redox reactions
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Redox Reactions
Or How Batteries Work
REDOX Reactions• The simultaneous transfer of electrons between chemical
species.– Actually 2 different reactions occurring at the same time.
Oxidation: a loss of electronsReduction: a gain of electrons
ReDox
Na +Cl NaCl
Nao +Clo Na+Cl-
Which element was Oxidized?Na
Which element was Reduced?Cl
How do we remember ReDox?
mnemonics
5
LEO growls GER
Losing Electrons Oxidation
Gaining Electrons Reduction
Turn to Page 5
7
OIL RIG
OxidationIsLosing
ReductionIsGaining
How do we know if there is aRedOx Reaction occurring?
We first must find the Oxidation states.
What is the oxidation state of each of the following species?
•Cl in Cl2
• 0•Mg in MgO• +2•Na in NaBr• +1
•F in SrF2
• -1•Sr in SrF2
• +2•O in O2
• 0
10J Deutsch 2003
Regents Question
Which particles are gained and lost during a redox reaction?
(1)Electrons
(2) Protons
(3) Neutrons
(4) Positrons
11J Deutsch 2003
Regents QuestionAs a Ca atom undergoes oxidation to Ca2+, the number
of neutrons in its nucleus
(1)Decreases
(2) Increases
(3) Remains the same
Turn to Page 6
Rules for Determining Oxidation Numbers
1. Free Elements have an oxidation number of zero. Ex: Na, S8, H2
2. All Metals in group 1 have a +1 Oxidation # in compounds. Ex: Na, K, Li
3. All Metals in group 2 have a +2 Oxidation # in compounds. Ex: Mg, Ca, Ba
4. Oxygen has a –2 Oxidation Number1. Unless it is in a Peroxide like H2O2
Continued Rules
5. Hydrogen has a +1 oxidation #.6. Halogens have a -1 oxidation #.7. For any neutral compound, the sum of the
oxidation numbers of the atoms in the compound must equal zero. For ex: H2SO4
8. For a polyatomic ion, the sum of the oxidation #’s must equal the ionic charge of the ion. Ex: SO4 2-
CuF2 PBr3
HNO3 C2O4-2
SO4-2 CO3-2
C12H22O11 H2O
Turn to Page 7
Now we will take a deeper look into Oxidation.
• Can be defined as:– The loss of electrons– The gain of oxygen– The loss of hydrogen
OXIDATION• The loss of electrons
Na(s) Na+ + e-
The sodium ion has been oxidized to the sodium cation.
Na + Cl
Sodium loses electron to chlorine
-ClNa +
+
OXIDATION
• Gain of oxygen– Combustion reactions
are classic examplesC(s) + O2(g) CO2(g)
(burning of coal)
2Fe(s) + 3O2(g) 2Fe2O3(s)
(rusting of iron)
OXIDATION
• Loss of hydrogen– Oxidation can sometimes be best seen as the loss
of hydrogenCH3OH(l) CH2O(l) + H2(g)
Methyl alcohol has been oxidized into formaldehyde
Which one of the following elements were oxidized?
•Cl in Cl2
• 0 → NO•Mg in MgO• +2 → YES•Na in NaBr• +1 → YES
•F in SrF2
• -1 → NO•Sr in SrF2
• +2 → YES•O in O2
• 0 → NO
Turn to Page 8
Now we will take a deeper look into Reduction.
• 3 Definitions
– The gain of electrons– The loss of oxygen– The gain of hydrogen
Gain of electrons– The process of silver
electroplating
Ag+ + e- Ag
Silver cation has gained an electron and has been reduced to silver metal.
REDUCTION
REDUCTION
Loss of oxygen – Reduction can also be seen as the loss
of oxygen in going from reactant to product.
Fe2O2 (s) + 3 CO (g) 2 Fe(s) + 3 CO2(g)
Iron ore is reduced to iron metal in a blast furnace with carbon monoxide
REDUCTION• Gain of hydrogen
A reduction can also be described as the gain of hydrogen atoms going from reactant to product.
CO(g) + 2 H2(g) CH3OH(l)
Carbon monoxide has been reduced to methyl alcohol
One’s loss is another’s gain
• Neither oxidation or reduction can take place without the other.– When those electrons are lost something else has
to gain them.
When trying to remember which is which think of LEO the lion goes GER OIL RIG
Lose Electrons Oxidation Oxidation is LosingGain Electrons Reduction Reduction is Gaining
Turn to Page 9
Use a chart to determineOxidation States.
Elements
Oxidation State
Number of Atoms
Total Charge
First we will do K2PtCl6
Elements K Pt Cl Sum of Oxidation States
Oxidation State
Number of Atoms
Total Charge
Now do the following:
• What is the oxidation state of S in MgSO4?
• What is the oxidation state of N in NaNO3?
• What is the oxidation state of Cl in KClO3?
Now do the following:
• What is the oxidation state of C in CO3-
• What is the oxidation state of Cl in ClO2-
• What is the oxidation state of Cr in CrO42-
• What is the oxidation state of Cr in CrO72-
Packet Page 10-11
Review of Terms
• oxidation-reduction (redox) reaction:–involves a transfer of electrons from the reducing agent to the oxidizing agent.
• oxidation: loss of electrons• reduction: gain of electrons•Reducing Agent get oxidized•Oxidizing agent get reduced
Packet Page 12-13
An oxidation-reduction, (redox), reaction involves the transfer of electrons.
The oxidation numbers of the atoms will change…. one goes up (oxidation) and one goes down (reduction)
Sodium transfers its electrons to chlorine
Reduction is the gain of electrons.
Nonmetals gain electrons to form – ionsThe oxidation number goes down (reduces)
A half-reaction can be written to represent reduction.
Cl0 + 1e- Cl-1
In reduction half reactions, electrons are written on the left because electrons are gained
Oxidation is the loss of electrons.
Metal atoms lose electrons to become + ionsThe oxidation numbers go up (increases)
A half-reaction can be written to represent
oxidation.Na0 Na1+ + 1e-
In oxidation half reactions, electrons are written on the right because electrons are lost
Half-Reactions
• Show either the oxidation or reduction portion of a redox reaction and the electrons gained or lost.
Steps for Writing Half-Reactions
Cu0 + Ag+1N+5O-23 Cu+2 (N+5O-23)2 + Ag0
1. Assign an oxidation number to each element.2. Write a partial half-reaction to show the
change in oxidation state:Oxidation: Cu0 Cu+2
Reduction: Ag+1 Ag0
Steps Continued…
3. Show the number of electrons needed to explain how the oxidation number changed.
Oxidation: Cu0 Cu+2 + 2e-Reduction: Ag+1 +1e- Ag0
4. Last, but not least, achieve conservation of charge: 2Ag+1 +2e- 2Ag0
Practice Problems
1. Mg + Cl2 MgCl2
2. Cu + 2Ag+ Cu2+ + 2Ag3. Al0 + Cr3+ Al3+ + Cr0
Mg + Cl2 MgCl2
Cu + 2Ag+ Cu2+ + 2Ag
Al0 + Cr3+ Al3+ + Cr0
Regents Question:
Given the reaction:Mg(s) + 2H+(aq) + 2Cl–(aq) Mg2+(aq) + 2Cl–(aq)
+ H2(g)
Which species undergoes oxidation?(1) Mg(s)(2) H+(aq) (3) Cl– (aq)(4) H2 (g)
LEO growls GERLEO
Regents Question:
Given the equation:C(s) + H2O(g) CO(g) + H2 (g)
Which species undergoes reduction?(1) C(s)(2) H +
(3) C2+
(4) H2 (g)LEO growls GERGER
Regents Question:In any redox reaction, the substance that
undergoes reduction will
(1) lose electrons and have a decrease in oxidation number
(2) lose electrons and have an increase in oxidation number
(3) gain electrons and have a decrease in oxidation number
(4) gain electrons and have an increase in oxidation number
Regents Question:
Given the equation:2 Al + 3 Cu2+ 2 Al3+ + 3Cu
The reduction half-reaction is(1) Al Al 3+ + 3e – (2) Cu 2+ + 2e – Cu (3) Al + 3e – Al 3+
(4) Cu 2+ Cu + 2e –
Worksheet Page 16-17
Table J tells us if a redox reaction can occur between an atom and an
ion.• A more active metal will replace an ion
below it on Table J.• A more active nonmetal will replace an ion
below it on Table J.• Any metal above H is more active than H
and will react with an acid to produce H2(g)– The higher up the table, the more readily the
replacement will take place.
Regents Question:
According to Reference Table J, which of these metals will react most readily with 1.0 M HCl to produce H2(g)?
(1) Ca(2) K (3) Mg(4) Zn
A more active metal will replace a less active metal from its compound.
• Zn + CuSO4 Cu + ZnSO4
Zinc replaces copper because zinc is more active than copper.
• Cu + ZnSO4 No ReactionCopper cannot replace zinc
Oxidation is on topReduction is on bottom
Review of Terms
• oxidation-reduction (redox) reaction:– involves a transfer of electrons from the reducing agent to the oxidizing agent.
•oxidation: loss of electrons•reduction: gain of electrons
Half-Reactions
•The overall reaction is split into two half-reactions, one involving oxidation and one reduction.
8H+ + Mn+7O4 + 5Fe2+ Mn2+ + 5Fe3+ + 4H2O
Reduction: Mn+7 + 5e Mn2+
Oxidation: 5Fe2+ 5Fe3+ + 5e
AGENTS
• To recap– Oxidizing agent = the species that’s being reduced
– Reducing agent = the species that’s being oxidized
Page 18
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