1
$\begingroup$

Somewhat recently, The Action Lab made a video called "Why does this powder only dissolve in cold water?" , saying the powder which was calcium acetate dissolves in cold water better than in warm water.

He also said that gases get less soluble in hot water than in cold water, which is true, by demonstrating it with a cold and hot soda can, noticing that the hot soda can bubbled up more violently than the cold one.

But the real point is what he said:

"All exothermic reactions slow down as it gets hotter."

A video reacting to The Action Lab from That Chemist, in which he says :

"His rationale in the video-which is wrong is that because it's exothermic when it's dissolved, if you add heat it becomes less soluble."

Sodium hydroxide when dissolving in water is exothermic, but it still gets more soluble as it gets hotter.

And not only that, but he had also said that dissolving sugar in water was a chemical change, even though it is strictly physical.

In my opinion, what I think That Chemist said was true, but correct me below.

$\endgroup$
1
  • 4
    $\begingroup$ The action Lab is wrong. The reason is that his knowledge of chemistry is weak and he leapt to a plausible conclusion based on too few examples (and din't check his conclusion by asking people with better chemistry knowledge). $\endgroup$
    – matt_black
    Sep 28, 2022 at 9:27

1 Answer 1

6
$\begingroup$

okay, I'll bite. They're both wrong.

The Action lab is specifically incorrect in conflating kinetics (reaction rate) with thermodynamics (equilibrium). Pretty much all reactions go faster with higher temp, but in the case of reversible exothermic reactions, the reverse reaction increases in rate more quickly than the forward reaction, so the final equilibrium state has more reactant remaining.

Where Action Lab is correct and The Chemist is wrong is in the application of Le Chatelier's principle to heat. The van't Hoff equation shows clearly that the change in the equilibrium constant for a reaction with temperature is dependent on the enthalpy change of the reaction (and nothing else):

$\ln {\frac {K_{2}}{K_{1}}}={\frac {\Delta _{r}H^{\ominus }}{R}}\left({\frac {1}{T_{1}}}-{\frac {1}{T_{2}}}\right)$

So if $\Delta_r H^\circ < 0$ and $T_2 > T_1$, then $K_2 < K_1$.

Where The Chemist is correct is when he points out that the rection can change as the temperature increases, which can affect this result. In the van't Hoff equation, we've assumed that $\Delta_r H^\circ$ is constant over the temperature range of interest. This isn't always true, but in most cases is a good approximation.

Strangely, after pointing this out, he makes exactly that mistake himself. In his counterexample of NaOH increasing in solubility with temperature, he wrongly assumes that the published enthalpy of solution of NaOH is the relevant value in his experiment. That value unfortunately is not applicable to this situation.

For any compound the "enthalpy of solution" is defined for the case of infinite dilution. So for NaOH, that means adding solid NaOH into pure neutral water in such a large volume that the added NaOH does not appreciably change the pH. In that case, there are two reactions going on:

(1) The separation of the NaOH lattice and hydration of $\ce{Na+}$ and $\ce{HO-}$ ions: $$\ce{NaOH(s) <=> Na+(aq) + HO-(aq)}$$ This reaction actually has a positive enthalpy change.

(2) The reaction of hydroxide ion with hydronium ions: $$\ce{HO- + H3O+ <=> 2H2O}$$ This reaction has a large negative enthalpy change.

In the case of infinite dilution (pH=7), both reactions are occurring, so the large negative enthalpy change of (2) results in a net negative enthalpy change, ie the reaction is exothermic.

However, when we get to the point that the solution is saturated, the pH is extremely high and there is very little hydronium left to react, so reaction (2) contributes much less, and the positive enthalpy change of reaction (1) dominates, making the reaction endothermic. (More accurately, the amount of hydronium is so low relative to hydroxide that the decrease in concentration of hydronium necessary to restore Kw for a given amount of added hydroxide is relatively very small.) Since the maximum solubility is determined by the behavior of a saturated solution, it is this enthalpy change that determines the change in solubility with temperature. That means that NaOH exhibits the expected behavior of an endothermic salt dissolution, ie increasing solubility with temperature.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.