Ionic Reactions in Solution

Types of Chemical Reactions: Double Replacement

Double Replacement:

Basic form: AX + BY --> AY + BX
occurs between ions in aqueous solution. A reaction will occur when a pair of ions come together to produce at least one of the following:
1. a precipitate
2. a gas
3. water or some other non-ionized substance.
4. formation of a complex ion

Driving Forces for Metathesis Reactions

During a double replacement or metathesis reaction, two new combinations of ions are produced. The following four reasons why these NEW combinations are more stable than the original combos.

1. formation of a precipitate (know your solubility rules)

Ex. NaCl_(aq) + AgNO_3(aq) --> NaNO_3(aq) + AgCl_(s)

Ex. BaCl_2(aq) + Na₂ SO_4(aq) --> 2NaCl_(aq) + BaSO_4(s)

The Solubility Rules

-Always Always Soluble

compounds with alkali metal ions (Li⁺, Na⁺, K⁺, Cs⁺, Rb⁺), NH₄⁺

NO₃–, C₂H₃O₂^–, ClO₃^–, & ClO₄^–

-Usually Soluble

Cl^–, Br^–, I^– [except-- Ag⁺, Pb²⁺, Hg₂²⁺]

SO₄^2– [except-- Ca²⁺, Ba²⁺, Sr²⁺, Hg₂²⁺& Pb²⁺]

-Usually NOT Soluble

O^2–, OH^– [except alkali metals -- Ba²⁺, Ca²⁺, Sr²⁺ are slightly soluble]

-Never Soluble

CO₃^2–, SO₃^2–, S^2–, PO₄^3– [except NH₄⁺ & alkali metals]

NOTE: some of these insoluble compounds WILL dissolve in acid solutions because of gas formation.

2. formation of a gas which leaves the system- be able to identify a gas

Gases that commonly form from chemical reactions will be small, typically nonpolar molecules.

If you see the following substances formed during metathesis, realize that they will decompose into gases and leave the system (preventing re-formation of the reactants).

H₂CO_3(aq) --> CO_2(g) + H₂O
H₂SO_3(aq) --> SO_2(g)+ H₂O
HCl_(aq) + FeS_(s) --> FeCl_2(aq) + H₂S_(g) [rotten egg smell]
NH₄OH_(aq) --> NH_3(g) + H₂O
2 HNO_2(aq) --> NO_(g)+ NO_2(g) + H₂O

NOTE: these compounds are formed from acids with carbonates, sulfites, sulfides, nitrites, and bases with ammonium compounds.

Gas formation is a very strong driving force. Even compounds that exist as insoluble solids will react (slowly) to form gases because gases leave the system and CANNOT re-form reactants.

CaCO_3(s) + 2HCl --> CO_2(g) + H₂O + 2 CaCl_2(aq)

The tendency to form H₂O is very strong. Insoluble oxides will react with acids.

ZnO_(s) + 2HCl --> ZnCl_2(aq) + H₂O

Sometimes, one insoluble solid can change into another even MORE insoluble solid, but you need more than the solubility rules to predict this (you need K_sp’s).

AgCl_(s) + Br^– --> AgBr_(s) + Cl^–

3. formation a weak electrolyte forms -(memorize the strong acid list so you will recognize weak acids, also H₂O and NH₄OH)

Ex. HCl_(aq) + NaOH_(aq) --> NaCl_(aq) + H₂O_(l)

Weak electrolytes are substances that break up into ions only a LITTLE in solution... therefore, the two ions are MOSTLY in a combined state... not likely to re-form the reactants. H₂O, weak acids, NH₄OH
Memorize the 8 strong acids so you can recognize a weak acid when you see one:

-- HCl, HBr, HI, HNO₃, H₂SO₄, HClO₃, HClO₄, HIO₄

Acids (forming H⁺ ions) and bases (forming OH^– ions) combine to form a salt (an ionic compound) and H₂O... the very weak electrolyte. Neutralization of the acid and base occur because the H⁺ & OH^– ions are “tied up” as H₂O.

4. formation a complex ion forms -(learn the structure of complex ions and common ligands)

A complex ion is an ion that contains a metal cation bound to one or more small molecules or ions (such as NH₃, CN^- or OH^-). These are usually formed from a transition metal surrounded by ligands (polar molecules or negative ions). As a "rule of thumb" there are twice the number of ligands around a cation as the charge on the cation.

Example: the dark blue Cu(NH₃)₄²⁺ (ammonia is used as a test for Cu²⁺ ions), and Ag(NH₃)²⁺.

Memorize the common ligands.

Ligands Names used in the ion

H₂O aqua

NH₃ ammine

OH^- hydoxy

Cl^- chloro

Br^- bromo

CN^- cyano

SCN^- thiocyanato (bonded through sulphur)
isothiocyanato (bonded through nitrogen)

Watch out for:

Aluminum also forms complex ions as do some post transitions metals. Ex: Al(H₂O)₆³⁺

The names are similar to writing ionic compounds. Cu(NH₃)₄²⁺ is the tetraamminecopper(II) ion, Ag(NH₃)²⁺is the diamminesilver(I) ion, and Al(H₂O)₆³⁺ is hexaaquoaluminum(III) ion. Zn(OH)₄^2- is the tetrahydroxyzinc(II) ion, the charge is the sum of the parts (2+)+4(-1)= -2.

Acid-base reactions may change NH₃ into NH₄⁺ (or vice versa) which will alter its ability to act as a ligand.

Visually, a precipitate may go back into solution as a complex ion is formed. For example, Cu²⁺ + a little NH₄OH will form the light blue precipitate, Cu(OH)₂. With excess ammonia, the complex, Cu(NH₃)₄²⁺, forms.

Keywords such as "excess" and "concentrated" of any solution may indicate complex ions. AgNO₃ + HCl forms the white precipitate, AgCl. With excess, concentrated HCl, the complex ion, AgCl₂^-, forms and the solution clears.

The odd complex ion, FeSCN²⁺, is one that is commonly used in standard complex ion problems.

Transitional metals, such as Iron, Zinc and Chromium, can form complex ions. Aluminum can form complex ions as well.

Cations that form complex ions with excess NH₃ are:

Cations that form complex ions with excess OH^- are: