The half-life of a reaction is generally denoted by t1/2. The half-life of reactions depends on the order of reaction and takes different forms for different reaction orders. From the integrated rate equations, concentration of reactants and products at any moment can be determined with the knowledge of time, initial concentration and rate constant of the reaction. Similarly, we can determine time too, with the knowledge of other two variables. The time in which the concentration of the reactant is reduced to one-half of the initial value is known as the half-life of a reaction.

Zero order reaction

In zero order reaction, the rate of reaction depends upon the zeroth power of concentration of reactants. From the integrated rate equation for a zero order reaction with rate constant, k, concentration at any time, t is expressed as,

A → B

[A] = −kt + [A]0

From the definition of half-life of a reaction, at t = t12; [A] = [A]02

⇒ = −kt12 + [A]0

⇒ −kt12 = −[A]02

⇒ t12 = [A]02k

Hence, from the above equation we can conclude that the half life of a zero order reaction depends on initial concentration of reacting species and the rate constant, k. It is directly proportional to initial concentration of the reactant whereas it is inversely proportional to the rate constant, k.

• Transition element:

1. A transition element is defined as the one which has incompletely filled d orbitals in its ground state or in any one of its oxidation states.
2. Zinc, cadmium, mercury are not regarded as transition metals due to completely filled d – orbital.

S-block elements are the elements with valence electrons in the s orbital. Elements in column 1 have one valence electron. Elements in column 2 have two valence electrons. S-block elements are very reactive. Elements in column 1 (the alkali metals) always lose their one valence electron to form a +1 ion. The p-block is the region of the periodic table that includes columns 3A to column 8A and does not include helium. There are 35 p-block elements, all of which are in p orbital with valence electrons. The p-block elements are a group of very diverse elements with a wide range of properties.

1. The electronic configuration for them are ns²np⁶

• Helium 1s ²2s²
• Neon 1s²,2s²,2p⁶
• Argon 1s²,2s²,2p⁶,3s²,3p⁶
• krypton1s²,2s²,2p⁶,3s²,3p64s²3d¹⁰4p6
• xenon 1s²,2s²,2p⁶,3s²,3p64s²3d¹⁰4p⁶4d¹⁰5s²5p⁶
• radon 1s²,2s²,2p⁶,3s²,3p64s²3d¹⁰4p⁶4d¹⁰5s²5p⁶4f¹⁴5d¹⁰6s²6p⁶

2.Atomic size : : It increases down the group as every time a new shell is added as we move down. They actually have Vander wall radii.

1. Ionization energy: They have highest ionization energy due to complete octet.
2. 4. Electron gain Enthalpy: It is positive as they have complete octet so they have no attraction for incoming electron.
3. Melting and boiling point: It is low due to weak force that exists that is Vander wall force.

Down the group size increases therefore Vander wall force also increases so as melting and boiling point increase.

1. All noble gases are odorless and colorless and tasteless.
2. All noble gases are sparingly soluble in water.
3. All are inert gases as they have complete octet.
4. All of them are monatomic.

1. The electronic configuration for them are ns²np⁶

• Helium 1s ²2s²
• Neon 1s²,2s²,2p⁶
• Argon 1s²,2s²,2p⁶,3s²,3p⁶
• krypton1s²,2s²,2p⁶,3s²,3p64s²3d¹⁰4p6
• xenon 1s²,2s²,2p⁶,3s²,3p64s²3d¹⁰4p⁶4d¹⁰5s²5p⁶
• radon 1s²,2s²,2p⁶,3s²,3p64s²3d¹⁰4p⁶4d¹⁰5s²5p⁶4f¹⁴5d¹⁰6s²6p⁶

The group 18 elements are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). These elements are non-reactive and are called noble gases as they have their outermost orbit complete. Due to stable electronic configuration they hardly react with other elements.

Preparation of Bleaching Powder

Bleaching powder is synthesized by the action of chlorine gas (produced from the chlor-alkali process) on dry slaked lime (Ca(OH)₂).

Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Uses of Bleaching Powder

• It is used for bleaching dirty clothes in the laundry, as a bleaching agent for cotton and linen in the textile industry.
• It is a strong oxidizing agent, hence used as an oxidizer in many industries.
• It is used as a disinfectant which is used for disinfecting water to make potable water.

Dry slaked lime with reacts with chlorine to prepare bleaching powder.

Cl₂ ​+ Ca(OH)_2​​⟶CaOCl₂​+CaCl₂​+HCl

     Dry slaked lime

The mixture is known as bleaching powder.

Properties of Chlorine:

• It is a greenish yellow gas with pungent and suffocating odour.
• It is soluble in H₂O

The word “electrolysis” was introduced by Michael Faraday in the 19th century. In chemistry, electrolysis is a method that uses a DC to drive a non-spontaneous chemical reaction. This technique is commercially significant as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell.

The process by which ionic substances are decomposed into simpler substances when an electric current is passed through them.

Deacon's process is a process used for preparation of molecular chlorine.

It involves the use of hydrogen chloride and oxygen as reactants and copper chloride as catalyst at high temperatures.

In this process, hydrogen chloride gas is oxidized by atmospheric oxygen in the presence of  CuCl₂ as catalyst at 723 K.

The balanced reaction for deacon's process is written as :

4HCl + O₂ →2Cl₂ + 2 H₂O

The reason F-F bond is weak is that when a fluorine tries to combine with another fluorine atom through a covalent bond, even though it has a very high electronegativity, since fluorine atom has a very small radius the electrons in the atoms repel each other and make the bond weak.

1. Chlorine dioxide is used as oxidizer or disinfectant.

2. Chlorine dioxide is used for control of tastes and odors associated with algae and decaying vegetation.

3. For water treatment, chlorine dioxide has several advantages over chlorine and other disinfectants.

According to Ellingham diagram the lower the position of a metal line in the Ellingham diagram, the greater is the stability of its oxide i.e lower metal line will have most negative Gibbs free energy due to which metal will be most stable in the form of its oxide. Bromine being in between lacks both these characteristics. Thus, the stability of oxides of halogens decreases in the order : I > Cl > Br > F. Higher oxide halogens are more stable than the lower ones because the higher ones are less reactive than the lower ones and also the size of the atoms are more of higher so they are less reactive and hence the oxides are more stable.

Uses of Interhalogen Compounds

• These are used as non-aqueous solvents.
• They are used as a catalyst in few reactions.
• UF₆ which is used in the enrichment of ²³⁵ U is produced by using ClF₃ and BrF₃.

U (s) + 3ClF₃ (l) → UF₆ (g) + 3ClF (g)

• These are used as fluorinating compounds.

While both Oxygen and Chlorine are chemically-active, they cannot form compounds with the inert gasses, while Fluorine is the most chemically-active of all the elements. Elements grouped together on the periodic table have similar electron configurations and therefore similar chemical properties and reactivity.

Oxygen fluorides are compounds of elements oxygen and fluorine with the general formula OnF2, where n = 1 to 6. Many different oxygen fluorides are known:

oxygen difluoride (OF2)
dioxygen difluoride (O2F2)
trioxygen difluoride or ozone difluoride (O3F2)[1][2]
tetraoxygen difluoride (O4F2)[3]
pentaoxygen difluoride (O5F2)
hexaoxygen difluoride (O6F2)[4]
dioxygen monofluoride(O2F)

tetraoxygen difluoride
Oxygen fluorides are strong oxidizing agents with high energy and can release their energy either instantaneously or at a controlled rate. Thus, these compounds attracted much attention as potential fuels in jet propulsion systems.

In oxides of halogen, the bonds are mainly covalent due to small difference in electronegativity between the halogens and oxygen: the bond polarity, however, increases as we move from F to I. The stability of oxides of iodine is greater than those of chlorine while bromine oxides are the least stable. For fluorine F this is the only possible oxidation number, as fluorine is the most electronegative element and doesn't loose electrons in any chemical reactions. Other halogens can have oxidation numbers: -1, +1, +3, +5 and +7.

Preparation of Hydrogen Chloride

Muriatic acid is prepared by warming NaCl crystals with concentrated H2SO4 (Sulphuric acid).

NaCl+H₂SO₄→NaHSO₄+HCl

Usually, most of the hydrogen chloride/hydrochloric acid that is formed is a co-product of some other chemical reactions. HCl is also formed by the chlorination of hydrocarbons.

Properties of Hydrogen Chloride

• HCl is an uncoloured gas and has a pungent aroma.

• Hydrochloric acid is the aqueous solution of hydrogen chloride.
• HCl is soluble in water.
• It liquefies at 189K to form a colourless liquid and freezes at 159k to form a white solid.

Uses of Hydrogen Chloride

• HCl is used in the preparation of chlorine, aqua regia, and other chlorides.
• It is used as a solvent to dissolve noble gases.
• It acts as a reagent in laboratories.

Hydrochloric acid is an inorganic chemical. It is a strong corrosive acid with a chemical formula HCl. It is also known as hydrogen chloride or muriatic acid.

Chemical properties of chlorine gas

1. Effect on litmus: Dry chlorine gas has no effect on litmus but the moist chlorine do have the effect, as it turns blue litmus red due to formation of HCl.
2. Reaction with metals and non metals: It reacts with metals and non metals to form respective chlorides that is given below:

Uses (Chlorine)

• It is used to get rid of the smell of putrefaction
• It is used as a disinfectant
• Chlorine is used in the treatment of drinking water to kill bacteria
• It is used to clean swimming pools
• It is used in the production of paper and paper products
• It is used as an antiseptic
• It is used to produce drugs
• It is used in the manufacture of dyes and plastics

The halogens are located on the left of the noble gases on the periodic table. These five toxic, non-metallic elements make up Group 17 of the periodic table and consist of: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Although astatine is radioactive and only has short-lived isotopes, it behaves similar to iodine and is often included in the halogen group. Because the halogen elements have seven valence electrons, they only require one additional electron to form a full octet. This characteristic makes them more reactive than other non-metal groups.

Introduction
Halogens form diatomic molecules (of the form X2​, where X denotes a halogen atom) in their elemental states. The bonds in these diatomic molecules are non-polar covalent single bonds. However, halogens readily combine with most elements and are never seen uncombined in nature. As a general rule, fluorine is the most reactive halogen and astatine is the least reactive. All halogens form Group 1 salts with similar properties. In these compounds, halogens are present as halide anions with charge of -1 (e.g. Cl-, Br-, etc.). Replacing the -ine ending with an -ide ending indicates the presence of halide anions; for example, Cl- is named "chloride." In addition, halogens act as oxidizing agents—they exhibit the property to oxidize metals. Therefore, most of the chemical reactions that involve halogens are oxidation-reduction reactions in aqueous solution. The halogens often form single bonds, when in the -1 oxidation state, with carbon or nitrogen in organic compounds. When a halogen atom is substituted for a covalently-bonded hydrogen atom in an organic compound, the prefix halo- can be used in a general sense, or the prefixes fluoro-, chloro-, bromo-, or iodo- can be used for specific halogen substitutions. Halogen elements can cross-link to form diatomic molecules with polar covalent single bonds.

Chlorine (Cl2) was the first halogen to be discovered in 1774, followed by iodine (I2), bromine (Br2), fluorine (F2), and astatine (At, discovered last in 1940). The name "halogen" is derived from the Greek roots hal- ("salt") and -gen ("to form"). Together these words combine to mean "salt former", referencing the fact that halogens form salts when they react with metals. Halite is the mineral name for rock salt, a natural mineral consisting essentially of sodium chloride (NaCl). Lastly, the halogens are also relevant in daily life, whether it be the fluoride that goes in toothpaste, the chlorine that disinfects drinking water, or the iodine that facilitates the production of thyroid hormones in one's body.

The halogens are located on the left of the noble gases on the periodic table. These five toxic, non-metallic elements make up Group 17 of the periodic table and consist of: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Although astatine is radioactive and only has short-lived isotopes, it behaves similar to iodine and is often included in the halogen group. Because the halogen elements have seven valence electrons, they only require one additional electron to form a full octet. This characteristic makes them more reactive than other non-metal groups. Halogens form diatomic molecules (of the form X₂​, where X denotes a halogen atom) in their elemental states. The bonds in these diatomic molecules are non-polar covalent single bonds. However, halogens readily combine with most elements and are never seen uncombined in nature. As a general rule, fluorine is the most reactive halogen and astatine is the least reactive. All halogens form Group 1 salts with similar properties. In these compounds, halogens are present as halide anions with charge of -1 (e.g. Cl^-, Br^-, etc.). Replacing the -ine ending with an -ide ending indicates the presence of halide anions; for example, Cl^- is named "chloride." In addition, halogens act as oxidizing agents—they exhibit the property to oxidize metals. Therefore, most of the chemical reactions that involve halogens are oxidation-reduction reactions in aqueous solution. The halogens often form single bonds, when in the -1 oxidation state, with carbon or nitrogen in organic compounds. When a halogen atom is substituted for a covalently-bonded hydrogen atom in an organic compound, the prefix halo- can be used in a general sense, or the prefixes fluoro-, chloro-, bromo-, or iodo- can be used for specific halogen substitutions. Halogen elements can cross-link to form diatomic molecules with polar covalent single bonds.

Fluorine differs from rest of the members of its group because of its small size, high electro negativity and non availability of d orbitals in the valence shell.

The halogen are reactive because of the outermost electrons of its shell, all atom wants to complete the electrons of the outermost shell with eight electrons or two. For the halogens it has 1 electron at it’s outermost shell and it wants to get rid of it that means it loses electrons easily to complete the octet rule. When it loses electrons easily, other atoms reacts or gains the electrons easily. Remember that gaining electrons releases energy.