Electrochemistry Chapter 18

Redox Review: Oxidation numbers = oxidation states Just like charges Use periodic table as needed Overall charge is 0 unless something indicated Oxidation = loss of electrons; number gets more positive Reduction = gain of electrons; number gets less positive LEO GER Half Reactions = separate the oxidation and reduction into equations Balance separately and then combine

Balancing Redox BASE: For each half reaction: Balance all elements not H and O Balance O by adding H2O Balancing H by adding H+ Balance charges by adding electrons Need electrons to be equal so cancel out – multiply one half reaction or both by an integer to be equal Add both half reactions together, cancel out anything same on both sides of arrow. Add OH- ions to both sides of equation (to equal H+ ion) combine these to make water cancel out water from both sides if needed ACID: For each half reaction: Balance all elements not H and O Balance O by adding H2O Balancing H by adding H+ Balance charges by adding electrons Need electrons to be equal so cancel out – multiply one half reaction or both by an integer to be equal Add both half reactions together, cancel out anything same on both sides of arrow.

Examples

Electrochemistry Study of the interchange of chemical and electrical energy Generation of an electric current from a spontaneous reaction Or a spontaneous reaction that generates an electric current Galvanic cells Device which changes chemical energy to electrical energy Uses a spontaneous redox reaction to produce a current that can be used to do work Electron transfer or flow of electrons causes current Need connection for electrons to flow between half reactions Separate the half reactions - one oxidation and one reduction

Components of Cell Need two solutions which will house the half reactions Oxidation = anode Reduction = cathode Electrons flow from anode to cathode Solutions will be connected Salt bridge – U tube filled with an electrolyte Porous disk Wires Measure the flow of electrons – electrical potential or cell potential (Ecell) Measured as voltage (V) 1 volt = 1 joule of work per columb ( 1V = 1 J/C)

Standard Reduction Potentials p845 Table 18.1 – Standard Reduction Potentials All based on hydrogen as standard potential = 0 All given as Reduction potentials (pay attention) Each half reaction will be assigned a cell potential To find the potential of the overall reaction, need to add both half reaction potentials together E°cell = E°cathode – E°anode When a half reaction is reversed, the sign of E° is reversed also When the half reaction is multiplied by an integer, E° remains the same Cell runs in the direction that gives a positive E value

Describing a Cell Line Notation – shorthand Instead of drawing a cell diagram use this shorthand way cathode on the right and the anode on the left. phases of all reactive species are listed and their concentrations or pressures are given if not in standard states (1 atm. for gasses and 1M for solutions). phase interfaces are noted with a single line ( | ) and multiple species in a single phase are separated by commas. salt bridge or porous disk is shown as a double line ( || ). EX: Mg (s) | Mg2+ (aq) || Al3+ (aq) | Al (s) Pt (s) | H2 (g) | H+ (aq) || Cu2+ (aq) | Cu (s)

Describing a Cell Drawing a diagram: (Schematic diagram) Label all parts Show half reactions Indicate direction of electron flow

Describing a Cell Write out or tell what is going on:

Relating free energy ∆G° = -nFE° Standard conditions n = number of electrons F = Faraday’s constant = 96,485 C/mol e- ∆G negative and Ecell positive = conditions for spontaniety

Batteries Battery = galvanic cell, or group of galvanic cells connected Potentials of individual cells added together to give battery potential Types of batteries: Lead storage battery – car battery Contains lead (both cathode and anode) and sulfuric acid Dry cell battery – AA, AAA, C, D Acid: zinc (anode); carbon rod (cathode); moist paste MnO2 & NH4Cl Alkaline: instead of NH4Cl has KOH or NaOH Lithium-ion or Nickel-cadmium batteries Fuel cells – reactants continuously supplied

Corrosion Involves oxidation of a metal prevent corrosion Natural process for many metals prevent corrosion Galvanizing - coating metals to protect them from oxidation Similar to plating Alloying – mixture of metals Cathodic protection – active metal attached by a wire to something, usually pipes

Electrolyisis Opposite of what did so far Forcing an electric current through a cell to produce a chemical change Cell potential would be negative Non-spontaneous reaction Requires an electrolytic cell (not galvanic cell) Electrolysis of water Electroplating