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Chapter 7
Chemical
Equations
Chemical Reaction
one or more substances are
changed to new substances
(a chemical change occurs)
2H2(g) + O2(g) 
2H2O(l)
reactants
products
-substances
-new substance
about to react
formed

(l)=liquid, (g)=gas, (cr)=crystalline solid, (aq)=aqueous
solution
Law of Conservation of Mass
The starting mass of
reactants equals the
final mass of the
products
“Mass is neither created nor destroyed”
Chemical Equations

Represents the
substances involved in a
chemical reaction
2H2 + O2  2H2O
Chemical Equations
2H2 + O2  2H2O
–coefficient- represents the
relative amounts of substance
taking part in a reaction
–subscript- tells the # of atoms
–Arrow () - “yields” or equals
Balancing Equations

The goal is to get the # of
atoms of each element
equal on both sides of the
reaction!
#1 fix *diatomic molecules
 Check
all diatomic
molecules!!
 add a subscript of 2 to
diatomic molecules ONLY
if they are alone-single
element
 Ex:
H + O
H2O
Make your paper Safe!
NOF
Cl
Br
I
1st
H
#2 Check for balancing
 Example:
NaCl
Na 1
Cl 1

Na
+
Na 1
Cl 2
Cl2
#3 Add coefficients where needed
–Do not change subscripts …
Ever, never ever
–Coefficient is multiplied through
the formula it is in front of
–Use lowest possible coefficient
 Example:
2 NaCl
Na
Cl

12
12
2 Na
Na
Cl
+
12
2
Cl2
Helpful Hints



Treat polyatomic ions as a unit
IF it appears on both sides.
Leave elements like hydrogen,
oxygen, etc. until last.
If there is an even # on one
side and an odd # on the other,
look for the lowest common
multiple.
Practice
1.
Mg
+
HCl
MgCl2
+
H
2.
(NH4)2SO4 +
Ba(NO3)2
BaSO4 +
NH4NO3
3. H2O2

H2O
+
O2
4. AgNO3 +
KCl 
AgCl +
KNO3
You do the last two on your own!
Star Questions
 When
one or more substances are
changed to new substances we call this
a?
 Chemical reaction
 represents the relative amounts of
substance taking part in a reaction
 Coefficients
 Examples are H2, N2, O2, F2
 Diatomic molecules
Types of Chemical Reactions
Synthesis –2 or more substances
combine to form another substance
(one product). Can be single
elements or compounds!
A
+
B  AB
+
Cl  NaCl
Example:
Na
“boy dates girl = happy couple”
In the cartoon, the skinny bird (reactant)
and the worm (reactant) combine to make
one product, a fat bird.
Decomposition – one substance
breaks down into 2 or more
simpler substances
AB

A
+
B

Na
+
Cl
Example:
NaCl
“happy couple breaks up into just guy and girl again”
In this cartoon the egg (the
reactant), which contained the
turtle at one time, now has
opened and the turtle (product)
and egg shell (product) are now
two separate substances.
Single replacement – one
single element replaces
another in a compound
A
+
BC

AC
+
B
HCl

ZnCl +
H
Example:
Zn +
“home wrecker”
Notice, the guy in the orange shirt
steals the date of the other guy.
So, a part of one of the reactants
trades places and is in a different
place among the products.
Double replacement –Two
compounds exchange positive
ions to form two new products
AB
+
CD

AD
+
CB
Example:
AgNO3+ NaCl

AgCl +
NaNO3
“JBHS Prom, change of hearts, swingers,
dating Jerry Springer style”
Star Questions
In a synthesis reaction how many
products are formed?
1
What type of reaction is it when one
single element replaces another in a
compound?
Single replacement
Neutralization reaction


Reaction between an acid & a
base
Products are a salt & water
Acid + Base  Salt + Water
HCl + NaOH  NaCl + H2O
 *Double replacement
reaction
Combustion Reaction


A reaction in which a substance
reacts rapidly with oxygen, often
produces heat & light
*Products are always carbon
dioxide and water
CH4 + 2O2  CO2 + 2H2O
Energy & Reaction Rates




Chemical reactions break
chemical bonds in reactants &
form new bonds in products.
Chemical energy—energy
stored in chemical bonds
Breaking bonds absorbs energy
Forming bonds releases energy
Endothermic reactions



More energy is needed to break
chemical bonds, so it absorbs
energy from its surroundings!
It’s surroundings would feel cold,
because heat (energy) is being
taken from it.
Example: Melting ice….. Energy is
absorbed from the surroundings to
melt the ice, which makes the air
nearby feel cold.
Exothermic reactions



Forming chemical bonds releases
energy into its surroundings
The energy released by forming
products is GREATER than the
energy required to break the
reactant’s bonds
It’s surroundings would feel hot,
because heat (energy) is being
released.
Conservation of energy

the amount of total
energy before & after
the reaction is the
same.
Reaction rate


The rate at which reactants
change into products (how fast
the reaction is going).
Rate affected by:
1.Temperature
2. Pressure
3.Surface area
4.Catalysts
5.Concentration
6. Stirring, etc.
Catalysts
a substance
that
affects the
reaction
rate w/o
being
used up in
the
reaction.
Star Questions
Explain a neutralization reaction.
Acid & base combine to form salt & water,
double replacement reaction
Explain a combustion reaction.
A reaction in which a substance reacts
rapidly with oxygen, often produces heat
& light, *Products are always carbon
dioxide & water