Download Chemical Equations and Reactions

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Chemical Equations
and Reactions
What are they?
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• chemical reaction- one or more substances
are changed into new substances.
2Na + Cl2
reactants
2NaCl
products
Reactant – substances that react
Product – new substances produced
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Evidence of Reactions
• Bubbling (gives off a gas)
• Precipitate-solid that falls
out of solution
• Temperature change
• Color change
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Chemical Equation
• Expression that describes a chemical
reaction using chemical formulas and
other symbols
Symbol
Meaning
(cr)
Crystalline solid
(l)
Liquid
(g)
Gas
(aq)
Aqueous-solid
dissolved in water
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Symbol for Chemical
Equations
Pb(NO3)4 + 4KI  PbI4 + 4KNO3
“Pb(NO3)4” is a formula.
 Means produces (or yields)
+ Means added together
Subscripts - Number of a particular atom in a molecule
Coefficients – Number of molecules
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Father of Modern Chemistry
1743 - 1794
First Described the “Law of Conservation of Mass”
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Conservation of Mass
Antoine Lavoisier found that the mass of the reactants
and the products are equal, even when the states of
matter change.
HgO
Hg + O2
He started with:
He ended up with:
10g of Mercury
Oxide (HgO)
and 9.3g Mercury…
10 g. = 0.7 + 9.3 g.
...But what
happened to the O2?
Matter is neither created nor destroyed.
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• Conservation of atoms-the number of
each type of atom on the reactants side
of the chemical equation MUST be equal
to the number of each type of atom on
the products side of the equation.
• Coefficient-represent the number of units
of each substance taking part in the
reaction
• Balanced chemical equation-the same
number of atoms of each element on
both sides of the equation
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Lavoisier’s Chemical Equation:
2HgO

2 Hg
+ O2
Hg (mercury) can exist by itself...but, oxygen will need to bond
with another oxygen to make O2 (diatomic)
To balance the atoms we need to:
Put the coefficient of 2 in front of reactant HgO.
Put the coefficient of 2 in front the product Hg.
This balances the equation!
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Is this balanced?
H2
N2
NH3
Why or Why Not?
Let’s Count the Atoms:
N
N
H
H
H
N
H
H
There are 2
nitrogen atoms
There are 2
hydrogen atoms
1 nitrogen and
3 hydrogen…
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Atoms can only bond in certain ways…
remember the criss-cross method.
H2
H2
N2
N2
H2
H2
That’s why we can’t
change the subscripts.
N2
H2
N2
N2
N
N
H
H
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Here is what it means...
Subscripts - Small #’s
below an element.
H2
Coefficients - Large #’s in
front of the formulas.
I can’t live
without you!
2H2
When balancing equations,
we can only change the coefficients!
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Now, back to the
balancing...
1H2
1N2
1NH3
We can only change coefficients
before the symbols.
N
N
H
H
H
N
H
1 molecule of
nitrogen
1 molecule of
hydrogen
1 molecule of
ammonia
H
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Do both sides have the same amount of atoms?
N2 + H 2
NH3
2
N
1
2
H
3
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Do both sides have the same amount of atoms?
N2 + 3 H 2
(6)
N 2 + 3 H2
2
NH3
2
N
1
(2)
2
H
3
(6)
2NH3
Then it is a balanced equation.
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Four Steps to Balance
Equations:
1. List the metals, nonmetals, oxygen,
and hydrogen below arrow.
2. Count the number of atoms you have
on both sides.
3. Balance by changing the coefficients
and recounting.
4. Start the process again if it still does
not balance.
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1. Set up your equation.
List the elements in this order below the equation:
Metals, Nonmetals, Oxygen, and Hydrogen
H2 + O2  H2O
Metals
Nonmetals
Oxygen
Hydrogen
O
H
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2. Count the number of atoms you have of each
on both sides.
H2 + O2  H2O
2 O 1
2 H 2
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3. Balance by changing the coefficients and
recounting.
2 H2 + O2  2H2O
2 O 1 (2)
(4) 2 H 2 (4)
How are you going to
make “H” add up to 4?
Is this balanced?
Yes!
Need to have at
least 2 “O”
But it changes
the number of
“H”
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Let’s try another:
Mg + 2 HCl  H2 + MgCl2
Need to have at
least 2 “Cl”
1 Mg 1
(2) 1 Cl 2
Changing the Cl
changes the
“H”?
(2) 1 H 2
Is this balanced?
Yes!
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Let’s try another:
Na +2 HCl  NaCl + H2
1 Na 1
In this case, we
will start with
1 Cl 1 hydrogen
since it
is the only one
H
(2) 1
2
unbalanced.
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But, changing the hydrogen in HCl affects the
number of chlorine atoms.
Na + 2 HCl  2 NaCl + H2
1 Na 1
(2) 1 Cl 1 (2)
(2) 1 H 2
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Changing the chlorine on the product side affects the
sodium (Na) on the reactants side. So we must
now change sodium as well.
2 Na + 2HCl  2NaCl + H2
(2) 1 Na 1 (2)
(2) 1 Cl 1 (2)
(2) 1 H 2
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Independent Practice
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5 Types of Chemical Reactions
Synthesis
Decomposition
Single Displacement
Double Displacement
Combustion
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Synthesis
A + B  AB
2H2 + O2  2H2O
Two or more substances combine to form a new one.
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Decomposition
AB  A + B
2MgO  2Mg + O2
One substance breaks down or decomposes
into 2 or more simpler substances.
Most reactions require heat, light or electricity.
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Single Displacement
A + BC  AC + B
Zn + Cu(NO3)2  Zn(NO3)2 + Cu
One element replaces another in a compound
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Double Displacement
AB + CD  AD + CB
HCl + NaOH  HOH + NaCl
(H2O)
The negative ion of one compound replaces the negative
ion of the other compound to form 2 new compounds.
Usually forms a precipitate, water or a gas.
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Combustion
CH4 + 2O2
CO2 + 2H2O + energy
Takes place when both CO2 and H2O are the
ONLY products
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Independent Practice
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Energy in Reactions
• In a chemical reaction energy is either
released or absorbed.
– The energy can be
• Heat
• Light
• Sound
• Electricity
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Energy in Reactions
• Exothermic reactions
– Some form of energy is given off by the
reaction
• Heat given off causes reaction mixture to feel hot
• Examples-burning wood, dynamite explosion
• Endothermic reactions
– Energy must be provided for the reaction to
take place
• Absorbs so much heat that the container feels cold
• Example-frequently used to obtain a metal from its
ore, using an electric current, chemical cold packs