Download Chem 1411 Chapter 4

CHEM - 1411 CHAPTER - 4
TYPES OF CHEMICAL REACTIONS & SOLUTION
STOICHIOMETRY
Solution
A solution is a homogeneous mixture of two or more substances. The component of solution that
is present in larger amount is called the solvent and the one that is present in smaller amount is
called the solute.
Solutions in which water is the dissolving medium (solvent) are called aqueous solutions.
Electrolyte
A substance (such as NaCl) that dissociate in aqueous solution to form ions and hence
conduct electricity is an electrolyte. They are usually ionic compounds. Ionic compounds
are usually the compounds of metals and nonmetals, (such as NaCl, FeSO4, MgBr2).
Electrolytes conduct electricity due to the presence of mobile ions.
NaCl(aq)
Na+(aq) + Cl–(aq)
Strong and Weak electrolytes
A strong electrolyte is the one that has a high degree of dissociation and a weak electrolyte is the one
that has a low degree of dissociation.
Ex. NaCl, HCl, MgBr2 (Strong Electrolytes), Ca(OH)2, NH4OH(Weak Electrolytes)
Acids and bases are also electrolytes.
Non-electrolyte
A substance that does not dissociate into ions in solution (such as sucrose, Glucose) is a
Non- electrolyte. Such solutions do not conduct electricity due to the absence of ions.
Water is a very effective solvent for ionic compounds because of its polar nature.
The O--H bond in water is polar due to the electro negativity (electron attracting power) of
‘O’atom. The ‘O’atom of water is rich in electrons and thus possesses a partial negative charge (δ-).
The H atom has a partial δ+. Positives ions (cations) of the salt attract “O” of water and negative
ions (anions) attract “H”. Thus water molecules surround each ion. This process is known as
hydration.
Precipitation Reactions.
These reactions result in the formation of insoluble substances known as the precipitate.
Precipitation reactions usually involve ionic compounds.
AgNO3(aq) + KCl ( aq)
AgCl (s) + KNO3 (aq)
(Precipitate)
Metathesis Reactions (Double Displacement reactions)
Reactions involving the exchange of ions in solutions are called metathesis reactions or
double displacement reactions.
K2SO4(aq) + BaCl2 ( aq)
BaSO4 (s) + 2KCl (aq)
Pb(NO3)2(aq) + 2KI(aq)
PbI2 (s) + 2KNO3(aq)
Spectator ions
Ions, that are present in the solution but do not involve in the overall reaction, are called
spectator ions.
Solubility Guidelines for Common Ionic Compounds in H2O
Ionic compounds soluble in H2O


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
Compounds containing alkali metal ions and ammonium salts
Nitrates and Acetates
Sulfates (exception Ca, Sr, Ba, Pb)
Chlorides, Bromides, Iodides (exception Pb, Hg (I), Ag)
Ionic compounds insoluble in H2O



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Hydroxides (exception Li, Na, K, Rb, Ca, Sr, Ba)
Phosphates, carbonates and Sulfides (exception Li, Na, K, Rb, NH4+)
Compounds containing S2–
Compounds containing OH– (Exception: compounds of NH4+ and the alkali
metals and alkaline earth metals)
Net ionic equation
When spectator ions are omitted from the ionic equation, the equation is called a net ionic
equation:
BaCl2(aq) + K2SO4(aq)
BaSO4(s) + 2 KCl (aq) (in aqueous medium)
*{BaCl2, K2SO4 and KCl are soluble and form ions in H2O, while BaSO4 insoluble}
so, Ba2+ (aq) + 2 Cl- (aq) + 2 K+ (aq) + SO42- (aq)
BaSO4 (s) + 2 K+ (aq) + 2 Cl- (aq)
Canceling common ions on both sides,
Ba2+ (aq) + SO42- (aq)
BaSO4 (s)
(Net ionic equation)
A net ionic equation includes only the ions and molecules directly involved in the reaction.
Acid Base Reactions
According to Arrhenius concept Acids are substances that are able to ionize in aqueous solution to
form H+ ions and Bases are substances that ionize to form OH- ions.
Acids that ionize to form one H+ ion are called monoprotic acids. Common monoprotic acids: HCl,
HNO3, vinegar.
Acids that ionize to form two H+ ions are called diprotic acids. Common diprotic acids: H2SO4,
vitamin C,(Ascorbic acid: H2C6H6O6).
The ionization of H2SO4, and other diprotic acids occurs in two steps:
H2SO4(aq)
H+(aq) + HSO4-(aq)
HSO4-(aq)
H+(aq) + SO42-(aq)
An aqueous solution of H2SO4 contains: H+, HSO4-, and SO42-
Examples of Bases
NaOH(aq)
Na+(aq) + OH- (aq)
Bronsted - Lowry concept of Acids and Bases
According to Bronsted Lowry concept, acids are proton donors and bases are proton
acceptors.
Weak Acids and Weak bases: They are acids or bases with low degree of dissociation.
Neutralization Reactions
Reaction between an acid and a Base to form salt and water is a neutralization reaction. It
involves the combination of H+ ions of acid with OH- ions of base to form water molecules.
HCl(aq) + NaOH(aq
(acid) + (base)
NaCl(aq) + H2O(l)
(salt)
+ (water)
The complete ionic equation is :
H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq)
Na+(aq) + Cl-(aq) + H2O(l)
The net ionic equation is: H+(aq) + OH- (aq)
H2O(l)
+
Thus neutralization involves the combination of H and OH to form water molecules
Acid-Base Reactions with Gas Formation
2HCl(aq) + Na2S(aq)
Net ionic equation:
2H+(aq) + S2-(aq)
NaHCO3(aq) + HCl(aq)
H2S(g) + 2NaCl(aq)
H2S(g)
NaCl(aq) +H2CO3(aq)
Oxidation and Reduction Reactions
An oxidation reaction is the one that involves loss of electrons and a reduction reaction
involves the gain of electrons.
A reaction in which oxidation and reduction occurs simultaneously is called a redox reaction.
The species that undergoes oxidation is the reductant (reducing agent); and the species that
undergoes reduction is the oxidant (oxidizing agent).
Oxidation involves an increase in the positive charge of the species and reduction involves an increase
in the negative charge.
In a redox reaction, the number of e- gained by the oxidant is equal to the number of e- lost by the
reductant.
Ex.
2 Na0
+
Cl20
2 Na+Clreductant
oxidant
Each Na atom loses one electron and each Cl atom gains one electron. Thus, Na undergoes
oxidation and Cl2 undergoes reduction.
Oxidation number (Oxidation state)
The number of charges the atom would have in a molecule if that many electrons were transferred
completely is the oxidation number of that atom in the molecule.
Rules to assign oxidation numbers:
1. The oxidation number of any free element is zero.
2. Sum of the oxidation numbers of all the atoms in a species is equal to the net charge of that
species. For compounds it is zero.
3. Group I metals are assigned an oxidation number of +1, Group II ,(+2) and Group III (+3).
4. F is always assigned an oxidation number of -1, Hydrogen +1(But, –1 when bonded with
Group I or II metals) and Oxygen –2(unless bonded with F).
Types of redox reactions
1.Combination reactions
Two or more substances combine to form a single product.
H2 + O2
H2 O
2.Decomposition reactions
The breakdown of a compound to two or more simple compounds
KClO3
KCl + O2
3.Combustion reactions
Reaction of a substance with oxygen, usually with the liberation of heat and light along with a flame
CH4 + O2
CO2 + H2O
4.Displacement reaction
This is the reaction in which an atom or an ion is replaced by another atom or ion.
CuSO4 + Zn
ZnSO4 + Cu
The Activity Series:
A series of metals arranged in the order of decreasing ease of oxidation is called the activity series.
Active metals are the metals at the top of the activity series.
Less active metals: the metals at the bottom of the activity series.
A metal in the activity series can be oxidized only by a metal ion below it:
More active metals displace less active metals from the less active metal’s salt solution
If we place Cu into a solution of Ag+ions, then Cu2+ ions can be formed and Ag is displaced because
Cu is more active than Ag in the activity series:
Cu(s) + 2Ag+(aq)àCu2+(aq) + 2Ag(s)
The metals above H2 displace hydrogen gas from water or dilute acids.
Expressing Concentration of Solutions
Molarity: Molarity of a solution is the number of moles of solute per liter of solution:
Molarity (M) =
Number of moles of solute
Volume of solution in liter
Number of moles = Mass in grams / Molar mass
For a 1.0 M solution of NaCl: the solution contains 1.0 M of Na+ ions and 1.0 M of Cl– ions.
Similarly for a 1.0 M solution of Na2SO4: the solution contains 2.0 M of Na+ ions and 1.0 M of
SO42– ions.
For an aqueous solution containing the same substance,
Minitial Vinitial = moles = Mfinal Vfinal
Acid Base Titration
The process to determine the volume of a solution(acid or base) which is just sufficient to react
completely with another solution of known strength using a suitable indicator is called Titration. At
the end of reaction the indicator changes its color. The point of completion of reaction is called End
Point or Equivalence point.
Balancing of Chemical Equations by Oxidation Number method
1.
2.
3.
4.
5.
6.
Write the unbalanced equation.
Determine the oxidation states of all atoms in the reactants and products.
Show electrons gained and lost using “tie lines.”
Use coefficients to equalize the electrons gained and lost.
Balance the rest of the equation by inspection.
Add appropriate states.