Chapter 8 Chemical Reactions

Chapter 8 Section 1: Writing and Balancing Chemical Reactions

Describing a Chemical Reaction Indications of a Chemical Reaction – Evolution of heat, light, and/or sound – Production of a gas – Formation of a precipitate – Color change

Chemical Equations Reactants – the substances that exist before a chemical change (or reaction) takes place. Products – the new substance(s) that are formed during the chemical changes. CHEMICAL EQUATION indicates the reactants and products of a reaction. REACTANTS  PRODUCTS

Chemical Equations reactantsproducts Depict the kind of reactants and products and their relative amounts in a reaction. 4 Al(s) + 3 O 2 (g) 2 Al 2 O 3 (s) The numbers in the front are called stoichiometric coefficients stoichiometric coefficients. The letters (s), (g), and (l) are the physical states of compounds. reactants product aluminum oxide

Chemical Equations This equation means: 4 Al(s) + 3 O 2 (g) 2 Al 2 O 3 (s) 4 Al atoms + 3 O 2 molecules yield 2 molecules of Al 2 O 3 4 Al moles + 3 O 2 moles yield 2 moles of Al 2 O 3 or 4 g Al + 3 g O 2 yield 2 g Al 2 O 3 4 mol 3 mol O 2 mol Al 2 O 108 g + 96 g = 204 g aluminum oxide sandpaper

Meaning of Chemical Formula Chemical Symbol Meaning Composition H 2 O One molecule of water: Two H atoms and one O atom 2 H 2 O Two molecules of water: Four H atoms and two O atoms H 2 O 2 One molecule of hydrogen peroxide: Two H atoms and two O atoms

Word Equations A WORD EQUATION describes chemical change using the names of the reactants and products. Write the word equation for the reaction of methane gas with oxygen gas to form carbon dioxide and water. methane + oxygen Reactant Product CH 4 O2O2 CO 2 H2OH2O carbon dioxide + water

Showing Phases in Chemical Equations Solid Phase – the substance is relatively rigid and has a definite volume and shape. NaCl(s) Liquid Phase – the substance has a definite volume, but is able to change shape by flowing. H 2 O(l) Gaseous Phase – the substance has no definite volume or shape, and it shows little response to gravity. Cl 2 (g) H 2 O(s) H 2 O(l) H 2 O(g)

Additional Symbols Used in Chemical Equations “Yields”; indicates result of reaction Used to indicate a reversible reaction A reactant or product in the solid state; also used to indicate a precipitate Alternative to (s), but used only to indicate a precipitate A reactant or product in the liquid state A reactant or product in an aqueous solution (dissolved in water) A reactant or product in the gaseous state (s)(s) (l)(l) (aq) (g)(g)

Additional Symbols Used in Chemical Equations Alternative to (g), but used only to indicate a gaseous product Reactants are heated Pressure at which reaction is carried out, in this case 2 atm Pressure at which reaction is carried out exceeds normal atmospheric pressure Temperature at which reaction is carried out, in this case 0 o C Formula of catalyst, in this case manganese (IV) oxide, used to alter the rate of the reaction 2 atm pressure 0 o C MnO 2 

Chemical Equations conservation of matter Because of the principle of the conservation of matter, equation must be balanced An equation must be balanced. It must have the same number of atoms of the same kind on both sides. Lavoisier, 1788

Cl H H H HH H H H 2 + Cl 2  HCl H 2 + Cl 2  2 HCl reactants products H Cl reactants products H Cl (unbalanced) (balanced) Unbalanced and Balanced Equations

Balancing Chemical Equations Balanced Equation – one in which the number of atoms of each element as a reactant is equal to the number of atoms of that element as a product What is the relationship between conservation of mass and the fact that a balanced equation will always have the same number of atoms of each element on both sides of an equation? Determine whether the following equation is balanced. 2 Na + H 2 O  2 NaOH + H 2 2 Na + 2 H 2 O  2 NaOH + H 2

Balancing Chemical Equations Write a word equation for the reaction. Write the correct formulas for all reactants and products. Determine the coefficients that make the equation balanced.

1) Write a word equation for the reaction. Write a balanced equation for the reaction between chlorine and sodium bromide to produce bromine and sodium chloride. 2) Write the correct formulas for all reactants and products. 3) Determine the coefficients that make the equation balance. chlorine + sodium bromide  bromine + sodium chloride Cl 2 + NaBr  Br 2 + NaCl Cl NaBr  Br NaCl

1) Write a word equation for the reaction. Write a balanced equation for the reaction between chlorine and sodium bromide to produce bromine and sodium chloride. 2) Write the correct formulas for all reactants and products. 3) Determine the coefficients that make the equation balance. chlorine + sodium bromide  bromine + sodium chloride Cl 2 + NaBr  Br 2 + NaCl Cl NaBr  Br NaCl

Balancing Chemical Equations An important point to remember 2 NO(g) + O 2 (g)  2NO 2 (g) The 2 to the left of NO(g) and NO 2 (g) refers to the number of molecules present in the balanced equation. It is a “multiplier” for every atom in the molecule. The subscript 2 in O 2 (g) and NO 2 (g) refers to the number of atoms of this type that are present in each molecules (or ionic compound).

Guidelines for Balancing Chemical Equations 1) polyatomic ions first 2) even / odd (make all even) 3) H 2 O Mg(OH) 2 2 4) single elements last Example: need 13 oxygen atoms Multiply by O 2 = “ ” 3X + O 2 2Y + Z X + O 2 2Y + Z X + 13 O 2 4Y + 2Z H-OH vs. ?

CH O 2  CO H 2 O Reactants Products 1 C atom 1 C atom 4 H atoms 4 H atoms 4 O atoms 4 O atoms

H 2 (g) + O 2 (g) H 2 O (l) hydrogen oxygen water Reactants  Products balanced catalyst – speeds up reaction C (s) + O 2 (g) CO 2 (g) carbon oxygen carbon dioxide Reactants Product 1 carbon atom 2 oxygen atoms Reactants Product 2 hydrogen atoms 2 oxygen atoms 1 oxygen atoms Pt Un Reactants Product 2 hydrogen atoms 4 hydrogen atoms 2 oxygen atoms Reactants Product 4 hydrogen atoms 2 oxygen atoms

“At-Seat Self-quiz” Directions: Complete the following brief assignment at your seat, quietly. 1.Balance the following reactions: a.SO 2 + O 2  SO 3 b.Fe 2 O 3 + H 2  Fe +H 2 O c.P + O 2  P 4 O 10 d.Al + N 2  AlN e.Write Reaction a, Reaction c, and Reaction d in words. 2.Express in symbol form: a.sulfur trioxide gas b.zinc chloride as a catalyst c.liquid mercury d.heat supplied to a chemical reaction e.solid copper f.potassium nitrate dissolved in water 3.How is the Law of Conservation of Mass related to the balancing of a chemical equation?

Chapter 8 Section 2: Types of Chemical Reactions

SynthesisSynthesis (Combination) reaction DecompositionDecomposition reaction A Single-replacement reaction Single-replacement B Double-replacement reaction Combustion reaction (of a hydrocarbon) A + B  AB AB  A + B A + BC  AC + B AB + CD  AD + CB CH + O 2  CO 2 + H 2 O A use activity series to predictactivity series B driving force…water, gas, or precipitate elementcompoundelementcompound

Combination Reaction Direct combination reaction (Synthesis) General form: A + B  AB element orelement or compoundcompound Na Cl Na Cl 2 Na + Cl 2  2 NaCl  Cl Na Cl

Combination Reaction Direct combination reaction (Synthesis) General form: A + B  AB element orelement or compoundcompound Na + Cl - Na ClNa Cl Na + Cl - 2 Na + Cl 2  2 NaCl

H H H H Decomposition Reaction H H H H O Decomposition reaction 2 H 2 O 2 H 2 O2O2 General form: ABAB compoundtwo or more elements or compounds OO O

H 2 O H 2 + O 2 electricity Decomposition Reactions H 2 O 2 H 2 O + O 2 NI 3 N 2 + I 2 22 Hydrogen Peroxide Electrolysis of water 2 2 Nitrogen triiodide AB A + B General Form 23

NI 3 N 2 + I 2 Nitrogen triiodide 2 3

Single and Double Replacement Reactions Double-replacement reaction CaCO HCl  CaCl 2 + H 2 CO 3 General form: AB + CD  AD + CB Single-replacement reaction Mg + CuSO 4  MgSO 4 + Cu General form: A + BC  AC + B

Single Replacement Reaction Example P O W !

Ca Activity Series (use with single-replacement) Foiled again – Aluminum loses to Calcium Element Reactivity Li Rb K Ba Ca Na Mg Al Mn Zn Cr Fe Ni Sn Pb H 2 Cu Hg Ag Pt Au Halogen Reactivity F 2 Cl 2 Br 2 I 2 Printable Version of Activity Series Printable Version of Activity Series

Single-Replacement Reactions FeCl 2 + Cu MgBr 2 + Cl 2 “Magic blue-earth” Zinc in nitric acid 2 A + BC AC + B General Form Zn(NO 3 ) 2 + H 2 Can Fe replace Cu? Yes Li Rb K Ba Ca Na Mg Al Mn Zn Cr Fe Ni Sn Pb H 2 Cu Hg Ag Pt Au F 2 Cl 2 Br 2 I 2 Can Zn replace H? Yes Can Br replace Cl? No NO REACTION Fe + CuCl 2 Zn + HNO 3 MgCl 2 + Br 2 Activity Series

Double Replacement Reaction K 2 CO 3 (aq) Potassium carbonate BaCl 2 (aq) Barium chloride 2 KCl (aq) Potassium chloride BaCO 3 (s) Barium carbonate ++

Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. PbI 2 K 1+ Pb 2+ lead (II) chloride + potassium iodide Cl 2 (aq) I (aq) Pb 2+ Pb K 1+ K Cl 1- I 1- Cl 1- I 1- KCl potassium chloride lead (II) iodide + (aq) (s) + +

“At-Seat Self-quiz” Directions: Complete the following brief assignment at your seat, quietly. 1.Balance each equation. Also, identify what type of reaction best describes the chemical equation. a.Pb(NO 3 ) 2 + K 2 CrO 4  PbCrO 4 + KNO 3 b.Cl 2 + KI  KCl + I 2 c.C 3 H 6 + O 2  CO 2 + H 2 O d.Al(OH) 3  Al 2 O 3 + H 2 O e.Li + O 2  Li 2 O f.HCl + Fe 2 O 3  FeCl 3 + H 2 O g.MgCO 3  MgO + CO 2 h.Ba(CN) 2 + H 2 SO 4  BaSO 4 + HCN 2.Predict the products of the following reactions, then complete them by balancing. a.CaI 2 + Hg(NO 3 ) 2  b.Al + Cl 2  c.Ag + HCl  d.C 2 H 2 + O 2  e.MgCl 2  f.H 2 O 2 