Chapter opener with space shuttle launch Chemical Reactions
Chemical Reactions 7.1 Kindergarten Volcanoes, Automobiles, and Laundry Detergents 7.2 Evidence of a Chemical Reaction 7.3 The Chemical Equation 7.4 How to Write Balanced Chemical Equations 7.5 Aqueous Solutions, Solubility: Compounds Dissolved in Water 7.6 Precipitation Reactions: Reactions in Aqueous Solution that Form Solids 7.7 Writing Chemical Equations for Reactions in Solution: Molecular, Complete Ionic, and Net Ionic Equations 7.8 Acid Base and Gas Evolution Reactions 7.9 Oxidation-Reduction Reactions 7.10 Classifying Chemical Reactions
7.1 Kindergarten Volcanoes, Automobiles, and Laundry Detergents 3
Some chemical reactions Kindergarten Volcano with baking soda and vinegar shows bubbles Gasoline burning (next slide) releases heat Soap scum precipitation out of hard water Soap scum dissoilved by laundry detergent All these are chemical reactions changes in composition 4
Combustion Reactions Reactants + O 2 Products (also heat) Figure 7.1 octane burning in and engine with oxygen make carbon dioxide and water 5
7.2 Evidence of a Chemical Reaction 6
Evidence of Chemical Change Five figures on page 202 Release or Absorption of Heat Color Change Emission of Light Formation of a Gas Formation of Solid Precipitate 7
Evidence of Chemical Change, Continued Water boiling Figure 7.5 Is boiling water a chemical change? No, there is no change in composition this Is a physical change. 8
Practice Decide Whether Each of the Following Involve a Chemical Reaction. Photosynthesis Heating sugar until it turns black Heating ice until it turns liquid Digestion of food Dissolving sugar in water Yes, CO 2 and H 2 O combine into carbohydrates Burning of alcohol in a flambé dessert Yes, sugar decomposing No, molecules still same Yes, food decomposing and combining with stomach acid No, molecules still same Yes, alcohol combining with O 2 to make CO 2 and H 2 O 9
7.3The Chemical Equations 10
Chemical Equations A Chemical Equation consists of Reactants on the left An arrow Products on the right 2Na(s) + Cl 2 (g) 2NaCl(s) reactants products The number 2 are called coefficient, which multiply the number of molecules or formula units 11
The Combustion of Methane Methane burning Figure on page 205 Methane gas burns to produce carbon dioxide gas and gaseous water. Whenever something burns it combines with O 2 (g). 12
Combustion of Methane, Balanced Tthe equation must be balanced. Adjust the numbers of molecules so there are equal numbers of atoms of each element on both sides of the arrow. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O(g) Molecular equation at bottom of Page 204 13
Symbols Used in Equations Symbols used to indicate state. (g) = gas; (l) = liquid; (s) = solid. (aq) = aqueous = dissolved in water. Energy symbols used above the arrow for decomposition reactions. D = heat. hn = light. shock = mechanical. elec = electrical. 14
7.4 How to Write Balanced Chemical Equations 15
Balancing Equations There are some tips on balancing equations in your text. On page 206. Most important: Do not change subscruipts, only change coefficients. The goal is to have the same number of each type of atom on both sides. Some examples: H 2 (g) + Cl 2 (g) HCl(g) There are 2 H on left, but 1 on right. 2 in front of HCl H 2 (g) + Cl 2 (g) 2 HCl(g) The equation is balanced. 16
More examples Example Cr(s) + O 2 (g) Cr 2 O 3 (s) Balance the Cr with a 2 2 Cr(s) + O 2 (g) Cr 2 O 3 (s) The right has 2 O but the left has 3 O. Balance by switch those and there will be 6 O on both sides. 2 Cr(s) + 3O 2 (g) 2Cr 2 O 3 (s) This unbalances the Cr, fix by changing the 2 to a 4 4 Cr(s) + 3 O 2 (g) 2Cr 2 O 3 (s) Always check at the end. 4 Cr 4 Cr 6 O 6 O 17
Another Example FcCl 3 (aq) + NaOH Fe(OH) 3 (s) + NaCl(aq) Balance the Cl with a 3 FcCl 3 (aq) + NaOH Fe(OH) 3 (s) + 3 NaCl(aq) Balance the Na with a 3 FcCl 3 (aq) + 3 NaOH Fe(OH) 3 (s) + 3NaCl(aq) All other elements are balanced 18
Predicting Whether a Reaction Will Occur in Aqueous Solution Forces that drive a reaction: Formation of a solid. (Precipitation) Formation of water. (Happens in acid + base) Formation of a gas. Transfer of electrons. (Called oxidationreduction) We will look at each type of reaction 19
Aqueous Solutions, Solubility: Compounds Dissolved in Water 20
Aqueous Solutions Aqueous solutions are a substance (solute) dissolved in water (solvent). The resulting mixture is a solution. Ionic solids that dissolve in water dissociate (or break up) into ions That are surrounded by water in solution. Do nmot break apart Polyatomic ions. Subbscipts Coefficents NaCl(s) MgCl 2 (s) Na + (aq) + Cl - (aq) Mg 2+ (aq) + 2 Cl (aq) Al 2 (SO 4 ) 3 (s) 2Al 3+ (aq) + 3SO 4 2- (aq) 21
Dissociation of soluble ionic compounds Figure of NaCl and AgNO 3 in solution on page 209 22
Dissociation, Continued Potassium iodide dissociates in water into potassium cations and iodide anions. KI(aq) K +1 (aq) + I -1 (aq) Copper(II) sulfate dissociates in water into copper(ii) cations and sulfate anions. CuSO 4 (aq) Cu +2 (aq) + SO 4-2 (aq) 23
Dissociation, Continued Potassium sulfate dissociates in water into potassium cations and sulfate anions. K 2 SO 4 (aq) 2 K +1 (aq) + SO 4-2 (aq) 24
Electrolytes Electrolytes are substances whose water solution is a conductor of electricity. All electrolytes have ions dissolved in water. Figures left side Page 209 25
Electrolytes, Continued In strong electrolytes, 100% of formula units are separated into ions. In nonelectrolytes, none of the molecules are separated into ions. In weak electrolytes, a small percentage of the molecules are separated into ions. 26
Types of Electrolytes Salts = Water soluble ionic compounds. Strong electrolytes. Acids = Form H + ions and anions in water solution. In binary acids, the anion is monoatomic. In oxyacids, the anion is polyatomic. Sour taste. React and dissolve many metals. Strong acid = strong electrolyte, weak acid = weak electrolyte. Bases = Water-soluble metal hydroxides. Bitter taste, slippery (soapy) feeling solutions. Increases the OH - concentration. 27
When Will a Salt Dissolve? A compound is soluble in a liquid if it dissolves in that liquid. The compound is a solute, the liquid is a solvent, and the mixture is a solution. NaCl is soluble in water, but AgCl is not soluble. NaCl top beaker Page 209 AgCl bottom beaker Page 209 28
Solubility Rules A set of solubility rules have been developed to help predict when a compound will dissolve Two tables show these rules Soluble compounds with exceptions Insoluble compounds with exceptions These tables will be provided with the quiz
Solubility Rules: Compounds that Are Generally Soluble in Water Compounds containing the following ions are generally soluble Li +, Na +, K +, NH 4 + NO 3, C 2 H 3 O 2 Exceptions (when combined with ions on the left the compound is insoluble) none none Cl, Br, I Ag +, Hg 2 2+, Pb 2+ SO 4 2 Ca 2+, Sr 2+, Ba 2+, Pb 2+ 30
Compounds containing the following ions are generally insoluble Solubility Rules: Compounds that Are Generally Insoluble Exceptions (when combined with ions on the left the compound is soluble or slightly soluble) OH Li +, Na +, K +, NH 4+, Ca 2+, Sr 2+, Ba 2+ S 2 Li +, Na +, K +, NH 4+, Ca 2+, Sr 2+, Ba 2+ CO 3 2, PO 4 3 Li +, Na +, K +, NH 4 + 31
Using the Solubility Rules to Predict an Ionic If a Precipitation Reaction occurs in Water Dissociate soluble compounds into ions Check combinations of cations and anions for solubility using the rules. If cation is Li +, Na +, K +, or NH 4+, then the compound will be soluble in water. If another cation, follow the rules for the anion. If a precipitate forms, use charges to write correct formula for the precipitate and the other product. 32
Determine if Each of the Following Is Soluble in Water Compounds ions KOH K + and OH - soluble, all K + compounds are soluble AgBr Ag + and Br - insoluble, all Br - are soluble, but Ag + is an exception. CaCl 2 Ca 2+ and Cl - Soluble, all Cl - are soluble, and Ca 2+ is not an exception Pb(NO 3 ) 2 Pb 2+ and NO - 3 Soluble, All NO 3- are soluble. PbCO 3 Pb 2+ and CO 2-4 Insoluble, CO 2-3 are usually insoluble, but Pb 2+ is not an exception. 33
7.6 Precipitation Reactions: Reactions in Aqueous Solution that Form Solids 34
Precipitation Reactions A precipitate forming 35
Precipitation Reactions, Continued 2 KI(aq) + Pb(NO 3 ) 2 (aq) 2 KNO 3 (aq) + PbI 2 (s) Figure 7.7, the above reaction 36
Precipitation Reactions, Continued 2 KI(aq) + Pb(NO 3 ) 2 (aq) 2 KNO 3 (aq) + PbI 2 (s) Beakers on page 212 illustrating this precipitation 37
No Precipitate Formation = No Reaction KI(aq) + NaCl(aq) KCl(aq) + NaI(aq) All ions still present, no reaction. Figure 7.8 illustrating no reaction 38
Predicting if a precipitate will form Dissociate soluble ionic compounds into ions. Look at possible combinations of cations and anions, and use the solubility rules top determine if a precipitate will form. If a precipitate forms determine its formula from ioninc charges. Alos the formula of soluble products. Write a balanced equations. A mixture is made of solutions of potassium phosphate and nickel(ii)chloride. Will a precipitate form? K 3 PO 4 (aq) + NiCl 2 (aq) precipitate?? 39
Predicting Precipitates (continued) K 3 PO 4 (aq) + NiCl 2 (aq) precipitate?? Dissoiciate (number of ions is not important at this point K + PO 4 3- Ni 2+ Cl - insoluble soluble Inner combination is insoluble because PO 3-4 compounds are insoluble and Ni 2+ is not an exception. The outer combination is soluble because K+ compounds are always soluble. 40
Predicting Precipitates (continued) K + and Cl - make KCl(aq) Ni 2+ and PO 4 3- make Ni 3 (PO 4 ) 2 (s) Reactants and products: K 3 PO 4 (aq) + NiCl 2 (aq) And balance: 2K 3 PO 4 (aq) + 3NiCl 2 (aq) KCl(aq) + Ni 3 (PO 4 ) 2 (s) 6KCl(aq) + Ni 3 (PO 4 ) 2 (s) 41
7.7 Writing Chemical Equations for Reactions in Solution: Molecular, Complete Ionic, and Net Ionic Equations 42
Ionic Equations The equations listed in the previous section identify the precipitate and soluble compounds. These equations do not reflect the actual structure of the species in solution. Ionic equations reveal the actual state of ionics. 43
Ionic Equations molecular equations. All compounds complete: 2 KOH(aq) + Mg(NO 3 ) 2 (aq) 2 KNO 3 (aq) + Mg(OH) 2 (s) complete ionic equations. Aqueous electrolytes are written as ions. Soluble salts, strong acids, strong bases. Insoluble substances and nonelectrolytes written in intact form. Solids, liquids, and gases are not dissolved, therefore, molecule form. 2K + (aq) + 2OH - (aq) + Mg +2 (aq) + 2NO - 3 (aq) K + (aq) + 2NO - 3 (aq) + Mg(OH) 2(s) 44
Ionic Equations spectator ions. Species that are identical on both sides 2K + (aq) + 2OH - (aq) + Mg +2 (aq) + 2NO 3 - (aq) K + (aq) + 2NO 3 - (aq) + Mg(OH) 2(s) When spectator ions are removed the result is the net ionic equation: 2OH -1 (aq) + Mg +2 (aq) Mg(OH) 2(s) 45
Another example Molecular equations. All compounds complete: K 2 (SO 4 ) (aq) + Ba(NO 3 ) 2 (aq) 2 KNO 3 (aq) + BaSO 4 (s) Ionic Equation : As the supecies actually appear in solution: 2K + (aq) + SO 4 2- (aq) + Ba +2 (aq) + 2NO 3 - (aq) K + (aq) + 2NO 3 - (aq) + BaSO 4(s) Net Ionic Equation: Spectator ions removed Ba +2 (aq) + 2NO 3 - (aq) BaSO 4(s) 46
Summary A molecular equation is a chemical equation showing the complete, neutral formulas for each compound in a reaction. A complete ionic equation is a chemical equation showing all of the species as they are actually present in solution. A net ionic equation is an equation showing only the species that actually participate in the reaction. 47
7.8 Acid Base and Gas Evolution Reactions 48
Properties of Acids Sour taste. Change color of vegetable dyes. React with active metals, not noble metals to produce hydrogen. i.e., Al, Zn, Fe, but not Cu, Ag or Au. Zn + 2 HCl ZnCl 2 + H 2 Corrosive. React with carbonates, producing CO 2. Marble, baking soda, chalk, limestone. CaCO 3 + 2 HCl CaCl 2 + CO 2 + H 2 O React with bases to form ionic salts. And often water. A woman eating a lemon Litmus paper changing to red 49
Common Acids Chemical name Formula Old name Strength Nitric acid HNO 3 Aqua fortis Strong Sulfuric acid H 2 SO 4 Vitriolic acid Strong Hydrochloric acid HCl Muriatic acid Strong Phosphoric acid H 3 PO 4 Moderate Chloric acid HClO 3 Moderate Acetic acid HC 2 H 3 O 2 Vinegar Weak Hydrofluoric acid HF Weak Carbonic acid H 2 CO 3 Soda water Weak Boric acid H 3 BO 3 Weak 50
A.k.a. alkalis. Taste bitter. Feel slippery. Properties of Bases Change color of vegetable dyes. Different color than acid. Litmus = blue. React with acids to form ionic salts. And often water. Neutralization. Litmus paper changing to blue 51
Common Bases Chemical name Sodium hydroxide Potassium hydroxide Calcium hydroxide Magnesium hydroxide Ammonium hydroxide Formula Common name Strength NaOH Lye, Strong caustic soda KOH Caustic potash Strong Ca(OH) 2 Slaked lime Strong Mg(OH) 2 Milk of magnesia Weak NH 4 OH, {NH 3 (aq)} Ammonia water, aqueous ammonia Weak 52
Acid Base Reactions Also called neutralization reactions. the H + from the acid and OH - from the base make water. The cation from the base combines with the anion from the acid to make the salt (salt = ionic compound). acid + base salt + water 2 HNO 3 (aq) + Ca(OH) 2 (aq) Ca(NO 3 ) 2 (aq) + 2 H 2 O(l) Ionic Equation: 2H + (aq) + 2NO 3 - (aq) + Ca +2 (aq) + 2OH - (aq) H + (aq) + 2NO 3 - (aq) + H 2 O (l) The net ionic equation for an acid-base reaction often is: H + (aq) + OH - (aq) H 2 O(l) 53
Gas Evolution Reactions gas evolution reactions. Directly from the ion exchange. Gas evolving page 218 K 2 S(aq) + H 2 SO 4 (aq) K 2 SO 4 (aq) + H 2 S(g) decomposition of an unstable ion exchange products into a gas and water. K 2 SO 3 (aq) + H 2 SO 4 (aq) K 2 SO 4 (aq) + H 2 SO 3 (aq) The quotes indicate the compound is unstable. H 2 SO 3 H 2 O(l) + SO 2 (g) Final equation: K 2 SO 3 (aq) + H 2 SO 4 (aq) K 2 SO 4 (aq) + H 2 O(l) + SO 2 (g) 54
Compounds that Undergo Gas Evolving Reactions Reactant type Reacting with Ion exchange product Decompose? Gas formed Example Metal n S, metal HS Acid H 2 S No H 2 S K 2 S(aq) + 2HCl(aq) 2KCl(aq) + H 2 S(g) Metal n CO 3, Acid H 2 CO 3 Yes CO 2 K 2 CO 3 (aq) + 2HCl(aq) metal HCO 3 2KCl(aq) + CO 2 (g) + H 2 O(l) Metal n SO 3 Acid H 2 SO 3 Yes SO 2 K 2 SO 3 (aq) + 2HCl(aq) metal HSO 3 2KCl(aq) + SO 2 (g) + H 2 O(l) (NH 4 ) n anion Base NH 4 OH Yes NH 3 KOH(aq) + NH 4 Cl(aq) KCl(aq) + NH 3 (g) + H 2 O(l) 55
Molecular, Total Ionic, and Net Ionic for Gas Evolving Reaction (Example 1) Initial Equation (before carbonic acid breaks down) K 2 SO 3 (s) + 2 HCl(aq) 2KCl(aq) + H 2 SO 3 (aq) Molecular Equation K 2 SO 3 (s) + 2 HCl(aq) 2KCl(aq) + + H 2 O(l) + SO 2 (g) Ionic Equation 2K + (aq) + SO 3 2- (s) + 2H + (aq) + 2Cl - (aq) K + (aq) + Cl - (aq) + H 2 O (l) + SO 2(g) Net Ionic Equation + SO 3 2- (s) + 2H + (aq) H 2 O (l) + SO 2(g) 56
Molecular, Total Ionic, and Net Ionic for Gas Evolving Reaction (Example 2) Initial Equation (before carbonic acid breaks down) CaCO 3 (s) + 2 HCl(aq) CaCl 2 (aq) + H 2 CO 3 (aq) Molecular Equation CaCO 3 (s) + 2 HCl(aq) CaCl 2 (aq) + H 2 O(l) + CO 2 (aq) Ionic Equation CaCO 3 (s) + 2H + (aq) + 2Cl - (aq) Ca 2+ (aq) + 2Cl - + H 2 O (l) + CO 2(g) Net Ionic Equation CaCO 3 (s) + 2H + (aq) + Ca 2+ (aq) + H 2 O (l) + CO 2(g) 57
7.9 Oxidation-Reduction Reactions 58
Oxidation/Reduction Metal + a non-metal Ca(s) + Cl 2 (g) Combustion with oxygen 2C 4 H 8 (g) + 6O 2 (g) CaCl 2 (s) 4CO 2 (g) + 4H 2 O(l) More general: electrons are transferred LEO says GER Loss of Electrons is Oxidation Gain of Electreons is Reduction 59
Oxidation Reduction Reactions The element that loses electrons in the reaction is oxidized. Substance that gains electrons in the reaction is reduced. You cannot have one without the other. In combustion, the O atoms in O 2 are reduced, and the non-o atoms in the other material are oxidized. 60
Combustion as Redox In the following reaction: 2 Mg(s) + O 2 (g) 2 MgO(s) The magnesium atoms are oxidized. 2Mg 0 2Mg 2+ + 4 e The oxygen atoms are reduced. O 20 + 4 e 2O 2 Adding the oxidation and reduction cancels the electrons and gives the overall reaction 61
Combustion as Redox, Continued Even though the following reaction does not involve ion formation, electrons are still transferred. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O(g) The carbon atoms are oxidized. C 4 C 4+ + 8 e These are not charges, they are called oxidation numbers, but they help us see the electron transfer. The oxygen atoms are reduced. 4O 0 + 8 e 4O 2-62
Reactions of Metals with Nonmetals (Oxidation Reduction) Metals react with nonmetals to form ionic compounds. Ionic compounds are solids at room temperature. The metal loses electrons and becomes a cation. The metal undergoes oxidation. The nonmetal gains electrons and becomes an anion. The nonmetal undergoes reduction. In the reaction, electrons are transferred from the metal to the nonmetal. 2 Na(s) + Cl 2 (g) NaCl(s) 63
Ionic Compound Formation as Redox In the reaction: Mg(s) + Cl 2 (g) MgCl 2 (s) The magnesium atoms are oxidized. Mg 0 Mg 2+ + 2 e The chlorine atoms are reduced. 2Cl 0 + 2 e 2Cl 64
Recognizing Redox Reactions O 2 is a reactant or a product. Any reaction between a metal and a nonmetal. Any reaction where electrons are transferred is redox. When a free element gets combined N 2 (g) + H 2 (g) NH 3 (g) Nitrogen ( -3) reduced and hydrogen (0 +1) oxidized When a metal cation changes its charge, it will be either oxidized if its charge increases or reduced if its charge decreases. Cu(s) + 2AgCl(aq) CuCl 2 (aq) + 2Ag(s) Net equation: Cu 0 (s) + 2Ag + (aq) Cu 2+ (aq) + 2Ag 0 (s) Copper (0 2+) is oxidized, and silver (1+ 0) is reduced. A free element (uncombined) like Cu(s), Ag(s), or O 2 (g) has charge zero 65
Practice Decide Whether Each of the Following Reactions Is a Redox Reaction. 2 Al(s) + 3 Br 2 (l) 2 AlBr 3 (s) redox (metal + nonmetal) Ba(NO 3 ) 2 (s) + 2 KCl(aq) BaCl 2 (s) + KNO 3 (aq) Not redox Fe 2 O 3 (s) + C(s) 2 Fe(s) + 3 CO(g) redox (Fe 3+ Fe 0 ) SO 2 (g) + O 2 (g) + H 2 O(l) H 2 SO 4 (aq) redox (O 2 combines) 66
Classifying Reactions One way is based on the process that happens. Precipitation, neutralization, formation of a gas, or transfer of electrons. Figure on page 222 67
Classifying Reactions, Continued Another scheme classifies reactions by what the atoms do. We studied this in lab. Type of reaction Synthesis Decomposition Displacement Double displacement General equation A + B AB AB A + B A + BC AC + B AB + CD AD + CB Figure on Page 225 68
Synthesis Reactions composition or combination reactions. Two (or more) reactants combine together to make one product. Simpler substances combining together. 2 CO + O 2 2 CO 2 2 Mg + O 2 2 MgO HgI 2 + 2 KI K 2 HgI 4 69
Decomposition Reactions A large molecule is broken apart into smaller molecules or its elements. Have only one reactant, make 2 or more products. 2 FeCl 3 ( s) elec 2 FeCl 2 ( l) Cl 2 ( g) 2 HgO (s) D 2 Hg(l) O 2 ( g) 2 O 3 h 3 O 2 70
Decomposition of Water elec 2 H2O(l) 2 H2(g) O2(g) Figure upper left page 224 71
Single Displacement Reactions One atom displacing another and replacing it in a compound. Zn(s) + 2 HCl(aq) ZnCl 2 (aq) + H 2 (g), Zn displaces H. Other examples of displacement reactions are: Fe 2 O 3 (s) + Al(s) Fe(s) + Al 2 O 3 (s) Cu(s) + 2AgNO 3 (aq) Cu(NO 3 ) 2 (aq) + 2Ag(s) 72
Double Displacement Reactions Two ionic compounds exchange ions. May be followed by decomposition of one of the products to make a gas. X Y q (aq) + A B q (aq) XB + AY Precipitation, acid base, and gas evolving reactions are also double displacement reactions. 73
Examples of Double Displacement Precipitation CaCl 2 (aq) + Na 2 CO 3 (aq) CaCO 3 (s) + 2 NaCl(aq) Acid base Ba(OH) 2 (s) + 2 HNO 3 (aq) Ba(NO 3 ) 2 (aq) + 2 H 2 O(l) Gas evolving Li 2 CO 3 (aq) + 2 HCl(aq) 2 LiCl(aq) + CO 2 (g) + H 2 O(l) 74