Chemical Reactions: Classification and Mass Relationships

1
Chapter 6

• Chemical Reactions Classification and Mass
  Relationships

2
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)

3
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)

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

5
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)

6
Avogadros 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)

7
Avogadros Number and the Mole

• Molecules mass the sum of the atomic masses of
  the atoms making up the molecule.
• m(C2H4O2) 2mC 4mH 2mO
• 2(12.01) 4(1.01) 2(16.00)
• 60.06 amu

8
Avogadros Number of the Mole

• One mole (mol) of any substance contains 6.02 x
  1023 (Avogadros 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 amus for a molecule

9
Avogadros Number and the Mole

• Calculate the molar mass of the following
• Fe2O3 (Rust)
• C6H8O7 (Citric acid)
• C16H18N2O4 (Penicillin G)

10
Avogadros 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)

11
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
• Avogadros - 6.02 x 1023 of X 1 mole of X
• Molar mass how many grams of a substance 1
  mole of that substance

12
Stoichiometric Calculations
13
Avogadros 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?

14
Avogadros Number and the Mole

• Calculate the number of oxygen atoms in 29.34 g
  of sodium sulfate, Na2SO4. 
• A.  1.244 1023 O atoms
• B.  4.976 1023 O atoms
• C.  2.409 1024 O atoms
• D.  2.915 1024 O atoms
• E.  1.166 1025 O atoms

15
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.  9.490 1025 Cr atoms
• B.  2.248 1024 Cr atoms
• C.  1.124 1024 Cr atoms
• D.  3.227 1023 Cr atoms
• E.  1.613 1023 Cr atoms

16
Stoichiometry Chemical Arithmetic
17
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)

18
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?

19
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.  0.800 mol
• B.  0.400 mol
• C.  0.200 mol
• D.  0.100 mol
• E.  0.0500 mol

20
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.  1070 g
• B.  535 g
• C.  267 g
• D.  178 g
• E.  108 g

21
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.

22
Problem

• What is the percent yield for the
  reactionPCl3(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.  195
• B.  85.0
• C.  66.3
• D.  51.4
• E.  43.7

23
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

24
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)

25
Types of Reactions

• AcidBase 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.

26
Types of Reaction

• Metathesis Reactions (Double Displacement
  Reaction) These are reactions where two
  reactants just exchange parts.
• AX BY ? AY BX

27
Types of Reactions

• OxidationReduction (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.

28
Precipitation Reactions

• Develop the reaction equation
• Balance the reaction equation
• Predict the state of matter of each species
  present

29
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

30
Solubility Rules

• Salts - soluble
• All alkali metal and ammonium ion salts
• All salts of the NO3, ClO3, ClO4, C2H3O2, and
  HCO3 ions

31
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

32
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

33
Precipitation Reactions and Solubility Rules

• Predict the solubility of
• (a) CdCO3 (b) MgO (c) Na2S (d) PbSO4 (e)
  (NH4)3PO4 (f) HgCl2

34
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.

35
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) ?

36
Problem

• Select the precipitate that forms when aqueous
  ammonium sulfide reacts with aqueous copper(II)
  nitrate. 
• A.  CuS
• B.  Cu2S
• C.  NH4NO3
• D.  NH4(NO3)2
• E.  CuSO4

37
Problem

• Select the precipitate that forms when the
  following reactants are mixed.Mg(CH3COO)2(aq)
  LiOH(aq) ? 
• A.  LiCH3COO
• B.  Li(CH3COO)2
• C.  MgOH
• D.  Mg(OH)2
• E.  CH3OH

38
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

39
Acids, Bases and Neutralization Reactions
40
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) ?

41
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

42
Oxidation Reduction Reactions

• Oxidation
• Is
• Loss (of electrons)
• Anode Oxidation
• Reducing Agent

43
Oxidation Reduction Reactions

• Reduction
• Is
• Gain (of electrons)
• Cathode Reduction
• Oxidizing Agent

44
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.

45
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.

46
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

47
Problem

• Sodium tripolyphosphate is used in detergents to
  make them effective in hard water. Calculate the
  oxidation number of phosphorus in Na5P3O10. 
• A.  3
• B.  5
• C.  10
• D.  15
• E.  none of these is the correct oxidation number

48
Problem

• The oxidation numbers of P, S and Cl in H2PO2-,
  H2S and KClO4 are, respectively 
• A.  -1, -1, 3
• B.  1, -2, 7
• C.  1, 2, 7
• D.  -1, -2, 7
• E.  -1, -2, 3

49
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.

50
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)

51
Problem

• Identify the oxidizing agent in the following
  redox reaction.Hg2(aq) Cu(s) ? Cu2(aq)
  Hg(l) 
• A.  Hg2(aq)
• B.  Cu(s)
• C.  Cu2(aq)
• D.  Hg(l)
• E.  Hg2(aq) and Cu2(aq)

52
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.  I2(aq)
• b.  I-(aq)
• c.  S2O32-(aq)
• d.  S4O62-(aq)
• e.  S2O32-(aq) and I-(aq)

53
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

54
Required Homework

• Assignment 6