Balancing Chemical Equations

What does a chemical equation look like? Reactants Products Examples 2Na + Cl 2 2NaCl H 2 + O 2 H 2 O 2 2H 2 + O 2 2H 2 O Why do we have Cl 2, H 2, and O 2, and not Cl, H, O Diatomic molecules = only exist by themselves in pairs Br 2, I 2, N 2, Cl 2, H 2, O 2, F 2

Balancing Chemical Equations Law of Conservation of Mass Total mass of reactants equals total mass of products In a chemical reaction total number of atoms in reactants equals total number of atoms in products. 2H 2 + O 2 2H 2 O Reactants = 4 Hydrogen Atoms, 2 Oxygen Atoms Products = 4 Hydrogen Atoms, 2 Oxygen Atoms

Balancing Chemical Equations 2H 2 + O 2 2H 2 O 2 = Coefficient, There are 2 Hydrogen gas Molecules (H 2 ). Or 2 Water molecules (H 2 O) 2 = Subscript, How many atoms there are in a molecule. In 1 hydrogen gas molecule there are 2 Hydrogen atoms In 1 water molecule there are 2 Hydrogen atoms

Balancing Chemical Equations Reading Coefficients and Subscripts 2H 2 O 2 Water molecules 4 Hydrogen Atoms 2 Oxygen Atoms 4AlCl 3 4 Aluminum Chloride molecules 4 Aluminum Atoms 12 Chlorine atoms 3Mg(OH) 2 3 Magnesium Hydroxide Molecules 3 Magnesium Atoms 6 Oxygen atoms 6 Hydrogen atoms

Balancing Chemical Equations Rules 1. Determine the correct formulas for all the reactants and products 2. Write the skeleton equation separating the reactants on the left and products on the right 3. Determine the number of atoms of each element or polyatomic ions in the reactants and products 4. Balance the elements one at a time by using coefficients, when no coefficient is written it is assumed to be Check each atom or polyatomic ion to be sure they are equal on both sides of the equation. 6. Make sure all the coefficients are in the lowest possible ratio 7. Never change the subscripts!!!!

Balancing Chemical Equations Examples H 2 + O 2 H 2 O Reactants = 2 hydrogen atoms, 2 Oxygen atoms Products = 2 hydrogen atoms, 1 Oxygen atoms What is out of balance? Oxygen, add coefficients to make Oxygen equal H 2 + O 2 2H 2 O Now what is out of balance? Hydrogens, add coeffiecients, to where? 2H 2 + O 2 2H 2 O

Balancing Chemical Equation Examples NaCl + AgNO 3 AgCl + NaNO 3 Reactants = 1 sodium atom, 1 chlorine atoms, 1 silver atom, 1 nitrogen atom, 3 oxygen atoms Products = 1 Sodium, 1 chlorine, 1 nitrogen, 3 oxygen Is it balanced? NaCl + AgNO 3 AgCl + NaNO 3

Balancing Chemical Equations Example AlCl 3 + O 2 Al 2 O 3 + Cl 2 Reactants = 1 aluminum atoms, 3 chlorine atoms, 2 oxygen atoms Products = 2 aluminum atoms, 3 oxygen atoms, 2 chlorine atoms Start with aluminums, add coefficient 2 in front of AlCl 3 2AlCl 3 + O 2 Al 2 O 3 + Cl 2 Now we have 6 chlorines in reactants and 2 in products Add coefficient of 3 in Cl in products 2AlCl 3 + O 2 Al 2 O 3 + 3Cl 2 Now Oxygens are out of balance how can we balance them? Add 1.5 in front of O 2 2AlCl O 2 Al 2 O 3 + 3Cl 2 Can’t have a decimal so we multiply it all by 2 4AlCl 3 + 3O 2 2Al 2 O 3 + 6Cl 2

Balancing Chemical Equations Example AgNO 3 + H 2 S Ag 2 S + HNO 3 Reactants = 1 silver atom, 1 nitrogen atom, 3 oxygen atoms, 2 hydrogen atoms, 1 sulfur atom Products = 2 silver atoms, 1 sulfur atoms, 1 hydrogen atom, 1 nitrogen atoms, 3 oxygen atoms Lets balance out the silvers first Add coefficient 2 in front of AgNO 3 in reactants 2AgNO 3 + H 2 S Ag 2 S + HNO 3 Now the Nitrogens and oxygens are out of balance. Whats next? Add coefficient 2 in front of HNO 3 2AgNO 3 + H 2 S Ag 2 S + 2HNO 3

Practice 1. H 2 + O 2  H 2 O 2 2. H 2 + O 2  H 2 O 3. LiCl + Mg(OH) 2  LiOH + MgCl 2 4. Pb(OH) 4  Pb + OH 5. C 2 + H 2 + O 2  C 6 H 12 O 6 6. NaCl  Na + Cl 2 7. MgCl 2 + KI  KCl + MgI 2 8. Ca(CN) 2 + Cs 2 S  CaS + CsCN 9. CoF 2 + LiOH  Co(OH) 2 + LiF

Balancing and Formula Practice! Write out and balance each chemical equation. 1. Sodium Sulfite reacts with Magnesium Oxide to form Magnesium Sulfite and Sodium Oxide 2. Mercury (II) Oxide breaks apart to form Mercury (II) and Oxygen (O 2 ) 3. Rubidium Silicate reacts with Calcium Flouride to form Rubidium Flouride and Calcium Silicate 4. Tin (II) Iodide reacts with Sodium Carbonate to form Tin (II) Carbonate and Sodium Iodide