Transcript Chapter 6

Chapter 6
Chemical Reactions:
Classification and Mass
Relationships
Balancing Chemical Equations
• Alphabet – elemental symbols
• Words – chemical formulas
• Sentences – chemical equations (chemical reactions)
reactants  products
limestone  quicklime + gas
Calcium carbonate  calcium oxide + carbon dioxide
CaCO3(s)  CaO(s) + CO2(g)
Balancing Chemical Equations
• Chemical reactions include
– Reactants
– Products
– Balanced – Law of Conservation of Mass
• # of atoms of an element on the reactant side must
equal the # of atoms of that element on the product
side.
– Indicate the state of matter of each chemical
in the reaction (Chapter 4)
Balancing Chemical Equations
• Write the equation without coefficients
• List the elements in each equation
– Secret: if the same polyatomic ion exists on both sides, keep it
together
• Determine the # of each kind of atom on both sides
• Balance atoms one element at a time by adjusting
coefficients
– DO NOT ALTER THE FORMULA OF THE COMPOUND!!!!!
• Only coefficients can be altered
– Secret:
• Balance atoms appearing only once on each side first.
• Save compounds comprised of only one type of element till last.
• Reduce to lowest terms if necessary
Examples
• Balance the following equations:
– Al(s) + Fe2O3(s) → Al2O3 (s) + Fe (l)
– Solid copper reacts with aqueous silver nitrate
to form aqueous copper (II) nitrate and silver
solid
– H3PO4 (l) → H2O (l) + P4O10 (s)
– C4H10(g) + O2 (g) → CO2(g) + H2O (g)
Avogadro’s Number and the Mole
•
Meaning of a chemical reaction
2 C4H10(g) + 13 O2(g) → 8 CO2(g) + 10 H2O (g)
–
2 molecules of C4H10(g) reacts with 13 molecules of O2(g)
to form
8 molecule of CO2(g) and 10 molecules of H2O(g)
Avogadro’s Number and the Mole
• Molecule’s mass = the sum of the atomic
masses of the atoms making up the
molecule.
• m(C2H4O2) = 2·mC + 4·mH + 2·mO
»
= 2·(12.01) + 4·(1.01) + 2·(16.00)
»
= 60.06 amu
Avogadro’s Number of the Mole
• One mole (mol) of any substance contains 6.02
x 1023 (Avogadro’s Number) units of that
substance.
• One mole (mol) of a substance is the gram
mass value equal to the amu mass of the
substance.
– Calculated the same as amu’s for a molecule
Avogadro’s Number and the Mole
• Calculate the molar mass of the
following:
– Fe2O3 (Rust)
– C6H8O7 (Citric acid)
– C16H18N2O4 (Penicillin G)
Avogadro’s Number and the Mole
• Methionine, an amino acid used by organisms
to make proteins, is represented below. Write
the formula for methionine and calculate its
molar mass. (red = O; gray = C; blue = N;
yellow = S; ivory = H)
Stoichiometry
• 4 Conversion units
– Chemical formula
– Balanced chemical equation
• Coefficients can read as;
– # of molecules
– # of moles of that molecules
• Allows conversion between compounds in an equation
– Avogadro’s # - 6.02 x 1023 of X = 1 mole of X
– Molar mass – how many grams of a substance = 1
mole of that substance
Stoichiometric Calculations
Avogadro’s Number and the Mole
• How many grams of oxygen are present in
5.961 x 1020 molecules of KClO3? How
many atoms of oxygen are present?
Avogadro’s Number and the Mole
• Calculate the number of oxygen atoms in
29.34 g of sodium sulfate, Na2SO4.
– A.
– B.
– C.
– D.
– E.
1.244 × 1023 O atoms
4.976 × 1023 O atoms
2.409 × 1024 O atoms
2.915 × 1024 O atoms
1.166 × 1025 O atoms
Problem
• Potassium dichromate, K2Cr2O7, is used in
tanning leather, decorating porcelain and water
proofing fabrics. Calculate the number of
chromium atoms in 78.82 g of K2Cr2O7.
–
–
–
–
–
A.
B.
C.
D.
E.
9.490 × 1025 Cr atoms
2.248 × 1024 Cr atoms
1.124 × 1024 Cr atoms
3.227 × 1023 Cr atoms
1.613 × 1023 Cr atoms
Stoichiometry: Chemical Arithmetic
Stoichiometry: Equation Arithmetic
• Balance the following, and determine how
many moles of CO will react with 0.500
moles of Fe2O3.
Fe2O3(s) + CO(g)
→
Fe(s) + CO2(g)
Stoichiometry: Chemical Arithmetic
•
Aqueous sodium hydroxide and chlorine
gas are combined to form aqueous sodium
hypochlorite (household bleach), aqueous
sodium chloride and liquid water.
–
How many grams of NaOH are needed to react
with 25.0 g of Cl2?
Problem
•
Sulfur dioxide reacts with chlorine to produce thionyl chloride (used
as a drying agent for inorganic halides) and dichlorine monoxide
(used as a bleach for wood, pulp and textiles).
SO2(g) + 2Cl2(g) → SOCl2(g) + Cl2O(g)
If 0.400 mol of Cl2 reacts with excess SO2, how many moles of Cl2O
are formed?
–
–
–
–
–
A.
B.
C.
D.
E.
0.800 mol
0.400 mol
0.200 mol
0.100 mol
0.0500 mol
Problem
• Nitrogen gas and hydrogen gas are combined to form
ammonia (NH3), an important source of fixed nitrogen
that can be metabolized by plants, using the Haber
process.
How many grams of nitrogen are needed to produce 325
grams of ammonia?
–
–
–
–
–
A.
B.
C.
D.
E.
1070 g
535 g
267 g
178 g
108 g
Percent Yields
• Yields of Chemical Reactions: If the actual
amount of product formed in a reaction is less
than the theoretical amount, we can calculate
a percentage yield.
Actual product yield
% yield 
 100%
Theoretica l product yield
Problem
• What is the percent yield for the reaction
PCl3(g) + Cl2(g) → PCl5(g)
if 119.3 g of PCl5 ( MM = 208.2 g/mol) are formed when
61.3 g of Cl2 ( MM = 70.91 g/mol) react with excess
PCl3?
–
–
–
–
–
A.
B.
C.
D.
E.
195%
85.0%
66.3%
51.4%
43.7%
Types of Chemical Reactions
• Chemical Reactions discussed in College
Chemistry can be broken down into 3 main
categories
– Precipitation reactions
– Acid-Base reactions
– Oxidation-Reduction (redox) reactions
Types of Chemical Reactions
• Precipitation Reactions: A process in
which an insoluble solid (precipitate) drops
out of the solution.
– Clear solutions of two ionic compounds when
mixed form a cloudy solution (cloudiness
indicates solid)
Types of Reactions
• Acid–Base Neutralization: A process in
which an acid reacts with a base to yield
water plus an ionic compound called a
salt.
– The driving force of this reaction is the
formation of the stable water molecule.
Types of Reaction
• Metathesis Reactions (Double
Displacement Reaction): These are
reactions where two reactants just
exchange parts.
AX + BY  AY + BX
Types of Reactions
• Oxidation–Reduction (Redox) Reaction:
A process in which one or more electrons
are transferred between reaction partners.
– The driving force of this reaction is the
decrease in electrical potential.
Precipitation Reactions
• Develop the reaction equation
• Balance the reaction equation
• Predict the state of matter of each species
present
Precipitation Reactions and
Solubility Rules
• To predict whether a precipitation reaction
will occur must be able to predict whether
a compound is soluble or not
– Solubility rules
Solubility Rules
• Salts - soluble:
• All alkali metal and ammonium ion salts
• All salts of the NO3–, ClO3–, ClO4–, C2H3O2–,
and HCO3– ions
Solubility Rules
• Salts which are soluble with exceptions:
• Cl–, Br–, I– ion salts except with Ag+, Pb2+, &
Hg22+
• SO42– ion salts except with Ag+, Pb2+, Hg22+,
Ca2+, Sr2+, & Ba2+
Solubility Rules
• Salts which are insoluble with exceptions:
• O2– & OH– ion salts except with the alkali metal ions,
and Ca2+, Sr2+, & Ba2+ ions
• CO32–, PO43–, S2–, CrO42–, & SO32– ion salts except
with the alkali metal ions and the ammonium ion
Precipitation Reactions and
Solubility Rules
• Predict the solubility of:
– (a) CdCO3 (b) MgO
(d) PbSO4
(e) (NH4)3PO4
(c) Na2S
(f) HgCl2
Precipitation Reaction
• Precipitation reactions only occur if a solid
is produced as a product.
• If all products are aqueous compounds
then no reaction has taken place.
Precipitation Reactions and
Solubility Guidelines
• Predict whether a precipitate will form for:
– (a) NiCl2(aq) + (NH4)2S(aq) 
– (b) Na2CrO4(aq) + Pb(NO3)2(aq) 
– (c) AgClO4(aq) + CaBr2(aq) 
Problem
• Select the precipitate that forms when
aqueous ammonium sulfide reacts with
aqueous copper(II) nitrate.
– A.
– B.
– C.
– D.
– E.
CuS
Cu2S
NH4NO3
NH4(NO3)2
CuSO4
Problem
• Select the precipitate that forms when the
following reactants are mixed.
Mg(CH3COO)2(aq) + LiOH(aq) →
–
–
–
–
–
A.
B.
C.
D.
E.
LiCH3COO
Li(CH3COO)2
MgOH
Mg(OH)2
CH3OH
Acids, Bases and Neutralization
Reactions
• Acid / Base Definitions
– Arrhenius
• Acid – donates a H+ (H3O+)
• Base – donates an OH-
– Bronsted-Lowry
• Acid – donates a H+
• Base – H+ acceptor
Acids, Bases and Neutralization
Reactions
Strong Acids (all)
Strong Bases (all)
Hydrochloric acid – HCl (aq)
Lithium hydroxide – LiOH (aq)
Hydrobromic acid – HBr (aq)
Sodium hydroxide – NaOH (aq)
Hydroiodic acid – HI (aq)
Potassium hydroxide – KOH (aq)
Sulfuric acid – H2SO4 (aq)
Calcium hydroxide – Ca(OH)2 (aq)
Nitric acid – HNO3 (aq)
Strontium hydroxide – Sr(OH)2 (aq)
Perchloric acid – HClO4 (aq)
Barium hydroxide – Ba(OH)2 (aq)
Weak Acids (examples)
Acetic acid – HC2H3O2 (aq)
Cyanic acid – HCN (aq)
Phosphoric acid – H3PO4 (aq)
Organic acids – contain ending group
–COOH
Weak Bases (examples)
Ammonia (ammonium hydroxide) – NH3
(aq) actually NH4OH (aq)
Organic amines – contain ending group
-NHx
Acids, Bases and Neutralization
Reactions
• Neutralization Reaction: produces salt & water.
– HA(aq) + MOH(aq)  H2O(l) + MA(aq)
• Write a balanced chemical equation for the
following:
– (a) HBr(aq) + Ba(OH)2(aq) 
– (b) HCl(aq) + NH3(aq) 
Oxidation-Reduction Reactions
• Redox reactions are those involving the
oxidation and reduction of species (element or
ion of an element).
• Oxidation and reduction must occur together.
They cannot exist alone.
• Two important types
– Single displacement reactions (activity series)
– Combustions – reaction of a substance with O2
Oxidation Reduction Reactions
Oxidation
Is
Loss (of electrons)
Anode Oxidation
Reducing Agent
Oxidation Reduction Reactions
Reduction
Is
Gain (of electrons)
Cathode Reduction
Oxidizing Agent
Redox Reactions
• Assigning Oxidation Numbers: All atoms have an
“oxidation number” regardless of whether it carries an
ionic charge.
1. An atom in its elemental state has an oxidation number
of zero.
2. An atom in a monatomic ion has an oxidation number
identical to its charge.
Redox Reactions
3. An atom in a polyatomic ion or in a molecular
compound usually has the same oxidation
number it would have if it were a monatomic ion.
– A. Hydrogen can be either +1 or –1.
– B. Oxygen usually has an oxidation number of –2.
• In peroxides, oxygen is –1.
– C. Halogens usually have an oxidation number of –1.
• When bonded to oxygen, chlorine, bromine, and iodine have
positive oxidation numbers.
Redox Reactions
4. The sum of the oxidation numbers must be zero for a
neutral compound and must be equal to the net charge
for a polyatomic ion.
– A. H2SO4
2(+1) + (?) + 4(–2) = 0 net charge
? = 0 – 2(+1) – 4(–2) = +6
– B. ClO4–
(?) + 4(–2) = –1 net charge
? = –1 – 4(–2) = +7
Problem
• Sodium tripolyphosphate is used in detergents
to make them effective in hard water. Calculate
the oxidation number of phosphorus in
Na5P3O10.
–
–
–
–
–
A.
B.
C.
D.
E.
+3
+5
+10
+15
none of these is the correct oxidation number
Problem
• The oxidation numbers of P, S and Cl in
H2PO2-, H2S and KClO4 are, respectively
– A.
– B.
– C.
– D.
– E.
-1, -1, +3
+1, -2, +7
+1, +2, +7
-1, -2, +7
-1, -2, +3
Redox Reactions
• Whenever one atom loses electrons (is
oxidized), another atom must gain those
electrons (be reduced).
– A substance which loses electrons (oxidized) is called a
reducing agent. Its oxidation number increases.
– A substance which gains electrons (reduced) is called the
oxidizing agent. Its oxidation number decreases.
Redox Reactions
• For each of the following, identify which
species is the reducing agent and which is
the oxidizing agent.
• Ca(s) + 2 H+(aq)  Ca2+(aq) + H2(g)
• 2 Fe2+(aq) + Cl2(aq)  2 Fe3+(aq) + 2 Cl–(aq)
• SnO2(s) + 2 C(s)  Sn(s) + 2 CO(g)
• Sn2+(aq) + 2 Fe3+(aq)  Sn4+(aq) + 2 Fe2+(aq)
Problem
• Identify the oxidizing agent in the following redox
reaction.
Hg2+(aq) + Cu(s) → Cu2+(aq) + Hg(l)
–
–
–
–
–
A.
B.
C.
D.
E.
Hg2+(aq)
Cu(s)
Cu2+(aq)
Hg(l)
Hg2+(aq) and Cu2+(aq)
Problem
• Sodium thiosulfate, Na2S2O3, is used as a "fixer"
in black and white photography. Identify the
reducing agent in the reaction of thiosulfate with
iodine.
2S2O32-(aq) + I2(aq) → S4O62-(aq) + 2I-(aq)
–
–
–
–
–
a.
b.
c.
d.
e.
I2(aq)
I-(aq)
S2O32-(aq)
S4O62-(aq)
S2O32-(aq) and I-(aq)
Optional Homework
• Text – 6.28, 6.29, 6.30, 6.33, 6.34, 6.38,
6.40, 6.42, 6.50, 6.54, 6.56, 6.60, 6.62,
6.66, 6.68, 6.72, 6.74, 6.76, 6.80, 6.82,
6.88, 6.90,6.92, 6.98, 6.100, 6.102, 6.106
• Chapter 6 Homework - website
Required Homework
• Assignment 6