Electrochemistry

Electrochemistry Electrochemistry is “the study of interchange between chemical change and electrical work” Redox reactions are reactions involving the transfer of electrons

OIL RIG Oxidation: Is Loss of electrons Reduction: Is Gain of electrons -------------------------------------------- Oxidation and reduction are the two halves of a redox reaction and the transfer of electrons.

Oxidation Numbers: 1, 2, 3…. Pure Element: Oxidation number = 0 Monatomic Ion: Oxidation number = ion’s charge Ca2+ Oxidation number = +2 Cl- Oxidation number = -1 Neutral Compound: Oxidation number = 0 Polyatomic Ion: Oxidation number = sum of all oxidation numbers in ion and/or the ion’s charge ClO4+ Oxidation number = +1 Oxidation numbers sometimes reffered to as oxidation states

Oxidation / Reduction Agents Reduction Agent --> Oxidized Product Causes reduction for other product, therefore is the reduction agent. Oxidising Agent --> Reduced Product Causes reduction for other product, therefore is the oxidising agent.

Balancing a Redox Reaction Separate and write half-reactions for reduction and oxidation. Balance elements and electrons (charges) in each half reaction. Balance number of electrons between half reactions. Add half reactions and cancel shared components

Balancing a Redox Reaction: Part II Redox with acidic conditions Balance oxygen using H2O Balance hydrogen using H+ Redox with basic conditions Balance oxygen using OH- Balance hydrogen using H2O Be sure to check that all charges and atoms that balance

Cells (not the ones you’re made of) Voltaic / galvanic vs. Electrolytic Cells A galvanic cell is spontaneous An electrolytic cell is non-spontaneous and requires an outside source of energy Spontaneous nature of a galvanic cell relates to the energies utilized in oxidation and reduction

Cell Components

Cathode / Anode and Reduction / Oxidation Vowels with vowels, and consonants with consonants ORCA

Build a Cell Workshop!

E°Cell = cell potential at standard conditions All elements exist in their standard states Solutions are 1M Temperature is 298 K Pressure at 1 ATM Degree symbol denotes “standard”

Cell Potential cont. E°Cell = E°Cathode - E°Anode Calculation of standard cell potential using reduction potentials of cathode and anode E°Cell determines spontaneity of the redox reaction being utilized in the cell E°Cell > 0 ----> Spontaneous E°Cell < 0 ----> Not Spontaneous E°Cell = 0 ----> At Equilibrium

Cell Potential and Work Cell potential can be related to work using the equation for charge Charge (q) = nF n = moles of electrons F = Faraday’s constant F = 96,485 C / mol e- C = Coulombs, unit of charge

Cell Potential and Work cont. Given that w = -qE°Cell The equation w = -nFE°Cell can be derived using the equation for charge Gibb’s Free Energy (ΔG) Because ΔG is defined as energy available to do work: ΔG°= -nFE°Cell ΔG° determines spontaneity ΔG° < 0 -----> Spontaneous ΔG° > 0 -----> Not Spontaneous ΔG° = 0 -----> At Equilibrium

Cell Potential and Equilibrium

Nernst Equation

Sources https://chem.libretexts.org/Core/Analytical_Chemistry/Electrochemistry/Redox_Ch emistry/Balancing_Redox_reactions Gates’ powerpoint

Answers For Handout Oxidation state of sulfur in S2O3 = +3 Oxidation state of oxygen in S2O3 = -2 Oxidation state of calcium in Ca(OH)2 = +2 Oxidation state of nitrogen in N2 = 0 Free Response Answer: The sign of the cell potential will be positive because (any one is sufficient): K is greater than 1 the reaction is spontaneous (occurs) E° for Sr2+ is more positive Standard reduction potential for Sr more negative E° = + 0.52 V The oxidizing agent is Mg2+ The cell potential would increase Since all ions are at 1 M, Q for the system is 1 and E° = (RT/nF) ln K so as T increases, so should E° Ecell will increase In the equation Ecell = E° - (0.0592 / n) log Q Q = 0.1 therefore log Q is negative therefore term after E° is positive therefore Ecell increases OR with the concentration of Mg2+ larger than that of Sr2+, Le Chatelier's principle predicts the reaction will have a larger driving force to the right and a more positive Ecell At equilibrium, Ecell = 0 EoCell = 0.460V Drawing of the cell requires electrodes (matching the reaction or platinum), porous disk or salt bridge with ions going the correct direction, electron flow the correct direction, salt solution labeled