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Chemical Reactions

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Title: Chemical Reactions


1
Chapter 7
  • Chemical Reactions

2
Homework
  • Assigned Problems (odd numbers only)
  • Questions and Problems 7.1 to 7.31 (begins on
    page 200)
  • Additional Questions and Problems 7.41 to 7.49
    (page 221)
  • Challenge Questions 7.51-7.57 (page 222)

3
Evidence of a Chemical Reaction
  • Physical changes
  • Involves no changes in chemical identity of a
    substance
  • No changes in physical properties (color,
    physical state, freezing point, boiling point)
  • Chemical changes
  • A chemical reaction in which one or more
    substances changes to a different substance
  • Properties that matter exhibits as it undergoes
    changes in chemical composition

4
Evidence of a Chemical Reaction
  • As a result of the chemical change, a chemical
    reaction has occurred which produces at least one
    new substance
  • Changes that identify chemical reactions (visual
    clues)
  • color change
  • precipitate formation
  • gas bubbles
  • flames
  • heat release

5
Evidence of a Chemical Reaction
  • A chemical change is a process that causes a
    change in the chemical composition of a substance
  • The creation of one or two new substances is
    characteristic of a chemical change
  • A chemical change occurs when new substances are
    made
  • Conversion of material(s) into one or more new
    substances
  • These substances will have different properties
    from the original material

6
Evidence of a Chemical Reaction
Fe Fe2O3
Li LiOH, H2
HCO3- CO2
Na NaOH, H2
7
The Chemical Equation
  • Chemical properties determine whether or not a
    substance can be changed to another substance
  • Chemical Reactions
  • are processes that involve chemical changes in
    matter resulting in new substances
  • involve a rearrangement and exchange of atoms to
    produce new molecules
  • Elements are not changed during a reaction
  • Chemists have developed a shorthand way of
    stating chemical changes on paper The chemical
    equation

8
The Chemical Equation
  • A chemical equation is a written statement that
    uses symbols and formulas (or words) to describe
    the changes during a chemical reaction
  • It shows substances at the beginning of a
    reaction (reactants)
  • It shows substances formed in the reaction
    (products)
  • The reactants and products are separated by an
    arrow

Reactants ? Products
9
The Chemical Equation
  • For a chemical equation to be valid
  • Only the reactants and products involved in the
    reaction are shown in the equation
  • The accurate formulas must be used for each of
    the substances
  • The law of conservation of mass must be obeyed
    Equal numbers of the same kind of atom on the
    reactant and product side of the equation

10
The Chemical Equation
  • The law of conservation of mass is complied with
    during a chemical reaction
  • Matter is not created nor destroyed during a
    chemical reaction
  • At least one new substance is produced but the
    quantity of matter does not change
  • Every atom present as a reactant has to be
    present as a product
  • The sum of the masses of the products is always
    equal to the sum of the masses of the reactants

11
How to Write Balanced Chemical Equations
  • A balanced chemical reaction has the same number
    of atoms of each element involved on each side of
    the equation
  • A balanced equation is consistent with the law of
    conservation of mass
  • Chemical reactions can be written as
  • Word equations
  • Formula equations


reactants
products
12
How to Write Balanced Chemical Equations
  • A balanced chemical reaction has the same number
    of atoms of each element on both sides of the
    arrow
  • Every atom must be accounted for (atoms are
    neither created nor destroyed)
  • Equations are balanced by placing a coefficient
    in front of one or more of the substances in the
    equation

13
How to Write Balanced Chemical EquationsSymbols
Used in Equations
  • To convey more information than just the type of
    chemical involved, symbols are used after a
    chemical formula to indicate its physical state
  • (g) gas
  • (l) liquid
  • (s) solid
  • (aq) aqueous, dissolved in water

14
How to Write Balanced Chemical Equations
  • When magnesium metal burns in air it produces a
    white, powdery compound magnesium oxide
  • Burning in air means reacting with O2
  • Write the word equation
  • The reactants are to the left of the arrow
  • The products are to the right of the arrow
  • Two or more reactants or products are separated
    by a plus sign

magnesium oxygen magnesium oxide
15
How to Write Balanced Chemical Equations
  • Indicate the physical state of each substance
  • Use the correct chemical symbol to indicate
    liquids and solids
  • Metals are solids, except for Hg which is liquid
  • Use molecular form for gases (H2, O2, N2, all
    halogens)
  • Identify polyatomic ions

magnesium(s) oxygen(g) magnesium
oxide(s)
16
How to Write Balanced Chemical Equations
  • Convert the word equation into a formula equation
  • Use the correct chemical symbol to indicate
    liquids and solids
  • There must be the same number of each kind of
    atom on the reactant and product side of the
    equation
  • Determine if the equation is balanced
  • If not equal, must BALANCE

___Mg (s) ___O2 (g) ___MgO(s)
17
How to Write Balanced Chemical Equations
  • Unbalanced equations can be adjusted by the use
    of a coefficient placed to the left of a
    substance
  • Coefficients will adjust the number of reactant
    and/or products molecules or formula units
  • NEVER change the subscripts of a compound to
    balance an element
  • It changes the identity of the compound
  • Can change coefficients but never subscript
    numbers

18
How to Write Balanced Chemical Equations
  • Guidelines when writing balancing chemical
    equations
  • Write the unbalanced chemical equation
  • Pick the element that occurs in only one compound
    on both sides of the equation
  • The element that occurs as a free element on
    either side of the equation, balance it last

19
Balancing a Chemical EquationExample 1
2
2
Coefficient
1 Mg
1 Mg
2 O
1 O
20
Balancing a Chemical Equation Example 2
  • When solid ammonium nitrite is heated it produces
    nitrogen gas and water vapor
  • Write the formula equation

21
Balancing a Chemical Equation Example 2
2
2 x N
2 x N
2 x O
1 x O
4 x H
2 x H
22
Balancing a Chemical Equation Example 3
  • Nitrogen monoxide gas decomposes to produce
    dinitrogen monoxide gas and nitrogen dioxide gas
  • Write the formula equation

23
Balancing a Chemical Equation Example 3
3
1 x N
3 x N
1 x O
3 x O
24
Balancing a Chemical Equation Example 4
  • Liquid nitric acid decomposes to reddish-brown
    nitrogen dioxide gas, liquid water and oxygen
    gas.
  • Write the formula equation

25
Balancing a Chemical Equation Example 4
2
2
2
1 x N
1 x N
3 x O
5 x O
1 x H
2 x H
26
Aqueous Solutions and SolubilityCompounds
Dissolved in Water
  • An aqueous solution
  • A homogeneous mixture of a substance with water
  • It contains one visible phase, cannot
    differentiate the various components
  • Same composition throughout with one substance
    dissolved into another
  • A uniform mixture requires complete interaction
    of the components

27
Aqueous Solutions and SolubilityCompounds
Dissolved in Water
  • Two parts solvent and solute
  • Solute substance being dissolved
  • Solvent substance that dissolves the solvent
  • Most solutions are liquid but can be gaseous or
    solid

28
Aqueous Solutions and SolubilityCompounds
Dissolved in Water
  • Water is an effective solvent for dissolving
    ionic compounds
  • Water is neutral but the O atom is surrounded by
    many electrons and gives it a partial negative
    charge
  • The other end which contains the H atom has a
    partial positive charge
  • The positive ions are attracted to the O atom and
    negative ions to the H atom of water
  • As an ionic compound dissolves, it becomes
    surrounded by water molecules

29
Aqueous Solutions and SolubilityCompounds
Dissolved in Water
  • When sodium chloride crystals are placed in
    water, they begin to dissolve
  • The attractive forces between the ions and water
    are stronger than forces between the ions in the
    crystal
  • Water molecules surround each ion, keeping them
    apart
  • The ions become uniformly dispersed throughout
    the solution

30
Aqueous Solutions and SolubilityCompounds
Dissolved in Water
  • When ionic compounds dissolve in water the ions
    dissociate and separate into ions floating in
    water
  • Potassium chloride dissociates in water into
    potassium cations and chloride anions

KCl(aq) K (aq) Cl- (aq)
31
Aqueous Solutions and Solubility Compounds
Dissolved in Water
  • Copper(II) sulfate dissociates in water into
    copper(II) cations and sulfate anions

CuSO4(aq) Cu2(aq) SO42-(aq)
32
Aqueous Solutions and Solubility Compounds
Dissolved in Water
  • Potassium sulfate dissociates in water into
    potassium cations and sulfate anions

K2SO4(aq) 2 K (aq) SO42-(aq)
33
Electrolytes and Nonelectrolytes
  • Electrolytes
  • A substance that exists as ions in solution
  • Formed from an ionic compound that dissociates in
    water forming a solution with cations and anions
  • Strong electrolytes are ionic compounds that
    dissociate (nearly) completely in solution and
    exist as ions
  • Strong electrolyte solutions conduct electricity
    due to the presence of ions

34
Electrolytes and Nonelectrolytes
  • Nonelectrolytes
  • Solute is a molecular substance (not ionic)
  • It does not form ions in solution (no
    dissociation)
  • Substance remains intact, dispersed throughout
    the solvent as individual molecules
  • Each molecule is separated by molecules of the
    solvent
  • These substances do not conduct electricity

35
Electrolytes and Nonelectrolytes
  • Strong electrolyte
  • Dissociates completely into ions
  • Conducts electricity
  • NaCl, HCl
  • Weak electrolyte
  • Mainly whole molecules
  • Very few separate (into ions)
  • Conducts electricity less than strong
    electrolytes
  • Acetic acid, HF
  • Nonelectrolyte
  • No dissociation into ions
  • Do not conduct electricity
  • Methanol, sucrose

36
Electrolytes and Nonelectrolytes
  • Strong electrolytes are completely ionized when
    dissolved in water
  • Sodium chloride dissociates to form Na and Cl-
  • Good conductor of electricity

37
Electrolytes and Nonelectrolytes
  • Weak electrolytes are only partially ionized when
    dissolved in water
  • Hydrofluoric acid only partially dissociates to
    form H and F-
  • Poor conductor of electricity

38
Electrolytes and Nonelectrolytes
  • Nonelectrolyes are not ionized when dissolved in
    water
  • e.g. sugar and ethanol do not dissociate into
    ions in water
  • Do not conduct electricity

39
SolubilitySoluble and Insoluble Salts
  • Not all ionic compounds are soluble in water
  • If water cannot overcome the ionic forces to
    separate the ions in solution insoluble compound
  • The factors to determine solubility are complex
    which make predictions difficult
  • Through observation a series of statements or
    rules have developed to guide predictions called
    the solubility rules (See Table 7.2, page 210 and
    handout)

40
Solubility Rules
  • Most compounds that contain NO3- and C2H3O2- ions
    are soluble in water
  • Most compounds that contain Li, Na, K, or NH4
    ions are soluble in water
  • Most compounds that contain Cl-, Br-, I- ions are
    soluble, except AgX, PbX2, and Hg2X2
  • X Cl-, Br-, I-
  • Most compounds that contain SO42- ions are
    soluble, except SrSO4, BaSO4, PbSO4, CaSO4

41
Solubility Rules
  • Most compounds that contain OH- ions are slightly
    soluble (will precipitate).
  • Exceptions NaOH, KOH, are soluble and
  • Ba(OH)2, Ca(OH)2 are moderately soluble
  • Most compounds that contain S2-, CO32-, or PO43-
    ions are slightly soluble (will precipitate)

42
Precipitation Reactions
  • Upon mixing two aqueous solutions, if two ions of
    an insoluble salt come into contact, a solid
    forms
  • This is a precipitation reaction
  • The solid that forms is called a precipitate
  • Can use the solubility rules to predict whether a
    solid will form when two ionic solutions are mixed

43
Precipitation Reactions
  • Two aqueous clear solutions are mixed together
    lead (II) nitrate and potassium iodide
  • The reaction between two solutes produces an
    insoluble product lead (II) iodide

44
Predicting Precipitation Reactions
  • There are no simple (basic) rules to predict
    which ionic compounds will be insoluble
  • Experimental observations have led to the
    guidelines for predicting solubility (see text
    table 7.2, page 210 and handout)
  • These solubility rules will be used to predict
    the products formed in precipitation reactions

45
Predicting Precipitation Reactions
  • In solution, the cations of the two reactants
    exchange anions
  • The chemical formulas are based on the charges on
    the ions
  • Will the new interactions of cations and anions
    form an insoluble compound?

46
Predicting Precipitation Reactions
  • To predict the identities of the products
  • Write the formulas of the two compounds being
    mixed as reactants in a chemical equation
  • Separate all soluble ionic compounds into ions

AB CD
A B- C D-
Reactants
47
Predicting Precipitation Reactions
  • Write the formulas of the potentially insoluble
    products that could form from the new combination
    of the ions
  • Exchange pairs
  • Charge balance each compound
  • A B-
  • C D-

A D- C B-
Products
48
Predicting Precipitation Reactions
  1. Use the solubility rules to determine whether any
    of the new products are insoluble
  2. If all of the potentially insoluble products are
    soluble, there will be no precipitate (NO
    REACTION)
  3. If any of the potential products are insoluble,
    use symbol (s) to indicate a solid forms
  4. Balance the equations with coefficients

49
Writing Equations for Precipitation Reactions
  • Aqueous solutions of calcium chloride and sodium
    carbonate are mixed
  • Write the formula of the two compounds
  • Write the formulas of the potentially insoluble
    products that could form from the reactants

Ca2
2 Cl-
2 Na
CO32-
CaCO3 and NaCl
50
Writing Equations for Precipitation Reactions
  • Use the solubility rules to determine whether any
    of the new products are insoluble
  • NaCl is soluble (Rule 2, Rule 3)
  • CaCO3 is insoluble (Rule 6)
  • Since one of the potential products is insoluble,
    write the formulas of the products of the reaction

51
Writing Equations for Precipitation Reactions
  • Include the symbol (s) to indicate solid and (aq)
    to indicate aqueous products
  • Balance the equation
  • Use coefficients, do not change the subscripts

2
52
Molecular, Complete Ionic, and Net Ionic
Equations
  • Example
  • Two aqueous clear solutions are mixed together
    lead (II) nitrate and potassium iodide
  • The reaction between two solutes produces an
    insoluble product lead (II) iodide

53
Molecular, Complete Ionic, and Net Ionic
Equations
  • Molecular Equations
  • Equations which show the complete chemical
    formulas for all reactants and products
  • Uses neutral formulas or symbols without
    indicating ionic character

54
Molecular, Complete Ionic, and Net Ionic
Equations
  • Complete Ionic Equations
  • Separates the neutral formulas into the ions they
    will actually form in solution
  • Equations which describe the actual ions and
    molecules in the solutions as well as the
    molecules of solid, liquid and gas not dissolved

55
Molecular, Complete Ionic, and Net Ionic
Equations
  • Spectator Ions
  • Ions that appear in identical forms on both sides
    of the equation
  • Although present, they play no role in the
    reaction
  • Net Ionic Equation
  • An ionic equation in which the spectator ions are
    omitted
  • It shows only the ions that are directly involved
    in a chemical reaction

56
Predicting Reactions
  • When chemicals (dissolved in water) are mixed and
    one of these four events can occur, the reaction
    will generally happen
  • Forces that drive a reaction
  • formation of a solid
  • formation of water
  • transfer of electrons
  • formation of a gas

57
Predicting Reactions
  • Determine if product(s) are
  • Insoluble (precipitation reaction)
  • Water (acid/base reaction)
  • Electron transfer (redox reaction)
  • Gas (gas-forming reaction)

58
Acid-Base (Neutralization) Reactions
  • Acids are substances that form H ion in water
  • Bases are substances that form OH- ion in water
  • A Neutralization reaction is the reaction between
    an acid and a hydroxide base to form a salt and
    water

59
Acid-Base (Neutralization) Reactions
  • The reaction of hydrochloric acid and sodium
    hydroxide
  • A double replacement reaction
  • Ionic Equation (remove spectator ions)
  • Net Ionic Equation (water forms)

water
acid
base
salt
60
Predicting Reactions
  • Determine if product(s) are
  • Insoluble (precipitation reaction)
  • Water (acid/base reaction)
  • Electron transfer (redox reaction)
  • Gas (gas-forming reaction)

61
Gas Evolution Reactions
  • Other anions that will react with H are sulfide,
    carbonate, and bicarbonate ion
  • These anions react with acids to form gases
  • Sulfuric acid reacting with aqueous lithium
    sulfide
  • Molecular equation

salt
base
gas
acid
62
Gas Evolution Reactions
  • Complete ionic equation
  • Net ionic equation

(balanced)
63
Predicting Reactions
  • Determine if product(s) are
  • Insoluble (precipitation reaction)
  • Water (acid/base reaction)
  • Electron transfer (redox reaction)
  • Gas (gas-forming reaction)

64
Oxidation-Reduction Reactions
  • When an atom, ion, or molecule becomes more
    positively charged, a loss of electrons occurs
    oxidation
  • When an atom, ion, or molecule becomes more
    negatively charged, a gain of electrons occurs
    reduction
  • When one reactant loses electrons another
    reactant must gain electrons
  • Reactions in which this electron transfer occurs
    between reactants is called an oxidation-reduction
    (redox) reaction

65
Oxidation-Reduction Reactions
  • A substance reacting with elemental oxygen
  • A metal reacting with a nonmetal
  • Electron transfer between two reactants
  • Be able to identify these types of redox
    reactions

66
Combustion Reactions
  • Combustion occurs when a hydrocarbon (or other)
    combines with oxygen to produce
  • carbon dioxide
  • water
  • heat (flame)
  • The reaction of oxygen with any substance
  • If a combustion reaction is possible then the
    substance will burn

67
Combustion Reactions
  • Examples
  • The combustion reaction of hydrocarbons produces
    carbon dioxide, water, and heat
  • The combustion of propane
  • The combustion of glucose
  • The combustion of sulfur
  • Also a combination reaction, also produces heat

heat
hydrocarbon
heat
hydrocarbon
heat
68
Classifying Chemical Reactions
  • Chemical reactions can be separated into groups
    of similar reactions
  • There are several ways to group reactions but
    most can be placed into one of four classes
  • Synthesis
  • Decomposition
  • Single-displacement
  • Double-displacement

69
Classifying Chemical Reactions
  • Synthesis Reactions
  • Reactions in which two or more substances combine
    to form a third substance
  • One new product forms
  • General form of equation

A B
AB
70
Synthesis Reactions
  • The combinations can include
  • Two elements
  • An element and a compound
  • Two compounds
  • Examples

71
Classifying Chemical ReactionsDecomposition
  • Decomposition Reactions
  • Reactions in which one reactant breaks down into
    simpler (smaller) substances
  • Generally initiated by addition of light energy,
    electric current, or heating substances to high
    temperature
  • Opposite of a Synthesis Reaction
  • General Form of Equation

AB
A B
72
Decomposition Reactions
  • Can be broken down to
  • Smaller compounds
  • Elements
  • Both
  • Examples

73
Classifying Chemical ReactionsSingle-Displacement
Reactions
  • Single-Displacement Reactions
  • One element displaces another element
  • Forms a new compound which frees the replaced
    element
  • Most reactions occur in an aqueous solution
  • General Form of Equation

A BC
AC B
74
Single-Displacement Reactions
  • In this type of single-displacement reaction
    there are two types
  • Metal displaces a metal
  • Metal displaces hydrogen
  • Examples

metal
metal
displaces
metal
displaces
hydrogen
75
Classifying Chemical ReactionsDouble-Replacement
Reactions
  • Double-Displacement Reactions
  • Two compounds exchange ions or atoms to form new
    compounds
  • Also called exchange reactions
  • Shows the exchange of associates when comparing
    the reactants and products
  • General Form of Equation

AB CD
AD BC
76
Double Replacement Reactions
  • Most of these reactions occur in aqueous solution
  • Most involve acids, bases, and ionic compounds
  • Products formed
  • Precipitate (a solid that is insoluble)
  • A gas
  • Water

77
Double Replacement Reactions
  • Examples

precipitate
gas
water
78
Summary of Reaction Types
79
  • End
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