Lesson 10: Balanced Equations (Obj.10-11)

(a) Potassium bromide. In order to write a balanced equation that represents the synthesis of potassium bromide, we need the formulas of the elements potassium and bromine on the left and the formula of potassium bromide on the right side of the equation.

We need to know the formula for potassium bromide. Well, potassium bromide consists of potassium and bromine; potassium is a metal, bromine is a nonmetal. We can figure that potassium will lose electrons. By looking at the periodic table we can see that potassium has one valence electron, so it will lose one electron and form a +1 charge. Bromine is a nonmetal which will gain electrons. By looking at its position on the periodic table, we can see that it will gain one electron and form a -1 charge. So with a +1 charge and a -1 charge, the correct formula for potassium bromide is KBr. So we write that as the product on the right side of the arrow.

Now what is potassium bromide made from? It's made from potassium and bromine. On the left side of the arrow we have to have the proper formulas for potassium and bromine. Potassium is a metal with metallic bonding and therefore it is a network material. So we will just use the symbol to represent potassium and write down K.

Bromine, on the other hand, is one of those diatomic elements and so we need to write down Br₂. There are a couple ways of coming up with this. One is to just memorize that bromine is one of the diatomic elements. The other is to go through the thought process that since bromine is a nonmetal, it might form a network or molecules. It does form molecules (because it is not one of the few covalent networks) and the molecules that are formed consist of two bromine atoms hooked together. So Br₂ is the correct formula.

So we know have the skeleton equation of  K + Br₂ KBr. In order to balance that, we have to make note of what is not balanced and as written. We have one potassium on each side of the arrow, and we have two bromines on the left and one on the right. If we start with two bromines we need to finish with two bromines, so we put a 2 in front of the KBr. That gives us two potassiums on the right, so we also need to put a 2 in front of the K on the left side of the equation. So that balances the equation and gives us 2 K + Br₂ 2 KBr.

(b) Aluminum oxide. The formation of aluminum oxide will start from aluminum and oxygen so we have aluminum plus oxygen gives aluminum oxide.

First, let's figure out the formula of aluminum oxide. Aluminum oxide is an ionic material because aluminum is a metal and oxygen is a nonmetal. Aluminum will lose electrons, and since it has three valence electrons it will lose three electrons and form an Al⁺³ ion. Oxygen is a nonmetal and will gain electrons. From its position on the periodic table, you can tell that it has room to gain two electrons. So when it gains electrons, it will form an oxide ion with a -2 charge. Because we have a +3 charge and a -2 charge, the correct formula for aluminum oxide is Al₂O₃.

On the left side of the equation we have aluminum, which is represented simply by Al. We also have O₂ because oxygen  is one of those diatomic elements that you need to remember. We now have Al + O₂ Al₂O₃ as the unbalanced skeleton equation.

Let's balance the oxygen first because it is the most complicated. If we start with oxygen in pairs and we end up with oxygen in triplets, the least number of oxygen atoms that we can do this with will be six oxygen atoms. So we need a 3 in front of O₂ and a 2 in front of Al₂O₃. That gives six oxygens on both sides. The 2 in front of Al₂O₃ also gives us four aluminum atoms on the right, so we need a 4 in front of the Al on the left. That gives us 4 Al + 3 O₂ 2 Al₂O₃.

(a) Sodium chloride is an ionic compound because it contains sodium, a metal and chlorine, a nonmetal. By this point you should be able to just write down a formula for sodium chloride, but let me go through the process again.

Sodium, being a metal, will lose electrons; it has one valence electron so it loses one electron and forms a +1 charged ion. Chlorine is a nonmetal, and will gain electrons. It has room to gain one electron, so when it does that, it forms a -1 charged ion called the chloride ion. So with a +1 charge and a - 1 charge, the formula for sodium chloride is NaCl.

The elements that it will decompose into are sodium and chlorine. For sodium we just use its symbol Na. Because chlorine is one of the diatomic elements, we write down the formula for its molecule, Cl₂. So now we have the skeleton equation, NaCl Na + Cl₂.

What's not balanced about this? The chlorine. We have Cl₂ on the right so we need to have a 2 in front of NaCl on the left. That also gives us two sodiums so we need to put a 2 in front of the Na on the right. And that gives us the balanced equation 2 NaCl 2 Na + Cl₂.

(b) Sulfur hexachloride. This is a prefix name, so we don't have to sort out the charges or oxidation states.

Sulfur hexachloride has the formula SCl₆. It decomposes into the elements sulfur and chlorine. Sulfur is represented by the symbol S; chlorine is one of the diatomic elements so it is represented by its formula Cl₂.

To balance this equation, we need to put a 3 in front of Cl₂. That gives us six chlorines on the right as well as the left. So the balanced equation for the decomposition of sulfur hexachloride is SCl₆ S + 3 Cl₂.