In this lesson, we will learn:
- To understand the difference between acid and base strength and concentration.
- To identify strong and weak acids/bases by their degree of dissociation.
- To understand how degree of dissociation leads to varying acid and base strength.
- In acid-base chemistry, there’s an important difference between strength and concentration. It is possible to have a highly concentrated ‘weak acid’ and a very dilute, quite harmless ‘strong acid’. Recall that:
Concentration measures the amount of substance in a sample, compared to the amount of solvent (e.g. water) it is dissolved in. This can be changed easily: add more solvent to decrease concentration or add more solute to increase concentration.
Acid and base strength (acidity or basicity) measures dissociation, where the degree of dissociation is how much a chemical compound splits from the complete compound (e.g. HCl) into its ions or components (e.g. H+ and Cl-). This can’t be changed because degree of dissociation is a core chemical property of something:
- A strong acid or base experiences 100% dissociation into its ions when put in water. This means, in theory, every single molecule of the substance becomes aqueous ions: every HX molecule becomes H+ and X- (the conjugate base) while the base B becomes the conjugate acid HB+which accepted H+ (aq) in solution. Many mineral (inorganic) acids are strong acids, including:
- Hydrochloric acid, HCl.
- Sulfuric acid, H2SO4.
- Nitric acid, HNO3.
- Some strong bases include:
- Potassium hydroxide, KOH.
- Sodium hydroxide, NaOH.
- Calcium hydroxide, Ca(OH)2.
- A weak acid or base experiences partial dissociation into aqueous ions. This means that less than 100% of the molecules of acid or base dissociates into aqueous ions in water.
- Carboxylic and other organic acids are weak acids.
- Ammonia is an example of a weak base as it does not completely dissolve forming ammonium and hydroxide ions.
- Recall that in our lesson on conjugate acids and bases, we learned that in a conjugate pair, the stronger the acid, the weaker the base. You will notice the effect of this in any chemistry information tables showing acid or base strength:
- For the strongest acids, the dissociation into H+ and the conjugate base is shown by a single headed reaction arrow → instead of the equilibrium arrows. This is because the conjugate base of a strong acid is extremely poor at accepting a proton, so it will not go back to being the conjugate acid. This process won’t reach equilibrium like weak acids and bases will.
- For the strongest bases, the addition of H+ to form the conjugate acid is also depicted by a single headed reaction arrow → because the reverse process of the poor conjugate acid returning to the original strong base simply will not occur.
- This is true of CONJUGATE PAIRS, not of individual molecules; for example if compound A is a poor base, it doesn’t mean it is automatically a strong acid.
- Be careful with polyprotic acids – those that have more than one proton to donate (e.g. H2SO4). Protons dissociate one at a time, and the second proton has a much lower degree of dissociation than the first proton. As far as sulfuric acid, a strong acid, is concerned, it has 100% dissociation of the first proton only:
H2SO4 → H+ + HSO4-
The effects of the second proton dissociating will be dealt with when looking at the acid dissociation constant, Ka.
- As stated above, the degree of dissociation is how acidity and basicity is measured. With this however, once you have two strong acids/bases which both experience 100% dissociation, they are considered to have identical strength. This is because they are equivalent to solutions of H3O+ (aq) (or OH- for bases). All strong acids/bases have identical strength as far as the degree of dissociation goes because they all completely dissociate to H3O+ or OH- solutions.
- One of the other differences between strong and weak acids and bases is in measurements like the enthalpy of neutralization. Remember that neutralization is the reaction:
H+ (aq) + OH- (aq) → H2O (l)
Enthalpy change of neutralization is the enthalpy change when an acid and base react in a neutralization to produce 1 mole of water. Strong acids have 100% dissociation into H+ (aq) and X- (aq), and strong bases will completely react to form OH- (aq) and B-H+ (aq). This means that all strong acids and bases have the same reaction to the same degree which is why their enthalpy of neutralization is a very similar exothermic value.
In a weak acid however, not all the substance ionizes in solution (usually less than 1% does). Most of the acid isn’t ionized and there may be other enthalpy changes occurring rather than just H+ reacting with OH-. This leads to the enthalpy of neutralization being less exothermic in weak acids and bases than in strong acids and bases.