Haloalkanes and Haloarenes: Key Chemistry Formulae for NEET Preparation

1. Stoichiometry and Chemical Reactions

1.1 Molar Mass Calculation

• Formula: $\text{Molar Mass}=\sum (\text{atomic mass of each element}\times \text{number of atoms of that element})$
• Explanation: The molar mass of a compound is calculated by summing the atomic masses of all the atoms in its chemical formula. For example, the molar mass of chloromethane (CH3_33​Cl) is calculated as: ${\text{Molar Mass of CH}}_3\text{Cl}=(12.01\times 1)+(1.008\times 3)+(35.45\times 1)=50.49,\text{g/mol}$

Common Mistake: Forgetting to multiply the atomic mass by the number of atoms can lead to incorrect molar mass calculation.

1.2 Law of Conservation of Mass

• Formula: $\text{Mass of reactants}=\text{Mass of products}$
• Explanation: In any chemical reaction, the total mass of reactants equals the total mass of products, reflecting the conservation of mass. This principle is crucial when balancing chemical equations.

NEET Tip: Always check the balance of atoms on both sides of a reaction to apply the law correctly.

2. Thermodynamics

2.1 Enthalpy Change (ΔH)

• Formula: $\Delta H=\sum \text{(enthalpy of products)}-\sum \text{(enthalpy of reactants)}$
• Explanation: The enthalpy change of a reaction indicates whether the reaction is exothermic (ΔH < 0) or endothermic (ΔH > 0).

Example: For the reaction of methane combustion: ${\text{CH}}_4+2{\text{O}}_2\to {\text{CO}}_2+2{\text{H}}_2\text{O}$ The enthalpy change can be calculated using standard enthalpies of formation.

2.2 Gibbs Free Energy (ΔG)

• Formula: $\Delta G=\Delta H-T\Delta S$
• Explanation: Gibbs free energy determines the spontaneity of a reaction. A negative ΔG indicates a spontaneous process.

Mnemonic: "Goldfish Are Horrible Without Tartar Sauce" helps remember the relationship between Gibbs free energy (G), enthalpy (H), temperature (T), and entropy (S).

Common Mistake: Confusing ΔH with ΔG can lead to incorrect conclusions about reaction spontaneity.

3. Chemical Kinetics

3.1 Rate Law

• Formula: $\text{Rate}=k[A{]}^m[B{]}^n$
• Explanation: The rate of a chemical reaction depends on the concentration of the reactants, where mmm and nnn are the orders of the reaction with respect to each reactant, and kkk is the rate constant.

Real-life Application: Understanding reaction rates is essential in controlling industrial chemical processes.

3.2 Arrhenius Equation

• Formula: $k=A{e}^{-\frac{E_a}{RT}}$
• Explanation: This equation relates the rate constant $K$ to the activation energy $E_a$ and temperature $T$, where $A$ is the frequency factor.

NEET Problem-Solving Strategy: For questions involving temperature change, remember to convert temperature to Kelvin and apply the Arrhenius equation to find the new rate constant.

4. Chemical Bonding

4.1 Bond Enthalpy

• Formula: $\Delta H=\sum \text{Bond enthalpies of bonds broken}-\sum \text{Bond enthalpies of bonds formed}$
• Explanation: Bond enthalpy is the energy required to break one mole of a bond in a molecule in the gaseous state. This concept helps in calculating the overall enthalpy change during a reaction.

Common Misconception: Bond enthalpy values are averages and may not accurately represent bond energy in specific molecules.

4.2 Dipole Moment

• Formula: $\mu =q\times d$
• Explanation: Dipole moment (μ) is the product of the charge (q) and the distance (d) between the charges. It indicates the polarity of a molecule.

NEET Tip: Molecules with a higher dipole moment are generally more polar and soluble in polar solvents like water.

Quick Recap

• Molar Mass: Sum of atomic masses in a compound.
• Enthalpy Change (ΔH): Indicates heat absorbed or released.
• Gibbs Free Energy (ΔG): Determines spontaneity.
• Rate Law: Depends on concentration and reaction order.
• Arrhenius Equation: Links rate constant with temperature.
• Bond Enthalpy: Energy to break bonds.
• Dipole Moment: Measure of molecular polarity.

This summary provides a concise yet comprehensive overview of essential chemistry formulae and concepts relevant to NEET UG preparation, with a focus on stoichiometry, chemical reactions, thermodynamics, and physical chemistry.