Chapter 4

Reactions in Aqueous Solutions

ByDr AYESHA MOHY-UD-DIN

IN THE NAME OF ALLAH, THE BENEFICENT THE MERCIFUL

AQUEOUS SOLUTIONS

A solution is a homogeneous mixture of a solute dissolved in a solvent. In aqueous

solutions solvent is WATER.

The solvent is generally in excess.ExampleThe solution NaCl(aq) is sodium chloride NaCl(s) dissolved in water H2O(l)

The solute is NaCl(s) and the solvent is H2O(l)

Aqueous Reactions

Aqueous reactions can be grouped into three general categories, each with its own kind of driving force:

1.Precipitation reactions2.Acid base neutralization reactions3.Oxidation-reduction reactions.

Electrolyte and Non-electrolyte

• Electrolyte: a substance that conducts electricity when dissolved in water. – Acids, bases and soluble ionic

solutions are electrolytes.

• Non-electrolyte: a substance that does not conduct electricity when dissolved in water. – Molecular compounds and insoluble

ionic compounds are non-electrolytes.

Electrolytes

• Some solutes can dissociate into ions.

• Electric charge can be carried.

Types of solutes

Na+

Cl-

Strong Electrolyte -100% dissociation,all ions in solution

high conductivity

Types of solutes

CH3COOH

CH3COO-

H+

Weak Electrolyte -partial dissociation,molecules and ions in solution

slight conductivity

Types of solutes

sugar

Non-electrolyte -No dissociation,all molecules in solution

no conductivity

Table 4.1 Electrolyte Classification of Some Common SubstancesStrong Electrolytes Weak ElectrolytesNonelectrolytes

HCl, HBr, HI CH3COOH H2O

HClO4 HF CH3OH

HNO3 C2H5OH

H2SO4 C12H22O11(sucrose)

KBr Most organic compd

NaCl

NaOH, KOHOther soluble ionic compounds

Ionic Equations

MgCl2(s) → Mg2+(aq) + 2 Cl- (aq)A strong electrolyte:

A weak electrolyte:

CH3COOH(aq) ← CH3COO -(aq) +H+(aq)→

CH3COOH(aq), CH3OH(aq), H2O(l), CO2(g), Zn(s), MgCO3(s)

A non-electrolyte:

The molecular equation does not tell us that the reaction actual involves ions in solution. So the soluble ionic substances in solution should be represented by their separate ions.Other ions are SPECTATOR IONS that do not take part in reaction.

Ag+(aq) + NO3-(aq) + Na+(aq) + I-(aq) →

AgI(s) + Na+(aq) + NO3-(aq)

Spectator ionsAg+(aq) + NO3

-(aq) + Na+(aq) + I-(aq) →

AgI(s) + Na+(aq) + NO3-(aq)

Writing Net Ionic Equation

AgNO3(aq) +NaI (aq) → AgI(s) + NaNO3(aq)

Overall /Molecular Equations: Complete formulas are written for all the reactants and products, no ions are written.

Complete ionic equation: Strong electrolytes are written in their ionized forms and weak/non-electrolytes as unionized form.

Ag+(aq) + I-(aq) → AgI(s)

Net ionic equation: Write only those chemical species which are involved in a chemical reaction. All spectator ions are eliminated.

Example of Ionic equations

Suppose copper (II) sulfate reacts with sodium sulfide. a) Write out the chemical reaction and name the

precipitate.

CuSO4 (aq) + Na2S (aq) CuS (s) + Na2SO4 (aq) a) Write out the net ionic equation.

Cu+2 (aq) SO4

-2 (aq) + 2Na+

(aq) + S-2 (aq) CuS (s) + 2Na+ + SO4

-2 (aq)

Cu+2 (aq) + S-2 (aq) CuS (s)

Precipitation Reactions

2Ag NO3 (aq) + Na2CO3 (aq) AgCO3(s) + 2NaNO3

Precipitation reactions are process in which soluble reactants yield an insoluble solid product that falls out of solution. Most precipitations take place when certain cations and anions combined to produce an insoluble ionic solid called a precipitate.

Soluble Cations

Alkali metals: Li+, Na+, K+, Rb+, Cs+,

NH4+

Soluble Anions:

Halides: Cl-, Br-, I- except of Ag+, Hg2

+2, Pb+2

NO3-, ClO4

-, CH3CO2-, SO4

-2 except SO4

-2 of Ba+2, Hg2+2,

Pb+2

Mostly insoluble

Metal (other than alkali metals) sulfides, hydroxidescarbonates, phosphates

Example

(a) Al2(SO4)3 + NaOH

i) write down the reactants and interchange of anions to get

product

Al2(SO4)3 + 6NaOH 2Al(OH)3 + 3Na2SO4

All common Na compounds are water soluble Na+ remain in

solution. The combination of Al3+ and OH- produce insoluble

Al(OH)3. Then the ionic equation is

2Al3+ +3SO4

2- + 6Na+ + 6OH-2Al(OH)3(s)+ 6Na+ + 3SO4

2-

The net ionic equation is :

Al3+ + 3OH-Al(OH)3(s)

Acid-Base ReactionsAn acid is a substance that provides hydrogen ions (H+) (increase the concentration of H+) in aqueous solution. H+ is too reactive to exit by itself, it attaches to water to give the more stable hydronium ion, H3O

+.

A base is a substance that produces hydroxide ions (OH-) (increase the conc. of hydroxide ions) in aqueous solution.

HA (aq) H+(aq) + A-(aq)an acid HA is a general formula

for an acid

MOH(aq) M+(aq) + OH-(aq)a base MOH is a general

formula for a base

Strong and Weak Acids and BasesA strong acid is an acid that is almost completely ionized in aqueous solution. A weak acid is an acid that only partially ionized (as result of an equilibrium reaction with water) in aqueous solution.

HCl(aq) H+(aq) + Cl-(aq) strong acid

CH3CO2H(aq) H+(aq) + CH3CO2-(aq) weak acid

A strong base is a base that dissociate nearly completely in aqueous solution.A weak base is a base that is only partially ionized (as result of an equilibrium reaction with water) in aqueous solution.

NaOH(s) Na+(aq) + OH-(aq) strong base

NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) weak base

Weak BasesNH3

CH3NH2  

Weak AcidsCH3COOH    HCOOH        HF                   HCN               HNO2             H3PO4

-       

Acid-Base Neutralization ReactionIn a neutralization reaction, an acid and a base react to form water and an aqueous solution of an ionic compound called a salt.

A neutralization reaction:

HA(aq) + MOH(aq) H2O(l) + MA(aq)

acid base water salt

HCl(aq) + NaOH(aq) NaCl(aq) +H2O(l)

H+(aq) + Cl-(aq) +Na+(aq) + OH-(aq) Na+(aq) +Cl-(aq) + H2O(l)

By eliminating the spectator ions, we discover the actual Reaction of the neutralization of strong acid by a strong base

The net ionic equation:

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

or H3O+(aq) + OH-(aq) 2H2O(l)

Oxidation-Reduction ReactionHistorically, OXIDATION referred to the combination of an element with oxygen to yield an oxide, and the word REDUCTION referred to the removal of oxygen from an oxide to yield the element.Today, by using broader definitions, we look at the change of oxidation state of the element involved in the reaction.

An oxidation is defined as the lose of one or more electrons by a substance. (Increase in Oxidation number)

A reduction is the gain of one or more electrons by a substance. (Decrease in Oxidation number)

As a remembrance LEO says GER•Loss Electrons = Oxidation•Gain Electrons = Reduction

What is Redox?What is Redox?

• REDOX stands for REDuction/Oxidation

• An oxidation and a reduction must occur together, and such a reaction is called an oxidation-reduction reaction or REDOX reaction. A redox reaction is a process in which electrons are transferred between substance or in which atoms change oxidation number.

• Not all the redox reactions involve oxygen.

Oxidizing and Reducing Agents

An oxidizing agent: contains an element whose oxidation number decreases in a redox reaction by gaining electrons (it make possible for some other substance to be oxidized and itself reduced).

A reducing agent: contains an element whose oxidation number increases in a redox reaction by losing electrons (it make possible for some other substance to be reduced and itself oxidized).

In general, a substance with an element in one of its highest possible oxidation state is an oxidizing agent. If an element is in its lowest possible oxidation state, the substance is a reducing agent.

An OXIDATION NUMBER is the APPARENT charge on an atom in a molecule or a compound.

1. The ON of any element in free (uncombined) state is zero. e.g. Na0, H2

0, O20 etc

2. The ON for any simple, monatomic ion is equal to the charge on the ion and it is same in compound as was in monoatomic ion e.g. +1 for Sodium in Na+,Na2CO3and NaCl3. The sum of all the oxidation numbers of the atoms for neutral species must be equal to zero and for ions must be equal to the charge on the ion.4. In its compounds, fluorine always has an ON of –1.5. In its compounds, hydrogen has an ON of +1 except for metal hydrides, where the ON is -1.6. In its compounds, oxygen has an ON of -2 except for the peroxides (-1), superoxides (-1/2) and OF2 (+2).

Rules for determining oxidation numbers (ON) Calculate the unknown “x”