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 they eat/drink/swallow something.

The answer to that question is the very heart of predicting the products of a chemical reaction. Predicting the products of chemical reactions is a process by which potential reactants are scrutinized to determine whether 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 take place only in the lab; it is actually 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.
  
  
• Chemical reactions follow the law of conservation of mass such that…

• The number and type of each atom on the reactant side are equal to the number and type of each atom 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) For ordinary chemical reactions, any mass-energy difference is far too small to matter in our calculations

• 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.

If the compound formed is a neutral compound, the overall charge must equal zero. 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. To be a little more specific, they are combining to achieve more stable valence-electron arrangements.

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 a subscript tells how many atoms of an element—or how many polyatomic ions—are represented in a chemical formula. Changing the subscripts changes what you have:

H2O is not the same as H2O2. The first is water. The second is hydrogen peroxide. You can die if you DON'T drink the first. You can 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 substances. You CANNOT change the subscripts of the things on the reactant side.

When they react (if they react), they are going to 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 formulas and subscripts of the products must be determined from the identities and properties of the new substances—they do NOT simply carry over from the reactants.

Look at a couple of balanced reactions as an example:

2H2 + O2 → 2H2O

6CO2 + 6H2O → C6H12O6 + 6O2

SiCl₄ + 4H₂O → H₄SiO₄ + 4HCl

2Al + 6HCl → 2AlCl3 + 3H2

Na2CO3 + 2HCl → 2NaCl + H2O + CO2

So… 

Determine the correct formulas of the PRODUCTS. For neutral ionic compounds, choose subscripts so that the positive and negative charges balance to zero.

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 the products of a chemical reaction, all the same!