Chemical Equations
and
Reactions
Including counting atoms
Day 1: Chemical reactions
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List evidence that suggests that a chemical
reaction has occurred
Counting atoms = conservation of mass
Describe a chemical reaction by using a word
equation and a formula equation
Interpret notations in formula equations, such
as those relating to states of matter or
reaction conditions
Demo: Check This Out!
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10g of baking soda
10 mL of vinegar
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Total mass=_______
Now I will combine them and we will see what
happens
Final Total mass=________
Demo:
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
What just happened?
Is the Law of Conservation of Mass wrong?
What was some evidence that a chemical
reaction occurred?
Chemical Change


Chemical reaction is the process by
which one or more substances change to
produce one or more different substances
Atoms do not become other types of
atoms nor do they appear or disappear
 The bonding patterns among the atoms
are merely rearranged
Reactants vs. Products
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Reactants are what you start with or the
“original substances”
Products are what you end up with or the
“new substances”
Example:

Camp fire- reactants: wood and fire/match…
products: carbon dioxide and water
Brainstorm

Take 5: Work with a partner to come up with as
many examples of CHEMICAL reactions and
chemical change as you can.
Evidence of a Chemical
Change

Change in energy
 Release of heat
 Release of light
 Production of
sound
 Change in
temperature
 Change in electrical
energy

Formation of a new
substance
 Formation of a gas
 Formation of a
precipitate (solid)
 Change in color
 Change in odor
Chemical Reaction vs.
Physical Change

Only way to know for sure that an actual
chemical reaction has taken place is to do a
chemical analysis on the products

These test would show that the new substances’
physical properties such as density, melting point
or boiling point are different then the reactants’
Examples

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Even though you have a color change when
you mix paints, the final chemical
composition is still the same as when you
started (physical change)
Remember back to first day of school when
you saw…Copper reacting with nitric acid.
There was a color change, but it is a
chemical change because you end up with
two totally different products
Reactions and Energy
Changes
Reactions can gain or lose energy
 Exothermic: when Energy is released
during a reaction it is a product.

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methane + oxygen -> carbon dioxide + water
+ energy
Endothermic: when Energy is absorbed
during a reaction it is a reactant.

Dinitrogen tetroxide + energy -> nitrogen
dioxide
More on Energy Changes

Exothermic reactions yield stronger bonds
 This is because the bonds holding the
products together are stronger than
those in the reactants
 The stronger the bond the more energy
released when bond forms and more
energy to break more bonds in the
reactants
Chemical Equations
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A representation of a chemical reaction that
uses symbols to show the relationship
between the reactants and the products
Must obey the law of conservation of mass
Number of atoms on each side of the arrow
must be the same
Think of yourself as an “atomic accountant” who
has to show every penny (atom) that comes in
and where it has gone to.
Example: Haber Process
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The Haber process, is a great example of
conservation of mass with chemical reactions.
Despite the fact that 78.1% of the air we breathe
is nitrogen, the gas is nutritionally unavailable.
It was not until the early 20th century that Fritz
Haber developed the first practical process to
convert atmospheric nitrogen to ammonia, which
is nutritionally available.

The Haber process is the industrial
implementation of the reaction of nitrogen
gas and hydrogen gas. It is the main
industrial route to ammonia:
N2 + 3 H2 → 2 NH3
Haber Process

Prior to the discovery of the Haber process,
ammonia had been difficult to produce on an
industrial scale

Fertilizer generated from ammonia produced
by the Haber process is estimated to be
responsible for sustaining one-third of the
Earth's population
https://ed.ted.com/lessons/the-chemicalreaction-that-feeds-the-world-daniel-d-dulek
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COUNTING
ATOMS
Why are we doing this
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We need to know how many of each type of
atom we have in order to
-> Balance Chemical Equations!
SUBSCRIPTS
1.
SUBSCRIPTS only refer to the atom
that they are BEHIND. For example…
H2S
There are TWO atoms of HYDROGEN
and only ONE atom of SULFUR.
COEFFICIENTS
2.
COEFFICIENTS apply to the entire
compound. You MULTIPLY the
coefficients and SUBSCRIPTS.
2 H2S
ATOMS OF HYDROGEN: 2*2 = 4
ATOMS OF SULFUR:
2*1 = 2
IF THERE ISN’T A SUBSCRIPT BEHIND AN
ELEMENT, ASSUME THERE IS ONLY ONE ATOM OF
THAT ELEMENT!
PARENTHESES with
Polyatomics
3. If elements or compounds are inside of
PARENTHESES, then the SUBSCRIPT
behind the parentheses applies to
everything inside.
Ba(OH)2
ATOMS OF BARIUM: 1
ATOMS OF OXYGEN:
1*2 = 2
ATOMS OF HYDROGEN: 1*2 = 2
PRACTICE
MgCl2
1
Atoms of Magnesium:
Atoms of Chlorine:
2
Al2S3
Atoms of Aluminum: 2
3
Atoms of Sulfur:
PRACTICE
H2SO4
Atoms of Hydrogen:
Atoms of Sulfur:
Atoms of Oxygen:
2
1
4
CH3OH
Atoms of Carbon:
1
Atoms of Hydrogen: 3+1 = 4
1
Atoms of Oxygen:
MORE PRACTICE
Ca3(PO4)2
Atoms of Calcium:
Atoms of Phosphorus:
Atoms of Oxygen:
3
1*2 = 2
4*2 = 8
Al2(SO4)3
Atoms of Aluminum: 2
1*3 = 3
Atoms of Sulfur:
4*3 = 12
Atoms of Oxygen:
YOU TRY
NaHCO3
Mg(OH)2
3H3PO4
C6H12O6
2H2O
MgO
ANSWERS
NaHCO3
Atoms of Sodium: 1
Atoms of Hydrogen: 1
Atoms of Carbon: 1
Atoms of Oxygen: 3
C6H12O6
Atoms of Carbon: 6
Atoms of Hydrogen: 12
Atoms of Oxygen: 6
Mg(OH)2
2H2O
Atoms of Magnesium: 1
Atoms of Oxygen: 1*2 = 2
Atoms of Hydrogen: 1*2 = 2
Atoms of Hydrogen: 2*2 = 4
3H3PO4
MgO
Atoms of Hydrogen: 3*3 = 9
Atoms of Phosphorus: 3*1 = 3
Atoms of Oxygen: 3*4 = 12
Atoms of Magnesium: 1
Atoms of Oxygen: 1
Atoms of Oxygen: 2*1 = 2
PUT IT ALL TOGETHER
2Ca3(PO4)2
Atoms of Calcium: ?
Atoms of Phosphorus:
Atoms of Oxygen: ?
?
ANSWER
2Ca3(PO4)2
Atoms of Calcium: 2*3 = 6
Atoms of Phosphorus: 2*2 = 4
Atoms of Oxygen: 2*4*2 = 16
Check your ANSWERs
1) NaOH
4) 4Li2O
Atoms of Sodium: 1
Atoms of Oxygen: 1
Atoms of Hydrogen: 1
Atoms of Lithium: 4*2 = 8
Atoms of Oxygen: 4*1 = 4
2) 4HNO3
5) 2NaOH
Atoms of Hydrogen: 1*4 = 4
Atoms of nitrogen: 1*4 = 4
Atoms of Oxygen: 3*4 = 12
Atoms of sodium: 2
Atoms of Oxygen: 2
Atoms of Hydrogen: 2
3) MgCl2
Atoms of Magnesium: 1
Atoms of Chlorine: 2
6) Li2SO4
Atoms of lithium: 2
Atoms of sulfur: 1
Atoms of Oxygen: 4
Day 2: Balancing Equations
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Now that we know how to count the number
of atoms
We can move onto writing and balancing
equations!
http://ed.ted.com/lessons/if-molecules-werepeople-george-zaidan-and-charles-morton
Writing and Balancing a
Formula Equation
Follow these steps
1) Write the word equation
2) Replace the words with molecular formulas.
Be sure to leave space in front of each
chemical formula.
3) Count the number of atoms of each element
on the left side and compare to the left side.
Make a chart if it helps you.
Writing and Balancing a
Formula Equation
4) Insert coefficients in front of appropriate formulas so
that the left side has the same number of atoms as
the right side for EACH element.

Be careful…sometimes it’s not as easy as it appears.
5) Check your answer to ensure a balanced equation.
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The numbers of atoms on both sides of the equation must
be equal.
The coefficients for each reactant & product must be in the
lowest possible whole number ratio. If they are not, divide
the coefficients by their greatest common factor to obtain
the smallest possible whole-number coefficients.
Check for Understanding

Work on writing the chemical formulas and
balancing the equations on the included
worksheet
Check your answers
(6) H2 + Cl2  2HCl
(7) 2H2 + O2  2H2O
(8) 3H2 + N2  2NH3
(9) CH4 + O2  CO2 + 2H2O
(10) 3NO2 + H2O  2HNO3 +NO
Let’s try a few harder ones
(a) Balance the equation for the reaction of iron (III) oxide with
hydrogen to form iron and water.
1) Write the words
iron (III)oxide + hydrogen (diatomic)  iron + water
2) Replace the names with molecular formulas
Fe2O3 + H2 -> Fe + H2O
3) Count the number of atoms
=> Make a chart
Fe- 2
Fe-1
O-3
O-1
H-2
H-2
4) Insert coefficients to balance
___Fe2O3 + ___H2 -> ___Fe + ___H2O
5) Check final answer
Fix chart to match the coefficients
you used in step 3 and make sure
everything is even
Fe- 2
Fe-1
O-3
O-1
H-2
H-2
(b) The reaction of ammonia with oxygen
produces nitrogen monoxide and water vapor.
1) Word:
ammonia + oxygen (diatomic)  nitrogen monoxide + water
2) Replace
___NH3 + ___O2 -> ___NO + ___H2O
3) Count
N- 1
N-1
H-3
H-2
O-2
O-2
3) Balance
4NH3 + 5O2 -> 4NO + 6H2O
4) Check
Now all of the steps!
(c) Methane reacts with oxygen to form carbon
dioxide and water
Word: methane + oxygen -> carbon dioxide +
water
Formula:
CH4 + O2 -> CO2 + H2O
Balanced: CH4 + 2O2 -> CO2 + 2H2O
Try this one…
(d) Mercury and oxygen react to form mercury
(II) oxide.
Word: mercury + oxygen -> mercury (II) oxide
Formula:
Hg +
O2 
HgO
Balanced: 2Hg + O2  2HgO
Try this one…
(e) Iron reacts in air to form rust otherwise
known as iron (III) oxide
Word: iron + oxygen  iron(III) oxide
Formula:
Fe + O2  Fe2O3
Balanced: 4Fe + 3O2  2Fe2O3
Try this one…
(f) Potassium chlorate decomposes into
potassium chloride and oxygen.
Word: potassium chlorate  potassium
chloride + oxygen
Formula:
KClO3  KCl + O2
Balanced: 2KClO3  2KCl + 3O2
Try this one…
(g) Copper metal placed in a silver nitrate
solution will react to form copper (II) nitrate
and silver metal.
Word: copper + silver nitrate  copper(II)
nitrate + silver
Formula: Cu + AgNO3  Cu(NO3)2 + Ag
Balanced: Cu + 2AgNO3  Cu(NO3)2 + 2Ag
So…
What Do All the Number Mean?
Numbers with Formulas
Coefficients
Relative amounts of
reactants/ products
in a balanced equation
Subscripts
Number of each type
of atom in a formula
Example: 3NaOH
Superscripts
Amount and type of
charge on an ion
Example: H2O
Example: SO4-2
Example: K+
Important Information
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Chemical equations tell more than just what
is used and made in a reaction
Shows how much of everything is in a
reaction
Tells temperature and/or pressure needed for
the reaction
Check This Out!
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NaHCO3(s) + HC2H3O2(aq) -> NaC2H3O2(aq)
+ CO2(g) + H2O(l)

N2(g) + 3H2(g) 350˚C, 25,000kPa 2NH3(g)
catalyst
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What do all of these letters and symbols
mean?
State Symbols and
Reaction Conditions
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The physical states of the reactants and products are
provided in the equation.
(g)
(s)
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gas
solid
(l)
(aq)
liquid
aqueous
(aq) = aqueous, which means “mixed with water” (for a
substance dissolved in water)
If special conditions are necessary for a reaction to take
place, they are often specified above the arrow.
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Some examples of special conditions are electric current, high
temperature, high pressure, or light.
Never Ever Ever!!!!!
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DO NOT CHANGE SUBSCRIPTS
If you change these your are changing the
compounds in the reactions!!!
We can only change the coefficients to make
the number of atoms balance

When we change these we are just changing the
number of moles of the substance needed in the
reaction
A Few Balancing Tips
1) Delay the balancing of elements (often H and O)
that occur in several reactants or products
2) If the same polyatomic ions appear on both
sides of the equation, treat them as single units,
like monatomic ions
3) You can rewrite water as H(OH) if the other side
has the hydroxide polyatomic ion.
4) Remember, balancing one element may
unbalance others (So ALWAYS double check at
the end!!!)
Polyatomic Ions and Balancing

Sometimes a polyatomic ion will stay intact
throughout the reaction so when we go to
balance the equation we can think of it as its
own “atom”
(a)
___Al + ___H2(SO4) -> ___Al2(SO4)3 + ___H2
2Al + 3H2(SO4) -> Al2(SO4)3 + 3H2
Aluminum reacts with arsenic acid(HAsO3), to
form H2 and aluminum arsenate. Write a
balanced equation for this reaction.
(b)
___Al + ___H(AsO3) -> ___H2 + ___Al (AsO3)3

2Al + 6HAsO3 -> 3H2 + 2Al (AsO3)3
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Magnesium nitrate reacts with copper to form
copper nitrate and magnesium solid.
(c)
___Mg(NO3)2 + ___Cu  ___ CuNO3 + ___ Mg
Mg(NO3)2 + 2Cu  2CuNO3 + Mg
How you feeling?
Let’s practice some more:
 Complete ChemQuest 29 and hand in
 Complete worksheets in note taking guide:
Word Equations and Chem worksheet 10-2
 Balancing Race!
Chem Quest 29 (11)
a)
b)
c)
d)
e)
f)
 magnesium chloride + hydrogen gas
 sodium hydroxide + beryllium sulfide
 barium nitrate + sodium fluoride
 calcium sulfate + potassium phosphate
 zinc + magnesium nitrate
 calcium carbonate + aluminum nitrate