Acid and Base Strength
- Construct models and/or animations to communicate the differences between strong, weak, concentrated and dilute acids and bases
- Recall that Brønsted-Lowry theory explains that acids are substances which donate protons and bases are substances which accept protons.
$$HF(aq) + H_2O(l) \leftrightharpoons H_3O^+(aq) + F^-(aq)$$
$$NH_3(aq) + H_2O(l) \leftrightharpoons NH_4^-(aq) + OH^-(aq)$$
- In the top equation, we can see there is the formation of the hydronium ion which must have been formed from the acceptance of a proton which is donated from HF. This means that HF must be an acid
- In the bottom equation, there is the formation of an ammonium ion from the acceptance of a proton by the ammonia. This means that ammonia must be a base.
Brønsted-Lowry Theory – Conjugate Acid and Base Pairs
The Brønsted-Lowry theory:
- defines acids as proton donors
- defines bases as proton acceptors
- is independent of the state of the reactants or the solvent
- demonstrate how acid-base reactions in reversible reactions involve the transfer of proton(s)
The theory introduces the concept of conjugate acid-base pairs:
- an acid forms a base when it loses a proton in a reversible reaction i.e. when the acid is weak
- a base forms an acid when it gains a proton in a reversible reaction
- the acid and the base it forms are called a conjugate acid-base pair
- the base and the acid it forms are called a conjugate acid-base pair
The acid and base of a conjugate acid-base pair differ from each other in structure by only 1 proton. For example, the reaction between acetic acid and water:
$$CH_3COOH(aq) + H_2O(l) \leftrightharpoons CH_3COO^-(aq) + H_3O^+(aq)$$
- A limitation to Humphrey Davy's hydrogen theory of acids was that not all hydrogen–containing molecules are acids. Molecules are only considered acids if the hydrogen atoms can be ionised or given away. Only hydrogen atoms that can be ionised are described as acidic.
- The acidity of a particular hydrogen is dependent on the stability of the conjugate base. This is because if the conjugate base is more stable, then that will also mean that it is going to be less reactive and less likely to hold onto the acidic hydrogen.
- For example, hydrogen atoms in methane, CH4, are not easily ionisable because the carbanion which would be formed from methane's deprotonation is highly unstable and thus methane is not considered an acid.
Strength of Acids and Bases
- Strength of acids and bases are dependent on their degree of ionisation. In the Brønsted-Lowry theory of acid and bases, the degree of ionisation is also referred to as the degree of deprotonation (acids) or protonation (bases).
- Strong acids completely dissociate in water to produce hydrogen ions.
- Strong bases completely dissociate in water to produce hydroxide ions.
- Strong acids completely deprotonate and react with water to produce the maximum concentration of hydronium ions.
- Strong bases completely protonate and react with water to produce the maximum concentration of hydroxide ions.
In the diagram on the left, the hydrochloric completely dissociates into H+ and Cl-. This means that HCl is a strong acid.
In the diagram on the right, the acetic acid only partially dissociates into H+ and CH3COO-.This means that CH3COOH is a weak acid.
Strength vs Concentration
- Strength and concentration of acids are independent concepts. Strength describes the percentage of acid molecules that ionise whilst concentration describes the number of acid molecules (ionised and unionised) per volume of solvent.
In the diagram on the left, there are more molecules of HCl than on the right. This means the solution on the left can be described as being concentrated while the solution on the right can be described as being dilute.
- The conventions we use to describe acid dissociation are also applied to bases. This means that a strong base is one which completely dissociates, and a weak base is one which only partially protonates.
- Although Group 1 and 2 metal hydroxides are mostly strong bases, Group 2 hydroxides e.g. Ca(OH)2 are less soluble in water. As a result, some undissociated solid Ca(OH)2 will be found floating in the water or lying at the bottom of the solution. In contrast, Group 1 hydroxides e.g. NaOH are very soluble in water which means only hydrated metal cations (Na+) and hydroxide ions will be found in the solution.
- Thus, it is likely for Group 1 metal hydroxides to have a higher pH than Group 2 metal hydroxides of equal concentration.
Strength, Ka, Kb
- Acid strength is proportional to the value of Ka. A higher value indicates higher concentration of hydrogen ions and more acids ionised. This corresponds to a stronger acid.
- Base strength similarly is proportional to the value of Kb. A higher value indicates a higher concentration of hydroxide ions and more base ionised. This corresponds to a stronger base.