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CIE A Level Chemistry復(fù)習(xí)筆記1.7.9 Acid & Base Dissociation

Category: A-level課程, 教材筆記, 福利干貨 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), HNO_3?(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
- NH_3?(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.