Predicting Reactions: Overview

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.

Predicting Reactions: Overview 
What happens if I mix this with that?

This comes up every week. Someone asks me what happens if I mix two things together. Or they ask what happens if the eat/drink/swallow something.

The answer to that question is the very heart of predicting the products of a chemical reaction. Predicting products of chemical reactions is a process by which potential reactants are scrutinized to determine if they will react and if so what product(s) will be formed.

What happens if I mix this baking soda with vinegar?
What happens if I let this spilled gasoline sit on the painted garage floor?
What happens if I pour bleach directly onto my clothes?

Predicting chemical reactions does not only take place in the lab, but actually is a part of everyday life! However, in the lab, we can be more specific and isolate the this and the that more.

So… here we go! This is going to be long, so get comfy. Maybe a soda or cup of coffee?

So, a very quick review of chemical reactions… 

• You start off with some reactants.
• Something happens.
• There are some products.
  
  
• Reactions are dictated by the law of conservation of matter such that…

• The number of and type of atoms on the reactant side is equal to the number of and type of atoms on the product side.
• The total mass on the reactant side is equal to the total mass on the product side (ignoring mass/energy E=mc² stuff)

• Compounds form in fixed, specific ratios of atoms. 

Okay, back to that what happens stuff…

You have a couple of reactants. You want to know what happens if they combine (assuming they will). If they DO combine, then a few things need to be considered.

The compound formed must end up with a neutral charge. That goes back to all those bonding types and such. So, at a simplistic level, you can think of it as they are trying to fill their valence electron orbitals. You are going to rely on the periodic table to provide information about those charges! 

Oh… and those polyatomic ions… A CHART would be nice!

Okay, this is a little shifty, here… About those subscripts… Remember that the subscript gives the number of each atom (or polyatomic ion) in the molecule. Changing the subscripts changes what you have:

H2O is not the same as H2O2. The first is water. The second is peroxide. You die if you don't drink the first. You die if you DO drink the second.

So, you have two things on the table. You know what they are. (The bottle is labeled!) So, you KNOW the formula for those molecules. You CANNOT change the subscripts of the things on the reactant side.

When they react (if the react), they are going form NEW things. (Pretty much the definition of a chemical reaction.) So, the NEW things have their own formulas—new formulas with their own symbols and subscripts. Hence, the subscripts from the reactant side do NOT carry through to the product side.

Look at a couple of balanced reactions as an example:

2H2 + O2 → 2H2O

6CO2 + 6H2O → C6H12O6 + 6O2

SiCl4 + 6H2O → H4SiO4 + 6HCl

2Al + 6HCl → 2AlCl3 + 3H2

Na2CO3 + 2HCl → 2NaCl + H2O + CO2

So… 

End up with neutral molecules no matter what you have to do to the subscripts on the PRODUCT side.

Now, just how do we do that?

The actual process of predicting the products of a chemical reaction is to look at what you have and find ways of rearranging it into new substances. Arguably, it is "one of those things" you grow into, perhaps as much "just getting it" as it is having a rubric to do so. 

BUT! We shall try to come up with a system to predict products of a chemical reaction, all the same!