Download Unit5C - OCCC.edu

Recap
Precipitation Reactions:
ions combine to form insoluble products
Neutralization Reactions:
H+ ions and OH- ions combine to form H2O
Next:
Oxidation-Reduction (Redox) Reactions:
Atoms or ions donate or accept electrons
Redox Reaction
• Corrosion of a metal
battery terminal
iron rusting
metal
Oxygen,
acid
Metal Corrosion
• metal loses electron
forms a cation
= metal is oxidized
AND another reactant gains electrons
= is reduced
Oxidation-Reduction Reactions
• An oxidation occurs
when an atom or ion
loses electrons.
• A reduction occurs
when an atom or ion
gains electrons.
• One cannot occur
without the other.
Redox Reactions
• Oxidation-Reduction Reactions (Redox Reactions)
– reactions that involve the transfer of electrons
between two reactants
– an element in one reactant is oxidized while an
element in another reactant is reduced
Mg (s) + 2 H+ (aq)  Mg2+ (aq) + H2 (g)
oxidized
reduced
Redox Reactions
• Oxidation:
– the loss of electrons
• chemical species becomes more positively
charged
– the gain of oxygen
– the loss of hydrogen
Redox Reactions
• Reduction:
– the gain of electrons
• the chemical species becomes more negatively
charged
– the gain of hydrogen
– the loss of oxygen
Redox Reactions
GER
LEO
LEO says GER
• LEO:
– Lose Electrons
Oxidation
• GER:
– Gain Electrons
Reduction
Redox Reactions
Oil :
 Oxidation Involves
Loss of eRig :
 Reduction Involves
Gain of eOil Rig
Redox Reactions
• Electrons are not explicitly shown in chemical equations.
• Oxidation Numbers are used to keep track of electrons
gained and lost during redox reactions.
• Oxidation number
– a hypothetical number assigned to an individual atom
present in a compound using a set of rules.
• May be positive, negative, or zero
Rules for Oxidation Numbers
• Oxidation numbers are always reported for individual
atoms or ions not groups of atoms or ions!!!!!!!!!!!
• For an atom in its elemental form, the oxidation number
is always zero.
– H2: oxidation # = 0 for each H atom
– Cu: oxidation number = 0
– Cl2: oxidation # = 0 for each Cl atom
Rules for Oxidation Numbers
• For any monoatomic ion:
oxidation # = charge on ion
• K+ oxidation # = +1
• Cl- oxidation # = -1
• S2- oxidation # = -2
Rules for Oxidation Numbers
• Group 1A Metal Cations:
Always +1
• Group 2A Metal Cations:
Always +2
• Hydrogen (H)
+1 when bonded to nonmetals
-1 when bonded to metals
Rules for Oxidation Numbers
• Oxygen (O)
-1 in peroxides (O22-)
-2 in all other compounds
• Fluorine (F)
always -1
Rules for Oxidation Numbers
• The sum of the oxidation numbers of all atoms in any
chemical species (ion or neutral compound) is equal to
the charge on that chemical species
H2O:
MgCl2:
1 + 1 + (-2) = 0
2 + (-1) + (-1) = 0
MnO4-: 7 + (-2) + (-2) + (-2) + (-2) = -1
Oxidation Numbers
• For many compounds, you will be able to directly apply
the rules to determine the oxidation number of all atoms
except for one.
– Use the last two rules to determine the oxidation
number of that last element.
Practice determining oxidation numbers
Example: Determine the oxidation state of all elements in
SO3.
Is it elemental?
No
Are any monoatomic ions present?
Which elements have rules?
Set up an equation to find the remaining
oxidation number. S + 3(-2) = 0 
No
O = -2
S = +6
Oxidation Numbers
Example: Determine the oxidation number of Mn and O in
MnO4-.
Is it elemental?
No
Are any monoatomic ions present?
Which elements have rules?
No
O = -2
Set up an equation to find the remaining
oxidation number.
Mn + 4(-2) = -1
so
Mn = +7
Oxidation Numbers
Example: Determine the oxidation state of all elements in
NaNO3
Is it elemental?
No
Na+
Are any monoatomic ions present?
Which elements have rules?
Set up an equation to find the remaining
oxidation number. 1 + N + 3(-2) = 0
Na = +1,
O = -2

N = +5
Practice
1) Determine the oxidation number of P in
HPO42-.
2) Determine the oxidation state of all
elements in Cr2O72-.
3) Determine the oxidation state of Sn in
SnBr4.
Redox Reactions
• There are many different kinds of redox reactions.
– Combustion
CH4 (g) + 2 O2 (g)  CO2 (g) + 2 H2O (g)
– Oxidation of Metals by Acids or Metal Salts
Cu (s) + H2SO4 (aq)  CuSO4 (aq) + H2 (g)
Redox Reactions
Oxidation of Metals by Acids or Metal Salts
• The reaction between a metal and an acid or between
a metal and a metal salt is called a displacement
reaction.
– a reaction in which an element reacts with a
compound and displaces an element from that
compound
A
+
BX

AX
+
B
Redox Reactions
Oxidation of Metals by Acids or Metal Salts
Examples:
Mg (s) + 2HCl (aq)  MgCl2 (aq) + H2 (g)
Zn (s) + 2 HBr (aq)  ZnBr2 (aq) + H2 (g)
Mn (s) + Pb(NO3)2 (aq)  Mn(NO3)2 (aq) + Pb (s)
Redox Reactions
Oxidation of Metals by Acids or Metal Salts
• How do you know if a redox reaction has occurred?
• You must examine the oxidation number of each of the
elements present in the reactants and products.
– If the oxidation number changes, then a redox
reaction has occurred.
Redox Reactions
Oxidation of Metals by Acids or Metal Salts
• When oxidation occurs:
– Electrons are lost
– Oxidation number increases
e-
• When reduction occurs:
– Electrons are gained
– Oxidation number is reduced (decreases)
e-
Redox Reactions
Oxidation of Metals by Acids or Metal Salts
Mg(s) + 2HCl (aq)  MgCl2 (aq) + H2 (g)
-1
0
+1
Oxidation # of Mg
increased
oxidation
Oxidation # of H+ ion
reduced
reduction
-1
+2
0
Redox Reactions
Oxidation of Metals by Acids or Metal Salts
Example: Identify the element that has been
oxidized and the one that has been reduced.
Zn (s) + 2 HCl (aq)  ZnCl2 (aq) + H2 (g)
Redox Reactions
Oxidation of Metals by Acids or Metal Salts
• Find the oxidation numbers:
Zn (s) + 2 HCl (aq)  ZnCl2 (aq) + H2 (g)
-1
0
+2
-1
0
+1
oxidized
reduced
Zn is the reducing agent.
HCl is the oxidizing agent.
Redox Reactions
Oxidation of Metals by Acids or Metal Salts
• Oxidizing agent:
– The reactant that causes another reactant to be
oxidized
– The reactant that contains the element that was
reduced
• Reducing agent:
– The reactant that causes another reactant to be
reduced
– The reactant that contains the element that was
oxidized.
Redox Reactions
Oxidation of Metals by Acids or Metal Salts
Example: Identify the oxidizing and
reducing agents in the following reaction.
CH4 (g) + 2 O2 (g)  CO2 (g) + 2 H2O (g)
Redox Reactions
Oxidation of Metals by Acids or Metal Salts
• You can also write a net ionic equation to describe a
redox reaction:
– Write all soluble strong electrolytes as ions
– Eliminate the spectator ions
Zn (s) + 2 HCl (aq)  ZnCl2 (aq) + H2 (g)
Zn (s) + 2 H+ (aq) + 2 Cl- (aq)  Zn2+ (aq) + 2 Cl- (aq) + H2
(g)
Zn (s) + 2H+ (aq)  Zn2+ (aq) + H2 (g)
Redox Reactions
Oxidation of Metals by Acids or Metal Salts
Example: Write the complete ionic and net
ionic equations for the reaction. Which
element is oxidized? What is the oxidizing
agent?
2 Al (s) + 3 Ni(NO3)2 (aq)  2 Al(NO3)3 (aq) + 3 Ni (s)
2 Al (s) + 3 Ni2+(aq) + 6 NO3- (aq)  2 Al3+ (aq) +
6 NO3- (aq) + 3 Ni (s)
2 Al (s) + 3 Ni 2+(aq)  2 Al3+ (aq) + 3 Ni (s)
Redox Reactions
Oxidation of Metals by Acids or Metal Salts
+5
+5
2 Al (s) + 3 Ni(NO3)2 (aq) 2 Al(NO3)3 (aq) + 3 Ni (s)
0
oxidized
+2
-2
+3
-2
0
Redox Reactions
Oxidation of Metals by Acids or Metal Salts
• Based on the previous equation, we wouldn’t want to
store a solution of Ni(NO3)3 in an aluminum container.
– The aluminum container would react and dissolve!!!
• Metals differ in the ease with which they are oxidized.
– Al(s) is oxidized by Ni(NO3)3 (aq)
– Ag(s) is NOT oxidized by Ni(NO3)3 (aq)
Redox Reactions
Oxidation of Metals by Acids or Metal Salts
• Activity series:
– A list of metals arranged in order of
decreasing ease of oxidation
– Used to predict whether a metal will react with
an acid or with a metal salt
– See Table 4.5
– Any metal on the list can be oxidized by, i.e.
will lose electrons to, the ions of a metal
below it.
Activity Series
Activity series:
Any metal on the list can be oxidized by, i.e. will lose
electrons to, the ions of a metal below it.
Cu can lose electrons to Ag+