Metathasis Reactions

Metathesis Reaction A reaction where the cations and anions exchange partners:

AX + BY AY + BX

Metathesis reactions are driven by the formation of a product, and can be divided into three different categories according to the type of product that is formed.

1. Formation of a Precipitate

Precipitate An insoluble solid formed by a reaction in solution.

A precipitation reaction occurs when two soluble ionic compounds are mixed together, they exchange partners and one of the products is an insoluble compound so it drops out of solution as a solid.

Example

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

The net ionic equation is:

Ag+(aq) + Cl-(aq) AgCl(s)

 

The key to recognizing precipitation reactions is to examine all combination of anion-cation partners and know which compounds are soluble and which ones are insoluble (solubility rules). The solubility rules are given in table 4.2 of your text, and are given below in modified form.

An ionic solid will be soluble if:

An ionic solid will be insoluble if:

You will not be expected to memorize all of the solubility rules, however, you are expected to know that, with no exceptions, the alkali metals and ammonium are soluble cations, while nitrate and acetate are soluble anions.

2. Formation of a Weak or Non-electrolyte

One example of this type of reaction is one that we have already discussed, the neutralization reaction between an acid and a soluble metal hydroxide to form water and a salt.

Example

H2SO4(aq) + 2NaOH(aq) 2H2O(l) + Na2SO4(aq)

The net ionic equation is:

2H+(aq) + 2OH(aq) 2H2O(l)

Insoluble metal hydroxides will also react with acids to form water.

Example

3HCl(aq) + Al(OH)3(s) 3H2O(l) + AlCl3(aq)

The net ionic equation is:

3H+(aq) + Al(OH)3(s) 3H2O(l) + Al3+(aq)

Certain (basic) metal oxides can also react with acids to form water.

Example

2HCl(aq) + Na2O(s) H2O(l) + 2NaCl(aq)

The net ionic equation is:

2H+(aq) + Na2O(s) H2O(l) + 2Na+(aq)

3. Formation of a Gas

This is the least common of the metathesis reactions. It occurs when one of the metathesis products is a gas with a low solubility in water, such as H2S or CO2. Generally one of the reactants will be an acid.

Examples

BaCO3(aq) + 2HNO3(aq) CO2(g) + H2O(l) + Ba(NO3)2(aq)

K2S(aq) + 2HCl(aq) H2S(g) + 2KCl(aq)

Net ionic equations:

CO3(aq) + 2H+(aq) CO2(g) + H2O(l)

S2-(aq) + 2H+(aq) H2S(g)

 

As an aid to rationalize reactions between acids and carbonates to form CO2, think of the reaction proceeding in two steps:

CO32-(aq) CO2(g) + O2-(aq)

O2-(aq) + 2H+(aq) H2O(l)

Also remember that H2CO3(aq) is an unstable acid that will partially dissolve in water to give H2O(l) + CO2(g)

Summary Metathesis Reactions

The task of predicting the outcome of a metathesis reaction may seem daunting and/or confusing at first. To help you with this process I suggest that you try the following approach:

1. Exchange the cation-anion pairs found in the reactants

AX + BY AY + BX

2. Look for one of the following products:

For example consider the following reactions

(a) Pb(NO3)2(aq) + Na2SO4(aq)

(b) FeO(s) + HClO4(aq)

(c) CuBr2(aq) + NaOH(aq)

(d) Na2CO3(s) + HCl(aq)

by exchanging the cation-anion pairs I get

(a) Pb(NO3)2(aq) + Na2SO4(aq) PbSO4(s) + NaNO3(aq)

Formation of an insoluble solid precipitation rxn

(b) FeO(s) + 2HClO4(aq) H2O(l) + Fe(ClO4)2(aq)

Formation of water rxn to form a non-electrolyte

(c) CuBr2(aq) + 2NaOH(aq) Cu(OH)2(s) + 2NaBr(aq)

Formation of an insoluble solid precipitation rxn

(d) Na2CO3(s) + 2HCl(aq) H2CO3(aq) + 2NaCl(aq)

H2CO3(aq) H2O(l) + CO2(g)

Formation of CO2 rxn to form a gas