Honors Chemistry, Chapter 8

1
Chapter 8 Chemical Equations and Reactions
2
Chemical Equations

• A chemical equation represents, with symbols and
  formulas, the identities and relative amounts of
  the reactants and products in a chemical
  reaction.
• Reactants Starting substances on the left side
  of the equation
• Products Final substances on the right side of
  the equation
• Obeys the Law of Conservation of Mass

3
Indications of a Chemical Reaction

• Evolution of heat and light. (Examples
  Decomposition of ammonium dichromate, burning of
  methane.)
• Production of a gas. (Example Carbon dioxide
  released when baking soda and vinegar react.)

4
Indications of a Chemical Reaction

• Formation of a precipitate. (Example Reaction
  of ammonium sulfide and cadmium nitrate ? Yellow
  ppt.)
• Color Change. (Example When iron and water react
  the iron changes from metallic color to
  red/brown.)

5
Characteristics of Chemical Equations

• The equation must represent the known facts (all
  reactants and products must be identified by
  chemical analysis.)
• The equation must contain the correct formulas
  for the reactants and products.
• The law of conservation of mass must be
  satisfied. (Coefficients added as necessary.
  Coefficient a small whole number in front of a
  formula in a chemical equation.)

6
Elements That Normally Appear as Diatomic or
Polyatomic Molecules
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Elements That Normally Appear as Diatomic or
Polyatomic Molecules
8
Word Equations

• Word equation An equation in which the reactants
  and products of a chemical reaction are
  represented by words.
• Methane oxygen ? carbon dioxide Water
• Arrow ( ?) is read as react to yield, yield,
  produce, or form

9
Formula Equations

• Formula equation An equation which represents
  the reactants and products of a chemical reaction
  by their symbols or formulas.
• CH4(g) O2(g) ? CO2 (g) H2O(g) (not balanced)
• CH4(g) O2(g) ? CO2 (g) 2H2O(g) (partially
  balanced)
• CH4(g) 2O2(g) ? CO2 (g) 2H2O(g) (balanced)

10
Symbols Used in Chemical Equations
11
Symbols Used in Chemical Equations
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Reaction Symbol Definitions

• Catalyst A catalyst is a substance that changes
  the rate of a chemical reaction but can be
  recovered unchanged.
• Reversable Reaction A chemical reaction in which
  the products re-form the original reactants.

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Practice Writing Formulas
Barium Acetate Ammonium Sulfate Aluminum
Carbonate Iron (III) Chromate Nickel (II)
Hydroxide Copper (II) Nitrate Rubidium
Chloride Lithium Permanganate Strontium
Peroxide Calcium Phosphate Mercury (I)
Sulfate Magnesium Bromide Zinc Thiosulfate
Tin (IV) Chlorate
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Example Reactions

• 3Fe(s) 4 H2O(g) Fe3O4 4 H2(g)
• 2HgO(s) 2Hg(l) O2(g)
• C2H4(g) H2(g) C2H6(g)

D
Pressure Pt
15
Significance of a Chemical Equation

• The coefficients of a chemical reaction indicate
  relative, not absolute amounts of reactants and
  products.
• H2(g) Cl2(g) ? 2HCl(g)
• In other words
• 1 molecule H2 1 molecule Cl2 2 molecules HCl
• Or
• 1 mole H2 1 mole Cl2 2 mole HCl

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Significance of a Chemical Equation

• The relative masses of the reactants and products
  of a chemical reaction can be determined from the
  reactions coefficients.
• 1 mol H2 x 2.02 g H2/mol H2 2.02 g H2
• 1 mol Cl2 x 70.90 g Cl2/mol Cl2 70.90 g Cl2
• 2 mol HCl x 36.46 g HCl/mol HCl 72.92 g HCl

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Significance of a Chemical Equation

• The reverse for a chemical reaction has the same
  relative amounts of substances as the forward
  reaction.

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Balancing Chemical Equations

• Identify the names of the reactants and the
  products and write the word equation.
• Water ? hydrogen oxygen
• Write a formula equation by substituting correct
  formulas for the names of the reactants and the
  products.
• H2O(l) ? H2(g) O2(g) (not balanced)

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Balancing Chemical Equations

• Balance the equation according to the law of
  conservation of mass
• Balance different types of atoms one at a time.
• First balance the atoms of elements that are
  combined and that appear only once on each side
  of the equation.
• Balance polyatomic ions that appear on both sides
  of the equation as single units.
• Balance H atoms and O atoms after all other
  elements are balanced.
• 2H2O(l) ? H2(g) O2(g) (Partially balanced)
• 2H2O(l) ? 2H2(g) O2(g) (balanced)

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Balancing Chemical Equations

• Count atoms to be sure the equation is balanced.
• 2H2O(l) ? 2H2(g) O2(g) (balanced)
• (4H 2O) (4H) (2O)

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When Balancing . . .

• Always write the correct formulas for both the
  reactants and the products
• Never try to balance an equation by changing
  subscripts(!)

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Balancing Exercise 1

• Using the four steps, write a balanced chemical
  equation for the reaction between zinc and
  aqueous hydrochloric acid which produces zinc
  chloride and hydrogen gas.

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Exercise 1 - Solution

• zinc hydrochloric acid ? zinc chloride
  Hydrogen
• Zn(s) HCl(aq) ? ZnCl2(aq) H2(g)
• Zn(s) 2HCl(aq) ? ZnCl2(aq) H2(g)
• (1 Zn)(2H 2Cl) (1Zn2Cl) (2H)

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Balancing Exercise 2
Using the four steps, write a balanced chemical
equation for the reaction between aluminum
carbide (Al4C3) and water which produces methane
gas (CH4) and aluminum hydroxide.
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Exercise 2 - Solution

• aluminum carbide water ? methane aluminum
  hydroxide
• Al4C3(s) H2O(l) ? CH4(g) Al(OH)3(s)
• Al4C3(s)12H2O(l)?3CH4(g)4Al(OH)3(s)
• (4Al3C)(24H12O)
• (3C12H) (4Al12O12H)

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Balancing Exercise 3
Using the four steps, write a balanced chemical
equation for the reaction between calcium carbide
(CaC2) and water which produces ethyne gas (HCCH)
(also known as acetylene) and calcium hydroxide.
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Exercise 3 - Solution

• calcium carbide water ? ethyne calcium
  hydroxide
• CaC2(s)H2O(l) ? HCCH(g)Ca(OH)2(s)
• CaC2(s)2H2O(l) ?
  HCCH(g)Ca(OH)2(s)
• (1Ca2C)(4H2O)
• (2H 2C) (1Ca2O2H)

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Chapter 8, Section 1 Review

• List four observations that indicate that a
  chemical reaction has taken place.
• List three requirements for a correctly written
  chemical equation.
• Write a word equation and a formula equation for
  a given chemical reaction.
• Balance a formula equation by inspection.

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Broad Types of Chemical Reactions

• Synthesis A X ?AX
• Decomposition AX ? A X
• Single-Replacement A BX ? AX B or Y
  BX ? BY X
• Double-Replacement AX BY? AY BX
• Combustion 2A O2 ? 2AO
• (coefficients and subscripts may vary)

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Synthesis Reactions

• In a synthesis reaction, also known as a
  composition reaction, two or more substances
  combine to form a new compound.
• A X ?AX
• Three Types
• Elements Oxygen or Sulfur
• Metals with Halogens
• Reactions with Oxides

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Reactions of Elements with Oxygen or Sulfur

• Mg(s) O2(g) ? ? 2Mg(s) O2(g)?
  2MgO(s)
• Rb(s) S8(s) ? ? 16Rb(s)S8(s)?
  8Rb2S(s)
• Fe(s) O2(g) ? ? 2Fe(s) O2 ?
  2FeO 4Fe(s) 3O2 ? 2Fe2O3

32
Reactions of Elements with Oxygen or Sulfur

• C(s) O2(g) ? ? C(s) O2(g) ?
  CO2(g) 2C(s) O2(g)? 2CO(g)
• S8(s) O2 ? ? S8(s) 8O2 ? 8 SO2(g)
• H2(g) O2(g) ? ? 2H2(g) O2(g) ?2H2O(g)
  

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Reactions of Metals with Halogens

• Na(s) Cl2(g) ? ? 2Na(s) Cl2(g) ?
  2NaCl(s)
• K(s) I2(s) ? ? 2K(s) I2(s) ?
  2KI(s)
• Mg(s) F2(g) ? ? Mg(s) F2(g) ? MgF2(s)
• Sr(s) Br2(l) ? ? Sr(s) Br2(l) ?
  SrBr2(s)

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Reactions of Metals with Halogens

• Na(s) F2(g) ? ? 2Na(s) F2(g) ?
  2NaF(s)
• Co(s) F2(g) ? ? 2Co(s) 3F2(g) ?
  2CoF3(s)
• U(s) F2(g) ? ? (Hint Uranium
  (VI)) U(s) 3F2(g) ? UF6(g)

35
Synthesis Reactions with Oxides

• CaO(s) H2O(l) ? ? CaO(s)H2O(l)?Ca(OH)2
  (s)
• SO2(g) H2O(l) ? ? SO2(g)H2O(l) ?
  H2SO3(aq)
• H2SO3(aq) O2(g) ? ? 2H2SO3(aq)
  O2(g) ? 2H2SO4(aq)
• CaO(s) SO2(g) ? ? CaO(s) SO2(g) ?
  CaSO3(s)

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Decomposition Reactions

• In a decomposition reaction, a single compound
  undergoes a reaction that produces two or more
  simpler substances.
• AX ?A X
• Five Types
• Decomposition of Binary Compounds
• Decomposition of Metal Carbonates
• Decomposition of Metal Hydroxides
• Decomposition of Metal Chlorates
• Decomposition of Acids

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Decomposition of Binary Compounds

• H2O(l) electricity ? 2H2O(l)
  electricity 2H2(g) O2(g) (electrolysis)
• HgO(s) D ? 2 HgO(s) D 2 Hg(l)
  O2(g) (thermolysis)

38

• Decompostion of Metal Carbonates
• CaCO3(s) D ? CaCO3 D CaO(s)
  CO2(g)
• Decomposition of Metal Hydroxides
• Ca(OH)2(s) D ? Ca(OH)2(s) D
  CaO(s)H2O(g)

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• Decompostion of Metal Chlorates
• KClO3(s) D ? 2KClO3(s) D
  2KCl(s)3O2(g)
• Decomposition of Acids
• H2CO3(aq) ? ? H2CO3(aq) ? CO2(g)H2O(l)
• H2SO4(aq) D ? H2SO4(aq) D
  SO3(g)H2O(l)
• (Sulfurous Acid ? ?)

40
Single-Replacement Reactions

• In a single-replacement reaction, also known as a
  displacement reaction, one element replaces a
  similar element in a compound.
• A BX ?AX B or
• Y BX ? BY X
• Four Types
• Replacement of a Metal by Another Metal
• Replacement of H2 in Water by a Metal
• Replacement of H2 in an Acid by a Metal
• Replacement of Halogens

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• Replacement of a Metal with another Metal
• Al(s) Pb(NO3)2(aq) ? ? 2Al(s)3Pb(NO3)2(aq)?3
  Pb(s)2Al(NO3)3
• Replacement of H2 in Water by a Metal
• Na(s) H2O(l) ? ? 2Na(s) 2H2O(l) ?
  2NaOH(aq) H2(g)
• Fe(s) H2O(g) ? ? 3Fe(s) 4H2O(g) ?
  Fe3O4(s) 4H2(g)
• (Oxidation State of Fe?)

42

• Replacement of H2 in an Acid by a Metal
• Mg(s) HCl(aq) ? ?
• Mg(s) 2HCl(aq) ? MgCl2(aq) H2(g)
• Replacement of Halogens
• Cl2(g) 2KBr(aq) ? 2KCl(aq) Br2(l)
• F2(g) 2NaCl(aq) ? 2NaF(aq) Cl2(g)
• Br2(l) KCl(aq) ? no reaction

43
Double-Replacement Reactions

• In double-replacement reactions, the ions of two
  compounds exchange places to make two new
  compounds.
• AX BY ?AY BX
• Three Types
• Formation of a Precipitate
• Formation of a Gas
• Formation of Water

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• Formation of a Precipitate
• KI(aq) Pb(NO3)2(aq) ? ?
• 2KI(aq)Pb(NO3)2(aq)?PbI2(s) 2KNO3(aq)
• Formation of a Gas
• FeS(s) 2HCl(aq) ? H2S(g) FeCl2(aq)
• Formation of Water (acid-base reactions)
• HCl(aq) NaOH(aq) ? NaCl(aq) H2O(l)

45
Combustion Reactions

• In a combustion reaction, a substance combines
  with oxygen, releasing a large amount of energy
  in the form of light and heat.
• 2H2(g) O2(g) ? 2H2O(g)
• C3H8(g) 5O2 ? 3CO2(g) 4H2O(g)

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Chapter 8, Section 2 Review

• Define and give general equations for
• Synthesis reactions
• Decomposition reactions
• Single-replacement reactions
• Double-replacement reactions
• Combustion reactions
• Classify a reaction into one of the five
  categories above.

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Chapter 8, Section 2 Review continued

• List three types of synthesis reactions and five
  types of decomposition reactions.
• List four types of single replacement reactions
  and three types of double replacement reactions.
• Predict the products of simple reactions given
  the reactants.

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Activity Series

• An Activity series is a list of elements
  organized according to the ease with which the
  elements undergo certain chemical reactions.
• Each element in the series will replace all
  elements below it in the series in a
  single-replacement reaction.

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Activity Series of Metals
50
Activity Series of Halogens

• Fluorine
• Chlorine
• Bromine
• Iodine

51
Chapter 8, Section 3 Review

• Explain the significance of an activity series.
• Use an activity series to predict whether a given
  reaction will occur and what the products will
  be.