Chemistry Mid semester Exam 2Dec 2024

 

       CHEMISTRY  MID SEMESTER EXAM 2024

In this following answer questions any 5 come in your exam

Q1) Explain the  VSEPR Model and draw the, structure of CH4, NH3 and H2O molecule?

The Valence Shell Electron Pair Repulsion (VSEPR) model is a chemical model that predicts the 3D geometry of molecules based on the number of electron pairs around the molecule's central atom: 

Explanation

• The VSEPR model assumes that electron pairs arrange themselves to minimize repulsion from each other. The greater the repulsion, the higher the energy of the molecule. 
• Electron pairs

  Some electron pairs form covalent bonds and are called bond pairs, while others do not form bonds and are called lone pairs. 
• Repulsion order

  The order of repulsion between electron pairs is lone pair-lone pair, lone pair-bond pair, and bond pair-bond pair. 
• Shapes

  >The VSEPR model describes five main shapes of simple molecules: linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral.
• Examples

  Some examples of VSEPR models include water (bent), ammonia (trigonal pyramidal), and boron trifluoride (trigonal planar). 
• Developers

  The VSEPR model is also known as the Gillespie-Nyholm theory after its developers, Ronald Gillespie and Ronald Nyholm. 

Q2) Write the postulates of Molecular orbital theory and explain oxygen is paramagnetic

 by energy level diagram?

Molecular Orbital Theory postulates that:

• Atomic orbitals combine to form molecular orbitals:

  When atoms approach each other to form a molecule, their atomic orbitals overlap to create new molecular orbitals that encompass the entire molecule.
• Molecular orbitals have different energies:

  These molecular orbitals can be lower in energy (bonding orbitals) or higher in energy (antibonding orbitals) compared to the original atomic orbitals, depending on the constructive or destructive interference of the wave functions during overlap.
• Electron filling follows Aufbau principle:

  Electrons are filled into the molecular orbitals in increasing order of energy, following the same rules as in atomic orbitals (Hund's rule, Pauli exclusion principle).
• Molecular orbital shape depends on atomic orbital shapes:

  The shape of a molecular orbital is determined by the shapes of the atomic orbitals that combined to form it.

Explanation of Oxygen's Paramagnetism using Molecular Orbital Diagram:

• Electronic configuration of Oxygen atom:

  Each oxygen atom has 8 electrons with electronic configuration 1s² 2s² 2p⁴.
• Molecular Orbital Diagram:

  When two oxygen atoms come together to form O2, their 2p orbitals overlap to create molecular orbitals.
• Key feature:

  The π* (antibonding) orbitals on the oxygen molecule are degenerate (same energy) and each contains one electron, resulting in two unpaired electrons.
• Paramagnetic nature:

  Due to the presence of these unpaired electrons, an oxygen molecule is attracted to a magnetic field, exhibiting paramagnetic behavior.

Energy Level Diagram for O2:

[Image: A molecular orbital diagram for O2, showing the σ(1s), σ*(1s), σ(2s), σ*(2s), σ(2pz), π(2px), π(2py), π*(2px), and π*(2py) orbitals. The π*(2px) and π*(2py) orbitals are degenerate and each contain one electron, indicating the paramagnetic nature of O2].

Key points to remember:

• Molecular Orbital Theory provides a more accurate explanation for the paramagnetic nature of oxygen compared to the simple valence bond theory.
• The presence of unpaired electrons in the π* orbitals is crucial for the paramagnetic behavior of oxygen.
• This is a classic example of how molecular orbital theory can explain the magnetic properties of molecules.

Q3)  Write the postulates of arrhenius theory of electrolytic dissociation?

According to the Arrhenius theory of electrolytic dissociation, when an electrolyte dissolves in water, it dissociates into positively charged cations and negatively charged anions, and these ions are responsible for the solution's electrical conductivity; the key postulates are:

• Ion Formation:

  When an electrolyte dissolves in water, its molecules split into positively charged ions (cations) and negatively charged ions (anions).
• Equilibrium:

  A dynamic equilibrium exists between the undissociated electrolyte molecules and the ions in solution.
• Ion Mobility:

  The ions are free to move through the solution towards the electrode with the opposite charge, allowing for the conduction of electricity.
• Conductivity Dependence:

  The electrical conductivity of a solution is directly proportional to the number of ions present, meaning more ions lead to greater conductivity.
• Charge Neutrality:

  The total positive charge of the cations in a solution always equals the total negative charge of the anions, ensuring overall electrical neutrality.

Example: When sodium chloride (NaCl) dissolves in water, it dissociates into sodium ions (Na+) and chloride ions (Cl-).

Important points to remember:

• This theory successfully explained the conductivity of electrolyte solutions and the behavior of acids and bases in aqueous solutions.
• While useful, the Arrhenius theory has limitations as it does not fully account for the role of the solvent in the dissociation process and cannot explain the behavior of non-aqueous solutions.

Q4) Differentiate between electrochemical cell and concentration cell?

The main difference between an electrochemical cell and a concentration cell is the type of energy produced and how matter is transferred:

• Electrochemical cell

  Uses a chemical reaction to produce electrical energy. The chemical reaction involves the electrolyte, electrodes, and sometimes an external substance.
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Concentration cell

• Uses physical changes to produce electrical energy. Matter is transferred from one part of the cell to another. 

Here are some other differences between electrochemical cells and concentration cells: 

• Construction

  An electrochemical cell has two half-cells, each with an electrode and an electrolyte. A concentration cell has two half-cells with the same electrodes, but different concentrations. 
• Energy source

  An electrochemical cell uses a reaction between a fuel and an oxidizing agent to produce energy. A concentration cell generates electricity from the reduction in the thermodynamic free energy of the electrochemical system. 
• Corrosion

  Concentration cells can corrode when the metal is in contact with different concentrations. This can be prevented by sealing off the cell and keeping it clean. 

Q5) Write the phase diagram of single component system?

The phase diagrams (P-V, P-T, V-T) typically used to represent single-component phase behavior are 2-D projections of a 3-D surface. A liquid can be converted to a vapor without the co-existence of two phases by circumnavigating the critical point.

Q6) Write the difference between order of reaction and molecularity?

Order of a reaction is the sum of the coefficients of the reacting species involved in the rate equation. Molecularity is the number of reacting species involved in simultaneous collisions in an elementary or simplest reaction. Order is an experimentally determined quantity.

Q7) Define the corrosion of metal by Oxygen and H2O contact and also write the prevention of corrosion?

General corrosion occurs when most or all of the atoms on the same metal surface are oxidized, damaging the entire surface. Most metals are easily oxidized: they tend to lose electrons to oxygen (and other substances) in the air or in water. As oxygen is reduced (gains electrons), it forms an oxide with the metal.

Corrosion is a process that occurs when metal is exposed to oxygen and water, causing it to oxidize. This can happen due to a number of factors, including:

• Weather conditions: Exposure to water, wind, and moisture can cause metals to oxidize.
• Harmful chemicals: Corrosive substances like acids, bases, and sulfur oxide can promote corrosion.
• Biological substances: Dirt and bacteria can also cause corrosion. 

Some examples of corrosion include rusting of iron, tarnishing of silver, and green coating on copper. 

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Here are some ways to prevent corrosion:

• Use non-corrosive metals

  Use metals like stainless steel or aluminum that are less likely to corrode. 
• Keep the metal clean and dry

  Keep the metal surface clean and dry to prevent corrosion. 
• Apply a coating

  Apply a coating of oil, grease, paint, varnish, or a thin layer of a non-corrosive metal to the surface of the metal. 
• Galvanization

  Coat the metal with zinc to create a protective layer. The zinc corrodes instead of the metal it's protecting. 
• Alloying

  Mix the metal with a less reactive metal to create a non-rusting alloy. For example, brass is an alloy of copper that is non-reactive. 
• Cathodic protection

  Use an electrochemical technique to convert the active sites on the metal surface to passive sites. This is often done by attaching galvanic anodes to the metal.

Q8) Define the Following

a) Polymer 

b) Monomer 

c) Polymerization 

Polymer: poly·​mer ˈpäl-ə-mər. : a chemical compound or mixture of compounds that is formed by combination of smaller molecules and consists basically of repeating structural units.

A polymer is a substance made up of long chains or networks of smaller molecules called monomers. The process of monomers linking together to form a polymer chain is known as polymerization

Monomer: A monomer is a molecule or small group of atoms that can bond with other monomers to form a larger structure called a polymer. The word "monomer" comes from the prefix "mono-" meaning "one" and "-mer" meaning "part"

Monomers are atoms or small molecules that bond together to form more complex structures such as polymers. There are four main types of monomer, including sugars, amino acids, fatty acids, and nucleotides

Polymerization:  Polymerization is a chemical process that joins small molecules, called monomers, to form large chain-like or network molecules, called polymers.

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Q9) Difference between thermoplastic and thermosetting plastic engineering chemistry?

a thermoplastic can be repeatedly melted and reshaped when heated, while a thermosetting plastic undergoes a permanent chemical change when heated, solidifying into a fixed shape and cannot be remelted or reshaped without degradation; this difference is due to the distinct molecular structures, with thermoplastics having linear chains and thermosets having cross-linked networks of molecule.

Thermoplastics have secondary bonds between molecular chains. Thermosetting plastics have primary bonds between molecular chains and held together by strong cross-links. Thermoplastics have low melting points and low tensile strength. Thermosetting plastics have high melting points and tensile strength.

Q10)  What is natural rubber and synthetic rubber?

 "natural rubber" refers to a polymer obtained directly from the latex of plants, primarily the rubber tree (Hevea brasiliensis), while "synthetic rubber" is a man-made polymer with similar elastic properties, created through chemical synthesis using petroleum-based monomers, designed to have specific properties tailored for different applications; essentially, natural rubber comes from nature, while synthetic rubber is artificially produced in a lab. 

Key points about natural rubber:

• Source: Latex from rubber trees 
• Chemical composition: Primarily composed of polyisoprene, a polymer made up of isoprene units 
• Properties: Good elasticity, but can be sticky and susceptible to degradation in certain environments 

Key points about synthetic rubber:

• Source: Created from petroleum-based chemicals 
• Chemical composition: Varies depending on the desired properties, but can include monomers like butadiene, styrene, and chloroprene 
• Benefits: Can be engineered to have specific properties like high heat resistance, oil resistance, or improved durability compared to natural rubber .

The rubber which is obtained from natural sources such as plants and animals is called natural rubber. The rubber which is prepared artificially, which is man-made is termed synthetic rubber. Synthetic rubber holds a wide range of applications in daily life as well as in industries.