Electrochemistry: Introduction to Galvanic/Voltaic Cells

Electrochemistry - Electrochemistry: Galvanic/Voltaic cells traditionally are used as sources of DC (direct current) electrical power and derives its energy (current) from spontaneous redox reactions “Red Cat An Ox” shows that reduction occurs at the cathode and oxidation occurs at the anode (negative terminal) In order to go from reduction potential to oxidation potential, you just flip the sign ° = −𝑛𝐹𝐸° where n is the moles of electrons transferred, E° is the standard state EMF (V or J/C), and F is Faraday’s 𝛥𝐺 constant (105 C/mol e-) ° = −𝑅𝑇𝑙𝑛𝐾𝑒𝑞 allows us to find the equilibrium constant K where R = 8.314J/molK 𝛥𝐺 "# Therefore, 𝐸 ! = $% 𝑙𝑛(𝐾&' ) ° − 𝑅𝑇 𝑙𝑛(𝑄) where Q = 𝑛𝐹 Nernst Equation allows us to use current concentrations with the law of mass action 𝐸 = 𝐸 [)*!+,-./]! [*&1-.1$./]" and helps show the instantaneous cell potential at different points during a reaction (leave out solids in Q) → As Q increases, E decreases At 25°C, RT/F = 0.0257 J/C or V For other equations similar to this, look at the image to the right → Logarithm Approximate Value Actual Value log(1) 0 0 log(3) 0.5 0.48 log(5) 0.7 0.70 log(7) 0.9 0.85 log(10) 1.0 1.0 - Electrolytics cells an electrical current is used to drive a nonspontaneous redox reaction This cell can be used for the decomposition of compounds such as water, NaCl, into simpler compounds In aqueous solution, at the cathode, if a metal is more reactive than hydrogen, the reduced metal reacts with water to produce H2 gas and if the metal is less reactive than hydrogen, then the reduced metal does not react with water Electroplating, which occurs at the cathode, means we can calculate the amount of metal plated on the cathode from charge/current that passes through the cell → Qcharge of e- from battery = Qcharge of e- for reduction where It = nF where I is current, n is moles of electrons, t is time, and F is Faraday’s constant Redox Reaction from Dissolving Zinc in Copper Sulfate: Think electronegativities when comparing whether the atom wants electrons or to given them up When dealing with ionic compounds the oxidation state is the same as the charge of the ion Introduction to Galvanic/Voltaic Cells: Once a metal loses/gains electrons, it can now dissolve into water since it is now polar A salt bridge allows for neutralization of the generation of positive/negative charge forming in solution in order for current to keep flowing through the wire (positive will flow into the negative liquid and negative will flow into the positive liquid, effectively neutralizing the liquid - Electrodes & Voltage of Galvanic Cell: Negative terminal (anode) is where electrons are coming from/oxidation is occurring and positive terminal (cathode) is where electrons are going to/reduction is occurring (“Red Cat An Ox”) Standard electrode potentials is relative to hydrogen reactivity and measures different ions (gives you values based on reduction half reactions) Add up the potentials of the terminals in order to come up with the voltage of the entire battery E° = Ered + Eox Standard reduction potential of hydrogen is 0 V Positive standard reduction potentials lead to stronger oxidizing agent and negative standard reduction potentials lead to strong reducing agents Shorthand Notation for Galvanic/Voltaic Cells: Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s) ← anode is the left and cathode is the right (a comes before c) Voltage as an Intensive Property: ΔG° changes based on how many moles you have (extensive property) but voltage does not change (same voltage to form 2 moles of a substance as it is to form 1 mole of a substance and is therefore an intensive property) → NEVER MULTIPLE VOLTAGE BY 2 IN THE STANDARD ENERGY EQUATION EVER If E° is negative, then ΔG° is positive, and therefore nonspontaneous Concentration Cell: When the concentrations of liquid are different, the concentrations want to equilibrate and become equal, which is enough to generate x amount of current When concentrations are equal, then E = 0 Introduction to Electrolysis: For an Electrolytic cell, an electrical current is used to drive a nonspontaneous redox reaction (E° < 0) and therefore needs a battery in order to drive the reaction (you need a voltage greater than the E°) Reduction occurs at the cathode and oxidation occurs at the anode (always) Cathode in this case is negative and the anode for this case is positive (opposite for galvanic cells) Quantitative Electrolysis: Remember, A = Q/t in units of C/s and you can find the charge, then use Faraday’s (C/mol e-) constant in order to figure out the moles of electrons gained/lost