Rules for Balancing Equations

Rules for Balancing Equations

Mr. CittaName______

Honors ChemistryDate______

Rules for Balancing Equations. (not all of the steps will always apply)

  1. If given a word equation, make sure that you predict reasonable products based on your knowledge of chemistry. (ex. oxygen + hydrogen  water)
  1. Write out the proper chemical formula for every chemical compound present. (all subscripts must be correct for every chemical, including diatomic molecules), ex. ( O2 + H2  H20)
  1. There is an order to follow when balancing equations.
  1. If any lone element is present alone and not in combination with anything balance it last. (ex. Ca, Cu, Fe)
  2. If any diatomic molecule is present, balance it 2nd to last, (if O2 is present, always balance it after any other diatomic.) (ex. H2, N2, O2)
  3. If water is present, balance it 3rd to last, (of course followed by H2 then O2, diatomics)
  4. Always try to start with the elements or ions that are in the most complicated formula (ratio) ex. Start with a compound with a 2:3 ratio instead of 1:3 ratio. (do Al2(CO3)3 – aluminum and carbonate, before you would do AlCl3 (1:3 ratio – aluminum chloride) )

**following this order will help you balance greatly and is always recommended.

  1. If any polyatomic ion is present and does not decompose during the reaction, treat it as a whole, don’t try to break down the components. (ex. treat CO3-2 as if it is just a whole item, CO3-2 = X)
  1. The goal is then to change coefficients properly so the law of conservation of mass is followed, that you have an equal count of every different atom on both sides of the equation. (Once you have corrected subscripts in step #2, you cannot and must not change them as long as you are sure they are correct.)

Ex. hydrogen+oxygen ??

Step #1 hydrogen+ oxygen water

(based on observations in lab, remember water in test tubes after blowing up hydrogen)

Step #2 ____H2+_____O2_____H2O

(oxygen will be balanced last, hydrogen 2nd to last and water 3rd to last, which means start with H2O first) *now only coefficients may be changed*

Step #3+5 __2__H2+_____O2___2__H2O (step 4 didn’t apply)

Note, before balancing ReactantsProducts

2 hydrogens2 hydrogens

2 oxygens1 oxygen

(law of conservation of mass is being violated, reactants and products must be equivalent.)

Note, after balancing, ReactantsProducts

4 hydrogens4 hydrogens

2 oxygens2 oxygens

law of conservation of mass is now satisfied and equation is balanced….

If you are given an equation where you have all the proper products and proper formulas(subscripts), you may skip all steps and start off by correcting coefficients only The other steps where done for you. (as in your homework tonight) (just step #4 and # 5 needed here)

Ex. ____Cu(NO3)2(aq)+____NaOH(aq) ______Cu(OH)2(s)+ ______NaNO3(aq)

before balancing

ReactantsProducts

1 Cu+21 Cu+2

2 NO3-11 NO3-1

1 Na+11 Na+1

1 OH-12 OH-1

** all polyatomics will be treated as complete units (step #4), not broken down to atomic components, (treat them like whole ions, they have not decomposed)

** you can start and finish with any ion as none of the requirements where met to do anything in a particular order, (no pure element, diatomics or water present, so no order to go by)

____Cu(NO3)(aq)+__2__NaOH(aq) ______Cu(OH)2(s)+ ____2__NaNO3(aq)

after balancing

ReactantsProducts

1 Cu+21 Cu+2

2 NO3-12 NO3-1

2 Na+12 Na+1

2 OH-12 OH-1

always check that reactants = products nothing is lost, remember you do not and should not always break everything down to individual atoms, if polyatomics are present and they don’t decompose(break down to components).

** be aware that if they do either decompose or form then you will have to account for elements contained in them individually….

Ex. H2CO3 H2O+CO2

In this example you’d have to count individual carbons, hydrogens and oxygens as the carbonate ion (CO3-2) has decomposed and is not longer present in products.

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