Bridging the Gaps Between the Three Worlds of Chemistry William C. Deese, Ph.D. Louisiana Tech University

Molecular World Chemistry Observed World Symbolic World Johnstone, JChemEd, 1993, p.301

Aqueous Solutions: Properties and Reactions Electrical Properties of Solutions Acids, Bases, and Indicators Neutralization Reactions Solubility Rules for Common Salts Precipitation Reactions

Electrical Properties of Solutions

Electrolytes Salt (NaCl) Strong electrolyte Sugar (C 12 H 22 O 11 ) Non-electrolyte

Symbolic World Description of Sugar and Salt Solutions C 12 H 22 O 11 (s) + H 2 O(l)  C 12 H 22 O 11 (aq) NaCl(s) + H 2 O(l)  Na + (aq) + Cl - (aq)

Svante Arrhenius 1859 – Proposed the dissociation theory in Discovered in 1896 a link between global temperature and CO 2. Received Nobel Prize in 1903.

Electrolytes HCl(aq) Strong electrolyte (Strong acid) HC 2 H 3 O 2 (aq) Weak electrolyte (Weak acid)

Symbolic World Description of HCl and Vinegar Solutions HCl(aq)  H + (aq) + Cl - (aq) HC 2 H 3 O 2 (aq)  H + (aq) + C 2 H 3 O 2 - (aq)

Arrhenius Acid A compound that produces hydronium (or hydrogen) ions when dissolved in water.

Short and Long Version of HCl(aq) Dissociation HCl(aq)  H + (aq) + Cl - (aq) HCl(aq) + H 2 O(l)  H 3 O + (aq) + Cl - (aq)

Short and Long Version of HC 2 H 3 O 2 (aq) Dissociation HC 2 H 3 O 2 (aq)  H + (aq) + C 2 H 3 O 2 - (aq) HC 2 H 3 O 2 (aq) + H 2 O (l)  H 3 O + (aq) + C 2 H 3 O 2 - (aq)

Symbolic World Description of NaOH Solution 100% dissociation into ions – Strong electrolyte NaOH(aq)  Na + (aq) + OH - (aq)

Arrhenius Base A compound that produces hydroxide ions when dissolved in water.

Symbolic World Description of NH 3 Solution NH 3 (aq) + H 2 O(l)  NH 4 + (aq) + OH - (aq) Partial dissociation into ions – Weak electrolyte

Some Properties of Acids and Bases

Symbolic Description of Reaction of Zinc with HCl(aq) Zn(s) + 2 HCl(aq)  ZnCl 2 (aq) + H 2 (g)

Symbolic Description of Reaction of Limestone with HCl(aq) CaCO 3 (s) + 2 HCl(aq)  CaCl 2 (aq) + H 2 O(l) + CO 2 (g)

Acid/Base Indicators AcidBase Phenol RedYellowRed PhenolphthaleinColorlessPink Red CabbagePinkYellow/Green

Acid / Base Neutralization Reaction HCl(aq) + NaOH(aq)  NaCl(aq) + H 2 O(l)

Neutralization H + (aq) + OH - (aq)  H 2 O(l)

Properties of Acids 1) Taste sour. 2) React with certain metals to produce H 2 (g). 3) Cause certain organic dyes to change color. 4) React with limestone to liberate CO 2 (g). 5) React with bases to form salts and water. (Neutralization)

Properties of Bases 1)Taste bitter. 2)Feel slippery or soapy. 3)Cause certain organic dyes to change color. 4)React with acids to form salts and water. (Neutralization)

Precipitation Reactions

Some Solubility Rules All Group I and NH 4 + salts are soluble. All nitrates and acetates are soluble. Most chlorides, bromides, and iodides are soluble. (except Ag +, Pb +2, Hg 2 +2 ) Most sulfates are soluble. (except Pb +2, Hg 2 +2, Sr +2, Ba +2, Ca +2 ) Most sulfides, sulfites, hydroxides, carbonates, and phosphates are insoluble.

Mixing Solutions of Sodium chloride and Lead(II) nitrate NaCl(aq) + Pb(NO 3 ) 2 (aq)  White Precipitate

Results of Double Displacement NaCl(aq) + Pb(NO 3 ) 2 (aq)  PbCl 2 + NaNO 3

Balanced Reaction with Phases 2 NaCl(aq) + Pb(NO 3 ) 2 (aq)  PbCl 2 (s) + 2 NaNO 3 (aq)

Mixing Solutions of Potassium iodide and Lead(II) nitrate KI(aq) + Pb(NO 3 ) 2 (aq)  Yellow Precipitate

Results of Double Displacement KI(aq) + Pb(NO 3 ) 2 (aq)  PbI 2 + NaNO 3

Balanced Reaction with Phases 2 KI(aq) + Pb(NO 3 ) 2 (aq)  PbI 2 (s) + 2 NaNO 3 (aq)

Molecular World Chemistry Observed World Symbolic World Johnstone, JChemEd, 1993, p.301

Aqueous Solutions: Properties and Reactions Electrical Properties of Solutions Acids, Bases, and Indicators Neutralization Reactions Solubility Rules for Common Salts Precipitation Reactions

Demonstration Assessments

1)A short demonstration is presented. 2)Students make visual observations. 3)Students write their observations. 4)Students write an explanation of what was observed. 5)The scoring rubric is presented.

General Rubric for Demonstration Assessments ScoreStudent Achievement 0Makes no accurate observations. 1-4Makes accurate observations without including inferences. 5-7Makes some correct inferences. 8-10Fully explains observations using correct vocabulary.

Rubric for Conductivity Titration Demonstration Assessment (Observations: points) A few drops of sulfuric acid solution were added to the pink barium hydroxide solution. The solution became cloudy. As more acid was added, the cloudiness increased and the light bulb grew dimmer. Upon adding more acid, the bulb grew gradually dimmer until it went completely dark. A few more drops of acid were added and the pink color disappeared leaving a white suspension. As more acid was added, the light bulb began to glow until it was bright.

Rubric for Conductivity Titration Demonstration Assessment (Partial Explanation: points) The acid neutralized the base and formed a precipitate. This reduced the number of ions in solution, which caused the bulb to dim. When all the base was neutralized, the indicator changed from pink to colorless leaving a white mixture.

Rubric for Conductivity Titration Demonstration Assessment (Further Explanation: 8-10 points) The products of the reaction were the precipitate, barium sulfate, and the weak electrolyte, water. Ba(OH) 2 (aq) + H 2 SO 4 (aq) ===> BaSO 4 (s) + 2 H 2 O(l) After the equivalence point, further addition of the strong acid, H 2 SO 4, increased the conductivity of the solution. This caused the bulb to gradually become bright again.

Molecular World Chemistry Observed World Symbolic World Johnstone, JChemEd, 1993, p.301