Introduction
Introduction to Chemistry and Agricultural Chemistry
Science and chemistry, Pure chemistry and analytical chemistry:
Science: Science is the systematic knowledge of material universe.
Chemistry: Chemistry is the science of properties of matter and changes in materials.
Pure chemistry: Pure chemistry deals with the basic principles of chemistry. It includes-
a) Inorganic chemistry- deals with the compounds formed mainly by electrovalent bond.
b) Organic chemistry- deals with the compounds formed mainly by covalent bond.
c) Biochemistry- deals with the compounds of biological organisms.
d) Physical chemistry- deals with the physical properties of matter related to the chemical changes.
Analytical chemistry: Analytical chemistry deals with the chemical characterization, identification and estimation of matter.
Applied Chemistry: Applied chemistry deals with the application of basic principles of chemistry to produce useful commodities (goods).
Branches of applied chemistry:
a) Industrial chemistry- deals with the production of industrial goods by applying basic principles of chemistry.
b) Agricultural chemistry- deals with the production of agricultural commodities by applying the basic principles of chemistry.
Analysis and Chemical Analysis: Analysis is a non technical term but chemical analysis is the technical term to describe the method of determining the properties of matter and changes in materials. Analysis means discussion, observation, experimentation, investigation and determination where as chemical analysis means characterization, identification and estimation of a matter.
Needs of chemical analysis
a) To identify the samples and its constituents of a matter.
b) To determine the amounts of constituents of a matter.
c) To establish the suitability and potentiality of a matter.
d) To diagnose the condition of the matter investigated.
Types of chemical analysis
a. Qualitative analysis - To determine the presence of a constituent.
b. Quantitative analysis - To determine the amount of constituen
Classification of qualitative analysis
The analytical method to find out the presence of an element or compound is called qualitative analysis. The element or compound present in a sample is identified by qualitative method of analysis. The substance like acids, bases, salts, element and radical (group) are identified by qualitative analysis.
There are two methods which are generally used for the identification of these radicals. They are -
i) Dry test: Identification of dry salt by the action of heat and chemical.
ii) Wet test: Identification of a clear solution by the action of various chemicals.
Classification of quantitative analysis
a. Gravimetric method of analysis: Estimation by precipitation, separation and Weighing.
b. Volumetric method of analysis: Estimation by volume measurement.
c. Instrumental method of analysis: Estimation by measuring physical properties.
d. Electrical method of analysis: Estimation by measurement of electrical quantity.
Gravimetric method of analysis
Gravimetric method of analysis is the measure of the amount of a substance present in a sample from the weight of the precipitate obtained. In gravimetric analysis a sample is brought into solution and the element or compound to be determined is precipitated as an insoluble stable compound then the precipitate filtered, washed, dried, ignited and weighed accurately. With the weight of precipitate, the amount of the element or substance is determined by its formula and atomic weight of its constituents.
Volumetric method of analysis
Volumetric analysis is the measure of volume of a solution to determine the amount of solid in a unknown solution. In volumetric analysis it is necessary to determine the equivalence point with sufficient accuracy.
Based on the nature of reactions, volumetric analysis can be subdivided into the following methods:
a) Acid-base titration method
b) Oxidation-reduction titration method
c) Precipitation titration method
d) Complexometric titration method
Instrumental method of analysis: The method of determining the amount of solid in a solution by measuring any physical properties with the help of an instrument.
Electrical analysis: Amount determination by measuring electrical properties.
Atomic Structure
Old and modern concept of Atom
Atom: Atom is the smallest possible particle of matter. Atom means ‘not to cut’. In other words atom means "incapable of being divided". Atoms are the basic building blocks of all matter.
In ancient time it was believed that "matter is continuous". Up to John Dalton (1803) the concept of atom was-
-Atoms are smallest possible particle
-Atoms are indivisible
-Atoms of different materials were different (unlike)
-Atoms of same matter are alike
After the discovery of proton and electron, neutron and isotopes, concepts of Dalton changed and proposed that-
1) Atoms can be divided into smaller particles like proton, electron, neutron, pion, meson; α, β, etc. can be obtained from an atom.
2) Atoms of the same element not alike (unlike)
The concept of isotopes ('iso' means same and 'topus' means place) proved that atoms of the same element are exist as differently in their mass. This is called isotopes of the same element.
Isotopes and radio isotopes:
The atoms of same the element having the same atomic number but different atomic masses and occupy the same place in the periodic table are called isotopes. The isotopes which emit α, β and γ radiation from nucleus are called radioisotopes. A neutron in the nucleus sometimes is transformed into an electron and proton. The transformed electrons escape(emitted) from the nucleus. This type of emitted electron is called β rays. (i.e. n p + e), when two protons and two neutrons escape from the uncles is called α rays. The atomic number is the number of protons in nucleus and the atomic mass is the total of protons and neutrons. After the discovery of isotope, it is established that the atomic weight (mass) of an element is an average of the weights of the isotopes of that elements occur in nature.
For example, chlorine, with an atomic weight of 35.46 is composed of two kinds of chlorine atoms containing masses 35 and 37. But both types of the atoms have the atomic number 17. This means that both have 17 protons in the nucleus and the serial number of chlorine is 17 and also placed in the 17th place of the periodic table. The difference in atomic mass (weight) is due to the number of neutrons in the nuclei of the different isotopic atoms. Chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons. It may also note that chlorine-35.46 indicates that the atoms of isotopic weight or mass number 35 are more abundant than the isotope of chlorine having mass number 37.
Isotopes of an element are generally written by the following symbols 17Cl37 and 17Cl35. The subscripts stand for the atomic number of the elements and superscripts denotes the mass number.
Atomic Model
J. J. Thomson (1898) at first proposed that atoms are uniform spheres of positively charged matter in which electrons are embedded; much like a fruit cake is situated with nuts.
Fig. 2.1. Thomson model
Rutherford’s atomic model: Rutherford in 1911 assumed that –
a) Positive charge is concentrated in a very small region at the centre called nucleus.
b) Electron is situated outside the nucleus in some sorts of configuration.
c) Electron revolving around the nucleus as the planets revolves around sun in solar system.
Fig. 2.2. Rutherford atom model
Limitation of Rutherford's atomic model: According to Rutherford's atomic model, an atom has a nucleus and the negative electrons which are revolving round the nucleus in the same way as the planets revolve round the sun in the solar system. This model could not say anything as to how and where these electrons were arranged. This model could also not explain how the spectral lines are produced by the atom when an electron jumps from one orbit to another.
In order to explain a) why an electron revolving round the nucleus, does not lose energy and consequently does not fall into the nucleus, b) how the spectral lines of the emission from atom are produced when an electron jumps from one energy level to the other; Niels Bohr, a Danish Physicist, in 1913, put forward a new atomic model which is based on Planck's quantum theory of radiation.
Quantum Theory of Emission of Radiation (Quantum concepts of radiation):
When energy is applied on an atom then the electron jumped from lower energy level to a higher energy level by absorbing energy. This process is called excitation. The electron at lower energy level is called ground state electron and the jumped electron at higher energy level is called excited electron. After certain period the excited electron come back from higher energy level to lower energy level. Excited atom emits radiation of definite wave. This type of discontinuous radiation from electronic structure is called quantum (plural is quanta) or photon.
In short, energy emitted from an excited atom is called quantum. The term quanta comes from the concepts “the energy of electron in an energy level is definite (specified) or quantized and all the electrons are arranged in four energy levels. But at present it is proved that electrons are arranged in more than four energy levels. So the term quantum may be discarded.
Planck’s quantum theory explain how the emission of radiation takes place from electronic structure of an atom and calculate the energy of an electron at an energy level.
According to Planck’s quantum theory the amount of energy absorbed or emitted by the electron is coming from energy level-1 to energy level -2 is equal to E2-E1=hυ where h is Planck’s constant and υ is the frequency of the radiation.
Excited state electron
Energy emitted (=E2-E1=hυ)
Ground state electron
Lower energy level Heat/electric energy absorbed
(E2-E1=hυ)
Higher energy level
Fig. 2.3. Showing excitation and emission of radiation from electronic structure of an atom
Explanation of Planck's Quantum Theory
In 1900 Max Planck studied the spectral lines obtained from thermal radiations emitted by a hot (black) body at different temperatures and put forward a theory which, after his name, is known as Planck's quantum theory of radiation ('quantum' is a Latin word which means 'how much'). Various postulates of this theory are:
1. When heat or energy supply to a matter, then it emits radiant energy not continuously but discontinuously as small packets or bundles or discrete (separate) units of the waves. Each of these units is called a quantum (plural is quanta) which can exist independently. The emission of radiant energy in continuous waves and in discontinuous waves as individual quantum from a heated iron ball is shown bellow:
Iron ball
(a) (b)
In case of light which is also a form of radiation, light energy is emitted or absorbed in the form of packets or bundles each of which is called Photon (instead of quantum). Photon is not a material body. It is considered to be a mass- less bundle of energy.
Thus according to this theory, light is composed of mass- less particles which are called photons. The presence of photons in light can not be detected readily under normal conditions, since even light of low intensity consists of billions of photons. It was in 1905, when Einstein, while explain the photoelectric effect, could prove the existence of photons.
2. The energy associated with each quantum or photon of a given radiation or light is proportional to the frequency (v/nu) of the emitted radiation or light, i.e.,
E ∞ v,
or, E = hv.
Where, h = a constant known as Planck's constant whose numerical value is 6.624 10-27 erg. s (in C. G. S. units) or 6.624 10-34 j-s (in S. I. units).
E = energy associated with each quantum or photon of a given radiation. E is in ergs or in KJ.
v = frequency of the emitted radiation or light.
This equation is applicable to all types of radiation and is called Planck's equation. This equation shows that the energy associated with a quantum or a photon, E is equal to hv.
Now since, ; [Here, c = velocity and = wave length of radiation]
As E = hv. So, we can write, E = h. ; This equation shows that smaller the wavelength (or higher the frequency) of radiation, larger the energy associated with a quantum or a photon. For example a photon transmitted by violet light, which has higher frequency, has more energy than that of transmitted by red light which has lower frequency.
In 1905 Einstein said that the energy in a photon (E) is associated with mass m and velocity c is also given by: E = mc2. This equation is called Einstein's equation or Einstein mass-energy relationship.
3. The energy emitted or absorbed by a body can be either equal to one quantum of energy (= hv) or any whole number, say, n multiple of this unit, i. e.,
Energy emitted or absorbed = n hv.
Thus the energy emitted or absorbed by a body can be equal to 1 hv, 2 hv, 3 hv etc. but never equal to any fractional value of hv like 1.5 hv, 2.4 hv, 4.9 hv etc. Thus we find that the energy emitted or absorbed by a body is quantized and this is called the concept of quantization of energy.
It is clear from the study of quantum theory of radiation as given above that the atoms would transfer energy in photon units. The absorption of a photon by an atom increases its energy by a definite quantity which is equal to hv. An atom which has absorbed energy in this way is said to be an excited atom or in the excited state. When an excited atom radiates energy, energy is given out in photon units.
The description given above makes it evident that this theory is also known as photon theory, since according to this theory, light radiations are supposed to be composed of photons each of which is associated with energy equal to hv.
Bohr's atomic model: Bohr’s atomic model retains the two essential features of Rutherford’s atomic model which are:
i) The atom has a very small positively charged nucleus at its center. All the protons and neutrons are contained in the nucleus. Thus most of the mass of atom is concentrated in the nucleus.
ii) Negatively charged electrons are revolving round the nucleus in the same way as planets are revolving round the sun.
However he applied Plank’s quantum theory to the revolving electrons and thus explained a) why the electrons are revolving round the nucleus b) Why not the electrons lose energy and consequently do not fall into the nucleus.
Bohr made the following postulates.
1. Fixed circular orbits: Bohr assumed that an electron is a material particle which is revolving round the nucleus in concentric circular orbits situated at definite (i.e., fixed) distance from the nucleus and with a definite velocity.
2. Stationary energy levels: As long as an electron remains in a particular orbit, it neither emits (i.e., radiates or loses) nor absorbs (i.e., gains) energy. Thus in a particular orbit the energy of a revolving electron remains constant or stationary. Hence each of the fixed orbits is associated with a definite amount of energy, i.e., with a definite whole number of quanta of energy. The orbits are, therefore also called stationary energy levels or simply energy levels or energy shells. This concept of stationary energy levels explains the stability of the atom, since an electron cannot lose energy gradually and so does not fall into the nucleus.