Chapter 20-Oxidation Reduction Reactions
During a reaction between a metal and a nonmetal
electrons are transferred from atoms of the metal to atoms of the nonmetal.
When oxygen bonds with an atom of a different element(other than fluorine)
electrons from that atom shift toward oxygen.
Oxidation-loss of electrons/gain of oxygen
Reduction-Gain of electrons/loss of oxygen
In general, all chemical reactions can be assigned to one of two classes
-Redox reactions-electrons are transferred from one reacting species to another; many of single combination decomposition and combustion are redox. -No electron transfer occurs. Double replacement and acid base are not redox.
What are the processes leading to reduction?
-complete gain of electrons(ionic reactions) -Shift of electrons toward an atom in a covalent bond -loss of oxygen -gain of hydrogen by a covalent compound -decrease in oxidation number
What are the processes leading to oxidation?
-complete loss of electrons (ionic reactions) -Shift of electrons away from an atom in a covalent bond -Gain of oxygen -Loss of hydrogen by a covalent compound -Increase in oxidation number
5th rule For any neutral compound, the sum of the oxidation numbers of the atoms in the compound must equal
0
4th rule The oxidation number of an atom in uncombined(elemental) form is
0. For example, the oxidation number of the potassium atoms in potassium metal (K) or of the nitrogen atoms in nitrogen gas (N2) is 0.
How to use oxidation number change method
1) Start with assigning oxidation numbers to all atoms in the eq.(PER atom not total atom ex: +6) 2) Identify which are oxidized and which are reduced. 3) Use bracketing lines to connect the atoms that undergo oxidation and another such line to connect those that undergo reduction. WRITE THE OXIDATION NUMBER CHANGE AT MIDPOINT OF EACH LINE. 4) Make the total increase in oxidation number equal to the total decrease in oxidation number by using appropiate coefficients. (the formula Fe2O3 does not need a coefficient because the formula already indicates 2 Fe.) 5) Balance for both atoms and charge
Reduction: 2Ag+ + 2e- ------->
2Ag (gain of electrons)
Write an equation to describe the corrosion of iron to iron hyroxides in moist conditions.
2Fe(s) + O2(g) + 2H2O (l) -------> 2Fe(OH)2 (s) 4Fe(OH)2 (s) + O2(g) + 2H2O (l) -------> 4Fe(OH)3 (s)
Why are reactions that involve oxidation and reduction called oxidation reduction reactions or REDOX?
Because no oxidation occurs without reduction. likewise, no reduction occurs without oxidation.
What is an example of oxidation that does not involve burning?
Bleaching stains in fabrics
Oxidation: Cu------->
Cu2+ + 2e-(loss of electrons)
How do you specifically deal with spectator ions in half reactions?
For each atom that appears on both sides of the reactants and products of the original equation is a spectator. Based on the net ionic equation; match up the atom that was in the original equation and place that same coefficient onto the spectator ion. Then add to both sides.
What is an example of a metal that does not corrode easily?
Gold and platinum; they are called noble metals because they are very resistant to losing their electrons by corrosion.
How can you tell if a redox reaction has occurred?
If the oxidation number of an element in a reacting species changes, then that element has undergone either oxidation or reduction. Therefore, the reaction as a whole must be a redox reaction.
What is another name for oxidation reduction reactions?
Redox reaction
LEO GER
Losing electrons is oxidation Gainining electrons is reduction.
Steps for half life reaction
Step 1: Write the unbalanced equation in ionic form (seperate aquous solutions) Step 2: Now write seperate half reactions for the oxidation and reduction processes. Make sure to include the compound the atom is in as well. Step 3: Balance the oxidation half reaction. FIRST MAKE SURE THE ATOM THAT IS BEING OXIDIZED IS BALANCED ON BOTH SIDES. THEN Add H2O or H+. Step 4: Add enough electrons to one side of each half reaction to balance the charges. (Always add the electrons to the reactant in reduction half; add electrons to product in oxidation) Step 5: Repeat for reduction. Step 6: Multiply each half reaction so the number of electrons lost in oxidation equals the amount of electrons gained in reduction. Step 7: Add the balanced half reactions to show an overall equation. Step 8: SUBTRACT TERMS THAT appear on both sides of the equation. (FOR BASE, ADD THE SAME AMOUNT OF (OH-) ions as the amount of H+ Ions on both sides of the equation. ON THE SIDE WHERE YOU HAVE OH AND H, add them TOGETHER TO FORM H20. Then subtract H20's.) Step 9: Add spectator ions and balance the equation.
Mg+S----> Mg2+ + S2-
The magnesium ion is said to be oxidized to a magnesium ion(Loss of electrons). But the sulfur atom Gains two electrons and is reduced to a sulfide ion.
Why does corrosion occur more rapidly in the presence of salts and acids?
These substances produce electrically conduction solutions that make electron transfer easier. Sometimes corrosion of metals is actually a desirable feature.
Magnesium and sulfur are heated together and undergo an oxidation reduction reaction to form magnesium sulfide. Where did the electrons lost by magnesium go?
To the Sulfur because it is more electronegative and thus, pulls the less electronegative atoms electrons towards itself.
It is easier and cheaper to replace a block of magnesium or zinc than to replace
a bridge or a pipeline.
Def. Oxidation Number
a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction.
What is the result of 2H2(g) + O2(g) -------> 2H2O (l) ?
a shift of bonding electrons away from hydrogen; even though there is not a complete transfer. Hydrogen is oxidized because it undergoes a partial loss of electrons.
Reduction of metal oxides by the removal of oxygen leads to
a smaller mass of the metal compared to the original metal oxide.
The burning of a fuel is
an oxidation reaction that uses oxygen
Oxidation numbers are often written
above the chemical symbols in a formula
How does aluminum protect itself from corrosion?
aluminum oxidizes quickly in air to form a coating of very tightly packed aluminum oxide particles. The coating protects the aluminum object from further corrosion.
A Half reaction is...
an equation showing just the oxidation or just the reduction that takes place in a redox reaction.
Redox reactions are currently understood to involve
any shift of electrons between reactants.
Because water is a molecular compound, no ionic charges are
associated with its atoms
When is the half life reaction method useful?
balancing ionic reactions
Why does magnesium prevent oxidation to iron ions?
because magnesium is a better reducing agent than iron and is more easily oxidized. so the magnesium immediatley transfers electrons to the iron; preventing their oxidation to iron ions. So magnesium is sacrificed by oxidation and protects iron.
Although iron forms a coating when it corrodes, why does it seem to be ineffective?
because the coating of iron oxide that forms is not tightly packed. WATER AND AIR can PENETRATE THE COATING and attack the Iron metal below it. Corrosion continues until the iron object becomes a pile of rust.
The bursts of bright white light produced by fireworks are the result of metals being
burned
Not all oxidation processes that use oxygen involve
burning ex: when elemental iron turns to rust, it SLOWLY oxidzes to compounds such as Iron (III) oxide
In redox reactions involving covalent compounds
complete electron transfer does not occur ex: 2H2(g) + O2(g) -------> 2H2O (l)
Def. Reduction
complete or partial gain or electrons or loss of oxygen
Def. Oxidation
complete or partial loss of electrons or gain of oxygen
When a metal and a nonmetal react and form ions, it is easy to identify
complete transfers of electrons.
What is an example of oxidation reduction?
corrosion of metallic parts of cars.
In a balanced redox equation, the total increase in oxidation number of the species oxidized must be balanced by the total
decrease in the oxidation number of the species reduced.
You can use oxidation numbers to keep track of
electron transfers-a type of chemical bookkeeping.
As a general rule, a bonded atoms oxidation number is the charge that it would have if the electrons in the bond were assigned to the atom of the more
electronegative element
6th rule For a polyatomic ion, the sum of the oxidation numbers must
equal the ionic charge of the ion.
The reaction equation above is highly
exothermic-releases a great deal of energy.
How to write separate component of oxidation/reduction: ex: 4Al ----> 4Al 3+ + 12e- 6O+ 12e- ------> 6O2-
for oxidation: always write the reactant of the atom being oxidized. Then write an arrow. Now on the products side, there should be the same atom and keep its ionic charge(or write it in) and everything. Now add the difference of the electrons between the starting reactants and end product of the atom; making sure that you include the charge of the subscripts as well. For reduction: Add the electron on the reactant side; based on the difference between product and reactant.
Reduction: S + 2e- ---> S2-
gain of electrons
A second method for balancing redox equations involves the use of
half reactions
which ones the reducing agent?
hydrogen
oxygen, the oxidizing agent in corrosion is reduced to oxide ions or to
hydroxide ions
When do you use oxidation number change?
if the oxidized and reduced species appear only once on each side of the equation and no acids or bases are involved.
What is a good way to attach oxidation numbers to a covlently bonded molecule ex: water?
imagine as if it were a complete transfer. ex: H2O ( H would have +1 and O would have -2)
2nd Rule The oxidation number of hydrogen
in a compound is +1, except in metal hydrides, such as NaH where it is -1.
3rd Rule The oxidation number of oxygen
in a compound is -2, except in peroxides such as H2O2 where it is -1. And in compounds with the more electronegative fluorine, where it is positive.
What happens in the bonding electrons in the formation of a water molecule?
in each reactant hydrogen molecule, the bonding electrons are shared equally between the hydrogen atoms. IN water, the bonding electrons are pulled toward oxygen because it is more electronegative than hydrogen.
Where does corrosion occur more rapidly?
in the presence of salts and acids.
2+
ionic charge (sign is placed last)
How does iron, a common construction metal often used in the form of the alloy steel, corrode?
iron corrodes by being oxidized to ions of iron by oxygen.
Iron reacts with water and oxygen to form
iron(III) oxide, or rust.
1st rule for assigning oxidation numbers The oxidation number of a monatomic ion is...
is equal in magnitude and sign to its ionic charge For example, the oxidation number of the bromide ion (Br1-) is -1. that of Fe3+ ion is +3. NOTICE THAT FOR OXIDATION NUMBERS, THE SIGN COMES FIRST.
Oxidation: Mg---> Mg2+ + 2e-
loss of electrons
After balancing the atoms in each half reaction; balance electrons gained in the reduction with the electrons
lost in oxidation.
By losing electrons to sulfur,
magnesium reduces the sulfur. As a result it is the reducing agent.
The modern concepts of oxidation and reduction have been extended to include
many reactions that don't even include oxygen.
What is another method of corrosion control?
one metal is "sacrificed" or allowed to corrode in order to save a second metal.
An increase in oxidation number is
oxidation
Losing electrons is
oxidation
Reduction is the opposite of
oxidation
To balance a redox reaction using half-reactions, write seperate half reactions for the
oxidation and the reduction.
+2
oxidation number (sign is placed in front)
What are two ways to balance a redox equation?
oxidation number changes Half reactions
What causes corrosion?
oxidation reduction reactions
Zinc blocks are attached to the steel iron hull of a ship. The zinc blocks _____(corrode) instead of the iron, preventing the hull from corroding.
oxidize
What accelerates the rate of corrosion?
water
The substance gaining oxygen is _________, while the substance losing oxygen is ________.
oxidized; reduced.
A bunsen burner _____ the methane in natural gas to carbon dioxide and water.
oxidizes
By accepting electrons from magnesium, sulfur ______ the magnesium.
oxidizes
Sulfur is the
oxidizing agent
Other than fluorine, which element is the most electronegative?
oxygen
Which ones the oxidizing agent?
oxygen
For carbon compounds the addition of ______ or the removal of ______ is always oxidation.
oxygen; hydrogen
Sacrificial zinc and magnesium blocks are sometimes attached to
piers and ship hulls to prevent corrosion in areas submerged under water. underground pipelines and storage tanks may be connected to magnesium blocks for protection.
The oxide of hydrogen
water H2O
Chromium metal also serves as a
protective coating and imparts an attractive mirrorlike finish. Like aluminum, chromium forms a corrosion resistant oxide film on its surface.
The combustion of gasoline in an automobile engine and the burning of wood in a fireplace are
reactions that require oxygen as they release energy.
Many reactions in which color changes occur are
redox reactions
Oxygen is _____ in the formation of water
reduced
An decrease in oxidation number is
reduction
Gaining electrons is
reduction
NaClO is a substance that
releases oxygen which oxidizes stain to a colorless form.
Oxidation causes the complete corrosion of
some metals
Simplest way to identify oxidizing and reducing agents
that the species that is reduced is the oxidizing agent and the species oxidized is the reducing agent.
What happens in the other reactant, oxygen?
the bonding electrons are shared equally between oxygen atoms in the oxygen molecule. However, when oxygen bonds to hydrogen in the water molecule , there is a shift of electrons toward oxygen. SO ITS REDUCED BECAUSE ITS UNDERGOING A PARTIAL GAIN OF ELECTRONS.
Early chemists saw oxidation only as
the combination of an element with oxygen to produce an oxide. ex: When methane (CH4) burns in air, it oxidizes and forms oxides of carbon and hydrogen (CO2 and H2O)
Early chemists saw REDUCTION as
the loss of oxygen from a compound. ex: 2Fe2O3(s) + 3C(s) -----> 4Fe(s) +3CO2(g) an oxygen is removed from iron
In some reactions involving covalent reactants or products,
the partial electron shifts are less obvious.
in this reaction: 2Fe2O3(s) + 3C (s) --------> 4Fe(s) +3CO2(g)
the reduction of iron also includes an oxidation process. Iron 3 oxide is reduced to iron by losing oxygen, CARBON oxidezes to Carbon dioixide by gaining oxygen.
Def. Reducing Agent
the substance that loses electrons
As elements burn,
their oxidation numbers change.
Why do some metals not corrode easily?
they lose electrons but are protected from extensive corrosion by the oxide coating formed on their surface.
An example of the "sacrifice method" for corrosion control?
to protect an iron object, a piece of magnesium or another active metal may be placed in electrical contact with iron. When oxygen and water attack the iron object, the iron atoms lose electrons as the iron begins to be oxidized.
a change in oxidation number represents the number of electrons
transferred
Oxidation and reduction always occur simultaneously; true or false?
true
In real life, how do we prevent corrosion? (1st method)
we coat the metal surface with oil, paint, plastic, or another metal. These coatings exclude air and water from the surface, thus preventing corrosion. If scratched or worn out, the exposed metal will begin to corrode....
How is patina formed?
when copper on the roof reacts with water vapor carbon dioxide and other substances in the air
When do you use half reaction?
when the same element is both oxidized and reduced. hard coeffecients. Reactions that take place in acidic or alkaline solution.
Def. Oxidation Number change method
you balance a redox equation by comparing the increases and decreases in oxidation numbers.
Half Reaction Method
you write and balance the oxidation and reduction half rections seperatley before combining them into a balanced redox equation.
