Chemical Reactions and Balancing Equations (I) Counting and Accounting

Examples of Correct Counting Sodium phosphate, Na3(PO4)3 Ammonium carbonate, (NH4)2CO3 Aluminum dichromate, Al2(Cr2O7)3 Silicon formate, Si(HCOO)4 Bismuth azide, Bi(N3)3 Chromium (III) sulfate, Cr2(SO4)3 Gold (III) Perchlorate, Au(ClO4)3 3 Na, 3 P, 12 O 2 N, 8 H, 1 C, 3 O 2 Al, 6 Cr, 21 O 1 Si, 4 H, 4 C, 8 O 1 Bi and 9 N 2 Cr, 3 S, 12 O 1 Au, 3 Cl, 12 O

Introduction Chemical reactions occur when bonds in the reacting species are rearranged to form one or more different substances. Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes. Symbols represent elements, formulas describe compounds, and chemical equations describe a chemical reaction.

Parts of a Reaction Equation Chemical equations show the conversion of reactants (the molecules shown on the left of the arrow) into products (the molecules shown on the right of the arrow). A “+” sign separates molecules on the same side The arrow is read as “react(s) to yield” Example C + O2  CO2 This reads “carbon plus oxygen react to yield carbon dioxide”

Chemical Equations Lavoisier, 1788 Because of the principle of the conservation of matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides.

Non-Chemical Symbols Used in Chemical Equations Solid (s) Liquid (l) Gas (g) Aqueous solution (aq) Catalyst H2SO4 Escaping gas () Change of temperature ()

Balancing Equations When balancing a chemical reaction you may add COEFFICIENTS in front of the compounds to balance the reaction, but you may not change the subscripts. Changing subscripts changes the compound. Subscripts are determined by (1) charge balance for ionic compounds and (2) the correct sharing of electrons for covalent compounds.

Subscripts vs. Coefficients The subscripts tell you how many atoms of a particular element are in a compound. The coefficients tell you about the quantity, or number, of molecules of the compound.

Rules for Counting When Using Coefficients A coefficient to the right of a chemical symbol acts as a multiplier for each of the elements contained within the chemical symbol. If there is no coefficient, then it means there is “one” of the species. As before, Ga2(SO4)3 would have 2 Ga’s, 3 S’s, and 12 O’s. Similarly, 3 Ga2(SO4)3 would have 6 Ga’s, 9 S’s, and 36 O’s.

Chemical Equations 4 Al(s) + 3 O2(g) → 2 Al2O3(s) This equation means 4 Al atoms + 3 O2 molecules ---produces→ 2 formula units of Al2O3

Another View of What a Balanced Chemical Equation Means

As a Result of Learning About Covalent Bonding, One Should Know that 7 Elements Exist Naturally as Diatomic Molecules. These diatomic molecules include: Chlorine Cl2 , Bromine Br2, Fluorine F2, Iodine I2, Oxygen O2, Nitrogen N2, and Hydrogen H2 In Balanced Chemical Equations, members of this “7” must be written as diatomic molecules. Li 3 He 2 C 6 N 7 O 8 F 9 Ne 10 Na 11 B 5 Be 4 H Al 13 Si 14 P 15 S 16 Cl 17 Ar 18 K 19 Ca 20 Sc 21 Ti 22 V 23 Cr 24 Mn 25 Fe 26 Co 27 Ni 28 Cu 29 Zn 30 Ga 31 Ge 32 As 33 Se 34 Br 35 Kr 36 Rb 37 Sr 38 Y 39 Zr 40 Nb 41 Mo 42 Tc 43 Ru 44 Rh 45 Pd 46 Ag 47 Cd 48 In 49 Sn 50 Sb 51 Te 52 I 53 Xe 54 Cs 55 Ba 56 Hf 72 Ta 73 W 74 Re 75 Os 76 Ir 77 Pt 78 Au 79 Hg 80 Tl 81 Pb 82 Bi 83 Po 84 At 85 Rn 86 Fr 87 Ra 88 Rf 104 Db 105 Sg 106 Bh 107 Hs 108 Mt 109 Mg 12 *

Rules for Balancing Chemical Equations Make sure all compounds and elements on both sides of the equation are written correctly. Check to be sure that the “7” diatomic molecules are written properly. Make a list for each element on each side of the equation. Add coefficients to balance the equation so you have the same number of atoms of each element on both the reactant and product sides of the equation. Adjust the lists as changes in coefficients are made.

Example #1: Initial Condition __Fe + __H2O → __Fe3O4 + __H2 Fe 1 3 O 4 H 2

Example #1: Balance Metal 3 Fe + __H2O → __Fe3O4 + __H2 Fe 3 O 1 4 H 2

Example #1: Balance Oxygen 3 Fe + 4 H2O → __Fe3O4 + __H2 Fe 3 O 4 H 8 2

Example #1: Balance Hydrogen 3 Fe + 4 H2O → __Fe3O4 + 4 H2 Fe 3 O 4 H 8

__Al + __HCl → __AlCl3 + __H2 Example #2: Initial Condition __Al + __HCl → __AlCl3 + __H2 Al 1 Cl 3 H 2

Example #2: Balance Chlorine __Al + 3 HCl → __AlCl3 + __H2 Al 1 Cl 3 H 2

Example #3: Balance Hydrogen __Al + 3 HCl → __AlCl3 + 1.5 H2 Al 1 Cl 3 H

Example #2: Get all whole numbers by multiplying all of the coefficients by 2. 2 Al + 6 HCl → 2 AlCl3 + 3 H2 Al 2 Cl 6 H

Example #3: Initial Condition __H2 + __O2 → __H2O O 2 1 H

Example #3: Balance the Oxygens and then Re-Balance the Hydrogens 2 H2 + __O2 → 2 H2O O 2 H 4