Transcript Chapter 5: Introduction to Reactions in Aqueous Solutions

Chapter 5: Introduction to Reactions in Aqueous Solutions
Electrolyte.
A substance, such as sodium chloride, that dissolved in water and
produces ions to give an electrically conducting solution is called
an electrolyte.
A substance, such as sucrose, or table sugar (C12H22O11), that is not
ionized and does not conduct electric current when dissolved in
water is called a Nonelectrolyte.
Strong electrolyte: is a substance that is almost completely ionized
in aqueous solution, and the solution is a good electrical conductor.
Weak electrolyte: is partially ionized in aqueous solution, and the
solution is only a fair conductor of electricity. (is an electrolyte that
dissolves in water to give an equilibrium between a molecular
substance and a relatively small quantity of ions.)
NaCl(s)
NH3(aq) + H2O(l)
Na+(aq) + Cl-(aq)
Strong electrolyte
NH4+(aq) + OH-(aq)
Weak electrolyte
~ Essentially all soluble ionic compounds and only a relatively few
molecular compounds are strong electrolytes.
~ Most molecular compounds are either nonelectrolytes or weak
electrolytes.
Table 1 Electrolyte Classification of Some Common Substances
Strong Electrolytes
Weak Electrolytes
Nonelectrolytes
HCl, HBr, HI
CH3COOH
H2O
HClO4
HF
CH3OH
HNO3
C2H5OH
H2SO4
C12H22O11(sucrose)
KBr
Most organic compounds
NaCl
NaOH, KOH
Other soluble
ionic compounds
E.g. Calculating ion concentrations in a solution of strong electrolyte. What are the
concentration (in molarity) of Mg2+ and Cl- ions in 5.0g/L of MgCl2 solution?
[Mg2+] = 5.0g x 1 molMgCl2 x 1 mol Mg2+ =5.3 x10-2 mol/L
L
95.21gMgCl2 1 molMgCl2
[Cl-] =2 x 5.3x10-2 = 0.11 mol/L
Aqueous Reactions
Aqueous reactions can be grouped into three general categories,
each with its own kind of driving force: precipitation reactions,
acid base neutralization reactions, and oxidation-reduction
reactions.
Precipitation Reactions
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.
E.g. the reaction of silver nitrate and sodium iodide in an aqueous water
solution yields sodium nitrate in solution and a yellow precipitate of
silver iodide. We can write the equation for the reaction as follow:
AgNO3(aq) + NaI(aq) AgI(s) + NaNO3(aq)
we called this a molecular equation, an equation in which the
substances are written as if they were molecular substances, even
though they may exist as ions in the solution. Molecular equation is
useful because it is explicit about what solutions have been added and
what products are obtained; and to calculate the amount of reactants or
products.
Ionic Equations
The molecular equation does not tell us that the reaction actual
involves ions in solution. However, we know that soluble ionic
substances in solution should be represented by their separate ions.
To represent this, the above reaction as an ionic equation, in which all
the ions are explicitly shown:
An ionic equation:
Ag+(aq) + NO3-(aq) + Na+(aq) + I-(aq) AgI(s) + Na+(aq)+NO3-(aq)
This is an example of ionic equation, which is a chemical equation for
a reaction involving ions in solution in which soluble substances are
represented by the formulas of the predominant species in that solution.
Net Ionic Equations
Note that some ions appear on both side of equation. These ions
go through the reaction unchanged- does not take part in the
reaction. We called them spectator ions. We can cancel them
from the equation. The resulting equation is a net ionic
equation.
Ag+(aq) + I-(aq) AgI(s)
Net ionic Equation
A net ionic equation is an equation that includes only the actual
participants in a reaction, with each participant denoted by the
symbol or formula that best represent it.
rules for converting molecular equations to ionic
equations
• The rules for converting molecular equations to ionic equations
follow:
• 1) Make sure the molecular is balanced
• 2) Ionic substances indicated in the molecular equation as
dissolved in solution, such as NaCl(aq), are normally written as
ions.
• 3) Ionic substances that are insoluble (do not dissolve) either as
reactants or products (such as precipitate) are represented by
formulas of the compounds
• 4) Molecular substances that are strong electrolytes, such as
strong acids, are written as ions. Thus, HCl(aq) is written as
H3O+(aq) + Cl-(aq) or as H+(aq) + Cl-(aq).
• 5) Molecular substances that are weak electrolytes or
nonelectrolytes are represented by their molecular formulas.
Predicting Precipitation Reaction
Empirical Rules for the solubilities of Common Ionic Compounds
Soluble compounds
Sodium, potassium and ammonium
Exceptions
compounds
Acetate and nitrates
Halides (chlorides, bromides, and iodides)
Lead(II), silver and mercury(I) halides are insoluble
Sulfates
Calcium, strontium(Sr), barium and lead(II)
sulfate are insoluble
Insoluble compounds
Carbonates and phosphates
Exceptions
Group 1 metals and ammonium compounds
are soluble
Hydroxides
Group 1 metals compounds are soluble and
calcium, Sr2+ and Ba2+ are slightly soluble
Sulfides
group 1 and group 2 metals and
ammonium compounds are soluble
_________________________________________________________________________________________________
E.g. Predict whether a reaction will occur in each of the
following case. If so, write a net ionic equation for the reaction.
If no reaction occurs, write NR after arrow.
(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+ +3SO42- + 6Na+ + 6OH-2Al(OH)3(s)+ 6Na+ + 3SO42The net ionic equation is :
Al3+ + 3OH-Al(OH)3(s)
(b) K2SO4(aq)+FeBr3(aq)
(c) CdCl2(aq) + (NH4)2S(aq)
Acid-Base Reactions
Consider the production of ions in pure water. It produces a small
percentage of ions (about 2x10-7% of the molecules react to gives ions).
H2O(l)+ H2O(l)  H3O+(aq) + OH-(aq)
Arrhenius definition
An acid is a substance that provides hydrogen ions (H+) (increase the
concentration of H+) in aqueous solution. The symbol H+(aq) does not
really represent the structure of the ion present 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)
an base
MOH is a general formula for a base
Strong Acid and Weak Acid
A 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.
E.g. HCl(aq)  H+(aq) + Cl-(aq)
strong acid
HC2H3O2(aq)  H+(aq) + C2H3O2-(aq)
weak acid
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Acids such as HCl and HNO3 that have only one acdic hydrogen atom per
acid molecule are called monoprotic acids.
A polyprotic acids such as H2SO4 and H3PO4 are acids that yield two or
more acidic hydrogens per molecules.
E.g. sulfuric acid, dissociate twice
H2SO4(aq) + H2O(l) HSO4-(aq) + H3O+(aq)
HSO4-(aq) + H2O(l)  SO42-(aq) + H3O+(aq)
Acid-Base Neutralization Reaction
When a base is added to an acid solution, the acid is said to be
neutralized. In 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
E.g. 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 natural 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)
Strong Base and Weak Base
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.
E.g. NaOH(s) Na+(aq) + OH-(aq)
strong base
NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq)
weak base
As polyprotic acids can give more than one H+, some base yield
more than one hydroxide ions.
In a neutralization involving a strong acid and a weak base NH3
In a neutralization involving a weak acid and a strong base
E.g NaOH(aq) + CH3COOH(aq) 
Reactions with Gases Formation
Some reactions are driven to products by the formation of a gas.
E.g carbonates react with acids to produce gases product.
1) NaHCO3(s) + HC2H3O2(aq) NaC2H3O2 (aq)+ H2CO3(aq)
2) Na2SO3(aq) + 2HCl(aq) 2NaCl + H2SO3(aq)
3) CaCO3(s) + 2H+(aq) Ca2+(aq) + H2CO3(aq)
Oxidation-Reduction Reaction
Historically, 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.
E.g. Rusting of iron: an oxidation of Fe
4Fe(s) + 3O2(g)  2Fe2O3(s)
Manufacture of iron: a reduction of Fe
2Fe2O3 + 3C(s)  4Fe(s) + 3CO2(g)
An oxidation and a reduction must occur together, and such a
reaction is called an oxidation-reduction reaction or redox reaction.
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.
A reduction is the gain of one or more electrons by a substance.
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.
How can you tell when a redox reaction is taking place?
Oxidizing and Reducing Agents
A species that is oxidized loses electrons or contain an atom that
increases in O.S. #. Similarly, a species that is reduced gains
electrons or contains an atom that decreases in oxidation #.
An oxidizing agent: contains an element whose oxidation state
decreases in a redox reaction (it make possible for some other
substance to be oxidized and itself reduced.) gains electrons
(electrons are found on the left side of its half-equation).
A reducing agent: contains an element whose oxidation state
increases in a redox reation loses electrons (electrons are found on
the left side of its half-equation).
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.
Balancing Redox Reactions
1 The Oxidation-Number Method: the key to this method is to realize
that the net change in the total of all oxidation numbers must be zero.
That is, any increase in oxidation number for the oxidized atoms must
be match by the corresponding decrease in oxidation number for the
reduced atoms.
E.g. Balance the following equation:
MnO4¯(aq) + Br-(aq)  Mn2+(aq) + Br2(aq)
The next step is to find the net increase in oxidation number for oxidized
atoms and the net decrease in oxidation number for the reduced atoms.
Then, multiply the net increase and net decrease by suitable factors so that
the two become equal.
2 Half Reactions Method
A half reaction is one of two parts of an oxidation-reduction
reaction, one of which involves a loss of electron and the other
which involves a gain of electrons.
Oxidation is a process in which the O.S. of some element increases
(lose e-) and in which electrons appear on the right side of a halfequation.
Reduction is a process in which the O.S. of some element decreases
and in which electrons appear on the left side of a half-equation.
Oxidation and reduction half-reactions must always occur together,
and the total number of electrons associated with the oxidation must
equal the total number associated with the reduction.
1) Write and balance separate half-equations for oxidation and
reduction, and balance the equation.
2) Adjust coefficients in the two half-equation so that the same
number of electrons appears in each half equation.
3) Add together the two half-equation, then cancel the species
common to both side of the equation to obtain the balanced overall
equation.
Balancing the equation for redox reaction in acid solution.
E.g. (a) Indicate whether each of the following is an oxidationreduction reaction.
(b) balancing the equations.
i) Fe2+(aq) + MnO4(aq) Fe3+(aq)+ Mn2+(aq)
ii) H2PO4(aq) + OH-(aq) HPO42(aq) + H2O(l)
Balance the equation basic solution
E.g. MnO4  (aq)+ SO32 (aq)  MnO2(s) + SO42 (aq) in basic solution
3
Disproportionation Reactions
In some redox reactions, called disproportionation reactions, the same
substance is both oxidized and reduced.
E.g. Decomposition of hydrogen peroxide
E.g. thiosulfate
Stoichiometry of Reaction in Aqueous Solution:
Titrations
An important method for determining the amount of a particular
substance is base on measuring volumes of reactant solution.
Titration is a reaction carried out by the carefully controlled addition
of one solution to another. The trick is stop at the point where both
reactants have reacted completely, a condition called the equivalent
point of the titration.
E.g. A flask contains a solution of unknown amount of HCl. This
solution is titrated with 0.101M NaOH. It take 3.35mL of NaOH to
complete the reaction with HCl. What is the mass of HCl?
E.g. A 0.235g sample of a solid that is 92.55 NaOH and 7.5% Ca(OH)2, by
mass, requires 45.6mL of HCl(aq) solution for its titration. What is the
molarity of the HCl(aq)?