Balancing Chemical Equations

Balancing Chemical Equations

Law of Conservation of Mass Do you remember this law? The Law of Conservation of Mass states: that mass is neither created nor destroyed in any chemical reaction. Can you provide an example that illustrates this law?

Law of Conservation of Mass Because mass can be neither created nor destroyed during a chemical change… the balancing of equations requires the same number of atoms on both sides of a chemical reaction.

But Before We Balance Equations…. we must learn how to count the number of atoms in a given formula!

Multiply the total number of atoms inside the parentheses by the Totaling Atoms Using PSEC Parentheses Count up the total number of atoms inside the parentheses Subscripts Multiply the total number of atoms inside the parentheses by the subscript value Extras Add any extra atoms that were not yet accounted for to the total Coefficients Multiply the total number of atoms (from PSE) by the coefficient

Examples Parentheses Subscripts Extras Coefficients 2Ba(NO3)2 4(NH4)3PO4 P – 4 atoms total N 1 O 3 4 atoms Parentheses P – 5 atoms total N 1 H 4 5 atoms Subscripts S – 8 atoms total (4)(2) = 8 atoms S – 15 atoms total (5)(3) = 15 atoms Extras E – 9 atoms total 8 + 1 = 9 atoms E – 20 atoms total 15 + 5 = 20 atoms C – 18 atoms total (9)(2) = 18 atoms Coefficients C – 80 atoms total (20)(4) = 80 atoms

Law of Conservation of Mass The mass of all the reactants (the substances going into a reaction) must equal the mass of the products (the substances produced by the reaction). Reactant + Reactant = Product

Example: Iron (II) Sulfide iron + sulfur iron (II) sulfide Fe + S FeS Note that in a chemical equation, by convention, we use the arrow “ " instead of the equals “ = ".

Balancing Equations 2 3 ___ Al(s) + ___ Br2(l) ---> ___ Al2Br6(s)

Learning Through Example Magnesium Oxide Mg + O2  MgO Then all you do is list the atoms that are involved on each side of the arrow Mg + O2  MgO Mg O Mg O

Mg + O2  MgO Mg + O2  MgO 2 Then start balancing: [1] Just count up the atoms on each side Mg + O2  MgO Mg O 1 1 2 1 [2] The numbers aren’t balanced so then add “BIG” numbers to make up for any shortages Mg + O2  MgO Mg O 1 2 2 2 2 And adjust totals

WE SAY THAT THE EQUATION IS BALANCED!! But the numbers still aren’t equal, so add another “BIG” number 2 Mg + O2  MgO Mg O 1 2 2 And adjust totals again NOW BOTH SIDES HAVE EQUAL NUMBERS OF ATOMS WE SAY THAT THE EQUATION IS BALANCED!!

Try to balance these equations using the same method: [1] Na + Cl2  NaCl [2] CH4 + O2  CO2 + H2O [3] Li + HNO3  LiNO3 + H2 [4] Al + O2  Al2O3

How did you do?? Here are the answers: [1] 2 Na + Cl2  2 NaCl [2] CH4 + 2 O2  CO2 + 2 H2O [3] 2 Li + 2 HNO3  2 LiNO3 + H2 [4] 4 Al + 3 O2  2 Al2O3