Thermochemistry Overview

The key concepts of thermochemistry were introduced today and will be explored over the next week.

Polarity Videos

Molecule Polarity

This was just an introduction to polarity. We will be exploring this more in the next lesson.

Our task was to determine the polarity of the molecules we did ball-and-stick diagrams for in yesterday's lesson.

Here are a couple of observations we need to be able to explain, using molecular polarity:

1. Polar solutes dissolve better in polar solvents. Non-polar solutes are only sparingly soluble in polar solvents. Why? The example we were given was ammonia (NH₃) dissolving in water.

   

   SOURCE: UC Davis ChemWiki



SOURCE: http://www.ausetute.com.au/lewisstr.html

2. Polar molecules have higher melting and boiling points than non-polar molecules with a similar-sized electron cloud. For example, methane (M_R = 16.0 gmol^-1) is a gas at room temperature while water (M_R = 18.0 gmol^-1) is a liquid. Why?

   

   SOURCE: http://chemsite.lsrhs.net/bonding/lewis_dots.html

   
   

SOURCE: http://2012books.lardbucket.org/books/principles-of-general-chemistry-v1.0/s15-02-intermolecular-forces.html

VSEPR Theory and Shapes

In Level 2 Chemistry, we only worry about molecules with a maximum of four regions of electron density (bonds and/or lone pairs of electrons) around the central atom.

NOTE: Error at 8:30. Tetrahedral bond angles are 109.5°, not 107°.

However, the bond angle in water is indeed around 107° due to the increased

repulsion caused by the two lone pairs of electrons.

Our task today was based around p p127-8 in Beginning Chemistry, but we used marshmallows, lollies and toothpicks to model the molecules, instead :)

Lewis Dot (Electron) Diagrams

The book explains these very well, but uses a slightly different method for working them out than shown here:

They are most useful for showing the covalent bonding in molecules:

We will spend the next lesson working through examples, including those in our homework on Moodle.

Electronegativity and Substance Type

Ionic compounds have ionic bonds between the ions. Molecular compounds and elements have covalent bonds between the atoms (but inter-molecular forces between the molecules). How do we know if the bonds are covalent or ionic?

We use the electronegativity difference between the atoms involved in the bond.

SOURCE: http://images.tutorcircle.com/cms/images/44/electronegativity-of-elements.png

Sometimes, the electronegativity difference is unexpected and does not match the physical properties, such as electrical conductivity or melting point. In these cases, we need to justify our choice (ionic or covalent bond; ionic or molecular substance) based upon all of the evidence we have.

We are not always given electronegativity values in an assessment. Therefore. we need to understand the trend in electronegativity (on the Periodic Table), so we can infer electronegativity difference in a compound:

NCEA Question - Solids

We struggled a bit with the homework tasks over the holidays, so had to do the following question in 15 minutes in class today:

If we struggled, the following video has been made available for us, explaining how to answer the questions:

NOTE: The video did not capture the entire screen, so some of the text is missing.

The full answers can be seen in our class OneNote.