Transcript chem10chp7spr08

Chemical Reactions
Chapter 7
Tro, 2nd ed.
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SSME’s on SSTS
Combustion Reaction
H2(white) & O2(red)  2 H2O
Plus LOTs of energy!!!
A typical chemical reaction
powers the liftoff of a space
shuttle, along with the more
complex solid rocket boosters
combustion.
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Evidence of Chemical Reactions
There are five main observations that denote a
chemical reaction may have occurred.
Try to list them yourself first:
Color change
Precipitate
Gas or odor detected
Heat evolved or absorbed
Change in pH (later in semester)
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Chemical Equations for Chemical
Reactions
Chemical reactions always involve change
Atoms, molecules or ions rearrange to form new
substances: chemical bonds are broken and new bonds
are formed
Chemical equations describe reactions observed in the
laboratory or in nature; we use the chemical symbols
and formulas of the reactants and products and other
symbolic terms to represent a chemical reaction.
Chemical equations provide us with the means to:
- summarize the reaction
- display the substances that are reacting (reactants)
- show the products
- indicate the amounts of all component substances in
a reaction
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Chemical Reactions
(Unbalanced) Chemical Equation
Al + Fe2O3  Fe + Al2O3
reactants  products
iron oxygen
aluminum oxygen
bonds break
bonds form
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Chemical Reactions
Coefficients (whole numbers) are placed in front of
substances to balance the equation and to indicate the
number of units (atoms, molecules, moles, or ions) of
each substance that are reacting.

2 Al + Fe2O3  2 Fe + Al2O3
Coefficient
Coefficient
Conditions required to carry out the reaction may be placed
above or below the arrow.  = Heat added.
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Law of Conservation of Mass
Matter cannot be created or destroyed
- Therefore the total mass cannot change,
and the total mass of the reactants will be
the same as the total mass of the products
In a chemical reaction, all the atoms
present at the beginning are still
present at the end
- if all the atoms are still there, then the
mass will not change
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Symbols used in chemical
equations
Learn the state symbols: s, l, g, aq
Delta, , means heat
 means “yields” or makes or produces
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THREE STEPS TO WRITING CHEMICAL
EQUATIONS:
1. PREDICT WHAT PRODUCTS WILL BE
If two elements react, you can decide what
compound will form based on whether it’s ionic
or covalent.
EXAMPLE: Al metal reacts with Cl2 gas (nonmetal)
to make an ionic cmpd. What ions are likely to
form? Al3+ and Cl-, so the product will be AlCl3.
If two compounds react, you need to know about
types of chemical reactions (coming up soon).
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THREE STEPS TO WRITING CHEMICAL
EQUATIONS:
2. WRITE CORRECT CHEMICAL FORMULA
FOR EACH REACTANT AND PRODUCT
**Remember the seven diatomic elements
**Remember the naming rules
This is the step where most of the mistakes
are made!!!
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THREE STEPS TO WRITING CHEMICAL
EQUATIONS:
3. BALANCE EQUATION USING STOICHIOMETRIC
COEFFICIENTS
**Never change formulas’ subscripts to massbalance an equation!
Start with the most complex formula or with the
formula that contains the most of an atom
**Treat polyatomic ions as single units unless they
have broken up in the reaction
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THREE STEPS TO WRITING CHEMICAL
EQUATIONS: An Example
Write the chemical equation for the reaction of solid
aluminum with chlorine gas to form aluminum chloride
salt
Predict the product – has already been given, but we’ll
learn how to do this later
Write the correct chemical formulas – keep working on this
__Al(s) + __Cl2(g)  __AlCl3(s) Not mass balanced
Balance equation using the correct stoich coefficients
1 Al & 2 Cl
 1 Al & 3 Cl
2 Cl vs. 3 Cl: Find least common denominator (6)
___Al(s) + 3 Cl2(g)  2 AlCl3(s) Not balanced yet
1 Al & 6 Cl
 2 Al & 6 Cl
2 Al(s) + 3 Cl2(g)  2 AlCl3(s) Now it’s balanced!
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Practice writing and balancing
chemical equations
1. Glucose and oxygen produce carbon
dioxide and water
2. Sulfuric acid and potassium hydroxide
produce potassium sulfate and water
1. C6H12O6(s) + 6 O2(g)  6 CO2(g) + 6 H2O(l)
2. H2SO4(aq) + 2KOH(aq)  K2SO4(aq) + 2 H2O(l)
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The meaning of a formula
is context-dependent.
The formula H2O can mean:
1.
2.
3.
4.
5.
2 H atoms and 1 O atom
1 molecule of water
1 mole of water
6.0221 x 1023 molecules of water
18.015 g of water
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In an equation formulas can represent units of individual chemical entities or
moles.
Formulas
Number of
molecules
Number
of atoms
Number
of
moles
Molar
masses
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Aqueous Solutions
Some chemicals in a reaction are dissolved
in water; they are called aqueous
solutions
Dissolving chemicals in water helps them
to react together faster
- water separates chemicals into individual
molecules or ions, called dissociation
- the particles come in contact more
frequently speeding the reaction up
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Dissociation
When ionic compounds dissolve
in water, the anions and
cations are separated from
each other - this is called
dissociation
- however not all ionic compounds
are soluble in water!
When compounds containing
polyatomic ions dissociate, the
polyatomic group stays
together as one ion
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Dissociation
When potassium iodide dissociates in water into
potassium cations and iodide anions:
KI(aq)  K+1(aq) + I-1(aq)
Copper(II) sulfate dissociates in water into copper(II)
cations and sulfate anions
CuSO4(aq)  Cu+2(aq) + SO4-2(aq)
Potassium sulfate dissociates in water into potassium
cations and sulfate anions
K2SO4(aq)  2 K+1(aq) + SO4-2(aq)
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Electrolytes
Electrolytes are
substances whose
water solution is a
conductor of
electricity
All electrolytes have
ions dissolved in
water
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Electrolytes
Strong electrolytes: all electrolyte
“units” are separated into ions;
includes soluble salts, strong
acids, strong bases
Nonelectrolytes: none are
separated into ions; includes
covalent solutes
Weak electrolytes: a small
percentage are separated into
ions; includes weak acids & weak
bases
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Types of Electrolytes
Salts = ionic compounds; soluble salts are all
strong electrolytes
Acids = form H+ ions in water solution. Strong
acids dissoc 100%  strong electrolytes; weak
acids dissoc <10%  weak electrolytes
Bases = water soluble metal hydroxides.
Strong bases dissoc 100%  strong electrolytes;
weak bases dissoc <10%  weak electrolytes
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When will a Salt Dissolve?
Soluble: if it dissolves in a
specified solvent
- NaCl is soluble in water, but
AgCl is not
Insoluble: if a significant
amount does not dissolve in
the specified solvent
- AgCl is insoluble in water
(though there is a very small
amount dissolved, but not
enough to be significant)
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Solubility Rules
Compounds that are Generally Soluble in
Water
Compounds Containing
the following Ions are
Generally Soluble
Exceptions
Li+, Na+, K+, NH4+
none
NO3–, C2H3O2–
none
Cl–, Br–, I–
Ag+, Hg22+, Pb2+
SO42–
Ca2+, Sr2+, Ba2+, Pb2+
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Solubility Rules
Compounds that are Generally Insoluble
Compounds Containing
the following Ions are
Generally Insoluble
Exceptions
(slightly soluble)
OH–
Li+, Na+, K+, NH4+,
Ca2+, Sr2+, Ba2+
Li+, Na+, K+, NH4+,
Ca2+, Sr2+, Ba2+
Li+, Na+, K+, NH4+
S2–
CO32–, PO43–
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Five Main Types of Chemical Equations
Combination
Decomposition
Single-Replacement
Double-Replacement
Combustion (I added this!)
(Ignore sections 9 & 10 and use our lecture
notes!)
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TYPES OF CHEMICAL REACTIONS:
1. COMBINATION (OR SYNTHESIS):
A + B C
2 K + S  K2S
2 Na + Br2  2 NaBr
MEMORIZE THESE TWO:
H2O + CO2  H2CO3
NH3 + H2O  NH4OH
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TYPES OF CHEMICAL REACTIONS:
2. DECOMPOSITION:
CA+B
2 KClO3  2 KCl + 3 O2
2 H2O2  2 H2O + O2
Memorize these two:
H2CO3  H2O + CO2
NH4OH  NH3 + H2O
(What is similar about these two equations
and the ones in the first type?)
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TYPES OF CHEMICAL REACTIONS:
3. SINGLE REPLACEMENT: A + BC  B + AC
K(s) + ZnSO4(aq)  Zn(s) + K2SO4(aq)
Al(s) + FeCl3(aq)  Fe(s) + AlCl3(aq)
Replacement occurs only if solid metal element is
more reactive than metal ion which is part of the
compound
ACTIVITY SERIES (partial):
K Ca Mg Al Zn Fe Sn H Cu Ag Au
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An atom of an element in the activity series will displace an
atom of an element below it from one of its compounds .
increasing
activity
Metals
K
Ca
Na
Mg
Al
Zn
Sodium (Na) will displace an
Fe
atom below it from one of its
Ni
Sn
compounds.
Pb
H
Cu
Ag
Hg
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EXAMPLES USING ACTIVITY SERIES:
Metal Higher in Activity Series Displacing Metal Below It
Mg(s) + PbS(s)  MgS(s) + Pb(s)
Metals
Mg
Al
Zn
Fe
Ni
Sn
Pb
Magnesium is above lead
in the activity series.
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Metal Lower in Activity Cannot Displace Metal Above It
Ag(s) + CuCl2(aq)  no reaction
Metals
Pb
H
Cu
Ag
Hg
Silver is below copper in
the activity series.
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EXAMPLES USING ACTIVITY SERIES:
Halogen Higher in Activity Series Displaces Halogen Below It
Cl2(g) + CaBr2(s)  CaCl2(aq) + Br2(aq)
Halogens
F2
Cl2
Br2
I2
Chlorine is above bromine
in the activity series.
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Using the Activity Series
SEE IF THESE REACT: if they do, then determine
the product formulas and balance the equations!
___Ca(s) + ___MgCO3(aq) 
___Zn(s) + ___HCl(aq) 
___Mg(s) + ___AgNO3(aq) 
___Ca(s) + ___HOH(l) 
___Cu(s) + ___Fe(NO3)3(aq) 
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TYPES OF CHEMICAL REACTIONS:
(continued)
4. DOUBLE REPLACEMENT: trades partners cation to
anion!
AB + CD  AD + CB
A. Precipitation reactions: must learn how to use the
solubility rules!
NaCl(aq) + AgNO3(aq)  NaNO3(aq) + AgCl(s)
B. Gas-forming reactions:
2 HNO3(aq) + MgS(aq)  H2S(g) + Mg(NO3)2(aq)
C. Neutralization (acid-base) reactions:
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
IN EVERY DOUBLE REPLACEMENT REACTION, A SOLID,
LIQUID OR GAS HAS FORMED!
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TYPES OF CHEMICAL REACTIONS:
(continued)
4.A. A precipitate is two ions combining to
make a compound that is not soluble in
water
MgCl2(aq) + 2 NaOH(aq)  Mg(OH)2(?) + 2 NaCl(?)
One of the two products
will be insoluble
Use tables and rules to determine which is
insoluble and will precipitate.
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Types of Chemical Reactions
(continued)
Practice finding which product is insoluble
__Pb(NO3)2(aq) + __KI(aq)  __PbI2(?) + ___KNO3(?)
Practice: what compound precipitates when
aqueous solutions of silver nitrate and
lithium bromide are mixed?
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TYPES OF CHEMICAL REACTIONS:
(continued)
4.B. Gas-forming reactions: may be double
replacement, but also others.
Na2S(aq) + 2 HCl(aq)  2 NaCl(aq) + H2S(g)
All carbonate compounds react with any acid to
form CO2(g)
If two compounds react to form H2CO3,
then it decomposes: H2CO3(aq)  H2O(l) + CO2(g)
EXAMPLE:
NaHCO3(aq) + HCl(aq)  NaCl(aq) + H2CO3(aq)
 NaCl(aq) + H2O(l) + CO2(g)
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TYPES OF CHEMICAL REACTIONS:
(continued)
4.C. Acid-base (neutralization) reactions
usually form water and a salt
HCl(aq) + NaOH(aq)  HOH(l) + NaCl(aq)
H2SO4(aq) + 2 KOH(aq)  2 HOH(l) + K2SO4(aq)
You try:
___H3PO4(aq) + ___Ca(OH)2(aq)  _____
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TYPES OF CHEMICAL REACTIONS:
5. COMBUSTION: reacting very rapidly and
exothermally with O2 gas
S(s) + O2(g)  SO2(g)
(+ HEAT)
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g)
(+HEAT)
2 H2(g) + O2(g)  2 H2O(g)
(+ HEAT)
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g)
(+HEAT)
***Memorize: all hydrocarbon combustion
products are the same: CO2 and H2O
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PRACTICE: WRITE THE COMPLETE BALANCED
CHEMICAL EQUATION
& ID THE TYPE OF REACTION
1. Aqueous solutions of magnesium chloride &
sodium hydroxide form magnesium hydroxide
and sodium chloride (you decide on phases)
2. Decomp of HCl gas to its elements
3. Zinc metal reacts with aqueous nitric acid
4. Ammonia gas plus water (Hint: combination)
5. Aqueous solutions of sulfuric acid + calcium
hydroxide react
6. Aqueous solutions of aluminum chloride and
zinc bicarbonate react
7. Methane gas burns in air
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PRACTICE: WRITE THE COMPLETE BALANCED
CHEMICAL EQUATION
& ID THE TYPE OF REACTION
1. MgCl2(aq) + 2 NaOH(aq)  Mg(OH)2(s) + 2 NaCl(aq)
Double replacement - ppt
2. 2 HCl(g)  H2(g) + Cl2(g) Decomposition
3. Zn(s) + 2 HNO3(aq)  Zn(NO3)2(aq) + H2(g)
Single replacement: Zn > H
4. NH3(g) + H2O(l)  NH4OH(aq) Combination (memorize)
5. H2SO4(aq) + Ca(OH)2(aq)  CaSO4(aq) + 2 H2O(l)
Double replacement - neut
6. 2AlCl3(aq) + 3Zn(HCO3)2(aq)  2 Al(HCO3)3(s) + 3 ZnCl2(aq)
Double replacement - ppt
7. CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g) Combustion HEAT!
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NET IONIC EQUATIONS
REFER TO HANDOUTS!!
SUBSTITUTE “normal chemical equation” for
“molecular equation in your textbook
PRACTICE, PRACTICE, PRACTICE!
Memorize the six strong acids (HCl, HBr,
HI, HNO3, H2SO4, HClO4) and what makes
a strong base (Group I and Group II
oxides and hydroxides)
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Heat (Enthalpy) in
Chemical Reactions (take good notes, not in
your chapter)
Energy changes always accompany chemical
reactions.
When this occurs energy is released to the
surroundings.
The amounts of substances are expressed in
moles.
H2(g) + Cl2(g) → 2 HCl(g) + 185 kJ (exothermic)
N2(g) + O2(g) + 185 kJ → 2 NO(g) (endothermic)
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Energy of Activation
A certain amount of energy is always
required for a reaction to occur.
The energy required to start a reaction is
called the energy of activation.
CH4 + 2 O2 → CO2 + 2 H2O + 890 kJ
This reaction will not occur unless activation
energy is supplied.
The activation energy can take the form of a
spark or a flame.
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8.2
8.1
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