8 / 18 Acids and Bases

8.1.NoS1 Falsification of theories—HCN altering the theory that oxygen was the element which gave a compound its acidic properties allowed for other acid–base theories to develop. (2.5)

8.1.NoS2 Theories being superseded—one early theory of acidity derived from the sensation of a sour taste, but this had been proven false. (1.9)

8.1.NoS3 Public understanding of science—outside of the arena of chemistry, decisions are sometimes referred to as "acid test" or "litmus test". (5.5)

8.1.U1 A Brønsted–Lowry acid is a proton/H+ donor and a Brønsted–Lowry base is a proton/H+ acceptor.

8.1.U2 Amphiprotic species can act as both Brønsted–Lowry acids and bases.

8.1.U3 A pair of species differing by a single proton is called a conjugate acid-base pair.

8.1.AS1 Deduction of the Brønsted–Lowry acid and base in a chemical reaction.

8.1.AS2 Deduction of the conjugate acid or conjugate base in a chemical reaction.

8.1.G1 Lewis theory is not required here.

8.1.G2 The location of the proton transferred should be clearly indicated. For example, CH3COOH/CH3COO– rather than C2H4O2/C2H3O2–.

8.1.G3 Students should know the representation of a proton in aqueous solution as both H+ (aq) and H3O+ (aq).

8.1.G4 The difference between the terms amphoteric and amphiprotic should be covered.

8.1.IM1 Acidus means sour in Latin, while alkali is derived from the Arabic word for calcined ashes. Oxygene means acid-forming in Greek, and reflects the mistaken belief that the element oxygen was responsible for a compound’s acidic properties. Acid–base theory has been developed by scientists from around the world, and its vocabulary has been influenced by their languages.

8.1.ToK1 Acid and base behaviour can be explained using different theories. How are the explanations in chemistry different from explanations in other subjects such as history?

8.1.Aims1 Aim 9: Each theory has its strengths and limitations. Lavoisier has been called the father of modern chemistry but he was mistaken about oxygen in this context.

8.2.NoS Obtaining evidence for theories—observable properties of acids and bases have led to the modification of acid–base theories. (1.9)

8.2.U1 Most acids have observable characteristic chemical reactions with reactive metals, metal oxides, metal hydroxides, hydrogen carbonates and carbonates.

8.2.U2 Salt and water are produced in exothermic neutralization reactions.

8.2.AS1 Balancing chemical equations for the reaction of acids.

8.2.AS2 Identification of the acid and base needed to make different salts.

8.2.AS3 Candidates should have experience of acid-base titrations with different indicators.

8.2.G1 Bases which are not hydroxides, such as ammonia, soluble carbonates and hydrogen carbonates should be covered.

8.2.G2 The colour changes of different indicators are given in the data booklet in section 22.

8.2.Uz1 A number of acids and bases are used in our everyday life from rust removers to oven cleaners, from foods to toothpastes, from treatments for bee stings to treatment of wasp stings.

8.2.Aims1 Aim 6: The evidence for these properties could be based on a student’s experimental experiences.

8.3.NoS Occam’s razor—the pH scale is an attempt to scale the relative acidity over a wide range of H+ concentrations into a very simple number. (2.7)

8.3.U1 pH = -log[H+(aq)] and [H+] = 10^{-pH}

8.3.U2 A change of one pH unit represents a 10-fold change in the hydrogen ion concentration [H+].

8.3.U3 pH values distinguish between acidic, neutral and alkaline solutions.

8.3.U4 The ionic product constant, 𝐾𝑤 = [H+][OH−] = 10−14 at 298 K.

8.3.AS1 Solving problems involving pH, [H+] and [OH-].

8.3.AS2 Students should be familiar with the use of a pH meter and universal indicator.

8.3.G1 Students will not be assessed on pOH values.

8.3.G2 Students should be concerned only with strong acids and bases in this sub-topic.

8.3.G3 Knowing the temperature dependence of Kw is not required

8.3.G4 Equations involving H3O+ instead of H+ may be applied.

8.3.ToK1 Chemistry makes use of the universal language of mathematics as a means of communication. Why is it important to have just one “scientific” language?

8.3.Aims1 Aim 3: Students should be able to use and apply the pH concept in a range of experimental and theoretical contexts.

8.3.Aims2 Aim 6: An acid–base titration could be monitored with an indicator or a pH probe.

8.4.NoS1 Improved instrumentation—the use of advanced analytical techniques has allowed the relative strength of different acids and bases to be quantified. (1.8)

8.4.NoS2 Looking for trends and discrepancies—patterns and anomalies in relative strengths of acids and bases can be explained at the molecular level. (3.1)

8.4.NoS3 The outcomes of experiments or models may be used as further evidence for a claim—data for a particular type of reaction supports the idea that weak acids exist in equilibrium. (1.9)

8.4.U1 Strong and weak acids and bases differ in the extent of ionization.

8.4.U2 Strong acids and bases of equal concentrations have higher conductivities than weak acids and bases.

8.4.U3 A strong acid is a good proton donor and has a weak conjugate base.

8.4.U4 A strong base is a good proton acceptor and has a weak conjugate acid.

8.4.AS1 Distinction between strong and weak acids and bases in terms of the rates of their reactions with metals, metal oxides, metal hydroxides, metal hydrogen carbonates and metal carbonates and their electrical conductivities for solutions of equal concentrations.

8.4.G1 The terms ionization and dissociation can be used interchangeably.

8.4.G2 See section 21 in the data booklet for a list of weak acids and bases.

8.4.ToK1 The strength of an acid can be determined by the use of pH and conductivity probes. In what ways do technologies, which extend our senses, change or reinforce our view of the world?

8.4.Aims1 Aim 6: Students should have experimental experience of working qualitatively with both strong and weak acids and bases. Examples to include: H2SO4 (aq), HCl (aq), HNO3 (aq), NaOH (aq), NH3 (aq).

8.4.Aims2 Aim 7: Students could use data loggers to investigate the strength of acid and bases.

8.5.NoS Risks and problems—oxides of metals and non-metals can be characterized by their acid–base properties. Acid deposition is a topic that can be discussed from different perspectives. Chemistry allows us to understand and to reduce the environmental impact of human activities. (4.8)

8.5.U1 Rain is naturally acidic because of dissolved CO_{2} and has a pH of 5.6. Acid deposition has a pH below 5.6.

8.5.U2 Acid deposition is formed when nitrogen or sulfur oxides dissolve in water to form HNO3, HNO2, H2SO4 and H2SO3.

8.5.U3 Sources of the oxides of sulfur and nitrogen and the effects of acid deposition should be covered.

8.5.AS1 Balancing the equations that describe the combustion of sulfur and nitrogen to their oxides and the subsequent formation of H2SO3, H2SO4, HNO2 and HNO3.

8.5.AS2 Distinction between the pre-combustion and post-combustion methods of reducing sulfur oxides emissions.

8.5.AS3 Deduction of acid deposition equations for acid deposition with reactive metals and carbonates.

8.5.IM1 The polluter country and polluted country are often not the same. Acid deposition is a secondary pollutant that affects regions far from the primary source. Solving this problem requires international cooperation.

8.5.ToK1 All rain is acidic but not all rain is “acid rain”. Scientific terms have a precise definition. Does scientific vocabulary simply communicate our knowledge in a neutral way or can it have value-laden terminology?

8.5.Aims1 Aim 6: The effects of acid rain on different construction materials could be quantitatively investigated.

8.5.Aims2 Aim 8: A discussion of the impact of acid rain in different countries will help raise awareness of the environmental impact of this secondary pollutant and the political implications.

8.5.Aims3 Aim 8: Other means of reducing oxide production—bus use, car pooling, etc. could be discussed.

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18.1.U1 A Lewis acid is a lone pair acceptor and a Lewis base is a lone pair donor.

18.1.U2 When a Lewis base reacts with a Lewis acid a coordinate bond is formed.

18.1.U3 A nucleophile is a Lewis base and an electrophile is a Lewis acid.

18.1.AS1 Application of Lewis’ acid–base theory to inorganic and organic chemistry to identify the role of the reacting species.

18.1.G1 Both organic and inorganic examples should be studied.

18.1.G2 Relations between Brønsted–Lowry and Lewis acids and bases should be discussed.

18.1.IM1 Acid–base theory has developed from the ideas of people from different parts of the world through both collaboration and competition.

18.1.ToK1 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?

18.1.Aims1 Aim 6: Transition metal complexes could be experimentally explored.

18.1.Aims2 Aim 7: Animations can be used to distinguish between the different acid–base theories.

18.2.NoS Obtaining evidence for scientific theories—application of the equilibrium law allows strengths of acids and bases to be determined and related to their molecular structure. (1.9)

18.2.U1 The expression for the dissociation constant of a weak acid (Ka) and a weak base (Kb).

18.2.U2 For a conjugate acid base pair, Ka × Kb = Kw

18.2.U3 The relationship between Ka and pKa is (pKa = -log Ka), and between Kb and pKb is (pKb = -log Kb).

18.2.AS1 Solution of problems involving [H+ (aq)], [OH–(aq)], pH, pOH, Ka, pKa, Kb and pKb.

18.2.AS2 Discussion of the relative strengths of acids and bases using values of Ka, pKa, Kb and pK

18.2.G1 The value Kw depends on the temperature

18.2.G2 The calculation of pH in buffer solutions will only be assessed in options B.7 and D.4

18.2.G3 Only examples involving the transfer of one proton will be assessed.

18.2.G4 Calculations of pH at temperatures other than 298 K can be assessed.

18.2.G5 Students should state when approximations are used in equilibrium calculations.

18.2.G6 The use of quadratic equations will not be assessed

18.2.IM1 Mathematics is a universal language. The mathematical nature of this topic helps chemists speaking different native languages to communicate more objectively.

18.2.Aims1 Aim 6: The properties of strong and weak acids could be investigated experimentally.

18.3.NoS Increased power of instrumentation and advances in available techniques—development in pH meter technology has allowed for more reliable and ready measurement of pH. (3.7)

18.3.U1 The characteristics of the pH curves produced by the different combinations of strong and weak acids and bases.

18.3.U2 An acid–base indicator is a weak acid or a weak base where the components of the conjugate acid–base pair have different colours.

18.3.U3 The relationship between the pH range of an acid–base indicator, which is a weak acid, and its pKa value.

18.3.U4 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.

18.3.U5 The composition and action of a buffer solution

18.3.AS1 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.

18.3.AS2 Selection of an appropriate indicator for a titration, given the equivalence point of the titration and the end point of the indicator.

18.3.AS3 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.

18.3.AS4 Prediction of the relative pH of aqueous salt solutions formed by the different combinations of strong and weak acid and base.

18.3.G1 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.

18.3.G2 For an indicator which is a weak acid: HIn(aq) {Colour A} <-> H+(aq) + In-(aq) {Colour B}

18.3.G3 The colour change can be considered to take place over a range of pKa ± 1.

18.3.G4 Examples of indicators are listed in the data booklet in section 22.

18.3.G5 Salts formed from the four possible combinations of strong and weak acids and bases should be considered. Calculations are not required.

18.3.G6 The acidity of hydrated transition metal ions is covered in topic 13. The treatment of other hydrated metal ions is not required.

18.3.ToK1 Is a pH curve an accurate description of reality or an artificial representation of reality?

18.3.Aims1 Aim 6: Experiments could include investigation of pH curves, determination of the pKa of a weak acid, preparation and investigation of a buffer solution and the determination of the pKa of an indicator.

18.3.Aims2 Aim 7: Data logging, databases, spreadsheets and simulations can all be used. For example, the equivalence point could be determined by using a conductivity probe or a temperature probe.