The Problem: Students confuse subscripts and coefficients
In theory, I only help with the chemistry labs this semester. But, I or one of my tutors, teach a recitation for Gen. Chem 1 or Gen Chem 2 every other week. They have been learning how to balance chemical equations and it just wasn't clicking for them. So, at this week's recitation I explained the concept of having the same amount of each type of chemical on the left as you do on the right.
But, they kept making the same mistake. They were just slapping on 2's for coefficients where there should have been a subscript to balance the charge on the molecule.
Solution #1: Do they know how to properly balance charges on chemical compounds? If not, click here for the explanation and here for the worksheet on that topic.
So, I started drawing pictures. It wasn't pretty, but it started to get the idea across that there are rules for balancing equations. I explained coefficients and subscripts are not interchangeable. Coefficients are for making more of one type of molecule, like 2 H2O molecules. Subscripts can only be added at the beginning of the problem when you are balancing charges within a molecule, like within water there has to be 2 hydrogens to every 1 oxygen otherwise it could be peroxide!
Then I drew out an equation on the board and we made tally marks and counted carbons, oxygens, etc on both the right and left side of the equation.
After this I decided to create a neat and orderly worksheet that shows how to balance equations visually so students can see these differences easier. Click here for the free version that is one page long with answer key. Click here for the paid version that is 3 pages long and has more difficult problems as well. They both work well for homework, classwork, or chemistry class activities.
The other issue I've noticed is that if students are given a problem that says to balance the reaction of "hydrochloric acid and sodium hydroxide" They tend to freeze.
Their brain suddenly shuts off.
And they accuse you of giving them...A WORD PROBLEM.
Hi! I'm CoScine Creative. I have developed and run a tutoring center at a small college. I also teach some of their algebra and chemistry courses. And I will neither confirm nor deny pranking my students by pretending to be one.