Balancing Chemical Equations

Again…. BUT YAY!

Must remember

• In a closed chemical system, the reaction follows these– Conservation of mass– Conservation of electrical charge– Conservation of energy– Conservation of atoms

• In chemistry 11, to balance chemical equations, we only follow conservation of atoms.

Coefficients

• We put coefficients in front of chemical formulas to balance them

• You have done some balancing in science 10, but in chemistry 11, you will be asked to balance some tricky chemical equations.

• You will also be asked to balance chemical equations on your final exam so you can to develop strategies to save time!

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 be another element attached to that element.

• Balance that other element on the other side.• Repeat this step until you have balanced

everything, even 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 just so the 1, that we always ASSUME gives people something to balance with.

Strategy 4

• Balance polyatomic ions as one single group.• Not always the case, but 95% of the time.• Do not break it up into individual elements• So assume PO4, SO4, NH4, NO3, OH etc etc are

all a single element and balance them accordingly.

Strategy 5

• You can use fractions when you are nearing the end and something cannot be balanced with whole numbers.

• So you can put ½ in front of something or ⅓ or ¼ if you need to.

• This is usually done for diatomics as you may have 3 O’s on one side and a single O2 on the other side. You put 3/2 infront of the O2 to balance it for now.

Strategy 6

• If you do put fractions, multiply til whole. So if you end up with a fraction of ½ in your balanced equation.

• Multiply by 2 to get rid of the ½ so it becomes 1 instead.

• You can remove the 1’s at the very end to make it neat.

• At this point, we can not ASSUME that any blanks are 1

• Remember, if you get comfortable, you can skip strategy 3 and not assume it is ZERO step.

One more step!

• So we have balanced!• But 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)

What about (aq)?

• (aq) stands for aqueous. It is when something is a solution.

• Since a liquid is a state, a solution is a dissolved solid in a liquid. So we call it aqueous.

• Aqueous is usually used for anything dissolved in water.• So anything pure is not AQUEOUS, they are liquid instead• NH3 or H2O or H2O2 are all pure and thus are LIQUID.• NaOH or HCl or NaCl are AQUEOUS as they are dissolved

in water.

NaOH(aq) + HCl(aq) → NaCl(aq) + H2 O(l)

Homework

• Pg #110 #7-56 (even or odd, you choose