Download regents chapter 11 2009

Regents Chapter 11
Chemical Reactions
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Section 11.1
Describing Chemical Reactions
• Objectives:
– Describe how to write a word equation.
– Describe how to write a skeleton equation.
– Describe the steps for writing a balanced
chemical equation.
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Chemical Reactions
• Chemical reactions occur everyday, everywhere.
There are thousands of different reactions that
occur
Digestion
Baking Cookies
Photosynthesis
Respiration
Explosion of the Hindenburg
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All chemical reactions…
– Have two parts:
• Reactants - the substances you start with
• Products- the substances you end up with
– The reactants turn into the products.
– Reactants
Products
• The arrow is read as yields
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Symbols used in Equations
•
•
•
•
(s) after the formula = solid: AgCl(s)
(g) after the formula = gas: CO2(g)
(l) after the formula = liquid: H2O(l)
(aq) after the formula = dissolved in water, an
aqueous solution: NaCl(aq) salt water solution
• ↑ used after a product indicates a gas has
been produced: H2 ↑
• ↓ used after a product indicates a precipitate
has been produced: PbI2 ↓
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Symbols used in equations
• indicates a reversible reaction (more
later)

heat
  ,   
• shows that heat or energy is supplied
to the reaction
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Balanced Chemical Equations
• Before we begin to balance chemical
equations lets talk about what the term
balanced means by using an equation for
the production of a bicycle
– What are the basic components needed to
manufacture a bicycle?
frame + wheel + handlebar + pedal = bike
– Our equation shows the reactants and
products
• What if we were responsible for ordering
the parts to make the bike? Is this
equation sufficient?
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Balanced Chemical Equations
• We need to know how many of each component is
needed to manufacture the bike
– 1 frame, 2 wheels, 1 handlebar, and 2 pedals
• Using these symbols the formula for our bike would be
FW2HP2
• The equation for the manufacture of the bike would become:
F + W + H + P  FW2HP2
– There is a problem with our equation
• It is not a balanced equation – the number of parts do not
equal the whole
– There is not the same number of each part on the left as there is on
the right
• Our equation should be:
F+ 2W + H + 2P  FW2HP2
• This is now a balanced equation showing the same number
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of each part on the left and on the right
Balanced Chemical Equations
• The same is true for a balanced chemical equation
• Remember…
– Atoms can not be created or destroyed in an ordinary
reaction – the atoms are merely rearranged by the
breaking and forming of bonds
– In any chemical change, mass is conserved
• The atoms in the products are the same atoms that were in
the reactants – they are just rearranged
– Law of Conservation of Mass
• A balanced equation has the same number of atoms
of each element on both sides of the equation.
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Balanced Chemical Equations
•
Use coefficients IN FRONT of the entire formulas to balance
the equation so that it obeys the law of conservation of mass.
However, the coefficient of 1 is omitted from the front of the
formula.
• Carbon burns in the presence of oxygen to produce
carbon monoxide
The skeleton or base equation:
REMEMBER –
diatomic
molecules
2C +
O2  2CO
1. Count the number of atoms of each element on both
sides of the equation
This is a balance C = 2
C=2
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equation
O=2
O=2
Rules for Balancing:
1.
2.
3.
4.
5.
6.
7.
Assemble the correct formulas for all the reactants and
products
Use + in between more than one reactant and product if
necessary
Use → between reactants and products
Count the number of atoms of each type appearing on both
sides
Balance the elements one at a time by adding coefficients
where needed (the numbers in front) - save balancing the H
and O until last!
(Hint - save O until the very last)
RECHECK to make sure equation is balanced (equal amount
of each element on both sides)
Check to make sure all coefficients are in the
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lowest whole number ratio
• Never change a subscript to balance an equation.
– If you change the formula you are describing a
different reaction.
– H2O is a different compound than H2O2
• Never put a coefficient in the middle of a formula
2NaCl is okay, but Na2Cl is not.
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Balance the Following Chemical Equation
• When hydrogen and oxygen are mixed, a spark will
initiate a rapid reaction. The product of the reaction is
water.
Step 1: Write the proper equation - correct formulas for reactants and
products
Reactants – hydrogen and oxygen
Product - water
H 2 + O2
H 2O
Step 2: Count the number of atoms of each type appearing on both sides
H= 2
O= 2
H= 2
Step 3: Balance atoms one at a time by adding coefficients
2 H2 + O2
Placing a coefficient of 2 in
front of H2 rebalances the
hydrogen
2 H2O
O= 1
Placing a
coefficient of 2 in
front of water
balances the O but
unbalances the
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hydrogen
Balanced Chemical Equation
• This is a properly balanced chemical equation
2H2 + O2
2H2O
• There is the same number and kinds of each
element in the reactants and products
– 4 hydrogen and 2 oxygen atoms in the reactants
– 4 hydrogen and 2 oxygen atoms in the products
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Practice Examples/ Problems
• _AgNO
 _Cu(NO
_Ag
2
1
1
3 + _Cu
3)2 + 2
• _Mg
+ _N
3
1 2  1_Mg3N2
4 + _O
5 2  _P
1 4O10
• _P
• _Na
+ _H
2
2 2O  _H
1 2 + _NaOH
2
• _CH
_O2  _CO
_H2O
1
1
4 + 2
2 + 2
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Section 11.2
Types of Chemical Reactions
• Objectives:
–Describe the five general types
of reactions.
–Predict the products of the five
general types of reactions.
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Types of Reactions
• There are millions of reactions.
• We can’t remember them all, but luckily they
will fall into several categories.
• We will learn 5 major types.
• We recognize reaction type by the
reactants
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Types of Reactions
1. Synthesis Reaction:
A+B  AB
2. Decomposition Reaction:
AB  A + B
3. Single Replacement:
A + BC  B + AC
4. Double Replacement:
AB + CD  AC + BD
5. Combustion
Requires oxygen (O2) as a reactant and H2O and
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either CO or CO as products
Synthesis Reactions
• Combination reactions
• Combine = put together
• Two or more substances to form a single
new substance (a compound)
• 2Ca +O2  2CaO
• SO3 + H2O  H2SO4
• We can predict the products if the
reactants are two elements
– Example
Mg + N2  Mg3N2
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Decomposition Reactions
• decompose = fall apart
• Single compound breaks down into two or
more simpler products
– 1 reactant  2 products
electricity
• 2NaCl
• CaCO3  


2Na + Cl2
CaO + CO2
– Note that energy (heat, sunlight, electricity, etc.) is
usually required
electricity
2H2O(l)    2 H2(g) + O2(g)
http://www.youtube.com/watch?v=m_21KafrwRc&feature=related (3:18 min) 20
http://www.youtube.com/watch?v=__gHPblNumM&feature=related (6:23 min)
Single Replacement
• One element replaces another in a
compound
• Reactants must be an element and a
compound.
• Products will be a different element and a
different compound.
K + NaCl  KCl + Na
Zn + 2HCl  ZnCl2 + H2
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Double Replacement
• Reactants are always two compounds and
products are always two different compounds
• Involves the exchange of positive ions between 2
compounds
• Two things replace each other.
– Reactants must be two ionic compounds.
• NaOH + FeCl3  ??????
– The positive ions change place.
• NaOH + FeCl3  Fe+3 OH- + Na+1 Cl-1
• 3NaOH + FeCl3  Fe(OH)3 + 3NaCl
•
http://www.youtube.com/watch?v=opY3FLrPTa4 (1:30 min)
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Combustion
• Means add oxygen
– Normally, a compound composed of only C,
H, (and maybe O) is reacted with oxygen –
usually called burning
– Often releases energy in the form of heat and
light
– If the combustion is complete, the products
will be CO2 and H2O.
– If the combustion is incomplete, the products
will be CO and H2O.
– http://www.youtube.com/watch?v=q6eIqNpdvlw&NR=1 (1:32 min)
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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 (NEVER SUBSCRIPTS!)
• Has special symbols to indicate physical state, if
heat or energy is required, etc.
• Come in 5 major types.
• We can tell what type of reaction is occurring by
looking at the reactants.
•
http://www.youtube.com/watch?v=i-HHvx1VC_8&NR=1 (1:29 min)
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Identifying Reactions
• Synthesis
– Reactants will be 2 elements
– Product will be 1 compound
• Decomposition
– Reactant will be 1 compound
– Products will be 2 elements
• Single replacement
– Reactants will be 1 element and 1 compound
– Products will be a different element and a different compound
(if the reaction occurs)
• Double replacement
– Reactants will be 2 compounds
– Products will be 2 different compounds
• Combustion
– One of the reactants will be oxygen
– Water will always be one of the products – the other product will either
be CO or CO2
– http://www.youtube.com/watch?v=tE4668aarck 1:36 min
–
http://www.youtube.com/watch?v=ul4xRy8hcsQ (5:12 min)
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