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IB DP Chemistry: HL复习笔记18.2.1 Acid & Base Dissociation Constants

Category: IB课程, 教材笔记, 福利干货 Date: 2022年7月29日 下午12:39

Acid & Base Dissociation Constants

Weak acids

  • A?weak acid?is an acid that?partially?(or incompletely)?dissociates?in aqueous solutions
    • For example, most carboxylic acids (e.g. ethanoic acid), HCN (hydrocyanic acid), H2S (hydrogen sulfide) and H2CO3?(carbonic acid)
    • In general, the following equilibrium is established:

HA (aq) + H2O (l) ?? A-?(aq) + H3O+?(aq)

OR

HA (aq) ?? A-?(aq) + H+ (aq)

  • At equilibrium, the majority of HA molecules remain unreacted
  • The position of the equilibrium is more over to the?left?and an equilibrium is established
  • As this is an equilibrium, we can write an equilibrium constant expression for the reaction
  • This constant is called the?acid dissociation constant,?Ka, and has the units mol dm-3

18.2-Ka-expression

Acid dissociation constant expressions

  • Carboxylic acids are weak acids
    • For example, propanoic acid, CH3CH2COOH (aq), dissociates according to the following equation which leads to the?Ka?expression for propanoic acid:

CH3CH2COOH (aq) + H2O (l) ??CH3CH2COO–?(aq) +?H3O+?(aq)

OR

CH3CH2COOH (aq) ??CH3CH2COO–?(aq) +?H+?(aq)18.2-Ka-expression-for-propanoic-acid

Acid dissociation constant expressions for propanoic acid

  • Values of?Ka?are very small
    • For example,?Ka??for propanoic acid = 1.34 x 10-5?mol dm-3?
  • When writing the equilibrium expression for weak acids, we assume that the concentration of?H3O+?(aq)?due to the ionisation of water is negligible

Weak bases

  • A weak base will also ionise in water and we can represent this with the?base dissociation constant,?Kb
  • In general the equilibrium established is:

B (aq) + H2O (l) ???BH+?(aq) +?OH- (aq)

18.2-Kb-expression-

Base dissociation constant expression

  • Amines are weak bases
    • For example,?1-phenylmethanamine, C6H5CH2NH2?(aq), dissociates according to the following equation which leads to the?Ka?expression for 1-phenylmethanamine:

C6H5CH2NH2?(aq) + H2O (l) ? C6H5CH2NH3+?(aq) + OH-?(aq)18.2-Kb-expression-for-1-phenylmethanamine

Base dissociation constant expression for 1-phenylmethanamine

pKa?and pKb?

  • The range of values of?Ka?and?Kb?is very wide and for weak acids, the values themselves are very small numbers

Table of?Ka?values5.6.2-Table-of-Ka-values

  • For this reason, it is easier to work with another term called?pKa?for acids or?pKb?for bases
  • In order to convert the values we need to apply the following calculations:

pKa?= -logKa? ? ? ? ? ? ? ? ?Ka= 10-pKa

pKb?= -logKb? ? ? ? ? ? ? ? ?Kb= 10-pKb

Table of?pKa?values5.6.2-Table-of-pKa-values

  • The range of?pKa?values for most weak acids lies between 3 and 7

Relative Strengths of Acids and Bases

  • The larger the?Ka?value, the stronger the acid
  • The larger the?pKa?value, the weaker the acid
  • The larger the?Kb?value, the stronger the base
  • The larger the?pKb value, the weaker the base

18.2-relative-strengths-of-acids-and-bases

pKa?and pKb?tell us the relative strengths of acids and bases?

 

Relating Kw to Ka

The Ionic Product of Water and Temperature

  • In all aqueous solutions, an equilibrium exists in water where a few water molecules dissociate into protons and hydroxide ions
  • We can derive an equilibrium constant for the reaction:

2H2O (l)?? H3O+?(aq) + OH-?(aq)

  • The concentration of water is constant, so the expression for?Kw?is:

Kw?= [H3O+][OH-]

  • This is a specific equilibrium constant called the?ionic product for water
  • The product of the two ion concentrations is?1 x?10-14?mol2?dm-6?at 25 °C
  • For conjugate acid-base pairs,?Ka?and?Kb?are related to?Kw

KaKb?=?Kw

  • The conjugate base of ethanoic acid is the ethanoate ion,?CH3COO–?(aq)?

CH3COOH (aq) + H2O (l) ??CH3COO–?(aq) +?H3O+?(aq)

acid??? ? ? ? ? ? ? ? ? ? ? ? ??? conjugate base

  • We can then put this in to the?Ka expression

18.2-Ka-expression-for-Ethanoic-acid

Acid dissociation constant for ethanoic acid

  • The ethanoate ion will react with water according to the following equation

CH3COO–?(aq) +?H2O (l) ? CH3COOH (aq) + OH-?(aq)

  • We can then put this in to the?Kb expression

18.2-Kb-expression-for-ethanoate-ion

Base dissociation constant for the ethanoate ion

  • Now, these two expressions can be combined, which corresponds to
    • KaKb?=?Kw
    • KaKb?= 10-14
  • Or we could say that
    • pKa?+ pKb?= pKw
    • pKa?+ pKb?= 14
    • This makes the numbers much more easy to deal with as using?KaKb?= 10-14?will give very small numbers

18.2-Combining-Ka-and-Kb-expressions

Combining Ka?Kb?expressions

  • Or rearranging these:
    • Ka?=??Kw?/?Kb
    • Kb=??Kw?/?Ka

The ionic product of water,?Kw

  • The ionisation of water is an?endothermic?process

2H2O (l) ??H3O+? aq)?+ OH-?(aq)?

  • In accordance with Le Chatelier's principle, an increase in temperature will result in the forward reaction being favoured
    • This causes an increase in the concentration of the hydrogen and hydroxide ions
    • This leads to the?magnitude of?Kw?increasing
    • Therefore, the?pH will decrease
  • Increasing the temperature, decreases the pH of water (becomes more acidic)
  • Decreasing the temperature, increases the pH of water (becomes more basic)

Relationship-between-temperature-and-Kw

Relationship between Kw?and temperature?

 

 

 

 

 

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