Thursday, 16 October 2014


If we think of buffers as solutions made up of weak acids and their respective conjugate bases, these are not much harder than working with weak acids. The only real difference is that we know the (desired) pH, so know the H3O+ ion concentration. Also, [H3O+] is not equal to [conjugate base] in the Ka expression.

Tuesday, 14 October 2014

Equivalence Point

This is a very tough concept to get started on, but not too tough once you work out the concentration of the salt made at the equivalence point.

This video is very long, so it may be worth having a past exam question handy and regularly using the pause button:

Friday, 19 September 2014

Sketching Titration Curves

These look daunting, but it isn't actually very hard to get the general shape correct. We used a weak acid vs. strong base example:

Thursday, 18 September 2014

Titration Curves - Introduction

This was a whirlwind introduction to Titration Curves. Tomorrow, we will look at the calculations to determine the initial and final pH. Next term, we will look at how to calculate equivalence point volume and pH, and in the buffer region.

Tuesday, 16 September 2014

pH of Weak Acids

This is a very difficult concept. Every student should be able to write a hydrolysis equation, base dissociation expression (KB), and calculate KB from pKA or KA (provided in the question). Beyond that, it will take a lot of practice:

Thursday, 11 September 2014

Weak Acids

It is a bit trickier to calculate the pH of a weak acid. This is because they only partially dissociate. However, this means that these acids have an equilibrium constant (called the acid dissociation constant), which can be used to calculate the hydronium ion concentration:

The process for doing this is:
  1. Write the balanced equation for the dissociation with water.
  2. Write the acid dissociation expression.
  3. Substitute and solve
    • [hydronium] = [conjugate base] = x
    • [acid] = original concentration stated in the question (only partially dissociated)
  4. Calculate pH

Friday, 29 August 2014

Solubility and Solubility Product

The dissolving of a sparingly soluble solute can be represented by an equilibrium equation:

As this is an equilibrium, we can write an equilibrium expression, called the Solubility Product:

The smaller the Solubility Product, the less soluble a solute is.

Solubility is usually expressed in one of two units: grams per litre (g L-1); or moles per litre (mol L-1).

We can use the solubility of a solute to calculate the Solubility Product, and use the the Solubility Product to calculate the solubility of a solute. Once we master these two skills, we have a good understanding of solubility.

Ionic Product and The Common Ion Effect

Ionic Product is the same as Solubility Product, but when two solutions (of known concentration) are combined and the ions may form a precipitate. If Ionic Product is larger than Solubility Product, a precipitate does form. If they are equal, it is a saturated solution. If Solubility Product is larger, no precipitate will form.

Thursday, 28 August 2014

Aqueous Solutions Introduction - Species in Solution

This entire unit is about things dissolved in water - aqueous solutions. If they are not dissolved in water, we do not have to consider them (too much).

 The examples we need to work through by tomorrow are:

We also did a brief experiment to show why some salts are alkaliine or acidic:

Wednesday, 13 August 2014

Organic Chemistry Revision

We made hexagons to represent every functional group we studied in Organic Chemistry earlier in the year:

We are putting brief study notes on the back of these, and playing a few revision games with them:

Wednesday, 30 July 2014

Practical Electrochemistry

Today, we made some half-cells and combined them with a salt-bridge and a voltmeter. Then, we were asked to make sense of our observations.

Tuesday, 29 July 2014

Standard Electrode Potentials and EMF

This is a big idea and we will spend the rest of the week exploring it:

  1. What are standard electrode potentials?
  2. How are they used to find the EMF of a cell?
  3. What happens if a cell has a negative EMF?
  4. How can EMF be used to predict what would be observed in a redox reaction?
  5. How can standard electrode potentials be used to rank oxidant- and reductant-strength?

Friday, 25 July 2014

Electrochemical Cells - Introduction

We started the lesson by making an electrochemical cell using zinc chloride, a zinc electrode, copper (II) sulfate, a copper electrode. We found that this generated about 1.4V

Thursday, 24 July 2014


Electricity can be used to split compounds up into their elements. This is an example of oxidation-reduction.

Wednesday, 2 July 2014

Balancing Redox Half Equations

Another recap of Level 2 work...

Thanks to Sam for finding these FLASH CARDS
Logan found these FLASH CARDS which are also excellent.

Tuesday, 1 July 2014

Oxidation Numbers

This is just a recap of last year's work. We discussed the allocation of oxidation numbers:

Monday, 23 June 2014

Chemical Minds

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

Friday, 20 June 2014

Entropy and Gibb's Free Energy

I had a shocker trying to explain this. This video does a much better job:

As this has only been recently added to this topic, there are not many good questions out there. However, this one is pretty good:

Thank you to Logan for finding this good video too:

Wednesday, 18 June 2014

Hess' Law

After some debate about whether it should be Hess's Law or Hess' Law, we tried to get our head around this complicated concept:

Tuesday, 17 June 2014

Enthalpy of Formation

Here is the video I did last year, in case it is easier to understand:

Tuesday, 3 June 2014

Forces Between Particles

We started by looking at the types of solid we encountered last year. Then we focused on the forces between molecules. Last year, we called them "inter-molecular forces" but we need to be more specific this year.

Friday, 30 May 2014


This is only slightly more complicated due to the molecules/ions with 5 (or 6) regions of electron density around the central atom.

Wednesday, 28 May 2014

Lewis Dot Diagrams Recap

We went over the steps to draw these, and a few examples:

Shapes of Molecules

Today, we were introduced to the shapes of molecules/ions with 5 or 6 regions of electron density. Thank you to Ella, Bronwyn and Sarah for remembering to bring the lollies and toothpicks to make our models with!


Friday, 23 May 2014

Lewis Dot Diagrams

We started with a recap of Level 2 Chemistry:

Then, we learned about some examples which break the Octet Rule:

Finally, we learned about how to do Lewis Dot Diagrams for polyatomic ions:

Today was #PinkShirtDay in NZ; a day to raise awareness of, and express intolerance for bullying. #StopBullying

Thursday, 22 May 2014


Today was primarily a recap of Level 2 work. However, as well as knowing how to use E.N. to predict bond type, we also need to remember two trends and to justify them using our knowledge of atomic structure.

Below is the video of the key considerations being summarised for the class in 2013:

Friday, 16 May 2014

Transition Metals

Transition metals have some characteristic properties. We need to be able to explain these in terms of their atomic structure (radius and/or electron configuration).

Thursday, 15 May 2014

Ionisation Energy

Ionisation Energies are often used as further evidence to determine the structure of atoms, particularly their electron configurations. Trends in 1st Ionisation Energies also tell us a lot about the different orbitals and their relative distances from the nucleus.

Tuesday, 13 May 2014

Electron Configurations

The Bohr Model was useful in explaining the properties of the first 20 elements but falls down beyond Calcium. To understand the properties of the transition metals and other elements up to Krypton (36), we need to understand s, p and d orbitals.

  • s orbitals can hold up to two electrons
  • p orbitals can hold up to six electrons in 3 sub-orbitals
  • d orbitals can hold up to ten electrons in 5 sub-orbitals

The next thing to get our heads around is how to write the electron configurations for ions. We came up with the "penguin" theory (thanks Bronwyn!!). The penguins on the edge of the ice-pack get taken away by the seals first... The electrons farthest from the nucleus are removed first when cations are formed.

For homework, we have to write the electron configurations for:

  1. copper (II)
  2. iron (II)
  3. iron (III)
  4. zinc
We also have to infer why zinc ions form colourless/white compounds.

Monday, 12 May 2014

Planning Ahead

The image below shows a brief overview of what we will be covering in the next few weeks, and a more detailed plan of what content will be covered in the first few lessons:

Friday, 9 May 2014

Atomic and Ionic Radius

We started today by revisiting the Bohr Model of the atom, as it can be used to explain trends in atomic and ionic radii.

We are expected to be able to answer all of these types of questions before we move onto the next concept.