Chapter 4 Chemical Reactions
Arrhenius' Ionic Theory of Solutions states that certain substances produce
freely moving ions when they dissolve in water, and these ions conduct an electric
current in solution.
These substances can be divided into two groups:
Electrolytes - a substance that dissolves in water to give an electrically conducting
solution.
Nonelectrolytes - substance that dissolves in water to give a nonconducting or
very poorly conducting solution.
When electrolytes dissolve in water they produce
ions, but to varying extents
Strong electrolyte is an electrolyte that exists in
solution almost entirely as ions.
NaCl(s)
Na+(aq) + Cl-(aq)
Weak electrolyte is an electrolyte that dissolves in
water to give a relatively small percentage of ions.
NH3(aq) + H2O(l)
.
NH4+(aq) + OH-(aq)
Solubility Rules
Substances vary in solubility, or their ability to dissolve, in water. To predict the
solubility of ionic compounds, chemists have developed solubility rules.
For a reaction involving ions, we have a choice of chemical equations. Depending
on what information we want to convey we can represent a reaction with a
molecular equation, a complete ionic equation or a net ionic equation.
Molecular equations
Chemical equation in which the reactants and products are written as if they were
molecular substances, even though they may actually exist in solutions as ions.
Ca(OH)2(aq) + Na2CO3(aq)
CaCO3(s) + 2NaOH(aq)
Complete Ionic Equations
Shows what is happening at the ionic level.
Ca+2(aq) + 2OH-(aq) + 2Na+(aq) + CO32-(aq)
CaCO3(s) + 2Na+(aq) + 2OH-(aq)
Net Ionic Equations
An ionic equation from which spectator ions have been removed.
Spectator ion is an ion in an ionic equation that does not take part in the reaction.
The resulting equation is:
Ca+2(aq) + CO32-(aq)
CaCO3(s)
Examples:
Types of Chemical Reactions
1. Precipitation Reactions
Precipitate is an insoluble solid compound formed during a chemical reaction in
solution. To predict whether a precipitate will form when you mix two solutions
together - use your solubility rules.
MgCl2(aq) + AgNO3(aq)
Metathesis reaction a reaction between compounds, that, when written as a
molecular equation, appears to involve the exchange of parts between the two
reactants.
2. Acid Base Reactions
Chemical reactions that involves the transfer of a proton between reactants
Arrhenius definitions of acids and bases
Acid - is the substance that produces hydrogen ions, H+, when it dissolves in water
H+(aq) + NO3-(aq)
HNO3(aq)
Base - is a substance that produces hydroxide ions, OH-, when it dissolves in water
Na+(aq) + OH-(aq)
NaOH(s)
In both cases these can be a strong or weak electrolyte.
The second theory was by Bronsted and Lowry, they defined an:
An acid as a species that donates a proton to another species in a proton -transfer
reaction.
NH4+(aq) + OH-(aq)
NH3(aq) + H2O(l)
A base as the species that accepts a proton in a proton -transfer reaction.
H+ ions can also be thought of as a proton bonded to a water molecule to give a
H3O+ ion - a hydronium ion.
Strong Acids and Strong Bases dissociate completely in water.
HCl(aq)
H+(aq)
NaOH(aq)
Na+(aq)
+
Cl-(aq)
+
OH-(aq)
Table 4.3 Common Strong Acids and Bases
Weak Acids and Bases
Weak acid is an acid that only partly ionizes in water; it is a weak electrolyte
HCN(aq)
H+(aq)
+ CN-(aq)
Weak Bases is a base that is only partly ionized in water; it is also a weak
electrolyte.
NH3(aq) + H2O(l)
NH4+(aq) + OH-(aq)
Neutralization Reaction is a reaction of an acid and a base that results in an ionic
compound and possibly water.
HCl(aq) + Ca(OH)2(aq)
CaCl2(aq) + H2O(l)
Polyprotic acids are acids that yield more than one acidic hydrogen per molecule.
Phosphoric acid, H3PO4
Carbonates and sulfites also react with acids to form gases.
H2CO3, carbonic acid, and H2SO3, sulfurous acid can be produces in a metathesis
reaction but are unstable.
3. Oxidation - Reduction Reactions - is one in which one or more atoms change
oxidation numbers, implying that there has been a transfer of electrons.
Oxidation state of an atom in a substance is the actual charge of the atom if it exists
as a monatomic ion, or a hypothetical charge assigned to the atom in the substance
by simple rules.
2Ca(s) + O2(g)
2CaO(s)
Table 4.5 (page 146) Rules for Assigning Oxidation Numbers
Oxidation Reduction Reactions can be broken down into two half reactions
Fe(s) + Cu+2(aq)
Fe+2(aq) + Cu(s)
Oxidation is the loss of electrons by a substance
Many metal react with oxygen to form metal oxides
Reduction is the gain of an electron by a substance
Oxidizing agent is a species that oxidizes another species and a reducing agent is a
species that reduces another species
Fe(s) + Cu+2(aq)
Fe+2(aq) + Cu(s)
Common Oxidation Reduction Reactions
1.Combination Reaction - where 2 substances combine to form a third substance
2. Decomposition Reaction- where a single compound reacts to give two or more
substances
3. Displacement Reaction - a reaction in which an element reacts with a compound,
displacing another element from it
4. Combustion Reaction - in which a substance reacts with oxygen, usually with
the rapid release of heat to produce a flame. the product contains one or more
oxides
Balancing Half Reactions - NEW balancing charges in an equation
The general term, concentration, refers to the quantity of solute in a standard
quantity of solution.
Molarity (M) is defined as the mole of solute dissolved in one liter of solution
An aqueous solution of 0.15M NH3 contains 0.15 moles of NH3 per 1 Liter of
water
Diluting Solutions
First we need to obtain the relationship between the initial concentration (initial
molarity) and after the dilution (final molarity)
Mi x Vi = Mf x Vf
Types of Quantitative Analysis
The determination of the amount of a substance or species present in a
material
1. Gravimetric Analysis
The amount of the species in a material is determined by converting the
species to a product that can be isolated completely and weighed.
Example: Pb+2 ions in drinking water
2. Volumetric Analysis
A method based on titrations
Titration is a procedure for determining the amount of substance A by adding a
carefully measured volume with a known concentration of a substance B until the
reaction of a and B is complete.