In acid base neutralization, if the concentration of the base remains constant but the concentration of the acid is increased, is the amount of heat released or change in temperature affected?

Expert Answers
caledon eNotes educator| Certified Educator

The answer to this prompt depends strongly upon the initial conditions, i.e. the state of the reaction before the acid concentration was increased. If the acid and base were in equal proportion according to a stoichiometric ratio, then the results will be somewhat different than if the two were not in proportion.

Regardless of those conditions, we can discuss a few general things about the statements being investigated. First, we should acknowledge the difference between energy and temperature. Energy is more difficult to define, but we can understand it in this case as the effect of changing the bonds in the acid and base. Temperature, on the other hand, is a measure of molecular motion. The temperature of two objects of different masses do not necessarily tell us about their energy, or vice versa; energy applied to mass A may result in a greater temperature change than the same energy applied to greater mass B, and so temperature is not a proportional indicator of energy itself unless we know about the interaction between the molecules of the system and energy. 

Let's assume that the system is hydrochloric acid (HCl) and sodium hydroxide (NaOH). These will neutralize to produce sodium chloride (table salt, NaCL) and water (H2O). Let's assume the solvent is water. In this case, the neutralization will add water to the system, as well as dissolving salt in it, which slightly raises its heat capacity. Therefore, if we're being really precise, we should acknowledge that this reaction will raise the volume of water in the system as well as its heat capacity, and if there was no exothermic aspect of this reaction, then we would expect the temperature of the system to go down as the energy is distributed. 

Now let's assume that we simply added more HCl to this already-balanced condition from the outset. The only thing that would be accomplished by this is that there is a greater amount of HCl present, which increases the chances of the NaOH encountering it, thereby speeding up the neutralization. However, the total amount of energy released would not change, because we still have the same number of NaOH molecules reacting.

If, on the other hand, HCl was our limiting reagent, and we increased it, we would expect the total amount of energy to increase, since more reactions are taking place in total, and therefore the temperature should increase as well. 

Thus, we can summarize that the amount of energy released depends directly upon the number of neutralization reactions taking place. Temperature change depends on the amount of energy released, and the heat capacity and mass of molecules capable of distributing it. 

user2391260 | Student

The chemical equilibrium is affected by both sides of the equation, remaining constant accordingly to the stechiometric calculus of the chemical agents' concentration. To start calculating the minimum quantities of each chemical agent in the reaction, one easy option is to consider the minimum quantity of 1 mol of salt deducing the other components' values from that. This is a simple example.

NH4+ + OH- <=> 1NH3 + H2O

In the balanced reaction there are 1 atom of N, 5 atoms of H and 1 atom of O in each side of the reaction.

The stronger the acid/base, the more they'll dissociate leading the chemical equilibrium to the opposite side. The weaker they are, the less salt will be produced at the chemical equilibrium.

3NH4+ + 1OH- <=> 1NH3 + 1H2O

If you suppose there are for instance 3 mols of NH4+ and one mol of OH-, you suppose there is a limiting reactor, the base. In that situation, because the equation needs to be balanced in order to calculate chemical equilibrium, it won't be affected nor will those cited reaction variables, because in effect there will not be any changes in the reaction products' concentration.

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