In 2014 I wrote this post in order to discuss some of the ‘odd/new’ topics on the AP exam in 2014. What follows in this post is essentially identical to that one, but with a few tweaks here and there, introduced since we are now 12 months further down the road.
These are brief comments that are expanded upon in my notes, and in the REA AP Chemistry Crash Course book that I authored.
Mass Spectrometry – See AP worksheet 01c and 01n. Probably limited to the identification of isotopes of elements and their relative abundance (and not organic molecules and their identification as in AP worksheet 01o), look out for the possibility of questions regarding diatomic molecules (Cl & Br are particularly popular examples). All species detected in the mass spectrometer have a positive charge, and m/z is plotted on the y-axis. Since z (the charge) is usually +1, the m/z value is the mass of the species (p43-46 Crash Course).
Capillary Action & Surface Tension – Both phenomena are related to the resolution of the various forces that are present. In the case of capillary action, adhesive forces act between the particles of the substance and the walls of the container, and cohesive forces are the IMF’s that act between the substance particles themselves. In the case of surface tension, the forces are all between the particles of the substance (p78 Crash Course).
Chromatography – Separation technique with the components of a mixture distributed amongst a moving phase and a stationary phase. The separation of a mixture depends on the relative affinity for each component of the mixture for either phase. Know how to calculate an Rf value (p67 Crash Course).
Coulomb’s law – Probable for this to be a persistent and recurring theme throughout the exam, where in the past the idea of ‘opposite charges attracting’ used to be sufficient. Although there will be no quantitative calculations involving the law, look out for references to the sizes of q1 & q2, the fact that opposite charges give an attractive force, that similar ones give a repulsive force, and the relevance of the magnitude of r2 on the size of the force (p96 Crash Course).
Potential applications include IMF’s, ionization energy and PES, ionic and covalent bonding (including relative sizes of lattice energy in a Born-Haber cycle), melting points of metals and bond length/strength in the potential energy diagram of diatomic hydrogen and other diatomic elements (p82 & p173 Crash Course).
Work – When a gas in a cylinder expands it does work on the piston that can be calculated via PdV. The work and PdV terms should have different signs (since one thing is doing work on the other), but the magnitude of each will be the same, i.e., the amount of work done is the same as the amount of work received (analogous with conservation of energy). I cannot see any calculations being asked about this. Work is similar to heat (which is the transfer of energy via thermal interaction), since both transfer energy from one system to another. Gibbs Free Energy is defined as the energy available for doing work (p167 Crash Course).
Particulate diagrams – Expect to see multiple references to particulate diagrams on the exam. You could be asked to draw them, but perhaps more likely (because of the difficulty of grading diagrams that you have drawn yourself), asked to interpret them. There are many areas where this could manifest itself, with the common theme being, “do you know what is happening at the particulate level?”
Examples include gas particles interacting with one another and the walls of the container, dissociation of particles in weak acid/base scenarios, relative number of reactant and product particles in equilibrium positions, particles interacting via IMF’s (especially H-bonding in biological applications), dissolution of ionic salts, i.e., the ion to dipole interactions with water, and many others. Be prepared to talk about interactions at the particulate level. (p97 Crash Course).
Biological applications – It is unlikely that you will be required to know any biology as such, in order to answer any questions on the AP chemistry exam, but please be aware of the following;
- General formula for amino acids and that proteins are chains of amino acids
- -COOH (acid) & –NH2 (base) groups appear in many biological molecules and can exchange H+
- Coupling of biological reactions that are thermodynamically unfavored and favored (positive and negative ∆G) can create overall processes that are favored (merging equations/ adding ∆G’s)
- Enzyme catalysis (likely no more than catalysts in disguise with little or no relevance to the enzyme part of the phase)
- Hydrophobic (water hating and non-polar) and hydrophilic (water loving and polar) areas of molecules
It may be that a question looks like a biology one form the outside, but in reality it is chemistry (for example IMF’s), tied up in a biological context. The important thing is to read the question, discern the chemistry, and not be distracted by thinking that it is a biology question that you nothing about (e.g., p78-79 Crash Course).
The same is true of questions that appear to be organic in nature (see questions 6 & 7 from 2014) but are really about some other aspect of chemistry.
Semi-conductors – Know the common semi-conductors (e.g., silicon) and briefly understand the role of valence band, conduction bands, band gap and n-type/p-type via doping (p102 Crash Course).
Alloys – Know the difference between substitutional and interstitial alloys (p99 Crash Course).