Plants absorb Carbon Dioxide from the atmosphere and produce oxygen and sugars in the photosynthesis process. Conversely, animals breath in oxygen and breath out carbon dioxide. Living organisms obtain energy from their food by means of a slow form of combustion called respiration. In respiration, oxygen is used to break down foods, such as sugars, to release energy.
The combustion of fossil fuels (e.g. coal and peat) also releases carbon Dioxide into the atmosphere.
As a result of cosmic radiation, a small number of atmospheric nitrogen nuclei are continuously being transformed by neutron bombardment into radioactive nuclei, carbon14. Some of these radiocarbon atoms find their way into living trees and other plants in the form of Carbon Dioxide, as a result of photosynthesis. When the tree is cut down photosynthesis stops and the ratio of radiocarbon atoms to stable carbon atoms begins to fall as the radiocarbon decays. The ratio of Carbon14 to Carbon12 in the specimen can be measured and enables the time that has elapsed since the tree was cut down to be calculated.
Many long-chain carboxylic acids occur naturally as esters in fats and oils and are therefore also known as fatty acids.
2H2 + O2 ==> 2H2O
will proceed without a catalyst only at high temperatures. However, if finely platinum is introduced, the reaction occurs so rapidly that the platinum reaches white heat and the gases may explode.
The tube was connected to a vacuum pump and to an air induction coil to provide a high voltage. When the current was flowing and the tube full of air nothing was observed, nothing was observed, because the voltage supplied was not high enough to cause the electricity to jump the gap between the cathode and anode. However, as the air was removed from the tube, lilac coloured streamers of light were seen to pass between the cathode and the anode and as more air was removed, a red glow was observed in this area.
When nearly all the gas had been removed the glow disappeared and the walls of the tube became a green colour. This green colour is due to a phenomenon called fluorescence which is attributed to the presence of invisible rays from the cathode. These rays were named Cathode Rays.
The octet rule is a guide to understanding chemical bonding. In order to become stable, atoms which have an incompletely filled outer atomic orbitals join chemically with other atoms, either
In 1916AD, the two kinds of chemical bonds which were known were described either as
These ionic and covalent bonds arise from the tendency of atoms to attain a stable configuration of electrons for each atom in a molecule, by either the transfer or the sharing of electrons between atoms. Because a molecule consists of at least two atoms with positively charges nuclei and negatively electronic clouds about these atoms, there are electrostatic interactions between the various particles of the atoms in the molecule. There is a repulsive force between particles having the same charge, which gives rise to repulsion between the positively charges nuclei of the atoms, and repulsion between the negatively charged electrons in the electronic clouds about the atoms.
However, there is an attractive force between particles of opposite charge, which gives rise to attraction between the positively charged nuclei and the negatively charged electronic cloud.
The relative strengths of these forces and the balance between them determines the nature of the bonding, both within the molecule and between the molecules. If an electron can travel in the orbitals of two different atoms, it can experience an attractive force to the nuclei of both atoms and thus give rise to a chemical bond between these atoms. A detailed discussion of the different types of bonds are given separately. Metallic Bond Ionic Bond Covalent Bond Polar Bond Dative Subsid Van Der Walls Forces Hydrogen Bond
If a zinc rod is placed in a solution containing zinc ions, a potential difference is established between the rod and the solution. Ions from the rod go into the solution leaving the metal rod with a negative charge.
Zn ==> Zn(++) + 2e(-)
An electrical double layer is formed between the electrons in the rod and the ions in solution. The formation of this electrical double layer is accompanied by a potential difference between the rod and the solution called electrode potential.
If a copper rod is placed in a solution containing copper ions, the rod is found to be positively charged with respect to the solution. In this instance metal ions leave the solution and deposit on the copper rod where they combine with electrons.
Cu(++) + 2e(-) ==> Cu
The term half-cell is used to describe the zinc rod in the solution of zinc ions.
Similarly, the copper rod in the solution of copper ions is also called a half cell.
The zinc rod and the copper rod are termed electrodes. If the half-cells are left in isolation, a state of equilibrium is reached and no further reaction takes place. However, when the electrodes of each half-cell are linked externally by means of a wire and the cells are separated by a porous partition, the electrons which would have collected previously on the zinc electrode are now free to travel along the external wire to the copper electrode.
The process which occurs may be summarised as follows:
At the Zinc electrode : Zn ==> Zn(++) + 2e(-) At the Copper electrode : Cu(++) + 2e(-) ==> Cu
The zinc electrode is acting as a source of electron, which flow through the wire. On reaching the copper electrode these electrons from the zinc rod combine with copper ions from solution and copper is deposited on the copper electrode.
An equation for the overall chemical process is obtained by adding together the two half-cell reactions in such a way that the electrons "cancel out"
Zn + Cu(++) ==> Zn(++) + Cu
This particular example is called the Daniell Cell.
Since the electric current consists of a flow of electrons, it is found that reduction takes place at the electrode where the electrons enter the electrolyte and oxidation takes place at the electrode where the electrons leave the electrolyte.
In this reaction in the Daniell Cell, the copper ions gain electrons and are said to be reduced while the zinc loses electrons and is said to be oxidised. This type of reaction is called an oxidation-reduction reaction or a redox reaction.
Instead of separating the solutions of copper and zinc ions by a porous partition, a salt bridge usually containing a concentrated solution of potassium chloride in order to minimise diffusion, can be used.
The purpose of the salt bridge is to prevent copper ions from diffusing rapidly towards the zinc electrode where they would be reduced to metallic copper by the electrons on the zinc rod.