Oxidation and Reduction Notes

Oxidation and Reduction

What Are Oxidation and Reduction?

• Reactions that require oxygen release energy, such as:

  • Combustion of gasoline in an automobile engine.

  • Burning of wood in a fireplace.

  • Breakdown of food in the body, utilizing oxygen from the air.

Oxygen and Redox

• Methane (CH₄) combustion:

  • Methane oxidizes and forms carbon and hydrogen oxides.

  • Example: $CH<em>4(g) + 2O</em>2(g) \rightarrow CO<em>2(g) + 2H</em>2O(g)$

• Oxidation doesn't always involve burning:

  • Iron rusting: slow oxidation to iron(III) oxide (Fe₂O₃).

  • Example: $4Fe(s) + 3O<em>2(g) \rightarrow 2Fe</em>2O_3(s)$

• Household bleach (NaClO) releases oxygen, oxidizing stains to a colorless form.

• Hydrogen peroxide (H₂O₂) decomposes, releasing oxygen, acting as a bleach and antiseptic.

Reduction

• Reduction is the opposite of oxidation; originally defined as the loss of oxygen.

• Iron ore reduction to metallic iron involves removing oxygen from iron(III) oxide via heating with carbon (coke).

  • Example: $2Fe<em>2O</em>3(s) + 3C(s) \rightarrow 4Fe(s) + 3CO_2(g)$

• Reduction includes oxidation - as iron(III) oxide reduces to iron, carbon oxidizes to carbon dioxide.

• Oxidation and reduction always occur simultaneously.

• A substance oxidized gains oxygen; a substance reduced loses oxygen.

• No oxidation occurs without reduction, and vice versa.

• Reactions involving oxidation and reduction are called oxidation-reduction (redox) reactions.

Electron Shift in Redox Reactions

• Modern redox definition: reactions involving any shift of electrons between reactants, not just reactions involving oxygen.

• Oxidation: complete or partial loss of electrons or gain of oxygen.

• Reduction: complete or partial gain of electrons or loss of oxygen.

• Oxidation is Loss of electrons, Gain of oxygen. Reduction is Gain of electrons, Loss of oxygen.

Redox Reactions That Form Ions

• In metal-nonmetal reactions, electrons transfer from metal atoms to nonmetal atoms.

• Magnesium and sulfur reaction: $Mg(s) + S(s) \rightarrow MgS(s)$

• Two electrons transfer from a magnesium atom to a sulfur atom.

• Magnesium atoms become more stable by losing electrons; sulfur atoms become more stable by gaining electrons.

  • $Mg + S \rightarrow Mg^{2+} + S^{2-}$

• Magnesium atom is oxidized to a magnesium ion; sulfur atom is reduced to a sulfide ion.

• Oxidation: $Mg \rightarrow Mg^{2+} + 2e^-$

• Reduction: $S + 2e^- \rightarrow S^{2-}$

• Substance oxidized loses electrons; substance reduced gains electrons.

• Reducing agent: the substance that loses electrons.

• Oxidizing agent: the substance that accepts electrons.

• The species reduced is the oxidizing agent; the species oxidized is the reducing agent.

• Example: $Mg(s) + S(s) \rightarrow MgS(s)$

  • Magnesium (reducing agent), Sulfur (oxidizing agent)

Sample Problem 20.1

• $2Ag^+ + 2NO<em>3^- + Cu \rightarrow Cu^{2+} + 2NO</em>3^- + 2Ag$

• Copper atom (Cu) loses two electrons, becoming $Cu^{2+}$ ion.

• Silver ions ($Ag^+$) gain electrons, becoming neutral silver atoms.

• Copper (Cu) is the reducing agent; Silver ($Ag^+$) is the oxidizing agent.

• Oxidation: $Cu \rightarrow Cu^{2+} + 2e^-$

• Reduction: $2Ag^+ + 2e^- \rightarrow 2Ag$

Redox With Covalent Compounds

• Reactions involving covalent compounds: complete electron transfer does not occur.

• Example: $2H<em>2(g) + O</em>2(g) \rightarrow 2H_2O(l)$

• Hydrogen molecule: bonding electrons shared equally.

• Water: bonding electrons pulled toward oxygen (more electronegative).

• Hydrogen is oxidized (partial loss of electrons); oxygen is reduced (partial gain of electrons).

• Hydrogen is the reducing agent; oxygen is the oxidizing agent.

• This redox reaction is highly exothermic.

• 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.

• 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.

Corrosion

• Iron corrodes by oxidation with oxygen, forming iron ions.

• Water accelerates corrosion.

• Oxygen (oxidizing agent) reduces to oxide ions or hydroxide ions.

  • $2Fe(s) + O<em>2(g) + 2H</em>2O(l) \rightarrow 2Fe(OH)_2(s)$

  • $4Fe(OH)<em>2(s) + O</em>2(g) + 2H<em>2O(l) \rightarrow 4Fe(OH)</em>3(s)$

• Salts and acids accelerate corrosion by producing conductive solutions, making electron transfer easier.

• Washing salt off cars is important because salt mixed with water creates a conductive solution, accelerating corrosion of steel parts.

• Corrosion can be desirable: copper reacts to form a patina of basic copper(II) carbonate.

• Noble metals (gold, platinum) resist corrosion.

• Some metals are protected by oxide coatings.

Resistance to Corrosion

• Iron oxide coating is not tightly packed, allowing water and air to penetrate, causing further corrosion.

• Aluminum forms a tightly packed aluminum oxide coating, protecting against further corrosion.

Controlling Corrosion

• Coat metal surfaces with oil, paint, plastic, or another metal to exclude air and water.

• Painting surfaces (e.g., the Golden Gate Bridge) protects from environmental effects.

• Chromium forms a corrosion-resistant oxide film.

• Magnesium can be placed in electrical contact with iron to protect it; magnesium is a better reducing agent and transfers electrons to iron atoms, preventing their oxidation (sacrificial metal).

• Zinc blocks are attached to steel hulls of ships, oxidizing instead of the iron (sacrificial anode).

• Zinc-coated steel is used for metal trash cans because zinc corrodes first, protecting the steel.

Key Concepts

• Oxidation: gain of oxygen or loss of electrons.

• Reduction: loss of oxygen or gain of electrons.

• Salts and acids accelerate corrosion by producing conductive solutions.

Glossary Terms

• Oxidation-reduction reaction: electron transfer between reactants.

• Oxidation: complete or partial loss of electrons or gain of oxygen; increase in oxidation number.

• Reduction: complete or partial gain of electrons or loss of oxygen; decrease in oxidation number.

• Reducing agent: donates electrons, gets oxidized.

• Oxidizing agent: accepts electrons, gets reduced.

Big Idea Reactions

• Oxidation-reduction reactions occur simultaneously.

• Losing electrons is oxidation.

• Gaining electrons is reduction.

• Substance gaining oxygen is oxidized; substance losing oxygen is reduced.

• Reduced species is the oxidizing agent; oxidized species is the reducing