Transcript 3 Balancing Eqtns Notes

Balancing Chemical Equations
Parts of a Chemical Equation
Molecules vs Compounds
• Molecules are two or more atoms bonded
together…but compounds must have two
different elements
– Ex- O2, NaCl, etc.
REMEMBER:
– Not all molecules are compounds, but all compounds
are molecules…explain
Chemical reactions continued
– Chemical reactions occur when
bonds between the outermost parts
of atoms are formed or broken
• What do we call those “outermost parts”???
• Valence electrons
Symbols represent elements
Formulas represent the molecules
Chemical equations represent the reaction
Law of conservation of matter
“mass is neither created nor
destroyed in any ordinary chemical
reaction. “
• Or more simply, the mass of substances
produced (products) by a chemical reaction is
always equal to the mass of the reacting
substances (reactants).
Law of conservation of matter
• Ex’s
– 1.00g carbon + 5.34g sulfur  _____g carbon
disulfide?
• 6.34g carbon disulphide
– 2.00g carbon + 10.68g sulfur  _____g carbon
disulfide?
• 12.68g carbon disulphide
Balancing Equations
• To follow the law of conservation of mass, any
equation must be BALANCED
• (Mass of reactants must equal the mass of
the products)
BALANCING EQUATIONS
– When balancing a chemical reaction
you may add coefficients in front of the
compounds to balance the reaction, but
you may not change the …
• subscripts.
• Changing the subscripts changes the
compound.
– H2O is very different that H2O2
– (Water vs Hydrogen Peroxide!)
Subscripts
tell you how
many atoms
of each
element you
have
Coefficients
tell you how
many of
each
molecule
you have
Ex:
4 Al + 3 O2  2Al2O3
This equation means :
“4 Al atoms + 3 O2 molecules
produces 2 molecules of Al2O3”
Steps to Balancing Chemical Equations
• 1. Write out the equation leaving room to add
coefficients.
• Ex-
Na3PO4 + Fe2O3  Na2O + FePO4
Step 2: Record how many atoms of
each element
2. Find the number of atoms for each element on each side.
– YOU MUST COMBINE ALL of the same element together…
– ex
Na3PO4 + Fe2O3  Na2O + FePO4
» Na- 3
» P-1
» O- 7
» Fe-2
Na-2
P-1
Fe- 1
O-5
HINT- write them in the same order on both sides so that it is easier
to compare them
STEP THREE
Determine where to place coefficients in front of
formulas so that the left side has the same
number of atoms as the right side for EACH
element in order to balance the equation.
2 Na3PO4 +___ Fe2O3  3 Na2O + 2 FePO4
» Na- 3 6
» P-1 2
» Fe- 2
» O- 7 11
Na-2 6
P-1 2
Fe- 1 2
O-5 9 11
Step 4…
4. Check your answer to see if:
– The numbers of atoms on both sides of the
equation are now balanced.
– The coefficients are in the lowest possible
whole number ratios. (reduced)
Some Helpful Hints for balancing equations:
• Take one element at a time, working left
to right except for H and O. Save H for
next to last, and O until last.
• IF everything balances except for O, and
there is no way to balance O with a
whole number, double all the
coefficients and try again. (Because O is
diatomic as an element)
Example Illustration:
Let’s try a few
1. ___ C3H8 + ___ O2  ____ CO2 + ___ H2O
Ready for another?
2. _____B4H10 +___O2 __B2O3 + ____H2O
Balancing Equations when given the
name (not the formula)
• Use “reacts with” and “produces” or “yields”
• Ex• Hydrogen reacts with Oxygen to produce
dihydrogen monoxide.
• Be able to write the chemical equation from
the word equation or vice versa!
Let’s try a few
1. ___ C3H8 + ___ O2  ____ CO2 + ___ H2O
Tricarbon octohydride reacts with
oxygen to produce carbon dioxide
and dihydrogen monoxide
Ready for another?
2. _____B4H10 +___O2 __B2O3 + ____H2O
Tetraboron decahydride reacts with
oxygen to produce diboron trioxide
and dihydrogen monoxide
Ready for another?
3. _____MgCl2 +___NaF __MgF2 + ____NaCl
Magnesium chloride reacts with
sodium fluoride to produce
magnesium fluoride and sodium
chloride