Mr. Strickland's Chemistry     pi-sigma

Topic 18:  Acids & Bases

 

Topic 18.1 Lewis Acids & Bases

Essential idea: The acid–base concept can be extended to reactions that do not involve proton transfer.

Understandings:

  • A Lewis acid is a lone pair acceptor and a Lewis base is a lone pair donor.
  • When a Lewis base reacts with a Lewis acid a coordinate bond is formed.
  • A nucleophile is a Lewis base and an electrophile is a Lewis acid.

Applications and skills:

  • Application of Lewis’ acid–base theory to inorganic and organic chemistry to identify the role of the reacting species.

 

Guidance:

  • Both organic and inorganic examples should be studied.
  • Relations between Brønsted–Lowry and Lewis acids and bases should be discussed.

 

International-mindedness:

  • Acid–base theory has developed from the ideas of people from different parts of the world through both collaboration and competition.

 

Theory of knowledge:

  • The same phenomenon can sometimes be explored from different perspectives, and explained by different theories. For example, do we judge competing theories by their universality, simplicity or elegance?

 

Topic 18.2 Calculations Involving Acids & Bases

Essential idea: The equilibrium law can be applied to acid–base reactions. Numerical problems can be simplified by making assumptions about the relative concentrations of the species involved. The use of logarithms is also significant here.

Understandings:

  • The expression for the dissociation constant of a weak acid (Ka) and a weak base (Kb).
  • For a conjugate acid base pair, Ka × Kb = Kw.
  • The relationship between Ka and pKa is (pKa = -log Ka), and between Kb and pKb is (pKb = -log Kb).

 

Applications and skills:

  • Solution of problems involving [H+ (aq)], [OH (aq)], pH, pOH, Ka, pKa, Kb and pKb.
  • Discussion of the relative strengths of acids and bases using values of Ka, pKa, Kb and pKb.

 

Guidance:

  • The value Kw depends on the temperature.
  • The calculation of pH in buffer solutions will only be assessed in options B.7 and D.4.
  • Only examples involving the transfer of one proton will be assessed.
  • Calculations of pH at temperatures other than 298 K can be assessed.
  • Students should state when approximations are used in equilibrium calculations.
  • The use of quadratic equations will not be assessed.

 

International-mindedness:

  • Mathematics is a universal language. The mathematical nature of this topic helps chemists speaking different native languages to communicate more objectively.

 

Topic 18.3 pH Curves

Essential idea: pH curves can be investigated experimentally but are mathematically determined by the dissociation constants of the acid and base. An indicator with an appropriate end point can be used to determine the equivalence point of the reaction.

Understandings:

  • The characteristics of the pH curves produced by the different combinations of strong and weak acids and bases.
  • An acid–base indicator is a weak acid or a weak base where the components of the conjugate acid–base pair have different colors.
  • The relationship between the pH range of an acid–base indicator, which is a weak acid, and its pKa value.
  • The buffer region on the pH curve represents the region where small additions of acid or base result in little or no change in pH.
  • The composition and action of a buffer solution.

 

Applications and skills:

  • The general shapes of graphs of pH against volume for titrations involving strong and weak acids and bases with an explanation of their important features.
  • Selection of an appropriate indicator for a titration, given the equivalence point of the titration and the end point of the indicator.
  • While the nature of the acid–base buffer always remains the same, buffer solutions can be prepared by either mixing a weak acid/base with a solution of a salt containing its conjugate, or by partial neutralization of a weak acid/base with a strong acid/base.
  • Prediction of the relative pH of aqueous salt solutions formed by the different combinations of strong and weak acid and base.

 

Guidance:

  • Only examples involving the transfer of one proton will be assessed. Important features are:

– intercept with pH axis

– equivalence point

– buffer region

– points where pKa = pH or pKb = pOH.

  • For an indicator which is a weak acid:

H In(aq)      ↔      H+(aq) + In-(aq)

Color A                 Color B

The color change can be considered to take place over a range of pKa ± 1.

  • For an indicator which is a weak base:

BOH(aq)     ↔      B+(aq) + OH- (aq)

Color A                 Color B

  • Examples of indicators are listed in the data booklet in section 22.
  • Salts formed from the four possible combinations of strong and weak acids and bases should be considered. Calculations are not required.
  • The acidity of hydrated transition metal ions is covered in topic 13. The treatment of other hydrated metal ions is not required.

 

Theory of knowledge:

  • Is a pH curve an accurate description of reality or an artificial representation? Does science offer a representation of reality?