Strong and weak acids

In a nutshell

Strong acids fully dissociate and weak acids partially dissociate. Strong and weak acids can both be either strong or concentrated.

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Acids release protons into water

Acids are ionised in solution and they release $$H^{+}$$​ ions into solution.

Examples

$$hydrochloric\ acid\ \rightarrow hydrogen\ ion \ +\ chloride\ ion \\ \ \\HCl \rightarrow H^+ + Cl^-$$

$$sulphuric\ acid\ \rightarrow\ hydrogen\ ion\ +\ hydrogen\ sulphate\ ion \newline \ \\ H_{2}SO_{4} \rightarrow H^+ + HSO_{4}^-$$​​

​​Strong acids

Definition

Strong acids fully dissociate in water to produce the maximum number of $$H^{+}$$ions. 

Example

An example of a strong acid is hydrochloric acid $$(HCl)$$​. It fully dissociates into $$H^{+}$$​ and $$Cl^{-}$$​ ions.

$$hydrochloric\ acid\ \rightarrow\ hydrogen \ ion\ +\ chloride\ ion\newline \ \\ HCl \rightarrow H^+ + Cl^-$$​​

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Weak acids

Definition

Weak acids, such as acetic acid ($$CH_{3}COOH$$​), only partially dissociate. Some of the molecules will dissociate whilst most will not, therefore not contributing to the acidity of the solution.

Example

An example of a weak acid is acetic acid ($$CH_{3}COOH$$​). It partially dissociates into $$H^{+}$$ and $$CH_{3}COO^{-}$$ ions.

​$$acetic\ acid\ \rightleftharpoons\ hydrogen\ ion\ +\ acetate\ ion\newline \ \\ CH_{3}COOH \rightleftharpoons\ H^+\ + CH_{3}COO^-$$

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1. Strong acid (complete dissociation) 2. Weak acid (partial dissociation)

pH of acids

pH is a measure of the $$H^+$$ ion concentration in solution. For a decrease of one on the pH scale, there is a $$10$$​ times increase in $$H^+$$​ ion concentration and vice versa for an increase in pH. 

Examples

• A solution with a pH of $$3$$ has a $$H^+$$ concentration that is $$10$$ times greater than a solution with a pH of $$4$$ and $$100$$ times greater than a solution with a pH of $$5$$.​
• If the pH of a solution drops from $$14$$ to $$10$$, the difference is $$4$$ so the concentration of $$H^+$$ ions is $$10^4$$ greater.​

Concentrated and dilute acids

Concentrated and strong acids are not the same thing. Concentration refers to the amount of acid which is present in a particular volume of water. 

The greater amount of acid in a given volume of water, the more concentrated the solution. As acid concentration increases, the pH will decrease regardless of whether the acid is strong or weak.