Earth

1. the gaseous atmosphere,
2. the liquid hydrosphere, and
3. the solid lithosphere, which has three distinct regions
   • the crustEarthCrust>Subsid,
   • the mantleEarthMantle>Subsid and
   • the coreEarthCore>Subsid.

Earth's Atmosphere

Earth's Core

Earth's Crust

	Oxygen		47%	
	Silicon		28%	
	Aluminium	 8%	
	Iron		 4.5%	
	Calcium		 3.5%	
	Sodium 		 2.5%	
	Potassium	 2.5%	
	Magnesium	 2.2%

Earth's Mantle

Effervescence

An example of effervescence is the release of carbon dioxide which bubbles as a gas from the liquid when limestone chips, which are composed of calcium carbonate are added to dilute hydrochloric acid.

Efforescence

Effusion

Effusion is the flow of a gas through a small aperture. The relative rates at which gases effuse, under the same conditions, is approximately inversely proportional to the square roots of their densities.

Electrochemical Cell

An Electrochemical Cell consists of a pair of electrodes immersed in solutions of electrolytes and connected via an external metallic conductor such an arrangement is called an electrochemical cell.

If the cell is used as a source of electrical energy, (i.e. if it converts the free energy of a physical or chemical change into electrical free energy), it is called a Galvanic Cells.

If the cell is used to achieve a change in the physical or chemical composition of the constituents of the cell by the application of electrical energy from some external source, it is called an Electrolytic Cell.

Electrochemical Equivalent

The Electrochemical Equivalent of a substance is the mass of the substance, in grams, which is liberated by the passage of 1 coulomb of electricity.

Electrochemical Equivalent

The Electrochemical Equivalent of a substance is the mass of the substance, in grams, which is liberated by the passage of 1 coulomb of electricity.

Electrochemical Series

			Electrode	
	Element		Potential (Volts)	
	=======		=========
	Lithium		-3.04	
	Rubidium	-2.92	
	Potassium	-2.92	
	Calcium		-2.87	
	Barium		-2.80	
	Sodium		-2.71	
	Magnesium	-2.37	
	Aluminium	-1.67	
	Mangnesium	-2.34	
	Zinc		-0.76	
	Chromium	-0.74	
	Iron		-0.44	
	Nickel		-0.24	
	Tin		-0.14	
	Lead		-0.13	
	Hydrogen	+0.00	
	Copper		+0.34	
	Iodine		+0.54	
	Silver		+0.80	
	Gold		+0.80	
	Mercury		+0.80	
	Iodide		+0.54	
	Bromine		+1.07	
	Chlorine	+1.36	
	Fluorine	+2.87	

A strong oxidising agent has a large positive potential and a strong reducing agent a large negative potential.

The electrochemical series is extremely useful for predicting the chemical behaviour of an element. An electropositive element will displace a less electropositive element from a solution of its ions (e.g. zinc will displace copper from a copper solution).

However, aluminium which is more electropositive than copper does not displace copper from copper salt solutions, due to the presence of a very stable film of aluminium oxide on the surface of the aluminium which has a lower electrode potential. This oxide film can be destroyed by the presence of added chlorine ions. Thus, in the presence of added chloride ions in the solution, copper is deposited and the aluminium dissolves.

Electrochemistry

Electrode

An Electrode is a conductor that emits or collects electrons in a cell. The anode is the positive electrode, where ions lose electrons and thus, oxidation occurs at the anode.

The cathode is the negative electrode, where ions gain electrons, and thus, reduction occurs at the cathode.

Electrode Potential

The Electrode Potential (i.e. the Reduction Potential) of a half cell is a measure of the potential for an element to lose its electrons when in contact with a solution of its ions. It is not possible to measure the electrode potential of a single electrode, so two half-cells are joined and the difference between their electrode potentials is measured. If the electrode potential is known for one of the half cells, that of the other may be calculated.

The standard half-cell chosen as a reference is the hydrogen half cell, which is assigned an electrode potential of zero. The hydrogen cell consists of a platinum electrode coated with platinum black containing absorbed hydrogen which catalyses the half-cell reaction.

		2H(+)   +   2e(-)   ==>   H2

The ability of an element to be oxidised or reduced in a cell is used to obtain a value for the electrode potential. If an electrode is immersed in one molar hydrochloric acid and hydrogen gas at a pressure of 1 atmosphere is bubbled over the platinum black electrode to maintain equilibrium, it is possible to obtain a value for the electrode potential of any element.

For example, if a hydrogen half cell is connected to a copper half-cell, (i.e. a copper rod in a solution of copper ions) and the potential of the cell measured, it is found to be +0.34 volts, and assigned as the electrode potential of the copper half cell, using the following convention.

1. If the metal electrode loses electrons more readily than the hydrogen electrode, the metal eventually acquires a negative potential due to the accumulation of electrons on the metal.
2. If the metal electrode does not form ions as readily as the hydrogen electrode, the metal appears positive with respect to the hydrogen electrode.
3. Standard electrode potentials have been measured for most elements that form ions in solution.

A negative value for electrode potential means that such an element loses electrons more readily than hydrogen (i.e. it is a better reducing agent). A positive value means the element is a poorer reducing agent than hydrogen. Thus, strong oxidising agent has a large positive potential and a strong reducing agent a large negative potential.

Electrolysis

Electrolytic Cell

An Electrolytic Cell is one in which a physical or chemical change is caused by electrical energy applied from some external source.