ENERGETICS AS/A2 KEYWORDS

Keyword	Definition
Hess's Law	The enthalpy change for a reaction regardless of the route taken to get from reactants to products (and vice versa.)
Temperature	The kinetic energy of particles in a system. The kinetic energy that any one particle could have.
Heat	The total energy of a given amount of substance.
Standard Conditions	101,000 Pa and 298K
Mean Bond Enthalpy	The energy required to break/overcome 1 mole of gaseous covalent bonds - averaged across many other (different) compounds containing the same bond.
1st Ionisation Enthalpy	The heat energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous 1+ cations.
2nd Ionisation Enthalpy	The heat energy required to remove 1 mole of electrons from 1 mole of gaseous 1+ cations to form 1 mole of gaseous 2+ cations.
1st Electron Affinity	The enthalpy change when 1 mole of atoms gains 1 mole of electrons to form 1 mole of gaseous 1- anions.
2nd Electron Affinity	The enthalpy change when 1 mole of 1- anions gains 1 mole of electrons to form 1 mole of gaseous 2- anions.
Enthalpy Change	The heat energy change measured at a constant pressure.
Enthalpy of Formation	The heat energy change when 1 mole of a compound is formed from its constituent elements where both the reactants and products are in their standard states.
Enthalpy of Combustion	The heat energy change when 1 mole of a compound completely reacts with oxygen and all products and reactants are in their standard states.
Enthalpy of Atomisation *	The enthalpy change when an element in its standard state is transformed into one mole of gaseous atoms.
Lattice Enthalpy of Formation *	The enthalpy change when 1 mole of a solid ionic compound is formed from its constituent ions in the gas phase.
Lattice Enthalpy of Dissociation	The enthalpy change when 1 mole of an ionic solid compound dissociates/broken up into its constituent ions in the gas phase.
Enthalpy of Hydration	The enthalpy change when 1 mole of aqueous ions are formed from the same ions in the gas phase.
Enthalpy of Solution	The enthalpy change when 1 mole of an ionic solid compound completely dissolves in a large enough amount of water such that its dissociated/dissolved ions are well spread out/separated in the solution - i.e. they do not interact.

CHROMATOGRAPHY

So, chromatography, the first thing you'd want to consider (which will apply throughout the rest of this topic) is the property of solubility.
The whole point and use of chromatography is to be able to separate out different chemicals that you'd find in a mixture of substances i.e. ink. If you can understand that, understanding the topic won't be much harder.

How it works - Paper Chromatography

The way chromatography works is that you separate the mixture using the different chemicals' levels of solubility (i.e. how well they can dissolve in a solvent).

In simple terms, in paper chromatography, as the solvent (something that dissolves another substance) seeps up the paper (the paper absorbs it), the solvent carries the chemicals with them. The diagram below will help:

 

 

From the diagram, 'a' is the distance that the solvent (usually water) has travelled (or has been absorbed) up the paper. 'b' represents the distance that one of the chemicals has dissolved up the paper with the solvent.
The base line is where all the chemicals are placed evenly to dissolve and be tested.

And all of this happens inside a large measuring cylinder or container.


What I mean by 'be tested' is that the diagram also shows a formula regarding 'Rf', this is used to indicate what type of chemical it is.
You calculate the Rf value by dividing the distance 'a' by the distance 'b' (on the image) and that value can tell you (from a database) what type of chemical that is.

And that's how you distinguish chemicals from a mixture/substance.

Step by Step Paper Chromatography;

1. Draw a line near the bottom of a sheet of filter paper, not regular paper - (this is the Base line)
2. Add spots of the different chemicals/dyes to the base line at equal intervals (as shown in the picture above.)
3. Now place the filter paper in a container of shallow solvent (i.e.water - depending on how well the chemical can dissolve) - Make sure that the solvent doesn't touch the chemical spots because, the chemicals.
4. Place a lid on the container to stop the solvent from evaporating.
5. The solvent will seep up the paper carrying the chemicals with it.
6. The chemicals will move at different rates depending on how soluble they are/how well they can dissolve.
7. The end result will show a pattern of spots called a Chromatogram.

How Chromatograms are used to identify Dyes.

A diagram to show chromatograms for all dyes.

Scenario: say you have an unknown brown dye and you're trying to find out what type of chemicals are present in the dye, you would use chromatography!

1. Firstly you make a chromatogram for the brown dye - (you would repeat the steps above for the brown dye)
2. Then you would make chromatograms for other dyes that you suspect would be in the brown dye. (say you chose purple, green and yellow.)
3. Finally you'd compare all the chromatograms and see which dyes are in the brown dye.

So from the diagram above we can conclude that the yellow and the purple dyes are both in the brown dye because on the chromatogram the purple and yellow chemicals line up level with all of the brown chemicals and they are of the same colour.

Thanks for reading! Hope this clarified any uncertainties in chromatography. :)

References:

Paddy Gannon - 2005CGP - Edexcel Chemistry - Pg 6 - Chromatography.