How do you solve a single displacement equation?

Question

EX. Mg + Cu(NO3)2 => ___ +____

Zn + HCl => ___ +_____

Answer 1

You simply switch the reactants that will have the same sign of charge.

Mg + Cu(NO3)2 -->

Mg and Cu are both metals so they will both have positive charges. Two positive charges cannot join together since they would repell each other. The only ions that can join would be a positive and a negative. The NO3 is a 1- charge. It will end up with one of the positive metallic ions. Since Mg is higher on the activity series of metals (found in any chem text) it will switch places with the other metal - in this case Cu.

Mg + Cu(NO3)2 --> Mg(NO3)2 + Cu

Zn + HCl -->

Zn will have a positive charge since it is a metal. H will have a positive charge since it is in group I and is joined to a negative Cl ion. That means that Zn and H cannot join to each other. In other words, Zn must replace H in the formula for the products. Zn is higher than H on the activity series of metals.

Zn + 2 HCl --> ZnCl2 + H2 (balanced equation)

Answer 2

The metal (Magnesium or Zinc, respectively) since they both form cations and not anions will replace the cation of the other compound. Hence the cation that is currently in that compound will be left "alone"

Mg + Cu(NO3)2 -----> Mg(NO3)2 + Cu

2Zn + 2HCl ----> ZnCl2 + H2