Weak acids: distinction & examples

In chemistry, there are acids of different strengths. Strong acids, such as hydrochloric acid, and weak acids, such as acetic acid, are divided. But what are weak acids and how can weak and strong acids be differentiated in terms of acid strength?

Weak acids – definition

Weak acids are acids that are not fully dissociated in an aqueous solution.

In chemistry, there are both acids and bases. As with bases, acids are distinguished between different acid strengths. Based on the acid strength Strength and weak acids be distinguished.

Acid term according to Arrhenius

Arrhenius was a Swedish chemist and one of the first to in 1887 a Acid-Base Theory has set up.

Arrhenius assumed that an acid is still a substance that tastes sour. Dissociated in an aqueous solution, this substance breaks down into hydrogen ions (H+), also known as protons. As defined by Arrhenius an acid dissociates upon addition of water into hydrogen ions and acid residue ions.

Brönsted concept of acidity

The Danish chemist Johannes Brönsted and the Englishman Thomas Lowry also developed a theory of the acid-base concept in 1923. Accordingly, the acid is a substance that can give off protons both in aqueous solutions and without the addition of water. Therefore, an acid is considered proton donor.

In the reaction, however, a substance must be present that will take up the proton released by the acid. the Acid-Base Reactions according to Brönsted are reactions between acids and bases with a proton transition.

Bases form the substance that absorbs the formed protons (H+) and is therefore also called proton acceptor designated. In aqueous solutions, the water molecule forms the proton acceptor and an oxonium ion is formed.

An oxonium ion is formed when water accepts protons and looks like this: H3O+.

Weak Acid – pH

pH indicates how acidic or alkaline a solution is. pH measures the concentration of oxonium ions in the solution. The higher the concentration of oxonium ions in the solution, the lower the pH and the more acidic the solution.

The pH is in a scale classified by 0 (very acidic) to 14 (very alkaline) enough. Concentrated, weak acids usually have a pH between 3 and 6. The molar concentration of oxonium ions is greater than that of hydroxide ions at such a pH.

At a pH of 7, the concentrations of oxonium ions and hydroxide ions are identical.

Examples of weak acids from everyday life:

Weak acids – list

Here is a list of some weak acids with their corresponding pKa value. The larger the pKa value, the lower the acid strength and the weaker the acid.

The pKa value is the negative logarithm of the acidity KS. The following applies to the pKa value: the lower the value, the higher the acid strength. This means a weak acid has a high pKa value and a strong acid has a low pKa value.

It is important that acid strength is not confused with the reactivity of an acid. Although hydrofluoric acid is a weak acid with a pKa value of 3.14, it can also decompose glass.

Weak acids – dissociation

Acids dissociate in water. They give off protons and acid residue ions are formed. The acid strength is crucial here, because it depends on how strongly the equilibrium of the dissociation of an aqueous solution is on the side of the products. This also affects the pH value.

Dissociation describes the breakdown of a chemical compound into individual components. Most often, dissociation takes place in water.

While strong acids completely dissociate into their ions in water, a weak acid, such as hydrocyanic acid, only partially decomposes into ions. This is because a weak acid reacts to form a strong base.

This corresponding base of a weak acid is poor in stabilizing its negative charge. Because of this, the corresponding base reacts quickly with a positively charged proton back to the neutral undissociated acid. This tendency of the corresponding base makes it a strong base. In the case of a strong acid, the corresponding base is relatively weak, so this weak base hardly reacts back. The corresponding base of a strong acid can therefore stabilize its negative charge well.

Most of the weak acid remains unchanged. Thus, a weak acid only partially decomposes into ions. The equilibrium of a weak acid is therefore on the left side, i.e. the side of the educt.

Hydrochloric acid is a strong acid. As a strong acid, it completely dissociates in water to form protons and acid residue ions.

Acetic acid is a weak acid and does not fully dissociate in water. When acetic acid reacts with water, oxonium ions and so-called acetate ions are formed. The acetate ions formed react back to acetic acid. In the case of acetic acid, for example, an equilibrium is established that is more to the left, i.e. on the side of the educts.

Main differences from strong acids

Strong acids react more quickly with bases than weak acids, such as hydrocyanic acid. Acids with a pKa value below 3.75 are considered strong. Strong acids completely dissociate in water. This is not the case with a weak acid because a weak acid is only partially dissociated.

Weak acids are used in everyday life, for example acetic acid in the food industry.

This also explains why strong acids conduct electricity better than weak acids can. Because they dissociate completely in contrast to weak acids, there are significantly more ions in the solution when strong acids are dissolved. As mobile charge carriers, ions are responsible for the conductivity of a solution.

The pH of a weak acid

You can also calculate the pH value for weak acids. In this section, you will learn how to calculate the pH of a weak acid, such as acetic acid.

To calculate the pH value, you need the acidity constant KS or the pKa value, which gives you information about the acidity. The larger the pKa value, the lower the acid strength and the weaker the acid. To help you calculate pH, there is a general formula for calculating pH.

Calculate the pH of weak acids

You can recognize a weak acid by the equilibrium in an acid-base reaction. Only a small proportion of the weak acid donates the proton to the water. During protolysis, the concentration of the weak acid HA hardly changes. In the reaction with water, the equilibrium shifts almost completely to the starting material side.

The law of mass action will help you calculate the pH of weak acids. To do this, we will derive the formula for calculating the pH value from the law of mass action in the following section.

Water is present in a high concentration during the reaction, which is why the concentration of water can be considered to be almost constant. Because of this, the concentration of water is included in the equilibrium constant K and defined as the acid constant KS.

The concentration of the reacting oxonium ions and the concentration of the deprotonated weak acid are the same because the reaction produces both an oxonium particle and a deprotonated weak acid particle.

Now you rearrange the formula according to the concentration of the oxonium ions.

In the last step, the negative decimal logarithm is applied and the expressions obtained are replaced by the pH value and the pKa value. This gives the following equation for calculating the pH of a weak acid.

It is thus assumed that the residual concentration of the weak acid HA corresponds to the concentration at the beginning, since the chemical equilibrium is on the side of the reactants. For example, you can use this pH calculation formula to calculate the pH of a solution containing acetic acid or phosphoric acid.

You can also calculate the pKa value using the law of mass action or take it from a list.

Weak acids – the most important thing

  • Acids are proton donors and donate protons in an aqueous solution.
  • Acids that do not completely dissociate in water are called weak acids.
  • The reason is that the corresponding base of a weak acid is a strong base, which cannot stabilize its negative charge badly and therefore reacts backwards.
  • Weak acids have a larger pKa value than strong acids.
  • Examples of weak acids are acetic acid, hydrofluoric acid and hydrocyanic acid.
  • The pH of weak acids is calculated using the formula: .