Archive for category Chemistry

Chemistry Assumed Knowledge

The following knowledge is assumed in GAMSAT chemistry section 3 questions. This is not necessarily an exhaustive list, nor is it necessarily necessary to learn everything on this list to get a good mark, though I’d encourage it.

Learn all this and you’re doing well, though I would encourage you to go beyond this list as it’s not possible to guarantee what will be on the GAMSAT year by year.

Gibbs Free Energy in Exothermic and Endothermic Reactions

Gibbs free energy is a useful concept which lets us determine how likely a reaction is to take place spontaneously. It is defined using the following equation:

ΔG = ΔH – TΔS

  1. A reaction will take place if it creates a reduction in Gibbs free energy, ie ΔG < 0.
  2. Gibbs energy is reduced if H is reduced. H is the heat energy locked up in the system.
  3. Gibbs energy is reduced if S is increased. S is a measure of Entropy or Disorder.

A reaction is most likely to happen if ΔH < 0 and ΔS > 0, ie if the system enters a lower energy state and becomes more disordered.

Exothermic and Endothermic reactions

  1. In an exothermic reaction heat is given out so heat energy in the system is reduced. ΔH <0. This is favourable.
  2. In an endothermic reaction heat is taken in so heat energy in the system is increased. ΔH >0. This is unfavourable

An Endothermic reaction can still take place if it results in a large enough increase in entropy, ie ΔS > 0.

Water can melt even though this is an endothermic reaction since liquid water is less organised and therefore has a higher entropy than solid crystalline ice.

Polythene will shrink in an oven even though this is an endothermic reaction (it requires heat to take place). This is because the shrunken polythene is less ordered and has higher entropy.

For more information on answering these sorts of questions see the Bodner Research Web

Structural Isomers

Two molecules are structural isomers when they differ only in their shape.

Ionisation Energy

Ionisation Energy

The amount of energy needed to steel an electron away from an atom.

Ionisation energy increases from left to right as the atoms are heavier and the nuclei have more charge. Metals on the left of the periodic table tend to loose electrons as they have a low ionisation energy

Box notation can be used to show electrons in subshells.
Valence electrons are much easier to remove than core electrons

Review

http://www.s-cool.co.uk/alevel/chemistry/atomic-structure/ionisation-energy.html

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Enantiomers

Enantiomers are mirror image molecules which have different effect. For example spearmint and caroway are enantiomers.

Enthalpy

Enthalpy is heat

The thermodynamics song

Oh you can’t pass heat from a cooler to a hotter! You can try it if you you like but you’re far better notter!

Thermodynamics, entropy and Gibbs Free Energy introduction from the Chem Guy

Gibbs free energy and thermodynamics are topic which come up year on year. The Chemistry Guy gives a good introduction to Thermodynamics.

Ionisation Energy

The amount of energy needed to steel an electron away from an atom.

Ionisation energy increases from left to right on the periodic table as the atoms are heavier and the nuclei have more charge. Metals on the left of the periodic table tend to loose electrons as they have a low ionisation energy.

Box notation can be used to show electrons in subshells.
Valence electrons are much easier to remove than core electrons

See SCool for more information

Ionic and Covalent Bonding

Intermolecular forces

See S-Cool for more

Acid Base Theory

An acid plus a base gives a salt plus water

Valence Shell Electron Repulsion Theory (VSEPR)

Subshells

  1. Electrons inhabit regions of space surrounding the electron called subshells
  2. The outermost electrons are known as valence electrons and are can be used in covalent bonding.


Watch chemistry dissecting the atom 4 shells subshells and orbitals in How to Videos and Webisodes | View More Free Videos Online at Veoh.com

Equilibrium

Equilibrium constant k

Stereoisomers

Carbon Chains

Chemguy explains organic chemistry. We love Chem Guy! www.chemguy.com

  1. Organic Chemistry is Carbon Chemistry, Molecules made of chains or rings of carbon
  2. Organic molecules are commonly represented using a chemical formula, a structural formula or a condensed structural formula. These three forms are equivalent.
  3. The Structural Formula is referred to as the Fisher Structure in gamsat.

Aliphatics and Alkanes

  1. Meth = 1
  2. Eth = 2
  3. Prop = 3
  4. But = 4
  5. Penta = 5

Simple molecules like this are called Alkanes.

Representing Branches

Side chains can be represented using formulae like 2 ethyl butane or 2,4 diethyl pentane.

Models and Bonding

Alkenes

  1. Hydrocarbons with carbon double bonds are called Alkenes
  2. Alkenes are unsaturated as they are not maxed out with hydrogen molecules.
  3. Examples of Alkenes are Propene or Butene