chapter four - 國立臺北科技大學chpro/chem/chap4.pdf · chapter four: types of chemical...
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Chapter Four:
TYPES OF CHEMICALREACTIONS ANDSOLUTION STOICHIOMETRY
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Contents
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4-1 Water, the Common Solvent p127
A polar molecule
Figure 4.1
(left) The water molecule is polar.(right) A space-filling model of the watermolecule.
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Figure 4.2
Polar water molecules interact with the positive andnegative ions of a salts in the dissolving process.
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4-2 The mature of AqueousSolutions: Strong andWeak Electrolytes
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Solvent
Solute
Strong electrolyte
Weak electrolyte
Non-electrolyte
Concentrated
Dilute
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Dissolution of a Solid in a Liquid
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Strong electrolytes:
The result of ionizations in water.
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Weak Electrolytes p132
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Weak Electrolytes
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Nonelectrolytes p133
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Electrolyte Behavior
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Electrolytes
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4-3 The Composition ofSolutions
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M
M
molaritymoles of soluteliters of solution
HClmoles of HCl
liters of solution3
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Molarity (M) = moles of soluteper volume of solution in liters:
Ex 4.1 Calculation of Molarityp134
Calculate the molarity of a solution prepared
by dissolving 11.5 g of solid NaOH in enough
water to make 1.50 L of solution.Solution:
Ex 4.5 Concentration and Volumep135
Typical blood serum is about 0.14 M NaCl.
What volume of blood contains 1.0 mg NaCl?
Solution:
Ex 4.6 Solution of KnownConcentration
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To analyze the alcohol content of a certain wine, a chemist
needs 1.00 L of an aqueous 0.2 M K2Cr2O7 (potassium
dichromate) solution. How much solid K2Cr2O7 must be
weighed out to make this solution?Solution:
Dilutionp137
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Dilution
Ex 4.7 Concentration and Volumep138
What volume of 16 M sulfuric acid must be used to
prepare 1.5 L of a 0.1 M H2SO4 solution?
Solution:
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4-4 Types of chemical Reactionsp140
Types of Solution Reactions
Precipitation reactions
Acid-base reactions
Oxidation-reductions
4-5 Precipitation Reactions p140
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Figure 4.15
The reaction of K2CrO4(aq) and Ba(NO3)2(aq). (a) The molecular-level
“picture”of the mixed solution before any reaction has occurred. (b)
The molecular-level “picture”of the solution after reaction has to form
BaCrO4(s). Note: BaCrO4(s) is not molecular. It actually contains Ba2+
and CrO42- ions packed together in a lattice. (c) A photo of the solution
has occurred, showing the solid BaCrO4 on the bottom.
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Precipitation of Silver Chloride
Table4.1 Simple Rules for Solubility
1. Most nitrate (NO3) salts are soluble.
2. Most alkali (group 1A) salts and NH4+ are
soluble.3. Most Cl, Br, and Isalts are soluble
(NOT Ag+, Pb2+, Hg2
2+)4. Most sulfate salts are soluble (NOT BaSO4,
PbSO4, HgSO4, CaSO4)5. Most OHsalts are only slightly soluble
(NaOH, KOH are soluble, Ba(OH)2,Ca(OH)2 are marginally soluble)
6. Most S2, CO32, CrO4
2, PO43salts are
only slightly soluble.
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Solubility Rules
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Ex 4.8 Predicting Reaction Productsp144
Using the solubility rules in Table 4.1, predict what
will happen when the following pairs of solutions are
mixed. (a) KNO3(aq) and BaCl2(aq); (b)Na2SO4(aq)
and Pb(NO3)2(aq); (c) KOH(aq) and Fe(NO3)3(aq)Solution:
(a)
(b)
(c)
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4-6 Describing Reactions in Solution p145
Complete ionic equation:
Spectator ions
Net ionic equation:
p146Three Types of Equations Are Used toDescribe Reactions in Solution
The formula equation gives overall reaction
stoichiometry but not necessarily the actual forms of
the reactants and products in the solution.
The complete ionic equation represents as ions all
reactants and products that are strong electrolytes.
The net equations includes only those components
undergoing a charge. Spectator are not include.
Ex 4.9 Writing Equations for Reactions p146
For each of the following reactions, write the formula
equation, the complete ionic equation, and the net ionic
equation.
a. Aqueous potassium chloride is added to aqueous silver
nitrate to from a silver chloride precipitate plus aqueous
potassium nitrate.
b. Aqueous potassium hydroxide is mixed with aqueous
potassium nitrate.
Solution:p146
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Notice
The solution with the greatest number of ions is not
necessarily the one in which:
the volume of the solution is the largest.
the formula unit has the greatest number of ions.
Describing Reactions in Solution
1. Molecular equation (reactants and products as
compounds)
AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3 (aq)
2. Complete ionic equation (all strong electrolytes
shown as ions)
Ag+(aq) + NO3(aq) + Na+(aq) + Cl-(aq) AgCl(s) + Na+(aq)
+ NO3(aq)
Describing Reactions in Solution (continued)
3. Net ionic equation (show only components thatactually react)
Ag+(aq) + Cl(aq) AgCl(s)
Na+ and NO3are spectator ions.
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4-7 Stoichiometry of PrecipitationReactions
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Ex 4.10 Determining the Mass ofProduct Formed
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Calculate the mass of solid NaCl that must be added to
1.50 L of a 0.100 M AgNO3 solution to precipitate all the
Ag+ ions in the form of AgCl.
Solution:
Ex 4.11 Determining the Mass of Product Formedp148
When aqueous solutions of Na2SO4 and Pb(NO3)2 are
mixed, PbSO4 precipitates. Calculate the mass of
PbSO4 precipitates. Calculate the mass of PbSO4
formed when 1.25 L of 0.0500 M Pb(NO3)2 and 2.00
L of 0.050 M Na2SO4 are mixed.Solution:
4-8 Acid-Base Reactions p149
An acid is a proton donor An base is a proton acceptor
Key Titration Terms
Titrant - solution of known concentration
used in titration.
Analyte - substance being analyzed.
Equivalence point - enough titrant added
to react exactly with the analyte.
Endpoint - the indicator changes color
so you can tell the equivalence point has
been reached.
Performing Calculations for Acid-Base Reactions
1. List initial species and predict reaction.
2. Write balanced net ionic reaction.
3. Calculate moles of reactants.
4. Determine limiting reactant.
5. Calculate moles of required reactant or
product.
6. Convert to grams or volume, as required.
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Proton Transfer
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Neutralization of a StrongAcid by a Strong Base
Ex 4.12 Neutralization Reactions p150
What volume of a 0.100 M HCl solution is needed to
neutralize 25.0 mL of 0.350 M NaOH?Solution:
Acid-Base Titrations p152
1. The exact reaction between titrant and analyte mustbe known (and rapid)
2. The stoichiometric (equivalence) point must beaccurately.
3. The volume of titrant required to reachstoichiometric point must be accurately
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Acid-Base Titration
Ex 4.14 Neutralization Titration p153
A Student carries out an experiment to standardize (determine the
exact concentration of) a sodium hydroxide solution. To do this , the
student weighs out a 1.3009-g sample of potassium hydrogen
phthalate (KHC8H4O4, often abbreviated KHP). KHP molar mass
204.22 g/mol) has one acidic hydrogen. The student dissolves the
KHP in distilled water. Adds phenolphthalein as an indicator, and
titrates the resulting solution with the sodium hydroxide solution to
the phenolphthalein endpoint. The difference between the final and
initial buret reading indicates that 41.20 mL of the sodium hydroxide
solution is required to react exactly with the 1.3009 g KHP. Calculate
the concentration of sodium hydroxide solution.
Solution:P153
Ex 4.15 Neutralization Analysis P153
An Environmental chemist analyzed the effluent (the
released waste material) from an industrial process known
to produces the compounds carbon tertrachloride(CCl4)
and benzoic acids (HC7H5O2), a weak acid that has one
acidic hydrogen atom per molecule. A sample this effluent
weighting 0.3518 g was shaken with water, and the
resulting aqueous solution required 10.59 mL of 0.1546 M
NaOH for neutralization. Calculate the mass percent ofHC7H5O2 in the original sample.
Ex 4.15 Solution:p154
4-9 Oxidation-Reduction Reactions p154
Oxidation states
Reaction of Sodium and Chlorine p155
Table4.2 Rules for Assigning OxidationStates
1. Oxidation state of an atom in an element = 0
2. Oxidation state of monatomic element = charge
3. Oxygen = -2 in covalent compounds (except in peroxides
where it = -1)
4. H = +1 in covalent compounds
5. Fluorine = -1 in compounds
6. Sum of oxidation states = 0 in compounds
7. Sum of oxidation states = charge of the ion
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Ex 4.16 Assigning Oxidation Statesp157
Assign oxidation states to all atoms in the following.
(a) CO2; (b) SF6; (c) NO3¯Solution:(a)
(b)
(c)
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React
Find the oxidation states for each of the
elements in each of the following compounds:
K2Cr2O7
CO32-
HClO4
MnO2
PCl5
SF4
Ex 4.17 Oxidation-Reduction Reactions p160
When powdered aluminum metal is mixed with pulverized
iodine crystals and a drop of water is added to help the
reaction get started, the resulting reaction produces a great
deal of energy. The mixture bursts into flames, and a purple
smoke of I2 vapor is produced from the excess iodine. The
equation for the reaction is 2Al(s)+3I2(s) 2AII3(s)
For this reaction, identify the atoms that are oxidized and
reduced, and specify the oxidizing and reducing agents.
Solution: p160
4-10 Balancing Oxidation-ReactionEquations
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The Half-Reaction Method for Balancing Oxidation-
Reduction Reaction in Aqueous Solutions
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Zinc and Iodine
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Barking Dogs
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Dry Ice and Magnesium
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Reactions of Silver
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Balancing Oxidation-ReductionReactions
Cr2O72-(aq) + SO3
-(aq) Cr3+(aq) + SO42-(aq)
How can we balance this equation?
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Method of Half Reactions
Cr2O72-(aq) 2Cr3+(aq)
SO3-(aq) + SO4
2-(aq)
How many electrons are involved in each half
reaction?
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6e- + Cr2O72- (aq) 2Cr3+ (aq)
SO3- (aq) + SO4
2- (aq) + 2e-
How can we balance the oxygen atoms?
Method of Half Reactions(continued)
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6e- + Cr2O72- (aq) Cr3+ (aq) + 7H2O
H2O +SO3- (aq) + SO4
2- (aq) + 2e-
How can we balance the hydrogen atoms?
Method of Half Reactions(continued)
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This reaction occurs in an acidic solution.
14H+ + 6e- + Cr2O72-(aq) Cr3+(aq) + 7H2O
H2O +SO3- (aq) SO4
2- (aq) + 2e- + 2H+
How can we balance the electrons?
Method of Half Reactions(continued)
Method of Half Reactions(continued)
14H+ + 6e- + Cr2O72- (aq) Cr3+ (aq) + 7H2O
3[H2O +SO3- (aq) SO4
2- (aq) + 2e- + 2H+]
Cr2O72- (aq) + 3SO3
-(aq) + 8H+(aq) 2Cr3+(aq) +
3SO42-(aq) + 4H2O(l)
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Half-Reaction Method - Balancingin Base
1. Balance as in acid.
2. Add OHthat equals H+ ions (both sides!)
3. Form water by combining H+, OH.
4. Check elements and charges for balance.
Balance the following oxidation-reduction reactions
that occur in acidic solution.
ClO- (aq) + I- (aq) Cl- (aq) + I3- (aq)
Br- (aq) + MnO4- (aq) Br2 (l)+ Mn2+ (aq)
CH3OH(aq) + Cr2O72-(aq) CH2O(aq) + Cr3+(aq)
React
Ex 4.19 Balancing Oxidation-ReductionReactions (Acidic)
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Potassium dichromate (K2Cr2O7) is a bright orange
compound that can be reduced to a blue-violet solution of
Cr3+ ions. Under certain conditions, k2Cr2O7 reacts with
ethyl alcohol (C2H5OH) as follows:
Balance this equation using the half-reaction method.
)()()()()()( 223
522
72 lOHgCOaqCrlOHHCaqOCraqH
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Ex 4.20 Balancing Oxidation-Reductions(Basic)
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Silver is sometimes found in nature as large nuggets;
more often it is found mixed with other metals and
their ores. An aqueous solution containing cyanide
ion is often used to extract the silver using the
following reaction that occurs in basic solution:
Balance this equation using the half-reaction method.
)()()()()( 22 aqCNAgaqOaqCNsAg Basic
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