Unit 5- Topic 16
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Describe the term oxidation in terms of electron transfer and oxidation number
Oxidation is the loss of electrons An element in a species is oxidised when its oxidation number increases
Describe the term reduction in terms of electron transfer and oxidation number
Reduction is the gain of electrons An element in a species is reduced when its oxidation number decreases
To what elements of the Periodic table does the definitions of oxidation and reduction apply to
They apply to all elements whether they are in the s-, p- or d- block of the Periodic Table
Definition of standard electrode potential
The measured potential difference of a cell when no current is flowing (emf) when a half-cell is connected to a standard hydrogen electrode under standard conditions
What are the standard conditions that the standard electrode potential are measured by
298K temperature Gas pressure: 100kPa Concentration of ions: 1 mol dm-3 These standard conditions apply to all equilibria
What reactions are involved in the measurement of electrode potential
All redox reactions and therefore equilibria
What is the dynamic equilibrium formed of a metal immersed in water
The rate at which ions are leaving the surface of the metal to go into solution is the same as the rate at which they are joining it from the solution so: M x+ (aq) + xe- <——> M (s)
Why is the standard hydrogen electrode necessary
Because it is not possible to measure the absolute potential difference between a metal electrode and its solution since to establish a electrical connection with the solution a metal would be needed therefore you would be ensuring the potential difference between the two pieces of metal.
The conditions for only the SHE
Hydrogen gas at pressure: 100kPa Platinum foil dipped in solution Solution with concentration of [H+] = 1 mol dm-3 At 298K temperature
What is needed to connect a SHE to a metal half-cell
A high resistant voltmeter connected with platinum wire to metal and platinum foil of SHE A salt bridge
Why is the salt bridge needed in electrochemical cells
Allows movement of ions The ions in the salt ridge should not interfere with the ions in both half-cells
Why is the platinum foil covered in porous platinum
Porous platinum has a large surface area and allows an equilibrium between hydrogen ions in solution and hydrogen gas to be established quickly
Why is a high-resistance voltmeter used
To achieve no flow of electrons in the external circuit so that the voltmeter represents the difference in potential between the two half-cells when both reactions are in equilibrium
In electrochemical cells, inn which electrode does oxidation occur
Anode
Equation for emf (E cell)
E cell = E rhs - E lhs
How to measure E wit a systems involving gases
Gas is bubbled into a solution containing its ions. A piece of platinum covered in porous platinum is dipped into the solution to connect the gas molecules and its ions to the externa circuit through an electrical connection
How to calculate E cell of two non-metal elements and their ions
Each non- metal is compared to a SHE. A half-cell containing a solution of both ions (of one element) with each of concentration 1 mol dm-3 is connected to the SHE and E annotated. Same is done with the other non-metal, and redox reaction calculated.
Components needed to measure the E of ions of same element but different oxidation numbers
-platinum wire -platinum foil covered in porous platinum -solution with both ions at a concentration of 1 mol dm-3
What half cell goes to the left of the electrochemical cells
The negative electrode, the one with the most negative E value
What does the vertical solid line mean in a cell diagram
A phase boundary
What are the two rules to follow when drawing a cell diagram
1-the two reduced forms of the species are shown on the outside of the cell diagram 2-the positive electrode is shown on the right-hand side of the cell diagram
What does double vertical lines represent in cell diagram
Salt bridge
Draw the standard hydrogen electrode diagram
H+(aq) l 1/2 H2 (g) l Pt(s)
Draw the cell diagram of Fe 2+ and Fe 3+ half cell
Pt(s) l Fe 3+ (aq), Fe 2+ (aq)