Wednesday, 10 September 2014

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 Cr2O72- 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:

Sunday, 7 September 2014

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:

Thursday, 28 August 2014

Oxidation Reduction

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


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



Overall (Net) Equations

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

Tuesday, 12 August 2014

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.


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)