Oxidation Reduction Equilibria and Titrations 1 Oxidation Reduction

Oxidation Reduction Equilibria and Titrations 1

Oxidation - Reduction reactions (Redox rxns) involve the transfer of electrons from one species of the reactants to another. This results in an increase in oxidation number (O. N. ) of a specific species and a complementary decrease in oxidation number of another species. Example: Ce 4+ + Fe 2+ D Ce 3+ + Fe 3+ The O. N. of cerium was decreased while that of iron was increased. Cerium is reduced while iron is oxidized. A process that involves an increase in O. N. is an oxidation process and vice versa 2

Usually, a Redox reaction can be separated into two halves. Ce 4+ + e D Ce 3+ Reduction Fe 2+ D Fe 3+ + e Oxidation Electrons appear in each half reaction while they do not show up in the overall equations. 3

Identification of a Redox Reaction It is a good practice to indicate the O. N. of each species in a chemical reaction in order to check if it is a Redox reaction or not. If the O. N. of any species changes, then it is a definite indication of a Redox reaction. Example, 2 KMn. O 4 + 5 H 2 C 2 O 4 + 6 HCl D 2 Mn. Cl 2 + 2 KCl + 10 CO 2 + 8 H 2 O It is observed that in the left-hand part of the equation, manganese has an O. N. equals 7 and carbon has an O. N. equals 3. In the right-hand part, the O. N. of manganese is 2 and that of carbon is 4. Therefore, permanganate is reduced while oxalic acid is oxidized. 4

An example of a non-Redox reaction can be written where no change in O. N. occurs, Na 2 CO 3 + +1 +4 -2 2 HCl = 2 Na. Cl + CO 2 + H 2 O +1 -1 +4 -2 +1 -2 There is no change in O. N. of any species involved in the reaction, which indicates that this is not a Redox reaction. 5

Balancing Redox Reactions Balanced chemical equations are the basis of any reasonable quantitative calculations. Therefore, it is very important to learn balancing chemical equations, especially Redox equations that can be manipulated through definite steps. For example, dichromate reacts with iron yielding Fe 3+ and Cr 3+ Cr 2 O 72 - + Fe 2+ D Cr 3+ + Fe 3+ To balance this equation, the following steps should be followed: 6

1. Split the equation into two half reactions Fe 2+ D Fe 3+ Cr 2 O 72 - D Cr 3+ 2. Proceed with each half reaction separately starting with mass balance. Let us start with first half reaction Fe 2+ D Fe 3+ One mole of Fe 2+ yields one mole of Fe 3+ which is balanced. 7

3. Balance the charges on both sides. Fe 2+ D Fe 3+ It is clear that an electron (e) should be added to the right side in order for the charges to be equal Fe 2+ D Fe 3+ + e This is a straightforward process but now consider the second half reaction, which contains species that are not mass balanced 8

Cr 2 O 72 - D Cr 3+ 1. Adjust the number of moles of chromium on both sides. Cr 2 O 72 - D 2 Cr 3+ 2. For each oxygen atom place an H 2 O on the other side Cr 2 O 72 - D 2 Cr 3+ + 7 H 2 O 3. Adjust the number of hydrogen atoms on both sides of the equation by adding H+ 14 H+ + Cr 2 O 72 - D 2 Cr 3+ + 7 H 2 O 4. The charges on both sides should be balanced at this point. This can be done by addition of 6 electrons to the left side of the equation 6 e + 14 H+ + Cr 2 O 72 - D 2 Cr 3+ + 7 H 2 O 9

5. This is the case if the reaction is carried out in acidic solution. The combination of the two half reactions necessitates the multiplication of the half reaction involving the iron by a factor of 6 in order to cancel the electrons involved 6 Fe 2+ D 6 Fe 3+ + 6 e + 14 H+ + Cr 2 O 72 - D 2 Cr 3+ + 7 H 2 O _____________________ 6 Fe 2+ + 14 H+ + Cr 2 O 72 - D 6 Fe 3+ + 2 Cr 3+ + 7 H 2 O This is the balanced equation assuming acidic conditions. In basic solutions, balancing Redox equations requires extra effort where all previous steps are needed in addition to other steps. 10

Example C 2 O 42 - + Mn. O 4 - D Mn 4+ + CO 2 First, proceed as in case of acidic solution. 1. Split the equation into two half reactions C 2 O 42 - D CO 2 Mn. O 4 - D Mn 4+ 2. Balance the first half reaction by adjusting the number of atoms on both sides (mass balance) C 2 O 42‑ D 2 CO 2 11

3. Adjust the charges on both sides (charge balance) C 2 O 42 - = 2 CO 2 + 2 e The first half reaction is completely balanced. Now, turn to balance the second half reaction Mn. O 4 - D Mn 4+ 1. Mass balance shows 1 mole of Mn on both sides, therefore Mn is adjusted. 2. Adjust the oxygens on both sides by placing an H 2 O for each oxygen atom present. Mn. O 4 - D Mn 4+ + 4 H 2 O 12

3. Adjust the number of hydrogen atoms on both sides by additon of H+ 8 H+ + Mn. O 4 - D Mn 4+ + 4 H 2 O 4. Adjusting the charges on both sides gives 3 e + 8 H+ + Mn. O 4 - D Mn 4+ + 4 H 2 O Now, watch carefully. 5. Add an OH- on both sides for each H+ present 8 OH- + 3 e + 8 H+ + Mn. O 4 - D Mn 4+ + 4 H 2 O + 8 OH 6. Combine the OH- and H+ to form H 2 O 3 e + 8 H 2 O + Mn. O 4 - D Mn 4+ + 4 H 2 O + 8 OH- 13

7. Adjust the number of H 2 O molecules on both sides 3 e + 8 H 2 O + Mn. O 4 - D Mn 4+ + 8 OH 8. Combine the two half reactions 3 (C 2 O 42 - D 2 CO 2 + 2 e) 2 ( 3 e + 8 H 2 O + Mn. O 4 - D Mn 4+ + 8 OH-) _______________________ 3 C 2 O 42 - + 8 H 2 O + 2 Mn. O 4 - D 6 CO 2 + 2 Mn 4+ + 16 OHThe first half reaction was multiplied by 3 and the second was multiplied by 2 in order to cancel the electrons on both sides. 14

Electrochemical Cells There are two types of electrochemical cells in the concept whether the cell generates potential (called a galvanic cell) or consumes potential (called an electrolytic cell). A cell is simply constructed from two electrodes immersed in solution. The electrode at which reduction occurs is called the cathode (-) while that at which oxidation occurs is called the anode (+). 15

Anode (+) Voltmeter (-) Cathode Semipermeable barrier The cells we will study in this chapter are of the first type where electrons generated by one half reaction will be consumed by the other half reaction forcing the current to flow. 16

Determination of Standard Electrode Potential The standard electrode potential of a half reaction can be determined using the cell above but replacing one electrode with a standard electrode like the standard hydrogen electrode (SHE) for which an arbitrary potential zero is assigned. Therefore, the electrode potential is the reading of the voltmeter. For example, when a 1. 00 M Cu 2+ solution is placed in contact with a Cu wire and the cell is completed with a SHE at standard conditions of temperature and pressure, the potential will read 0. 521 V. This is due to reaction: Cu 2+ + 2 e D Cu (s) Eo = 0. 521 V 17

2 H+ + 2 e- = H 2 Eo = 0. 000 V. when PH 2 = 1 atm, [H+] = 1 M, 298 K Hydrogen electrode 18

On the other hand, when Zn 2+ solution is placed in contact with a Zn electrode in an electrochemical cell with a SHE as the second electrode, the potential will read -0. 762 V. The reaction is: Zn 2+ + 2 e D Zn (s) Eo = - 0. 762 V From these results we can predict that Cu 2+ is a better oxidizing agent since the standard electrode potential is more positive than that for Zn 2+. However, SHE is troublesome and more convenient reference electrodes are used. Saturated calomel electrode (SCE) and Ag/Ag. Cl electrodes are most common. 19

The more positive the Eo, the better oxidizing agent is the oxidized form (e. g. , Mn. O 4 -). The more negative the Eo, the better reducing agent is the reduced form (e. g. , Zn). 20

The S. C. E. is a common reference electrode. The cell half-reaction is: Hg 2 Cl 2 + 2 e- = 2 Hg + 2 Cl-. E = 0. 242 V for saturated KCl. Commercial saturated calomel electrode. 21

Reference electrode potentials are all relative. The measured cell potential depends on the one used. Schematic representation of electrode potential relative to different reference electrodes. 22

A complete cell consists of an indicating electrode that responds to the analyte and a reference electrode of fixed potential. The potential difference between the two is measured. 23 Cell for potentiometric measurements.

Effect of Concentration on Electrode Potential The IUPAC convention for writing half-cell reactions is to represent the process as a reduction. The more positive half-cell reaction is the oxidizing agent (where the anode is) and the less positive half-cell reaction is the reducing agent (where the cathode is). The relationship between the concentration and the electrode potential for a half-cell reaction is represented by Nernst equation where for the half-cell reaction we have: a. A + b. B + ne D c. C + d. D Eo = x V 24

Calculating the Cell Potential The process of calculating the cell potential is simple and involves calculation of the potential of each electrode separately, then the overall cell potential can be determined from the simple equation: Ecell = Ecathode - Eanode Sometimes, this relation is written as: Ecell = Eright – Eleft This is true since the convention is to place the cathode to the right of the cell while the anode is placed to the left of the cell 25

For example, look at the following cell representation: Cu(s)| Cu. SO 4(0. 100 M) || Zn. Cl 2 (0. 200 M)|Zn(s) This cell is read as follows: a copper electrode is immersed in a 0. 100 M Cu. SO 4 solution (this is the first half-cell, anode), 0. 200 M Zn. Cl 2 solution in which a Zn(s) electrode is immersed (this is the second half-cell, cathode). If the Ecell is a positive value, the reaction is spontaneous and if the value is negative, the reaction is nonspontaneous in the direction written and will be spontaneous in the reverse direction. 26