19 Redox processes Flashcards
What is a redox equilibria?
A redox equilibria is achieved when there is a constant addition and reduction of atoms (e.g. Magnesium, Copper, …). This creates a constant negative charge on the cathode while also leading a to constant number of atoms (e.g. Magnesium ions, Copper ions,…) in the surrounding solution.
when rate of forward and backward reaction are equal.
What do electrochemical cells do?
Energy conversion between electrical and chemical energy lie at the core of electrochemical cells.
Name the two fundamental types of electrochemical cells and their functions.
Galvanic cells:
Convert chemical potential energy into electrical energy.
Electrolytic cells:
Use electrical energy to power chemical non spontaneous redox reactions.
(e.g. Electrolysis, …)
What are half-cells?
A metal bathing in the solution of its own ions.
How is the potential of an element as a electrode determined?
(three factors?)
Factors of electrode elements:
the nature of the element
the concentration of its ion in solution
the temperature of the solution
What are the three types of commonly encountered half-cells.
A metal immersed in its own solution.
An inert electrode immersed in an aqueous solution containing two ions of the same element in diffrent oxidation states .
A gas bubbling over an inert electrode immersed in an aqueous solution containg the ions of the gas, for example the standard hydrogen electrode, which has an equilibrium between hydrogen ions and hydrogen molecules.
What is BOD
(Biochemical oxygen demand)
Biochemical oxygen demand is the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period.
What does BOD indicate?
BOD indicates the amount of putrescible organic matter present in water. Therefore, a low BOD is an indicator of good quality water, while a high BOD indicates polluted water.
How much oxygen is required in water for fish to survive.
What is the maximium capacity of oxygen in water?
Solubility of oxygen and oxygen compounds
Water solubility of oxygen at 25oC and pressure = 1 bar is at 40 mg/L water.
What is the winkler method used for?
The Winkler Method is a technique used to measure dissolved oxygen in freshwater systems.
Why do voltaic cells work?
A voltaic cell is a way of exploiting an exothermic reaction.l
A redox reaction proceeds spontaneosly because it is energetically (and entropically) favourable for one species to be reduced and another oxidized.
A voltaic cell seperates these two reactions and connects them in such a way that electrons must flow in order for oxidation and reduction to take place, electrons must flow through a wire!
Electrode Potential (E⁰) values
- E⁰value can be used to predict the feasibilty of redox and cell reactions
- This is reduction (remains as shown in the series)
- The other reaction is oxidation - reverse the reaction as shown in series now write both equations sown)
- Subtract the cell potential for the oxidation reaction from the cell potential for the reduction reaction
- REMEMBER, even though equations are flipped, cell potential remain with the same symbls (+/-)
Cell potential (V) = (E⁰red.) - (E⁰oxi.)
Electrode potential (E⁰) and free energy change (△G)
△G⁰= -nFE⁰
n = number of moles of electrons transfered
F = Faraday constant, charge per mole, 96500 C/mol
For non-standard conditions (Option C - We will do this soon!) we use the Nernst Equation (in data booklet):
E = E⁰ - (RT/nF)lnQ
How do can you use standard electrode potentials to determine products of (and writing redox equations for) electrolysis
- Identify all ions in solution
- Identify which ions could go to each electrode
- Cations (+) ions will go to the cathode (-) and be reduced
- Anions (-) ions will go to the anode (+) and be oxidised
- Identify which of the two cations will actually be reduced: the reaction with the more positive E⁰ (lower down on reactivity series) will be reduced
- identify which of the two anions will actually be oxidised: the reaction with the more negative E⁰ will be oxidized
- If a concentrated halide solution - halogen will be preferentially kl
