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CIE A Level Chemistry复习笔记1.7.9 Acid & Base Dissociation

Category: A-level课程, 教材笔记, 福利干货 Date: 2022年8月12日上午10:50

Acid & Base Dissociation

Strong acids

A strong acid is an acid that dissociates almost completely in aqueous solutions
- HCl (hydrochloric acid), HNO₃(nitric acid) and H₂SO₄ (sulfuric acid)
The position of the equilibrium is so far over to the right that you can represent the reaction as an irreversible reaction

1.7-Equilibria-Dissociation-of-a-Strong-Acid

The diagram shows the almost complete dissociation of a strong acid in aqueous solution

The solution formed is highly acidic due to the high concentration of the H⁺/H₃O⁺ ions
Since the pH depends on the concentration of H⁺/H₃O⁺ ions, the pH can be calculated if the concentration of the strong acid is known

1.7-Equilibria-pH-Formula pH is the negative log of the concentration of H⁺/H₃O⁺ ions and can be calculated if the concentration of the strong acid is known using the stoichiometry of the reaction

Weak acids

A weak acid is an acid that partially (or incompletely) dissociates in aqueous solutions
- Eg. most organic acids (ethanoic acid), HCN (hydrocyanic acid), H₂S (hydrogen sulfide) and H₂CO₃ (carbonic acid)
The position of the equilibrium is more over to the left and an equilibrium is established

1.7-Equilibria-Dissociation-of-a-Weak-Acid

The diagram shows the almost complete dissociation of a weak acid in aqueous solution

The solution is less acidic due to the lower concentration of H⁺/H₃O⁺ions
Finding the pH of a weak acid is a bit more complicated as now the concentration of H⁺ ions is not equal to the concentration of acid
To find the concentration of H+ ions, the acid dissociation constant (Ka) should be used

Acid & equilibrium position table

1.7-Equilibria-Table-1_Acid-Base-Dissociation

Strong bases

A strong base is a base that dissociates almost completely in aqueous solutionsE.g. group 1 metal hydroxides such as NaOH (sodium hydroxide)
The position of the equilibrium is so far over to the right that you can represent the reaction as an irreversible reaction

1.7-Equilibria-Dissociation-of-a-Strong-Base The diagram shows the almost complete dissociation of a strong base in aqueous solution

The solution formed is highly basic due to the high concentration of the OH^- ions

Weak bases

A weak base is a base that partially (or incompletely) dissociates in aqueous solutions
- NH₃(ammonia), amines and some hydroxides of transition metals
The position of the equilibrium is more to the left and an equilibrium is established

1.7-Equilibria-Dissociation-of-a-Weak-Base

The diagram shows the almost complete dissociation of a weak base in aqueous solution

The solution is less basic due to the lower concentration of OH^-ions

Base & equilibrium position table

1.7-Equilibria-Table-2_Acid-Base-Dissociation

Exam Tip

Hydrogen ions in aqueous solutions can be written as either as H₃O⁺ or as H+ however, if H₃O⁺ is used, H₂O should be included in the chemical equation:HCl(g) → H⁺(aq) + Cl^-(aq)orHCl(g) + H₂O(l) → H₃O⁺(aq) + Cl^-(aq) Remember that some acids are both strong and weak acids – for example, H₂SO₄ (sulfuric acid) has two hydrogen ions that can ionise.H₂SO₄ acts as a strong acid: H₂SO₄ → H⁺ + SO₄^-HSO₄^- acts as a weak acid: HSO₄^-⇌ H⁺ + SO₄^2- Also, don't forget that the terms strong and weak acids and bases are related to the degree of dissociation and not the concentration.The appropriate terms to use when describing concentration are dilute and concentrated.