Chemistry 123 Lectures

SOLUBILITY

In chapter 4 we defined solubility as the amount of a substance that dissolves in a given quantity of solvent at a given temperature to form a saturated solution. We approached solubility in a very simple manner by classifying salts as soluble or insoluble and solubility guidelines were established indicating if a precipitate would form when two solution are combined.

Overview of Solubility

Experimentally, we can consider solubility equilibria to make predictions about the amount of a given salt that will dissolve. First, we need to define what we mean by solubility and we need to consider what we already know about solubility and compare that to what a chemist thinks about when they hear the term solubility.
When chemists discuss solubility they do so in terms of the solubility product constant. Keep in mind that the solubility product constant and molar solubility are two completely different terms. As chemists, we are able to calculate one from the other, but directly comparing the two without a calculation can lead to some improper assumptions. It is very important to recognize the similarities/differences between solubility in terms of the amount (in grams) that dissolve per liter of solution, the molar solubility, and the K_sp.

The Solubility Product Constant

Example Problem: 1.4 x 10^-6 grams of ZnCO₃ dissolve in 1.000 mL of solution and 2.8 x 10^-6 grams of BaCrO₄ dissolve in 1.000 mL of solution, which one has the largest K_sp? Solution: They are the same. Click here to see the explanation.

One common mistake students make is to simply look at the Ksp and think that constant tells them everything they need to know about the solubility when two slightly soluble salts are compared. If you are thinking this way, then you certainly aren't thinking like a chemist. A chemist would think through the process shown in the example below.

Ranking the solubility of slightly soluble salts given the Ksp Part I

Ranking the Solubility of slightly soluble salts given the Ksp Part II

Homework:
Read Section 17.4 and the K_sp Lab
Mastering Chemistry Solubility Constant Expression Tutorial
Mastering Chemistry Introduction to Solubility and the Solubility Product Constant Tutorial
Mastering Chemistry The Solubility Product in Medicine Tutorial
*Mastering Chemistry K_sp Pre-Lab Assignment
*Solubility Graded Homework #1
*Solubility Graded Homework #2
Questions 1-18 from the Solubility homework set posted on Carmen.

LAB #1
DEVELOPING A MASTERY OF THE SOLUBILITY PRODUCT CONSTANT LAB

Once you have a handle on saturated aqueous solutions, we can start to analyze and manipulate the solubility of these solutions and determine the criteria for precipitation.

Criteria For Precipitation

With a background on precipitate formation, we can now predict if a precipitate will form when two solutions are combined.

If Two Solutions are Mixed Will a Precipitate Form?

In certain cases, it is beneficial to separate a mixture of ions in solution. By utilizing the K_sp values and performing calculations, we can calculate the concentration needed to give the best separation of ions in solution.


Order of Precipitation, Minimum Concentration Needed to Facilitate Precipitation, and Best Separation

Homework:
Read Section 17.6
Mastering Chemistry Precipitation Tutorial
Mastering Chemistry Selective Precipitation Tutorial
Mastering Chemistry Precipitation Calculation Tutorial
Mastering Chemistry Fractional Precipitation of Metal Carbonates Tutorial
*Solubility Graded Homework #3
Questions 58-69 from the Solubility homework set posted on Carmen.

A common laboratory experiment constructed to illustrate the principles of solubility is Qualitative Analysis. This experiment is designed to answer what is present. Unfortunately, there is no spot test for each individual cation, so a scheme was developed, which groups cations based on their solubility characteristics. Each ion is then isolated and identified.

Applied Qualitative Analysis Scheme

The Group I cations precipitate as chlorides under acidic conditions.

Group I Separations

Why does our HCl need to be cold and dilute?

Group I Analysis

APPLIED QUALITATIVE ANALYSIS LAB

Similar analysis is performed for the Group II and Group III cations and is investigated in detail in your lab manual.

Chemists love to manipulate things. K_sp expressions are equilibrium expressions and as Le Chatlier showed us, equilibria can be manipulated if you apply a stress. There are many factors that influence solubility and the factors we will investigate in this class are: Common Ion, pH, Complex Ion Formation, and Amphoterism. In all of these effects, it is very important to WRITE OUT THE EQUILIBRIUM EXPRESSION so you can properly analyze how the solubility will be influenced.

The Common Ion effect is simply a direct application of Le Chatlier's principle.

Common Ion Effect

Homework:
Mastering Chemistry Solubility of "Insoluble" Salts Tutorial
Mastering Chemistry The Common-Ion Effect Tutorial
Mastering Chemistry Common-Ion Effect on Solubility for Lead Thiocyanate Tutorial
Mastering Chemistry Common-Ion Effect on Solubility for a Metal Hydroxide Tutorial
*Solubility Graded Homework #4-#5

The pH of a solution has a dramatic influence of the solubility. When investigating pH effects, it is important for you to know the strong acids, which will allow you to identify all the neutral anions in solution.

pH Effects

How does adding acid/base influence solubility?

Homework:
Mastering Chemistry The Effect of Acid on Solubility Tutorial
Mastering Chemistry Effect of pH on Solubility Tutorial
Mastering Chemistry Acid Rain: Effect on Solubility of Calcium Carbonate Tutorial
*Solubility Graded Homework #6-#7
Mastering Chemistry Creating a Buffer Solution Tutorial
Mastering Chemistry pH of a Buffer Solution Tutorial
*Solubility Graded Homework #8-#9
Questions 24-32 from the Solubility homework set posted on Carmen

Le Chatlier's principle does a great job of explaining the solubility in terms of the common ion effect and pH effects, but in some instances some puzzling results are obtained. For instance, when concentrated NH₃ is added to a saturated solution of Zn(OH)₂ its solubility increases. Now we need to determine why this is, and if Le Chatlier's principle is still valid.

Does zinc hydroxide follow the rules we've discussed so far?

It turns out Le Chatlier's principle still applies, but there is something else going on in solution responsible for these results. The concept responsible for the difference in solubility is complex ion formation.

Complex ion formation and coordination complexes

This allows us to investigate zinc hydroxide in a more complex way and it allows us to perform several calculations based on complex ion formation.

Re-analyzing zinc hydroxide

Solubility of zinc hydroxide in 15 M NH3

Determining the concentration of free metal cations in solution

Amphoterism is a special case where a slightly soluble salt has its solubility increased due to pH effects in acidic solution and has its solubility increased due to complex ion formation under basic conditions.

Amphoterism

Solubility of Al(OH)₃ in 15 M NH₃

Molar Solubility of Al(OH)₃ in 15 M NH₃ continued

Amphoteric Effects on Solubility

Homework:
Mastering Chemistry Solubility of Zinc Hydroxide in Basic Solution
Mastering Chemistry Cyanide Poisoning
*Solubility Graded Homework #10-#12
Questions 33-46 from the Solubility homework set posted on Carmen

The theory behind the separations of the Group II and Group III encompass all these effects and utilizes the solubility of the sulfide ion in solution.

Group II and Group III Sulfide Solubility

Will FeS precipitate?

At what pH will FeS begin to precipitate?

Homework:
Read Section 17.7 and the Qualitative Analysis Lab
Mastering Chemistry Qualitative Analysis of Metal Cations Tutorial
Mastering Chemistry Selective Precipitation in an Acidic Solution Tutorial
*Solubility Graded Homework #13-#14
Questions 19-23 & 72-84 from the Solubility homework set posted on Carmen


THERMOCHEMISTRY

Review sections 5.2 – 5.7 in the textbook, especially topics including: The 1st Law of Thermodynamics, Enthalpy, and how q and ΔH are related.

In Chapter 5, the term enthalpy was introduced and in this chapter we will use enthalpy and introduce a new concept, entropy, to tell a more complete story of Thermodynamics.

First, we will look into combustion reactions (which are exothermic) and see that as we progress from reactants to products there is a lowering in the potential energy stored in the chemical bonds, and the potential energy is converted to thermal energy (release of heat).

Chapter 19 Thermodynamics

The first law of Thermodynamics provides the means for accounting for energy, but it gives no hint as to why a particular process occurs in a given direction. A process is considered to be spontaneous if it occurs without outside intervention, and the driving force for a spontaneous process is an increase in entropy. We can define entropy as the measure of randomness or disorder and entropy can be expressed mathematically using macro/microstates and we can also compare the entropy of various systems.

Spontaneous Process and Entropy

Mathematical Definition of Entropy

Show PHET tutorial of an ideal gas.

Macro and Micro States

Comparing Entropy of Various Systems

Homework:
Read Sections 19.1 (skip pages 804, 805, and the first two paragraphs of 806), 19.2, and 19.3.
Mastering Chemistry Entropy and Microstates Tutorial
Mastering Chemistry Qualitative Predictions about Entropy Tutorial
*Thermochemistry Quiz Questions #1 - #4

The second law of Thermodynamics states that in any spontaneous process there is always an increase in entropy of the universe.

2nd Law of Thermodynamics

The third law of Thermodynamics states that the entropy of a pure crystal at zero degress kelvin is zero. The change in entropy of a reaction can be calculated from the standard entropy of each substance.

3rd Law of Thermodynamics

Homework:
Read Section 19.4
Mastering Chemistry Entropy of Reaction for Nitrogen Dioxide Formation Tutorial
Mastering Chemistry Standard Entropy of Reaction Tutorial
Mastering Chemistry Entropy and the Second Law of Thermodynamics Tutorial
Mastering Chemistry Third Law of Thermodynamics Tutorial
*Thermochemistry Quiz Question #5 - #6

Gibbs Free Energy is a state function combining enthalpy and entropy in the form of G = H - TS. J. Willard Gibbs developed this equation, which provides a convenient way to use the change in enthalpy and change in entropy to predict whether a given reaction occurring at constant pressure and constant temperature will be spontaneous. The derivation of this equation can be seen below.

Free Energy (G)

We can also consider the various situations for the relative signs of ΔH and ΔS to predict how ΔG will change with temperature. We can use the chart generated in the video below to investigate the effect of temperature on the spontaneity of a chemical reaction.

Predicting the Sign of Delta G

DEVELOPING A MASTERY OF THERMODYNAMIC RELATIONSHIPS LAB

Homework:
Read Section 19.5
Mastering Chemistry Interactive Activity – Temperature Dependence of Entropy Tutorial
Mastering Chemistry Standard Free Energy of Formation Tutorial
Mastering Chemistry Gibbs Free Energy: Temperature Dependence Tutorial
Mastering Chemistry Melting and Boiling Points Tutorial
*Thermochemistry Quiz Question #7 - #9

Read Section 19.6
Mastering Chemistry Enthalpy, Entropy, and Spontaneity Tutorial
Mastering Chemistry Gibbs Free Energy: Spontaneity Tutorial
*Thermochemistry Quiz Question #10 - #11

Free Energy Under Non-standard Conditions

Thermochemistry of the Haber Process

Read Section 19.7
Mastering Chemistry A Molecular View of Thermodynamics Tutorial
Mastering Chemistry Free Energy and Chemical Equilibrium Tutorial
Mastering Chemistry Free Energy and the Reaction Quotient Tutorial
Mastering Chemistry Gibbs Free Energy and Equilibrium Tutorial
Mastering Chemistry Gibbs Free Energy: Equilibrium Constant Tutorial
Mastering Chemistry Isomerization of Glucose to Fructose Tutorial
*Thermochemistry Quiz Question #12 - #23

After completing these tutorials and quiz questions, you should now be prepared to handle the more challenging problems:
*Quiz Questions #24-#34.



ELECTROCHEMISTRY

Review section 4.4 of the textbook, especially the Oxidation numbers rules on page 137.

Read Sections 20.1, 20.3, and 20.4
Mastering Chemistry Oxidation-Reduction Reactions Tutorial
Mastering Chemistry Oxidation States Tutorial
Mastering Chemistry Identifying Oxidizing and Reducing Agents Tutorial

Chemistry is an experimental science where measurements are made. The theories and equations we see in the text are backed up by experimental results. The following example in the video below helped shape Electrochemistry.

Electrochemistry Observations

Electrochemistry is the study of chemical and electrical energy. The two processes we will discuss are the generation of an electrical current from a chemical (redox) reaction (this is a spontaneous process) and the use of an electric current to produce a chemical change (a nonspontaneous process).

Electrochemistry

Now that we have observed various electrochemical processes, we need a convention to describe each voltaic cell.

Silver-Zinc Voltaic Cell Part 1

Silver-Zinc Voltaic Cell Part 2

Silver-Iron Voltaic Cell

When you describe voltaic cells, be sure to keep a few things in mind:
*When a half reaction is reversed, the sign of the standard cell potential is reversed.
*The standard cell potential is an intensive property.
*To run spontaneously, the standard cell potential must be positive.
*A chemically inert conductor is required if none of the substances participating in the half reaction is a conducting solid.

Voltaic Cells

Homework:
Mastering Chemistry Introduction to Galvanic Cells Tutorial
Mastering Chemistry Animation – Analysis of Copper-Zinc Voltaic Cell Tutorial
Mastering Chemistry A Nickel–Aluminum Galvanic Cell Tutorial
Mastering Chemistry A Nickel-Silver Galvanic Cell Tutorial
Mastering Chemistry Cell Potential Tutorial
*Electrochemistry Quiz Questions #1 - #6

Now that we have investigated Voltaic Cells, we need at come up with a convention to balance the reactions occurring in these cells.

Balancing Redox Reactions

Read Section 20.2
Mastering Chemistry Balancing Redox Equations and Stoichiometry Tutorial
Mastering Chemistry Balancing Redox Equations: Half-reaction Method Tutorial

Work can be done when electrons are transferred through a wire. The amount of work depends on the potential difference between the anode and cathode, and we can determine the relationship between the Cell Potential and the Gibbs Free Energy.

Ecell and Delta G (part 1)

Ecell and Delta G (part 2)

All the calculations to this point have been calculated under standard conditions. When the concentrations of the solutions in the anode and cathode compartment are changed, the cell potential will change. This relationship, referred to as the Nerst equation, will allow us to calculate the cell potential under non-standard conditions.

Cell Potential and Concentration

Application of Nerst Equation

Homework:
Read Section 20.5 and 20.6
Mastering Chemistry Cell Potential and Free Energy of a Lithium–Chlorine Cell Tutorial
Mastering Chemistry Interactive Activity – The Relationship of E°cell, Keq, and Gibbs Mastering Chemistry Free Energy Tutorial
Mastering Chemistry Cell Potential and Free Energy Tutorial
Mastering Chemistry Introduction to the Nernst Equation Tutorial
Mastering Chemistry Cell Potential and Equilibrium Tutorial
Mastering Chemistry The Nernst Equation Tutorial
Mastering Chemistry The Nernst Equation and pH Tutorial
*Electrochemistry Quiz Question #7 - #13

DEVELOPING A MASTERY OF ELECTROCHEMICAL RELATIONSHIPS LAB

In a Galvanic/Voltaic Cell, a spontaneous redox reaction produces a current (electricity). In an electrolytic cell, electrical energy is needed to produce a chemical change. During electrolysis, the electrical energy forces a non-spontaneous reaction to occur.

Electrolysis

In an electrolytic process, we can use the stoichiometric relationships of an electrolytic process to calculate various electrolytic properties. In the first example shown below, we will calculate the mass of solid copper that is plated when a current of 10 amps is passed for 30 minutes through a Cu²⁺(aq) solution.

Stoich of Electrolytic Processes

The next example shows how long a current of 5 amps must be applied to a solution of Ag⁺ to produce 10.5 grams of Ag(s).

Electrolysis Example

Producing H₂(g) has gained widespread attention from its potential application as a source of alternate energy. The following example shows why the electrolysis of water is not an effective way to produce hydrogen gas and shows how much hydrogen gas is liberated during the passage of 2 amps for 30 minutes.

Electrolysis of H2O

We can also calculate the concentration of a particular ion remaining in a solution after a current has been passed through its solution. In this case, the [Cu²⁺] remaining in 335 mL of solution that was originally 0.215 M copper(II) sulfate after the passage of 2.17 amps for 235 seconds, can be calculated.

Calculating Concentration in Electrolysis

Homework:
Read Section 20.9
Mastering Chemistry Introduction to Electrolysis Tutorial
Mastering Chemistry Introduction to Electroplating Tutorial
Mastering Chemistry Electrolysis of Aqueous Salts Tutorial
Mastering Chemistry Analysis of Electroplating Tutorial
Mastering Chemistry Electrolysis and Current
*Electrochemistry Quiz Question #14 - #15

After completing these tutorials and quiz questions, you should now be prepared to handle the more challenging problems:
*Quiz Questions #16-#23.


TRANSITION METAL COMPLEXES

Transition Metals utilize their valence d-orbitals to form coordination complexes, which have characteristics important to industry, technology, and medicine. Coordination complexes exist in every color of the rainbow and can be found in jewelry, steel, paints, anticancer drugs, and photographic films. Most catalysts contain transition metal complexes and they are commonly used in the pharmaceutical industry. There two areas are vitally important to research chemists and are rapidly growing. A better understanding of the fundamentals of coordination complexes will help you understand current materials and will help chemists improve how these materials function.

In the early days, chemists were fascinated by transition metal complexes due to their color. The color of these complexes depends heavily on the d electron count of the transition metal, and the energy levels in the complex ions.

Transition Metals and Coordination Complexes

Being able to properly interpret the properties of the electromagnetic spectrum will allow us to interpret how our eyes detect color.

The Electromagnetic Spectrum and Color

The discreet nature of the energy levels in molecules allows us to determine the energy it would take to excite an electron from an energy level of lower energy to one of higher energy. If this excitation falls in the visible range of the electromagnetic spectrum, our eye will detect color.

Orbital Energies

Chemists are able to measure the wavelength of light a material absorbs using a UV-Vis Spectrometer, and by measuring which wavelengths of radiation are absorbed and which are not, we can gain valuable information on a molecules electronic excitations, or electronic structure.

UV-Vis Spectroscopy

UV-Vis Spectroxcopy (cont.)

Keep in mind that a transition metal atom has very different properties than when it is the central atom in a coordination complex. This is due to the difference in how the electrons fill the orbitals to achieve the lowest energy configuration possible.

Electron Configurations of Transition Metal Complexes

Coordination Compounds consist of a transition metal complex ion (transition metal center with ligands attached) and counter ions. The relative energies of the d orbitals in these transition metal complexes lead to some unique physical properties, including color and magnetism.

Chemistry of Coordination Complexes

In 1893 Alfred Werner proposed a theory that successfully explaining the difference in color for various transition metal complexes. This theory is still used today, but before we go into the modern day theory, lets take a look into how Werner developed his theory.

Coordination Complexes Before 1893

Modern Day Formulas for Transition Metal Complexes

Once the modern day formulas were introduced, chemists began to investigate how ligands surrounded the transition metal center. The term isomer is given to complexes that have the same overall composition, but different structures. This prompted chemists to introduce two new terms: coordination sphere and coordination number.

Arranging Ligands Around the Transition Metal Center: Introducing Isomers

Once Werner showed coordination complexes could exist as isomers, chemists began to investigate various isomers and in this class we will cover two types of structural isomers (linkage and coordination sphere) and two types of stereo isomers (geometric and optical).

Isomer Overview

Linkage Isomers

Coordination Sphere Isomers

Geometric Isomers

Optical Isomers

At this point we really havn't scratched the surface of why a transition metal wants to form a bond with a ligand.

Complex Formation: The Metal-Ligand Bond

The Chelate Effect

When you are given a transition metal complex you will need to know how to determine its oxidation state, its coordination number, and its geometry.

Transition Metal Complexes: Oxidation States, Coordination Number, and Geometry

When a transition metal complex has a coordination number of 4 it can exist as either a tetrahedral or square planar molecule. There are a set of empirical observations that can allow you to predict whether a complex with a coordination number of 4 will exist as a tetrahedral molecule or a square planar molecule.

Stereochemistry

Determining if an ML4 complex is Td or Square Planar.

Example Problem: Determining if an ML4 complex is Td or Square Planar.

Transition metal complexes bond to a variety of surrounding atoms/molecules which we refer to as ligands. The nature of the ligand that binds to the transition metal center has a large influence on the properties of the transition metal complex.

Ligands

We have discussed that transition metal complexes exhibit a wide variety of colors and have varying magnetic properties. In order to explain the similarities and differences in these properties we need to look into the bonding theories of transition metal complexes.

Bonding Theories of Transition Metal Complexes

Crystal Field Theory

In order to understand the bonding theories of transition metal complexes you need to visualize how the ligands interact with the 5 different d orbitals. You cannot visualize how the d orbitals interact with the ligands if you do not know the shapes of the d orbitals like the back of your hand.

Shapes of d Orbitals

By observing the colors of various transition metal complexes, we can start to see how the nature of the ligand bound to the transition metal center plays a big role in determining the resulting color of a transition metal complex.

Orbital Overlap and Orbital Energies in Crystal Field Theory

Crystal Field Theory

The Spectrochemical Series

High Spin/Low Spin Co Complexes

Octahedral Field Splitting vs. Tetrahedral Field Splitting

Octahedral Field Splitting vs. Square Planar Field Splitting