Determining the empirical formulas of ionic compounds based on charge balance is often a challenge for beginning chemistry students. Many visual aides have been developed for this purpose, from repurposing commercial interlocking bricks to custom 3-D printed bricks. This article describes yet another option– upcycled can carriers. These high-density polyethylene (HDPE) carriers, shown in Figure 1, are especially popular in in the craft brewing industry.
They have several characteristics that make them an attractive candidate for classroom upcycling.
- They stack like interlocking bricks (although they do not actually lock).
- The four- and six-can versions can generally be used interchangeably.
- They come in a wide variety of colors, allowing color-coding of charges.
- They can be easily modified to their desired size with a utility knife.
- The HDPE can be easily sanitized in between uses.
- Their use can be an introduction to a discussion about reusing vs. recycling.
Putting them together:
- Ask you friends, colleagues, neighbors, etc. to save their carriers for you. Maybe even agree on a particular local brewery to patronize. It turns out there are slight differences in the carriers from different manufacturers, which effects how well they stack.
- Whichever color you have the most of, those are the bottom ones. For the purposes of this post, these are red.
- A utility knife, glue (cyanoacrylates work nicely), and a permanent marker will be needed for assembly.
- The length of each ion corresponds to its charge.
- The bottom of cations are 2 caps wide, and the top 1 cap wide.
- The bottom of anions are 1 cap wide and the tops are 2 caps wide.
- Glue the top and bottom pieces for each ion together.
- Trim off and recycle the excess.
Figure 3 shows the construction of a 2+ ion. The white carrier is trimmed to a width of one cap and a length of 2 caps. The red carrier, which will be the bottom piece, has a length of 2 caps and a width of two caps. The two carriers are then glued together. Figure 4 shows the construction of a 2- ion.
Positive ions (cations) on the left, negative ions (anions) on the right. No judgment here- that just mimics the order we write them in ionic compound formulae.
Combine positive and negative ions by placing the top portions of the anions over the bottom portions of the cations until the total lengths of the cations and anions match. There should be no visible red caps showing from the cations, and each of the anion caps should have a red cap under it. The formula is then said to be balanced. Figures 5 and 6 show an incorrectly and correctly balanced CaCl2. Figures 7 and 8 show examples of 1+/2- and 2+/3- compounds, respectively.
Guided Inquiry Questions
Construct compounds with the following charge combinations:
1+/1- 2+/2- 1+/2- 2+/1- 3+/1- 3+/2- 1+/3- 2+/3-
- How many of each ion is present in the balanced formula?
- What is the total "length" of each balanced formula? For example, the “length” of CaCl2 is 2 caps.
- What is the relationship between the charges of the ions and the length of the balanced formulas?
- What formula would you expect for a compound composed of a Pb4+ ion and a PO43- ion?