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.
- To understand the chemical structures and properties that influence dissociation and acid/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.
- (AP) Analysing the structure of an acid or base helps to explain its relative strength. There are several structural factors to consider.
Before looking at them, remember the definition of a strong acid/base is 100% dissociation. A strong acid will have a weak conjugate base that, due to these factors, tends to dissociate and not re-form the strong acid once it does dissociate.
- Electronegative atoms: an electronegative atom can help to stabilize a conjugate base and make H atoms more likely to be lost as H+. Electronegativity will reduce the ability of lone pairs to attack and accept protons, so the conjugate base is weakened and in comparison, the acid is stronger.
- An example is HCl. The H-Cl bond is quite polar, with Cl commanding most of the electron density and hydrogen being quite + or ‘acidic. Once dissociated, Cl- is a stable ion due to the electronegativity of Cl and it will not accept protons to re-form HCl. This contributes to HCl’s strength as an acid.
- The inductive effect: this is where electron density is ‘pushed’ or drawn towards an atom in a chemical bond. If dissociation occurs, this can cause atoms to be more stable as ions (such as carbocations) than they would be without this effect, again stabilising them as conjugate bases.
- Resonance: resonance is when a system delocalizes its electron density. Instead of different atoms of higher and lower charge density (think of mountains and valleys), it is more evenly spread out over the whole system (now think of these mountains/valleys flattened out).
Resonance stabilizes molecules because electron density is less pronounced and available at any one atom. Less availability of electrons means less ability of a lone pair to attack a proton, so the conjugate base is less strong, more stable, and unlikely to change.
- An example is the strong acid HNO3. Once it dissociates into H+ and NO3-, the three resonance forms of NO3- stabilize the structure and it does not re-form HNO3. Because it doesn’t “re-associate”, it stays fully dissociated and qualifies as a strong acid. This is also true of H2SO4 or sulfuric acid.
Most species have a combination of these three effects that lead to their overall degree of dissociation and acid/base strength.
- Phosphoric acid (H3PO4) is an example. It has resonance forms that stabilize its conjugate base (H2PO4-) just like the strong acids nitric acid (HNO3) and sulfuric acid (H2SO4). Unlike nitric and sulfuric acid though, it has only two resonance forms instead of three. In this way, we say it has less resonance stabilization.
In addition to this, P is less electronegative than N and S. It does not pull electrons toward it with as much force as N or S, so the H atoms are less ‘acidic’, with a lower tendency to be lost as H+.
In summary, compared to nitric and sulfuric acid, phosphoric acid has a less stable conjugate base and less tendency to dissociate in the first place. This is why phosphoric acid is a weak acid unlike its nitric and sulfuric analogues.