A substance which increases the speed of a reaction without being consumed in the reaction is called a catalyst. The phenomenon of increasing the rate of reaction by the use of catalyst is called catalysis. Living cells contain thousands of different kinda of proteins called enzymes which act as catalysts.

Urea is a nitrogenous compound formed in the liver. It has a chemical formula of CH4N2O. It is also known as Carbamide or Ureophil. This compound is the final end product of protein metabolism. It is a waste product and has no physiological function. It dissolves in blood and kidney excretes it in urine. This organic compound has two NH2 groups connected by a functional group carbonyl. Urea dissolves in water and is non-toxic. It is colourless and has no smell. It is widely used as an important raw material in industries and commonly used in fertilizers.

• It is used in the manufacture of ammonia, to produce nitric acid and subsequently used as a fertilizer.
• Nitric acid salts include important compounds like potassium nitrate, ammonium nitrate, and nitric acid. Nitrated organic compounds such as nitro glycerine are often explosives.
• Liquid nitrogen is utilized as a refrigerant for transporting foodstuff and freezing purposes. Preservation of bodies and reproductive cells and stable storage of biological samples also makes use of liquid nitrogen.
• Nitrogen makes up 78 percent of the Earth ‘s atmosphere and is a part of all living tissue. Nitrogen is a crucial ingredient of life, since it is a constituent of DNA and as such is part of the genetic code.
• Nitrogen molecules often exist in soil. Nitrogen can be present in nitrates and nitrites in water and in soils. These compounds are all part of the nitrogen cycle and both are interconnected.

Group 15 elements include nitrogen, phosphorus, arsenic, antimony and bismuth. Nitrogen is the real constituent of the air and records for 78% of it by volume. It is the primary member of this group and happens in a free state as a diatomic gas, N₂.

Phosphorus is a fundamental constituent of animal and plant matter. Phosphate groups are constituents of nucleic acids, that is, DNA and RNA. Around 60% of bones and teeth are made out of phosphates. Phosphoproteins are available in egg yolk, milk, and bone marrow. The rest of the elements of the group, that is, arsenic, antimony, and bismuth, mostly happen as sulfides. For example, Stibnite, Arsenopyrite, and bismuth glance.

 Packing efficiency is defined as the percentage of space occupied by constituent particles packed inside the lattice. It can be calculated with the help of geometry in three structures namely: HCP and CCP structures. The packing efficiency of simple cubic lattice is 52.4%.

Matters exist in solid state because of close packing of their constituent particles. There are two types of close packing found in solids. These are Cubic Close Packed (ccp) and Hexagonal Close Packed (hcp) lattice.

Enthalpy is defined as the sum of internal energy of a system and the product of its pressure and volume. It is denoted by the symbol E. It is a state function. Units used to express are calorie, BTU, or joules. Below we have given the equation.

Enthalpy Equation:

E= U + PV where,

E is the enthalpy

U is the internal energy of a system

P is the pressure

V is the volume

Entropy is defined as the measure of the thermal energy of a system per unit temperature which is not available for doing useful work. It is denoted as S. The SI unit for Entropy is Joules per Kelvin. Entropy change at constant temperature is calculated as given below:

ΔS_system = q_rev / T

ΔS represents the change in entropy,

q_rev represents the reverse of the heat, and

T is the temperature in the Kelvin scale.

Normality in chemistry is one of the expressions used to measure the concentration of a solution. It is abbreviated as ‘N’ and is sometimes referred to as the equivalent concentration of a solution. It is mainly used as a measure of reactive species in a solution and during titration reactions or particularly in situations involving acid-base chemistry.

Example: 

n the following reaction calculate and find the normality when it is 1.0 M H₃PO₄

H₃AsO₄ + 2NaOH → Na₂HAsO₄ + 2H₂O

Solution:

If we look at the given reaction we can identify that only two of the H⁺ ions of H₃AsO₄ react with NaOH to form the product. Therefore,  the two ions are 2 equivalents. In order to find the normality, we will apply the given formula.

N = Molarity (M) × number of equivalents

N = 1.0 × 2 (replacing the values)

Therefore, normality of the solution = 2.0.

Centered Unit Cells

• If the constituent particles of a crystal lattice are present at positions other than corners in addition to those at corners, it is known as centered unit cell.
• Centered unit cells are of three types:
  • Body-Centred Unit Cells:

If the constituent particles of a unit cell are present at its body-centre besides the ones that are at its corners.

• Face-Centred Unit Cells:

If the constituent particles of a unit cell are present at the center of each face, besides the ones that are at its corners.

• End-Centred Unit Cells:

If the constituent particles of a unit cell are present the center of any two opposite faces besides the ones present at its corners.

Types of Unit Cell

Numerous unit cells together make a crystal lattice. Constituent particles like atoms, molecules are also present. Each lattice point is occupied by one such particle.

1. Primitive Cubic Unit Cell
2. Body-centered Cubic Unit Cell
3. Face centered cubic unit cell

The ‘crystal lattice’ is the pattern formed by the points and used to represent the positions of these repeating structural elements. The periodic structure of an ideal crystal is most easily described by a lattice. The crystal lattice is the array of points at the corners of all the unit cells in the crystal structure. The crystal lattice is the symmetrical three-dimensional structural arrangements of atoms, ions or molecules (constituent particle) inside a crystalline solid as points. It can be defined as the geometrical arrangement of the atoms, ions or molecules of the crystalline solid as points in space.

The ‘crystal lattice’ is the pattern formed by the points and used to represent the positions of these repeating structural elements. The periodic structure of an ideal crystal is most easily described by a lattice. The crystal lattice is the array of points at the corners of all the unit cells in the crystal structure. The crystal lattice is the symmetrical three-dimensional structural arrangements of atoms, ions or molecules (constituent particle) inside a crystalline solid as points. It can be defined as the geometrical arrangement of the atoms, ions or molecules of the crystalline solid as points in space.