Edexcel A Level Chemistry: 14: Redox II

Loss of electrons

Gain of electrons

Increase in oxidation number

Decrease in oxidation number

…equilibria

The electrons build up on the surface and result in a negative charge

The build up of electrons on the surface attracts them, forming a layer of positive ions

Some of the positive ions in the layer regain their electrons from the build up of electrons below, becoming part of the metal surface

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

The potential difference between the metal and the solution

By creating a reference electrode and measuring the difference in potential between this reference electrode and the metal electrode

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

An equilibrium between hydrogen ions in solution and hydrogen gas to be established quickly due to the porous platinum

100kPa (1 bar) gas pressure
298K
Ion concentration of 1 moldm-3

Two half cells

By connecting the standard hydrogen electrode to the Magnesium electrode via a circuit containing a high resistance voltmeter

To complete the circuit by allowing the movement of ions

Potassium Nitrate as a gel or liquid

Its ions should not interfere with the components of the half cells

To avoid any flow of current through the circuit

H+(aq) + e- ⇌ 1/2H2(g)

Mg2+(aq) + 2e- ⇌ Mg(s)

Because if the voltmeter was removed, electrons would flow from the magnesium electrode to the hydrogen electrode

That the equilibrium position of this reaction is further to the left than the equilibrium of the hydrogen electrode

That the equilibrium position of the reaction is further to the right than the equilibrium position of the standard hydrogen electrode

The more readily the metal looses electrons to form ions

The less readily the metal loses electrons to form ions

The potential difference when no current is flowing through the external circuit

The emf of a cell containing the half-cell connected to the standard hydrogen electrode

A reducing agent

F- as it is the ion with the least negative redox potential

The standard hydrogen electrode, set at zero

…the more the equilibrium lies to the left and the more readily the species to the right looses electrons. The more powerful the reducing agent.

…the more the equilibrium lies to the right and the more readily the species on the left loses electrons. The more powerful the oxidising agent

A phase boundary

A comma

On the outside of the notation

On the right hand side

By a double dotted line

By calculating the difference between the electrode potentials of the two half cells

On the left hand side

The reactants may be kinetically stable because the activation energy is very large
The reaction may not be taking place under standard conditions

The electrode potential of the half cell, because the position of equilibrium may change

If E0cell is positive

The more positive the E0cell value, the more negative the Gibbs ener

Proportional, so as one increases, so does the other

Proportional, so as one increases, so does the other

A cell that can be recharged by passing a current through it int he opposite direction to the flow of current generated by the cell.

The Nickel-Cadmium cell, NiCd

NiO(OH)

Cadmium

Aqueous Potassium Hydroxide

One which produces a voltage from the chemical reaction of a fuel with oxygen

Hydrogen

Platinum

H2 (g) + 1/2O2(g) ---> H2O(l)

H2 (g) + 1/2O2(g) ---> H2O(l)

Offer an alternative to the direct use of fossil fuels such as petrol and diesel
Products do not include pollutants, only water
Lighter and more efficient than other engines

Compressing the gas can be dangerous
Lots of energy is needed to get it from water by electrolysis, requiring the burning of fossil fuels
Can also be produced from non-renewable sources

Potassium Manganate (VII)

MnO4- + 8H+ + 5Fe2+ ---> Mn2+ + 4H2O +5Fe3+

Purple to colourless