Redox Reactions and Electrochemistry PDF

REDOX Reactions and Electrochemistry  Intended Learning Outcomes At the end of this topic, the students should be able to: Describe redox reactions; Balance redox reactions using change in oxidation method In accorda...

REDOX Reactions and Electrochemistry  Intended Learning Outcomes At the end of this topic, the students should be able to: Describe redox reactions; Balance redox reactions using change in oxidation method In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Intended Learning Outcomes At the end of this topic, the students should be able to: Describe Galvanic/Voltaic cells and solve related problems Describe electrolytic cells and solve related problems In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Intended Learning Outcomes At the end of this topic, the students should be able to: Explain the significance of the Nernst equation for different redox systems and solve problems related to the topic In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Oxidation Numbers charges assigned to atoms according to certain rules a measure whether an atom is neutral, electron-rich or electron-deficient In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Assigning Oxidation Numbers SOME Important Rules Atom in elemental state: ON = 0 For monoatomic ions, ON is equal to the charge of the ion F in all of its compounds: ON = -1 Oxygen usually has an ON = -2 except: ü when with F (eg. OF2), ON = +2 ü In peroxides (eg. H2O2, Na2O2), ON = -1 ü In superoxide ion, O2-, ON = -1/2 For H, ON = +1 except in hydrides (ON = -1) In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Assigning Oxidation Numbers Assign the oxidation number of each atom in the following compounds: 1. HNO2 2. Cr2O72- 3. SO2 4. FeSO4 5. KMnO4 6. ZnCO3 In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  REDOX Reactions characterized by the transfer of electron(s) from one substance to another OXIDATION reaction in which a substance loses electron(s) REDUCTION reaction in which a substance gains electron(s) In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  REDOX Reactions oxidation and reduction reactions occur simultaneously loss of 2 mol e- per mole Zn Cu2+ + Zn Cu + Zn2+ +2 0 0 +2 gain of 2 mol e- per mole Cu2+ The total no. of electrons lost must be equal to the total no. of electrons gained In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  REDOX Reactions REDUCING AGENTS tend to lose electrons easily (get oxidized) cause the reduction of another substance also called “reductants” OXIDIZING AGENTS tend to gain electrons easily (get reduced) cause the oxidation of another substance also called “oxidants” In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  REDOX Reactions Cu2+ + Zn Cu + Zn2+ OA RA REMEMBER! LEORA and GEROA LEORA: Loss of electrons, Oxidation, Reducing Agent GEROA: Gain of electrons, Reduction, Oxidizing Agent In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Balancing Redox Reactions Two Methods 1. Change in the oxidation number (CON) method 2. Half-reaction method In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Balancing Redox Reactions Change in ON Method 1. Assign ON to all atoms 2. Identify which atom was oxidized and which was reduced (show using line diagram) 3. Balance the total no. of electrons lost and the total no. of electrons gained by supplying coefficients. 4. Balance the rest of the atoms by inspection. In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Balancing Redox Reactions Change in ON Method gain of 5 mol e- per mol I gain of 10 mol e- per mol I2O5 +5 -2 +2 -2 0 +4 -2 I2O5 + 5 CO → I2 + 5 CO2 loss of 2 mol e- per mole C loss of 2 mol e- per mole CO (loss of 2 mol e- per mole CO) x 5 In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Balancing Redox Reactions Change in ON Method loss of 2 mol e- per mol S loss of 2 mol e- per mol H2S (loss of 2 mol e- per mole H2S) x 2 +1 -2 +4 -2 0 +1 -2 2H2S + SO2 → 3S + 2 H2O gain of 4 mol e- per mole S gain of 4 mol e- per mole SO2 In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Balancing Redox Reactions Exercises 1. K2Cr2O7 + S + H2O ⟶ KOH + SO2 + Cr2O3 2. MnSO4 + NaBiO3 + H2SO4 ⟶ NaMnO4 + Bi2(SO4)3 + H2O + Na2SO4 3. NH3 + O2 ⟶ NO2 + H2O In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution. Electrochemistry  Electrochemical Cell A physical arrangement such that the reducing and oxidizing agents do not come in direct contact. The electrons are transferred through an external circuit. In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Electrochemical Cell Basic Components of an Electrochemical Cell 1. Electrodes Immersed in electrolyte solutions ü Anode - oxidation ü Cathode – reduction ü Inert Electrode – electron bank, dispenses and receives electrons to and from the substances in the solution In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Electrochemical Cell Basic Components of an Electrochemical Cell 2. Electrolytic solutions 2. Salt Bridge ü maintains electrical neutrality In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Cell Notation/Diagram Consider the Daniell Cell OHR: Zn ⟶ Zn2+ + 2e- RHR: Cu2+ + 2e- ⟶ Cu Zn + Cu2+ ⟶ Cu + Zn2+ In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Cell Notation/Diagram phase boundary salt bridge Zn(s) Zn2+(aq) Cu2+(aq) Cu(s) anode cathode active electrolyte active electrolyte in the OHR in the RHR (anolyte) (catholyte) In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Cell Notation/Diagram Things to Remember ü chemical species (conventional symbols) ü phase boundary (I) ü salt bridge (II) ü separation between species of same phase (,) ü concentration/pressure (unit – in parenthesis) ü electrode polarity In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Cell Notation/Diagram Cu + Ag+ ⟶ Cu2+ + Ag Cu(s) Cu2+(aq) Ag+(aq) Ag(s) In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Cell Potential, Ecell the electromotive force (emf) of the Galvanic Cell ELECTROMOTIVE FORCE the force that drives the electrons to flow into the external circuit of a galvanic cell resulting to electric current measured in volts (V) In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Properties of Cell Potential, Ecell When a reaction is multiplied by a factor, emf is unchanged. The emf of the reverse reaction is equal in magnitude but opposite in sign to the potential of the forward reaction. Cu2+ + Zn ⇌ Zn2+ + Cu E°cell = 1.10 V Cu + Zn2+ ⇌ Cu + Cu2+ E°cell = -1.10 V In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Properties of Cell Potential, Ecell A positive overall potential for a reaction indicates that the reaction is spontaneous. Rxn 1: Cu2+ + Zn ⇌ Zn2+ + Cu E°cell = 1.10 V Rxn 2: Cu + Zn2+ ⇌ Cu + Cu2+ E°cell = -1.10 V Rxn 1 is spontaneous while rxn 2, it’s reverse is non- spontaneous. In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Standard Cell Potential, Eocell The emf obtained when: ü All ion concentrations are 1 M ü Partial pressure of gases are 1 atm ü Cell temperature is 25°C E°cell = E°cathode – E°anode E°cell = E°ox + E°red Can be calculated from the standard electrode potentials of the two half cells. In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Standard Reduction Potential, Eored Consider the given Electrochemical Cell The measured voltage is the electric potential difference between the anode and the cathode. The standard reduction potential, Eored for each half-cell is needed to predict the voltage that will be observed. In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Standard Reduction Potential, Eored The Standard Hydrogen Electrode (SHE) To find the relative electrode potentials, an electrode potential of zero is arbitrarily assigned to a particular half-cell and compare the electrode potentials of all other half-cells with this standard. By international agreement, the Standard Hydrogen Electrode (SHE) was chosen as the reference electrode. In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Standard Reduction Potential, Eored The Standard Hydrogen Electrode (SHE) The standard electrode potential of a half-cell is generated from a cell consisting of a SHE (anode) and the half-cell under standard conditions. STANDARD CONDITIONS Temperature: 298 K Gases: Partial Pressure = 1 atm Ionic Species: Concentrations = 1 M 2H+(aq) + 2e- ⇋ H2(g) Eo = 0.00 V In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Standard Reduction Potential, Eored The Standard Hydrogen Electrode (SHE) In the given cell, the measured potential is 0.799 V, hence it is the standard reduction potential for the reaction: 2Ag+(aq) + 2e- ⇋ 2Ag(s) The positive sign means that the electrons flow from left to right through the meter. In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  The Standard Reduction Potential Table The more positive Eo the greater the tendency for the substance to be reduced, therefore stronger OA The half-cell reactions are reversible The sign of Eo changes when the reaction is reversed Changing the stoichiometric coefficients of a half-cell reaction does not affect the value of Eo In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Standard Reduction Potential, Eored Consider the Eored of some reactions: Mg2+ + 2e– ® Mg E° = –2.37 V Zn2+ + 2e– ® Zn E° = –0.76 V 2H3O+ + 2e– ® H2 + 2H2O E° = 0.00 V Cu2+ + 2e– ® Cu E° = +0.34 V Cl2 + 2e– ® 2Cl– E° = +1.36 V Strength of OA: Cl2 > Cu2+ > H3O+ > Zn2+ > Mg2+ Strength of RA: Mg > Zn > H2 > Cu > Cl– In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Sample Problems 1. A galvanic cell consists of a Mn electrode in a 1.0 M Mn(NO3)2 solution and an Al electrode in a 1.0 M Al(NO3)3 solution. Calculate E°cell for this cell at 25°C Mn2+ + 2e– ® Mn E° = –1.05 V Al3+ + 3e– ® Al E° = –1.67 V In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Galvanic vs. Electrolytic Cell GALVANIC CELL - electrochemical cell in which the oxidation-reduction reactions proceeds spontaneously ELECTROLYTIC CELL – an electrochemical cell in which the oxidation-reduction reactions are driven by electricity In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Galvanic vs. Electrolytic Cell Galvanic Electrolytic Spontaneity of the reaction spontaneous Non-spontaneous Overall emf (Ecell) + - Energy conversion CE-EE EE-CE Anode reaction oxidation Cathode reaction reduction Polarity of anode - + Polarity of cathode + - Flow of electron Anode to cathode Flow of ions (a)Cation Cation goes to cathode (b)Anion Anion goes to anode In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution. Electrolysis and Electrolytic Cell  Electrolysis and Electrolytic Cells ELECTROLYSIS The process of applying electric current to effect a chemical change Redox reaction involved has a (–) cell potential, hence, non-spontaneous. In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Electrolysis ELECTROLYSIS OF WATER RHR: 2H3O+ + 2e– ® H2 + 2H2O OHR: 6H2O ® O2 + 4H3O+ + 4e– Overall: 2H2O(l) ⇌ O2(g) + 2H2(g) E°cell = –1.64 V In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Electrolysis ELECTROLYSIS OF MOLTEN SALTS Cations are reduced Anions are oxidized Electrolysis of pure NaCl RHR Na+ + e– ® Na E° = –2.71 V OHR 2Cl– ® Cl2(g) + 2e– E° = –1.36 V Overall 2Na+ + 2Cl– ⇌ Cl2(g) + Na(s) E°cell = –4.07 V In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Electrolysis ELECTROLYSIS OF MOLTEN SALTS Downs Cell Used in the commercial production of Cl2 and Na metal through electrolysis of molten NaCl In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Electrolysis ELECTROLYSIS OF AQUEOUS SALT SOLUTIONS Ions + water (can be oxidized or reduced) More than one possible reaction can occur at the electrodes (competing reactions) In predicting most probable reactions: ü Standard reduction potentials are used (standard conditions are assumed) In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Electrolysis Consider aqueous NaCl Species present: ü Na+, Cl-, H2O ü traces of H3O+ and -OH In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Electrolysis Consider aqueous NaCl Species that may undergo reduction are Na+, H2O, and H3O+ Possible Cathode Reactions: Na+(aq) + e– ® Na(s) E° = –2.71 V 2H2O(l) + 2e– ® H2(g) + 2OH– E° = –0.83 V 2H3O+(aq) (10–7 M) + 2e– ® H2(g) + 2H2O E° = –0.41 V In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Electrolysis Consider aqueous NaCl Species that may undergo oxidation are Cl–, H2O, and OH– Possible Anode Reactions: 2Cl–(aq) ® Cl2(g) + 2e– E° = –1.36 V 6H2O(l) ® O2(g) + 4H3O+ + 4e– E° = –1.23 V In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Electrolysis Consider aqueous NaCl Possible Cathode Reactions: Na+(aq) + e– ® Na(s) E° = –2.71 V 2H2O(l) + 2e– ® H2(g) + 2OH– E° = –0.83 V 2H3O+(aq) (10–7 M) + 2e– ® H2(g) + 2H2O E° = –0.41 V Highly negative potential for the reduction of Na+ [H3O+] is too low (10-7 M) In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Electrolysis Consider aqueous NaCl Possible Anode Reactions: 2Cl–(aq) ® Cl2(g) + 2e– E° = –1.36 V 6H2O(l) ® O2(g) + 4H3O+ + 4e– E° = –1.23 V Overvoltage/Overpotential Additional voltage that must be applied beyond the cell potential in order to effect electrolysis O2 production requires additional ~0.4-0.6 V In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Electrolysis Consider aqueous NaCl Reactions: RHR 2H2O(l) + 2e– ® H2(g) + 2OH– E° = –0.83 V OHR 2Cl–(aq) ® Cl2(g) + 2e– E° = –1.36 V Overall 2Cl– + 2H2O ⇌ Cl2(g) + H2(g) + 2OH– E°cell = –2.19 V In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Electrolysis SAMPLE PROBLEMS An aqueous solution of K2SO4 is electrolyzed by means of Pt electrodes. A. Which of the following gases should form at the anode: O2, H2, SO2, SO3? Explain. B. What product should form at the cathode? Explain. C. What is the minimum voltage required? Why is the actual voltage needed likely to be higher than this value? In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Quantitative Aspects of Electrolysis How much chemical change occurs when a given current flows for a specific period of time? Faraday’s Law The amount (in moles) of a product formed by an electric current is chemically equivalent to the amount (in moles) of electrons supplied Michaeal Faraday In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Quantitative Aspects of Electrolysis Useful relationships ü 1 mole e–s = 1 Faraday = 96,485 coulombs ü 1 coulomb = 1 ampere-sec (A-s) Note: electric current is measured in amperes In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Quantitative Aspects of Electrolysis In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.  Quantitative Aspects of Electrolysis SAMPLE PROBLEMS 1. In an electrolytic cell, how many grams of nickel (MM = 58.69 g/mol) will be deposited on a metal sheet from a solution containing Ni2+ when a current of 10.0 amperes flows for 1.50 hours? In accordance with Sec. 185. Fair Use of a Copyrighted Work of Republic Act 8293, the copyrighted works included in this material may be reproduced for classroom purposes only and not for commercial distribution.