CHAPTER 5 RESULTS SHOWING SCOPE AND QUALITY OF RESEARCH ON
6.1 INTRODUCTION
CHAPTER 6
SYNTHESIS OF STUDENT DIFFICULTIES AND PROPOSITIONAL KNOWLEDGE REGARDING SPECIES IN ACID-BASE CHEMISTRY
Table 6.1 Student difficulties with acid-base species with coded propositional statements
Difficulty
Number Difficulty Descriptions (in bold) linked to Propositional statements, with Codes
Difficulty Classification S1.1 Acid definitions are limited to operational definitions.
Operational and theoretical definitions are both necessary for scientific understanding. (1.1) Operational definitions indicate how a physical quantity might be recognised or measured. (1.1.1) Theoretical definitions show relationships between other concepts. (1.1.2)
Acidic and basic substances have characteristic properties. (1.2) Acidic substances give acidic aqueous solutions. (2.1)
Acidic solutions have a pH of less than 7 (2.1.1.1) Weakly acidic solutions taste sour. (2.1.1.3)
Acidic solutions react chemically with carbonates. (2.1.1.6.2) Interim see P19 in Chapter 7
4
S1.2 Base definitions are limited to operational definitions.
(1.1.1) (1.1.2) (1.2) from S1.1
Basic substances give basic aqueous solutions (3.1) Interim, see S4 Basic solutions have a pH greater than 7 (3.1.1.1)
3+
S1.3 Ionic compounds have no ions in solution
Ionic solids dissociate into cations and anions when they dissolve in water. (8.2.5.1) In reality, few salts dissociate completely in water. (8.2.5.1.1)
2
S2.1 Acids are substances not particles.
Different theoretical models conceive acids as substances or as particles. (1.1.3.3)
Brønsted acids are molecules or ions that can release a proton (hydrogen ion) (2.3.1.1) Interim see S6 4
S2.2 Bases are substances not particles.
(1.1.3.3) from S2.1
Brønsted bases are molecules or ions that can accept a proton (hydrogen ion) (3.3.1.1) Interim, see S6 2
S2.3 Examples of acids are limited to the Arrhenius model.
Brønsted acids include all Arrhenius acids (2.3.2.1)
Brønsted acids include the molecule H2O and ion NH4+ (2.3.2.2) Interim, see S6 Arrhenius acids do not include water (2.2.2.2.1.1)
2
S2.4 Examples of bases are limited to the Arrhenius model.
Arrhenius bases are limited to substances containing OH groups (3.2.2.0) Arrhenius bases include NaOH (3.2.2.1.1)
Arrhenius bases do not include Brønsted bases (3.2.2.2) such as water (3.2.2.2.1)
Brønsted bases include the molecules H2O, NH3 and, ions OH-, HCOO–, CH3COO–, CN–, and S2–
(3.3.2.1) Interim, see R6 in Chapter 8
Brønsted bases do not include Arrhenius bases (3.3.2.2) such as NaOH (3.3.2.2.1)
4
S2.5 Neutralization is limited to and always occurs between compounds having H and OH in the formula
Neutralization is a process whereby acidic and basic substances react chemically to produce new substances (7.1) including water, if in aqueous solution. (7.1.2.2)
CO2 and SO2 are acidic gases found in the atmosphere (2.1.2.2) Arrhenius bases do not include alcohols. (3.2.2.2.2)
3
S3 One model can explain all acid-base phenomena.
Definitions vary according to different models (1.1.3) Different models are useful in different contexts (1.1.3.1)
n/a
n/a: It is not appropriate to classify the difficulty only in the acid-base context.
Difficulty
Number Difficulty Descriptions (in bold) linked to Propositional statements, with Codes
Difficulty Classification S4 Acid and base definitions are not distinguished
Arrhenius acids are substances that release hydrogen ions in aqueous solution. (2.2.1) Arrhenius bases are substances that release hydroxide ions in aqueous solution. (3.2.1) Brønsted model: acids are molecules or ions that can release a proton (hydrogen ion). (2.3.1.1) Brønsted model: bases are molecules or ions that can accept a proton (hydrogen ion). (3.3.1.1) Neutral substances are neither acidic nor basic. (5.1)
Basic substances (or alkalis) give basic (or alkaline) solutions. (3.1)
2
S5 Alkali is another word for base.
Arrhenius bases are substances that release hydroxide ions in aqueous solutions (3.2.1) Alkali is an alternative term for Arrhenius bases (3.2.1.1)
Arrhenius bases do not include Brønsted bases (3.2.2.2) such as water (3.2.2.2.1)
Brønsted bases are molecules or ions that can accept a proton (hydrogen ion) (3.3.1.1) Interim, see S6 Brønsted bases include the molecules H2O, NH3 and, ions OH-, HCOO–, CH3COO–, CN–, and S2–
(3.3.2.1) Interim, see R6 in Chapter 8
Brønsted bases do not include Arrhenius bases (3.3.2.2)
2
S6 Amphoteric species are neither acid nor base.
Amphoteric species are those that can behave both as an acid and a base (4.1)
Amphoteric properties depend upon the context in which the species is investigated (4.1.1) In aqueous solutions, amphoteric hydroxides can form either hydrogen or hydroxide ions. (4.1.2) Amphoteric substances include Al(OH)3 and Zn(OH)2 (4.2.1)
Arrhenius acids include HCl (2.2.2.1.1) Interim, see R7 in Chapter 8 Arrhenius bases include NaOH, Al(OH)3 and Zn(OH)2 (3.2.2.1.1)
Molecules or ions are classified as Brønsted acids when they release a proton (hydrogen ion) to a base.
(2.3.1.1)
Molecules or ions are classified as Brønsted bases when they accept a proton (hydrogen ion) from an acid (3.3.1.1)
Brønsted acids: examples include the water molecule H2O and the ion NH4+
(2.3.2.2) Interim, see S6 Brønsted bases: examples include the water molecule H2O, and the ions: OH-, HCOO– (3.3.2.1) Interim,
see R6 in Chapter 8
3
S7 Acidic and basic substances are not relevant in everyday life.
Foods often contain acidic substances (2.1.2.1) Fruit, tea, milk contain acids (2.1.2.1.1) CO2 and SO2 are acidic gases found in the atmosphere (2.1.2.2)
Basic materials are found in cleaning materials such as oven cleaner, household ammonia, household bleach; and washing soda, Na2CO3 (3.1.2.1) Interim, see P3 in Chapter 7
Basic substances found in the laboratory include metal hydroxides such as limewater , Ca(OH)2 (3.1.2.4) Basic substances used in cooking include ‘bicarb’ or ‘baking soda’, NaHCO3 (3.1.2.3)
NaCl forms a neutral aqueous solution (5.1.2)
4
S7.1 Antacids are substances that do not react with acids.
Antacids are basic substances (3.1.2.2) used as a medicine that prevents or corrects excess acidity in the stomach (3.1.2.2.1)
3
S8 Neutrality is not understood.
Neutral substances are neither acidic nor basic (5.1) Interim, see P4 in Chapter 7 Neutral solutions have a pH of 7. (5.1.1)
1
S9 Salts are not a class of compound
NaCl forms a neutral aqueous solution (5.1.2)
The salt produced in neutralization reactions depends on the particular acid and base involved.
(7.1.2.1.1)
Acetic (ethanoic) acid and sodium hydroxide will produce sodium acetate (ethanoate). (7.1.2.1.3)
1
S10 Neutral solutions have neither H+(or H3O+) nor OH– ions
A neutral solution is one where [H+] = [OH–] (Arrhenius model) (5.2) or [H3O+] = [OH–] (Brønsted model) (5.3)
4