic. $\endgroup$
    – MaxW
    Mar 9 '20 at 7:59
  • 1
    $\begingroup$ @MaxW I thought it would be better to use isobaric open system, which is what found in our lab experiment . Therefore we could relate to real life practical situation. $\endgroup$ Mar 9 '20 at 8:34
  • 1
    $\begingroup$ @Edoardo I think you are forgetting that temperature can't be absorbed! Heat is absorbed and that heat is used to break bonds $\endgroup$ Mar 9 '20 at 10:09
  • 1
    $\begingroup$ @Edoardo You just can't add and subtract temperature like this! What gets added is heat. Heat is not temperature! $\endgroup$ Mar 9 '20 at 10:17

Energy is needed for separating positive from negative ions in the dissolution process. This energy is taken in the surrounding water. Water is loosing energy in the dissolution process. That is why the temperature of the water decreases. There is nothing special in using $NH_4Cl$. The same phenomena happens when dissolving a salt like $NaCl$ or any other one.

It may happen that a dissolution is exothermic. This happens with $NaOH$ for example. In this case a new phenomena is happening. One of the ions separated from the solid substance attracts an important number of water molecules that get adsorbed or fixed in the outer layer around each ion. This happens with ions "derived" from the water formula, like $OH^-$ ions or $H_3O^+$ ions, which get surrounded by a lot of oriented water molecules. This creation of new bonds is exothermic, as every bond formation.

  • $\begingroup$ This answer is only applicable for cases where lattice enthalpy is concerned ( ie. solution formation) , what about reaction between Ba(OH)2 and NH4OH where temperature of product is lower than reactant and the process is endothermic $\endgroup$ Mar 7 '20 at 10:14

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