Redox Experiments

A key part of the assessment of this unit is to carry out some experiments and infer what is happening. We then need to write balanced half-equations and overall equations, as well as identifying what is acting as the oxidant and what is the reductant.

Some ions are very difficult to identify due to their colours. For example, both Fe³⁺ and Cr₂O₇^2- are orange; Fe²⁺ is such a pale green that it often looks colourless. We need to use some other reagents/tests to confirm what has been produced:

Oxidation Numbers

Oxidation Numbers are a very convenient way to determine what has been oxidised and what has been reduced, particularly if you don't have half-equations (for example, if you have an overall equation).

There are some simple rules for assigning oxidation numbers:

1. All elements = 0   e.g. Fe, O₂
2. Monatomic ions = their charge   e.g. Fe²⁺ = +2, Fe³⁺ = +3
3. Oxygen (in a compound or ion) = -2, except in hydrogen peroxide (H₂O₂) = -1
4. Hydrogen (in a compound or ion) = +1, except in a metal hydride = -1 e.g. LiH
5. The sum of all oxidation numbers in a compound = 0
6. The sum of all oxidation numbers in a polyatomic ion = its charge

Here is the video from 2013, which covers it in a slightly different way:

Oxidation Reduction

Tournament Week sees the start of our Oxidation-Reduction topic. This will be assessed early next term via an Internal Assessment (AS91167).

KEY CONCEPTS:
OXIDATION
REDUCTION
OXIDANT
REDUCTANT
REDOX PAIRS
HALF EQUATION
OVERALL (NET) EQUATION

Redox Pairs

This is the first thing to do - learn which ions/molecules are paired up, and their respective colours. There is a great set of Flash Cards on the "No Brain Too Small" website which are worth printing out: HERE

Oxidation

Reduction

Overall (Net) Equations

Here is a video from 2013 going through a series of experiments:

Organic Chemistry Overview

We are trying a slightly different way of covering the last part of our topic, and preparing for our exam at the same time. Organic Chemistry is based around seven types of compounds and the relationships between them. Here is an overview of the process. This is a "living document" and will be added to over the rest of the week...

DESCRIBE the Compounds

I prepared some SOLO Taxonomy hexagons containing the names of the seven types of compounds we have studied (or need to study this week). There is a great hexagon generator on Pam Hook's website.

The students spent today cutting them out and writing "study notes" on the back on them. They have been given absolute freedom to choose:

• the order to fill them out in
• how much detail to include on each card
• whether to work individually or to collaborate

EXPLORE the Compounds

Our trolley of chemicals for the remainder of the unit.

I have given the students some page references from our textbook, Beginning Chemistry (Wignall and Wales) and some experiments they may choose to carry out. I have directed them to past examination papers and provided PowerPoints on Moodle, our Learning Management System. I have asked the technician to prepare a trolley with all of the possible chemicals (that I can think of) required to explore these compounds. The students have access to NetBooks, if they want them. I have made videos and blogposts to direct the learning.

My students have been challenged with the following "ultimate" outcomes:

1. Can you find (and explain) all the possible links between these compounds? CONNECT the hexagons in such a way that this is represented visually. I will challenge you to justify why certain hexagons are touching!
2. Can you EXPAND on every bullet-point/note on the back of each card? I will challenge you to do this over the next few lessons!
3. I am going to choose any two hexagons at random. Can you FIND a link between them, even if it is via another hexagon? Can you explain how I would tell those two compounds apart? Can you tell me how to turn one of them into the other?

One student's idea of the links between each type of compound.

REFLECTIONS of the Idea

I will write this next week, once I have talked to students about their learning and seen their assessment results... Watch this space.

Alcohol Derivatives - Amines

Amines are a very interesting group of weak bases. They have very similar properties to ammonia.

Different sources quote different ways of preparing amines. Most sources quote that amines are made by reacting ammonia with a haloalkane. This would make an acid (hydrogen halide, such as HCl) and a base (the amine). These would react to make an amino salt. You would need excess ammonia and/or high temperature (to drive off the hydrogen halide) for an aqueous solution of the amine to truly be produced. Please keep this in mind when looking at the video and other sources.

For more information, please click HERE (ChemGuide)

Alcohol Derivatives - Haloalkanes

Haloalkanes are a very useful intermediate compound, often used when converting one useful compound into another. Their own uses are limited, primarily due to the impact they have on the environment. Haloalkanes are generally immiscible (insoluble in water) as they are (technically) non-polar.

Haloalkanes are the products of:

• reacting a halogen (such as bromine) with hydrocarbons.
• reacting an alcohol with Lucas Reagent (chloroalkanes produced); remember that tertiary alcohols react very quickly, while primary alcohols may not react at all.
• reacting an alcohol with PCl₃, PCl₅ or SOCl₂

Haloalkanes can be converted into:

• alkenes, using NaOH (or KOH) dissolved in alcohol (elimination reaction)
• alcohols, using aqueous NaOH (or KOH) (substitution reaction)
• amines, using excess NH₃ (substitution reaction); if you use do not use enough NH₃, it will make an amino salt instead, such as ethyl ammonium chloride

For more information about haloalkanes, click HERE (ChemGuide)

Alcohol Derivatives - Carboxylic Acids

Carboxylic acids are made when primary alcohols are oxidised under reflux. They are weak acids, as they do not completely dissociate in water. This means that they have a lower hydronium ion concentration that inorganic acids, like hydrochloric acid, so have a higher pH (but still below 7) and slower reaction rate.

For more information about carboxylic acids and their reactions, click HERE (ChemGuide)

Reactions of Alcohols

Today, we did some experiments with primary, secondary and tertiary alcohols. There was no teaching done today, so here are some videos explaining some of our observations and how to use them to identify/classify alcohols.

Preparation of Butanol from Butene

There are a few ways to make alcohols:

• fermentation of sugars
• synthesis gas
• hydration of alkenes

We looked at the hydration of 1-butene using dilute sulfuric acid:

Isomers of Pentanol

We had to try to make and name as many isomers of pentanol as we could (using molecular models):

Some of the numbering in these is redundant, but it is a good habit to include them at this stage.

The Structure of Alcohols

The structure of alcohols tells us a lot about their physical properties and chemical reactions. This lesson was about the different isomers of C₄H₉OH, with the students starting by trying to make (and name) all of the ones that contain the alcohol functional group.

EXTENSION (not covered in Level 2, but worth seeing if you disagree about your structures of 2-butanol you made in class)

Reactions of Alkenes

This content will be covered over the rest of the week, so this post will be updated regularly.

Here is an introduction to two of the most simple reactions of alkenes: combustion and addition:

The first task we started with today was to draw and name as many isomers of C₆H₁₂ as possible...this was tough!! According to the internet, there are over 20!!

Then, we were taught about oxidation and polymerisation:

Alkenes - Introduction

Alkenes are unsaturated hydrocarbons. The functional group is a C=C bond (double bond). This lesson was based around the isomers and nomenclature of alkenes:

Here is the activity we started with that shows alkenes have the same general formula as cyclic alkanes:

Here is the activity we did at the start on Monday's lesson...just a little tougher:

Reactions of Alkanes

We were shown a couple of simple reactions of alkanes: combustion and substitution.

Then we had to do balanced equations for these and learn about "cracking":

Alkanes - Introduction to Organic Chemistry

I'm not a fan of teaching this topic the way Beginning Chemistry does. I prefer to "get into it" with the functional groups and cover types of reactions, nomenclature and isomerism over and over again, in context. So, we jumped straight into using MolyMods to make as many isomers of C₅H₁₂ as possible.

Here are the questions we tried at the start of the next lesson:

Acid Strength

Acids can be described as concentrated or dilute. This is about how much water is used to make them up. They can also be described as strong or weak. This is about the concentration of hydronium ions. Concentration has some effect, but the extent to which an acid dissociates (turns into ions) is more important:

Chemical Minds

Check out this wikispace, set up by Michele McMahon at St Cuthbert's College.

Calculating pH of Bases

Calculating pH - Strong Acids

Strong acids fully dissociate - all of their hydrogen atoms react with water to become hydronium ions. We use the concentration of hydronium ions to calculate the pH:

pH = -log[H₃O⁺]

Likewise, we can use the pH to calculate the hydronium ion concentration:

[H₃O⁺] = 10^-pH

Acids and Bases - Moving Beyond Year 11

Equilibrium Constant

Mr Nicoll was unwell so left these instructions:

1. Work through the notes and exercises about Equilibrium Constant in Beginning Chemistry.
2. Do Question Two, in the 2013 NCEA paper (AS91166).

Here is a short video talking about Equilibrium Constant and going through the answers to the exam question:

Concentration and Equilibrium

Dynamic Equilibia and le Chatelier's Principle

Today was an introduction to these ideas including a demonstration of the nitrogen dioxide/dinitrogen tetroxide equilibrium.

Explaining Rates of Reaction

We looked at the main conditions that can increase reaction rates. It led to a discussion about depressants and psychedelics, but here is the actual Chemistry content from today:

The Kinetic Theory

The Particle Model (Year 9) and the Collision Theory (Year11) are both important in understanding why reactions and phase changes occur. We are about to learn about factor that control the rate of reaction, but first we had to revisit these key ideas from previous years:

Bond Enthalpy

Bond enthalpy is the amount of energy required to break one mole of a particular bond. As bond breaking requires energy, these values are always exothermic.

Calculating Enthalpy Change

We do experiments using calorimeters to find enthalpy changes. The change in temperature of the solution is translated into an energy change for the water component of this solution. This is then inferred to be the equal (but opposite) value for the energy change for the reactants. This can then be used to calculate the enthalpy change.

Enthalpy Change Diagrams

We were asked to do the same for an endothermic reaction. For homework, we have to read pp140-142 and attempt #2, p144 in Beginning Chemistry.

Molecular Polarity

The last part of this unit is about molecular polarity. We combine the ideas of bond polarity and shape to predict and justify molecular polarity.

Here is the blog post etc. from last year:

Shapes of Molecules

Here is the video of me teaching it to his class last year:

There is also an excellent series of lessons in Khan Academy worth working through: HERE

I really like this guy's video too:

In class, we were also asked to work on a past NCEA question: Discuss the shapes and bond angles in NOCl and H₂S (the Lewis Dot Diagrams were provided in the question):

The Lewis diagram for NOCl shows there are three regions of electron density around the central atom (N). This means the electron cloud has a trigonal planar shape with bond angles of 120°. However, as there are only two atoms bound to the nitrogen atom (O & Cl), this is a bent molecule. Due to the repulsion of the lone pair of electrons on the nitrogen atom, the bond angle would be slightly lower than 120°.

The Lewis diagram for H₂S shows there are four regions of electron density around the central atom (S). This means the electron cloud has a tetrehedral shape with bond angles of 109.5°. However, as there are only two atoms bound to the sulfur atom (H in both cases) this is a bent molecule. Due to the repulsion of the two lone pairs of electrons on the sulfur atom, the bond angle would be slightly lower than 109.5°.

2013 Blog

While we have a Student-Teacher, it may be worth looking at the 2013 Level 2 Chemistry blog for additional videos etc. This topic was taught in April and May in 2013:

http://l2chem2013.blogspot.co.nz/

Titration Overview

Titration Calculations

Titration Techniques

We started learning about the practical part of this topic (and the key part of the assessment): titrations.

We will look at the calculations involved in a future lesson.

Making a Standard Solution

Aqueous Solutions Introduction

Today was our first introduction to solutions and quantitative analysis. We met some new units (grams per litre and moles per litre). We will learn more about the importance of these over the next two weeks.

Water of Crystallisation

Many ionic crystals have water molecules inside them. We can use a very simple technique to find the formula of these hydrated salt.

Empirical and Molecular Formula

Empirical Formula is the simplest ratio of the atoms in a compound. For ionic compounds, this is also its ionic formula. For molecules, it may be the molecular formula, but it is more likely to be a simplification of it.

Percentage Composition

Today we learned how to calculate the percentage composition of compounds, using anhydrous copper (II) sulfate as our example:

Stoichiometry

Having tried to master balancing equations, the importance of this skill became more apparent as we learned about the mole, molar mass and relative atomic mass, and how these are used to analyse what is happening in a chemical reaction.

Balancing Equations

We had a small disaster with the filming of this, but the content is there anyway...

Today, we reviewed how to balance acid-base equations. The same sequence can be used for any equations, but the current Internal Assessment only focuses upon this type of reaction.

Below is a better video from last year:

Introduction to Chemistry

After all of the Administration, we learned about the Chemists' "Bible": The Periodic Table of Elements. If you know how to use the Periodic Table, it helps you know more about the elements without having to rote-learn lots of facts.

One of the things we focused on was using the Groups (columns) to predict the valency of an element. There are some that we still need to learn, but the rules apply in most cases.

Key Exceptions: Boron "the moron"; Transition Metals (Fe, Mn, Zn, Ag, etc.)