Download Chemical Reactions

BALANCING CHEMICAL
REACTIONS
Reactants: Zn + I2
Product: Zn I2
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Introduction
– Chemical reactions occur when bonds
between atoms are formed or broken
– Chemical reactions involve changes in
matter, the making of new materials with
new properties, and energy changes.
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Chemical Equations
Depict the kind of reactants and
products and their relative amounts
in a reaction.
4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)
The numbers in the front are called
stoichiometric coefficients
The letters (s), (g), (l), and (aq) are
the physical states of compounds.
Symbols Used in Chemical Equations
+
Used to separate two reactants or two products
“Yields,” separates reactants from products
Used in place of  for reversible reactions
(s)
Indicates a solid state; placed after the formula
(l)
Indicates a liquid state: placed after the formula
(g)
Indicates a gaseous state; placed after the formula
(aq)
Indicates an aqueous solution; the substance is dissolved in
water; placed after the formula
Δ
Heat
Pt
Indicates that heat is supplied to the reaction
A formula written above or below the yield sign indicates its
use as a catalyst (in this example, platinum).
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Conservation of Mass
• Mass cannot be created or
destroyed
• The amount of mass you start out
with is the amount you end with
• Equation:
MASSREACTANTS = MASSPRODUCTS
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Examples:
H2 + O2 H2O
2.00g
+
2.00g
=
4.00g
7.00g
+
6.00g
=
13.0g
250g
+
450g
=
700g
Chemical Equations
Because of the principle of the
conservation of matter,
an equation
must be
balanced.
It must have the same
number of atoms of the
same kind on both sides.
Lavoisier, 1788
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Balancing Equations
– When balancing a chemical rxn, you may
add coefficients in front of the cpds to
balance the reaction, but
you may not change
the subscripts.
• Changing the subscripts changes the
compound.
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Subscripts vs. Coefficients
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• The subscripts
tell you how
many atoms of
a particular
element are in a
compound. The
coefficient tells
you about the
quantity, or
number, of
molecules of
the compound.
Coefficients
What is a coefficient?
XY2
2XY
XY2
A Number that goes in front of a variable
What is the distributive property?
3(X+Y) = 3X+3Y
3(X2Y)
2(XY3)2
X= 6
Y= 3
4(H2O)2
X= 4
Y = 12
H = 16
O= 8
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Steps to Balancing Equations
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There are four basic steps to balancing a chemical
equation.
1. Write the correct formula for the reactants and the products. DO
NOT TRY TO BALANCE IT YET! DO NOT CHANGE THE
FORMULAS!
2. Find the number of atoms for each element on the left side.
Compare those against the number of the atoms of the same
element on the right side.
3. 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.
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 Suggestions to Help You
– You can use a RAP chart to keep atoms
straight
– Take one element at a time. Save H 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)
– (Shortcut) Polyatomic ions that appear on
both sides of the equation should be balanced
as independent units
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Balancing Equations
2 H2 +
O2
2 H2O
Reactant
Atoms
Atoms’
Symbol
2 4
H
2 4
O
1 2
2
Product
Atoms
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Balancing Equations
2 Al(s) + 3 Br2(l)
Reactant
Atoms
Atoms’
Symbol
Al2Br6(s)
Product
Atoms
1 2
Al
2
2 6
Br
6
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Ca + 2 H2O →
Ca(OH)2 + H2
BALANCED!
Reactant
Atoms
Element
Symbol
1,2,1,1
Product
Atoms
1
Ca
1
2 4
H
4
1 2
O
2
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2 Al(OH)3 + 3 H2SO4 →
Reactant
Atoms
Al2(SO4)3 + 6 H2O
Element
Product
BALANCED!
Symbol
Atoms
12
Al
2
3 6
O
1 6
5 8 12
H
2 12
1 3
SO4
3
2,3,1,6
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Balancing
Equations
5 O2(g)-->___CO
3
4 2O(g)
__C3H8(g) + __
2(g) + __H
2 4H10(g) +__O
5 2O(g)
2
__B
11 2(g) --->__B
4 2O3(g) + 10
__H
Balancing Equations
Sodium phosphate + iron (III) oxide 
sodium oxide + iron (III) phosphate
2 Na3PO4 + Fe2O3 ---->3Na2O +2 FePO4
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