Transcript 9glencoe chem chap 9 students

Chemical Reactions
Objectives:
1.
Recognize evidence of
chemical change
2.
Represent chemical
reactions with equations
3.
Balance chemical equations
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Chemical reactions are
represented by balanced
chemical equations
Definition: a chemical
reaction is the process by
which the atoms of one or
more substance are
rearranged to form different
substances
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Evidence of chemical change
can be indicated by a
temperature change
Other examples include color
change, production of a gas
(usually indicated by
bubbles), and the formation
of a solid called a precipitate
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Definition: reactants are the
starting substances in a
chemical reaction
Reactants are separated from
each other by plus signs on
the left side of the equation
Definition: products are the
substances formed during the
reaction
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Products are separated from
each other by a plus sign on
the right side of the equation
Reactants and products are
separated from each other by
an arrow
The arrow means “yields”,
“react to produce”,
“produces”, or other similar
meanings
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Definition: word equations are
statements indicating reactant
and products of chemical
reactions
Ex: Aluminium + Hydrochloric
acid  Aluminium chloride +
Hydrogen
Compare the word equation to
the chemical equation for the
same compounds
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(skeleton equation): Al +
HCl  AlCl3 + H2
This skeleton chemical equation
is not balanced and does not give
the phase:
2Al(s) + 6HCl(aq) 2AlCl3(s) +
3H2(g) (this is balanced)
Definition: a skeleton equation
uses chemical formulas rather
than words
A skeleton equation includes the
phase but is not balanced
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Definition: a chemical
equation is a statement that
uses chemical formulas to
show the identities and
relative amounts of the
substances involved in a
chemical reaction
Definition: a coefficient in a
chemical equation is a whole
number written in front of a
reactant or a product
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1. Write the skeleton equation
2. Count the atoms of the
elements in the reactants
3. Count the atoms of the
elements in the products
4. Change the coefficients to
make the number of atoms of
each element equal on both
sides of the equation
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5. WRITE THE COEFFICENTS IN
THE LOWEST POSSIBLE RATIO
6. Check your work! Does it
pass the laugh test??
Finally, all chemical equations
obey the law of conservation of
mass
The mass of all the reactants
will always equal the mass of
all the products in a chemical
equation
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Objectives:
1. Classify chemical
reactions
2. Identify the characteristics
of different classes of
chemical reactions
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There are many types of chemical
reactions & not all fit neatly into a
pattern
By learning to recognize patterns
of chemical behavior, you can
learn to classify chemical
compounds
Recognizing patterns, however,
will help to learn to predict
products
Five types we will study include
synthesis, decomposition,
combustion, single replacement
and double replacement
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Definition: a synthesis
reaction is a chemical
reaction in which two or more
substances produce a single
product
Synthesis reactions are
sometimes called combination
reactions because the
reactants combine with each
other to produce a single
product
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The reactants of the most
common combination
reactions are either 2
elements or 2 compounds
The product of a combination
reaction is always a
compound
Fe(s) + S(s) → FeS(s) iron(II)
sulfide OR
2Fe(s) + 3S(s) → Fe2S3(s)
iron(III) sulfide
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Some nonmetal oxides react
with water to produce acid:
SO2(g) + H2O(l) → H2SO3(aq)
sulfurous acid
Some metallic oxides react
with water to produce a base:
CaO(s) + H2O(l) → CaOH2(aq)
calcium hydroxide
When two nonmetals
combine, more than one
product might be possible:
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S(s) + O2(g)→ SO2(g) sulfur
dioxide
2S(s) + 3O2(g)→ 2SO3(g)
sulfur trioxide
Other examples of 2
compounds combining to
become 1 product:
CaO(s) + H2O(l) →Ca(OH) 2(s)
2Na + Cl2(g) → 2NaCl(s)
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Some substances break down into
simpler compounds when they
react
These reactions may require heat
or energy (from heat, light or
electricity)
Definition: In decomposition
reactions, a single compound is
broken down into two or more
products
In some ways a decomp reaction is
the opposite of a synthesis
reaction
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The products can be any
combination of elements and
compounds
It can be difficult to predict the
products of decomposition
reactions
Rapid decomp reactions that
produce gaseous products can
cause explosions
Ex: CaCO3 → CaO(s) + CO2(g)
Ex: NH4NO3(s) →N2O(g) +2H2O(g)
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Definition: a combustion reaction
is when an element or compound
reacts with oxygen, often
producing energy as heat and light
If the supply of oxygen is
insufficient during a reaction,
combustion will be incomplete
Many reactions with oxygen as a
reactant are classified as
combustion reactions
Ex:C(s) + O2(g) → CO2(g)
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We will study the hydrocarbon plus
oxygen reactions
Combustion reactions can be tough to
balance because you have O2 as a
reactant (2 oxygens) and H2O as a
product (1 oxygen)
2C2H18(l) + 25 O2(g)→16CO2 + 18H2O(l)
The products will be carbon dioxide
and water
There are exceptions, but for now,
look for the oxygen on the reactant
side, & carbon dioxide and water on
the product side for the
hydrocarbon/oxygen reactions
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Definition: in single replacement reactions, the atoms of
one element replace the atoms
of a second element in a
compound
These are also called single
displacement reactions
Ex: potassium replaces
hydrogen (which is released as
a flammable gas)-2K(s) +
2H2O(l)→2KOH +H2(g)
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Whether one metal displaces
another metal or hydrogen is
determined by the activity series
The activity series lists metals in
order of decreasing reactivity
A reactive metal will replace any
metal listed below it in the
activity series
Look at the metallic element first
& see if the element replaces the
metal in the compound
Mg(s) +
Zn(NO3)2(aq) →
Mg(NO3)2(aq) + Zn(s)
Mg(s) + 2AgNO3(aq)
→ Mg(NO3)2(aq) +
2Ag(s)
Mg(s) + LiNO3 →
no reaction
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The halogens are their own
activity series in which
nonmetal replaces nonmetal
The reactivity is determined
by the periodic table
Go down the halogen column
with fluorine being the most
reactive and iodine the least
reactive
Ex: F2(g) + 2NaBr(aq)→2NaF +Br2(l)
Ex: Br2(g) + 2NaF(aq)→ NR
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Double replacement
(displacement) reactions involve
an exchange of positive ions
between two reacting compounds
All double replacement reactions
produce either water, a precipitate
or a gas
Without those products, you get NR
1. one product is slightly soluble
and precipitates:
Na2S(aq) + Cd(NO3)2(aq) → CdS(s) +
2NaNO (aq
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Definition: a precipitate is a solid
that settles out a liquid mixture
Precipitate formation involves a
change of state (liquid → solid) &
signifies a chemical reaction
2. One product is a gas that
bubbles out of the mixture:
2NaCN(aq) + H2SO4(aq) → 2HCN(g)
+ Na2SO4(aq)
3. one product is a molecular
compound such as water:
Ca(OH)2(aq) + 2HCl(aq) → CaCl2(aq)
+ 2H2O(l)
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Synthesis reactions (baking a
cake) have the general
equation:
A + B → AB 2 or more
substances become 1 product
Decomposition reactions (CSI
stinky dead stuff) have the
general equation:
RS → R + S 1 substance
becomes 2 or more
substances
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Combustion reactions (fiery) have
the general equation:
CxHy + (x + y/4)O2 → CO2 +
(y/2)H2O
Single replacement reactions
(activity series) have the general
equation:
T + RS → TS + R an element &
compound react
Double replacement reactions
(must have a change to state to
occur) have the general equation:
R+S- + T+U- → R+U- + T+S- 2
compounds react
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Objectives:
1. Describe aqueous solutions
2. Write complete ionic and net
ionic equations for chemical
reactions in aqueous solutions
3. Predict whether reactions in
aqueous solutions will produce a
precipitate, water or a gas
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Definition: an aqueous solution contains
one or more substances called solutes
dissolved in water
Water is the solvent
Definition: a solvent is the most plentiful
substance in a solution
Definition: a solute is the least plentiful
substance (often a solid salt) in a
solution
There will be no reaction in aqueous
solution unless a product has undergone
a phase change
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Some solutes are molecular such
as sucrose and ethanol
These solutes exist as molecules
in aqueous solutions
Some molecular solutes like
hydrogen chloride form ions in
aqueous solution
Molecular solids that form ions in
aqueous solutions are acids
Ionic compounds might be
solutes in aqueous solutions
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Definition: a precipitate is a solid
formed from mixing 2 aqueous
solutions
Aqueous double-replacement
reactions tend to form ionic
compounds by their formula units
AgNO3(aq) + NaCl(aq)→ AgCl(s) +
NaNO3(aq)
Definition: a complete ionic
equation is an ionic equation that
shows all the particles in a solution
as they exist
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Definition: net ionic equations are
ionic equations that include only
those particles that participate in
the reaction
Definition: spectator ions are
particles that do not participate in
the reaction
We can write the complete ionic
equation to show the cations and
anions:
Ag+(aq) + NO3-(aq) + Na+(aq) + Cl(aq) → AgCl(s) + Na+(aq) + NO -(aq)
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We can also eliminate ions that
do not directly participate in the
reaction
In order to be involved in the
reaction, the ion must undergo a
change of state (phase change)
Therefore, you must include the
state when writing these
equations
Spectators will appear on both
sides of the equation as aqueous
ions
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Here is the same reaction with
the spectator ions removed:
Ag+(aq) + Cl-(aq) → AgCl(s)
This is the called the net ionic
equation
The net ionic equation shows only
the particles that actually take
part in the reaction
You must balance the ionic
charge in these equations
The net ionic charge on each side
of the equation must be zero
Q: How are ionic equations
different from chemical
equations?
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Look at this reaction:
Pb(s) + AgNO3(aq) → Ag(s) +
Pb(NO3)2(aq)
The nitrate is a spectator
The net ionic equation is
Pb(s) +
Ag+ → Ag(s) + Pb2+(aq) ***this is
unbalanced***
The atom numbers are ok, but the ionic
charges are not, so coefficients are
used to balance the charges:
Pb(s) + 2Ag+ → 2Ag(s) + Pb2+(aq) **now
it is balanced**
Generally single and double
replacement reactions can be written
as ionic equations
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Use a solubility rules chart to determine
precipitate formation
Q: Will we get a precipitate when
aqueous solutions of Ba(NO3)2(aq) and
Na2SO4(aq) are mixed?
The new compounds might be NaNO3
and BaSO4
The equation looks like this: 2Na+(aq) +
SO42-(aq) + Ba2+(aq) + 2NO3-(aq) →
BaSO4(s) + 2Na+(aq) + 2NO3-(aq)
the sodium and nitrate are spectators,
so the net ionic equation is
2+
2-
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Ba2+(aq) + SO42-(aq)
→ BaSO4(s)
Q: Why is there no
concern about Na+
and SO42- or the Ba2+
and NO3- forming a
precipitate?
A: These
combinations of
ions will not form a
precipitate
according to the
solubility table
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Unlike precipitate reactions, no
reaction is observable when
water forms
Chem rxn: HBr(aq) + NaOH(aq) →
H2O(l) + NaBr(aq)
Complete ionic: H+(aq) + Br-(aq)
+Na+(aq) + OH-(aq) → H2O(l) +
Na+(aq) + Br-(aq)
Net ionic: H+(aq) + OH-(aq) →
H2O(l)
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Common gases produced are
HCN, CO2 & H2S
Chem rxn: 2HI + Li2S(aq)
→H2S(g) +2Li(aq)
Complete ionic: 2H+(aq) + 2I(aq) + 2Li+(aq) + S2-(aq) →
H2S(g) + 2Li+(aq) + 2I-(aq)
Net ionic: 2H+(aq) + S2-(aq) →
H2S(g)