Transcript CHEMICAL REACTIONS

B-2 1/3-4: 8.1/8.2 Chem and phys change, Bal Eq WS 1,2
M
1/7: Rev Bal, 8.3 Types chem eq WS 3, 4
WS 5 and 6 are extra work on balancing and typing equations
B-1 1/8-9: Quiz 8.1-8.3: Ratios, 9.1 3-Step process
B-2 1/10-11: 3-Step, 9.2 Lim react, % Yield
M
1/14: Quiz 3-Step, % Yield
B-1 1/15-16: Lab
B-2 1/17-18: Rev Lab
M
1/21: Rev Chap
B-1 1/22-23: Test
CHEMICAL REACTIONS
Reactants: Zn + I2
Product: Zn I2
Chemical Equations
• Show a chemical reaction
– Chem reactions describe a chemical change
• Chem reactions = chemical change
– Reactants converted into products
– Formation of new substance
– Particles must collide for a reaction to occur
• Old bonds broken in reactants (endo)
• New bonds formed in products (exo)
– Atoms (and mass) are conserved
– Energy changes can also be described
Collision Theory
• For reactant molecules to cause a
chemical change they must collide
• 2 criteria must be met for a collision to
cause a chem reaction to occur
–
–
–
–
–
1. proper energy to break old bonds
2. proper orientation to form new bonds
Collision alignment.MOVCollision
Effective.MOV
Collision no energy.MOV
Physical Change
Changes in physical properties
 melting
 boiling
 dissolving
 No change occurs in the identity of the substance
 Changes in energy occur in physical changes
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)
Learning Check
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
Solution
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. __2__ polishing silver
Chemical Reaction
A process in which at least one new
substance is produced as a result of
chemical change.
Evidence of Chem Rx
•
•
•
•
Formation of a gas
Formation of a precipitate
Color change
Large energy change
– Exothermic – feels warm
• Heat, sound, light
– Endothermic – feels cold
• Photosynthesis
• Know all examples from text pg 261
A Chemical Reaction
Reactants
Products
Chemical Reactions
– Chemical reactions occur when old bonds between
atoms are broken and new bonds between atoms are
formed
– Chemical reactions involve chemical changes in matter.
• New substances with new properties are created (products).
– Chemical reactions can absorb or give of energy.
– Symbols represent names of reactants and product
• formulas describe compounds
• chemical equations describe a chemical reaction
Heat in Chemical Reactions
 Energy changes always accompany
chemical and physical changes
 Heat is the primary form of energy
exchanged during chemical reactions
 One reason why reactions occur is that
the products attain a lower, more stable
energy state than the reactants.
Enthalpy
• Enthalpy = H
• total heat content in a substance
• Sum of potential and kinetic energies
• Can calc Ek = ½mv2
• Cannot calc Ep
• Cannot determined H for any system
• We can and do measure the change in
energy during a reaction = DH
Energy Changes in Chem Eq
• DH = change in energy (enthalpy)
– DH = energyprod – energyreactants
– DH (+) = endothermic rx = +200 kJ
– DH (-) = exothermic rx = -100 kJ
Endo Chem Rx
• Reactant + 200 kJ
Product
– Feels cold - energy absorbed
– Energy must be added to keep rx going
– Higher energy, less stable prod formed
Exo Chem Rx
• Reactant
Product + 100 kJ
– Feels warm - energy given off
– Most spontaneous reactions are exo
– Prod lower energy, more stable, than reactants
Chemical Equations
Their Job: Depict the kind of reactants
and products and their relative amounts
in a reaction.
4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)
The numbers in the front are called
stoichiometric coefficients
The letters (s), (g), (l) and (aq) are the
physical states of compounds.
Parts of a Chemical Equation
– Chemical equations shows the conversion of
reactants (the molecules shown on the left of the
arrow) into products (the molecules shown on the
right of the arrow).
• A + sign separates molecules on the same side
• The arrow is read as “yields” or “produces”
• Example
C + O2  CO2
• This reads “carbon plus oxygen react to yield
carbon dioxide”
Symbols Used in Equations
• Solid (s)
p 265
• Liquid (l)
table 2
• Gas (g)
• Aqueous solution (aq)
• Formation of precipitate () or (s)
• Escaping gas () or (g)
H2SO4
• Catalyst
• Heating D
Special Conditions
•
•
•
•
•
•
•
produces or yields
reversible reaction. Both reactants
and products present at end of reaction
D
reactants are heated
10˚C
rx carried out at specific temp
1 atm
pressure rx is conducted under
Pd
catalyst used in the reaction
State symbols table p. 265
Chemical Equations
Because of the principle of
the conservation of matter,
an equation must be balanced.
It must have the same
number of atoms of the
same kind on both sides.
Lavoisier, 1788
Subscripts vs. Coefficients
• The subscripts
tell you how
many atoms of a
particular
element are in a
compound. The
coefficient tells
you about the
quantity, or
number, of
molecules of the
compound.
Balancing Equations
– When balancing a chemical reaction you
may add coefficients in front of the
compounds to balance the reaction (get
the same number of atoms of each element
on both side of the equation.
– You may
not change the subscripts.
• Changing the subscripts changes the
compound. The subscripts are
determined by the # of electrons
needed to obtain an octet
Chemical Equations
4Al(s) + 3O2(g)-->2Al2O3(s)
This equation means
4 Al atoms + 3 O2 molecules
---produces--->
2 molecules of Al2O3
AND/OR
4 moles Al + 3 moles O2
---produces--->
2 moles of Al2O3
Meaning of Chem Equation
Steps to Balancing Equations
There are four basic steps to balancing a chemical equation.
1. Write the correct formula for the reactants and the
products. DO NOT TRY TO BALANCE IT YET! Once you
write the formulas correctly DO NOT CHANGE them!
2. Determine the order of elements to use in order to balance
the equation. Hints will follow.
3. Place coefficients in front of formulas so that the left side
of the equation has the same number of atoms of each
element as the right side of the equation.
4. Check your answer to see if:
– The numbers of atoms of each element, on both each side
of the equation should be the same.
– The coefficients are in the lowest possible whole number
ratio (reduced).
Balancing Equations
2 H2(g) + ___ O2(g) ---> ___
2 H2O(l)
___
What Happened to the Other Oxygen Atom?????
This equation is not balanced!
Two hydrogen atoms from a hydrogen molecule (H2) combines with
one of the oxygen atoms from an oxygen molecule (O2) to form
H2O. Then, the remaining oxygen atom combines with two more
hydrogen atoms (from another H2 molecule) to make a second H2O
molecule.
Some Suggestions to Help You
Some Helpful Hints for balancing equations:
• Take one element at a time usually starting with the most
complex substance.
• It is usually better to balance in this order: metals,
nonmetals, hydrogen, oxygen.
• If everything balances except for O2, and there is no
way to balance O2 with a whole number, use a fraction or
mixed number. Then, multiply all coefficients by 2 to
remove the fraction.
• Polyatomic ions that appear on both sides of the equation
should be balanced as whole units, not individual atoms.
Balancing
Equations
2 Al(s) + ___
3 Br2(l) ---> ___ Al2Br6(s)
___
Balancing
Equations
5
____C3H8(g) + _____
O2(g) --->
3
_____CO
2(g)
2 4H10(g) +
____B
2 B O (g)
4
__
2 3
+
4
_____
H2O(g)
11 O (g) ---->
11/2
_____
2
+
10
5
_____
H2O(g)
Balancing Equations
Sodium phosphate + iron (III) oxide 
sodium oxide + iron (III) phosphate
2 Na3PO4 +
3 Na2O
Fe2O3 ---->
+
2 FePO4
Balancing Chem Equations
• Al
+
HCl
-->
AlCl3
• FeCl3 + Li2SO4 -->
• Cu2S
+
Cu2O
-->
+
H2
Fe2(SO4)3 + LiCl
Cu
• C3H8 + O2 --> H2O + CO2
• C2H6 + O2 --> H2O + CO2
+
SO2
Type of Reactions
Chemical reactions are classified into
five general types
 Combination (synthesis)
 Decomposition (analysis)
 Single Displacement
 Double Displacement (metathesis)
 Combustion
Types of Chemical Reactions
1. Combination or Synthesis Reaction:
Two or more reactants combine to
form (synthesize) one product.
A+B
AB
2Mg + O2
2MgO
2Na + Cl2
2NaCl
CO2 + H2O
H2CO3 DEMO
Types of Chemical Reactions
2. Decomposition Reaction:
A single substance is or broken down
(decomposed) to give two or more
different substances:
AB
A + B
2PbO2
2PbO + O2
2NaHCO3
Na2CO3 + H2O + CO2
2CuCO3
2CuO + CO2
2KClO3
2KCl + 3 O2
DEMO
Quiz
Classify the following reactions as
1) combination or 2) decomposition:
___A. H2 + Br2
2HBr
___B. Al2(CO3)3
Al2O3 + 3CO2
___C. 4 Al
+ 3C
Al4C3
Solution
Classify the following reactions as
1) combination or 2) decomposition:
_1_A. H2 + Br2
_2_B. Al2(CO3)3
_1_C. 4 Al
+ 3C
2HBr
Al2O3 + 3CO2
Al4C3
Types of Chemical Reactions
3.
Single Displacement Reaction:
One element reacts with a compound to take
the place of one of the elements of that
compound.
A + BC
Zn + 2HCl
Mg + 2CH3COOH
Fe + CuSO4
B + AC
H2 + ZnCl2
DEMO
Zn(CH3COO)2 + H2
FeSO4 + Cu
Types of Chemical Reactions
• 4. Double Displacement or Metathesis:
Two compounds exchange partners with each
other to produce two different compounds.
One product is in a different form (phase).
AB + CD
2NaOH + Fe(NO3)2
Na2CO3 + CaCl2
ZnS + 2HCl
AD + CB
Fe(OH)2(s) + 2NaNO3
2NaCl + CaCO3(s)
ZnCl2 + H2S(g)
DEMO
Quiz
Classify the following reactions as
1) single displacement
2) double displacement
A. 2Al + 3H2SO4
Al2(SO4)3 + 3H2
B. Na2SO4 + 2AgNO3
C. 3C + Fe2O3
Ag2SO4 + 2NaNO3
2Fe
+ 3CO
Solution
Classify the following reactions as
1) single replacement
2) double replacement
1 A. 2Al + 3H2SO4
Al2(SO4)3 + 3H2
2 B. Na2SO4 + 2AgNO3
1 C. 3C + Fe2O3
Ag2SO4 + 2NaNO3
2Fe
+ 3CO
Types of Chemical Reactions
• 5. Combustion
All hydrocarbon (molecules containing
C, H, and O) reacts with oxygen to form
carbon dioxide and water
HCO + O2
H2O + CO2
Combustion
A reaction in which a compound (often
carbon) reacts with oxygen
CH4 + 2O2
C3H8 + 5O2
C6H12O6 + 6O2
CO2 + 2H2O
3CO2 + 4H2O
6CO2 + 6H2O
Quiz
Balance the combustion equations
__C5H12 + __O2
__C4H10O
+ __O2
__ CO2 + __H2O
__CO2 + __H2O
Solution
Balance the combustion equations
1 C5H12 + 8 O2
1 C4H10O
+ 6 O2
5 CO2 + 6 H2O
4 CO2 + 5 H2O
Mol Factors
And
Energy changes
In chemical equations
Mole-Mole Factor
 Shows the mole-to-mole ratio between
two of the substances in a balanced
equation
 Derived from the coefficients of any
two substances in the equation
Writing Mole Factors
Fe and O2
4 Fe +
4 mol Fe
3 mol O2
3 O2
2 Fe2O3
and
3 mol O2
4 mol Fe
and
2 mol Fe2O3
4 mol Fe
and
2 mol Fe2O3
3 mol O2
Fe and Fe2O3
4 mol Fe
2 mol Fe2O3
O2 and Fe2O3
3 mol O2
2 mol Fe2O3
Quiz
3 H2(g) + N2(g)
2 NH3(g)
A. A mol factor for H2 and N2 is
1) 3 mol N2 2) 1 mol N2
1 mol H2
3) 1 mol N2
3 mol H2
2 mol H2
B. A mol factor for NH3 and H2 is
1) 1 mol H2 2) 2 mol NH3 3)
2 mol NH3
3 mol H2
3 mol N2
2 mol NH3
Solution
3 H2(g) + N2(g)
2 NH3(g)
A. A mol factor for H2 and N2 is
2) 1 mol N2
3 mol H2
B. A mol factor for NH3 and H2 is
2) 2 mol NH3
3 mol H2
Chemical Calculations
4 Fe + 3 O2
2 Fe2O3
How many moles of Fe2O3 are produced when
6.0 moles O2 react?
6.0 mol O2 x 2 mol Fe2O3 = 4.0 mol Fe2O3
3 mol O2
Quiz
4 Fe + 3 O2
2 Fe2O3
How many moles of Fe are needed to react
with 13.5 mol of O2?
1) 9.00 mol Fe
2) 10.1 mol Fe
3) 18.0 mol Fe
Solution
4 Fe + 3 O2
2 Fe2O3
13.5 mol O2 x 4 mol Fe = 18.0 mol Fe
3 mol O2
End
Mol : Energy Conversions
3 H2(g) + N2(g)
2 NH3(g) + 42 KJ
The conversion factors for energy are
1) 3 mol H2
42 KJ
2) 1 mol N2
42 KJ
3) 2 mol NH3
42 KJ
Quiz
• How many KJ are given off by the
reaction of 2.50 mol of H2 gas?
2.50 mol H2
42 KJ = 35.0 KJ
3 mol H2
Quiz
• How many moles of ammonia are
produced by a reaction that gives off
100 KJ of energy?
100 KJ
2 mol NH3 = 4.76 mol NH3
42 KJ
Net Ionic Equations
• NIE show
– 1. Balance the formula equation
– 2. Dissociate (pull apart) all (aq) compounds
• Subscripts for R and P become coefficients
• CaCl2(aq)
Ca2+(aq) + 2Cl-(aq)
– 3. Remove all spectator ions
• spectator ions do nothing in the reaction and are
excluded from the equation
• NaOH(aq) + Cu(NO3)2(aq) --> NaNO3(aq) + Cu(OH)2(s)
First, balance the equation
2NaOH(aq) + Cu(NO3)2(aq) -->
2NaNO3(aq) + Cu(OH)2(s)
Write ionic eq (pull apart all aq compounds) and
cancel the spectator ions
2Na+ + 2OH- + Cu2+ + 2NO3- -->
2Na+ + 2NO3- + Cu(OH)2(s)
Write NIE (write (+) ion first)
Cu2+(aq) + 2OH-(aq) ---> Cu(OH)2(s)
This is all that really happened!!!
Net Ionic Equations
– BaCl2 and Na2SO4 are mixed. Write NIE
for the reaction if a precipitate forms.
• Ions present are Ba2+ Cl- Ag+ SO42-
– Only + and - ions join together, therefore
– Possible ppts are (Ba2 + and SO42-) or (Na+ and Cl-)
» All chlorides are soluble (ex Ag+ and any Pb ion)
» All sulfates are soluble (ex Ba2+)
– Barium and sulfate ions will form a ppt
Ba2+ + 2Cl- + 2Na+ + SO42-
Ba+(aq) + SO42-(aq)
BaSO4 (s) + 2Na+ + 2Cl-
BaSO4(s)
NIE from Reactants
• Write NIE for any reaction (or no rx)
that occurs when (aq) solutions of
NaCl(aq) + AgNO3(aq) are mixed
Na+ + Cl- + Ag+ + NO3- -->
• possible ppt = AgCl or NaNO3
– All nitrate are soluble
– Most Ag compounds are insoluble therefore ppt is
– Ag+(aq) + Cl-(aq)
AgCl(s)
Al + HCl ---> AlCl3 + H2
•
Cu2S + Cu2O ---> Cu + SO2
•
WC + O2 ---> WO3 + CO2
•
Al4C3 + H2O ---> Al(OH)3 + CH4
•
TiCl4 + H2O ---> TiO2 + HCl
•
NH3 + O2 --->
N2 + H2O
Review
• Write NIE for any rx between aq sol’n of
– AgNO3
(aq)
+ Ca(OH)2 (aq)
– AlCl3 (aq) + Pb(NO3) 2 (aq)
Energy Expressed as Mol Ratio
• 2H2(g) + O2(g)
2H2O(l) + 572 kJ
2 mol H2 = 572 kJ therefore
2 mol H2
572 kJ
or
572 kJ
2 mol H2
How many kJ of energy are given off when
12.0 mol of H2 is reacted with an excess of O2?
• Convert 12.0 mol H2 to mol to kJ
12.0 mol H2 x
572 kJ =
2 mol H2