Introduction
Environmental Chemistry is a multidisciplinary field that combines principles of chemistry and environmental science to study the chemical processes occurring in the environment. This subject is critical for understanding the interactions between natural systems and the chemical substances that influence them, whether they are naturally occurring or introduced by human activities.
Environmental Chemistry encompasses the study of the sources, reactions, transport, effects, and fates of chemical species in the air, water, and soil environments. It plays a vital role in identifying and solving environmental problems, such as pollution, climate change, and the degradation of natural resources.
Objectives
By studying Environmental Chemistry, students will gain:
1. A deep understanding of the Fundamental Chemistry which includes Elements, Chemical bonding, chemical reactions and equations, Organic functional groups and classes of organic compounds;
2. Skills in identifying and addressing Chemical kinetics and Chemical equilibrium;
3. The ability to analyse whole electrochemistry.
Elements
The chemical elements, the fundamental substances, are made from atoms. And atom is the smallest particle of an element which can take part in a chemical reaction. The main components of an atom are protons, which are positively charged, electrons, which are negatively charged and neutrons, which are uncharged. The all atoms of an element have the same atomic number (Z), which is equal to the number of protons in an atom. In an atom the number of protons and electrons is equal and therefore an atom is electrically neutral. The elements may exist in mono-atomic or polyatomic forms. For example in air, argon exists as a single atom, Ar, and nitrogen (N) exists as N2.
A given element may exist in more than one polyatomic (molecular) forms. These forms are known as allotropes. For example, in atmosphere oxygen is found both as dioxygen, O2, and ozone,O3.
The elements and their compounds are building block of every material found in the universe. The relative abundance of the elements by mass in whole Earth is: 35% iron, 30% oxygen, 15% silicon. On the other hand Earth’s crust has 40% oxygen, 28% silicon, 8% aluminum 6% iron, etc.
Classification of Elements on the Basis of Their Properties
Metals
These elements have low ionization energies, low electron affinities and low electronegativities, and form cations. Out of 118 elements known at present, about 81 are metals. Metals are further classified as follows.
1. Alkali Metals
Alkali metals, viz., Li, Na, K, Rb, Cs are very reactive and react with O2 , S, and other nonmetals to form compounds. That is why these are never found in nature in native state. There outermost electronic configuration is [noble gas], ns1. These elements are easily ionized to form cations. Alkali form strongly alkaline oxides, e.g., Na2O and hydroxides, NaOH, all of which react with water to produce hydroxide ions, OH- , which make the aqueous solution alkaline.
2. Alkaline Earth Metals
The elements belonging to Group 2 are also reactive metals and commonly known as alkaline earth metals. The alkaline earth metals have the outermost electronic
configuration, ns2. Thus these metals form di positive ions. The oxides and hydroxides and carbonates of Mg, Ca, Sr and Ba are alkaline and their aqueous solutions are alkaline. The carbonates of these metals are found in Earth crust. The last member of the group, Radium, is radioactive and has several applications.
3. Transition metals
There are four series of these elements. The first series includes Sc, Ti, V, Cr, Mn, Fe, Co, Ni and Cu. Chemistry of these elements is of great environmental significance. These elements exhibit the variable oxidation states. For example Mn shows all the oxidation states starting from -1 to +7. These elements form large number of complexes and also act as catalysts in all types of chemical systems.
4. Rare earth elements
The elements of lanthanide group are called the rare earths, which comprise the fourteen elements from Ce(z = 580 to Lu (Z = 71), but sometimes La, Sc and Y are also included. The prominent member of actinide series are U and Pu, which are used in production of nuclear energy.
5. Heavy metals
Because of their high relative atomic masses, As, Be, Cd, Pb, Mn, Hg, Ni and Se are called heavy metals. These elements concern us because of occupational or residual exposure. They persist in nature and can cause damage or death in animals, humans and plants, even at a very low concentration (1 or 2 microns in some cases). Industrial processes release these into air and water. Since heavy metals have a property to accumulate in the selected body organs, such as brain and liver, long term exposure may result in slowly progressing physical, muscular and neurological degenerative processes that mimic Alzheimer's diseases, Parkinson’s diseases, Muscular dystrophy and multiple sclerosis, Allergies are not uncommon. In all water analyses, measurement of heavy metals is necessary.
6. Base metals
The non-ferrous metals, excluding precious metals, are called base metals, e.g., iron, steel, aluminium, tin, tungsten, molybdenum, tantalum, cobalt, bismuth, cadmium, titanium, zirconium, antimony, manganese, beryllium, chromium, germanium, vanadium, gallium, hafnium, indium, niobium, rhenium, thallium, Copper
and Lead.
7. Ferrous and Nonferrous metals
Ferrous metals contain iron, for example carbon steel, stainless steel (both alloys; mixtures of metals) and wrought iron, Non-ferrous metals are metals that do not
contain iron, for example aluminium, brass, copper and titanium brass. Al, Be, Cu, Pb, Mg, Ni, Sn, Zn, and precious metals.
8. Metalloid
There is no rigorous definition of metalloids, but the elements having the properties of both metals and nonmetals are called metalloids. The metalloids often form
amphoteric oxides (B, Si, Ge, As and Sb and often behave as semiconductor (B, Si, Ge, As).
Non-Metals
About 22 elements, which are members of p-block, behave as non-metals. They are usually poor conductor. They are found as gases, liquids or solids.
1. Carbon Family
The elements of Group 14, C, Si, Ge, Sn, Pb constitute the carbon family. Carbon is nonmetal, Si and Ge are semimetal, and tin and lead are metals. The chemistry of carbon is vast and studied separately as organic chemistry. This is due to the property of catenation, i.e., ability to carbon chains. The carbon cycle is an important cycle in environment.
2. Nitrogen family
Nitrogen family consists of group 15 elements, viz., N, P, As, Sb, Bi, etc. The first two elements, N and P are very important and their biogeochemical cycles play a very important role in the chemistry of the environment.
3. Oxygen family
Group 16 contain 6 elements, viz., O, S, Se, etc. O and S are nonmetals whereas Se is a metalloid. The importance of oxygen and sulfur cycles in environment shall be presented in a subsequent module.
4. Halogen
The elements of group 17 are known as halogen, viz., F, Cr, Br, I. The halogens have a very strong tendency to pick up one electron to acquire the stable noble gas configuration. These are among the most reactive elements. Chlorine easily undergoes photochemical dissociation and forms Cl atom/free radical, which is highly reactive and responsible for stratospheric ozone depletion. Chlorine is widely used for disinfection drinking water.
5. Noble/Inert/Rare Gases
Group 18 elements such as He, Ne, Ar, Kr, Xe and Rn are known as noble, rare or inert gases. Due to their stable ns2, np6 electronic configuration, they have very little tendency to form compounds with other elements. For this reason they exist in mono-atomic form, e. g., He, Ar, Ne, etc. In air, argon is the third most significant gas (0.9%) after N2 and O2. Radon is a radioactive element having serious environmental concerns as discussed another module.
Chemical Bonding
Chemical bonding is the process by which atoms or molecules combine to form more complex structures, held together by attractive forces. These forces arise from the interactions between electrons and nuclei of different atoms. Chemical bonds can be categorized into several types, including ionic bonds, where electrons are transferred between atoms, covalent bonds, where electrons are shared between atoms, and metallic bonds, which involve a 'sea' of shared electrons among a lattice of metal atoms. The nature of the chemical bond determines the physical and chemical properties of the resulting compounds.
Ionic bonds are formed through the transfer of electrons from one atom to another, resulting in the formation of ions. This typically occurs between metals and non-metals. Metals lose electrons to become positively charged cations, while non-metals gain those electrons to become negatively charged anions. The electrostatic attraction between oppositely charged ions creates the ionic bond.
Example Reaction: Formation of Sodium Chloride (NaCl)
Covalent bonds involve the sharing of electrons between atoms, typically between non-metals. The shared electrons allow each atom to achieve a stable electron configuration, similar to that of noble gases.
Example Reaction: Formation of Water (H₂O)
Metallic bonds occur between metal atoms. In metallic bonding, atoms in a metal lattice share a 'sea' of delocalized electrons, which are free to move throughout the structure. This electron sharing gives metals their characteristic properties, such as conductivity, malleability, and ductility.
Example: Metallic Bonding in Copper (Cu)
Hydrogen Bonding
Example: Hydrogen Bonding in Water (H₂O)
Van der Waals Forces
Example: Interaction Between Noble Gas Atoms
Chemical reactions are processes in which substances, known as reactants, undergo chemical changes to form new substances, called products. These reactions involve the breaking and forming of chemical bonds, resulting in the transformation of substances with different chemical properties. The representation of these chemical reactions using symbols and formulas is known as chemical equations.
Combination (Synthesis) Reactions
Decomposition Reactions
Single Displacement (Replacement) Reactions
Double Displacement (Metathesis) Reactions
Combustion Reactions
Redox (Oxidation-Reduction) Reactions
Chemical equations use chemical symbols and formulas to represent reactants and products in a chemical reaction. A balanced chemical equation has the same number of atoms of each element on both sides, ensuring the law of conservation of mass is obeyed.
Parts of a Chemical Equation:
Example of a Balanced Equation:
Steps to Balance a Chemical Equation:
Write the unbalanced equation.
Count the number of atoms of each element on both sides.
Use coefficients to balance each element.
Check to ensure the equation is balanced.
Word Equations
Formula Equations
Ionic Equations
Net Ionic Equations
Law of Conservation of Mass: Mass is neither created nor destroyed in a chemical reaction.
Stoichiometry: The calculation of reactants and products in chemical reactions.
Reaction Conditions: Factors such as temperature, pressure, and catalysts that affect the rate and outcome of a reaction.
Organic functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. These functional groups play a crucial role in organic chemistry because they determine the properties and reactivity of the compounds in which they are found. Each functional group has a specific set of atoms arranged in a particular pattern, and this arrangement gives the group its unique chemical behaviour. Functional groups are used to classify organic compounds and predict their reactivity.
For example, compounds with hydroxyl groups (-OH) are typically alcohols and have certain properties such as solubility in water and the ability to form hydrogen bonds. Similarly, carboxyl groups (-COOH) are found in carboxylic acids, which are known for their acidity and ability to participate in esterification reactions.
Functional Group | Structure | Properties | Example |
Hydroxyl Group | -OH | Polar, forms hydrogen bonds | Ethanol (CH3CH2OH) |
Aldehyde | -CHO | Polar, reactive | Formaldehyde (HCHO) |
Ketone | >C=O | Polar, less reactive than aldehydes | Acetone (CH3COCH3) |
Carboxyl Group | -COOH | Highly polar, acidic | Acetic acid (CH3COOH) |
Amino Group | -NH2 | Basic, forms hydrogen bonds | Aniline (C6H5NH2) |
Thiol Group | -SH | More nucleophilic than hydroxyl group | Ethanethiol (CH3CH2SH) |
Phosphate Group | -PO4 | Highly polar, acidic | ATP |
Ether Group | R-O-R | Relatively non-polar | Diethyl ether (CH3CH2OCH2CH3) |
Ester Group | -COOR | Polar, pleasant odours | Ethyl acetate (CH3COOCH2CH3) |
Amide Group | -CONH2 | Polar, forms hydrogen bonds | Acetamide (CH3CONH2) |
Nitrile Group | -C≡N | Polar, high dipole moment | Acetonitrile (CH3CN) |
This table provides a concise overview of common organic functional groups, highlighting their structures, properties & examples.
This table provides an overview of various classes of organic compounds, their functional groups, and examples of each class.
Class of Organic Compounds | Functional Group | Example |
---|---|---|
Hydrocarbons | - | Methane (CH₄), Ethylene (C₂H₄), Acetylene (C₂H₂) |
Alcohols | Hydroxyl (-OH) | Methanol (CH₃OH), Ethanol (C₂H₅OH), Isopropanol (C₃H₇OH) |
Aldehydes | Carbonyl (C=O) at end | Formaldehyde (CH₂O), Propanal (CH₃CH₂CHO) |
Ketones | Carbonyl (C=O) within | Acetone (CH₃COCH₃), Butanone (CH₃COCH₂CH₃) |
Carboxylic Acids | Carboxyl (COOH) | Formic acid (HCOOH), Acetic acid (CH₃COOH), Benzoic acid (C₆H₅COOH) |
Esters | Carboxylate (COO-) | Methyl acetate (CH₃COOCH₃), Ethyl butanoate (CH₃CH₂COOCH₂CH₃) |
Amines | Amino (NH₂) | Methylamine (CH₃NH₂), Dimethylamine (CH₃NHCH₃), Trimethylamine (N(CH₃)₃) |
Ethers | Oxygen between two groups | Dimethyl ether (CH₃OCH₃), Diethyl ether (CH₃CH₂OCH₂CH₃), Anisole (CH₃OC₆H₅) |
Halogenoalkanes | Halogen (F, Cl, Br, I) | Chloromethane (CH₃Cl), Bromoethane (CH₃CH₂Br), Iodoform (CHI₃) |
Catalysis is a process by which the rate of a chemical reaction is increased by the presence of a substance called a catalyst. Catalysts work by providing an alternative reaction pathway with lower activation energy, thus enabling the reaction to proceed more rapidly. Catalytic processes play a crucial role in various industries, including petrochemicals, pharmaceuticals, and environmental remediation.
Catalytic processes are essential for improving reaction efficiency, selectivity, and sustainability in chemical synthesis and environmental remediation. They enable the production of valuable chemicals, fuels, and pharmaceuticals while minimizing energy consumption and waste generation.
Solubility refers to the ability of a substance (solute) to dissolve in a solvent to form a homogeneous mixture called a solution. It is typically expressed as the maximum amount of solute that can dissolve in a given amount of solvent at a specified temperature and pressure. Solubility depends on various factors, including temperature, pressure, polarity, and the nature of the solute and solvent.
Factors Affecting Solubility
Types of Solutions
Solubility Rules
Electrochemistry deals with the study of chemical reactions involving the transfer of electrons between reactants. It encompasses redox reactions, electrolysis, electrochemical cells, and corrosion.
Redox Reactions
Electrochemical Cells
Electrolysis
Electrode Potentials
Corrosion
Chemical kinetics is the branch of chemistry concerned with the study of the rates of chemical reactions and the factors that affect reaction rates. It provides insight into the mechanisms by which reactions occur and allows scientists to predict reaction rates under various conditions.
Reaction Rate
Rate Laws
Reaction Mechanisms
Factors Affecting Reaction Rate
Chemical equilibrium occurs in a reversible reaction when the rates of the forward and reverse reactions are equal, resulting in the concentrations of reactants and products remaining constant over time. It is a dynamic state where reactions continue to occur, but there is no net change in the overall composition.
Equilibrium Constant
Le Chatelier's Principle
Types of Equilibria
Applications
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