Chapters 6-7a Chemical Reactions

Chapter 6 Table of Contents Evidence for a Chemical Reaction 6.2 Chemical Equations 6.3 Balancing Chemical Equations 7.1 Predicting Whether a Reaction Will Occur 7.2 Reactions in Which a Solid Forms

Section 6.1 Evidence for a Chemical Reaction Return to TOC 3 Chemical reactions often give a visual signal. But reactions are not always visible. What are the clues that a chemical change has taken place?

Section 6.1 Evidence for a Chemical Reaction Return to TOC Copyright © Cengage Learning. All rights reserved 4 Some Clues That a Chemical Reaction Has Occurred

Section 6.1 Evidence for a Chemical Reaction Return to TOC Copyright © Cengage Learning. All rights reserved 5 Exercise What is a clue that a chemical reaction has occurred? a)The color changes. b)A solid forms. c)Bubbles are present. d)A flame is produced.

Section 6.1 Evidence for a Chemical Reaction Return to TOC Copyright © Cengage Learning. All rights reserved 6 Exercise What is a clue that a chemical reaction has occurred? “Colorless hydrochloric acid is added to a red solution of cobalt(II) nitrate, turning the solution blue.” a)The color changes. b)A solid forms. c)Bubbles are present. d)A flame is produced.

Section 6.1 Evidence for a Chemical Reaction Return to TOC Copyright © Cengage Learning. All rights reserved 7 Exercise What is a clue that a chemical reaction has occurred? “A solid forms when a solution of sodium dichromate is added to a solution of lead nitrate.” a)A gas forms. b)A solid forms. c)Bubbles are present. d)A flame is produced.

Section 6.2 Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 8 Chemical reactions involve a rearrangement of the ways atoms are grouped together. A chemical equation represents a chemical reaction.  Reactants are shown to the left of the arrow.  Products are shown to the right of the arrow.

Section 6.2 Chemical Equations Return to TOC 9 In a chemical reaction atoms are not created or destroyed. All atoms present in the reactants must be accounted for in the products.  Same number of each type of atom on both sides of the arrow.

Section 6.2 Chemical Equations Return to TOC 10 Taking 20 kids to the zoo? What if you came home with only 18 kids? Parents are funny that way! What if you came home with 22 kids? At who’s house would you drop them off?

Section 6.2 Chemical Equations Return to TOC 11 Unbalanced Equation: Balancing the Equation: The balanced equation: CH 4 + 2O 2  CO 2 + 2H 2 O Balancing a Chemical Equation

Section 6.2 Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 12 Physical states of compounds are often given in a chemical equation. These are sometimes called descriptors. Physical States

Section 6.2 Chemical Equations Return to TOC 13 Example

Section 6.2 Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 14 Exercise When blue light shines on a mixture of hydrogen and chlorine gas, the elements react explosively to form gaseous hydrochloric acid. What is the unbalanced equation for this process? a)H 2 (g) + CH 4 (g) HCl(g) b)HCl(g) H(g) + Cl(g) c)H(g) + Cl(g) HCl(g) d)H 2 (g) + Cl 2 (g) HCl(g)

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 15 The principle that lies at the heart of the balancing process is that atoms are conserved in a chemical reaction. Atoms are neither created nor destroyed. The same number of each type of atom is found among the reactants and among the products.

Section 6.3 Balancing Chemical Equations Return to TOC 16 Chemists determine the identity of the reactants and products of a reaction by experimental observation. The identities (formulas) of the compounds must never be changed in balancing a chemical equation.

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 17 1.Read the description of the chemical reaction. What are the reactants, the products, and their states? Write the appropriate formulas. Hydrogen gas (H 2 ) and oxygen gas (O 2 ) combine to form liquid water (H 2 O). 2.Write the unbalanced equation that summarizes the information from step 1. H 2 (g) + O 2 (g)  H 2 O(l) How to Write and Balance Equations

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 18 3.Balance the equation by inspection, starting with the most complicated molecule. Equation is unbalanced by counting the atoms on both sides of the arrow. How to Write and Balance Equations

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 19 3.Balance the equation by inspection, starting with the most complicated molecule. We must balance the equation by adding more molecules of reactants and/or products. How to Write and Balance Equations

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 20 How to Write and Balance Equations 4.Check to see that the coefficients used give the same number of each type of atom on both sides of the arrow. Also check to see that the coefficients used are the smallest integers that give the balanced equation. The balanced equation is: 2H 2 (g) + O 2 (g)  2H 2 O(l) or could be: 4H 2 (g) + 2O 2 (g)  4H 2 O(l) preferred

Section 6.3 Balancing Chemical Equations Return to TOC 21 Another Balancing Example:

Section 6.3 Balancing Chemical Equations Return to TOC 22 Balancing using the underline method. Na 2 O(s) + H 2 O(l) NaOH(aq) 2 CH 4 (g) + O 2 (g) CO 2 (g) + H 2 O(g) 2 2 Fe(s) + O 2 (g) Fe 2 O 3 (s) LiOH(s) + CO 2 (g) LiHCO 3 (s) KClO 3 (s) KCl(s) + O 2 (g)  MnO

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 23 Exercise Balance the following equation in standard form (lowest multiple integers) and determine the sum of the coefficients? FeO(s) + O 2 (g)  Fe 2 O 3 (s) a)3 b)4 c)7 d)14 24

Section 6.3 Balancing Chemical Equations Return to TOC 24 Exercise Which of the following correctly balances the chemical equation given below? There may be more than one correct balanced equation. If a balanced equation is incorrect, explain what is incorrect about it. CaO + C  CaC 2 + CO 2 I. CaO 2 + 3C  CaC 2 + CO 2 II. 2CaO + 5C  2CaC 2 + CO 2 III. CaO + (2.5)C  CaC 2 + (0.5)CO 2 IV. 4CaO + 10C  4CaC 2 + 2CO 2

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 25 Exercise Of the three that are correct, which one is preferred most (the most accepted convention)? Why? CaO + C  CaC 2 + CO 2 I. CaO 2 + 3C  CaC 2 + CO 2 II. 2CaO + 5C  2CaC 2 + CO 2 III. CaO + (2.5)C  CaC 2 + (0.5)CO 2 IV. 4CaO + 10C  4CaC 2 + 2CO 2

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 26 Concept Check When balancing a chemical equation, which of the following statements is false? a)Subscripts in the reactants must be conserved in the products. b)Coefficients are used to balance the atoms on both sides. c)When one coefficient is doubled, the rest of the coefficients in the balanced equation must also be doubled. d)Phases are often shown for each compound but are not critical to balancing an equation.

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 27 The number of atoms of each type of element must be the same on both sides of a balanced equation. Subscripts must not be changed to balance an equation. A balanced equation tells us the ratio of the number of molecules which react and are produced in a chemical reaction. Coefficients can be fractions, although they are usually given as lowest integer multiples. Notice

Section 6.3 Balancing Chemical Equations Return to TOC 28 1.Formation of a solid 2.Formation of water 3.Transfer of electrons 4.Formation of a gas Four Driving Forces Favor Chemical Change Section 7.1

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 29 Ukrainian Wolves Partially charged water molecules act like Ukrainian Wolves in desloving fully charged NaCl ion. iology1111/animations/dissolve.swf iology1111/animations/dissolve.swf Section 7.1

Section 6.3 Balancing Chemical Equations Return to TOC 30 A reaction in which a solid forms is called a precipitation reaction.  Solid = precipitate Precipitation Section 7.2

Section 6.3 Balancing Chemical Equations Return to TOC 31 The ions separate and move around independently. Strong electrolyte – each unit of the substance that dissolves in water produces separated ions. What Happens When an Ionic Compound Dissolves in Water? Section 7.2

Section 6.3 Balancing Chemical Equations Return to TOC 32 K 2 CrO 4 (aq) + Ba(NO 3 ) 2 (aq)  Products What Happens When an Ionic Compound Dissolves in Water? Section 7.2

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 33 K 2 CrO 4 (aq) + Ba(NO 3 ) 2 (aq)  Products The mixed solution contains four types of ions: K +, CrO 4 2–, Ba 2+, and NO 3 –. Determine the possible products from the ions in the reactants. The possible ion combinations are: How to Decide What Products Form Section 7.2

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 34 Decide which is most likely to be the yellow solid formed in the reaction. K 2 CrO 4 (aq) reactant Ba(NO 3 ) 2 (aq) reactant The possible combinations are KNO 3 and BaCrO 4.  KNO 3 white solid  BaCrO 4 yellow solid How to Decide What Products Form Section 7.2

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 35 Using Solubility Rules Section 7.2

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 36 Using Solubility Rules Predicting Precipitates  Soluble solid  Insoluble solid  Slightly soluble solid Section 7.2

Section 6.3 Balancing Chemical Equations Return to TOC 37 Let’s Practice Determining Solubility soluble soluble Which of the following are soluble in water?soluble Na 2 CO 3 yes CaCl 2 yes AgClno BaSO 4 no (NH 4 ) 2 Syes Cu(OH) 2 no Ba(OH) 2 Ca 3 (PO 4 ) 2 no yes Pb(NO 3 ) 2 yes PbCl 2 no Section 7.2

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 38 1.Write the reactants as they actually exist before any reaction occurs. Remember that when a salt dissolves, its ions separate. 2.Consider the various solids that could form. To do this, simply exchange the anions of the added salts. 3.Use the solubility rules to decide whether a solid forms and, if so, to predict the identity of the solid. How to Predict Precipitates When Solutions of Two Ionic Compounds Are Mixed Section 7.2

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 39 Concept Check Which of the following ions form compounds with Pb 2+ that are generally soluble in water? a)S 2– b)Cl – c)NO 3 – d)SO 4 2– e)Na + Section 7.2

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 40 Concept Check A sodium phosphate solution reacts with a lead(II) nitrate solution. What precipitate, if any, will form? a)Pb 3 (PO 4 ) 2 b)NaNO 3 c)Pb(NO 3 ) 2 d)No precipitate will form. Section 7.2

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 41 Concept Check Consider a solution with the following ions present: When all are allowed to react (and there is plenty available of each), how many different solids will form? List them. Five different solids will form. PbCl 2, PbSO 4, Pb 3 (PO 4 ) 2, AgCl, Ag 3 PO 4 Section 7.2

Section 6.3 Balancing Chemical Equations Return to TOC 42 Chapters 6-7b Chemical Reactions Vv

Section 6.3 Balancing Chemical Equations Return to TOC Describing Reactions in Aqueous Solutions 7.4 Reactions That Form Water: Acids and Bases 7.5 Reactions of Metals with Nonmetals (Oxidation–Reduction) 7.6Ways to Classify Reactions 7.7Other Ways to Classify Reactions

Section 6.3 Balancing Chemical Equations Return to TOC 44 Net Ionic Equations AgNO 3 (aq) + NaCl(aq) 1. Divorce Ag + +NO Na + + Cl - 2. Change Partners Ag + +NO Na + + Cl - AgCl + NaNO 3 3. Soluble? Ag + +NO Na + + Cl - AgCl(s) + Na + + NO Cross out Spectator Ions Ag + + Cl - AgCl(s)5. Balance Ag + + Cl - AgCl(s) Total Ionic Equation Molecular Equation Net Ionic Equation AgCl(s) + NaNO 3 (aq) Section 7.2

Section 6.3 Balancing Chemical Equations Return to TOC 45 Pb(NO 3 ) 2 (aq) + NaI(aq) 1. Divorce Pb +2 +NO Na + + l - 2. Change Partners Pb +2 +NO Na + + l - Pbl 2 + NaNO 3 3. Soluble? Pb +2 +NO Na + + l - Pbl 2 (s) + Na + NO Cross out Spectator Ions Pb l - Pbl 2 (s)5. Balance Pb +2 + l - Pbl 2 (s) Total Ionic Equation Molecular Equation Net Ionic Equation Pbl 2 (s) + NaNO 3 (aq) Section 7.2 Net Ionic Equations

Section 6.3 Balancing Chemical Equations Return to TOC 46 BaCl 2 (aq) + Na 2 SO 4 (aq) 1. Divorce Ba +2 + Cl - + Na + + SO Change Partners Ba +2 + Cl - + Na + + SO 4 -2 BaSO 4 + NaCl 3. Soluble? Ba +2 + Cl - + Na + + SO 4 -2 BaSO 4 (s)+ Na + + Cl - 4. Cross out Spectator Ions Ba +2 + SO 4 -2 BaSO 4 (s)5. Balance Ba +2 + SO 4 -2 BaSO 4 (s) Total Ionic Equation Molecular Equation Net Ionic Equation BaSO 4 (s) + NaCl(aq) Section 7.2 Net Ionic Equations

Section 6.3 Balancing Chemical Equations Return to TOC 47 1.Molecular Equation  Shows the complete formulas of all reactants and products.  It does not give a very clear picture of what actually occurs in solution. Types of Equations for Reactions in Aqueous Solutions Section 7.3

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 48 2.Complete Ionic Equation  All strong electrolytes are shown as ions.  Notice: K + and NO 3 – ions are present in solution both before and after the reaction. Types of Equations for Reactions in Aqueous Solutions Section 7.3

Section 6.3 Balancing Chemical Equations Return to TOC 49 2.Complete Ionic Equation  Spectator ions – ions which do not participate directly in a reaction in solution. Types of Equations for Reactions in Aqueous Solutions Section 7.3

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 50 3.Net Ionic Equation  Only those components of the solution that undergo a change.  Notice: Spectator ions are not shown in the net ionic equation. Types of Equations for Reactions in Aqueous Solutions Section 7.3

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 51 Concept Check Write the correct molecular equation, complete ionic equation, and net ionic equation for the reaction between cobalt(II) chloride and sodium hydroxide. Molecular Equation: CoCl 2 (aq) + 2NaOH(aq)  Co(OH) 2 (s) + 2NaCl(aq) Complete Ionic Equation: Co 2+ (aq) + 2Cl  (aq) + 2Na + (aq) + 2OH  (aq)  Co(OH) 2 (s) + 2Na + (aq) + 2Cl  (aq) Net Ionic Equation: Co 2+ (aq) + 2OH  (aq)  Co(OH) 2 (s) Section 7.3

Section 6.3 Balancing Chemical Equations Return to TOC 52 Types of Reactions 1.Combination (one product) A + B C 2.Decompostion (one reactant) A B + C 3.Single Replacement A + BC AC + B 4.Double Replacement AB + CD AD(s) + CB(l) 5.Acid-Base (Neutralization) HA + BOH H 2 O + BA 6.Combustion- Organic + O 2 CO 2 + H 2 O 7.No Reaction- both products are (aq).

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 53 A strong acid is one in which virtually every molecule dissociates (ionizes) in water to an H + ion and an anion. Arrhenius Acids and Bases Section 7.4

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 54 Strong Acids Behave as Strong Electrolytes Section 7.4

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 55 A strong base is a metal hydroxide that is completely soluble in water, giving separate OH  ions and cations.  Most common examples: NaOH and KOH Arrhenius Acids and Bases Section 7.4

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 56 The products of the reaction of a strong acid and a strong base are water and a salt.  Salt  Ionic compound Net ionic equation  H + (aq) + OH − (aq)  H 2 O(l) Reaction of H + and OH − is called an acid- base reaction.  H +  acidic ion  OH −  basic ion Arrhenius Acids and Bases Section 7.4

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 57 1.The common strong acids are aqueous solutions of HCl, HNO 3, and H 2 SO 4. 2.A strong acid is a substance that completely dissociates (ionizes) in water (into H + ions and anions). 3.A strong base is a metal hydroxide compound that is very soluble in water (and dissociates into OH – ions and cations). Summary of Strong Acids and Strong Bases Section 7.4

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 58 4.The net ionic equation for the reaction of a strong acid and a strong base is always the same: it shows the production of water. 5.In the reaction of a strong acid and a strong base, one product is always water and the other is always an ionic compound called a salt, which remains dissolved in the water. This salt can be obtained as a solid by evaporating the water. Summary of Strong Acids and Strong Bases Section 7.4

Section 6.3 Balancing Chemical Equations Return to TOC 59 6.The reaction of H + and OH – is often called an acid-base reaction, where H + is the acidic ion and OH – is the basic ion. Summary of Strong Acids and Strong Bases Section 7.4

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 60 Concept Check The net ionic equation for the reaction of HNO 3 and LiOH is a)H + + NO 3 – + LiOH → H 2 O + LiNO 3 b)HNO 3 + LiOH → H 2 O + LiNO 3 c)H + + OH – → H 2 O d)Li + + NO 3 – → LiNO 3 Section 7.4

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 61 Reactions between metals and nonmetals involve a transfer of electrons from the metal to the nonmetal. A reaction that involves a transfer of electrons.  2Mg(s) + O 2 (g)  2MgO(s) Oxidation–Reduction Reaction Section 7.5

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 62 Concept Check Which of the following best describes what is happening in the following representation of an oxidation–reduction reaction: a)Metal Al gains 3 e – and O 2 – in Fe 2 O 3 loses these 3e –. b)Metal Al gains 3 e – and Fe 3+ in Fe 2 O 3 loses these 3e –. c)Metal Al loses 3 e – and O 2 – in Fe 2 O 3 gains these 3e –. d)Metal Al loses 3 e – and Fe 3+ in Fe 2 O 3 gains these 3e –. Section 7.4Section 7.5

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 63 1.A metal–nonmetal reaction can always be assumed to be an oxidation–reduction reaction, which involves electron transfer. 2.Two nonmetals can also undergo an oxidation–reduction reaction. At this point we can recognize these cases only by looking for O 2 as a reactant or product. When two nonmetals react, the compound formed is not ionic. Characteristics of Oxidation–Reduction Reactions Section 7.4Section 7.5

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 64 Formation of a solid when two solutions are mixed. Notice this is also a double– displacement reaction.  AB + CD  AD + CB Precipitation Reaction Section 7.4Section 7.6

Section 6.3 Balancing Chemical Equations Return to TOC 65 Involves an H + ion that ends up in the product water.  H + (aq) + OH − (aq)  H 2 O(l)  HCl(aq) + KOH(aq)  H 2 O(l) + KCl(aq) Acid–Base Reaction Section 7.4Section 7.6

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 66 Transfer of electrons  2Li(s) + F 2 (g)  2LiF(s) Oxidation–Reduction Reaction Section 7.4Section 7.6

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 67 Oxidation–reduction reaction Single–replacement reaction  A + BC  B + AC Formation of a Gas Section 7.4Section 7.6

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 68 Involve oxygen and produce energy (heat) so rapidly that a flame results.  CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g)  Special class of oxidation–reduction reactions. Combustion Reactions Section 7.4Section 7.7

Section 6.3 Balancing Chemical Equations Return to TOC 69 A compound forms from simpler materials.  C(s) + O 2 (g)  CO 2 (g)  Special class of oxidation–reduction reactions. Synthesis (Combination) Reactions Section 7.4Section 7.7 Only one product!

Section 6.3 Balancing Chemical Equations Return to TOC 70 Occurs when a compound is broken down into simpler substances.  2H 2 O(l)  2H 2 (g) + O 2 (g)  Special class of oxidation–reduction reactions. Decomposition Reactions Section 7.4Section 7.7 Only one reactant!

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 71 Summary Section 7.4Section 7.7