Now that we have finished Module 3, I can finally write about how we tackled the module. There are two important laws covered early in the module, The Law of Mass Conservation and The Law of Definite Proportions. I thought it was odd that these laws were covered before the periodic table, but it seems that they are covered early in most textbooks that I glanced over.
The Law of Mass Conservation states that matter can not be created or destroyed; it can only change forms. Great! This makes sense when you think of a pot of water. Place a pot of water on the stove, add some heat, and notice what happens. The water boils away, but only because it changed forms. We didn’t destroy the water. We converted it to water vapor using heat. Wile uses the example of a fireplace. The math is easy because it is simple addition and subtraction. The totals must be equivalent on both sides of the equation. Simple algebra will solve for any unknown.
I have to stop here. Since I am talking specifically about the Apologia book, I need to make a note about his problem 3.1. He asks how much oxygen was used up by the wood while it burned. Ok. Fine. We know that fire needs oxygen and fuel to burn. But the words “used up” in his question are very misleading. The oxygen is used, but in the reaction it is converted to carbon dioxide and water vapor. Here he is saying that matter can’t be destroyed and then asking the kids how much oxygen is “used up”. It was very confusing to my daughter. I think it is confusing because they haven’t had any exposure to chemical equations. So, while the fire place is a good example that she could easily grasp, I needed to explain that oxygen was used and changed not “used up”. (Plus, it is highly likely that there was still oxygen in the room and the wood just burned away completely making it the limiting reactant which is a topic they will learn later. But geez, the whole thing bugs me.) I think it is pretty important that you make a couple more practice problems. You can use the same problem. Change the numbers and the amount of substance you are trying to find.
He then goes on to talk about the periodic table, which if you are like me, you have already done.
We then talked about the Law of Definite Proportions which states that the proportion of elements in any compound is always the same. This law is based on ratios. Hopefully, your student(s) has had some exposure to ratios and scale factor in their algebra because it becomes very helpful to do the problems.
I don’t think the math is difficult for these problems, but what helped my daughter was thinking in terms of scale factor. For example, if you have picked 3 bushels of apples, but have to pick 9 bushels of apples by the end of the day, you need to increase by a scale factor of 3. In Chemistry, if you know what you need to make 21 g of lime, but you want really want 55 g of lime, you need to increase each of your reactants by a scale of 2.6. My daughter was confused thinking she had to increase it by adding more calcium or something rather than just a scale – a number without a unit. Once I had her thinking in terms of scale factor it made much more sense.
Again. You need to give them more problems. Doing one on your own problem is not enough to fully understand the concept. Practice!
The rest of the module is fairly easy, but I expand on it a bit. It is increasingly frustrating to my kids when he says “you will learn more about this later”. He gives them a basic knowledge of ionic and covalent. Then tells them there are some exception, but don’t worry about it now. Gah! I guess it is necessary, but it is frustrating.
In this module, he gives the very basic idea of naming compounds. I love his paragraph about naming covalent compounds. (not really) NaCl will always be NaCl not Na2Cl or anything else. Why? Why, mom? Why not? Ugh. Of course they want to know. If he took the time to explain about how the compounds form, it makes it pretty easy that explains it pretty well. Since I took the time in the first module to explain the idea of electrons and charges of the elements, it wasn’t difficult. Ionic compounds want to be stable. The non-metal will grab electrons from the metal to fill up their outer shells. The metal is happy to give them away. Usually the metal now is happy because it doesn’t have any extra electrons in its outer shell and the non-metal is happy because its outer shell is filled up. It makes a lovely ionic compound. In NaCl, the sodium has one electron in its outer shell. It has a +1 charge. Chlorine has 7 electrons in its outer shell. It has a -1 charge. If Chlorine takes that one electron from sodium, everyone is happy and it is a stable compound. THAT is why NaCl is formed.
Take the time to practice this with the kids. Make other compounds. Try Potassium flouride. Potassium (a +1 charge) has one it wants to give away and flouride (a -1 charge) wants one. Ta-da! This is how to write ionic compounds. It gets trickier with the transistion metals. Here is a handy-dandy video for you to watch.
In his example, he gives the students Al2O3 to name. It is aluminum oxide. Why are there 2 aluminums and 3 oxygens? Because Aluminum has a +3 charge and Oxygen has a -2 charge. Aluminum is a metal in the 3rd group. That is how I know. Oxygen is a non-metal in group 6. It has a -2 charge. That is how I know. In order to make everyone happy, oxygen wants wants two to be full, but aluminum has three to give away. So another oxygen comes along to take that one, but it isn’t happy because it still needs one more. So, aluminum brings that one but still has two left over. This gives us our third oxygen that will take the last two. Does that even make any sense? Thus, there are two aluminums and three oxygens.
Make lots of practice problems. Lots and lots. They need the practice. To make it easy, stay away from the transistion metals. But if you are really brave, this video explains it for you.
I do not think it is too much to ask of the kids to understand how compounds are named. It isn’t difficult, and makes things easier later on.
Good luck!! One more website for you.
We did not go to module 4. We have moved into module 7. I will write about that when we are finished.