Transcript Precipitation and Redox Reactions

Precipitation and Redox Reactions
How Reactions Occur
Objectives
1. Understand How:
1. Precipitation Reactions Occur
2. Redox Reactions Occur
2. Be Able to Predict:
1. The Products of a Reaction.
2. Whether a Reaction will Occur or not.
OBJECTIVE 1: Precipitation Reactions
• Understand:
• What a Precipitation Reaction is.
• What makes a Precipitation Reaction Occur.
• How the compounds swap Negative Ions.
• Use Formula Writing and Solubility Rules to
predict products of a Precipitation Reaction
Precipitation Reactions
• What is a Precipitant?
• A Product of a chemical reaction that is a:
• Solid
• Gas
• Or Water (H2O) NOT AQUEOUS!!!!!!!!!!
• A Product must be INSOLUABLE.
• How do you know if a product is insoluble?
• Look on your solubility chart.
Remember?
1. Chemical reactions start with REACTANTS and make
PRODUCTS
2. Five Signs of a Chemical Change
1. Gas Given Off (Bubbles)
2. Color Change (BIGGIE!!!!!!)
3. Heat is Produced or Absorbed
4. Odor Change
5. Precipitate Forms (Solid is Made)
Precipitation Reactions
1. Also known as Double Replacement Reactions
2. Two or more aqueous Reactants are mixed to
form one or more Products that at least one is:
1. A solid (insoluble)
2. A gas
3. Or WATER!!
How Do I Know the
States of Matter
1. Find the Ion that is in the
compound that is on the list in the
first column.
2. Determine if that ion is normally
soluble (aq) or Insoluble (s)
3. Look to see if the exception is
present in the compound. If the
exception is present then the
opposite is true.
4. Example:
1. CaCO3: CO3-2 is insoluble,
Ca is not an exception so the
compound is written
CaCO3(s)
2. Na2CO3: CO3-2 is insoluble,
Na is an exception so the
compound is written
Na2CO3(aq)
Precipitation Reactions occur
1. when metals in two different compounds swap
nonmetals.
2. Compound + compound  compound + compound
AX + BY  AY + BX
3. You MUST write NEW Formulas on every compound
involved!
4. Determine if a product is a Solid, Gas or H2O
1. If all products are aqueous (aq) then NOTHING
HAPPENS.
Let’s Review that Formula Writing Thing
• Copper (II) Chloride
• Copper (II) means?
• Copper has a charge of +2
• SO: Cu+2
• Chloride means?
• Chlorine has a charge of -1
• How do I know? Chlorine is in Group 17 and all of
them are -1.
• It takes two Chlorines to cancel out Copper’s +2 charge:
• CuCl2
• Remember: Drop and Swap the Numbers
Let’s look at a reaction
• Copper (II) Chloride is mixed with Silver Nitrate.
• Again, Copper (II) Chloride is CuCl2
• Silver Nitrate is:
• Silver is always +1 so: Ag+
• Nitrate is always: NO3-1
• So: AgNO3
• AX
+
BY

AY
+
BX
• CuCl2
+ 2 AgNO3 
Cu(NO3)2 + 2 AgCl
• Copper (II) Chloride + Silver Nitrate 
Copper (II) Nitrate + Silver Chloride
• The last names swap
Does this reaction happen (STATES OF
MATTER – MATTER!!!!)
•
•
•
•
•
•
•
•
Check your states of matter
CuCl2 is soluble so, CuCl2 (aq)
AgNO3 is soluble so, AgNO3 (aq)
Cu(NO3)2 is soluble so Cu(NO3)2 (aq)
AgCl is insoluble so AgCl (s)
CuCl2(aq) + 2 AgNO3(aq)  Cu(NO3)2(aq) + 2 AgCl(s)
A solid formed so a reaction will occur
NOTICE: Cl had a 2 subscript in the reactants but not in
the products, why?
• Every compound is different!!
• You must Drop and Swap everytime!
Let’s try another
• Lead (II) Nitrate is mixed with Sodium Bromide
• Write the correct formulas for the reactants:
• Pb(NO3)2
• NaBr
• Swap last names:
• Lead (II) Bromide
• Sodium Nitrate
• Write the correct formulas for the products:
• PbBr2
• NaNO3
• Pb(NO3)2 + 2 NaBr  PbBr2 + 2 NaNO3
Continue
• Pb(NO3)2 + 2 NaBr  PbBr2 + 2 NaNO3
• Check your states of matter
• WAIT A MINUTE, where did the in front 2’s come
from?????
• BALANCING
• Pb(NO3)2 + 2 NaBr  PbBr2 + 2 NaNO3
• All Nitrates are soluble
• All Bromides are soluble except Ag+, Pb+2, Hg+2
• So,
• Pb(NO3)2 (aq) + 2NaBr (aq)  PbBr2 (s) + 2NaNO3 (aq)
• A solid formed so a reaction occurs
Precipitation Reactions
• Think about it like “foil”ing in algebra, first and last ions go
together + inside ions go together
• Example:
AgNO3(aq) + NaCl(s)  AgCl(s) + NaNO3(aq)
Another example:
K2SO4(aq) +
Ba(NO3)2(aq)  2 KNO3(aq) + BaSO4(s)
Practice
•
1.
2.
3.
4.
5.
6.
Predict the products. Balance the equation
HCl(aq) + AgNO3(aq) 
CaCl2(aq) + Na3PO4(aq) 
Pb(NO3)2(aq) + BaCl2(aq) 
FeCl3(aq) + NaOH(aq) 
H2SO4(aq) + NaOH(aq) 
KOH(aq) + CuSO4(aq) 
Ionic Equations
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•
•
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Compounds that are (aq) have been torn apart by water.
AgNO3 (aq) is really Ag+ (aq) + NO3- (aq)
CaCl2 (aq) is really Ca+2 (aq) + 2 Cl- (aq)
In Ionic Equations we show this, the ions are shown
separately
• Solids and gases are kept together
• There are two types:
• Complete Ionic and Net Ionic
Complete Ionic Equations
• 2 AgNO3 (aq) + CaCl2 (aq) 
2 AgCl (s) + Ca(NO3)2 (aq)
• Really Like This:
2 Ag+(aq) + 2 NO3-(aq) + Ca+2(aq) + 2 Cl-(aq) 
2 AgCl (s) + Ca+2(aq) + 2 NO3-(aq)
• NOTICE:
• The AgCl (s) is not torn apart.
• Solids, Gases and Water are kept together.
• Only things that are (aq) are torn apart.
Net Ionic Equations
• Products that are (aq) don’t make the reaction
happen.
• They are there, but they just watch the action.
• Spectator Ions: Ions that are (aq) on both
sides of the reaction. They are there but they
just watch the game.
• The Solid, Gas, or Water as a PRODUCT make
the reaction happen.
• In a Net Ionic Equation we focus on the solid,
gas or water, and what make them. Ignore the
spectators.
• Forget the Crowd, focus on the players.
Net Ionic Equations
• Let’s go back to our Complete Ionic Equation.
2 Ag+(aq) + 2 NO3-(aq) + Ca+2(aq) + 2 Cl-(aq) 
2 AgCl (s) + Ca+2(aq) + 2 NO3-(aq)
• Let’s find sort out the actors and the spectators.
• Look at the Products, find the solid, gas, or water.
• 2 AgCl (s)
• Look at the reactants, find what made it.
• 2 Ag+ (aq) and 2 Cl- (aq)
• Put them together.
• 2 Ag+ (aq) and 2 Cl- (aq)  2 AgCl (s)
• Forget the Spectator Ions, they just watch and complain about
the players and refs anyway.
Steps for Writing Ionic Equations
1.
2.
3.
4.
5.
6.
Write the complete reaction.
Break apart everything that is (aq).
Write the Complete Ionic Equation.
Find the Solid, Liquid, or Water.
Find what made the solid, liquid or water.
Write the Net Ionic Equation.
Practice
Predict the products. Balance the equation. Write the net
ionic equation
1. HCl(aq) + AgNO3(aq) 
2. CaCl2(aq) + Na3PO4(aq) 
3. Pb(NO3)2(aq) + BaCl2(aq) 
4. FeCl3(aq) + NaOH(aq) 
5. H2SO4(aq) + NaOH(aq) 
6. KOH(aq) + CuSO4(aq) 
Objective 2: Oxidation-Reduction
(REDOX) Reactions
LEO SAYS GER
OBJECTIVES
1. Define oxidation and reduction in terms of the
loss or gain of oxygen, and the loss or gain of
electrons.
2. State the characteristics of a redox reaction and
identify the oxidizing agent and reducing agent.
Oxidation and Reduction (Redox)
Early chemists saw “oxidation” reactions only as the
combination of a material with oxygen to produce
an oxide.
For example, when gasoline burns in air, it oxidizes
and forms oxides of carbon and hydrogen (oxides
are compounds containing Oxygen, duh)
2 C8H18 + 25 O2  16 CO2 + 18 H2O
BUT……..
• Not all oxidation processes that use oxygen
involve burning:
• Elemental iron slowly oxidizes to compounds
such as iron (III) oxide, commonly called “rust”
6 Fe (s) + 3 O2 (g)  2 Fe2O3 (s)
• Hydrogen peroxide also releases oxygen
when it decomposes
2 H2O2 (aq)  2 H2O (l) + O2 (g)
Oxidation and Reduction (Redox)
A process called “reduction” is the opposite
of oxidation, and originally meant the loss of
oxygen from a compound
Oxidation and reduction always occur
simultaneously
The substance gaining oxygen (or losing
electrons) is oxidized, while the substance
losing oxygen (or gaining electrons) is
reduced.
Oxidation and Reduction (Redox)
Today, many of these reactions may not
even involve oxygen
Redox currently says that electrons are
transferred between reactants
(MgS)
Mg + S → Mg2+ + S2-  MgO (s)
•The magnesium atom (which has zero charge) changes to a
magnesium ion by losing 2 electrons, and is oxidized to Mg2+
•The sulfur atom (which has no charge) is changed to a
sulfide ion by gaining 2 electrons, and is reduced to S2-
Oxidation and Reduction (Redox)
0
1
0
1
2 Na  Cl 2  2 Na Cl
Each sodium atom loses one electron:
1
0
Na  Na  e

Each chlorine atom gains one electron:
0

1
Cl  e  Cl
How do I know if REDOX occurs
• If one (or more) element(s) in the reaction loses
electrons (LEO) and one (or more) element(s) in
the reaction gains electrons (GER) then REDOX
has occurred.
• It is that simple:
• Write the equation
• Determine the charges on every element
• See if LEO-GER has occurred
Assigning Oxidation Numbers
• An “oxidation number” is a positive or
negative number assigned to an atom to
indicate its degree of oxidation or reduction.
• Generally, a bonded atom’s oxidation
number is the charge it would have if the
electrons in the bond were assigned to the
atom of the more electronegative element
Rules for Assigning Oxidation Numbers
1) The oxidation number of any
uncombined element is zero.
2) The oxidation number of a
monatomic ion equals its charge.
0
0
1
1
2 Na  Cl 2  2 Na Cl
Rules for Assigning Oxidation Numbers
3) The oxidation number of oxygen in
compounds is -2, except in
peroxides, such as H2O2 where it is -1.
4) The oxidation number of hydrogen in
compounds is +1, except in metal
hydrides, like NaH, where it is -1.
1
2
H2O
Rules for Assigning Oxidation Numbers
5) The sum of the oxidation numbers of the
atoms in the compound must equal 0.
1
2
H2O
2(+1) + (-2) = 0
H
O
2
2 1
Ca(O H ) 2
(+2) + 2(-2) + 2(+1) = 0
Ca
O
H
Rules for Assigning Oxidation Numbers
6) The sum of the oxidation numbers in
the formula of a polyatomic ion is equal
to its ionic charge.
? 2
N O3

X + 3(-2) = -1
N
O
thus X = +5
? 2
S O4
2
X + 4(-2) = -2
S
O
thus X = +6
Reducing Agents and Oxidizing Agents
•An increase in oxidation number = oxidation
• A decrease in oxidation number = reduction
1
0
Na  Na  e

Sodium is oxidized – it is the reducing agent
0

1
Cl  e  Cl
Chlorine is reduced – it is the oxidizing agent
LEO says GER :
Lose Electrons = Oxidation
1
0
Na  Na  e

Sodium is oxidized
Gain Electrons = Reduction
0

1
Cl  e  Cl
Chlorine is reduced
LEO says GER :
- Losing electrons is oxidation, and the substance
that loses the electrons is called the reducing agent.
- Gaining electrons is reduction, and the substance
that gains the electrons is called the oxidizing
agent.
Mg(s) + S(s) →
Mg is the
reducing
agent
S is the
oxidizing
agent
MgS(s)
Mg is
oxidized:
loses e-,
becomes a
Mg2+ ion
S is reduced:
gains ebecomes a S2ion
Identifying Redox Equations
In general, all chemical reactions can be
assigned to one of two classes:
1) oxidation-reduction (REDOX), in which
electrons are transferred:
•
•
•
•
Single-replacement – Where an element replaces another
element in a compound (AX + B  BX + A)
Combination – Where two or more elements combined to form
a single compound (A + X  AX)
Decomposition – Where a single compound breaks down into
it’s individual elements (AX  A + X)
Combustion – Where something burns in the air.
Non REDOX
This second class has no electron transfer, and
includes all others:
• Precipitation Reactions (Double-replacement)
(AX + BY  AY + BX)
• Acid-base reactions (HX + AOH  AX + H2O)
REDOX Reactions - Single Replacement
(AX + B  BX + A)
•
•
•
•
•
•
•
Copper (II) Chloride reacts with Lithium to produce
Lithium Chloride and Copper
+2 -1
0
+1 -1
0
CuCl2 (aq) + 2 Li (s)  2 LiCl (aq) + Cu (s)
You have to use the Activity Series on the back of your
Periodic Table to determine if this will happen.
If the element by itself is above the element in the
compound then the reaction will occur.
If the element in the compound is above the element by
itself, then no reaction will occur.
Remember, METALS REPLACE METALS,
NONMETALS REPLACE NONMETALS.
REDOX Reactions - Single Replacement
(AX + B  BX + A)
• Practice:
• Solid Copper is mixed with aqueous Silver Nitrate
• Copper is above Silver on the Activity Series, so a
reaction will occur (a copper (II) compound is formed)
• Cu (s) + 2 AgNO3 (aq)  Cu(NO3)2 + 2 Ag
• Check
• Reactants Cu = 0
Products
Cu = +2
Ag = +1
Ag = 0
N=+5
N=+5
O=-2
O=-2
• Notice N and O didn’t change only Cu and Ag
ACTIVITY SERIES
CHART
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•
•
•
•
In a Single Replacement reaction,
you look at the metals in the
Reactants.
If the individual metal is higher on
the Activity Series Chart then the
reaction WILL Occur.
If the individual metal is lower on
the Activity Series Chart, then the
reaction WILL NOT occur.
I always have students say, what
do you mean higher on the chart?
I MEAN HIGHER, ABOVE, FIRST.
Lithium is higher (above) Barium.
REDOX Reactions - Combination
(A + X  AX)
• Sodium and Chlorine react to produce Sodium Chloride
•
0
0
+1 -1
• 2 Na (s) +
Cl2 (g) 
2 NaCl (s)
• These normally will occur, so if I tell you two elements
combine, then place them together and drop and swap
the Oxidation numbers (charges).
REDOX Reactions - Combination
(A + X  AX)
• Practice
• Ca (s) + O2 (g) 
• Pb (s) + N2 (g) 
REDOX REACTIONS – Decomposition
(AX  A + X)
• Water is decomposed to form Hydrogen and Oxygen
Gas
• +1; -2
0
0
• 2 H2O (l) 
2 H2 (g) + O2
REDOX REACTIONS – Decomposition
(AX  A + X)
• These will not normally occur, but if I tell you that a
compound decomposes into it’s elements, then break
the compound apart.
REDOX REACTIONS – Decomposition
(AX  A + X)
• Practice
• Lead (II) Oxide is decomposed into it’s elements
• Copper (I) Nitride is decomposed into it’s elements