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Chapter 5 Chemical Reactions and Quantities Physical Change In a physical change, § The identity and composition of the substance do not change. § The state can change, or the material can be torn into smaller pieces. 5.1 Chemical Changes 1 2 Some Examples of Chemical and Physical Changes Chemical Change In a chemical change, n Reacting substances form new substances with different compositions and properties. n A chemical reaction takes place. TABLE 5.1 Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 3 4 Learning Check Chemical Reaction Classify each of the following as a In a chemical reaction, 1) physical change or 2) chemical change. § A chemical change produces one or more new substances. A. ____Burning a candle. § There is a change in the composition of one or more substances. B. ____Ice melting on the street. C. ____Toasting a marshmallow. D. ____Cutting a pizza. E. ____Polishing a silver bowl. 5 6 1 Chemical Reaction Learning Check In a chemical reaction, Identify the visible evidence of a chemical reaction in each of the following: A. Methane gas in an outdoor heater burns with a blue flame. B. Bleach removes stains from a shirt. C. Bubbles of CO2 are released when baking soda is mixed with vinegar. § Old bonds are broken, and new bonds are formed. § Atoms in the reactants are rearranged to form one or more different substances. § Fe and O2 form rust (Fe2O3). 7 Chapter 5 Chemical Reactions and Quantities 8 Chemical Equations A chemical equation 5.2 Chemical Equations § Gives the chemical formulas of the reactants on the left of the arrow and the products on the right. Reactants Product O2 (g) CO2 (g) C(s) 9 Symbols Used in Equations Symbols are used in chemical equations to show 10 Chemical Equations are Balanced In a balanced chemical reaction, TABLE 5.2 § Atoms are not gained or lost. § The states of the reactants. § The number of atoms in the reactants is equal to the number of atoms in the products. § The states of the products. § The reaction conditions. 11 12 2 A Balanced Chemical Equation Learning Check In a balanced chemical equation, State the number of atoms of each element on the reactant side and the product side for each of the following balanced equations: § There must be the same number of each type of atom on the reactant side and on the product side. § Numbers called coefficients are used in front of one or more formulas. Al + S 2Al + 3S Al2S3 Not Balanced Al2S3 Balanced 2Al = 2Al 3S = 3S A. P4(s) + 6Br2(l) → 4 PBr3(g) B. 2Al(s) + Fe2O3(s) → 2Fe(s) + Al2O3(s) 13 Learning Check 14 Equation for A Chemical Reaction Determine if each equation is balanced or not. A. Na(s) + N2(g) → Na3N(s) B. C2H4(g) + H2O(l) → C2H5OH(l) 15 Checking a Balanced Equation 16 Guide to Balancing Equations Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings Reactants 1 C atom 4 H atoms 4 O atoms = = = Products 1 C atom 4 H atoms 4 O atoms 17 18 3 Balancing Chemical Equations Learning Check STEP 1 Write the equation with the correct formulas. NH3(g) + O2(g) NO(g) + H2O(g) STEP 2 Determine if the equation is balanced. No, not all atoms are balanced. STEP 3 Balance with coefficients in front of formulas. 4NH3 + 5O2 4NO + 6H2O Check the balance of atoms in the following: Fe3O4(s) + 4H2(g) 1) 2 1) 2 reactants and products. = = = 2) 4 3) 8 B. Number of O atoms in reactants. STEP 4 Check that atoms of each element are equal in 4 N (4 x 1N) 12 H (4 x 3H) 10 O (5 x 2O) 3Fe(s) + 4H2O(l) A. Number of H atoms in products. 2) 4 3) 8 C. Number of Fe atoms in reactants. 4 N (4 x 1N) 12 H (6 x 2H) 10 O (4O + 6O) 1) 1 2) 3 3) 4 19 Learning Check 20 Equations with Polyatomic Ions Balance each equation and list the coefficients in the balanced equation going from reactants to products: A. __Mg(s) + _N2(g) __Mg3N2(s) 1) 1, 3, 2 B. __Al(s) 1) 3, 3, 2 2) 3, 1, 2 + __Cl2(g) 3) 3, 1, 1 __AlCl3(s) 2) 1, 3, 1 3) 2, 3, 2 21 Balancing with Polyatomic Ions Na3PO4(aq) + MgCl2(aq) Balance PO43- as a unit 2Na3PO4(aq) 2PO43Balance Mg 3MgCl2(aq) 3Mg2+ = Learning Check Balance and list the coefficients from reactants to products: NaCl(aq) + Mg3(PO4)2(s) = 22 A. __Fe2O3(s) + __C(s) Mg3(PO4)2(s) 2PO43- 1) 2, 3, 2,3 Mg3(PO4)2(s) B. __Al(s) + __FeO(s) 3Mg2+ 1) 2, 3, 3, 1 Balance Na and Cl to complete balanced equation 3MgCl2(aq) + 2Na3PO4(aq) 6NaCl(aq) + Mg3(PO4)2(s) 6Na+ = 6Na+ 6Cl= 6Cl- 1) 3, 2, 1, 2 2) 2, 3, 1, 3 + __CO2(g) 3) 1, 1, 2, 3 __Fe(s) + 2) 2, 1, 1, 1 C. __Al(s) + __H2SO4(aq) 23 __Fe(s) 2) 2, 3, 4, 3 __Al2O3(s) 3) 3, 3, 3, 1 __Al2(SO4)3(aq) + __H2(g) 3) 2, 3, 2, 3 24 4