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How can I figure out which two solutions will emit the largest heat, when they are mixed in same volumes? Then let's assume that there are five pairs of solutions given:

\begin{align} \ce{2NaOH + H2SO4 &-> Na2SO4 + 2H2O} \tag1\\ \ce{Ba(OH)2 + H2SO4 &-> BaSO4 + 2H2O} \tag2\\ \ce{2NH4OH + H2SO4 &-> (NH4)2SO4 + 2H2O} \tag3\\ \ce{NaOH + H3CCOOH &-> H3CCOONa + H2O} \tag4\\ \ce{2NaOH + H2C2O4 &-> Na2C2O4 + 2H2O} \tag5 \end{align}

Which one emits the most heat, when we mix same volumes of given pairs?

These are the balanced reaction equation, that occur when the above solutions are mixed. All of these reactions are acid base reactions. And I know when there is an acid base reaction a salt is formed. So how can I proceed with it?

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You should just think about a fact: all chemical processes will correspond to enthalpy changes.

The basic process of acid-base reaction is $$\ce{H+ + OH- -> H2O}$$

This process will have an enthalpy change of $\pu{-55.8 kJ mol-1}$ at $\pu{298 K}$. (I don’t know why, but in different books or articles this value will be slightly different.) However,

  1. Precipitate formation is usually exothermic, since there are some ion bonds formed.
  2. Weak acid and base usually need some energy, for they are not completely ionized, during the reaction, they will to ionize more since $\ce{H+}$ or $\ce{OH-}$ is consumed. Ionization will be endothermic.

Therefore the answer would be the second reaction, since $\ce{BaSO4}$ is precipitated, which is exothermic, while weak acid and base reactions are endothermic.


(However, there seems to be always some exception, for example, ionization of $\ce{HF}$ will be exothermic. But to discuss this may be out of topic, and to be frankly, I’m not quite sure what its mechanism is.)

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  • $\begingroup$ Can you figure out whether the reaction is endothermic or exothermic by just looking at the reaction? $\endgroup$ – On the way to success May 1 '15 at 15:12
  • $\begingroup$ This question is too broad and I should say I can't. But you will meet some typical types of reaction and you can expect it. Usually we think it by thinking bonds breaking/forming, equilibrium shifting when heating/cooling, or sometimes but "common senses". $\endgroup$ – Asydot May 1 '15 at 15:27

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