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Honors Chemistry, Chapter 8Page 1
Chapter 8 – Chemical Equations and Reactions
Honors Chemistry, Chapter 8Page 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
Honors Chemistry, Chapter 8Page 3
Indications of a Chemical Reaction
1. Evolution of heat and light. (Examples: Decomposition of ammonium dichromate, burning of methane.)
2. Production of a gas. (Example: Carbon dioxide released when baking soda and vinegar react.)
Honors Chemistry, Chapter 8Page 4
Indications of a Chemical Reaction
3. Formation of a precipitate. (Example: Reaction of ammonium sulfide and cadmium nitrate Yellow ppt.)
4. Color Change. (Example: When iron and water react the iron changes from metallic color to red/brown.)
Honors Chemistry, Chapter 8Page 5
Characteristics of Chemical Equations
1. The equation must represent the known facts (all reactants and products must be identified by chemical analysis.)
2. The equation must contain the correct formulas for the reactants and products.
3. 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.)
Honors Chemistry, Chapter 8Page 6
Elements That Normally Appear as Diatomic or Polyatomic Molecules
Element Symbol Molecular Formula
Physical State
Hydrogen H H2 Gas
Nitrogen N N2 Gas
Oxygen O O2 Gas
Fluorine F F2 Gas
Chlorine Cl Cl2 Gas
Honors Chemistry, Chapter 8Page 7
Elements That Normally Appear as Diatomic or Polyatomic Molecules
Element Symbol Molecular Formula
Physical State
Bromine Br Br2 Liquid
Iodine I I2 Solid
Sulfur S S8 Solid
Phosphorous P P4 Solid
Honors Chemistry, Chapter 8Page 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”
Honors Chemistry, Chapter 8Page 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)
Honors Chemistry, Chapter 8Page 10
Symbols Used in Chemical Equations
Symbol Explanation
Yields; indicates result of a reaction
Indicates a reversible reaction
(s) Reactant or product in the solid state
Precipitate
(l) Reactant or product in the liquid state
(aq) Reactant or product in aqueous solution
(g) Reactant or product in the gaseous state
Honors Chemistry, Chapter 8Page 11
Symbols Used in Chemical Equations
Symbol Explanation
Product in the gaseous state
Reactants are heated ( or heat)
2 atm Reaction at 2 atmospheres pressure
pressure Reaction at elevated pressure
0oC Reaction at 0oC
MnO2 Reaction in the presence of a catalyst
Honors Chemistry, Chapter 8Page 12
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.
Honors Chemistry, Chapter 8Page 13
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
Honors Chemistry, Chapter 8Page 14
Example Reactions
3Fe(s) + 4 H2O(g) Fe3O4 + 4 H2(g)
2HgO(s) 2Hg(l) + O2(g)
C2H4(g) + H2(g) C2H6(g)
Pressure
Pt
Honors Chemistry, Chapter 8Page 15
Significance of a Chemical Equation
1. 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
Honors Chemistry, Chapter 8Page 16
Significance of a Chemical Equation
2. The relative masses of the reactants and products of a chemical reaction can be determined from the reaction’s 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
Honors Chemistry, Chapter 8Page 17
Significance of a Chemical Equation
3. The reverse for a chemical reaction has the same relative amounts of substances as the forward reaction.
Honors Chemistry, Chapter 8Page 18
Balancing Chemical Equations
1. Identify the names of the reactants and the products and write the word equation.
Water hydrogen + oxygen
2. 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)
Honors Chemistry, Chapter 8Page 19
Balancing Chemical Equations3. Balance the equation according to the
law of conservation of massa. Balance different types of atoms one at a time.b. First balance the atoms of elements that are
combined and that appear only once on each side of the equation.
c. Balance polyatomic ions that appear on both sides of the equation as single units.
d. 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)
Honors Chemistry, Chapter 8Page 20
Balancing Chemical Equations
4. Count atoms to be sure the equation is balanced.2H2O(l) 2H2(g) + O2(g) (balanced)
(4H + 2O) = (4H) + (2O)
Honors Chemistry, Chapter 8Page 21
When Balancing . . .
• Always write the correct formulas for both the reactants and the products
• Never try to balance an equation by changing subscripts(!)
Honors Chemistry, Chapter 8Page 22
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.
Honors Chemistry, Chapter 8Page 23
Exercise #1 - Solution
1. zinc + hydrochloric acid zinc chloride + Hydrogen
2. Zn(s) + HCl(aq) ZnCl2(aq) + H2(g)
3. Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)
4. (1 Zn)+(2H + 2Cl) = (1Zn+2Cl) + (2H)
Honors Chemistry, Chapter 8Page 24
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.
Honors Chemistry, Chapter 8Page 25
Exercise #2 - Solution
1. aluminum carbide + water methane + aluminum hydroxide
2. Al4C3(s) + H2O(l) CH4(g) + Al(OH)3(s)
3. Al4C3(s)+12H2O(l)3CH4(g)+4Al(OH)3(s)
4. (4Al+3C)+(24H+12O)
= (3C+12H) + (4Al+12O+12H)
Honors Chemistry, Chapter 8Page 26
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.
Honors Chemistry, Chapter 8Page 27
Exercise #3 - Solution
1. calcium carbide + water ethyne + calcium hydroxide
2. CaC2(s)+H2O(l) HCCH(g)+Ca(OH)2(s)
3. CaC2(s)+2H2O(l) HCCH(g)+Ca(OH)2(s)
4. (1Ca+2C)+(4H+2O)
= (2H + 2C) + (1Ca+2O+2H)
Honors Chemistry, Chapter 8Page 28
Chapter 8, Section 1 Review
1. List four observations that indicate that a chemical reaction has taken place.
2. List three requirements for a correctly written chemical equation.
3. Write a word equation and a formula equation for a given chemical reaction.
4. Balance a formula equation by inspection.
Honors Chemistry, Chapter 8Page 29
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)
Honors Chemistry, Chapter 8Page 30
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
Honors Chemistry, Chapter 8Page 31
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 2FeO4Fe(s) + 3O2 2Fe2O3
Honors Chemistry, Chapter 8Page 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)
Honors Chemistry, Chapter 8Page 33
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)
Honors Chemistry, Chapter 8Page 34
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)
Honors Chemistry, Chapter 8Page 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)
Honors Chemistry, Chapter 8Page 36
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
Honors Chemistry, Chapter 8Page 37
Decomposition of Binary Compounds
• H2O(l) electricity ? 2H2O(l) electricity 2H2(g) + O2(g)
(electrolysis)
• HgO(s) ? 2 HgO(s) 2 Hg(l) + O2(g)
(thermolysis)
Honors Chemistry, Chapter 8Page 38
• Decompostion of Metal Carbonates
CaCO3(s) ?CaCO3 CaO(s) +
CO2(g)
• Decomposition of Metal Hydroxides
Ca(OH)2(s) ? Ca(OH)2(s) CaO(s)
+H2O(g)
Honors Chemistry, Chapter 8Page 39
• Decompostion of Metal Chlorates
KClO3(s) ?2KClO3(s) 2KCl(s)+3O2(g)
• Decomposition of Acids
H2CO3(aq) ? H2CO3(aq) CO2(g)+H2O(l)
H2SO4(aq) ? H2SO4(aq) SO3(g)
+H2O(l)
(Sulfurous Acid ?)
Honors Chemistry, Chapter 8Page 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
Honors Chemistry, Chapter 8Page 41
•Replacement of a Metal with another Metal
Al(s) + Pb(NO3)2(aq) ? 2Al(s)+3Pb(NO3)2(aq)3Pb(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?)
Honors Chemistry, Chapter 8Page 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
Honors Chemistry, Chapter 8Page 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
Honors Chemistry, Chapter 8Page 44
•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)
Honors Chemistry, Chapter 8Page 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)
Honors Chemistry, Chapter 8Page 46
Chapter 8, Section 2 Review
1. Define and give general equations for:a) Synthesis reactions
b) Decomposition reactions
c) Single-replacement reactions
d) Double-replacement reactions
e) Combustion reactions
2. Classify a reaction into one of the five categories above.
Honors Chemistry, Chapter 8Page 47
Chapter 8, Section 2 Review continued
3. List three types of synthesis reactions and five types of decomposition reactions.
4. List four types of single replacement reactions and three types of double replacement reactions.
5. Predict the products of simple reactions given the reactants.
Honors Chemistry, Chapter 8Page 48
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.
Honors Chemistry, Chapter 8Page 49
Activity Series of MetalsRx with water
Rx with steam
Rx with acids
Rx with oxygen
Unreactive
Li
Rb
K
Ba
Sr
Ca
Na
Mg
Al
Mn
Zn
Cr
Fe
Cd
Co
Ni
Sn
Pb
H2
Sb
Bi
Cu
Hg
Ag
Pt
Au
Honors Chemistry, Chapter 8Page 50
Activity Series of Halogens
Fluorine
Chlorine
Bromine
Iodine
Honors Chemistry, Chapter 8Page 51
Chapter 8, Section 3 Review
1. Explain the significance of an activity series.
2. Use an activity series to predict whether a given reaction will occur and what the products will be.