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

Mouse Embryos

This question requires you to work out which embryos are male and which are female based on the information about surrounding embryos. It’s essentially a minesweeper style problem. Embryo 10 is the most feminine of those given because it is female :)

Primary sexual characteristics are still determined by genotype and are not overridden by hormones.

Electronegativity

This question is about hydrogen bonding. A larger atom when covalently bonded to a hydrogen atom can pull the hydrogen’s electron away leaving the hydrogen nucleus unshielded creating a polar molecule. The degree to which this happens is detemined by electronegativity.

Patient on a bed

This is a simple center of mass problem hat requires very little maths. It requires you to realise that the center of mass is not the center of the bed, in fact it lies closer to point Y with a ratio equal to the ratio between the force at point X and point Y.

1 KG = Approximately 10 Newtons (N)

Find the Redox

This question simply requires you to count positives and negatives and notice any changes.

Heat and Respiration in Insects

This question requires you to correctly interpret the graph and to understand that it’s angle corresponds to rate of change.

Saturated Fatty Acids

  1. This test is a simple logic puzzle couched in organic chemistry terms but you’ll still need a basic knowledge of organic chemistry to understand the terms.
  2. The formula for Palmitic acid can be rewritten C15H31COOH.
  3. From this we can infer that to be saturated a fatty acid must have twice as many plus one hydrogen atoms as oxygen atoms.
  4. For each double bond that is present, 2 hydrogens can be removed from the chain.

Kinematics in 100M Run

This question is about kinematics and motion with constant acceleration. For a good introduction to kinematics see the Physics Fundamentalized series of podcasts.

Activation Energy

Activation Energy is the energy you need to put into a system before the reaction will take place.

Unit 15 – Osmosis

In pure water water will be quickly taken across a semi-permeable membrane until an equilibrium is reached or the cell bursts. This is a statistical phenomenon, water moves both ways but moves most quickly from the region of high concentration to the region of low concentration.

Unit 16 – heterozygousity and cystic fibrosis

The chance that a couple will both be heterozygous is 1/20 x 1/20 or 1/400. The chance that a baby from such a couple will be homozygous is 1/4 so the combined chance is 1/400 x 1/4 = 1/1600.

Unit 34 – Logarithmic Decline in Salmonella Population

This question requires you to realise that the results are presented on a log scale. The greatest decline occurs on day 4 even though the line is not as steep because the difference between 10<sup>7</sup> and 10<sup>6</sup> is 10 times greater than the difference between 10<sup>6</sup> and 10<sup>5</sup>.

The reduction in size of the normal bacterial population is much closer to 100 000 000 than to 1 000 000

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.

Carboxylic Acids

  1. Carboxylic acids have the Carboxylic acid functional group at the end
  2. End in -oic acid, eg Methanoic Acid.
  3. Methanoic Acid is otherwise known as Formic Acid
  4. Ethanoic Acid is otherwise known as Acetic Acid

Redox (Reduction Oxidation) Reactions

  1. Oxidation is the loss of electrons.
  2. Reduction is the gain of electrons (reduction in charge)
  3. An oxidising agent gains electrons and so causes oxidation. Oxygen is an Oxidising agent as it can compete it’s outer shell by gaining two electrons.

Inverse Square Law

  1. The inverse square law applies to gravity and magnetic charges.
  2. force = sum of charges / distance2

Fischer and Haworth Projections

Note the following molecular diagrams are NOT equivalent.

Fischer projections

  1. Generaly represent all the atoms
  2. use lines to represent bonds

eg.

D-glucose_Fischer

see http://en.wikipedia.org/wiki/Haworth_projection for more.

Haworth Projections

  1. Visualise the molecule in three dimentions so more commonly used for ring molecules.
  2. Carbon and Hydrogen atoms are assumed and are not drawn

eg

Glucose_Haworth

See http://en.wikipedia.org/wiki/Haworth_projection for more detail.

This image is used under the Creative Commons Attribution ShareAlike 3.0 License.

Chiral Carbon

A Chiral Carbon is one which does not exhibit symmetry. For more information and a test see:

http://www2.vuw.ac.nz/staff/paul_teesdale-spittle/organic/chiral_web/main.htm

Bell Jar Lung Model

This question requires a basic understanding of lung/chest/diaphragm anatomy.