How to figure out products in a reaction

​Balancing Reactions- just follow the rules!

By Robert Ripperdan

There are a couple of important rules that apply to balancing reactions.

1. Compounds that are solids, liquids, or gases must be charge balanced. The total cation charge must equal the total anion charge.

2. The Law of Conservation of Mass must be followed: same number of atoms of each element on the reactant and product sides.

The previous reaction is obviously not balanced, since there are 2 OH^- on the reactant side and 3 on the product side. That's OK: this is only a skeleton reaction. The ingredients have to be charged balanced individually, but the overall reaction is not required to follow the Law of Conservation of Mass. That's why skeleton reactions aren't real reactions; they're just ingredient lists without the proper proportions.

So (obviously) the next step is to balance the skeleton reaction.

​Now we have 6 (OH), 2 Al³⁺, and 3 Ca²⁺ on each side of the reaction. Boom. Done.

​The compound is balanced but the reaction is not. That's OK; that's expected in a skeleton reaction.

​That brings us to a general strategy for solving these types of problems:

• Figure out the products of the skeletal reaction using the general form (A,B, etc) of the reaction type and remembering that any solid, liquid, or gas product must be electrically neutral.

• THEN, balance the reaction to obey the Law of Conservation of Matter.

Multiple Choice

What is the balanced reaction between Pb(NO₃)₂ (aq) and KI (aq) ?

Pb(NO₃)₂ + KI $$\rightarrow$$ ?

• Pb(NO₃)₂ (aq) + KI (aq) $$\rightarrow$$ PbI (s) + KNO₃ (aq)

• Pb(NO₃)₂ (aq) + 2KI (aq) $$\rightarrow$$ PbI (s) + KNO₃ (aq)

• Pb(NO₃)₂ (aq) + 2KI (aq) $$\rightarrow$$ PbI₂ (s) + 2KNO₃(aq)

• Pb(NO₃)₂ (aq) + KI (aq) $$\rightarrow$$ PbI (s) + K₂NO₃ (aq)

To summarize:

• You already know about making compounds. Cations first. Always. Compounds must be electrically neutral (i.e. 'charge-balanced')

• Identify which of the 5 reaction types you are working with and then follow the general form of the reaction type.

• Good news/bad news. Bad news first: there are more rules to follow in order to better represent reality of reactions (especially double replacement). Good news: you don't have to know them. Yet.

​ You already know about writing compounds. Cations first. Always.