Chemical Equations and Reactions
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Transcript of Chemical Equations and Reactions
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Chemical Equations and Reactions
What are they?
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The Two Sides of a Chemical Reaction
• Definition: In a chemical reaction one or more substances are changed into new substances.
Na + Cl NaCl
Reactant – About to be changed
Product – The new substance
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Symbol for Chemical Equations
“Pb(NO3)4” is a formula.
Means produces (or yields)
+ Means added together
Coefficients – Number of molecules Subscripts - Number of a particular atom in a
molecule
Pb(NO3)4 + 2KI PbI2 + 2KNO3
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First Described the “Law of Conservation of Mass”
Father of Modern Chemistry
1743 - 1794
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5 Conservation of MassAntoine Lavoisier found that the mass of the
reactants and the products are equal, even when the states of matter change.
This is the Law of the Conservation of Mass –Matter is neither created nor destroyed.
HgHg + + OO22HgHgOO
He started with: 10g of Mercury Oxide (HgO)
He ended up with:and 9.3g Mercury…
0.7
g O
xygen b
eca
me a
gas
10 g.10 g. = = 0.7 + 9.3 0.7 + 9.3 g.g.
...But what happened to the
O2?
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Lavoisier’s Chemical Equation:
Hg (mercury) can exist by itself... but, oxygen will need to bond with another oxygen to make O2
HgO Hg + O222
To balance the atoms we need to:
This balances the equation!
Put the coefficient of 2 in front the product Hg.
Put the coefficient of 2 in front of reactant HgO.
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Not that easy?
Let’s try...
Easy, Right?
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VISUALIZING to Balance VISUALIZING to Balance an Equation…an Equation…
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9 Equations Must Be Balanced
HH22 + + OO22
HH22 OO
Law of Conservation of Mass
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10 Equations Must Be Balanced
2 2 HH22 + + OO222 2 HH22
OO
Matter is neither created nor destroyed.
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Is this reaction possible?
NN NN HH HHNN
HH
HHHH
N2 NH3Why or Why Not?
Let’s Count the Atoms:
There are 2 nitrogen atoms
There are 2 hydrogen
atoms
1 nitrogen and 3 hydrogen…
H2
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Before we begin balancing, we will look at a couple of rules.
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13 Atoms can only bond in certain ways..
That’s why we can’t change the subscripts.subscripts.
HHHH
NNNN
NN
NN
NNNN
NNNN
HH
HHHH
HH
HHHH
HH
HH
NN
NN
NN NN HH HH
H2
N2
N2
N2
N2
N2
H2
H2
H2
H2
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Here is what it means...
Subscripts - Small #’s below an element.
Coefficients - Large #’s in front of the formulas.
2H2H2When balancing equations,
we can only change the coefficients!
HHHH
HHHH
HH
HH
I can’t live without
you!
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Now, back to the balancing...
NN NN HH HHNN
HH
HHHH
N2 NH3We can only change coefficients
before the symbols.
1 molecule of nitrogen
1 molecule ofhydrogen
1 molecule of ammonia
H21 1 1
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NN NN HH HHNN
HH
HHHH
After counting the atoms we know...
There are too many nitrogen atoms
and not enough hydrogen atoms.
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NN NN
HH HH NN
HH
HHHH
A little common sense math goes a long way in making this equation
balance…
Count the atoms. Is it balanced?
There are 2 nitrogen atoms. We will get 2 more
molecules for a total of 6 hydrogen
atoms. That will make 2 NH3.
HH HH
HH HHNN
HH
HHHH
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N2 + 3 H2 2NH3
NN NN
HH HH NN
HH
HHHH
HH HH
HH HHNN
HH
HHHH
Then it is a balanced equation.
Do both sides have the same amount of atoms?
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Four Steps to Balance Equations:
1. Set up your equation.Write the metals, nonmetals, oxygen, and
hydrogen.
2. Count and then write 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.
HH22 + O + O2 2 H H22OOHH22 + O + O2 2 H H22OOWrite the elements in this order:
Metals, Nonmetals, Oxygen, and Hydrogen
OOHH
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2. Count and write the number of atoms you have of each on both sides.HH22 + O + O2 2 H H22OOHH22 + O + O2 2 H H22OO
OOHH
22 112222
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HH22 + O + O2 2 H H22OOHH22 + O + O2 2 H H22OO
OOHH
22 112222
(2)(2)22
(4)(4)
22
(4)(4)
3. Balance by changing the coefficients and recounting.
Need to have at least 2 “O”
But it changes the number of
“H”
How are you going to make “H” add up
to 4?
Is this balanced?Yes!
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Mg + HCl Mg + HCl H H2 2 + MgCl + MgCl22Mg + HCl Mg + HCl H H2 2 + MgCl + MgCl22
MgMgClCl
11 112211
(2)(2)
22
(2)(2)
Let’s try another:
Need to have at least 2 “Cl”
Changing the Cl changes the “H”?
Is this balanced?Yes!
HH 2211
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Na + HCl Na + HCl NaCl + H NaCl + H22Na + HCl Na + HCl NaCl + H NaCl + H22
NaNa
ClClHH
11 11
112211
11(2)(2)
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In this case, we will start with
hydrogen since it is the only one
unbalanced.
Let’s try another:
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Na + Na + HCl HCl NaCl + H NaCl + H22Na + Na + HCl HCl NaCl + H NaCl + H22
NaNa
ClClHH
11 11
112211
11(2)(2)
22
(2)(2) (2)(2)
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But, changing the hydrogen in HCl affects the number of chlorine atoms.
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26Changing the chlorine on the product side affects the sodium (Na) on the reactants side. So we must now change sodium as well.
Na + Na + 22HCl HCl NaCl + HNaCl + H22Na + Na + 22HCl HCl NaCl + HNaCl + H22
NaNa
ClClHH
11 11
112211
11(2)(2)(2)(2) (2)(2)
2222(2)(2) (2)(2)
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http://www.chem.vt.edu/RVGS/ACT/notes/scripts/bal_eq1.html
http://www.wfu.edu/~ylwong/balanceeq/balanceq.html
Click on the web sites below and balance the equations.Click on the web sites below and balance the equations.
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4 Types of Chemical Reactions
SynthesisDecompositionSingle DisplacementDouble Displacement
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Synthesis
AA + + BB AABB
22HH22 + + OO22 2 2HH22OO
Two or more substances combine to form a new one.
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Decomposition
AAB B A A + + BB
22MgMgOO MgMg ++ OO22
Decomposition the opposite of synthesis; a substance breaks down into 2 or more simpler
substances.
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Single Displacement
AA + + BBC C A ACC + + BB
ZnZn + + CuCu(NO(NO33))22 ZnZn(NO(NO33))22 ++
CuCuThis occurs when one element replaces another in a
compound; a zinc strip is put into a copper nitrate solution and the
copper precipitates out (it turns solid).
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Double Displacement
AAB B + + CCDD A ADD + + CCBB
HHClCl + + NaNaOHOH HHOHOH ++ NaNaClCl(H2O)
This occurs when two elements “switch” with one another to form two new compounds.