Enthalpy of Neutralization of Strong Acid and Strong Base

Dhristi JEE 2022-24

Introduction of Enthalpy of Neutralization

The standard enthalpy change of neutralization reaction is the enthalpy change that occurs when the solutions of an acid and an alkali react together under conditions to produce 1 mole of water. This reaction is also said to be an exothermic reaction as a high amount of energy is being given out when the neutralization reaction takes place. Enthalpy is also called the heat that is involved when a reaction takes place.

Neutralization Enthalpy of Strong Base and Strong Acid

For a strong acid and a strong base, the neutralization enthalpy is still constant. This is because both strong acids and strong bases are fully ionized in a dilute solution. Neutralization changes in enthalpy are often negative-when an acid and alkali react, heat is released

What is Standard Enthalpy Change?

As solutions of an acid and an alkali react together under normal conditions to produce 1 mole of water, the standard enthalpy change of neutralization is the enthalpy change. Note that the neutralization shift in enthalpy is always measured per mole of water produced. Neutralization alterations in enthalpy are often negative - when an acid and alkali react, heat is released. The values are often very nearly similar for reactions involving strong acids and alkalis, with values between -57 and -58 kJ mol-1. Which varies slightly depending on the combination of acid and alkali.

Neutralization Reaction

When an acid and a base react to form water and salt, a neutralization reaction requires the combination of H+ ions and OH- ions to produce water. There is a pH equal to 7 for the neutralization of a heavy acid and strong base. Neutralizing a strong acid and a weak base would have a pH of less than 7 and, conversely, the resultant pH will be greater than 7 when a strong base neutralizes a weak acid.

It means that salts are formed from equal weights of acid and base when a solution is neutralized. The amount of acid required is the amount that one mole of protons (H+) would give and the amount of base needed is the amount that one mole of protons would give (OH-). Since salts are formed from neutralization reactions of equal acid and base weight concentrations, N parts of the acid will always neutralize N parts of the base.

Why do Strong Acids that React with Strong Alkalis Produce Similar Values?

We assume that strong acids and strong alkalis in the solution are completely ionized and that the ions work independently of each other. In solution, dilute hydrochloric acid, for example, contains hydrogen ions and chloride ions. The sodium hydroxide solution in the solution consists of sodium ions and hydroxide ions. In essence, the equation for any strong acid being neutralized by a strong alkali is just a reaction to make water between hydrogen ions and hydroxide ions. The other ions present (for example, sodium and chloride) are merely spectator ions, which do not participate in the reaction.

The equation of reaction between hydrochloric  acid and sodium hydroxide solution is:

                        NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

But the actual happening is different:

                       OH(aq) + H+(aq) → H2O(l)

If the reaction is the same in both a strong acid and strong alkali, then it is not surprising that enthalpy change is similar.

Anything about 99% of the acid is not naturally ionized in a weak acid such as acetic acid at ordinary concentrations. This implies the other enthalpy terms involved in ionizing the acid as well as the reaction between the hydrogen ions and hydroxide ions would include the enthalpy shift of neutralization. And ammonia is also present primarily as ammonia molecules in solution in a weak alkali like ammonia solution. 

Again, apart from the basic form of water from hydrogen ions and hydroxide ions, there may be other enthalpy modifications involved. The calculated enthalpy shift of neutralization for reactions involving acetic acid or ammonia is a few kJ less exothermic than with solid acids and bases.

One source that provides the enthalpy shift of sodium hydroxide solution neutralization with HCl as-57.9 kJ mol-1:

                 NaOH(aq) + HCl(aq) → Na+(aq) + Cl(aq) + H2O

The neutralization enthalpy change for acetic acid-neutralizing sodium hydroxide solution is -56.1 kJ mol-1:

                 NaOH(aq) + CH3COOH(aq) → Na+(aq) + CH3COO(aq) + H2O

For very weak acids, such as cyanide hydrogen solution, the neutralization shift of enthalpy can be much less. The value of the hydrogen cyanide solution being neutralized by potassium hydroxide solution as -11.7 kJ mol-1, for example, is given by another source.

                    NaOH(aq) + HCN(aq) → Na + (aq) + CN − (aq)+H2O

Experiment to understand the Enthalpy of Neutralization of Strong Acid and Strong Base

The experiment can be conducted between a strong acid and a strong base by titration process. The temperature that is evolved while the reaction is proceeding to equilibrium is noted down and then the heat value is calculated from the same.

Precautions to be taken while Performing the Experiment for Neutralization Reaction

When performing the experiment in the lab it is necessary to take some precautions which can be provided as follows:

  1. Due to radiation, there is some heat lost to the environment and hence the reaction flask can be a bit hot. Handling hot things must be done carefully.

  2. The solution density must be equal to 1 g/ml.

  3. The hydrochloric acid that is the strong acid and the sodium hydroxide that is the strong base ionization is considered to be 100 per cent.

  4. The specific heat of the solution is taken as 4.189 J/g.

  5. The mixture that contains both HCl and NaOH must be stirred properly to get accurate results.

Book your Free Demo session
Get a flavour of LIVE classes here at Vedantu
Vedantu Improvement Promise
We promise improvement in marks or get your fees back. T&C Apply*
FAQs (Frequently Asked Questions)

1. Find out whether exothermic or endothermic reactions are neutralization reactions?

A chemical reaction in which heat is released is known as an exothermic reaction. A chemical reaction in which heat energy is absorbed is called an endothermic reaction. Once an alkali neutralizes an acid, the reaction is exothermic. The other ions are bystanders. The formation of bonds is an exothermic operation, so the response is exothermic as bonds are formed. The value of ‘H’ is positive i.e. the heat from the atmosphere is taken in. It investigates the neutralization of a strong acid with a strong base and discovers that the environment is still heat-added. It is important to note that bonds are made and during an exothermic reaction, energy is released into the atmosphere. The reaction of neutralization is also exothermic.

2. Why the heat of neutralization of strong acid and strong base is constant?

For a strong acid and a strong base, the neutralization enthalpy is still constant: this is because both strong acids and strong bases are fully ionized in a dilute solution. Neutralization changes in enthalpy are often negative-when an acid and alkali react, heat is released. For a strong acid and a strong base, the neutralization enthalpy is still constant: this is because both strong acids and strong bases are fully ionized in a dilute solution. Neutralization changes in enthalpy are often negative-when an acid and alkali react, heat is released.

3. How does a neutralization reaction take place?

A neutralization reaction takes place when the acid which is present in the solution tends to donate a proton and the base tends to take up the proton. The by-product that is obtained in the neutralization reaction is water where the positive hydrogen ions react with the negative hydroxyl anions to provide the product. The resulting salt will always be of neutral charge as there is the presence of both the negative and positive charges that balance out each other.

4. How many steps are involved in the neutralization reaction?

There is only a single step that is involved in the neutralization reaction. This reaction that proceeds in a single step forms the product and the by-product together. Depending on the strength of the acids and bases the reaction rate will proceed further. While a weak acid does not dissociate completely the strong acid tends to dissociate faster and hence the reaction rate will be higher. Based on the reaction rate the enthalpy is also measured. This measured enthalpy has a negative sign due to the exothermic effect or the heat is being given out.

5. What indicators are used in the neutralization reaction to understand the endpoint of the reaction?

The indicator that is used in the neutralization reaction of most of the acid-base titrations is phenolphthalein. This phenolphthalein is a colourless weak acid indicator that remains entirely colourless when it is mixed in an acidic solution. However, when the reaction starts and the titrant is added to the strand the colour slowly starts changing to a pale pink or magenta color. As the pH goes on increasing the colour of the solution also keeps on getting darker and becomes more visible. This change in colour is the phenomenon that is used to determine the endpoint of the neutralization reaction.  

6. Is it possible for strong acids and weak bases to react with each other?

Yes, it is possible for strong acids to react with weak bases. While there is a theory that states that the strong acid can only react with strong bases and the weak acids can only react with weak bases however it is found that strong acids and weak bases can also react with each other. One of the common examples is the reaction between hydrochloric acid which is a strong acid and ammonia which forms a conjugate salt as the product. In this type of reaction, the pH will always be lower than 7 and will hence be an acidic solution which will be very weak.

7. What is the importance of learning regarding the Enthalpy of Neutralization of Strong Acid and Strong Base?

Enthalpy of Neutralization of Strong Acid and Strong Base provides a deep knowledge regarding the reactions that take place between any acid and a base. Students also get to understand the mechanism of the reaction that is taking place. It also has various real-life applications such as when a person has acidity a basic solution is prepared which will help relieve the acidity. This is due to the neutralization reaction that takes place in the body. 

Comment