Slide 1 / 144 Electrochemistry Slide 2 / 144 Electrochemistry · Electrochemistry deals with relationships between reactions and electricity · In electrochemical reactions, electrons are transferred from one species to another. · Provide insight into batteries, corrosion, electroplating, spontaneity of reactions Slide 3 / 144 Electrochemical Reactions · In electrochemical reactions, electrons are transferred between various reactant and product species in reactions. · As a result, oxidation state/number of one or more substances/species change · Oxidation number is the formal charge on the atom when it is connected to other atoms. · In order to keep track of what species loses electrons and what gains them, we assign oxidation numbers/oxidation states to individual atoms.
Slide 4 / 144 Oxidation Numbers Take a look at this reaction between Zn metal and acid with assigned oxidation numbers. How do we know what number goes with each atom? Where do these numbers came from? Zn(s) + 2H + (aq) ➝ Zn 2+ + H 2 (g) 0 +2 0 +1 Slide 5 / 144 How do we assign oxidation numbers ? Rules for Assigning Oxidation Numbers Elements in their elemental form have an Elements oxidation number of 0. Compounds The sum of the oxidation numbers in a neutral compound is 0. The oxidation number of a monatomic Monoatomic ion is the same as its charge. ions The sum of the oxidation numbers in a Polyatomic ions polyatomic ion is the charge on the ion. Slide 6 / 144 Rules for Assigning Oxidation Numbers Hydrogen -1 when bonded to a metal +1 when bonded to a nonmetal Fluorine always has an oxidation Fluorine number of -1. Usually -1. Other May have positive oxidation numbers in halogens oxyanions. For example, Cl has an oxidation number of +5 in ClO 3 - .
Slide 7 / 144 Rules for Assigning Oxidation Numbers Nonmetals tend to have negative Nonmetals oxidation numbers although some are positive in certain compounds or ions. Oxygen has a oxidation number of -2, Oxygen except in the peroxide ion, when its oxidation number is -1. Slide 8 / 144 1 What is the oxidation number of each oxygen atom in the compound MnO 2 ? A -2 B -1 C 0 D +1 E +2 Slide 9 / 144 2 What is the oxidation number of the manganese atom in the compound MnO 2 ? A +3 +2 B C +1 D +4 E +7
Slide 10 / 144 3 What is the oxidation number of oxygen atom in MnO 4 1- , the permanganate ion? A -2 B -1 C 0 D +2 E +4 Slide 11 / 144 4 What is the oxidation number of the manganese atom in MnO 4 1- , the permanganate ion? A +1 B +2 C +5 D +4 E +7 Slide 12 / 144 5 What is the oxidation number of sulfur in HSO 4 1- , the hydrogen sulfate ion? A -2 +1 B +2 C D +4 E +6
Slide 13 / 144 Oxidation and Reduction Zn(s) + 2H + (aq) ➝ Zn 2+ + H 2 (g) 0 0 +2 +1 Oxidation-loss of electrons A species is oxidized when it loses electrons. Here, zinc loses two electrons to go from neutral Zn metal to the Zn 2+ ion. Zn is also a reducing agent- provides electrons (reductant) Reducing agent loses electrons. LEO OIL The lion says RIG GER Slide 14 / 144 Oxidation and Reduction Zn(s) + 2H + (aq) ➝ Zn 2+ + H 2 (g) +2 0 0 +1 Reduction- gaining of electrons A species is reduced when it gains electrons. Here, each of the H + gains an electron, and they combine to form H 2 . H is an oxidizing agent- accepts electrons (oxidant) An oxidizing agent gains electrons. Slide 15 / 144 Oxidation and Reduction Zn(s) + 2H + (aq) ➝ Zn 2+ + H 2 (g) 0 +2 0 +1 · What is reduced is the oxidizing agent. · H + oxidizes Zn by taking electrons from it. · What is oxidized is the reducing agent. · Zn reduces H + by giving it electrons.
Slide 16 / 144 Oxidation and Reduction Redox Reactions Zn(s) + 2H + (aq) ➝ Zn 2+ + H 2 (g) 0 +2 0 +1 An electrochemical reaction in which oxidation and reduction occurs is known as a REDOX reaction Slide 17 / 144 6 Which of the following is/are an oxidation- reduction (redox) reactions? (a) K 2 CrO 4 + BaCl 2 ➝ KCl + BaCrO 4 (b) Pb 2+ + 2 Br 1- ➝ PbBr 2 (c) Cu + S ➝ CuS A a only b only B C c only D a and c b and c E Slide 18 / 144 7 Which substance is oxidized in the following reaction? (First, assign oxidation numbers.) Cu + S ➝ CuS A Cu B S C Cu and S D CuS E This is not a redox reaction.
Slide 19 / 144 8 Which substance is the reducing agent below? Cu + S ➝ CuS A Cu B S C Cu and S D CuS E This is not a redox reaction. Slide 20 / 144 9 Which substance is oxidized in the following reaction? (First, assign oxidation numbers.) Ca + Fe 3+ ➝ Ca 2+ + Fe A Ca Fe 3+ B Ca 2+ C D Fe E This is not a redox reaction. Slide 21 / 144 10 Which substance is the oxidizing agent below? Ca + Fe 3+ ➝ Ca 2+ + Fe A Ca Fe 3+ B Ca 2+ C D Fe E This is not a redox reaction.
Slide 22 / 144 11 Which substance is reduced in the following reaction? (First, assign oxidation numbers.) 3 K + Al(NO 3 ) 3 ➝ Al + 3 KNO 3 A K B Al C N D O E This is not a redox reaction. Slide 23 / 144 12 Which substance is the reducing agent? 3 K + Al(NO 3 ) 3 ➝ Al + 3 KNO 3 A K B Al(NO 3 ) 3 C KNO 3 D This is not a redox reaction. Slide 24 / 144 Redox Practice 1 H 2 S (g) + Cl 2 (g) --> 2HCl (g) + S (s) a) Assign oxidation numbers to each element above. b) Which element is oxidized? c) Which element is reduced? d) Name the reducing agent. e) Name the oxidizing agent.
Slide 25 / 144 Redox Practice 2 SnCl 2 (aq) + 2HgCl 2 (aq) --> SnCl 4 (aq) + Hg 2 Cl 2 (s) a) Assign oxidation numbers to each element above. b) Which element is oxidized? c) Which element is reduced? d) Name the reducing agent. e) Name the oxidizing agent. Slide 26 / 144 13 Which element is oxidized in the reaction below? 2- ➝ Fe 3+ + Cr 3+ + H 2 O Fe 2+ + H + + Cr 2 O 7 Slide 27 / 144 14 H 2 S (g) + Cl 2 (g) --> 2HCl (g) + S (s) Which is oxidized? Which is reduced?
Slide 28 / 144 15 SnCl 2 (aq) + 2HgCl 2 (aq) --> SnCl 4 (aq) + Hg 2 Cl 2 (s) Which is oxidized? Which is reduced? Slide 29 / 144 Redox reactions in aqueous solutions A large number of redox reactions occur in aqueous solutions. Unlike acid base nutralization and precipitation reactions,most of the reaction proceed slowly. Each redox reaction is the sum of two half reactions: Consider the reaction of iodide ions and hydrogen peroxide. 2I- (aq) + H 2 O 2 (aq) + 2e- ⇒ I2 + 2OH- (aq) + 2e- Slide 30 / 144 Redox reactions in aqueous solutions This reaction involves two parts as represented below. 2I- (aq) + H 2 O 2 (aq) + 2e- ⇒ I 2 + 2OH- (aq) + 2e- 1. Oxidation half reaction 2. Reduction half reaction. 2I - (aq) ⇒ I 2 + 2e - oxidation H 2 O 2 (aq) + 2e - ⇒ 2OH - (aq) reduction Add the two half reactions to get the overall reaction. 2I- (aq) + H 2 O 2 (aq) + 2e- ⇒ I 2 + 2OH- (aq) + 2e- How do we balance a redox reaction?
Slide 31 / 144 Balancing Redox reactions Half-reaction method (oxidation # method) · Assign oxidation numbers to determine what is oxidized and what is reduced. · Identify the oxidation and reduction process. · Write down the individual oxidation and reduction equations. · Balance these half reactions · Combine them to attain the balanced equation for the overall reaction. This method can be used in general to balance any redox reaction unless any specific condition such as acidic or basic is mentioned Slide 32 / 144 Half-reaction method (oxidation # method) Let us consider the simple replacement reaction of Mg with AgCl Mg + AgCl ➝ Ag + MgCl 2 -1 0 -1 +1 0 +2 Oxidation: Mg --> Mg 2+ + 2 e - --------(1) Reduction: Ag + + 1 e - --> Ag ---------(2) Since all the atoms are balanced, we need to balance only electrons. Multiply equation (2) x 2 Oxidation: Mg --> Mg 2+ + 2 e- --------(1) Reduction: 2Ag + + 2 e- --> 2Ag ---------(3) Slide 33 / 144 Half-reaction method (oxidation # method) Adding the half-reactions (1) and (3) yields the following: Oxidation: Mg --> MgCl 2 + 2e - Reduction: 2AgCl + 2e - --> 2Ag Overall: Mg + 2AgCl + 2e - --> MgCl 2 + 2e - + 2Ag and we cancel out electrons from both sides: Overall: Mg + 2AgCl + 2e - --> MgCl 2 + 2e - + 2Ag Net equation: Mg + 2AgCl --> MgCl 2 + 2Ag Since the original equation is given with chlorine you would keep it here in the final balanced equation too.
Slide 34 / 144 Redox reactions -balancing Practice Fe 3 O 4 +C --> Fe + CO Fe3O4 + 4C --> 3Fe + 4CO Slide 35 / 144 Redox reactions -balancing Practice: SnO 2 + C --> Sn + CO SnO 2 + 2C --> Sn + 2CO Slide 36 / 144 The Half-Reaction Method In acidic medium: Other atoms This diagram shows the steps involved in balancing half-reactions. · Write down the individual half reaction. O · First balance atoms other than H and O. · Balance oxygen atoms by adding H 2 O. · Balance hydrogen atoms by adding H + . H · Balance charge by adding electrons. · Multiply the half-reactions by integers so e- that the electrons gained and lost are the same.
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