Archive for category Reasoning in Biological and Physical Sciences

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

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.

Introduction to parallel and series circuits

A common question in Gamsat is to estimate the current flowing in a portion of an electrical circuit.

The video below (not embeddable) is a good introduction to parallel and serial circuits

http://www.youtube.com/watch?v=E8AZBR8Zz04

Ohms Law

Current = Voltage / Resistance

Currents in Parallel and Series Circuits

To calculate the current flowing across each globe, for each circuit:

  1. Calculate the total resistance by reducing the circuit using ohms law.
  2. Resistors in series will all have the same current flowing through them.
  3. Resistor in parallel will have a portion of the current flowing through them, so if there are 3 resistors in parallel and they are all the same then each will have 1/3 of the total current flowing through it.

Stereoisomers

Alcohols

  1. Alcohols contain an OH group.
  2. This usually makes them polar creating strong intramolecular hydrogen bonds so they have much higher melting and boiling points.
  3. A Primary Alcohol has the OH group at the end of the molecule.
  4. A Secondary Alcohol has the OH chain as a branch. The carbons either side are often referred to as R for Rest.
  5. A Tertiary Alcohol is a secondary alcohol which has another group attached to the same carbon as the OH.

Ketones, Aldehydes, Ethers and Amines

  1. An Aldehyde has a double bonded Oxygen at the end of the Carbon chain
  2. Ketones have a double bonded Oxygen side group at a position other than the end of the carbon chain.
  3. Ketones have -one at the end of them, eg Butanone
  4. Ethers have an Oxygen in the middle of the carbon chain, eg Methyethylether
  5. An Amine has an NH2 side group. Eg. Amino Ethane.

Mendelian Genetics

Log scale