I’m starting a series of blog entries on the Big Ideas of the College Board’s new AP Chemistry Curriculum [link] with Big Idea 2 because it’s a little easier to tackle off the bat than Big Idea 1. Big Idea 2 states:
“Chemical and physical properties of materials can be explained by the structure and the arrangement of atoms, ions, or molecules and the forces between them.” – APCCF
A theme throughout this Big Idea is that students can move back and forth between particulate level representations, symbolic representations and macroscopic observations (which is Science Practice 1). In other words, Johnstone’s triangle − the idea that expert students can move back and forth between the various representations − becomes very important.
For example, if the student is given LiBr as the formula of a substance they are expected to determine that it will be an ionic compound (LO 2.17), which means it will be a brittle solid (LO 2.19) that dissociates when it dissolves in water because of attraction between the ions and polar water molecules (LO 2.14), forming a solution that conducts electricity (LO 2.19 again). Given NH3 as a formula a student is expected to determine it will be a molecular compound that dissolves in water because it is polar and forms hydrogen bonds with water (LO 2.15) but does not conduct electricity because it does not form ions but remains discreet molecules containing one N and three H atoms. The student could also be asked to work with or draw pictures of all of this (LO 2.8) or design an experiment to determine the type of bonding present in an unknown solid (LO 2.22). In other words, they would have to know you could try to dissolve the compound in water and, if it dissolved, test the conductivity to determine whether the compound is molecular or ionic.
Forces play a large role in the Big Idea, and it’s clear that student need to be able to differentiate between various strengths of forces and the directionality of those forces. I’m particularly intrigued here by learning objective 2.3:
“The student is able to use aspects of particulate models (i.e., particle spacing, motion, and forces of attraction) to reason about observed differences between solid and liquid phases and among solid and liquid materials.”
I could see a question such as “Why can you move your hand through air and liquid water but not NaCl. Justify your answer by discussing the interparticle interactions in each substance.” Air is a mixture of gas molecules and water is composed of H2O molecules. In the case of air the interaction between particles are weak dispersion forces, easily overcome by the motion of your hand. In water the interparticle attraction is still relatively weak hydrogen bonds, but enough that you can feel the viscosity. For solid NaCl however, the particles are much closer together than in air or liquid water and more strongly attracted to one another because each ion is a full +1 charge and they are stacked in a lattice structure. This means the particles cannot easily move when you wave your hand at them and viola!, it behaves as a solid.
The one part of this Big Idea I am still trying to wrap my head around teaching is LO 2.25 which deals with alloys:
“LO 2.25 The student is able to compare the properties of metal alloys with their constituent elements to determine if an alloy has formed, identify the type of alloy formed, and explain the differences in properties using particulate level reasoning.”
The part of me that studied a little materials science in graduate school feels this is too much for AP/general chemistry level knowledge. I also don’t have a good way to teach this yet. The key word is yet! I am going to work on a POGIL activity that addresses alloys, which I will post and discuss on this blog. Please feel free to bug me if I haven’t done it yet.
Comments, questions, and (polite) arguments are encouraged. Please leave me a note in the comments below.