How Compounds Form

General Chemistry Index

Where are we going with this? This page will assist in developing the ability to predict products of simple reactions as listed in of reactions: synthesis (i.e., combination), decomposition, single displacement, double displacement, acid/base, and combustion.

How Compounds Form 
What happens if I mix this with that?

Not everything will combine with each other. There are certain and specific rules regarding what will combine with what.

In chemistry, with regard to "charge" opposites attract. Something with a negative charge will be attracted to (and could theoretically) combine with a different thing having a positive charge.

For example, Na has a charge of +1. O has a charge of -2. Therefore, these two elements will combine. 

Likewise, the polyatomic molecule, OH has a charge of -1. It will also combine with something having a positive charge, such as K which is +1

Figuring out if something will actually combine is the first step in finding the subscripts that produce a neutrally charged molecule.

Let's go through this a little…

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Synthesis / Combination Reactions

So in this kind of reaction you have the model form

A + B --> AB

If this is going to work A has to be positive and B has to be negative. (Chemistry grammar prefers that the positives are written first. Generally, this is also going left to right across the periodic table.)

So, let's mess with this!

Can Na combine chemically with K?

Finding the charge of Na on a chart or on the periodic table tells us that it is +1.

Finding the charge of K on a chart or on the periodic table tells us that it is +1.

Well… remember that "opposites attract" rule? So NO. No reaction.

How about Mg and Cl?

Chart… Periodic table… Mg is +2. Cl is -1

YES! These two will combine!

Okay, synthesis is easy!

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Single Replacement / Displacement

These reactions look like this…

Single replacement

AB + C --> AC + B

or

AB + C --> CB + A

Think about that opposites attract thing. A is positive (chemistry grammar). B is negative. C can be either positive or negative. The compound that might form¹ will be the one where C replaces which ever part of AB has the same charge.

Let's say this…

A is +1

B is -1

C is +2

C and A CANNOT combine because they are both positively charged. Therefore C might replace A and combine with B.

Real example…

K2O + Li -->

What could Li replace? Chart… Periodic table… charges…

K is +1

O is -2

Li is +1

Therefore, the Li might² replace the K. It could combine with the O.

SUMMARY

So, in single replacement, the single thing can only replace the thing in the compound that has the same charge.

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¹ Whether or not it will form depends on if C is more highly active than the thing it is trying to replace.

² Have to look it up in the activity series table…

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Double Replacement / Displacement

These reactions look like this…

AB + CD --> AD + BC

The double replacement reaction looks more complex than synthesis or single replacement. However (ironically), the possible products are a lot easier to predict. 

Remember that opposites attract thing? Also, that chemistry grammar thing of putting positives first? Well, for double replacement, those two things add up to making this really easy.

So, look at this…

MX + NY --> ???

Grammar says M and N are positive and X and Y are negative. (But you should actually look it up!)

Presuming whoever wrote the question followed the grammar rules, since opposites attract, the only thing that might be possible is for the X and Y to swap places. 

Example? Sure. And how about those polyatomic ions? Okay, that too.

AgNO3 + K2SO4 --> ????

The trick is to realize that SO4 and NO3 are polyatomic ions. How about some coloring?

AgNO3 + K2SO4 --> ????

Blue is positive. Red is negative (don't hate… just a rough life)

Opposites attract. Blue cannot be with blue. Red cannot be with red.

There's only one possible product. Swap the negative parts: 

AgNO3 + K2SO4 = Ag2SO4 + KNO3

The only thing left to do is figure out IF IT ACTUALLY WILL react. That… That's a whole 'nother thing. (The reaction above does, by the way.) 

There's a lot of rules related to this, and then one of the rules has rules. Yeah… I know.

SUMMARY

So, in double replacement, the negatives swap places.