Choosing Indicators

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.

Buffers

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...

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.

Weak Acid vs. Strong Base

Weak Base vs. Strong Acid

pH - 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 K_b expression.
3. Calculate K_b from K_a
4. Calculate [OH^-] from the K_b expression (assuming [HB⁺] = [OH^-]
5. Calculate [H₃O⁺] from [OH^-], using K_W
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:

pH - 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.

pH - 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:

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

Predicting Precipitation

This was an overview of Ionic Product and we use it to predict if a precipitate will form. Then we looked at what happens when a common ion is added to a saturated solution, for example making a limewater solution more alkaline with sodium hydroxide.

Solubility Product and Solubility

Aqueous Systems Overview

Today was a bit overwhelming - an introduction to the entire Aqueous Systems unit:

Electrolysis

This is the last concept we need to learn about in this topic. It is the opposite to the theory of cells in every way except one:

Oxidation occurs at the Anode

Reduction occurs at the Cathode

Electrochemical Cells

Today we made a Zinc-Copper cell and measured its voltage. We then put a group of them in series to amplify the voltage - this is called a battery.

There is a convention for how we write a "cell diagram" for this, and we will explore that more next week.

Standard electrode potentials are measured by setting up the hydrogen half-cell as the anode. The redox pair that you want to know the electrode potential for is then set up as the cathode, and the voltage recorded - this is the electrode potential for this particular redox pair.

Standard Electrode Potential

Oxidation-Reduction

Today we recalled the work we had done on: oxidants and reductants; observations and half equations; and oxidation numbers:

Entropy

This video sums it up very nicely:

It is worthwhile knowing the sorts of questions we may be asked about entropy as well:

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Enthalpy Calculations - Enthalpy of Formation and Hess's Law

USING ENTHALPIES OF FORMATION

USING HESS'S LAW

The following is how we could use Hess's Law to solve the same problem as used for showing using Enthalpies of Formation:

Enthalpy Calculations - Experimental Data

Molar Heat of Fusion

We did an experiment to find the molar heat of fusion for water, by melting ice and recording temperature changes.

We found our results did not match the theoretical values due to our inability to control certain variables, such as energy transfer with the environment.

We then explored the concept of molar heat of fusion:

Students used this template to explore molar heat of vaporisation, next...

Energy Recap

The background of the work we are about to get into includes:

COLLISION THEORY

Specifically, we need to be aware that changing temperature has an effect upon the energy of the system. To keep things "fair", we need to compare things at the same temperature (or at a specific temperature).

ENTHALPY CHANGES

One of the key concepts we need to remember from last year is enthalpy change:

Intermolecular Forces

Today, we had the following tasks:

• Reading pp93-4; 96 in Continuing Chemistry
• Graphng task pp95-6 in Continuing Chemistry
• Homework sheets (Lewis structures etc.)

Lewis Structures and Shapes - Level 3

EXAMPLE QUESTION

This is how I would lay out my answer, allowing me to make a concise conclusion about the polarity of each molecule, so linking my knowledge to my decision about their respective polarity:

Lewis Structures and Shapes - Level 2 Recap

We need to remember how to draw Lewis Dot Diagrams:

Then, we need to understand the shapes that molecules (and polyatomic ions) can be:

https://figures.boundless.com/10397/full/vsepr-geometries.png

Once we have these mastered, we can look at Lewis Structures for polyatomic ions, exceptions to the Octet Rule, and shapes with more 5 and 6 for their Steric Number.

Periodic Trends

There are a couple of properties of atoms that have characteristic trends. We need to be able to discuss these trends in terms of atomic structure, specifically the forces acting within an atom that affect these properties.

ELECTRONEGATIVITY

1ST IONISATION ENERGY