Who’s Ready for Combination Reactions!!??

Seriously though, we’ll be talking about Combination reactions today and believe it or not they’re not that bad. Let me show you.



Combination reactions always follow this pattern:

1 Compound + H2O → 1 Compound


They’re really easy to balance, but you have to know which type of combination reaction you’re working with before you can start.

1. Metals – If the compound in the reactants is a metal and Oxygen, these are the steps you follow:

1. You must determine the charge of the metal. To do this, separate the compound and use your periodic table or the crossing rule (If the compound has subscripts, that may be the charge of the opposite atom). Lets look at this example:

CaO + H2O →
↓   ↓
+2  -2

          Looking at the periodic table we see that Ca has a charge of +2 and O has a charge of -2

2. Now that we know the charge of the metal, we “Combine” that metal with Hydroxide (OH-). The number of hydroxides is determined by the charge of the metal.

CaO + H2O →  Ca(OH)2
↓   ↓                      ↑      ↑
+2  -2               Ca+2  (OH)-1

In our example, we need 2 (OH) ions to balance the -2 charge of Ca

3. That’s it! It’s that simple. And all you would have to do now is balance but even that is pretty easy. In fact, this one is already balanced!

1. Nonmetals – If the compound in the reactants is a Nonmetal and Oxygen, these are the steps you follow:

1. When a nonmetal oxide reacts with water it forms an Oxoacid (Remember, oxoacids start with H and end with a polyatomic ion). So to do this you simply add all the H’s, add all the nonmetal atoms, and add all the O’s. Those numbers become the subscripts of your Oxoacid. For example:

CO2 + H2O →

Just add the H’s (2), the non metal in this case is C (1), and the O’s (3). Then combine them into one Oxoacid:

CO2 + H2O → H2CO3


The only exception would be if the answer does not have a Polyatomic Ion, but the subscripts all have a common denominator, you make the common denominator the coefficient and reduce the subscripts. For example:

N2O5 + H2O → H2N2O6

Well there is no Polyatomic Ion that is N2O6 But all the subscripts are divisible by 2! So 2 becomes the Coefficient!

N2O5 + H2O → 2 HNO3

So that is Combination Reactions. Hope you learned something and don’t end up like this guy…


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