G12 HL Chemistry Mad!LAB!

March 2, 2018

2/28/2018

C8: Photovoltaic & Dye-Sensitized Solar Cells

Conjugation is the interaction of alternating double bonds, for example, in organic molecules, to produce a delocalized array of pi electrons all over the atoms. Molecules with conjugated bonds can absorb visible light, with longer conjugated systems absorbing light of longer wavelength. All the carbon atoms in such systems have sp2 hybridization: the pi-electron clouds of adjacent double bonds overlap with one another and form a large cloud of delocalized electrons. This type of multi-center chemical bonding is known as electron conjugation. It is similar to the electron delocalization seen in benzene and produces a chain of carbon-carbon bonds with a bond order of 1.5.

Looking at the examples on page 711, those compounds with alternating double and single bonds have conjugation. A and B show conjugation but C and D do not. For conjugated alkenes, the higher the degree of conjugation, the longer the wavelength of light that can be absorbed.

Semiconductors have electrical conductivity midway between that of conductors and insulators. The conductivity of a semiconductor increases with temperature, in contrast to that of conductors. Conductors are typically metals with low ionization energies and therefore, freely moving electrons.

Photovoltaic cells made of semiconductors can absorb photons of light, resulting in electrons being knocked free from atoms and creating a potential difference. Semiconductor materials for such cells are often Group 14 elements such as silicon and germanium. Conductivity can be increased by doping the semiconductor with impurities from Group 15 elements such as phosphorous to create n-type semiconductors, or Group 13 elements such as boron to create p-type semiconductors. Page 712-713 go into detail about this. The below images come from the Pearson textbook.

Photovoltaic cells absorbs photons in a semiconducting material, which causes some valence electrons to be removed, resulting in some ionization in the cell. A charge separation occurs in the semiconductor which allows for a one-way flow of electrons. The cell can then be linked to an external circuit where the flow of electrons provides electrical power.

In a dye-sensitized solar cell (DSSC), photons are absorbed by a dye in a way similar to the absorption of photons by chlorophyll in photosynthesis. Electrons in the dye are then injected into a titanium (IV) oxide nanoparticle layer, which conducts the electrons to the anode. Once a dye molecule as emitted its excited electron, it needs to gain another electron. To do this, dye-coated titanium (IV) oxide nanoparticles are immersed in a solution of I- ions. The I- ions release electrons to the dye on the titanium (IV) oxide, becoming oxidized to tri-iodide ions. They can accept electrons at the cathode, being reduced back to iodide ions. See page 714, Figure 7.

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March 1, 2018

2/28/2018

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C6 Part 2: Fuel Cells

A fuel cell is an electrochemical device that converts chemical potential energy in a fuel into electrical energy. In a hydrogen fuel cell, the fuel is hydrogen (obviously), which is oxidized by oxygen and produces water. There is therefore no pollution and fuel cells are very efficient. The key components of a fuel cell are (1) the electrolyte or separator which prevents components from mixing. The proton exchange membrane is a polymer which allows H+ ions to diffuse but not electrons or molecules, (2) the oxidizing and reducing electrodes which are catalysts that allow the chemical reaction to occur, and (3) the bipolar plate which collects the current and builds up voltage in the cell.

Alkali Fuel Cells. The electrolyte in these cells is a solution of potassium hydroxide, providing a source of OH- ions. As the OH- ions migrated towards the anode, they reacted with H+ ions, producing water. If an acidic electrolyte (such as phosphoric acid) is used, then the H+ ions will migrate towards the cathode.

Image courtesy of alkammonia.eu.

Hydrogen fuel cells use hydrogen and oxygen as fuel. The heat formed can be used as a heat source. However, the hydrogen has to be pure and often platinum or other catalysts are added to graphite electrodes, which makes them expensive to run on a commercial scale. The oxygen comes from the air. The hydrogen comes from (1) the electrolysis of water, or (2) reforming hydrocarbons or biofuels.

Direct Methanol Fuel Cells. Methanol is used as a fuel instead of hydrogen. Methanol provides H+ ions at the anode. The fuel cell has the same components as the hydrogen PEM fuel cell. The anode reaction requires water, so a dilute solution of 1M methanol is used.

Images courtesy of wikipedia.org and americanhistory.si.edu

Page 695 in your text compares different fuel cells based on their energy density and specific energy. The table below, from the Pearson textbook, compares the advantages and disadvantages of different batteries and fuel cells.

While octane has a high energy density, fuel cells tend to have a higher thermodynamic efficiency. This is the ratio of the Gibbs energy change to the enthalpy change. (delta G/delta H).

It is possible to alter the EMF of a cell by changing the concentration of the mobile ions. When standard conditions exist, it is possible to predict the EMF of a voltaic cell by adding their half-cell potentials under standard conditions.

The Nernst equation can be used to calculate the potential of an electrochemical cell under non-standard conditions.

Mr. Thornley is on the case.

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Option C: Energy

2/26/2018

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C6: Electrochemistry & Rechargeable Batteries

In a primary electrochemical cell, the materials are consumed and the reaction is not reversible. Either the anode or the electrolyte (or both) need to be replaced, or the battery is thrown away. Typically the anode is oxidized and can no longer be used. Furthermore, the ions traveling through the cell can polarize the cell, which causes the reaction to stop. Polarization can cause a buildup of hydrogen bubbles on the anode, which increases the cell's internal resistance and reduce its output. In a secondary electrochemical cell (aka a rechargeable battery), the chemical reactions that generate electricity can be reversed by applying an electric current to them. Secondary cells can deliver stronger current demands than primary cells.

Secondary Cells: Lead-Acid Batteries
Rechargeable batteries are used in cars, for energy storage in the electric grid, in motorized electric vehicles, as emergency backup and for other uses. The typical lead-acid battery in a car is recharged while driving. In the lead-acid battery, the electrolyte is sulfuric acid.

Secondary Cells: Nickel-Cadmium Batteries
The NiCd rechargeable cell (used in toys and electronics) has a nickel (III) oxide hydroxide cathode, which reduces to nickel (II) hydroxide during discharge and a cadmium metal anode, which is oxidized to cadmium hydroxide. The electrolyte is aqueous potassium hydroxide.

Secondary Cells: Lithium-Ion Batteries
Lithium-Ion batteries used lithium atoms absorbed into a lattice of graphite electrodes rather than lithium metal for the anode. The cathode is a lithium cobalt oxide complex. The lithium atoms are oxidized to lithium ions during discharge. Lithium is an ideal material for lightweight batteries. Lithium-ion batteries are used in cell phones, laptops and cameras.

Pages 690-691 contain tables that compare and contrast the different types of batteries.

The voltage of a battery depends on the nature of the cathode and anode. The further apart the electrode potentials of the oxidizing and reducing materials, the more voltage per cell is available. Putting cells in series provides increased voltage.

The total number of electrons moving along with the energy given to them by the cell give a measure of how much work can be done by the current. This in turn depends on the nature and quantity of the materials (mass and surface area) as well as the specific energy density.

The moving electrons in the external circuit that provide us with useful energy, but each electrochemical cell also has to move cations and anions inside the cell. A battery's internal resistance depends on ion mobility, electrolyte conductivity, and the electrode's surface area.

Reactions occur faster at higher temperatures. At lower temperatures, reactions slow down. Ion mobility is reduced and the battery's internal resistance is increased. While batteries have lower resistance at higher temperatures, they also have an increased rate of self-discharge, so storing batteries at high temperatures is not a good idea.

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Chapter 20: Organic Chemistry

10/8/2017

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Types of Organic Reactions

Mission 1: Types of Organic Reactions
Mission Objectives. You should be able to...
1. Explain why OH- is a better nucleophile than water.
2. Deduce the mechanism of the nucleophilic substitution reactions with aqueous NaOH in terms of SN1 and SN2 mechanisms.
3. Explain how the rate depends on the identity of the halogen (leaving group) whether the halogenoalkane is primary, secondary or tertiary.
4. Outline the difference between protic and aprotic solvents.

Nucleophiles are species that are electron-rich, which means that they have a lone pair of electrons and may carry a negative charge. They are attracted to carbon and act as Lewis bases and can donate electrons. Examples include: OH-, H2O, NH3, and CN-.

Electrophiles are reactants that are electron-deficient, as they have a positive charge or partial positive charge. They are attracted to benzene and act as Lewis acids. Examples include: H+, Br+, and NO2+.

When dealing with these kinds of reactions, we must determine (1) what happens, which is the type of reaction, and (2) how it happens, which is the mechanism of the reaction.

The partial positive charge makes carbon atoms electron-deficient and therefore susceptible to attack by nucleophiles, which are electron-rich species capable of donating a pair of electrons to form covalent bonds. There are two types of nucleophilic substitution: SN1 and SN2. During these reactions, the carbon-halogen bond breaks and the halogen atom is released as a negative ion. This type of bond breakage is known as heterolytic fission.

The halogen, because it detaches, is known as the leaving group. The mechanism depends on whether the halogenoalkane is primary, secondary or tertiary.

Recall that primary compounds are attached to the functional group and at least two hydrogens. Secondary molecules are attached to the functional group, one hydrogen and two alkyl groups. Tertiary molecules are attached to the functional group and three alkyl groups, but no hydrogen atoms.

SN2 Reactions and primary halogenoalkanes. Nucleophilic substitution in primary halogenoalkanes proceed in one step. The slow step involves both the halogenoalkane and the nucleophile so that the rate of reaction is dependent on the concentrations of both reactants: rate = k[halogenoalkane][nucleophile] The molecularity is bimolecular, as there are two molecular entities.

When drawing mechanisms for SN2 reactions, please follow the rules on page 440. They are serious about those curly arrows. Below is a video showing the mechanism for a SN2 reaction using bromomethane in a solution with hydroxide ions.

Quick summary of SN2 mechanism: Bimolecular nucleophilic substitution. Involves heterolytic fission and nucleophilic substitution with primary halogenoalkanes. An unstable state is created, which makes the reaction bimolecular. Both incoming and leaving groups happen simultaneously (one step) and both are responsible for the rate-determining step.

SN1 Reactions and tertiary halogenoalkanes. Tertiary halogenoalkanes undergo nucleophilic substitution reactions that involve more than one step. The rate-determining step involves only the halogenoalkane; the bond to the leaving group breaks, forming a carbocation. A carbocation is a carbon atom with a slightly positive charge. The reaction is first order: rate = k[halogenoalkane]. The reaction is unimolecular.

When drawing mechanisms for SN1 reactions, please follow the rules on page 441.
Below is a video that goes through the SN1 mechanism using terbutylbromide and methanol as a solvent.

Summary of steps for SN1 mechanism: (1) formation of the carbocation, (2) nucleophilic attack, and (3) deprotonation.

Below, Richard Thornley briefly explains rates of nucleophilic substitution using the halogens. The second video reviews the steps in SN1 and SN2 reactions.

Factors that affect the rate of nucleophilic substitution:

1. The identity of the halogen. The quicker the rate-determining step, the higher the rate of reaction and a better leaving group achieves this. As you move down G17, the strength of the carbon-halogen bond decreases as the size of the halogen increases. A larger halogen results in long, weak bonds. Therefore iodine is the better leaving group out of all the halogens.

2. The classes of halogenoalkane. The class of halogenoalkane has a direct effect on the rate of formation. Tertiary carbocations have greater stability than primary carbocations. They rapidly form and reacts with the nucleophile immediately.

3. The choice of solvent. SN2 reactions are best performed in aprotic, polar solvents and SN1 reactions are best performed in protic, polar solvents. Aprotic polar solvents are ideal for SN2 reactions because they don't possess O-H or N-H groups and cannot solvate the nucleophile. Aprotic solvents include ethyl ethanoate and propanone. Protic polar solves are suitable for SN1 reactions because they are polar in nature, possess either an O-H or N-H groups so H bonds can be formed, and solvate the nucleophile, thus inhibiting its ability to attack electrophiles.

What are protic and aprotic solvents? Let's find out.

Read up on electrophilic reactions (p. 445-447). Below is a short video on Markovnikov's rule.

Synthetic Routes

Mission 2: Synthetic Routes
Mission Objectives. You should be able to...
1. Deduce a multi-step synthetic route given starting reagents and the products.

Mission 3: Stereoisomers
Mission Objectives. You should be able to...
1. Construct 3D models of a wide range of stereoisomers
2. Explain stereoisomerism in non-cyclic alkenes and C3 C4 cycloalkanes.
3. Compare physical and chemical properties of enantiomers.
4. Describe and explain optical isomers in simple organic molecules.
5. Distinguish between optical isomers using a polarimeter.

Stereoisomers

Stereoisomers have an identical molecular formula and bond multiplicity but show different spatial arrangements of the atoms. Stereoisomers can be subdivided into two major classes: conformational isomers and configurational isomers. Conformational isomers can be interconverted by rotation about the sigma bond without breaking any bonds. But if there is a double bond present, the pi bond must be broken. Therefore conformational isomers differ from one another in the arrangement of atoms around a single bond. We will play with the ethane molecule and reference page 453.

Configurational isomers can only be interconverted by the breaking of the sigma or pi bond or through rearrangement of the stereocenters (an atom with 3 or more different attachments). Configurational isomers are subdivided into cis-trans and E/Z isomers on one end and optical isomers on the other.

In terms of nomenclature, a special set of rules are required. Read about it on page 454. Cis-isomers are the equivalent of Z isomers and trans-isomers are the equivalent of E isomers.

Cis-trans isomers are determined by the positions of substituents (terminal atoms or groups of atoms in a terminal position) relative to a reference plane. For alkenes, the reference plane is the C=C. Cis-isomers have substituents one the SAME SIDE of the reference plane whereas trans-isomers have substituents on OPPOSITE SIDES of the reference plane.

Optical Isomerism is a type of configurational isomerism determined by the presence of chiral carbon atoms, also known as a stereocenter or asymmetric center. A chiral carbon is bonded to four different atoms or groups of atoms. Optical isomers have the ability to rotate plane-polarized light and exist in pairs that are called enantiomers or diastereomers. Enantiomers are non-superimposable mirror images of each other. They have no plane or symmetry and their optical activity is readily assigned when the molecules are represented as 3-D images.

The physical and chemical properties of two enantiomers of a particular substance are the same (BP, MP, viscosity, density, and solubility). Many chemical properties are the same except for their chemical reactions with other optical isomers. Enzymes in the body are chiral. Read about the examples of + and - limonene and the drug thalidomide on page 457.

Diastereomers are non-superimposable but they do not form mirror images. They have two or more stereocenters and differ in the configuration of at least one center. Diastereomers with the same general formula have different physical and chemical properties.

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Chapter 19: Electrochemical Cells

9/17/2017

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Mission 1: Electrochemical Cells
Mission Objectives. You should be able to...
1. Construct and annotate both types of electrochemical cells.
2. Explain how a redox reaction is used to produce electricity in a voltaic cell and how current is conducted in an electrolytic cell.
3. Distinguish between electron and ion flow in both electrochemical cells.
4. Deduce the products of the electrolysis of a molten salt.
5. Explain the process of electroplating.

EMF: electromotive force, which is the energy supplied by a source divided by the electric charge transported through the source. In a voltaic cell, the EMF is equal to the electric potential difference for zero current. Electrons take the path of least resistance. Difference between high potential energy and low potential energy is the EMF.

In voltaic cells, a cell potential is generated, resulting in the movement of electrons from the anode to the cathode via the external circuit. The cell potential is defined as the potential difference between the cathode and the anode when the cell is operating and is always less that the cell's maximum voltage. Under standard conditions (1M, 298K) the cell potential is called the standard cell potential, represented by (Ecell). To calculate Ecell, you will need the potentials for the oxidation reaction and the reduction reaction.

Ecell = Ecathode - Eanode OR as your book states: Ecell = Erhe - Elhe. Rhe means right hand electrode and lhe means left hand electrode. I prefer the first equation. The cathode is usually more positive and the anode is usually less positive. You need to reference the standard cell potentials table in your data booklet.

Below is a series of Thornley vids that go into detail about electrochemical cells. We will work our way through them.

Relationship between Gibbs Free Energy and Cell Potential.

The Standard Hydrogen Electrode.

The HL content in Chapter 19 focuses on the electrolysis of aqueous solutions. Here are some relevant videos.

There are examples of the electrolysis of aqueous solutions (see below). You're required to understand the setup of the cells and the observations at each electrode, and you should be familiar with the industrial uses of these processes.

A. Electrolysis of aqueous sodium chloride (p. 422-424)
B. Electrolysis of aqueous copper (II) sulfate ( p. 425-428)
C. Electrolysis of water (p. 428-429)

The following factors affect the amount of product formed at the electrodes during electrolysis: current (I), duration of electrolysis (t), and charge on the ion (z).

Let's review with Mr. Weng!!!

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Chapter 18: Acids & Bases

8/15/2017

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Helpful Links:
ICE Charts
Calculating Ka
Calculating Kb from Ka and Ka from Kb

Mission 1: Lewis Acids & Bases
Mission Objectives. You should be able to...
1. Describe a Lewis acid and a Lewis base.
2. Explain how a coordinate bond is formed.
3. Differentiate between nucleophiles and electrophiles and provide examples.

Lewis' definition of acids and bases focuses on the behavior of electrons. Acids accept electron pairs and bases donate electron pairs. Lewis's theory is broader than Bronsted-Lowry's, which allows for a wider variety of substances to be included. Ammonia (NH3) and OH- act as Lewis bases. H+ is a Lewis acid. On page 396, you'll see the example of boron trifluoride sharing electron pairs with ammonia. Ammonia donates a lone pair of electrons to BF3 to form a coordinate bond. The arrow represents a coordinate bond in which one atom contributes both electrons involved in forming the covalent bond.

Ligands are ions/moleules attached to metal atoms by coordinate bonding OR a molecule that binds to another (larger) molecule. Transition metals have partially occupied d-subshells so they can form complex ions with ligands that have lone pairs. The metal atom/ion acts as the Lewis acid and the ligand acts as a Lewis base. Check page 397.

Nucleophiles are electron-rich with at leasst one lone pair. They act as Lewis bases. Electrophiles are electron-deficient that can accept a lone pair from a nucleophile. They act as Lewis acids.

Mission 2: Calculations
Mission Objectives. You should be able to...
1. Solve problem involving [H+], [OH-] (both aq), pH, pOH, Ka, pKa, Kb, and pKb.
2. Discuss the relative strengths of acids and bases using values of Ka, Kb, pKa and pKb.

Strong acids/bases ionize completely. Weak acids/bases ionize partially. What this means is that strong acids and bases break up into their component ions completely. Weak acids and bases do not. When writing equations demonstrating this, strong acids and bases are written with a solid arrow, whereas weak acids and bases are written using the equilibrium arrows.

We can determine the concentration [ ] of the disassociated weak acid using the relationship between [ ] of reactants and products. Look at the example problems on p. 399, 401 & 402.

Khan Academy has a lovely article relating Ka & Kb, and how they relate to pH. Ka is the acid disassociation constant and Kb is the base disassociation constant. They are related to the ion product constant: Kw = Ka * Kb. Basically, pKa is the -log (Ka) and pKb is the -log (Kb). These values predict whether a species will donate or accept protons at a specific pH value. pKa + pKb = 14

These values also describe the degree of ionization of an acid or base and are true indicators of acid base strength because adding water does not change the equilibrium constant.

Small pKa --> large Ka (strong acid). Small pKb --> large Kb (strong base). Below are the relationships.

Mission 3: pH Curves
Mission Objectives: You should be able to...
1. Sketch the general shape of graphs of pH against volume for titrations involving strong and weak acid and bases with an explanation of their features.
2. Select an appropriate indicator for a titration given the equivalence point of the titration and the end point of the indicator.
3. Understand the preparation of buffer solutions.
4. Predict the relative pH of aqueous salt solutions formed by different combinations of acids and bases.

A buffer is a solution that resists a change in pH. It can be a weak acid and its conjugate base OR a weak base and its conjugate acid. You will learn about the four kinds of pH curves: (1) titration of a strong acid and a strong base, (2) titration of a weak acid and a strong base, (3) titration of a weak base and a strong acid, and (4) titration of a weak base and a weak acid.

Indicators are typically a weak acid or a weak base that displays a different color in acidic or alkaline environments. Many indicators act as weak acids. An indicator can also be a weak base.

Selection of an indicator.

In the below video, Sal Khan reviews titration curves. You can watch the whole video if you like, but I would focus on the first 5-6 minutes, when he reviews the four titration curves.

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G12 Chem HL

March 2, 2018

C8: Photovoltaic & Dye-Sensitized Solar Cells

March 1, 2018

C6 Part 2: Fuel Cells

Option C: Energy

C6: Electrochemistry & Rechargeable Batteries

Chapter 20: Organic Chemistry

Types of Organic Reactions

Synthetic Routes

Stereoisomers

Chapter 19: Electrochemical Cells

Chapter 18: Acids & Bases

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