Friday, 16 October 2015

Equivalence Point pH

STEP ONE: Write a balanced equation for what has happened at the equivalence point. This is not an equilibrium reaction, it is neutralisation.

STEP TWO: Use the n, c, V relationship to calculate the concentration of the conjugate being formed in the neutralisation reaction.

STEP THREE: Write an equation for the dissociation of the conjugate with water.

STEP FOUR: Use the weak base (or weak acid) calculations, as appropriate, to find the concentration of hydronium ions.

STEP FIVE: Use this concentration to calculate pH.

Thursday, 15 October 2015

Titration Curves

During a titration, the pH does not change uniformly. There are key points that can be calculated, and the remainder of the curve can be sketched from these:

Initial pH

This example starts with a weak acid in the conical flask. If it were a weak base, the calculations are a lot harder, but you can refer to THIS for how to do that calculation.

Equivalence Volume and Half-Equivalence Point

This relies on a good memory of the titration topic from last year. Once you know the equivalence volume, you are ready to mark the pH of the half-equivalence volume (pKa = pH).

Final pH

This is very easy if you remember how to calculate the pH of a strong base (in this example). Put all of these points together to sketch the shape of the curve.

Equivalence Point pH

This is very hard and seldom asked in NCEA. Having said that, maybe this is the year that it will be asked...

We will cover this in Friday's lesson and dedicate an entire blog post to it.

Saturday, 10 October 2015


The first things we need to master about buffers:
  1. Define a buffer
  2. Explain how a buffer works
  3. Calculate the pH of buffer solutions

On Mondays lesson, we can use our workbooks (Continuing Chemistry), and the internet as our resources, as well as the following videos:

Our next step is to work out how to make a buffer of a desired pH...

Thursday, 17 September 2015

pH of Weak Bases

Whenever these questions are asked, they are worth Excellence. This is because there are six key steps to remember, and they need to be remembered in order!

  1. Write out the equation for the base acting as a an alkaline solution.
  2. Write a Kb expression.
  3. Calculate Kb from Ka
  4. Calculate [OH-] from the Kb expression (assuming [HB+] = [OH-]
  5. Calculate [H3O+] from [OH-], using KW
  6. Calculate pH

This video is very long, so use it to work through an example, rather than trying to learn it all in one go:

Monday, 14 September 2015

pH of Weak Acids

Weak acids only partially dissociate, so how do we calculate their pH?

We were encouraged to work through pp180-181 in Continuing Chemistry to check whether or not we have understood this.

Thursday, 10 September 2015

pH of Strong Acids and Bases

This is just a recap of last year, but these skills are vital for moving forward in this topic:

Here is the concept being taught to a Year 12 class:

Tuesday, 8 September 2015

Acids, Bases and Salts

We started with a recap of the Bronsted-Lowry definitions of acids, bases and amphiprotic species:

Then, we looked at the pH of some salts, comparing them to a control of NaCl (known to have a pH of 7.0):

We need to write chemical equations for the ions to justify the observed pH values (alkaline or acidic):

For example:
HCO3- + H2O <=> H2CO3 + OH-
This equation shows an increase in [OH-], which is expected as the pH > 7.0