✅14 - Redox II
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What is oxidation in terms of electrons?
Loss of electrons
What is reduction in terms of electrons?
Gain of electrons
What is oxidation in terms of oxidation number?
Increase in oxidation number
What is reduction in terms of oxidation number?
Decrease in oxidation number
Redox reactions are in…
…equilibria
What happens to the electrons when a metal is placed in water?
The electrons build up on the surface and result in a negative charge
How are positive ions attracted to a metal in water?
The build up of electrons on the surface attracts them, forming a layer of positive ions
How do some of the positive ions in solution form part of the metal?
Some of the positive ions in the layer regain their electrons from the build up of electrons below, becoming part of the metal surface
How is a dynamic equilibrium established when a metal is placed in water?
The rate at which ions are leaving the surface of the metal to go into solution is about the same as the rate at which they join the metal from solution
What is the absolute potential difference?
The potential difference between the metal and the solution
How can the absolute potential difference be measured?
By creating a reference electrode and measuring the difference in potential between this reference electrode and the metal electrode
Which reference electrode is usually used to calculate absolute potential difference?
Standard Hydrogen electrode
What is a standard hydrogen electrode?
Hydrogen gas at a pressure of 100kPa bubbling over a piece of platinum foil dipped into a solution of HCl with a hydrogen ion concentration of 1 moldm-3 at 298K
What does the platinum foil in a standard hydrogen electrode allow?
An equilibrium between hydrogen ions in solution and hydrogen gas to be established quickly due to the porous platinum
What are standard conditions?
100kPa (1 bar) gas pressure 298K Ion concentration of 1 moldm-3
What is a complete cell made up of?
Two half cells
How could the standard electrode potential of a Magnesium ion/metal system be measured?
By connecting the standard hydrogen electrode to the Magnesium electrode via a circuit containing a high resistance voltmeter
Why is a salt bridge needed?
To complete the circuit by allowing the movement of ions
What is a salt bridge usually made of?
Potassium Nitrate as a gel or liquid
What property should the salt bridge have?
Its ions should not interfere with the components of the half cells
Why is a high resistance voltmeter used?
To avoid any flow of current through the circuit
What are the two reactions which occur in a SHE?
H+(aq) + e- ⇌ 1/2H2(g) Mg2+(aq) + 2e- ⇌ Mg(s)
Why is magnesium the negative electrode of the cell in SHE?
Because if the voltmeter was removed, electrons would flow from the magnesium electrode to the hydrogen electrode
What does the negative sign in front of a standard electrode potential indicate?
That the equilibrium position of this reaction is further to the left than the equilibrium of the hydrogen electrode