Chapter 20 Oxidation-Reduction Reactions

Chapter 20Oxidation-Reduction Reactions

Some Common Reactions The combustion of gasoline in an automobile engine requires oxygen Burning of wood in a fireplace requires oxygen The reactions that break down food in your body and release energy use oxygen The oxide of hydrogen is water Charcoal oxidizes when it burn forming CO2 Bleaching stains in fabric is still oxidation even though it does not burn.

Oxidation When methane burns in air, it oxidizes and forms oxides of carbon and hydrogen. CH4(g) + 2O2(g) CO2(g) + 2H2O(g) When elemental iron turns to rust, it slowly oxidizes to compounds such as iron (III) oxide. 4Fe(s) + 3O2(g) 2Fe2O3(s)

Reduction Originally reduction meant a loss of oxygen from a compound 2Fe2O3(s) + 3C(s) 4Fe(s) + 3CO2(g) iron oxide carbon iron carbon dioxide Reduction of iron ore to metallic iron involves the removal of oxygen from iron (III) oxide. Involves heating the ore with carbon.

Question What happens to magnesium and oxygen when they react to form magnesium oxide? +2 -2 2Mg + O2 2MgO magnesium oxygen magnesium oxide Magnesium loses electrons to form Mg2+ Oxygen gains electrons to form O2-

Electron Shift in Redox Reactions The modern concept of oxidation and reduction have been extended to include many reactions that do not even involve oxygen. Oxygen is the most electronegative element (besides fluorine) When oxygen bonds with an atom of a different element (except fluorine), electrons from that atom shift toward oxygen.

Redox Reactions Redox reactions are currently understood to involve any shift of electron between reactants. • Oxidation – a process that involves a complete or partial loss of electrons or a gain of oxygen. • Results in an increase in the oxidation number of an atom • Reduction – a process that involves a complete or partial gain of electrons or the loss of oxygen. • Results in a decrease in the oxidation number of an atom

Redox Reactions Oxidation and reduction always occur simultaneously. The substance gaining oxygen or losing electrons is oxidized The substance losing oxygen or gaining electrons is reduced.

Redox Reactions During a reaction between a metal and a nonmetal, electrons are transferred from atoms of the metal to atoms of the nonmetal. Mg + S Mg2+S2- magnesium sulfurmagnesium sulfide 2 electrons are transferred from a magnesium atom to a sulfur atom. Magnesium atoms become more stable by the loss of electrons. Sulfur atoms become more stable by the gain of electrons

Redox Reactions Mg + S Mg2+S2- magnesium sulfurmagnesium sulfide Oxidation: Mg Mg2+ + 2e- (loss of electrons) Reduction: S + 2e- S2- (gain of electrons) Magnesium atom is said to be oxidized to a magnesium ion Sulfur atom is said to be reduced to a sulfide ion.

Redox Reactions When a metal combines with oxygen, it loses electrons When oxygen is removed from the oxide of a metal, the metal gains electrons. This knowledge is what led to the broader definition of oxidation and reduction as an exchange of electrons.

Redox Reactions Reducing agent – the substance that loses the electrons Mg + S MgS reducing agent oxidized Oxidizing agent – the substance that accepts electrons Mg + S MgS oxidizing agent reduced

Sample Problem Silver nitrate reacts with copper to form copper nitrate and silver. From the equation below, determine what is oxidized and what is reduced. Identify the oxidizing agent and the reducing agent. 2AgNO3 + Cu Cu(NO3)2 + 2Ag Rewrite the equation in ionic form 2Ag+ + 2NO3- + Cu Cu2+ + 2NO3- + 2Ag

Sample Problem 2Ag+ + 2NO3- + Cu Cu2+ + 2NO3- + 2Ag Oxidation: 2 e- are lost from copper when it becomes a Cu2+ Reduction: 2e- are gained by two silver ions which become neutral silver atoms. 2Ag+ + 2NO3- + Cu Cu2+ + 2NO3- + 2Ag oxidizing reducing Agent agent reduced oxidized

Sample Problem 2Na + S 2Na+ + S2- oxidized reduced reducing agent oxidizing agent 4Al + 3O2 4Al3+ + 3O2- Oxidized reduced Reducing agent oxidizing agent 2I- I2 + 2e- oxidation Zn2+ + 2e- Zn reduction

Redox with Covalent Compounds Some reactions involve covalent compounds. In these compounds complete electron transfer does not occur. 2H2(g) + O2 (g) 2H2O (l) In each reactant hydrogen molecule, the bonding electrons are shared equally between the hydrogen atoms. In water, however, the bonding electrons are pulled toward oxygen because it is much more electronegative than hydrogen.

Redox with Covalent Compounds 2H2(g) + O2 (g) 2H2O (l) There is 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.

Redox with Covalent Compounds 2H2(g) + O2 (g) 2H2O (l) In oxygen, the bonding electrons are share equally between oxygen atoms in the reactant oxygen molecule. When oxygen bonds to hydrogen in the water molecule, there is a shift of electrons toward oxygen. Oxygen is thus reduced because it undergoes a partial gain of electrons.

Redox with Covalent Compounds H H O O H O shift of bonding e- sharede- sharede- away from H equallyequally H and toward O H is reducing O is oxidizing agent agent

Redox with Covalent Compounds • Some reactions involving covalent reactants or products, the partial electron shifts are less obvious. • General guideline for covalent reactants or products: • for carbon compounds, the addition of oxygen or the removal of hydrogen is always oxidation

Processes Leading to Oxidation & Reduction

Corrosion • Iron, often used in the • form of the alloy steel, • corrodes by being • oxidized to ions of iron • by oxygen. • 2Fe(s) + O2(g) + 2H2O (l) 2Fe(OH)2(s) • 4Fe(OH)2(s) + O2(g) + 2H2O (l) 4Fe(OH)3(s) • Equations describe the corrosion of iron to iron hydroxides in moist conditions.

Corrosion Water in the environment accelerates the rate of corrosion. Corrosion occurs more rapidly in the presence of salts and acids. Salts and acids produce electrically conducting solutions that make electron transfer easier.

Resistance to Corrosion • Not all metals corrode easily. • Gold and platinum are called noble metals because they are very resistant to losing their e- by corrosion. • Other metals lose electrons easily but are protected from extensive corrosion by the oxide coating formed on their surface. • Aluminum oxidizes quickly in air to form a coating of very tightly packed aluminum oxide particles.

Resistance to Corrosion • Iron also forms a coating when it corrodes • But 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 only a pile of rust.

Controlling Corrosion To prevent corrosion, the metal surface can be coated with oil, paint, plastic or another metal. Coatings exclude air and water from the surface, preventing corrosion. If coating is scratched or worn away, however, the exposed metal will begin to corrode.

Controlling Corrosion Another method of corrosion control One metal is “sacrificed” or allowed to corrode, in order to save a second metal. To protect an iron object, a piece of magnesium (or other active metal) may be placed in electrical contact with the iron. When oxygen and water attack the iron object, the iron atoms lose electrons as the iron being to be oxidized.

Controlling Corrosion Because magnesium is a better reducing agent than iron and is more easily oxidized the magnesium immediately transfers electrons to the iron, preventing their oxidation to iron ions. Magnesium is “sacrificed” by oxidation and protects the iron in the process.

Controlling Corrosion Sacrificial zinc and magnesium blocks are sometimes attached to piers and ship hulls to prevent corrosion damage in areas submerged in water. Underground pipelines and storage tanks may be connected to magnesium block for protection It is easier and cheaper to replace a block of magnesium or zinc than to replace a bride or a pipeline.

Question Can you identify the common chemical characteristic of all metal corrosion? The transfer of electrons from metals to oxidizing agents.

Questions Define oxidation and reduction in terms of the gain or loss of oxygen. Oxidation is the gain of oxygen Reduction is the loss of oxygen Define oxidation and reduction in terms of the gain or loss of electrons. LEO the lion goes GER Loss of Electrons is Oxidation Gain of Electrons is Reduction

Questions What happens to the atoms in an iron nail that corrodes? Iron atoms are oxidized when iron corrodes How do you identify the oxidizing agent and the reducing agent in a redox reaction? The species reduced is the oxidizing agent. The species oxidized is the reducing agent.

Questions Use electron transfer or electron shift to identify what is oxidized and what is reduced in each reaction. (use electronegativity values for molecular compounds) 2Na(s) + Br2(l) 2NaBr(s) Na oxidized, Br2 reduced H2(g) + Cl2(g) 2HCl(g) H2 oxidized, Cl2 reduced

Questions Use electron transfer or electron shift to identify what is oxidized and what is reduced in each reaction. (use electronegativity values for molecular compounds) 2Li(s) + F2(g) 2LiF(s) Li oxidized, F2 reduced S(s) + Cl2(g) SCl2(g) S oxidized, Cl2 reduced

Questions Use electron transfer or electron shift to identify what is oxidized and what is reduced in each reaction. (use electronegativity values for molecular compounds) N2(g) + 2O2(g) 2NO2(s) N2 oxidized, O2 reduced Mg(s) + Cu(NO3)2(aq) Mg(NO3)2(aq) + Cu(s) Mg oxidized, Cu reduced

Questions Identify the reducing agent and the oxidizing agent for each reaction. 2Na(s) + Br2(l) 2NaBr(s) Na reducing agent, Br2 oxidizing agent H2(g) + Cl2(g) 2HCl(g) H2 reducing agent, Cl2 oxidizing agent

Questions Identify the reducing agent and the oxidizing agent for each reaction. 2Li(s) + F2(g) 2LiF(s) Li reducing agent, F2 oxidizing agent S(s) + Cl2(g) SCl2(g) S reducing agent, Cl2 oxidizing agent

Questions Identify the reducing agent and the oxidizing agent for each reaction. N2(g) + 2O2(g) 2NO2(s) N2 reducing agent, O2 oxidizing agent Mg(s) + Cu(NO3)2(aq) Mg(NO3)2(aq) + Cu(s) Mg reducing agent, Cu oxidizing agent

End of Section 20.1

Oxidation Numbers Oxidation number is a + or – number assigned to an atom to indicate its degree of oxidation or reduction. General Rule A bonded atom’s oxidation # is the charge that it would have if the e- in the bond were assigned to the atom of the more electronegative element.

Rules for Assigning Oxidation Numbers • The oxidation number of a monatomic ion is = in magnitude and sign to its ionic charge. • Bromide (Br1-) is -1Iron III (Fe3+) is+3 • The oxidation number of hydrogen in a compound is +1, except in metal hydrides, such as NaH, where it is -1 • 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.

Rules for Assigning Oxidation Numbers • The oxidation number of an atom in uncombined (elemental) form is 0. • Potassium metal (K) is 0Nitrogen Gas (N2) is0 • For any neutral compound, the sum of the oxidation numbers of the atoms in the compound must equal 0 • For polyatomic ion, the sum of the oxidation numbers must equal the ionic charge of the ion.

Some Thought Determining oxidation numbers of elements in compounds is a way for chemists to keep track of electron transfer during redox reactions What are other examples where items are numbered to keep track of movement? The numbers on a sports player’s jersey The area codes assigned to telephone numbers in different regions

Binary Ionic Compounds In binary ionic compounds, such as NaCl and CaCl2, the oxidation numbers of the atoms equal their ionic charges. Na1+ + Cl-1 NaCl oxidation # +1 -1 neutral Ca2+ + Cl-1 CaCl2 oxidation # +2 -1 neutral Note the sign I put before the oxidation number

Molecular Compounds No ionic charges are associated with atoms of molecular compounds. However, oxygen is reduced in the formation of water for example. In water the two shared e- in the H – O bond are shifted toward the O and away from the H. Imagine the e- contributed by the two H atoms are completely transferred to the O.

Molecular Compounds The charge that would result from the transfer are the oxidation numbers of the bonded elements. The oxidation number of O is -2 and the oxidation number of each hydrogen is +1 Oxidation numbers are often written above the chemical symbols in a formula. +1 -2 H2O

Multiple Oxidation Numbers Many elements can have several different oxidation numbers. +1 +6 -2 K2CrO4 – Potassium Chromate +1 +12 -2 K2CrO7 – Potassium Dichromate

Sample Problems What is the oxidation number of each kind of atom in the following ions and compounds? +4 -2 SO2 +4 -2 CO32- +1 +6 -2 Na2SO4 -3 +1 -2 (NH4)2S

Sample Problems Determine the oxidation number of each element in the following. +3 -2 S2O3 +1 -1 Na2O2 +5 -2 P2O5 +5 -2 NO3-

Sample Problems Determine the oxidation number of chlorine in each of the following substances. +1 +5 -2 KClO3 0 Cl2 +2 +7 -2 Ca(ClO4)2 +1 -2 Cl2O

Chapter 20 Oxidation-Reduction Reactions