Nivaldo J. Tro

http://www.cengage.com/chemistry/tro

Mark Erickson • Hartwick College

Chapter 14Oxidation and Reduction

Oxidation: Rust• Oxidized iron atoms (iron atoms

that lost electrons) bond with reduced oxygen atoms (oxygen atoms that gained electrons) to form iron oxide or RUST.

• Water is necessary; salt accelerates the reaction.

• We reduce iron ore (Fe2O3) for the iron content, and each year $10 billion worth of this iron oxidizes back to Fe2O3.

Definitions: Oxidation and Reduction

• Broadest definitions are– Oxidation: Loss of electrons– Reduction: Gain of electrons

Multiple Definitions• Oxidation

– The gain of oxygen

– The loss of electrons

– The loss of hydrogen

• Reduction

– The loss of oxygen

– The gain of electrons

– The gain of hydrogen

Oxidation and reduction MUST occur together.

Concept Check 14.1

• Thermite is used to weld pieces of metal together when electricity is not available as an energy source to melt iron. The thermite reaction produces molten through the following oxidation-reduction reaction:

Al + Fe2O3 → Fe + Al2O3

• For the thermite reaction, identify the elements being oxidized and those being reduced.

Concept Check 14.1 Solution• There are two ways to identify which substance is being

oxidized and which is being reduced in the thermite reaction.

2Al + Fe2O3 → 2Fe + Al2O3

1) Al is oxidized because it is gaining oxygen, Fe is being reduced because it is losing oxygen.

2) 2Al → 2Al3+ + 6 e− (loss of 3 e− for each Al metal) 2Fe3+ + 6 e− → 2Fe (gain of 3 e− for each Fe3+)

Al loses electrons (the reducing agent), it is being oxidized. Fe3+ gains electrons (the oxidizing agent), it is being reduced.

Common Oxidizing Agents

Oxidizing agents•Antiseptics and disinfectants (they kill microorganisms)

•Oxygen

•Hydrogen peroxide

•Benzoyl peroxide

•Iodine

•Chlorine

•Sodium hypochlorite

Common Reducing Agents• Reducing agents

– Less common in the household, more common on an industrial scale

– Hydrogen (Haber process)

• Nitrogen fixation

– Elemental carbon (coke)

• Reduction of metal ions

Concept Check 14.2

• For the following redox reactions, identify the oxidizing and reducing agents:

a) Cu2+ + Zn → Cu + Zn2+

b) 2Mg + O2 → 2MgO

Concept Check 14.2 Solution

a) Cu2+ + Zn → Cu + Zn2+

– Cu2+ gains electrons (reduced), and therefore is the oxidizing agent.

– Zn loses electrons (oxidized), and therefore is the reducing agent.

b) 2Mg + O2 → 2MgO

– O2 gains electrons and loses an oxygen atom (reduced), and therefore is the oxidizing agent.

– Mg loses electrons and gains oxygen (oxidized), and therefore is the reducing agent.

Respiration and Photosynthesis• Animals, including humans, use respiration to oxidize food

for energy.

• In photosynthesis, plants reduce carbon to form glucose and oxygen.

Animals and plants depend on each other for life.

Concept Check 14.3

• What substance is the reducing agent in respiration?

Concept Check 14.3 Solution

• A reducing agent is a substance in a chemical reaction that is oxidized. The carbons in glucose (C6H12O6) lose hydrogen during respiration. The loss of hydrogen is characteristic of an oxidation, therefore, glucose is oxidized and is the reducing agent in respiration. O2 is reduced and is the oxidizing agent.

C6H12O6 + 6O2 6CO2 + 6H2O

Batteries• Based on the natural

tendency of electrons to transfer between substances

• Physical separation of the substances to gain and lose electrons is required.

• Metals and ions are physically separated so that the only path available to electrons is through a wire.

Concept Check 14.4• Identify the oxidizing and reducing agents in the Zn/Cu

electrochemical cell.

Concept Check 14.4 Solution

Overall reaction: Zn(s) + Cu2+ → Zn2+ + Cu(s)

•Reaction at the anode: Zn(s) → Zn2+ + 2e−

– Zn is losing two electrons and therefore is being oxidized. The species that is oxidized is the reducing agent.

•Reaction at the cathode: Cu2+ + 2e− → Cu(s)– Cu2+ is gaining two electrons and therefore is

being reduced. The species that is reduced is the oxidizing agent.

Limited Lifetimes• Anode: The electrode where

oxidation occurs (−)

• Cathode: The electrode where reduction occurs (+)

• For the preceding battery example, the zinc electrode eventually dissolves away and the Cu2+ solution is depleted.

• Rechargeable batteries use an externally applied energy source to force electrons to travel in the opposite direction, regenerating the starting materials for the spontaneous redox reaction.

Automobile Batteries• Anode: Porous lead plates in sulfuric acid

• Cathode: Lead oxide, also in sulfuric acid

Automobile Batteries• The overall reaction is the sum of the oxidation and the

reduction.

• Recharge is accomplished during automobile operation.

Flashlight Batteries• Leclanché dry cell

– Zinc body functions as the anode– Carbon rod functions as the cathode but does not appear

in reaction.

• Alkaline batteries– Employs a base in the oxidation

of the zinc– Longer battery and shelf life– Less corrosion

Fuel Cells• Electrochemical reactions convert chemical energy directly

to electrical energy, eliminating intermediate conversions and losses of energy.

The Fuel Cell in Space• Fuel cell reactants and

products are not contained in the cell hardware; they flow in and then out.

• Astronauts consume the waste product of fuel cells during manned space flight.

Molten Carbonate Fuel Cell (MCFC)

• The molten carbonate fuel cell is the most promising of the designs to compete with fossil fuel electricity production.

• 54–85% efficient

• Small size

• Sulfur and nitrogen oxide emissions low

• BUT it relies on a fossil fuel

Chemistry in the MCFC• Reforming methane

• Oxidizing hydrogen

Chemistry in the MCFC• Reducing oxygen

• Overall

Corrosion• Rusting is a redox process.

• Iron(III) oxide has no structural integrity.

• 1/5 of domestic iron production goes to replace rusted iron.

• Rust prevention is an important industrial process.

• Paint and mixing or coating iron with another metal (Al, Zn) are preventative measures against corrosion.

Zinc in Rust Prevention

A galvanized nail has a thin layer of zinc on its surface which oxidizes instead of the underlying iron.

Zinc wire is attached to this underground pipe. The zinc loses electrons more easily and protects the iron pipe from oxidation.

Aging and Antioxidants• Antioxidants are reducing agents; they promote reduction

and retard oxidation by giving up hydrogen atoms to free radicals.

• Free radicals are highly reactive molecules and are oxidizing agents.

Oxidation of Human Cells• Theories of aging:

– Free radical attack changes cell walls and the immune system mistakes the cell as foreign.

• Free radical attack on DNA causes cells to divide and function improperly.– We call this cancer.

• Balanced diet or supplements?

Chapter SummaryMolecular Concept

• Oxidation

• Reduction

• Redox reactions

• Oxidizing agent

• Reducing agent

• How batteries work

• Metals that undergo redox reactions

Societal Impact

• Redox reactions are responsible for many things in our lives like batteries.

• Development in batteries has led to advancement in fuel cells, which in turn has led to the improvement of electric cars.