1Chapter 11Electrochemistry – Oxidation and Reduction Reactions2Oxidation-Reduction ReactionsAn oxidation and reduction reaction occurs in both aqueous solutions and in reactions where substances are burned in the presence of oxygen gas, involves a transfer of electrons as the driving force of the chemical reaction. | The reactants in these reactions will lose or gain electrons and change their charge as they form the products. | The driving force of these chemical changes is electron transfer.3Oxidation-Reduction Reactions| Oxidation- reduction reactions are a very important class of chemical reactions. | They occur all around us and even within us. z The bulk of the energy needed for the functioning of all living organisms, including humans, is obtained from food through oxidation-reduction processes. z Such diverse phenomena as the electricity obtained from a battery to start a car, the use of natural gas to heat a home, iron rusting, and the functioning of antiseptic agents to kill or prevent the growth of bacteria all involve oxidation-reduction reactions. z The knowledge of this type of reaction is fundamental to understanding many biological and technological processes.4Oxidation-Reduction ReactionsHistorically, the word oxidation was first used to describe the reaction of a substance with oxygen. | According to this historical definition, each of the following reactions involves oxidation:4 Fe (s) + 3 O2(g) Æ 2 Fe2O3(s)S (s) + O2(g) Æ SO2(g)CH4(g) + 2 O2(g) Æ CO2(g) + 2 H2O (g)The reactant on the far left (in bold) in each of these chemicalequations is said to have been oxidized.5Oxidation-Reduction ReactionsOriginally, the term reduction referred to processes where oxygen was removed from a compound. | A particularly common type of reduction reaction, according to the original definition, is the removal of oxygen from a metal oxide to produce free metal.CuO (s) + H2(g) Æ Cu (s) + H2O (g) 2 Fe2O3(s) + 3 C (s) Æ 4 Fe (s) + 3 CO2(g) The word reduction comes from the reduction in mass of the metal oxide; the free metal has less mass than the metal oxide.6Oxidation-Reduction ReactionsToday the words oxidation and reduction are used in a much broader sense. | Current definitions include the previous examples but also include reactions with numerous non-oxygen containing substances. | Reactions that involve the transfer of electrons from one reactant to another reactant, regardless of the substances involve, are collectively called oxidation-reduction reactions (redox).7Oxidation-Reduction ReactionsFor all redox reactions, the reactants must lose electrons and gain electrons during the chemical process.Redox Reactions: OIL RIG| OIL Î oxidation is loss of electrons (e.g., metals in salts)| RIG Î reduction is gain of electrons (e.g., nonmetals in salts)8Oxidation NumbersThe name given to the charge on an element, ion or element in a covalent bond is called the oxidation state or oxidation numbers.For example: | Na+has an oxidation state of +1. | F–has an oxidation state of –1.9Oxidation NumbersTo determine what substance is oxidized and which is reduced, we must determine the original and final oxidation states of each substance during the chemical reaction.| To determine the oxidation states of all elements in molecules or ionic compounds, it is necessary to follow a few rules.10Oxidation NumbersOxidation NumbersTo assign oxidation numbers to elements in compounds, there are a few simple rules:1. The oxidation number of any free, uncombined element is zero.z Na (s), Mg (s), Cu (s), Fe (s) and Zn (s)11Oxidation Numbers| All naturally occurring diatomic molecules have zero oxidation states. z Br2I2N2Cl2H2O2F2z This makes sense—there is no dipole (difference in Electronegativity) between the shared electrons in the diatomic molecules.| Nonmetals, in their natural uncombined state, have zero oxidation states. z E.g., P4(s) and S8(s)12Oxidation Numbers2. For simple Group A binary ionic compounds (salts), the oxidation state is the charge of the element:| Metals (Group I A, IIA, and IIIA) are assigned a positive oxidation state – determined by the number of electrons the element has lost.| Nonmetals (Group VA, VIA, VIIA) are assigned a negative oxidation state determined by the number of electrons the element has gained.13Oxidation NumbersUse the periodic table to help with assigning oxidation numbers to elements.a. IA metals have oxidation numbers of +1.b. IIA metals have oxidation numbers of +2.c. IIIA metals have oxidation numbers of +3.• There are a few rare exceptions.14Oxidation Numbersd. VA elements have oxidation numbers of –3 in binary compounds with H, metals or NH4+.e. VIA elements below O have oxidation numbers of –2 in binary compounds with H, metals or NH4+.f. VIIA elements have oxidation numbers of –1 in binary compounds with H, metals or NH4+.Oxidation NumbersF–, Cl–, Br–, I––1VII nonmetalsO2–, S2–, Se2–, Te2––2VI nonmetalsN3–, P3–, As3–, Sb3–, Bi3––3V nonmetalsAl3+, Ga3+, In3++3IIIA metalsMg2+, Ca2+, Sr2+, Ba2++2IIA metalsLi+, Na+, K+, Rb+, Cs++1IA metalsMonoatomic IonsOxidation numberGroups16Oxidation Numbers3. In the formula for any compound, the sum of the oxidation numbers of all elements in the compound is zero. Na2SO4= zero charge on formula unitIn a polyatomic ion, the sum of the oxidation numbers of the constituent elements is equal to the charge on the ion.SO42-= - 2 on the ion.17Oxidation Numbers4. Fluorine has an oxidation number of –1 in its compounds.5. Hydrogen, H, has an oxidation number of +1 unless it is combined with metals, where it has the oxidation number - 1.z Examples – LiH, BaH218Oxidation Numbers6. Oxygen usually has the oxidation number –2.z Exceptions:z In peroxides O has oxidation number of –1.• Examples: H2O2, CaO2, Na2O2z In OF2, O has oxidation number of +2. (Fluorine is the most electronegative element and has the greater share of electrons.)19Oxidation NumbersFor simple binary ionic compounds that are comprised of transition metals (with more than one possible oxidation state), the oxidation state of the metal is determined by balancing the charge:z For our purpose, all transition metals in salts (except zinc and silver) have oxidation states that are determined by the balancing the charge of the anion.Oxidation Numberse.g. FeO vs. Fe2O3-2FeOz Since the oxidation state of oxygen is −2 in the