Atom Inventory& Balancing Equations

Conservation

• Law of Conservation of Matter—in a chemical reaction, matter can neither be created nor destroyedneither be created nor destroyed – – just rearranged.

• The numbers & kinds of atoms The numbers & kinds of atoms present in the products are the same present in the products are the same as those in the reactants.as those in the reactants.

• Balanced equations illustrate this law.

Atoms are FOREVER!!

Coefficients—(#’s in front of compound) show how many of each compound is present. 2H2H22O =‘s O =‘s 2 waters2 waters

Atom InventoryList atoms in reactants and products under the

arrowDetermine the number of each atom.SubscriptsSubscripts—show a ratio of atomsatoms to each

other in a particular compound.

HH22O =‘s 2 hydrogens bonded to 1 oxygenO =‘s 2 hydrogens bonded to 1 oxygenSubscripts are NEVERNEVER changed when balancing!

H HO

H HO

H HO

Example of Atom Inventory:

CHCH44 + 2 O + 2 O22 CO CO22 + 2 H + 2 H22OO

Reactants Products

____ C ____ ____ H ____ ____ O ____What does it mean if there is What does it mean if there is

no coefficient in front of the no coefficient in front of the compound?compound?

It is an implied “1”!

1 1

4 4

4 4

Is this equation balanced?

Yes!!!

Steps to Balance

1.Do an atom inventoryatom inventory.

2.2.BalanceBalance by using coefficientsusing coefficients to show multiple number of elements or compounds.(NEVER change subscripts to balanceNEVER change subscripts to balance…is H2O the same

as H2O2? If so, would you like a big cold glass of

hydrogen peroxide?!) (Coefficients only inserted in front of compoundin front of compound, never

split compound or the ratio would )

3.Recount both sides.

Example:H2 (g) + O2 (g) H2O (l)

1. Atom Inventory:

Reactants Products

2 H 2

2 O 1

2 Insert coefficients to balance

3. Double check to make sure it is all BALANCED!

2 2

244

Reactants Products____ Al ____ ____ O ____

Balance

Al + O2 Al2O3

Do an atom Do an atom inventoryinventoryUse coefficient to multiply

COMPOUND with element in it by # that balances it.

1 2

2 3

Is this equation balanced? No

33

-- 6-- 6

2244

-- 6-- 6

-- 4-- 4-- -- 44

Additional Rules• Polyatomic ionsPolyatomic ions, (like NO3

- and CO32-) that

appear on both sides of equation should be balanced as units rather than balancing their atoms individually

• AgNONO33 + Cu ---> Cu(NONO33)2 + Ag

• Balance atoms in pure elements w/ no subscripts last

• Recount all atoms one last time to be sure you are correct!

2 2

Balancing Equations

• For labs, demos, and whenever possible, indicate the states of each element or compound. – (s) solid (g) gas– (l) liquid (aq) aqueous

• Diatomic Elements? • H O F Br I N ClH O F Br I N Cl• Remember these elements need a subscript of 2 if

in elemental state. If in compound then criss- cross charges to get subscripts

Practice• NaCl + H2SO4 Na2SO4 + HCl

2 NaCl + H2SO4 Na2SO4 + 2 HCl

3 BaF2 + 2 Al(OH)3 3 Ba(OH)2 + 2 AlF3

2C2H6 + 7O2 4CO2 + 6H2O(had to double everything to balance oxygen)

•BaF2 + Al(OH)3 Ba(OH)2 + AlF3

•C2H6 + O2 CO2 + H2O

Balanced Equations show Proportions

• Coefficients show how many of each compound reacts with, or creates another substance.

• Does that mean ONLY 2 hydrogen molecules will react with only 1 oxygen molecule?

• NO!! Could be 250 H2 react with 125 O2

• 2 H2 molecules produce 2 H2O molecules

• 1 O2 molecule produces 2 H2O molecules

• 2 H2 molecules react with 1 O2 molecules

Example:2 H2 (g) + O2 (g) 2 H2O (l)

• Does that mean ONLY 2 hydrogen molecules will react with only 1 oxygen molecule?

• NO!! Could be 250 H2 react with 125 O2

Example:2 H2 (g) + O2 (g) 2 H2O (l)

Types of Chemical ReactionsTypes of Chemical Reactions• SynthesisSynthesis

• DecompositionDecomposition

• Single replacement (displacement)Single replacement (displacement)

• Double replacement (displacement)Double replacement (displacement)

• CombustionCombustion

A + B AB

AB A + B

AA + BBC AAC + BB

AAB + CCD AAD + CCB

CxHy + O2 CO2 + H2O

NN22 + 3H + 3H22 2NH2NH33 HH22OO22 H H22 + O + O22

Zn + 2HCl Zn + 2HCl ZnCl ZnCl22 + + HH22

NaCl + AgNONaCl + AgNO33 NaNO NaNO33 + + AgClAgCl

CHCH44 + 2O + 2O22 CO CO22 + 2H + 2H22OO

ExampleExampless

AA + BDD BAA + DD