Chemical Equations
& Reactions
1
Learning Objectives
• Recognize observations indicating a chemical reaction
(heat, light, color change, precipitate) has occurred.
• Write and balance chemical equations with symbols,
formulas, identities and relative amounts of reactions
and products.
• Recognize types of chemical equations (synthesis,
decomposition, single and double replacement, etc.).
• Predict chemical reaction outcome using an activity
series list.
2
A Chemical Equation
• Represents with symbols and formulas, the
identities and relative molecular or molar
amounts of the reactants and products in a
chemical reaction.
3
Word Equation / Formula Equation
Methane + oxygen --> carbon dioxide + water
CH4(g)
+ 02(g)
Reactants
-->
CO2(g)
+ H20(g)
Products
The above formula equation is not balanced.
4
Chemical Reaction Indications
1. Production of energy as heat and/or light.
2. Production of a gas.
3. Formation of a precipitate – a solid produced
as a result of a chemical reaction in solution.
A precipitate separates from the solution.
4. A color change.
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Chemical Equation Requirements
1. The equation must represent known facts.
2. The formulas for the reactants and products
must be written correctly.
3. The law of conservation of mass must be
satisfied. The number of atoms of each
element must be the same on each side of
the yield sign. After a formula is written
correctly, coefficients can be placed in front
of a formula to show conservation of mass.
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Diatomic Molecules
Element
Symbol
Molecular Formula
Physical State at
Room Temp.
Hydrogen
H
H2
Gas
Nitrogen
N
N2
Gas
Oxygen
O
O2
Gas
Fluorine
F
F2
Gas
Chlorine
Cl
Cl2
Gas
Bromine
Br
Br2
Liquid
Iodine
I
I2
Solid
When writing a chemical equation including any of the above elements, they are
shown as diatomic molecules as in column 3 above.
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Coefficients
• When placed in front of a correctly written
chemical formula, a coefficient multiplies the
number of atoms of each element indicated in
the formula
• 2O2 means 4 O
• 2H20 means 4 H and 2 O
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9
Practice
Write word and balanced chemical equations
for: solid calcium reacts with solid sulfur to
produce solid calcium sulfide. Include symbols
for physical states.
Calcium(s) + sulfur(s) --> calcium sulfide(s)
Ca(s) + S(s) --> CaS(s)
Balanced: 1 Ca on each side
1 S on each side
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Write word and balanced chemical equation for:
Hydrogen gas reacts with fluorine gas to produce
hydrogen fluoride gas.
Hydrogen gas + fluoride gas --> hydrogen fluoride gas
H2(g) + F2(g) --> HF(g)
H2(g) + F2(g) --> 2HF(g)
Solid aluminum metal reacts with aqueous zinc chloride
to produce solid zinc metal and aqueous aluminum
chloride.
Aluminum + Zinc Chloride --> Zinc + Aluminum Chloride
Al(s) + ZnCl2(aq) --> Zn(s) + AlCl3(aq)
Al(s) + 3 ZnCl2(aq) --> Zn(s) + 2 AlCl3(aq)
2 Al(s) + 3 ZnCl2(aq) -->3 Zn(s) + 2 AlCl3(aq)
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Translate these chemical equations into sentences:
CS2(l) + 3O2(g) --> CO2(g) + 2SO2(g)
Liquid carbon disulfide reacts with oxygen gas to
produce carbon dioxide gas and sulfur dioxide gas.
NaCl(aq) + AgNO3(aq) --> NaNO3(aq) + AgCl(s)
Aqueous solutions of sodium chloride and silver
nitrate react to produce aqueous sodium nitrate
and a precipitate of silver chloride.
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Write & Balance:
Hydrazine, N2H4, reacts violently with oxygen to
produce gaseous nitrogen and water.
N2H4(l) + O2(g) --> N2(g) + H2O(l)
Is it balanced?
No. There are 4H on reactant side and 2H on
product side (H: 4/2) and O is 2/1.
N2H4(l) + O2(g) --> N2(g) + 2H2O(l)
N: 2/2
H:4/4
O:2/2
Now, it is balanced.
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Chemical Equation Indications
1. Coefficients indicate relative amounts of
reactants and products (proportions,
molecules, moles, grams, ratios).
H2(g) + Cl2(g) --> 2HCl(g)
1 molecule H2 : 1 molecule Cl2 : 2 molecules HCl
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2. Coefficients can be used to determine
relative masses of reactants and products
H2(g) + Cl2(g) --> 2HCl(g)
1mol H2 x 2.02g H2 = 2.02 g H2
mol
1 mol Cl2 x 70.90 g Cl2 = 70.90 g Cl2
mol
2 mol HCl x 36.46 g HCl = 72.92 g HCl
mol
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• 3. The reverse reaction for a chemical
equation has the same relative amounts of
substances as the forward reactions.
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Balancing Chemical Equations
1. Write equation.
2. Write chemical formulae for each compound.
3. Balance according to the Law of Conservation
of Mass by adjusting coefficients.
4. Count atoms to be sure the equation is
balanced.
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Practice - Write word, formula, and balanced
chemical equations for this reaction.
1. Magnesium and hydrochloric acid react to
produce magnesium chloride and hydrogen.
Word Equation: Magnesium + hydrochloric acid
--> magnesium chloride + hydrogen
Formula Equation: Mg(s) + HCl(aq) --> MgCl2(s) +
H2(g)
Adjust coeffs: Mg(s) + 2HCl(aq) --> MgCl2(s) +
H2(g)
Count atoms: Mg: 1/1 H:2/2 Cl:2/2
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Solid sodium combines with chlorine gas to produce
solid sodium chloride.
Sodium(s) + chlorine(g) --> sodium chloride
Na(s) + Cl2(g) --> NaCl(s)
Balance: Na(s) + Cl2(g) --> 2NaCl(s)
2Na(s) + Cl2(g) --> 2NaCl(s)
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Types of Chemical Reactions
1.
2.
3.
4.
5.
Synthesis
A + X --> AX
Decomposition
AX --> A + X
Single-displacement
A + BX --> AX + B
Double-displacement AX + BY --> AY + BX
Combustion – a substance combines with
oxygen releasing energy as light and heat.
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Activity Series – elements organized
according to how they react
Most active metals:
Li react w/cold
Rb H20 & acids
K replacing H2.
Ba React w/O2
Sr forming oxides
Ca
Na
Mg react w/steam
Al (not cold H20)
Mn and acids, replaZn cing H2. React
Cr with O2 forming
Fe oxides
Cd
Co
Ni
Sn
Pb
H2
Sb
Bi
Cu
Hg
Ag
Pt
Au
Do not react w/H20.
React w/acids, replacing H2. React w/O2
forming oxides.
React w/O2, forming
oxides
An element can replace any
element placed below it
BUT
It cannot replace any
element above it.
Zn can replace Cu but Au
cannot replace Mg
Fairly unreactive,
forming oxides only
indirectly.
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Nonmetal activity series:
Most active
F
Cl
Br
I
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Sample Problems – activity series
50oC
Zn(s) + H2O(l) --->
No reaction, water must be 100oC (steam) at least.
Sn(s) + O2(g) -->
Yes, any metal more active than Ag will react w/O2
to form an oxide. (Sn is above Ag)
Cd(s) + Pb(NO3)2(aq) -->
Yes, Cd is above Pb. Products: Cd(NO3)2 + Pb
Cu(s) + HCl(aq) -->
No, Cu is below H
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1. Synthesis Reaction
A + X --> AX
Samples:
Fe(s) + S(s) --> FeS(s)
2Mg(s) + O2(g) --> 2MgO(s)
H20 + SO3 --> H2SO4
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Synthesis with Oxides (see handout for
more information!!)
CaO(s) + H2O(l) --> Ca(OH)2(s)
Pollution:
SO2(g) + H2O(l) --> H2SO3(aq)
2H2SO3(aq) + H20(l) --> 2H2SO4(aq)
Oxides:
CaO(s) +SO2(g) --> CaSO3(s)
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2. Decomposition Reaction
AX --> A + X
electricity
H20(l) --------> 2H2(g) + O2(g) (electrolysis)
D
2HgO(s) ---> 2Hg(l) + O2(g)
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Decomposition of Metal Oxide
D
CaCO3 --->CaO + CO2
Decomp of Metal Hydroxide
D
Ca(OH)2 --->
CaO + H2O
Decomp of Metal Chlorate
D
2KClO3 ---->
MnO 2KCl + 3O2
Decomp of Acids
H2CO3 --> CO2 + H2O (occurs at room temp)
D
H2SO4 ---> SO3 + H2O
2
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3. Single Displacement Reaction
A + BX --> AX + B
Or Y + BX --> BY + X
1. Fe + CuSO4 --> FeSO4 + Cu
2. Cu + 2AgN03 --> Cu(NO3)2 + 2Ag
3. CI2 + 2KI --> 2KCl + I2
How is 3. different from 1. or 2.?
In 1.& 2. metals are being displaced.
In 3. a halogen is being displaced.
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Hydrogen displaced by a metal:
Mg + 2HCl --> H2 + MgCl2
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Double-Displacement Reaction
AX + BY --> AY + BX
A,X,B, and Y in reactants are ions.
AY and BX are ionic or molecular compounds.
1. Formation of a Precipitate
2KI(aq) + Pb(NO3)2(aq) --> PbI2(s) + 2KNO3(aq)
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Formation of a Gas
FeS(s) + 2HCl(aq) –> FeCl2(aq) + H2S(g)
Formation of Water
HCl(aq) + NaOH(aq) --> NaCl(aq) + H20(l)
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Combustion Reactions
2H2(g) + O2(g) --> 2H2O(g)
C3H8(g) + 5O2(g) --> 3CO2(g) + 4H20(g)
Other products are heat and light.
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Predicting Activity
(use the activity series handout)
Zn(s) + H20(l) --> ?
No, not hot enough. Steam needed (100oC)
Cr(s) + H2O(l) --> ?
Yes, Cr is above H on the chart.
Pt(s) + O2(g) --> ?
No.
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Cd(s) + 2HBr(aq) -->
Yes, Cd is above H
Mg(s) + steam -->
Yes
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Activity Series
Activity of metals:
Li
Rb
K
Ba
Sr
Ca
Na
Mg
Al
Mn
Zn
Cr
Fe
Cd
Co
Ni
Sn
Pb
H2
Sb
Bi
Cu
Hg
Activity of halogen nonmetals
<--Most active metal
react w/cold
H20 & acids
replacing H2.
React w/O2
forming oxides
F2
Cl2
Br2
I2
<-- Most active nonmetal
react w/steam
(not cold H20)
and acids, replacing H2. React
with O2 forming
oxides
Do not react w/H20.
React w/acids, replacing H2. React w/O2
forming oxides.
React w/O2, forming
oxides
Ag Fairly unreactive,
Pt forming oxides only
Au indirectly.
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Solubility Chart
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