Calculation of pH

The calculation of the pH of a solution involves finding the logarithm of the hydrogen ion concentration in the solution and reporting the negative value of the answer as the pH of the solution.


Calomel Half Cell

The calomel half cell is a half cells formed by a metal in contact with an insoluble salt or oxide and a solution. It is commonly used as a reference Half Cell.


Carbohydrates

Carbohydrates (i.e. sugars) are organic compounds composed of carbon, hydrogen, and oxygen only, with the general formula Cx(H2O)y. For example, sucrose has the formula C12H22O11. The nomenclature of carbohydrates usually have the termination "-ose". Thus, the words triose, tetrose, pentose, hexose, etc., denote the number of carbon atoms forming a straight chain in the carbohydrate. The carbohydrates may have a aldehydic or ketonic structure, and therefore the appropriate prefix aldo- or keto- is used to designate the structure. The simplest carbohydrates are the sugars including glucose and sucrose. Polysaccharides are carbohydrates of much greater molecular weight and complexity.


Carbon Compounds

No text prepared for this topic yet


Carbon Cycle

The carbon cycle is the natural process where carbon is circulated through animals, plants, soil and the air in the environment. The carbon cycle is one of several major cycles of chemical elements in the environment.

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.


Carbon Dating

Carbon dating is a method of estimating the ages of archaeological specimens of biological origin.

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.


Carbonates

Carbonates are the salts of the unstable carbonic acid which contain the di-negative carbonate ion, CO3(2--). The free carbonate ion has a planar triangular structure.


Carbonium Ion

The carbonium ion is an organic ion with a positive charge on the carbon atom which has only six electrons. Carbonium ions are intermediates in certain types of organic reactions.


Carboxyl Acid Group

The carboxyl acid group is the organic functional group, -C=O.OH, that is present in carboxylic acids.


Carboxylic Acid

Carboxylic acid are the organic compounds of carbon, hydrogen and oxygen containing the functional group -C=O.OH (i.e. the carboxyl group). In systematic chemical nomenclature, the names of carboxylic acid names end in the suffix "-oic". In general, organic acids are weak acids.

Many long-chain carboxylic acids occur naturally as esters in fats and oils and are therefore also known as fatty acids.


Carrier Gas

A carrier gas is the gas, (usually helium or nitrogen), which carries the sample undergoing analysis through the column in gas liquid Chromatography.


Catalysis

Catalysis is the process of changing the rate of a chemical reaction by use of a catalyst.


Catalyst

A catalyst is a substance that increases the speed of a chemical reaction, and is itself unaltered chemically at the end of the reaction. The catalyst alters the rates of chemical reactions by participating in the reaction, but is itself unaltered at the end of the reaction. A catalyst can
  1. accelerate the rate of reaction. For example, the reaction

    
    	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.

  2. decelerate the rate of reaction. The above reaction, in which water is formed using fine platinum as a catalyst, can be retarded by traces of hydrogen sulphide, ammonia, carbon disulphide or iodine. These inhibitors are adsorbed on to the platinum and prevent the access of the reacting substances. For this reason they are said to poison the catalyst and are termed negative catalysts or inhibitors.
A catalyst or inhibitor can have a very noticeable effect on the rate of reaction, even when it is present in low concentration. In general, the change in the rate of reaction is proportional to the concentration of the catalyst.


Catalytic Converters

Catalytic converters are the materials that catalyse the conversion of the exhaust emissions from cars into less harmful produces. These catalytic converters reduce carbon monoxide emissions and can also reduce the discharge of soot, smoke and nitrogen oxides. A catalytic converter can only work with lead free petrol, as lead prevents metals in the catalyst from working properly.


Catalytic Cracking

Catalytic cracking is the thermal cracking process in which a catalyst is used to lower the temperature required, or to modify the products obtained.


Cathode

A cathode is the negative electrode in electrolysis, where ions gain electrons. Thus, reduction occurs at the cathode.


Cathode Rays

Cathode rays arise from the passage of electrons from the cathode to the anode of a discharge tube. When scientists were experimenting with discharge tubes (i.e. long glass tubes, closed at both ends, with metal plates sealed into each end of the tube) at about the middle of the 19th Century, the anode is the name given to the plate connected to the positive terminal of the induction coil while the cathode is the name given to the plate connected to the negative terminal of the induction coil.

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.


Cathode Reactions

Cathode reactions are the chemical reactions that occur at the cathode of an electrochemical cell, involve the positive ions in solution which migrate to the cathode, where they gain an electron. These are oxidation reactions, as they involve the transfer of an electron to the ion.


Cation

A cation is that ion which goes to the cathode and is therefore positively charged ion.


Cation Hydrolysis

Cation hydrolysis involves the reaction of the cation of a salt with water to give excess hydrogen ions in solution.


Chemical Bonds

The chemical bond refers to the attractive forces which link atoms to each other within a molecule. These bonds keep the molecule intact as an entity.

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

atom to another to achieve the full outer shells.

In 1916AD, the two kinds of chemical bonds which were known were described either as

  1. the Ionic Bond by Walter Kossel (Germany), or
  2. the Covalent Bond by G.N.Lewis (University of California).

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


Chemical Cell

A cell is the assembly of electrodes and solutions in a suitable container, which are interconnected to produce electricity. A cell consists of two half cells.

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.


Start of Hypertext .... Elements .... Compounds .... Index
Hypertext Copyright (c) 2000 Donal O'Leary. All Rights Reserved.