Balancing Chemical Equations

Evidence of a chemical reaction The temperature may change (but could be from adding or removing energy) Different coloured materials may form (precipitate) New phases may be formed (precipitate/gas)

Conservation Laws In a closed chemical system, the reaction follows Conversation of mass Total mass does not change Conversation of electrical charge Total charge does not change Conversation of energy Total energy does not change Conversation of atoms Total number of atoms does not change What we use to balance chemical equations

Some terms to know Chemical reaction equation – equation shown during a chemical reaction Reactants – starting chemicals Product – resulting chemicals Coefficients – refer to the number of molecules in each species in the reaction System – The part of the universe being studied Closed – Nothing can enter or leave system Open – things can enter or leave system

Some terms to know Conserved – The quantity does not change during reaction Only possible in a closed system Why do we a closed system? We must our reaction follow the conservation laws Or else we will have skewed results

Coefficients We put coefficients in front of chemical formulas to state how many molecules of each species are present We put coefficients to balance our atoms Reactant atoms must equal product atoms You have done some balancing in grade 10

Strategy - 1 Scan the equation and balance ELEMENTS that only occur ONCE on both sides of our equation. Metal atoms usually show up once on both sides, so these should usually be your starting point If there are no metal atoms, such as an organic reaction, you should balance anything that is not H or O first. We usually balance H and O last as they show up multiple times.

Strategy - 2 After balancing the first element, there will usually be another element attached to the element you balanced Balance that other element Repeat until you have balanced everything except for H and O Balance H and O LAST!

Strategy - 3 Assume that if there are no coefficients, it is ZERO. If you get comfortable, you can forget this step. This step is used so that you have something to balance with if you write 1.

Strategy - 4 Balance polyatomic ions as one single group of atoms. Not always the case, but 95% of the time. Do not balance it as individual atoms. So assume PO4, SO4, NH4, NO3, OH etc etc are all a single element and balance them accordingly.

Strategy - 5 You can use fractions/decimals when you are nearing the end and something cannot be balanced with whole numbers. So you can put ½ or any fraction/decimal in front of a chemical species This is usually done for diatomics as you may have 3O on one side and 1O2 on the other side. You put 3/2 or 1.5 in front of the O2 to balance it for now.

Strategy - 6 If you do put fractions/decimals, you must multiply everything to get whole numbers. Multiply by the LCD to get rid of denominators So if it is ½, and the LCD is 2. Multiply by 2 and the fraction becomes 1.

Strategy - 7 You can remove the 1’s at the very end to make it neat. At this point, we can ASSUME that any blanks are 1 Remember, if you get comfortable, you can skip strategy 3 and not assume it is ZERO step.

Strategy - 8 We need to add the STATE into our chemical equation The state will tell us if something is solid, liquid, or gas. We show this by subscripts with parentheses. (s) for solid, (l) for liquid and (g) for gas 2H2(g) + O2(g) → 2H2O(l)

Strategy - 8 Metals are usually solid (except mercury which is a liquid) Non-metals can be solid, liquid, or gases Bromine is the only liquid Column 13-14 are usually solid Diatomic are gases except for bromine (liquid) and iodine (solid) Non-diatomics are solids Noble gases are gases

Diatomics Diatomics are molecules that exist as two atoms of the same element join together in nature There are 7 diatomics H, N, O, F, Cl, Br, I Two ways to remember 7 + H HOFBrINCl

What about aqueous? Aqueous means something is a solution and dissolved in the solvent water. NaOH, HCl, HCl are aqueous as they are dissolved in water Liquids are only for pure substances, aqueous is not a pure substance NH3 and H2O are pure substances and are liquids We use (aq) to represent this state NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

Example - 1 Balance the following H2S + PbCl2 → PbS + HCl (NH4)3PO4 + NaOH → Na3PO4 + NH3 + H2O C19H17NO3 + O2 → CO2 + H2O + N2 Cr2(SO4)3 + KI + KIO3 + H2O → Cr(OH)3 + K2SO4 + I2 MoCl3 + O2 + AgCl → MoCl4 + Ag2O

Practice - 1 Page 110 - #7-56