Types of Reactions II: Double Replacement, Neutralization, Combustion

Last time, we looked at the three simple types of reactions: synthesis, decomposition, and single replacement. Now, we will talk about double replacement, neutralization (a special type of double replacement) and combustion.

In double replacement, the ionic compounds in the reaction switch partners like a dance.
In general: AB + CD --> CB + AD



For example, if we have Na₂SO4 + CaCl₂ as reactants, the products will be NaCl + CaSO₄. Now how can we predict if a double replacement reaction occurs? We know that for single replacements, we can use the activity series to predict if the reaction will happen.

In double replacements, we use the Solubility Table. First, find the negative charged ion in the very left hand column. Then, look for the positive charged ion in the middle column. Finally, the last column will tell you if it is soluble or not soluble.

Here is something very important: if it is soluble, the state of the compound will stay the same. In many cases, this is aqueous (aq), meaning dissolved in water. However, if it says not soluble, this means that a precipitate (a solid) was formed, meaning there WAS a reaction.

So in summary: if both products are soluble and did not change state in the reaction, there is no reaction. If at least one of the products has changed states, then it works!

Finally, if a reaction DOES happen, we will express the solid formed in a Net Ionic Equation.
Basically, all the aqueous ions that are equal on both sides will get cancelled (like algebra in math), and the last ones remaining, along with the solid compound will be your equation.

So, for example, if we use the  Na₂SO4 + CaCl_{2 --------->} NaCl + CaSO₄, and then we balance it first...we'd get


_{1 Na2SO4(aq) + 1 CaCl2(aq)--------->} 2 NaCl_(aq) + 1 CaSO_4(s) 
We see that CaSO₄ produces becomes a solid state, so we would have to write a net ionic equation.

This equation is basically only going to involve the ions that form the solid compound.

So we would get: 

Ca²⁺_(aq) + 1 SO₄^2-_(aq) -----> 1 CaSO_4(s)


And that is your net ionic equation.

In neutralization, the reactants are an acid + a base, producing water and a salt. 
Acids have a positive H cation, and bases have a negative OH anion. Both are aqueous solutions, and the water produced is usually liquid. 
+ = + salt


In general: HA + BOH -----> H₂O + BA 
For example, HCl (aq) + NaOH (aq) will produce H₂O (l) and NaCl (aq). 


Finally, the last type of reaction is combustion. This is the burning of a gas, usually in the form of organic compounds or hydrocarbons. 



The general formula is: AB + O2 ----> AO + BO
While it is believed that the products of combustion are always CO₂ and H₂O, this is false, as there may not be H's or C's in the reactants, so C's and H's should not appear in the products.


An example of combustion would be: 
CH₄ + O₂ -------> CO₂ + H₂O

Here is a video explaining double replacements more in-depth: