Mr. Strickland's Chemistry     pi-sigma

Topic 8:  Acids & Bases

 

Topic 8.1 Theories of Acids & Bases

Essential idea: Many reactions involve the transfer of a proton from an acid to a base.

Understandings:

  • A Brønsted–Lowry acid is a proton/H+ donor and a Brønsted–Lowry base is a proton/H+ acceptor.
  • Amphiprotic species can act as both Brønsted–Lowry acids and bases.
  • A pair of species differing by a single proton is called a conjugate acid-base pair.

 

Applications and skills:

  • Deduction of the Brønsted–Lowry acid and base in a chemical reaction.
  • Deduction of the conjugate acid or conjugate base in a chemical reaction.

 

Guidance:

  • Lewis theory is not required here.
  • The location of the proton transferred should be clearly indicated. For example, CH3COOH/CH3COO rather than C2H4O2/C2H3O2.
  • Students should know the representation of a proton in aqueous solution as both H+ (aq) and H3O+ (aq).
  • The difference between the terms amphoteric and amphiprotic should be covered.

 

International-mindedness:

  • 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.

 

Theory of knowledge:

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

 

Topic 8.2 Properties of Acids & Bases

Essential idea: The characterization of an acid depends on empirical evidence such as the production of gases in reactions with metals, the color changes of indicators or the release of heat in reactions with metal oxides and hydroxides.

Understandings:

  • Most acids have observable characteristic chemical reactions with reactive metals, metal oxides, metal hydroxides, hydrogen carbonates and carbonates.
  • Salt and water are produced in exothermic neutralization reactions.

 

Applications and skills:

  • Balancing chemical equations for the reaction of acids.
  • Identification of the acid and base needed to make different salts.
  • Candidates should have experience of acid-base titrations with different indicators.

 

Guidance:

  • Bases which are not hydroxides, such as ammonia, soluble carbonates and hydrogen carbonates should be covered.
  • The color changes of different indicators are given in the data booklet in section 22.

 

Utilization:

  • 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.

 

Topic 8.3 The pH Scale

Essential idea: The pH scale is an artificial scale used to distinguish between acid, neutral and basic/alkaline solutions.

Understandings:

  • pH = − log[H+(aq)] and [H+] = 10−pH.
  • A change of one pH unit represents a 10-fold change in the hydrogen ion concentration [H+].
  • pH values distinguish between acidic, neutral and alkaline solutions.
  • The ionic product constant, Kw = [H+][OH] = 10−14 at 298 K

 

Applications and skills:

  • Solving problems involving pH, [H+] and [OH].
  • Students should be familiar with the use of a pH meter and universal indicator.

 

Guidance:

  • Students will not be assessed on pOH values.
  • Students should be concerned only with strong acids and bases in this sub- topic.
  • Knowing the temperature dependence of Kw is not required.
  • Equations involving H3O+ instead of H+ may be applied.

 

Theory of knowledge:

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

 

Topic 8.4 Strong and Weak Acids & Bases

Essential idea: The pH depends on the concentration of the solution. The strength of acids or bases depends on the extent to which they dissociate in aqueous solution.

Understandings:

  • Strong and weak acids and bases differ in the extent of ionization.
  • Strong acids and bases of equal concentrations have higher conductivities than weak acids and bases.
  • A strong acid is a good proton donor and has a weak conjugate base.
  • A strong base is a good proton acceptor and has a weak conjugate acid.

 

Applications and skills:

  • 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.

 

Guidance:

  • The terms ionization and dissociation can be used interchangeably.
  • See section 21 in the data booklet for a list of weak acids and bases.

 

Theory of knowledge:

  • 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?

 

Topic 8.5 Acid Deposition

Essential idea: Increased industrialization has led to greater production of nitrogen and sulfur oxides leading to acid rain, which is damaging our environment. These problems can be reduced through collaboration with national and intergovernmental organizations.

Understandings:

  • Rain is naturally acidic because of dissolved CO2 and has a pH of 5.6. Acid deposition has a pH below 5.6.
  • Acid deposition is formed when nitrogen or sulfur oxides dissolve in water to form HNO3, HNO2, H2SO4 and H2SO3.
  • Sources of the oxides of sulfur and nitrogen and the effects of acid deposition should be covered.

 

Applications and skills:

  • Balancing the equations that describe the combustion of sulfur and nitrogen to their oxides and the subsequent formation of H2SO3, H2SO4, HNO2 and HNO3.
  • Distinction between the pre-combustion and post-combustion methods of reducing sulfur oxides emissions.
  • Deduction of acid deposition equations for acid deposition with reactive metals and carbonates.

 

International-mindedness:

  • 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.

 

Theory of knowledge:

  • 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?