Redox Flashcards
Define galvanic cell.
Electrochemical cells in which spontaneous redox reactions occur. They convert chemical energy to electrical energy. Also known as voltaic cell or Daniel cell
Define electrode potential.
When there is a separation of charges and thus a potential difference between a metal electron and ion in solution arises.
Define standard electrode potential.
Potential of reduction half-equation under standard conditions of 298K, 1 bar and 1 mol dm^-3 measured relative to S.H.E.
Oxidation is when…
O.N of any element is increased, gain of oxygen or loss of electrons.
Reduction is when…
O.N of any element is decreased, loss of oxygen or gain of electron.
When is hydrogen’s charge not +1?
In metal hydrides, hydrogen has a -1 charge.
When is oxygen’s charge not -2?
In peroxides, oxygen has a -1 charge.
In a galvanic cell, where does oxidation occur ?
At the anode.
What charge do anode and cathode have in galvanic cells ?
Anode is the negative electrode, cathode is the positive electrode.
Why are the reactants separated in galvanic cells ?
If they are in direct contact, electrons will be directly transferred from one reactant to the other and can’t be used to provide current. Separating them creates a current.
What is the purpose of salt bridges ?
They maintain the charge balance in each half-cell. Ion mobility for cation and anion of salt bridge must be similar.
How are half-cells formed ?
Different chemical systems (ex: Cu and Zn), same solutions at different concentrations (ex: Cu at 1M, Cu at 2M) or the same gas at different pressure (hydrogen at 1 bar, hydrogen at 2 bar).
What are the standard conditions ?
298K/25 degrees Celsius, 1 bar, concentration of solution is 1M and gas is bubbled at 1 bar (if there is gas).
What is a standard hydrogen electrode made of ?
A platinum electrode coated with platinum black, solution containing 1M hydrogen ions, hydrogen gas at 100kPa pressure continuously bubbled through the solution over Pt electrodes.
Why use platinum black coating ?
It helps to increase surface area of electrode.
Define standard cell potential.
The potential difference between the cathode (reduction half-cell) and the anode (oxidation half-cell) of a cell.
Define electrochemical series.
A series in which elements (metals and non-metals) are arranged in order of their standard electrode potential.
Define redox series.
A series of all possible electrode potentials arranged in increasing values down the series (values get more positive down the series). Electrochemical series is part of the redox series.
Meaning of standard cell potential.
More positive standard electrode potential, the stronger oxidising agent it is. More negative standard electrode potential, the stronger reducing agent it is.
Define electrolytic cell.
Electrochemical cells in which non-spontaneous chemical reactions are made to occur by the forced input of electrical energy.
Define electrolysis.
The process of using an electric current to bring about chemical change.
What charge do anode and cathode have in electrolytic cells ?
Anode is the positive electrode. Cathode is the negative electrode.
In electrolytic cells, where do oxidation and reduction occur ?
Oxidation is at the anode and reduction is at the cathode.
Why is the voltage applied across the electrodes in an electrolytic cell higher than the standard cell potential ?
Because of electrical resistance and problem of overvoltage.
Define overvoltage.
Slow rate of attainment of equilibrium at the electrodes.
What is the formula of cryolite and why is it used in the electrolysis of aluminum oxide ?
Na3AlF6 is used to lower the high melting point of aluminum oxide.
State Faraday’s first law.
The mass of an element liberated at an electrode during electrolysis is directly proportional to the quantity of electricity that passes through the electrolyte solution.
State Faraday’s second law.
When the same quantity of electricity is passed through different electrolytes connected in series, the masses of different substances deposited or liberated are proportional to their chemical equivalent masses.
Define BOD.
It’s the amount of oxygen needed to oxidize the organic components of a water sample over 5 days at a specific temperature.
