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Topic 4 Chemical Reactions

Topic 4 Chemical Reactions

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Topic 4 Chemical Reactions

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  1. Topic 4 Chemical Reactions Chemical reactions are the heart of chemistry. Chemical reactions involve a change from reactant substances to product substances. The products have physical and chemical properties different from those of the reactants.

  2. Ions in Aqueous Solution Many ionic compounds (soluble salts) dissociate into independent ions when dissolved in water. • These compounds that “freely” dissociate into independent ions in aqueous solution are called electrolytes. • Their aqueous solutions are capable of conducting an electric current because they have charged ions capable of carrying a current. • Generally, ionic solids that dissolve (soluble) in water are electrolytes. Not all ionic compounds are soluble; depends on the charge and size of the ions involved in the ionic bond.

  3. Ions in Aqueous Solution Not all electrolytes are ionic compounds. Some molecular compounds (mostly acids) dissociate into ions. • The resulting solution is electrically conducting, and so we say that the molecular substance is an electrolyte.

  4. Ions in Aqueous Solution Most molecular compounds (except acids) dissolve but do not dissociate into ions; soluble but no ions formed. • These compounds are referred to as nonelectrolytes. They dissolve in water to give a nonconducting solution. • Covalent bonds are stronger than ionic bonds and do not dissociate in water resulting in neutral dissolved species.

  5. Ions in Aqueous Solution Electrolytes dissolve in water to produce ions, but do so at varying extents and dictates their conductivity; higher # ions, higher the conductivity. A strong electrolyteis an electrolyte that exists in solution entirely as ions (100% dissociation to saturation point). • Most ionic solids that dissolve in water do so almost completely as ions, so they are strong electrolytes. • strong electrolytes – strong acids, strong bases, soluble salts

  6. Ions in Aqueous Solution A weak electrolyteis an electrolyte that dissolves in water to give a relatively small percentage of ions. • Most soluble molecular compounds are either nonelectrolytes or weak electrolytes. • Weak electrolytes - weak acids, weak bases, insoluble salts • Solutions of weak electrolytes contain only a small percentage of ions. We denote this situation by writing the equation with a double arrow.

  7. We’ve stated the terms strong acid, strong base, soluble salt, insoluble salt, but we haven’t describe how to determine which species fall under these terms. To be able to write chemical reactions correctly, we will need to understand solubility and how strong and weak species dissociate in water. The first thing we will cover is the solubility rules for ionic compounds. You must know the solubility rules to distinguish between soluble and insoluble salts. Soluble salts (aq) dissociate 100% into ions up to their saturation point while insoluble salts (s) have very little dissociation.

  8. Solubility Rules for Ionic Compounds (Dissociates 100%) 1.) All compounds containing alkali metal cations (group I) and the ammonium ion (NH4+) are soluble. 2.) All compounds containing NO3-, ClO4-, ClO3-, and C2H3O2- anions are soluble. 3.) All chlorides, bromides, and iodides are soluble except those containing Ag+, Pb2+, or Hg22+. 4.) All sulfates are soluble except those containing Hg22+, Pb2+, Ba2+, Sr2+, or Ca2+. Ag2SO4 is slightly soluble. 5.) All hydroxides are insoluble except compounds of the alkali metals and Ca2+, Sr2+, and Ba2+ are slightly soluble. 6.) All other compounds containing PO43-, S2-, CO32-, CrO42-, SO32- and most other anions are insoluble except those that also contain alkali metals or NH4+. Generally, compounds that dissolve > 0.10 M are considered soluble (aq) < 0.01 M are considered insoluble (s) in between are slightly soluble (this class we will assume slightly soluble as soluble) Soluble or insoluble? Hg2Cl2 (s) insoluble KI (aq) soluble code: soluble Pb(NO3)2 (aq) soluble HW 26

  9. Memorize these Strong Acids (Ionizes 100%) HCl, HBr, HI, HClO4, HNO3, H2SO4 Strong Bases (Dissociates 100%) NaOH, KOH, LiOH, Ba(OH)2, Ca(OH)2, Sr(OH)2 Rest of acids and bases (including acidic cations and basic anions) are weak.

  10. Ions in Aqueous Solution A molecular/formula unit equationis one in which the reactants and products are written as if they were molecules/formula units, even though they may actually exist in solution as ions. Calcium hydroxide + sodium carbonate M.E. Ca(OH)2 Many ionic compounds undergo a displacement reaction between the cation of one species with the anion of another. Displacement involves switching atoms or ions between species, balancing charges between ions by adding subscripts, balancing atoms in reaction by changing coefficients on substances, and adding states (s, l, aq, g). + Na2CO3  CaCO3 + NaOH 2 (aq) (aq) (s) (aq) strong base soluble salt insoluble salt strong base s solid l liquid aq aqueous (strong and weak acid/bases and soluble salts dissolved in water) g gases

  11. Ions in Aqueous Solution An total ionic equationrepresents strong electrolytes as separate independent ions. This is a more accurate representation of the way electrolytes behave in solution. • A total ionic equation is a chemical equation in which strong electrolytes (such as soluble ionic compounds, strong acids/bases) are written as separate ions in solution. (note: g, l, insoluble salts (s), weak acid/bases do not break up into ions) M.E. Ca(OH)2 (aq) + Na2CO3 (aq)  CaCO3 (s) + 2 NaOH (aq) Total ionic strong base soluble salt insoluble salt strong base Ca2+(aq) + 2OH-(aq) + 2Na+(aq) + CO32- (aq)  CaCO3(s) + 2Na+(aq) + 2OH-(aq)

  12. A net ionic equationis a chemical equation from which the spectator ions have been removed. • A spectator ionis an ion in an ionicequationthat does not take part in the reaction. Species does not change form from reactant to product (Na+(aq) and OH-(aq) are unchanged in this reaction and are spectators). Net ionic equations. M.E. Ca(OH)2 (aq) + Na2CO3 (aq)  CaCO3 (s) + 2 NaOH (aq) Total Ionic Ca2+(aq) + 2OH-(aq) + 2Na+(aq) + CO32- (aq)  CaCO3(s) + 2Na+(aq) + 2OH-(aq) Net Ca2+(aq) + CO32- (aq)  CaCO3(s)

  13. Let’s try an example. First, we start with a molecular equation: switch atoms or ions between species, balance charges between ions by adding subscripts, balance atoms in reaction by changing coefficients on substances, and add states (s, l, aq, g). molecular equation HNO3(aq) + Mg(OH)2(s) Total ionic Net ( )2 Mg NO3 + H2O 2 2 (aq) (l) insoluble salt soluble salt strong acid Break up strong acid/bases and soluble salts. 2 H+(aq) + 2NO3-(aq) + Mg(OH)2(s) Mg2+(aq) + 2NO3-(aq) + 2H2O(l) Remove spectator ions. 2 H+(aq) + Mg(OH)2(s) Mg2+(aq) + 2H2O(l)

  14. Ions in Aqueous SolutionMolecular and Ionic Equations HCN (aq) + NaOH(aq) Total ionic Net NaCN + H2O (aq) (l) strong base soluble salt weak acid HCN (aq) + Na+(aq) + OH-(aq) Na+(aq) + CN-(aq) + H2O(l) HCN(aq) + OH-(aq) CN-(aq) + H2O(l)

  15. Types of Chemical Reactions • Most of the reactions we will study fall into one of the following categories • Precipitation (solid) Reactions • Acid-Base Reactions • Oxidation-Reduction Reactions (transfer of electrons)

  16. Types of Chemical ReactionsPrecipitation Reactions A precipitation reaction occurs in aqueous solution because one product is insoluble. • A precipitate (ppt)is an insoluble solid compound formed during a chemical reaction in solution. • For example, the reaction of sodium chloride with silver nitrate forms AgCl(s), an insoluble precipitate. Is this a ppt rxn? NaCl (aq) + Fe(NO3)2(aq) FeCl2(aq) + 2 NaNO3(aq) HW 27 code: ppt No ppt is formed; therefore, not a pptrxn - all aqions; basically, no reaction

  17. Types of Chemical Reactions Acid-Base Reactions • Acids and bases are some of the most important electrolytes. • They can cause color changes in certain dyes called acid-base indicators. • Many household acids and bases encountered everyday. • Red cabbage juice is used asan acid-base indicator. • Vitamin C – ascorbic acid • Aspirin – acetyl salicylic acid • Milk of Magnesia • Vinegar – acetic acid

  18. Types of Chemical ReactionsDifferent ways to define acids/bases The Arrhenius Concept • The Arrhenius concept defines acids as substances that produce hydrogen ions, H+, (also known as hydronium ions, H3O+) when dissolved in water. • An example is nitric acid, HNO3, a molecular substance that dissolves in water to give H+ and NO3-. HNO3(aq) + H2O(l) H3O+(aq) + NO3-(aq) Note: You will encounter acid ionization equations written both ways: as a dissociationand an ionization.

  19. Types of Chemical ReactionsDifferent ways to define acids/bases The Arrhenius Concept • The Arrhenius concept defines bases as substances that produce hydroxide ions, OH-, when dissolved in water. • An example is sodium hydroxide, NaOH, an ionic substance that dissolves in water to give sodium ions and hydroxide ions.

  20. Types of Chemical ReactionsDifferent ways to define acids/bases By Arrhenius concept is ammonia an acid or base? By Arrhenius concept, ammonia produces OH-; therefore, it is a base. However, this concept does not cover all acids/bases that exist which gets us to a more useful acid/base concept especially when dealing with aqueous solutions.

  21. Types of Chemical ReactionsDifferent ways to define acids/bases The Brønsted-Lowry Concept • The Brønsted-Lowry concept of acids and bases involves the transfer of a proton (H+) from the acid to the base. • In this view, acid-base reactions are proton-transfer reactions.

  22. Types of Chemical ReactionsDifferent ways to define acids/bases The Brønsted-Lowry Concept • The Brønsted-Lowry concept defines an acid as the species (molecule or ion) that donates a proton (H+) to another species in a proton-transfer reaction. • Abaseis defined as the species (molecule or ion) that accepts the proton (H+) in a proton-transfer reaction.

  23. H+ Types of Chemical ReactionsDifferent ways to define acids/bases The Brønsted-Lowry Concept base acid We obtained the same conclusion that NH3 is a base using this theory; however, we didn’t look at the production of OH- to make the decision. It was based on NH3 accepting a proton in the aqueous rxn. The H2O molecule is the acid because it donates a proton. The NH3 molecule is a base, because it accepts a proton. Using Brønsted-Lowry concept makes it easy to write acid-base reactions. You remove a proton off the acid component and place it on the base component giving you the products. Water is amphiprotic meaning it can act as an acid or base. Since NH3 is a better base, water is the acid in this reaction.

  24. Types of Chemical ReactionsDifferent ways to define acids/bases If you learn the acids and bases in water, it will inform you how water reacts in the reaction. We know that nitric acid is an acid in water; therefore, water must be the base and proton acceptor in the reaction. where HNO3 is an acid (proton donor) and H2O is a base (proton acceptor). H+ A consequence of the Bronsted-Lowry concept is that on the product side of the acid/base reaction there are also acid/base products. This leads us to the term of conjugate acid/base pairs. These are species that differ by a proton with the species containing more protons being the acid. acid/base acid/base acid/base acid/base HCN/CN-H3O+/H2OH2SO4/HSO4-H2PO3-/HPO32-

  25. Types of Chemical ReactionsDifferent ways to define acids/bases In summary, • The Arrhenius concept • acid: proton (H+) producer • base: hydroxide ion (OH-) producer • The Brønsted-Lowry concept • acid: proton (H+) donor • base: proton (H+) acceptor HW 28 code: acid

  26. Types of Chemical ReactionsAcid-Base Reactions Strong and Weak Acids and Bases • A strong acidis an acid that ionizescompletely in water; it is a strong electrolyte. 100% 100% none left; all ionized

  27. Types of Chemical ReactionsAcid-Base Reactions Strong and Weak Acids and Bases • A weak acidis an acid that only partially ionizes in water; it is a weak electrolyte. < 5% bulk does not ionize far less hydronium formed as compared to a strong acid of the same concentration

  28. Types of Chemical ReactionsAcid-Base Reactions Strong and Weak Acids and Bases • A strong baseis a base that is present entirely as ions, one of which is OH-; it is a strong electrolyte. 100%

  29. Types of Chemical ReactionsAcid-Base Reactions Strong and Weak Acids and Bases • A weak baseis a base that is only partially ionized in water; it is a weak electrolyte. < 5% bulk does not ionize far less hydroxide formed as compared to a strong base of the same concentration The main question is what are the weak bases? Weak acids are easier because if you learn the 6 strong acids, you know all the rest are weak.

  30. Weak bases are NH3, NH2-, NH- compounds and some anions. Salts may be acidic/basic/neutral and are composed of cations (positive ions) and anions (negative ions). cation anion acidic or neutral basic or neutral cations of strong bases anions of monoprotic strong are neutral; otherwise, acids are neutral; otherwise, cation contributes acidity anion contributes basicity Na+, K+, Li+, Ca2+, Sr2+, Ba2+ Cl-, Br-, I-, NO3-, ClO4- are are neutral; rest of cations are neutral; rest of anions are acidic to salt basic to salt Learn which cations and anions are neutral and the rest of the cations are acidic and the rest of the anions are basic.

  31. Predicting Whether a Salt is Acidic, Basic, or Neutral To predict the acidity or basicity of a salt, you must examine the acidity or basicity of the ions composing the salt. Depending on the two components (cation/anion) the overall salt will be acidic/neutral/basic : neutralcationneutral anion neutral salt acidic cationneutral anion  acidic salt neutral cationbasic anion  basic salt acidic cationbasic anion  depends on which is larger Ka or Kb

  32. Predicting Whether a Salt is Acidic, Basic, or Neutral acidicneutral acidic salt pH < 7 neutralbasic basic salt pH > 7 KC2H3O2(s)  K+(aq) + C2H3O2-(aq) neutral basic basic salt pH > 7

  33. Predicting Whether a Salt is Acidic, Basic, or Neutral acidic neutral acidic acidic neutralacidic • AlCl3 • Zn(NO3)2 • KClO4 • Na3PO4 • LiF • NH4F (Ka = 5.6x10-10 > Kb = 1.4x10-11) • NH4ClO (Ka = 5.6x10-10< Kb = 3.6x10-7) • NH4C2H3O2(Ka = 5.6x10-10 = Kb = 5.6x10-10) neutral neutralneutral neutralbasicbasic neutralbasicbasic acidic basic acidic acidic basic basic acidic basic neutral

  34. acid base salt Types of Chemical ReactionsAcid-Base Reactions • One of the chemical properties of acids and bases is that they neutralize one another. • A neutralization reaction is a reaction of an acid and a base that results in an ionic compound and water. • The ionic compound that is the product of a neutralization reaction is called a salt. Neutralization Reactions Is the salt product acidic, basic, or neutral? Based on the cation being neutral and the anion being basic, this is a basic salt.

  35. During a neutralization reaction an acid and base are mixed together to make a solution, the resulting solution will contain a salt and water Acid + Base salt + water One of the chemical properties of acids and bases is that they neutralize one another; however, that doesn’t mean that the product will be neutral. The misconception is that if the acid and base are in stoich proportions that the resulting solution is neutral. This is not true. The salt formed may be a acidic, basic, or neutral salt and will dictate the pH of the solution. Common sense can help you predict the pH of a mixture. SA + SB  WA + SB SA + WB  neutral salt only true neutralization pH =7 basic salt original acid and base neutralize but product has basic properties and basic pH acidic salt original acid and base neutralize but product has acidic properties and acidic pH strong base basic salt weak acid strong base neutral salt strong acid HW 29-30 code: neu 35

  36. Types of Chemical ReactionsAcid-Base Reactions • Carbonates react with acids to form CO2, carbon dioxide gas. Acid-Base Reactions with Gas Formation with certain salts H2CO3(aq) H2O(l) + CO2(g) • Sulfites react with acids to form SO2, sulfur dioxide gas. H2SO3(aq) H2O(l) + SO2(g) • Sulfides react with acids to form H2S, hydrogen sulfide gas.

  37. Types of Chemical ReactionsAcid-Base Reactions Other Acid-Base reactions Nonmetal oxides (or acid oxides or acid anhydrides) react with water to form acids SO2 + H2O  H2SO3 Metal oxides (or basic oxides or basic anhydrides) react with water to form bases Na2O + H2O  2NaOH

  38. Types of Chemical Reactions Oxidation-Reduction Reactions (Redoxrxn) • Oxidation-reduction reactionsinvolve the transfer of electrons from one species to another. • Oxidation is defined as the loss of electrons (higher oxidation state). • Reduction is defined as the gain of electrons (lower oxidation state). • Oxidation and reduction always occur simultaneously.

  39. Redox reactions – transfer of e- • reduction – oxidation reactions • Reduction – gain of e- / gain of H / lost of O • Fe3++ 1e- Fe2+ (lower ox state) • note: must balance atoms and charges; • electrons are always on reactants side.

  40. Oxidation - loss of e- / loss of H / gain of O Fe2+ Fe3++ 1e- (higher ox state) ox: H2O + BrO3-  BrO4- + 2H++ 2e- (Br oxidized: charge goes 5+  7+) red: 2H++ 2e-  H2 (H reduced: charge 1+  0) Oxidizing agent is species that undergoes reduction. Reducing agent is species that undergoes oxidation. Note: need both for reaction to happen; can’t have something being reduced unless something else is being oxidized. Electrons are always on the product side for ox. We must calculate charge on Br to decide if it is oxidized or reduced in charge (O: 2-; ion: 1-) Br + 4(-2) = -1 Br = -1+8 = +7 Br + 3(-2) = -1 Br = -1+6 = +5

  41. Types of Chemical Reactions Oxidation-Reduction Reactions Copper prefers to be reduced while iron prefers to be oxidized when present together. Lets write a net ionic reaction for iron and copper (II) sulfate. soluble salt soluble salt total ionic: Fe(s) + Cu2+(aq) + SO42-(aq) Fe2+(aq) + SO42-(aq)+ Cu(s) net ionic: Fe(s) + Cu2+(aq)  Fe2+(aq) + Cu(s)

  42. Types of Chemical Reactions In this oxidation-reduction reaction, Feo lost 2e- and was oxidized to Fe2+ while Cu2+ gained 2e- and was reduced to Cuo. the charge is basically an inventory of electrons as compared to protons: +# means neutral species lost e- while -# means neutral species gained e- decreased by 2e- - oxidation Reducingagent Oxidizing agent increased by 2e- - reduction Notice that not only the atoms are balanced on both sides, the charges are balanced as well reactants: 0 + 2+ = 2+ products: 2+ + 0 = 2+ which gets us to the topic of oxidation numbers.

  43. Types of Chemical ReactionsOxidation-Reduction Reactions Oxidation Numbers • The concept of oxidation numbers is a simple way of keeping track of electrons in a reaction. • The oxidation number(or oxidation state) of an atom in a substance is the actual charge of the atom if it exists as a monatomic ion. • Alternatively, it is hypothetical charge assigned to the atom in the substance by simple rules.

  44. Types of Chemical ReactionsOxidation-Reduction Reactions Oxidation Number Rules

  45. Oxidation-Reduction Reactions , S 6+ Na2SO4 has Na+, O 2- Cl2 ClO2ClO2-Cl2O5 SO42- S + 4 (-2) = -2 S-8 = -2 S = 8-2 = 6+ 2 (+1) + S + 4 (-2) = 0 2 + S-8 = 0 S = 8-2 = 6+ Cl : 0 +4 +3+5 HW 31 code: number You can determine charges based on whole compound being equal to zero and using the rules to determine charges with calculating unknown charges or you can do it based on the ions separately.

  46. Types of Chemical ReactionsOxidation-Reduction Reactions Some Common Oxidation-Reduction Reactions • Most of the oxidation-reduction reactions fall into one of the following simple categories: • Combination Reaction • Decomposition Reactions (breaks up) • Displacement Reactions (switches) • Combustion Reactions (reacts with O2)

  47. Types of Chemical ReactionsOxidation-Reduction Reactions Combination Reactions • A combination reaction is a reaction in which two substances combine to form a third substance. R1 + R2 P

  48. Types of Chemical ReactionsOxidation-Reduction Reactions Decomposition Reactions • A decomposition reaction is a reaction in which a single compound reacts to give two or more substances. R  P1 + P2

  49. A displacement reaction (also called a single- replacement reaction) is a reaction in which an element reacts with a compound, displacing an element from it. Types of Chemical ReactionsOxidation-Reduction Reactions Displacement Reactions

  50. Types of Chemical ReactionsOxidation-Reduction Reactions Combustion Reactions • A combustion reaction is a reaction in which a substance reacts with oxygen, usually with the rapid release of heat to produce a flame. • Common combustion reactions involve hydrocarbons • Hydrocarbon + O2 CO2 + H2O HW 32 code: rxns