I’ve just finished grading a test on bonding done by my AP students, and as usual, there are several issues relating to the phraseology and wording of their answers that need to be corrected. I’ve addressed these problems previously here (and you should read that post first), and I’ve also written similar posts in relation to periodicity and kinetics, too. Here are a few things that came up this time around, they may or may not have been dealt with in that earlier post.
- Phrases like, “water molecules are held together with hydrogen bonds“, and “carbon dioxide molecules are held together with LDF’s” are problematic to me because the words ‘held together with‘ are ambiguous. YES, I know that you (my AP student) are talking about the inter-molecular forces present, but the atoms within the molecules are of course, ‘held together with‘ covalent bonds. In short, when describing the forces attracting one water molecule to another, it is better to say, ‘water molecules are attracted to one another with hydrogen bonds’. Be careful!
- Just because two substances have the same type of inter-molecular force, don’t assume that by talking about one of the substances, that I automatically know that the same IMF exists between the particles of the other. A good example of this issue is illustrated by the AP question 1988, 5, that asks students to describe the difference in boiling point of neon and xenon. Just because you tell me that neon has smaller atoms, with less electrons, less surface are and that those atoms are less easily polarized leading to weaker LDF’s between the atoms so less energy is required to separate them, and therefore a relatively low boiling point results (all true, relevant and good stuff), none of that implies that Xenon also has LDF’s, or that those LDF’s are weaker than those in neon! This is why I often suggest that many of these answers start with the explicit identification of the IMF’s present in each substance before going on to talk about the differences that cause the differences in properties.
- Talking of the noble gases, they are monatomic gases, and hence one should refer to their atoms and NOT their molecules. This leads us to the another common issue i.e., the overuse of the word molecule. ‘Molecule’ is inappropriately used when talking about ionic compounds. I suppose that you may get away with it on the AP exam where a question is focusing on some other aspect, but I cannot abide phrases such as, “the sodium chloride molecules’ – it’s wrong and it drives me nuts.
- It is not a complete answer to describe differences in melting point of ionic substance by simply referring to the relative sizes of the ions, you have to say why that matters. For example, to say that NaCl has a higher melting point than KCl ‘because Na ions are smaller’ won’t cut it. Saying, ‘because Na ions are smaller and can therefore get closer to the chloride ion, reducing the distance between the ions in the Coulomb’s law relationship, and thus resulting in a larger electrostatic attraction’, will. I also like arguments based upon charge density.
- Don’t assume that the examiner knows the relative strengths of IMF’s. Of course they do, but you should clearly state that YOU know that, too!
If you put this post together with the one from 2013, many pitfalls relating to your answers to bonding questions should be avoided.