Why do acid base reactions cause the reacting or forming molecules to accelerate? (Basically, why do acid base reactions produce heat?) [duplicate]

This question already has an answer here:

My question is hopefully pretty clear, but to rephrase my question:

Why do acid base reactions produce heat? We know that heat is merely the transference of momentum (or velocity) between molecules. So, if that is the case, and acid base reactions do produce heat, then the reaction of (for example) H₂O and NaOH MUST be resulting in an increase of the molecules' velocity. The question is, why would there even be an increase of molecular velocity during acid base reactions?

Note: Using terms like 'potential energy', 'enthalpy', 'electrical potential', 'lower energy state', and 'energy' are forbidden, because the question looks for an answer that is purely based on 2 observable rules.

1- An object will accelerate if it is subject to a force

2- Electric charges can apply forces on each other.

marked as duplicate by Mithoron, Pritt Balagopal, M.A.R. ಠ_ಠ, Jan, Todd MinehardtNov 5 '17 at 17:54

• – Mithoron Nov 4 '17 at 17:14
• You aren't, in fact, in any position to forbid us anything. – Mithoron Nov 4 '17 at 17:16
• – Mithoron Nov 4 '17 at 17:21
• Watch your language - asking somebody to “doubt their intelligence” is extremely inappropriate and will not be tolerated here. Consider this a warning. – orthocresol Nov 4 '17 at 22:01
• @EmirhanG. The warning was about being insulting. You're continually justifying yourself when all you needed to do was not be insulting in the first place. We have a very strict Be Nice policy throughout Stack Exchange that strongly discourages such behavior. Ortho was not acting on "Ego," but instead emphasizing that moderators are well within their rights to suspend an account if a member becomes problematic. You're teeter-tottering on the edge right now, so please choose your words and your manner carefully from now on. – jonsca Nov 5 '17 at 2:26

Strong acids and bases like $\ce{HCl}$ and $\ce{NaOH}$ dissociate fully in water. Let us consider dissociation of $\ce{HCl}$. $\ce{HCl}$ in water dissociates as this. $$\ce{HCl <=> H+ + Cl-}.$$ Now, initially, the $\ce{H}$ and $\ce{Cl}$ are covalently bonded to each other. If we want to break this bond, we have to separate the positive and negative charges and this needs energy. Water molecules are in constant motion. They have kinetic energy. When you add $\ce{HCl}$ to water, some of the water molecules collide with the $\ce{HCl}$ molecules and break them. Now, the potential energy of the $\ce{H+}$ and $\ce{Cl-}$ system increases(becomes less negative). Thus the $\ce{HCl}$ molecules absorb some of the kinetic energy from the water molecules and some of this energy is stored as potential energy.

But as $\ce{H2O}$ is a polar molecule, the water molecules get attracted to the $\ce{H+}$ and $\ce{Cl-}$ ions. The water molecules hydrate the ions and this decreases potential energy of the system(water molecules + ion). This decrease in potential energy is marked by an increase in kinetic energy of the hydrated ion and the surrounding water molecules.

So, as dissociation takes kinetic energy of the system and converts it into potential energy, this process should decrease the temperature of the system. Similarly, hydration should increase the temperature of the system. To decide which effect is more dominant, we have to take into consideration $\Delta H_\text{Lattice Energy}$ and $\Delta H_\text{Hydration Energy}$ to decide which of the overall process of dissolution of $\ce{HCl}$ is exothermic or endothermic.

Similarly, using Hess' Law you can calculate if the acid-base reaction will be exothermic or endothermic.

• The first paragraph of your answer was pretty nice, but the second paragraph used some unnecessary and (personally) disliked terms, which I had forbidden to be used in the answer. Also, your answer suffices only to explain the production of heat when 'separate solutions of an acid and a base get mixed, but not to explain the production of heat when just an acid or a base is dissolved in water. A few weeks ago in the lab, I had dissolved some KOH in water and the solution had gotten pretty hot. – Emirhan G. Nov 4 '17 at 16:29
• I used the energy terms to supplement the force-momentum explanation. When you'll be doing calculations in chemistry, the the energy or enthalpy changes are easier to measure and calculate. So I added a paragraph on it. – Apoorv Potnis Nov 4 '17 at 16:34
• I've edited the answer. Please see it again. – Apoorv Potnis Nov 4 '17 at 16:41
• What happened to covalent bonding? If this is just about ions attracting each other and accelerating, then salt water should spontaneously heat up, since all the ions inside there are constantly being accelerated. And also the NaCl should spontaneously precipitate, since the Na+ and Cl- ions want to be next to each other. – orthocresol Nov 4 '17 at 16:50
• @orthocresol Yes. I made a mistake. There is no ionic bonding between $Na$ and $\ce{Cl}$ in solution or it'll precipitate. – Apoorv Potnis Nov 4 '17 at 17:29