1 Chapter 5 Chemical Reactions and Quantities 5.2 Chemical Equations

2 A chemical equation gives the chemical formulas of the reactants on the left of the arrow and the products on the right. ReactantsProduct C(s) O 2 (g) CO 2 (g)

3 Symbols Used in Equations Symbols used in chemical equations show the states of the reactants. the states of the products. the reaction conditions. TABLE 5.2

4 Chemical Equations are Balanced In a balanced chemical reaction atoms are not gained or lost. the number of reactant atoms is equal to the number of product atoms.

5 A Balanced Chemical Equation In a balanced chemical equation, there must be the same number of each type of atom on the reactant side and on the product side of a balanced equation. numbers called coefficients are used in front of one or more formulas. Al + S  Al 2 S 3 Not Balanced 2Al + 3S  Al 2 S 3 Balanced 2 Al = 2 Al 3 S = 3 S

6 Learning Check State the number of atoms of each element on the reactant side and the product side for each of the following balanced equations. A. P 4 (s) + 6Br 2 (l)  4 PBr 3 (g) B. 2Al(s) + Fe 2 O 3 (s)  2Fe(s) + Al 2 O 3 (s)

7 Solution A. P 4 (s) + 6Br 2 (l)  4 PBr 3 (g)4 P 12 Br 12 Br B. 2Al(s) + Fe 2 O 3 (s)  2Fe(s) + Al 2 O 3 (s)2 Al 2 Fe 3 O3 O

8 Learning Check Determine if each equation is balanced or not. A. Na(s) + N 2 (g)  Na 3 N(s) B. C 2 H 4 (g) + H 2 O(l)  C 2 H 5 OH(l)

9 Solution Determine if each equation is balanced or not. A. Na(s) + N 2 (g)  Na 3 N(s) No. 2 N on reactant side, 1 N on product side. 1 Na on reactant side, 3 Na on product side. B. C 2 H 4 (g) + H 2 O(l)  C 2 H 5 OH(l) Yes. 2 C = 2 C 6 H = 6 H 1 O = 1 O

10 Guide to Balancing a Chemical Equation Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

11 To balance the following equation, Fe 3 O 4 (s) + H 2 (g)  Fe(s) + H 2 O(l) work on one element at a time. use only coefficients in front of formulas. do not change any subscripts. Fe:Fe 3 O 4 (s) + H 2 (g)  3Fe(s) + H 2 O(l) O:Fe 3 O 4 (s) + H 2 (g)  3Fe(s) + 4H 2 O(l) H: Fe 3 O 4 (s) + 4H 2 (g)  3Fe(s) + 4H 2 O(l) Steps in Balancing an Equation

12 1. Write the equation with the correct formulas. NH 3 (g) + O 2 (g)  NO(g) + H 2 O(g) 2. Determine if the equation is balanced. No, not all atoms are balanced. 3. Balance with coefficients in front of formulas. 4NH 3 (g) + 5O 2 (g)  4NO(g) + 6H 2 O(g) 4. Check that atoms of each element are equal in reactants and products. 4 N (4 x 1 N) = 4 N (4 x 1 N) 12 H (4 x 3 H) = 12 H (6 x 2 H) 10 O (5 x 2 O)= 10 O(4 O + 6 O) Balancing Chemical Equations

13 Equation for A Chemical Reaction

14 Checking a Balanced Equation ReactantsProducts 1 C atom=1 C atom 4 H atoms=4 H atoms 4 O atoms = 4 O atoms

15 Check the balance of atoms in the following. Fe 3 O 4 (s) + 4H 2 (g)  3Fe(s) + 4H 2 O(l) A. Number of H atoms in products. 1) 22) 43) 8 B. Number of O atoms in reactants. 1) 22) 43) 8 C. Number of Fe atoms in reactants. 1) 12) 33) 4 Learning Check

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

17 Balance each equation and list the coefficients in the balanced equation going from reactants to products: A. __Mg(s) + __N 2 (g)  __Mg 3 N 2 (s) 1) 1, 3, 2 2) 3, 1, 2 3) 3, 1, 1 B. __Al(s) + __Cl 2 (g)  __AlCl 3 (s) 1) 3, 3, 22) 1, 3, 1 3) 2, 3, 2 Learning Check

18 A. 3) 3, 1, 1 3Mg(s) + 1N 2 (g)  1Mg 3 N 2 (s) B. 3) 2, 3, 2 2Al(s) + 3Cl 2 (g)  2AlCl 3 (s) Solution

19 Equations with Polyatomic Ions

20 Balancing with Polyatomic Ions MgCl 2 (aq) + Na 3 PO 4 (aq)  NaCl(aq) + Mg 3 (PO 4 ) 2 (s) Balance PO 4 3- as a unit MgCl 2 (aq) + 2Na 3 PO 4 (aq)  NaCl(aq) + Mg 3 (PO 4 ) 2 (s) 2 PO 4 3- = 2 PO 4 3- Balance Mg and Cl 3MgCl 2 (aq) + 2Na 3 PO 4 (aq)  6NaCl(aq) + Mg 3 (PO 4 ) 2 (s) 3 Mg 2+ = 3 Mg 2+ 6 Na + = 6 Na + 6 Cl - = 6 Cl -

21 Balance and list the coefficients from reactants to products. A. __Fe 2 O 3 (s) + __C(s)  __Fe(s) + __CO 2 (g) 1) 2, 3, 2, 3 2) 2, 3, 4, 3 3) 1, 1, 2, 3 B. __Al(s) + __FeO(s)  __Fe(s) + __Al 2 O 3 (s) 1) 2, 3, 3, 1 2) 2, 1, 1, 1 3) 3, 3, 3, 1 C. __Al(s) + __H 2 SO 4 (aq)  __Al 2 (SO 4 ) 3 (aq) + __H 2 (g) 1) 3, 2, 1, 2 2) 2, 3, 1, 3 3) 2, 3, 2, 3 Learning Check

22 A. 2) 2, 3, 4, 3 2Fe 2 O 3 (s) + 3C(s)  4Fe(s) + 3CO 2 (g) B. 1) 2, 3, 3, 1 2Al(s) + 3FeO(s)  3Fe(s) + 1Al 2 O 3 (s) C. 2) 2, 3, 1, 3 2Al(s) + 3H 2 SO 4 (aq)  1Al 2 (SO 4 ) 3 (aq) + 3H 2 (g) Solution