Transcript Chemical change and Chemical reactions

Physical Properties
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color
melting point
boiling point
electrical conductivity
specific heat
density
state (solid, liquid, or gas)
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Physical Change
Changes in physical properties
 melting
 boiling
condensation
No change occurs in the identity of the
substance
Example:
Ice , rain, and LecturePLUS
steam Timberlake
are all water
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Chemical Change
 Atoms in the reactants are rearranged to
form one or more different substances
 Old bonds are broken; new bonds form
Examples:
Fe and O2 form rust (Fe2O3)
Ag and S form tarnish (Ag2S)
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Learning Check E1
Classify each of the following as a
1) physical change or 2) chemical change
A. ____ a burning candle
B. ____ melting ice
C. ____ toasting a marshmallow
D. ____ cutting a pizza
E. ____ polishing silver
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Solution E1
Classify each of the following as a
1) physical change or 2) chemical change
A. __2__ a burning candle
B. __1_ melting ice
C. __2__ toasting a marshmallow
D. __1__ cutting a pizza
E. __1__ polishing silver
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Chemical Reaction
A process in which at least one new
substance is produced as a result of
chemical change.
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- Page
321
Products
Reactants
A Chemical Reaction
Reactants
Products
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Learning Check E2
A. How does an equation indicate a change in
the identity of the reacting substances?
B. How did the yellow and green reactants
combine?
C. Did all the reactants form product? Why or
why not?
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Learning Check E2
A. How does an equation indicate a change in
the identity of the reacting substances?
The formulas of the reactants are different
than the formulas of the products.
B. How did the yellow and green reactants
combine? 1 yellow combined with 1 green.
C. Did all the reactants form product? Why or
why not? No. There were more yellow
reactants than green.
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All chemical reactions…
•
have two parts:
1. Reactants = the substances you
start with
2. Products = the substances you
end up with
• The reactants will turn into the
products.
• Reactants  Products
In a chemical reaction
• Atoms aren’t created or destroyed (according
to the Law of Conservation of Mass)
• A reaction can be described several ways:
#1. In a sentence every item is a word
Copper reacts with chlorine to form copper (II)
chloride.
#2. In a word
equation some symbols used
Copper + chlorine  copper (II) chloride
Symbols in equations?
• the arrow (→) separates the reactants
from the products (arrow points to products)
–Read as: “reacts to form” or yields
• The plus sign = “and”
• (s) after the formula = solid: Fe(s)
• (g) after the formula = gas: CO2(g)
• (l) after the formula = liquid: H2O(l)
Symbols used in equations
• (aq) after the formula = dissolved
in water, an aqueous solution:
NaCl(aq) is a salt water solution
Now, read these equations:
Fe(s) + O2(g)  Fe2O3(s)
Cu(s) + AgNO3(aq)  Ag(s) + Cu(NO3)2(aq)
Pt
NO2(g)   N2(g) + O2(g)
Learning Check E3
12 oz of dough, 4 oz mushrooms, 12 slices
pepperoni, 8 oz cheese and 5 oz tomato sauce
are used to make a pizza. Write a recipe in
words for putting together a pizza.
How would you write the recipe as an
equation?
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Solution E3
Example: Combine 12 oz dough + 4 oz
mushrooms + 12 slices pepperoni + 8
oz cheese + 5 oz tomato sauce and heat
30 minutes at 350°C to produce 1 pizza
12 oz dough + 4 oz mshrm
+ 12 pep + 8 oz chse
1 pizza
+ 5 oz tom sauce
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Reading A Chemical Equation
4 NH3 + 5 O2
4 NO + 6 H2O
Four molecules of NH3 react with five
molecules O2 to produce four molecules NO
and six molecules of H2O
or
Four moles NH3 react with 5 moles O2 to
produce four moles NO and six moles H2O
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Balanced Chemical Equations
• Atoms can’t be created or destroyed
in an ordinary reaction:
–All the atoms we start with we must
end up with (meaning: balanced!)
• A balanced equation has the same
number of each element on both
sides of the equation.
A Balanced Chemical Equation
Same numbers of each type of atom on each
side of the equation
Al
+
S
Al2S3
Not Balanced
2Al
+
3S
Al2S3
Balanced
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Matter Is Conserved
H2
+
Cl2
2 HCl
+
+
Total atoms
2 H, 2 Cl
=
Total atoms
2H, 2 Cl
Total Mass
2(1.0) + 2(35.5)
73.0 g
=
Total Mass
2(36.5)
73.0 g
=
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Law of Conservation of Mass
In any ordinary chemical reaction,
matter is not created nor destroyed
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Rules for balancing:
1) Assemble the correct formulas for all the
reactants and products, using “+” and “→”
2) Count the number of atoms of each type
appearing on both sides
3) Balance the elements one at a time by
adding coefficients (the numbers in front)
where you need more - save balancing the
H and O until LAST!
(hint: I prefer to save O until the very last)
4) Double-Check to make sure it is balanced.
• Never change a subscript to balance an
equation (You can only change coefficients)
– If you change the subscript (formula) you are
describing a different chemical.
– H2O is a different compound than H2O2
• Never put a coefficient in the middle of a
formula; they must go only in the front
2NaCl is okay, but Na2Cl is not.
Steps in Balancing An Equation
Fe3O4 + H2
Fe + H2O
Fe: Fe3O4 + H2
3 Fe + H2O
O:
Fe3O4 + H2
3 Fe + 4 H2O
H:
Fe3O4 + 4 H2
3 Fe + 4 H2O
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Learning Check E4
Fe3O4 + 4 H2
3 Fe + 4 H2O
A. Number of H atoms in 4 H2O
1) 2
2) 4
3) 8
B. Number of O atoms in 4 H2O
1) 2
2) 4
3) 8
C. Number of Fe atoms in Fe3O4
1) 1
2) 3
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3) 4
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Solution E4
Fe3O4 + 4 H2
3 Fe + 4 H2O
A. Number of H atoms in 4 H2O
3) 8
B. Number of O atoms in 4 H2O
2) 4
C. Number of Fe atoms in Fe3O4
2) 3
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Learning Check E5
Balance each equation. The coefficients for
each equation are read from left to right
A.
Mg
+
1) 1, 3, 2
B.
Al
+
1) 3, 3, 2
N2
Mg3N2
2) 3, 1, 2
Cl2
AlCl3
2) 1, 3, 1
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3) 3, 1, 1
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3) 2, 3, 2
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Learning Check E5
C.
Fe2O3
+
1) 2, 3, 2,3
D.
Al
Al
+
+
Fe
2) 2, 3, 4, 3
Fe +
2) 2, 1, 1, 1
H2SO4
1) 3, 2, 1, 2
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CO2
Al2O3
3) 3, 3, 3, 1
Al2(SO4)3
2) 2, 3, 1, 3
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+
3) 1, 1, 2, 3
FeO
1) 2, 3, 3, 1
E.
C
+ H2
3) 2, 3, 2, 3
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Solution E5
A. 3 Mg
+ N2
Mg3N2
B. 2 Al
+ 3 Cl2
2 AlCl3
C. 2 Fe2O3 + 3 C
D. 2 Al
+ 3 FeO
E. 2 Al + 3 H2SO4
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4 Fe
+ 3 CO2
3 Fe +
Al2O3
Al2(SO4)3
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+ 3 H2
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Types of Reactions
• There are probably millions of reactions.
• We can’t remember them all, but luckily they
will fall into several categories.
• We will learn: a) the 4 major types.
• We will be able to: b) predict the products.
• For some, we will be able to: c) predict
whether or not they will happen at all.
• How? We recognize them by their reactants
#1 –Synthesis/ Combination
Reactions
• Combine = put together
• 2 substances combine to make one
compound (also called “synthesis”)
• Ca + O2 CaO
• SO3 + H2O  H2SO4
• We can predict the products, especially
Mg3N2 (symbols, charges, cross)
if the reactants are two elements.
• Mg + N2 
Complete and balance:
Ca + Cl2 
Fe + O2  (assume iron (II) oxide is the product)
Al + O2 
Remember that the first step is to write
the correct formulas – you can still
change the subscripts at this point, but
not later while balancing!
• Then balance by changing just the
coefficients only
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#2 - Decomposition Reactions
• decompose = fall apart
• one reactant breaks apart into two
or more elements or compounds.
electricity
• NaCl    Na + Cl2

• CaCO3   CaO + CO2
• Note that energy (heat, sunlight,
electricity, etc.) is usually required
#2 - Decomposition Reactions
• We can predict the products if it is
a binary compound (which means
it is made up of only two elements)
–It breaks apart into the elements:
electricity
• H2O   

• HgO  
#3 - Single Replacement Reactions
• One element replaces another
• Reactants must be an element and a
compound.
• Products will be a different element
and a different compound.
• Na + KCl  K + NaCl (Cations
switched)
• F2 + LiCl  LiF + Cl2 (Anions
switched)
#3 Single Replacement Reactions
• Metals will replace other metals (and they
can also replace hydrogen)
• K + AlN 
• Zn + HCl 
• Think of water as: HOH
– Metals replace the first H, and then
combines with the hydroxide (OH).
• Na + HOH 
#3 Single Replacement Reactions
Practice:
• Fe + CuSO4 
• Pb + KCl 
• Al + HCl 
#4 - Double Replacement Reactions
• Two things replace each other.
– Reactants must be two ionic
compounds, in aqueous solution
• NaOH + FeCl3 
– The positive ions change place.
• NaOH + FeCl3 Fe+3 OH- + Na+1 Cl-1
= NaOH + FeCl3 Fe(OH)3 + NaCl
Complete and balance:
• assume all of the following
reactions actually take place:
CaCl2 + NaOH 
CuCl2 + K2S 
KOH + Fe(NO3)3 
(NH4)2SO4 + BaF2 
How to recognize which type?
• Look at the reactants:
E + E = Combination
C
= Decomposition
E + C = Single replacement
C + C = Double replacement
Practice Examples:
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•
•
•
•
•
•
H2 + O 2 
H2O 
Zn + H2SO4 
HgO 
KBr + Cl2 
AgNO3 + NaCl 
Mg(OH)2 + H2SO3 
SUMMARY: An equation...
• Describes a reaction
• Must be balanced in order to follow the
Law of Conservation of Mass
• Can only be balanced by changing the
coefficients.
• Has special symbols to indicate the
physical state, if a catalyst or energy is
required, etc.
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