Writing Simple Chemical
Equations
Balancing and Classification of
Reactions
Chemical Reactions
• Balanced chemical equations represent
chemical reactions
• A substance which undergoes a chemical
change has taken part in a chemical reaction
• Remember the differences between chemical
and physical change?
Chemical Change
Physical Change
Burning
Melting
Rusting
Freezing
Signs of a Chemical Reaction
Indicators of a probable
chemical reaction
– These may be present in some
physical changes too
1. Color Changes
2. Precipitation of a solid from a
solution
3. Energy changes, heat or light
absorbed or released
4. Odor Changes; like baked bread
5. Gas release- sometimes occurs
in physical change too
Mini-Lab 6.1 Energy Change
Question: How can we observe energy changes?
Background Information:
• All chemical reactions involve an energy
change.
• Sometimes the change is so small that special
detection instruments are needed to observe
it.
• Sometimes they can be observed easily.
• Light, heat, sound, kinetic and potential are all
forms of energy.
Mini-Lab 6.1 Energy Change
Procedure:
1. Place 25g of iron powder and 1g of NaCl
in a reseal able bag
2. Add 30g of vermiculite to bag, seal bag
and shake
3. Open bag and add 5ml of water, reseal
and shake gently
4. Hold bag in your hands, note any
changes you observe
Mini-Lab 6.1 Energy Change
Analysis: (answer the following in your notebook
1. Did you observe energy changes? If so,
describe them
2. Did a chemical reaction take place? What
kind of reaction?
3. Can you think of anytime in the real world,
where this reaction might be useful? Explain
Exothermic Reaction
The Reaction has as one
of the products, energy,
in the form of heat!
Endothermic Reactions
These reactions use energy
from the surroundings, in the
form of heat, as one of the
reactants; creating a cold
product!
Chemical Equations
1. Describe all changes • Reactants: Iron and
that take place
Oxygen
• Product: Iron(III) Oxide
2. Identify the
Rust
substances that
react
a. Called Reactants
3. Identify the
substances formed
a. Called Products
The Description of Changes
• More than what can be observed
• Ex. “bubbles are formed”, does not tell what gas is
formed
• A Chemical Equation is how we represent the
changes taking place in a reaction
• Word equations are the simplest way to
describe reactants and products
• An arrow, between reactants and products,
represents change
• + signs are used to separate reactants and to
separate products
Word Equations
• Example:
The combining of Vinegar (acetic acid) and Baking
soda results in a vigorous reaction, forming a
bubbly product
The word equation:
Vinegar + Baking Soda  Sodium acetate + water +
Carbon Dioxide
Chemical Equations
• Shorter than word equations
• more exact and specific
• Replace the names of the substances with their
chemical formulas
Example: (the previous reaction)
HC2H3O2 + NaHCO3
NaC2H3O2 + H2O + CO2
(Vinegar)
Acetic acid
Sodium
Hydrogen
Carbonate
Sodium Acetate
water
Carbon dioxide
• By examining the equation you can determine EXACTLY
what elements are involved and how many atoms of
each
Writing Basic Chemical Equations
1. Replace the names of the substances with their
chemical formulas
– (remember; use oxidation numbers to form stable
octets and write compound formulas)
2. Write the chemical equation with all reactants
on one side, separated by a (+) sign
3. Draw an arrow pointing toward the products ,
also separated by a (+) sign. (recall diatomic
elements)
4. Add symbols next to each compound, indicating
physical state of matter (solid-s, liquid-l, gas-g)
Write Chemical Equations; Practice
1. Sodium metal + chlorine gas sodium
chloride crystals
Answer: Na (s) + Cl2 (g)  NaCl(s)
Reactants
Product
Write Chemical Equations; Practice
2. Propane gas + oxygen  carbon dioxide +
water + energy
Answer: C3H8(g) + O2(g)  CO2 +H2O(g) + energy
Write Chemical Equations; Practice
3. Zinc metal + hydrochloric acid  zinc
chloride solution + hydrogen
Answer: Zn(s) + HCl(aq)  ZnCl2(aq) +H2(g)
Additional Practice
•
•
•
•
Hand out #’s 26-30 - with partner (10 min)
Post answers!
Handout #’s 31-35 – with partner (8 min)
Post answers!
• Exit Ticket on your own!
• Complete #36-50 for homework!
Balancing Chemical Equations
What is Balancing Equations?
• Ensuring every atom in the reactants is
present in the products in the same
number and type!
Why?
Law of Conservation of Matter: matter is neither
created or destroyed, just transformed from one form
to another
No creation, no destruction, just
rearrangement!
Same Number and Type of Atoms
Balancing Chemical Equations
Steps of process:
1. Write the chemical equation with all reactants on
one side, separated by a (+) sign
2. Draw an arrow pointing toward the products , also
separated by a (+) sign. (recall diatomic elements)
3. Add symbols next to each compound, indicating
physical state of matter (solid-s, liquid-l, gas-g)
4. Count the numbers of each type of element from
each side of the equation.
5. List the symbols and numbers of each.
6. Change the coefficients as needed to ensure
conservation of matter. (numbers and types equal)
Example of Process
• Chemical equation for aqueous magnesium chloride
plus silver nitrate solution making aqueous magnesium
nitrate and solid silver chloride:
• Add symbols for physical state
• MgCl2(aq) + AgNO3(aq) Mg(NO3)2 (aq) + AgCl(s)
• Count number and type of elements on each side
Atoms in Reactants
Atoms in Products
Mg- 1
Mg- 1
Cl- 2
Cl- 1
Ag- 1
Ag- 1
N-1
N-2
O- 3
O- 6
Matter is not conserved! Numbers are different!
Practice: Count the number of Atoms
• Complete HO 1-10 (pairs) (20 min)
Last Step
Change the coefficients as needed to ensure
conservation of matter.
MgCl2(aq) + AgNO3(aq) Mg(NO3)2 (aq) + AgCl(s)
Atoms in reactants
Mg- 1
Cl- 2
Ag- 1 -2
N-1 - 2
O- 3 -6
Atoms in products
Mg- 1
Cl- 1 -2
Ag- 1 2
N-2
O- 6
How can I change the coefficients to make then the same?
1. A 2 added before the AgNO3(aq) in the reactants, will balance both N
and O
2. Another 2 added before AgCl(s) in the products will balance both the Ag
and Cl
MgCl2(aq) + 2 AgNO3(aq)  Mg(NO3)2 (aq) + 2 AgCl(s)
Predicting Oxidation Numbers
• The total charge on the ion is known as the Oxidation Number
of the atom
Some metals have the same oxidation number in all compounds
• Group 1 elements , oxidation number = 1+
• Group 2 elements , oxidation number = 2+
• Aluminum, oxidation number = 3+
– Groups 3-12 Many have more than one oxidation # depending on
the reaction
• Group 13 elements have 3 valence electrons, oxidation # =3+
• Group 14 may have 2+ or 4+ oxidation number
• Groups 15, 16 and 17 tend to gain electrons since they are
already ½ full
– Their oxidation numbers are 3-, 2-, and 1- respectively
Write a Balanced Equation for
Each of These Reactions
1. Sodium metal + chlorine gas sodium chloride crystals
2. Propane gas + oxygen  carbon dioxide + water + energy
3. Zinc metal + hydrochloric acid  zinc chloride solution +
hydrogen
Remember:
• use the oxidation numbers to construct octets and
determine formula compounds
• Use your reference sheets for names of ionic substances
and covalent molecules. (check for diatomic molecules)
Answers
1. balance equations for ppt #1.docx
2. balance equations for ppt #2.docx
3. balance equations for ppt #3.docx
Answers
1Na (s) + Cl2 (g)  NaCl(s)
Atoms reactants
Atoms products
•
Na – 1
Na – 1
•
Cl – 2
Cl - 1
• Add coefficient of 2 to NaCl to balance Cl
• Add coefficient of 2 to Na to balance Na
2 Na (s) + Cl2 (g)  2 NaCl(s)
Answers
Diatomic molecule
• 2. C3H8(g) + O2(g)  CO2 +H2O(g) + energy
•
Propane from reference sheet
• Atoms reactants
Atoms products
•
C–3
C–13
•
H–8
H–28
•
O – 2 10
O – 3 6 10
• Add coefficient of 4 to H2O to balance H
• Add coefficient of 3 to CO2 in product to balance C
• Add coefficient of 5 to O2 in reactants to balance O
•
C3H8(g) +5 O2(g)  3 CO2 + 4 H2O(g) + energy
• Now it is balanced! Matter was conserved!
Answers
• 3. Zn(s) + HCl(aq)  ZnCl2(aq) +H2(g)
•
Diatomic molecule
• Atoms reactants
Atoms
products
•
Zn– 1
Zn– 1
•
H–12
H–2
•
Cl – 1 2
Cl - 2
• Add coefficient of 2 to HCl to balance H and Cl
• Zn(s) + 2 HCl(aq)  ZnCl2(aq) +H2(g)
• Now it is balanced! Matter was conserved!
Let’s Use the Internet to Help us Practice!
• Using a computer, Your Brain and your Newly
Acquired Skills, practice balancing
– Complete at least 10 equations from the beginner
level, then 10 at the intermediate level.
– Then try 2 or three advanced ones!
• Track your progress and record the number of
trials it took for each equation in your notebook!
• Complete an exit ticket, indicating how many
tries it took for each equation
• Leave your exit ticket on the laptop cart when
you have put your computer away properly!
More Balancing!
• Access the Following Site:
• http://education.jlab.org/elementbalancing/index.html
1. At least 10 Beginner equations and 10 Intermediate!
Beginner
level
Number of
trials
Intermediate
level
Number of
Trials
1
1
2
2
3
3
4
4
Advanced
Level
5
5
1
6
6
2
7
7
3
8
8
9
9
10
10
Number of
Trials
Exit Ticket
1. Complete an exit ticket, indicating how many
tries it took for each equation.
Name___________________ EXIT TICKET
Total number of beginner equations you tried _____
Total number of trials on beginner equations ______
Total number of intermediate equations you tried ______
Total number of trials on intermediate equations ______
How many advanced equations did you try?
How well do you understand this process? Not at all, A little, mostly, I got it!
2. Leave your exit ticket in the box AFTER you
have put your computer away properly!
Other Sites You Can Use To Practice
• http://chemistry.csudh.edu/lechelpcs/rxnbala
ncingcsn7.html (pretty good, hard but shows
solutions after 3 tries)
• http://funbasedlearning.com/chemistry/chem
Balancer
• http://www.sciencegeek.net/Chemistry/taters
/EquationBalancing.htm (All at Once)
Types Of Reactions
• There are 5 major types of chemical
reactions
1. Synthesis
2. Decomposition
3. Single displacement
4. Double displacement
5. Combustion
• Reactions can be categorized by
recognizing patterns which occur.
Synthesis
• Pattern to recognize synthesis
• Whenever two or more substances
combine to form a single product, the
reaction is synthesis.
Decomposition
• Pattern to recognize synthesis
• Whenever a compound is broken down into
two or more simpler substances it is a
decomposition reaction.
Single-Displacement Reaction
• Pattern to recognize single displacement
• Whenever one element takes the place of
another in a compound, it is a single
displacement reaction.
Let’s Classify these reactions!
1.
2.
3.
4.
5.
6.
2 Na + Cl2  2NaCl
PCl5  PCl3 + Cl2
2Al + Fe2O3  2Fe + Al2O3
2Ag2O  4Ag + O2
Cl2 + 2KBr  2KCl +Br2
CaO + SiO2  CaSiO3
Answers
1. 2 Na + Cl2  2NaCl
2 substances combine so it is synthesis!
2. PCl5  PCl3 + Cl2
A single compound is broken down into two
simpler substances- it is decomposition!
3. 2Al + Fe2O3  2Fe + Al2O3
One element takes the place of another, it is
single displacement!
Answers Continued
4. 2Ag2O  4Ag + O2
Decomposition
5. Cl2 + 2KBr  2KCl +Br2
Single-displacement
6. CaO + SiO2  CaSiO3
Synthesis
Double-Displacement Reactions
• Pattern to recognize a double-displacement
• Whenever the positive ions of two ionic
compounds are interchanged, it is a double
displacement reaction.
• At least one product must be a precipitate of
water!
Combustion Reaction
• Pattern to recognize combustion
• Whenever a substance combines rapidly
with OXYGEN to form one or more oxides, is
a combustion reaction.
Let’s Apply what we have learned!
• Classify these reactions:
1. PbCl2 + Li2SO4  PbSO4 + 2LiCl
2. CH4 + 202  CO2 + 2H2O
3. C6H1206 + 602 6CO2 + 6H2O
4. BaCl2 + H2SO4  2HCl +BaSO4
Answers
1. PbCl2 + Li2SO4  PbSO4 + 2LiCl Interchanged
cations, so Double-displacement
2. CH4 + 202  CO2 + 2H2O substance combines
with Oxygen and forms an oxide, so Combustion
3. C6H1206 + 602 6CO2 + 6H2O Combustion
4. BaCl2 + H2SO4  2HCl +BaSO4 Doubledisplacement
Compare and Contrast Types Of Reactions
Reaction Type
Synthesis
General Equation
Element/compound + Element/compound  compound
Examples: 2 Na + Cl2  2NaCl
CaO + SiO2  CaSiO3
Decomposition Compound  two or more elements/compounds
Examples: PCl5  PCl3 + Cl2
2Ag2O  4Ag + O2
Single
Displacement
Element a + compound bc  element b + compound ac
Example: 2Al + Fe2O3  2Fe + Al2O3
Cl2 + 2KBr  2KCl +Br2
Double
Displacement
Compound ac + compound bd  compound ad + compound
bc
Example: PbCl2 + Li2SO4  PbSO4 + 2LiCl
BaCl2 + H2SO4  2HCl +BaSO4
Combustion
Element/Compound + Oxygen  Oxide
Example: CH4 + 202  CO2 + 2H2O
C6H1206 + 602 6CO2 + 6H2O
Identify these Reactions as one of the
five types studied
1.
2.
3.
4.
5.
6.
7.
8.
A + BX AX +B
AB  A+B
AXZ  AX + Z
AB + CD  AC + BD
BC + D  BD + C
BD + oxygen  B oxide + D + water
AD + XY  AY + DX
+

+
Answers
1.
2.
3.
4.
5.
6.
7.
8.
Single displacement (AKA single replacement)
Synthesis
Decomposition
Double Displacement (AKA Double replacement)
Single Displacement (AKA Single replacement)
Combustion
Double Displacement (AKA Double replacement)
Decomposition
Factors that Affect Direction Of Reactions
• External factors modify the direction of
reactions
• Many can change direction: Called Reversible
– Like charging and draining a battery
• Adding or removing energy as heat can affect
direction
– Endothermic reactions; added heat pushes reaction
to the right
– Exothermic reactions; added heat pushes the
reaction to the left
Reversibility of Reactions
• Not all reactions are reversible
– Like fuel burns, food is digested and paint hardens
– New products are formed and at least one reactant
is used up
• Some reactions automatically reverse to
establish equilibrium
– No net (overall) change
– Reactants and products change place, forming at
about the same rate
– Reactants are never used up because they are
always being used then reformed from products
– Ex. CaCo3  CaO + CO2
Initiating a Reaction
• For a reaction to occur, particles must collide
with sufficient force to cause electrons to
rearrange.
• The amount of energy needed to cause a
reaction is called Activation Energy
– Slow reactions have high activation energy
• To determine reaction rate, measure how
quickly one reactant disappears or one product
appears
Reaction Potential Energy Diagram
Shows the potential energy changes that occur as reactants
become products.
It has five distinct regions:
1.
2.
3.
4.
5.
the potential energy of the
reactants
the potential energy gain
that must take place in
order for old bonds to be
stretched to the breaking
point
the potential energy of the
transition state
the potential energy
released as new bonds form
during a chemical change
the potential energy of the
products.
Details
1. The flat region labeled "Reactants"
shows the potential energy of the
reacting particles relative to the
products.
•
The actual potential energy of the
reactants is an unknown
2. Moving particles possess kinetic
energy. When they collide, their
kinetic energy is converted to
potential energy.
3. The rising part of the graph
represents the increase in potential
energy that occurs when reactants
collide.
4. The minimum gain in potential
energy that results in the stretching
of reactant bonds to the breaking
point is called the activation energy
(Ea). It can be determined by
experiment.
Details
1.
2.
The top of the curve represents the point at
which the bonds of the colliding particles
are stretched to the breaking point.
The unstable group of atoms formed at this
point are neither reactants nor products
but something in between - a transitional
structure called the activated complex. (#3)
1. The potential energy of this structure is
very high because the bonds are
stretched as far as possible.
2. This structure exists for the shortest
amount of time imaginable.
3. In an instant, the particles either form
new bonds to give new products or
reform old bonds to give the original
reactants.
Details
1. The falling part of the curve represents
the energy released when new bonds
form between particles to make one or
more products.
2. The potential energy difference between
the reactants and the products is called
the heat of reaction (ΔH).
1.
It represents the net energy change of
the reaction.
3. If the potential energy of the products is
greater than that of the reactants, then
the reaction is classified as endothermic.
4. If the potential energy of the products is
less than that of the reactants, then the
reaction is classified as exothermic.
Details
1. The second flat region represents
the potential energy of the
products. The actual potential
energy of the products is also an
unknown.
2. The difference in energy between
reactant and products determines
endothermic vs exothermic
reactions
1. absorbs energy vs releases
energy
Important notes!
• Reactions with low activation energy are fast, while
those with high activation energy are slow.
• The higher the activation energy, the slower the rate of
the reaction.
• If the potential energy of the reactants is greater than
that of the products, the reaction is exothermic - it
results in the net release of potential energy as heat.
• If the potential energy of the reactants is less than that
of the products, the reaction is endothermic - it results
in the net gain of energy from some external source
(e.g. the sun) which is then stored in the products.
Factors that Affect Rate Of Reactions
1. Temperature; most reactions speed up
with higher temp
2. Concentration; raising the concentration
of a reactant can speed up a reaction
3. Catalysts; speeds up rate w/out being
changed by the reaction (enzymes in
cells)
4. Inhibitor; slows down a reaction
(preservatives)
Review Problems
• What are the correct coefficients of these reactions?
1.
2.
3.
4.
5.
•
KNO3 + H2CO3  K2CO3 + HNO3
SnO2 + H2  Sn + H2O
SeCl6 + O2  Se O2 + Cl2
N2 + H2  NH3
P4 + O2  P2O5
Answer the following;
1.
2.
3.
4.
5.
What is activation energy?
What happens if there is not enough activation energy?
Why do you need a match to start a fire?
What are the indicators that a reaction has taken place?
What does each of the following mean, when in a chemical
equation?
a)
b)
c)
d)
(g)
(s)
(l)
(aq)
Answers to Review Problems
1. 2, 1, 1, 2
2. 1, 2, 1, 2
3. 1, 1, 1, 3
4. 1, 3, 2
5. 1, 5, 2
Answer the following;
1. The amount of energy needed to start a reaction
2. The reaction will not start
3. It provides activation energy
4. Color change, precipitation, energy change, odor or gas
production
5. What does each of the following mean
a)
b)
c)
d)
(g) gas
(s) solid
(l) liquid
(aq) dissolved in water
Test Tomorrow!
• Study your notes and review Handout!
• Practice answering questions, and writing and
balancing equations
• Prepare by looking through your notes before
class!
• Visit the wiki for additional help!
• Organize your notebook, check your checklist!