Chapter 7 Chemical Reactions 7.3 Balancing a Chemical Equation Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Balancing Chemical Equations 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(g) + 5O2(g) 4NO(g) + 6H2O(g) STEP 4 Check that atoms of each element are equal in reactants and products. 4 N (4 x 1N) = 4 N (4 x 1N) 12 H (4 x 3H) = 12 H (6 x 2H) 10 O (5 x 2O) = 10 O (4O + 6O)

Steps in Balancing an Equation To balance the following equation, Fe3O4(s) + H2(g) Fe(s) + H2O(l) Work on one element at a time. Use only coefficients in front of formulas. Do not change any subscripts. Fe: Fe3O4(s) + H2(g) 3Fe(s) + H2O(l) O: Fe3O4(s) + H2(g) 3Fe(s) + 4H2O(l) H: Fe3O4(s) + 4H2(g) 3Fe(s) + 4H2O(l)

Learning Check Check the balance of atoms in the following: Fe3O4(s) + 4H2(g) 3Fe(s) + 4H2O(l) A. Number of H atoms in products. 1) 2 2) 4 3) 8 B. Number of O atoms in reactants. C. Number of Fe atoms in reactants. 1) 1 2) 3 3) 4

Solution Fe3O4(s) + 4H2(g) 3Fe(s) + 4H2O(l) A. Number of H atoms in products. 3) 8 (4H2O) B. Number of O atoms in reactants. 2) 4 (Fe3O4) C. Number of Fe atoms in reactants. 2) 3 (Fe3O4)

Equation for A Chemical Reaction Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Checking a Balanced Equation Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings Reactants Products 1 C atom = 1 C atom 4 H atoms = 4 H atoms 4 O atoms = 4 O atoms

Learning Check A. __Mg(s) + __N2(g) __Mg3N2(s) 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 2) 3, 1, 2 3) 3, 1, 1 B. __Al(s) + __Cl2(g) __AlCl3(s) 1) 3, 3, 2 2) 1, 3, 1 3) 2, 3, 2

Solution A. 3) 3, 1, 1 3Mg(s) + 1N2(g) 1Mg3N2(s) B. 3) 2, 3, 2 2Al(s) + 3Cl2(g) 2AlCl3(s)

Equations with Polyatomic Ions Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Balancing with Polyatomic Ions MgCl2(aq) + Na3PO4(aq) NaCl(aq) + Mg3(PO4)2(s) Balance PO43- as a unit MgCl2(aq) + 2Na3PO4(aq) NaCl(aq) + Mg3(PO4)2(s) 2PO43- = 2PO43- Balance Mg and Cl 3MgCl2(aq) + 2Na3PO4(aq) 6NaCl(aq) + Mg3(PO4)2(s) 3 Mg2+ = 3 Mg2+ 6 Na+ = 6 Na+ 6 Cl- = 6 Cl-

Learning Check Balance and list the coefficients from reactants to products: A. __Fe2O3(s) + __C(s) __Fe(s) + __CO2(g) 1) 2, 3, 2,3 2) 2, 3, 4, 3 3) 1, 1, 2, 3 B. __Al(s) + __FeO(s) __Fe(s) + __Al2O3(s) 1) 2, 3, 3, 1 2) 2, 1, 1, 1 3) 3, 3, 3, 1 C. __Al(s) + __H2SO4(aq) __Al2(SO4)3(aq) + __H2(g) 1) 3, 2, 1, 2 2) 2, 3, 1, 3 3) 2, 3, 2, 3

Solution A. 2) 2, 3, 4, 3 2Fe2O3(s) + 3C(s) 4Fe(s) + 3CO2(g) B. 1) 2, 3, 3, 1 2Al(s) + 3FeO(s) 3Fe(s) + 1Al2O3(s) C. 2) 2, 3, 1, 3 2Al(s) + 3H2SO4(aq) 1Al2(SO4)3(aq) + 3H2(g)