1. Includes student activities
Analysis of strong and weak electrolytes using a conductivity apparatus
Presentation to Accompany the Conductivity of Solutions Lecture Demonstration
Includes student activities
Chemistry Education Research Group
Department of Chemistry & Biochemistry
University of Oregon
Eugene, Oregon USA

2. Animal of the Day
Caribou?
Caribou B. A. C.
Antelope D.

3. H2
partial negative charge
H2O
partial positive charge
Because of the polar nature of water, it can dissolve substances that are polar, or have charged particles
In the H2O molecule, sharing is not equal. The electron density is centered around the oxygen nucleus.
Water is a polar molecule
(also referred to as a dipole)

4. Net dipole moment of water (physics)
Oxygen is more electronegative compared to hydrogen.
vectors

5. Net dipole moment of water (physics)

6. Forces of attraction between water molecules Physics: opposite charges attract
Intermolecular force (IMF)
A force of attraction between
molecules
The type of IMF: dipole-dipole

7. Forces of attraction between water molecules

8. Representation of liquid water molecules

9. Movement of liquid water molecules

10. Analysis of Aqueous Solutions
Many reactions occur in aqueous solutions, especially reactions of ionic compounds.
A solution consists of one or more solutes (minor component) dissolved in a solvent (the major component).

11. Supplementing with
electrolytes
Cells use electrolytes to maintain voltages across cell membranes and to carry electrical impulses for nerve signaling and muscle contractions.
Electrolytes, particularly sodium ions and potassium ions, are lost when you sweat.

12. Because of the polar nature of water, it is often able to dissolve other polar covalent substances or ionic substances. (Opposite charges attract)
Let’s consider a couple of common substances
Sodium chloride, NaCl (table salt)
Sucrose, C12H22O11 (sugar)

13. Substances that dissolve in water are “soluble”
Sodium chloride, NaCl (table salt) Sucrose, C12H22O11 (sugar) Once dissolved, do they have the same behaviors?

14. Detecting Ions in Aqueous Solution Using a Conductivity Tester
Chemistry: McMurry and Fay, 6th Edition
Chapter 4: Reactions in Aqueous Solution
7/29/2018 2:51:52 AM
Detecting Ions in Aqueous Solution Using a Conductivity Tester
Lecture Demonstration
Conductivity Apparatus
Copyright © 2011 Pearson Prentice Hall, Inc.

15. What does a battery do? e- e- e- e-
A cell or battery provides the electromotive force that pushes electrons out
of the negative terminal and pulls electrons into the positive terminal.

16. Physics: circuit diagram
Electrons are pulled into the positive terminal of the battery.
Electrons are pushed out of the negative terminal of the battery.
Direction of e- flow e- e- e-
Physics circuit diagram

17. Detecting Ions in Aqueous Solution using a Conductivity Tester
Chemistry: McMurry and Fay, 6th Edition
Chapter 4: Reactions in Aqueous Solution
7/29/2018 2:51:52 AM
Detecting Ions in Aqueous Solution using a Conductivity Tester
This solution does not contain ions.
This solution contains ions.
Conductivity Apparatus
Copyright © 2011 Pearson Prentice Hall, Inc.

18. What is electrical current?
Electrical current is electrical charges in motion.
What are the electrical charges?
Electrical charges can be electrons in a wire, or ions moving in a solution.

19. Conductivity Observations
Light Bulb
Inference
What is present in solution?
D.I. Water
NaCl(s)
NaCl(aq)

20. Will pure water conduct electricity?
A. Yes B. No C. Somewhat D. Not sure

21. D.I. water Solid NaCl NaCl(aq)
Experimental Observations: the light bulb does not light up
Deionized water is water that has had many of the ions removed.
Inference: the conductivity tester is not detecting any mobile ions.
D.I. water
Solid NaCl
NaCl(aq)
What is happening at the molecule level?
Pure water consists mostly of H2O molecules.

22. What about the water?
Deionized and distilled water are pure, with no ions present. Tap water contains ions and does conduct electricity.

23. Will NaCl(solid) conduct electricity?
A. Yes B. No C. Somewhat D. Not sure

24. Conductivity Observations
Light Bulb
Inference
What is present in the substance?
D.I. Water
NaCl(s)
NaCl(aq)

25. Conductivity Observations
Light Bulb
Inference
What is present in the substance?
D.I. Water
Light blub does not light up
no ions detected in the liquid
Water molecules
H2O
NaCl(s)
no ions detected in the solid
In the solid, there are Na+ cations and Cl- anions, but these ions are not mobile.
NaCl(aq)

26. Will NaCl(aq) conduct electricity?
A. Yes B. No C. Somewhat D. Not sure

27. Demo

28. When an aqueous solution of NaCl(aq) is tested for conductivity the light bulb glows brightly.
Draw a picture diagram of how solid NaCl dissolve in water to form a solution. Include the orientation of the water molecules. Save your diagram for later.

30. Conductivity Observations
Light Bulb
Inference
What is present in the substance?
D.I. Water
Light blub does not light up
no ions detected in the liquid
Water molecules
H2O
NaCl(s)
no ions detected in the solid
In the solid, there are Na+ cations and Cl- anions, but these ions are not mobile.
NaCl(aq)
Bright glow
Ions present in solution
H2O molecules solvent
Na+(aq), Cl-(aq) ions solute

31. D.I. water Solid NaCl NaCl(aq)
Experimental Observations What is happening at the atom level?
D.I. water
Solid NaCl
NaCl(aq)

32. Write a chemical equation representing solid NaCl dissolving in water
H2O

33. H2O Initial 100 NaCl 0 Na+ 0 Cl- Final 0 NaCl 100 Na+ 100 Cl-
Soluble ionic compounds dissolved in water will support a movement of current when subjected to why?
Solid NaCl
Na+(aq) and Cl-(aq) ions
H2O
All of the NaCl units in the solid are pulled apart
and become Na+ and Cl- ions.
Initial 100 NaCl Na Cl-
Final NaCl Na Cl-
100%
Dissociated
Soluble ionic compounds fully dissociate into ions.
(But, not all ionic compounds are soluble in water… more on that later.)

34. Dissociation and Dissolving
An electrolyte is a substances that dissociates into ions when dissolved in water.
A short computer animation illustration how positive and negative ions in a solid ionic compound dissolve in water might be used to accompany dissolving salts in water demonstration

35. Draw a particle diagram of the species existing in a solution of sodium chloride, NaCl(aq).

37. Conductivity Observations
Light Bulb
Inference
What is present in solution?
D.I. Water
Light blub does not light up
no ions detected in the liquid
Water molecules
H2O
NaCl(s)
no ions detected in the solid
In the solid, there are Na+ cations and Cl- anions, but these ions are not mobile.
NaCl(aq)
Bright glow
Ions present in solution
H2O molecules solvent
Na+(aq), Cl-(aq) ions solute

38. Dissociation and Dissolving
An electrolyte is a substances that dissociates into ions when dissolved in water.
A short computer animation illustration how positive and negative ions in a solid ionic compound dissolve in water accompanies the dissolving salts in water demonstration.

39. Soluble ionic compounds dissolved in water will support a movement of current when subjected to … why?
NaCl(aq) is a
strong electrolyte
Solid NaCl
Na+(aq) and Cl-(aq) ions
H2O
100
100
100
Soluble ionic compounds fully dissociate into ions.
(But, not all ionic compounds are soluble in water… more on that later.)

40. Draw a particle diagram of the species existing in a solution of sodium chloride, NaCl(aq).

41. The orientation of the water molecules toward the Na+ cation and Cl- anion matters! Think physics: opposite charges attract.
The partial negative oxygen end of the water molecule
will be attracted to the positive charge Na+ cation.

42. The partial positive hydrogen end of the water molecule will be attracted to the negative charge Cl- anion.

43. What occurs in the NaCl solution between the electrodes during a conductivity test?
Draw a particle diagram.

44. Experimental Observations What is happening at the atom level?
NaCl(aq)

45. To which electrode are the cations attracted
To which electrode are the cations attracted? To which electrode are the anions attracted

46. To which electrode are the cations attracted
To which electrode are the cations attracted? To which electrode are the anions attracted

47. To which electrode are the cations attracted
To which electrode are the cations attracted? To which electrode are the anions attracted

48. Anions
Cations

49. Insert URL computer animation of ion movement on solution.

50. There is a migration of cations and anions in opposite directions.

51. Caribou and antelope migrate in opposite directions.
What do caribou and antelope do? They migrate in opposite directions.
Caribou and antelope migrate in opposite directions.

52. Animation of migration of cations and anions in NaCl(aq)
Antelope
Caribou

53. Animation of migration of cations and anions in NaCl(aq) during conductivity testing- +
Anions
Cations
Caribou
Antelope

54. What occurs in the NaCl solution between the electrodes during a conductivity test?
Answer: a migration of cations toward the cathode and anions toward the anode.
Electrical current is electrical charges in motion.
Electrical charges can be electrons in a wire, or ions moving in a solution.

55. Red = O, grey = H, black = C How about covalent compounds?
for example, sucrose, C12H22O11, or table sugar
Red = O, grey = H, black = C
Covalent compounds must have a polar portion to be soluble in water. Covalent compounds can contain C, H, O, N, or P (non-metals).

56. Conductivity Observations
Light Bulb
Inference
What is present in the substance?
Water
Light blub does not light up
no ions detected in the liquid
Water molecules
H2O
Sucrose(s)
Sucrose(aq)

57. Conductivity Observations
Light Bulb
Inference
What is present in solution?
Water
Light blub does not light up
no ions detected in the liquid
Water molecules
H2O
Sucrose(s)
no ions detected in the solid
In the solid, there are no cations and anions
Sucrose(aq)

58. H2O Demonstration sucrose, C12H22O11
Sucrose is added to water, stirred and it dissolves. The solution is tested with the conductivity apparatus.
H2O
sucrose, C12H22O11

59. Two sucrose molecules (solid) dissolving in water

60. Sucrose dissolving in water

61. Sucrose dissolving in water

62. sucrose, C12H22O11 (s)  C12H22O11 (aq)
Sucrose dissolving in water
H2O(l)
sucrose, C12H22O11 (s)  C12H22O11 (aq)

63. A representation of solid sucrose dissolving in water
sucrose, C12H22O11 (s)
Insert URL sucrose dissolving in water computer animation

64. Sucrose dissolving in water

65. One Sucrose molecule dissolving in water

66. Sucrose dissolving in water

67. Sucrose dissolving in water

68. Sucrose dissolving in water

69. Sucrose dissolving in water
Glucose and fructose
are not ions!!

70. Strong electrolytes: fully dissociated into ions when dissolved in water (ionic solutions such as NaCl(aq)). Non-electrolytes: remain in molecular form when dissolved in water (covalent compounds such as sucrose) Note: dissolved ≠ dissociated

72. Conductivity Observations
Light Bulb
Inference
What is present in solution?
Water
Light blub does not light up
no ions detected in the liquid
Water molecules
H2O
Sucrose(s)
no ions detected in the solid
In the solid, there are no cations and anions
Sucrose(aq)
no ions detected in the solution
In the solution, there are no cations and anions

73. Role of the water molecules when sucrose dissolves in water
Electrostatic forces
of attraction
A sucrose solution is
a non-electrolyte.
Partial charges shown on selected atoms

74. Role of the water molecules when sucrose dissolves in water
URL computer animation dissolving sucrose in water

75. Representing a sucrose solution at the particle level

76. A sucrose solution does not conduct electricity. Why not
A sucrose solution does not conduct electricity. Why not? What must be missing?
No dissociation, no charged particles
Covalent compounds do not dissociate and do not conduct electricity.
When dissolved, covalent compounds do not dissociate and will exist as molecules.
No charged particles – no conduction of electricity

77. Demonstration Test the conductivity of a solution acetic acid
HC2H3O2(aq)

78. Conductivity Observations
Light Bulb
Inference
What is present in solution?
Acetic acid(aq)
NaCl(aq)
Bright glow
Ions present in solution
H2O solvent
Na+(aq), Cl-(aq) solute
Sucrose(aq)
No light
No ions present in solution
C12H22O11 (aq)
solute

79. Demonstration

80. Acetic acid CH3COOH(aq)
What do you observe about the light bulb?
What does this imply about the solution?

81. Consider the conductivity of aqueous acetic acid, HC2H3O2.
This indicates that acetic acid
fully dissociates into ions.
does not dissociate and remains in the molecular form.
partially dissociates to form a mixture of ions and molecules.

82. Conductivity Observations
Light Bulb
Inference
What is present in solution?
Acetic acid(aq)
dim
Some ions
H2O solvent, Mostly HC2H3O2 molecules; some H+ ions, C2H3O2- ion
NaCl(aq)
Bright glow
Ions present in solution
H2O solvent
Na+(aq), Cl-(aq) solute
Sucrose(aq)
No light
No ions present in solution
C12H22O11 (aq)
solute

83. Acetic acid CH3COOH(aq)
An acetic acid solution is a weak electrolyte.
What is present in the solution?
Draw a picture diagram.

84. Weak electrolytes - partially dissociated into ions
H2O
HC2H3O2 D H+ + C2H3O2-
~ 99%
~ 1%
Double reaction arrow indicates that the reaction is significant in both directions.
An acetic acid solution establishes a dynamic equilibrium

85. Weak electrolytes - partially dissociated into ions
H2O
HC2H3O2 D H+ + C2H3O2-
~ 99%
~ 1%
Double reaction arrow indicates that the reaction is significant in both directions.
Sketch a picture diagram representing an aqueous solution of acetic acid at the molecular level.

86. Draw a particle level diagram for a solution of acetic acid
Dynamic equilibrium of a weak acid solution
An animation of a strong acid solution dissociation.

87. Only some ions present

89. Demonstration Test the conductivity of a solution hydrochloric acid
HCl(aq)

90. Write a chemical equation representing dissolving gas phase HCl(g) in water to form a solution of hydrochloric acid

91. Demonstration hydrochloric acid reacts with water

92. Conductivity Observations
Light Bulb
Inference
What is present in the substance?
Water
Light blub does not light up
no ions detected in the liquid
Water molecules
H2O
HCl(aq)

93. Demonstration
HCl(aq) solution

94. Conductivity Observations
Light Bulb
Inference
What is present in the substance?
Water
Light blub does not light up
no ions detected in the liquid
Water molecules
H2O
HCl(aq)

96. Draw a particle level diagram representing a solution of hydrochloric acid

97. Particle level diagram of a solution of HCl(aq)

99. Draw a particle level diagram representing HCl(g) molecules reacting with water molecules
Dynamic equilibrium of a weak acid solution
An animation of a strong acid solution dissociation.
Insert computer animation of HCl(aq)

100. Electrolytes
A strong electrolyte dissociates completely when dissolved in water.
A weak electrolyte only dissociates partially when dissolved in water.

101. You need to memorize the following seven strong acids in life.
All other acids are considered as weak acids.
HCl – Hydrochloric acid
HBr – Hydrobromic acid
HI – Hydroiodic acid
HNO3 – Nitric acid
H2SO4 – Sulfuric acid
HClO3 – Chloric acid
HClO4 – Perchloric acid
H2SO4
is a diprotic acid.
Only the first H+ off is considered strong
Strong acids are strong electrolytes – They completely dissociate in water!

102. Demonstration Test the conductivity of a solution sodium hydroxide
NaOH(aq)

103. Conductivity Observations
Light Bulb
Inference
What is present in the substance?
Water
Light blub does not light up
no ions detected in the liquid
Water molecules
H2O
HCl(aq)
Light bulb lights up brightly
mobile ions in solution
H3O+ and Cl- ions
NaOH(aq)

104. Conductivity Demonstration sodium hydroxide solution

105. Demonstration
NaOH(aq) solution

106. Conductivity Observations
Light Bulb
Inference
What is present in the substance?
Water
Light blub does not light up
no ions detected in the liquid
Water molecules
H2O
HCl(aq)
NaOH(aq)

108. Write a chemical equation representing dissolving solid sodium hydroxide in water to form a solution of sodium hydroxide

109. Write a chemical equation representing dissolving solid sodium hydroxide in water to form a solution of sodium hydroxide

110. Draw a particle level diagram for a solution of sodium hydroxide

111. Particle level diagram for a solution of NaOH(aq)

112. Demonstration sodium hydroxide interacts with water

113. Selected Strong Bases and Weak Bases
Strong bases are strong electrolytes – they completely dissociate in water.
sodium hydroxide, NaOH
potassium hydroxide, KOH
calcium hydroxide, Ca(OH)2
strontium hydroxide, Sr(OH)2
barium hydroxide, Ba(OH)2
Weak Acids
Weak Bases
hydrofluoric acid, HF
ammonia, NH3
phosphoric acid, H3PO4
Methyl amine, CH3NH2
acetic acid, CH3COOH (or HC2H3O2)
and many more…

114. To recap…
Substances that dissolve in water are water soluble.
Substances that fully dissociate in water are strong electrolytes.
All ionic compounds, that are water soluble, are strong electrolytes.