What is the conjugate base of NH3

Acid-base reaction: definition, examples, conjugated pairs

The concept of acids and bases is a very important branch of chemistry, with which numerous effects and reactions can be explained.

Definition according to Brönsted

Brönsted defined one acid as a chemical compound that releases positively charged hydrogen ions (protons) during reactions. Because acids release hydrogen ions, they are also known as proton donors (Latin: "donare" - to give).

In parallel, Brönsted defined chemical compounds that absorb positively charged hydrogen ions as Bases. Bases are therefore proton acceptors (Latin "acceptare" - to accept, to take up).

From the Brönsted definitions we can conclude that an acid must necessarily have a hydrogen atom in order to be able to donate protons. A Brönsted base must also be able to absorb a positive hydrogen ion.

Examples of acid-base reactions

To illustrate the Brönsted acid-base concept, let us consider one of the most important and well-known acids in the chemical industry - hydrochloric acid (HCl).

As we can already see from the formula for hydrochloric acid, the acid consists of one hydrogen and one chlorine atom. If the acid releases the hydrogen (with a positive charge), a negatively charged chlorine anion remains.

This process can be recorded in the presence of water as follows:

From this equation we can therefore conclude that, according to Brönsted, HCl behaves like an acid, since it emits a proton. The water behaves like a Brönsted base because it absorbs a proton, which was the criterion for a base. Since HCl can only give off one proton, hydrochloric acid is called one monoprotonic acid.

However, there also exist multi-protonic acids. An example of such a multi-protonic acid would be phosphoric acid (H3PO4). As you can easily see, phosphoric acid contains 3 hydrogen atoms. If the phosphoric acid now releases one of these 3 protons, 2 remain.

The reaction to this is as follows:

Because phosphoric acid can release even more protons, the release of the first proton is also known as the first protolysis stage.

The second stage of protolysis would be the following:

And the third stage of protolysis:

So you get as a product , which is also called Phosphate ion referred to as.

As can be easily seen, the phosphate ion does not contain any other hydrogen atoms that could be given off as protons. Thus the phosphate ion is no longer an acid and it is known that phosphoric acid only has 3 protolysis levels. The number of initial protons that can be released also determines the number of protolysis stages. Usually the acidity decreases with the level of protolysis.

This means that the acid strength of phosphoric acid up to the phosphate ion can be described as follows:

Conjugated acid-base pairs

If we denote the acid with HX and its corresponding base as X-, the following reaction with water can be recorded:

However, since both the forward reaction and the reverse reaction take place, this reaction can also be written as follows:

The only noticeable difference between HX and X- is the missing proton. Consequently, the base is the deprotonated form the acid.

Such Acid-base pairsthat differ by only one proton are referred to as corresponding acid-base pair. Let us consider the already known reaction of hydrochloric acid with water as an example of a corresponding acid-base pair.

In this case, of course, the hydrochloric acid is the acid. Consequently, the water acts as a base. The hydrochloric acid releases a proton into the water. This creates a chloride anion and oxonium. The chloride anion corresponds exactly to hydrochloric acid - except for the missing proton.

For this reason, the chloride anion is the base corresponding to hydrochloric acid. On the other hand, the oxonium corresponds exactly to the water - except for the added proton. Since the oxonium is the protonated form of water, it is also the acid corresponding to the base water.

The relationship is illustrated in the graphic below.

Likewise, conjugated acid-base pairs can also be found for reactions in which the water reacts as an acid. In the following example, ammonia is used as the base. Ammonia (NH3) can take up another proton and reacts to form the ammonium ion. In addition, the water gives off a proton and forms hydroxide.

If you look at the reverse reaction, the NH4 + gives off a proton, which is why it reacts as a (conjugate) acid. OH-, on the other hand, takes on a proton in the reverse reaction, which is why it reacts as a (conjugated) base.

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