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4/19/15 Chapter 6A Chemical Reactions 1 CHAPTER OUTLINE § § § § § § § § Chemical Reactions Chemical Equation Balancing Equations Types of Chemical Reactions Double Replacement Reactions Oxidation-Reduction Reactions Redox in Biological Systems Activity Series of Metals 2 1 4/19/15 CHEMICAL REACTIONS q A chemical reaction is a rearrangement of atoms in which some of the original bonds are broken and new bonds are formed to give different chemical structures. q In a chemical reaction, atoms are neither created, nor destroyed. q A chemical reaction, as described above, is supported by Dalton’s postulates. 3 CHEMICAL REACTIONS a chemical reaction, atoms are neither 6In oxygen atoms = 6 oxygen atoms created, nor destroyed 4 2 4/19/15 CHEMICAL REACTIONS q A chemical reaction can be detected by one of the following evidences: 1. Change of color (formation of a solid) 2. Formation of a gas 3. Exchange of heat with surroundings 5 CHEMICAL EQUATIONS q A chemical equation is a shorthand expression for a chemical reaction. Word equation: Aluminum combines with ferric oxide to form iron and aluminum oxide. Chemical equation: Al + Fe2O3 → Fe + Al2O3 6 3 4/19/15 CHEMICAL EQUATIONS q Reactants are separated from products by an arrow. Al + Fe2O3 → Fe + Al2O3 q Coefficients are placed in front of substances to balance the equation. 2 Al + Fe2O3 → 2 Fe + Al2O3 Subscripts 7 CHEMICAL EQUATIONS q Reaction conditions are placed over the arrow. Δ Al + Fe2O3 → Fe + Al2O3 heat q The physical state of the substances are indicated by the symbols (s), (l), (g), (aq). Δ 2 Al (s) + Fe2O3 (s) → 2 Fe (l) + Al2O3 (s) solid liquid 8 4 4/19/15 BALANCING EQUATIONS q A balanced equation contains the same number of atoms on each side of the equation, and therefore obeys the law of conservation of mass. q Many equations are balanced by trial and error; but it must be remembered that coefficients can be changed in order to balance an equation, but not subscripts of a correct formula. 9 BALANCING EQUATIONS q The general procedure for balancing equations is: Write the unbalanced equation: CH4 + O2 → CO2 + H2O Make sure the formula for each substance is correct 10 5 4/19/15 BALANCING EQUATIONS q The general procedure for balancing equations is: Balance by inspection: CH4 + O2 → CO2 + H2O 1C Count and compare each element on 4H both sides of the equation2 O = 1C ! 2H ! 3O 11 BALANCING EQUATIONS q Balance elements that appear only in one substance first. Balance H 4 H present on each side CH4 + O2 → CO2 + H2O 1 CH4 + O2 → CO2 + 2 H2O 12 6 4/19/15 BALANCING EQUATIONS Balance O When finally done, check for the on 4 O present smallest coefficients possible each side 1 CH4 + O2 → CO2 + 2 H2O 1 CH4 + 2 O2 → CO2 + 2 H2O 13 Examples: 2 AgNO AgNO33 ++ H H22SS → → Ag Ag22SS ++ 2HNO HNO 3 3 2 Al(OH) Al(OH)3 3++3 H2SO4 → Al2(SO4)3 + 6H2H O2O FeFe 4H H22 → → 3Fe Fe ++ H42H O2O 3O 3O 4 4+ + 2 C4CH410 H10+ + 13 O O22 → → 8CO CO 10 2 2+ + H 2OH2O 14 7 4/19/15 CONCEPT CHECK q If red spheres represent oxygen atoms and blue spheres represent nitrogen atoms, write a balanced equation for the reaction shown below. 2 NO + O2 → 2 NO2 15 TYPES OF CHEMICAL REACTIONS q Chemical reactions are classified into five types: 1. Synthesis or combination 2. Decomposition 3. Single replacement 4. Double replacement 5. Combustion 16 8 4/19/15 SYNTHESIS or COMBINATION q In these reactions, 2 elements or compounds combine to form another compound. A + B → AB 17 DECOMPOSITION q In these reactions, a compound breaks up to form 2 elements or simpler compound. AB → A +B 18 9 4/19/15 SINGLE REPLACEMENT q In these reactions, a more reactive element replaces a less reactive element in a compound. A + BC → B + AC 19 DOUBLE REPLACEMENT q q The In these cation reactions, from onetwo compound compounds replaces combine the cation to in form another two new compound. compounds. + + AB + CD → AD + CB 20 10 4/19/15 COMBUSTION q A reaction that involves oxygen as a reactant and produces large amounts of heat is classified as a combustion reaction. CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g) 21 Examples: Classify each of the reactions below: 1. 2. 3. 4. Decomposition Mg + CuCl2 → MgCl2 + Cu CaCO3 → CaO + CO2 Synthesis Single replacement 2 HCl + Ca(OH)2 → CaCl2 + 2 H2O reactive 4 FeMg + 3isOmore Fe2O3 than Cu 2→ 2 Double replacement 22 11 4/19/15 DOUBLE REPLACEMENT REACTIONS q Double replacement reactions can be subdivided into one the following subgroups: 1. Precipitation: formation of a solid 2. Neutralization: formation of water 3. Unstable product: formation of a gas 23 PRECIPITATION REACTIONS q In these reactions one of the products formed is an insoluble solid called a precipitate. q For example, when solutions of potassium chromate, K2CrO4 , and barium nitrate, Ba(NO3)2 , are combined an insoluble salt barium chromate, BaCrO4 , is formed. K2CrO4 (aq) + Ba(NO3)2 (aq) BaCrO4 (s) + 2 KNO3 (aq) precipitate 24 12 4/19/15 NEUTRALIZATION REACTIONS q The Saltsmost are ionic important substances reaction withofthe acids cation anddonated bases is calledthe from neutralization. base and the anion donated from the acid. q In the these laboratory, reactions an neutralization acid combines reactions with a are base to form a salt observed byand an increase water. in temperature (exothermic reaction). HCl (aq) + NaOH (aq) æ æÆ NaCl (aq) + H 2O (l) Acid Base Salt 25 UNSTABLE PRODUCT REACTIONS q Some chemical reactions produce gas because one of the products formed in the reaction is unstable. q Two such products are: Carbonic acid: H2CO3 (aq) → CO2 (g) + H2O (l) Sulfurous acid: H2SO3 (aq) → SO2 (g) + H2O (l) 26 13 4/19/15 UNSTABLE PRODUCT REACTIONS q When either of these products appears in a chemical reaction, they should be replaced with their decomposition products. 2 HCl + Na2CO3 → 2 NaCl + H2CO3 2 HCl + Na2CO3 → 2 NaCl + CO2 (g) + H2O (l) 27 Examples: Complete and balance each neutralization reaction below: 2 HNO3 + Ba(OH)2 → Ba(NO3)2 + 2 H2O H2SO4 + 2 NaOH → Na2SO4 + 2 H2O HC2H3O2 + KOH → KC2H3O2 + H2O 28 14 4/19/15 OXIDATION-REDUCTION REACTIONS q Reactions In an oxidation-reduction known as oxidation reaction, and reduction electrons are (redox) havefrom transferred many one important substance applications to another.in our q everyday If one substance lives. loses electrons, another substance q must Rusting gain ofelectrons. a nail or the reaction within your car batteries are two examples of redox reactions. 29 OXIDATION-REDUCTION REACTIONS q Oxidation is defined as loss of electrons, and reduction is defined as gain of electrons. q One way to remember these definitions is to use the following mnemonic: Oxidation Is Loss of electrons OIL Reduction Is Gain of electrons RIG q Combination, decomposition, single replacement and combustion reactions are all examples of redox reactions. 30 15 4/19/15 OXIDATION-REDUCTION REACTIONS q In Forgeneral, example, atoms in the offormation metals loseofelectrons calcium to sulfide form cations, from calcium and are and therefore sulfur oxidized, while atoms of non-metals gain toCaS form anions, and are Caelectrons +S therefore reduced. Ca Ca2+ + 2 e- S + 2 e- S2- Oxidation Reduction q Therefore, the formation of calcium sulfide involves two half-reactions that occur simultaneously, one an oxidation and the other a reduction. 31 OXIDATION-REDUCTION REACTIONS q Similarly, in the reaction of magnesium metal with hydrochloric acid Mg + 2 HCl Mg MgCl2 + H2 Mg2+ + 2 e- 2 H+ + 2 e- H2 Oxidation Reduction In every oxidation-reduction reaction, the number of electrons lost must be equal to the number of electrons gained. 32 16 4/19/15 OXIDATION-REDUCTION IN BIOLOGICAL SYSTEMS q Many important biological reactions involve oxidation and reduction. q In these reactions, oxidation involves addition of oxygen or loss of hydrogen, and reduction involves Oxidation loss of oxygen or gain of hydrogen. (loss of q For example, poisonous methyl alcohol is hydrogen) metabolized by the body by the following reaction: CH3OH methyl alcohol H2CO + 2H• formaldehyde 33 OXIDATION-REDUCTION IN BIOLOGICAL SYSTEMS q The formaldehyde is further oxidized to formic acid and finally carbon dioxide and water by the Oxidation following reactions: (gain of oxygen) 2 H2CO + O2 2H2CO2 formaldehyde 2 H2CO2 + O2 formic acid CO2 + H2O formic acid 34 17 4/19/15 OXIDATION-REDUCTION IN BIOLOGICAL SYSTEMS q In many biochemical oxidation-reduction reactions, the transfer of hydrogen atoms produces energy in the cells. q For example, cellular respiration is an oxidationreduction process that transfers energy from the bonds in glucose to form ATP. C6H12O6 + 6 O2 6 CO2 + 6 H2O + ATP + Heat 35 OXIDATION-REDUCTION IN BIOLOGICAL SYSTEMS Loss of hydrogen atoms (becomes oxidized) C6H12O6 Glucose 6 O2 6 CO2 6 H 2O Gain of hydrogen atoms (becomes reduced) ATP + Heat 18 4/19/15 OXIDATION-REDUCTION IN BIOLOGICAL SYSTEMS q The oxidation of a typical biochemical molecule can involve the transfer of hydrogen atoms to a proton acceptor such as coenzyme FAD to produce its reduced form FADH2. 37 REDOX IN BIOLOGICAL SYSTEMS q In summary, the particular definition of oxidationreduction depends on the process that occurs in the reaction. q A summary of these definitions appears below: 38 19 4/19/15 Example 1: Linoleic acid, an unsaturated fatty acid, can be converted to a saturated fatty acid by the reaction shown below. Is linoleic acid oxidized or reduced in this reaction? C18H32 O2 + 2 H2 C18H36 O2 Gain of Reduction hydrogen 39 Example 2: The reaction of succinic acid (C4H6O4) provides energy for the ATP synthesis and is shown below: C4H6 O4 FAD + 2 H• C4H4 O4 + 2 H• FADH2 a) Is succinic acid oxidized or reduced? oxidation Loss of b) Is FAD oxidized or reduced? reduction hydrogen Gain of hydrogen 40 20 4/19/15 ENZYMES IN BIOLOGICAL SYSTEMS q In biochemical reactions, enzymes are necessary to oxidize glucose and other foods. q For example, oxidation of glucose involves the transfer of hydrogen atoms and electrons to an enzyme, such as NAD + to produce its reduced form NADH. 41 ENZYMES IN BIOLOGICAL SYSTEMS q Similarly, oxidation of methanol involves transfer of 2 hydrogen atoms and 2 electrons to NAD+ to form the reduced form NADH. 42 21 4/19/15 ENZYMES IN BIOLOGICAL SYSTEMS q Molecules such as NAD+ are called “electron carriers” since they carry electrons in their reduced form. q The electron carriers collectively are called electron transport chain. q As electrons are transported down the chain, ATP is generated. 43 ENZYMES IN BIOLOGICAL SYSTEMS 44 22 4/19/15 ACTIVITY SERIES OF METALS q Activity series is a listing of metallic elements in descending order of reactivity. q Hydrogen is also included in the series since it behaves similar to metals. q Activity series tables are available in textbooks and other sources. 45 ACTIVITY SERIES OF METALS q Elements listed higher will displace any elements listed below them. q For example Na will displace any elements listed below it from one of its compounds. 2 Na (s) + MgCl2 (aq) → 2 NaCl (aq) + Mg (s) Na (s) + AgCl (aq) → NaCl (aq) + Ag (s) 46 23 4/19/15 ACTIVITY SERIES OF METALS q Elements listed lower will not displace any elements listed above them. q For example Ag cannot displace any elements listed above it from one of its compounds. Ag (s) + CuCl2 (aq) → No Reaction Ag (s) + HCl (aq) → No Reaction 47 Example 1: Use activity series to complete each reaction below. If no reaction occurs, write “No Reaction”. Pb (s) + 2 HCl (aq) → PbCl2 (aq) + H2 (g)Metals Pb is more reactive than H Fe Ni Sn Pb H Cu Ag 48 24 4/19/15 Example 2: Use activity series to complete each reaction below. If no reaction occurs, write “No Reaction”. Zn (s) + MgCl2 (aq) → No Reaction Zn is less reactive than Mg Metals Na Mg Al Zn Fe Ni 49 Example 3: Use activity series to complete each reaction below. If no reaction occurs, write “No Reaction”. Ni (s) + CuCl2 (aq) → NiCl2 (aq) + Cu (s)Metals Ni is more reactive than Cu Fe Ni Sn Pb H Cu Ag 50 25 4/19/15 Example 4: Use activity series to complete each reaction below. If no reaction occurs, write “No Reaction”. 3 Mg (s) + 2 AlCl3 (aq) → 3 MgCl2 (aq) + 2Metals Al (s) Mg is more reactive than Al Na Mg Al Zn Fe Ni 51 THE END 52 26