Chemistry Notes: Hydrocarbons, Organic Chemistry, Thermodynamics, and More
Hydrocarbons
Reactions and Concepts
Wurtz Reaction
Alkyl halides react with metallic sodium in dry ether to form alkanes.
Sodium Acetate to Methane and Ethane
Sodium acetate heated with soda lime undergoes decarboxylation to form methane. It undergoes Kolbe’s electrolysis to yield ethane.
Conformations
Different spatial arrangements of atoms due to free rotation around a C-C single bond. Ethane exhibits eclipsed and staggered conformations, with staggered being more stable due to minimized repulsion.
Geometrical Isomerism
Spatial arrangement difference around a carbon-carbon double bond. Example: 2-butene.
Lindlar’s Catalyst
Partially deactivated palladised charcoal used for reducing alkynes to cis-alkenes.
HBr Addition and Peroxide Effect
Markovnikov’s rule determines major product formation in HBr addition to unsymmetrical alkenes. The peroxide effect allows addition against Markovnikov’s rule in the presence of peroxide.
Preparation of Acetone and Benzene
Calcium carbide with water yields acetylene gas. Passing acetylene through a red-hot iron tube at 873K results in cyclic polymerization to form benzene.
Hückel’s π Electron Rule and Gammexane Preparation
Planar, conjugated systems with (4n+2) π electrons are aromatic. Benzene adds chlorine in UV light to form benzene hexachloride (Gammexane).
Conversion of Benzene to Cyclohexane
Hydrogenation of benzene with nickel catalyst at high temperature and pressure yields cyclohexane.
Markovnikov’s and Anti-Markovnikov’s Rule
Markovnikov’s rule states that in an addition reaction of a hydrogen halide to an unsymmetrical alkene, the hydrogen atom adds to the carbon atom of the double bond that already has more hydrogen atoms attached. Anti-Markovnikov’s rule describes the opposite, often occurring in reactions involving peroxides or radical initiators.
Organic Chemistry
Tests and Techniques
Lassaigne’s Test and Sodium Fusion Extract
Lassaigne’s test detects nitrogen, sulphur, or halogens in organic compounds by fusing them with sodium. The resulting solution is called sodium fusion extract.
Silver Nitrate Test
Identifies the presence of halogens in organic compounds by adding silver nitrate solution.
Organic Compound Purification
Methods like distillation, crystallization, and chromatography are used to separate and purify organic compounds.
Sublimation and Crystallization
Sublimation is the transition of a substance directly from solid to gas phase. Crystallization is the formation of solid crystals from a solution or melt, used for purifying substances.
Distillation and Electromeric Effect
Distillation separates components of a liquid mixture based on boiling points. The electromeric effect describes electron movement in a molecule due to adjacent double bonds or electron-withdrawing/donating groups.
Dumas Method and Kjeldahl’s Method
Dumas method determines the molecular weight of a volatile liquid. Kjeldahl’s method determines nitrogen content in organic compounds.
Carius Method and Metamerism
Carius method involves complete oxidation of organic compounds. Metamerism is isomerism where different alkyl groups are attached to the same functional group.
Homolytic and Heterolytic Bond Cleavage
Homolytic cleavage results in free radicals, while heterolytic cleavage produces ions.
Thermodynamics
Systems and Laws
Open, Closed, and Isolated Systems
– Open system: Exchanges matter and energy with surroundings.
– Closed system: Exchanges energy but not matter with surroundings.
– Isolated system: Doesn’t exchange matter or energy with surroundings.
1st and 2nd Laws of Thermodynamics
– 1st Law: Energy cannot be created or destroyed, only converted.
– 2nd Law: Heat flows from hotter to cooler objects, and entropy (disorder) tends to increase.
Properties and Concepts
Intensive and Extensive Properties
– Intensive property: Independent of substance amount (e.g., temperature, pressure).
– Extensive property: Depends on substance amount (e.g., mass, volume).
Gibbs Free Energy and Hess’s Law
– Gibbs Free Energy: Measures spontaneity at constant temperature and pressure. ΔG < 0 indicates spontaneity.
– Hess’s Law: Total enthalpy change for a reaction is the same regardless of the pathway.
Lattice Enthalpy and Entropy
– Lattice Enthalpy: Energy to separate one mole of a solid ionic compound into gaseous ions.
– Entropy (S): Measure of disorder or randomness. Increases in spontaneous processes.
Born-Haber Cycle and Enthalpy
– Born-Haber Cycle: Represents formation of an ionic compound from its elements.
– Standard Enthalpy of Formation (ΔH°f): Heat change when one mole of a substance is formed from its elements.
– Standard Enthalpy of Reaction (ΔH°rxn): Heat change for a reaction under standard conditions.
Spontaneous Processes and State Functions
– Spontaneous Process: Occurs without external intervention. Criteria: ΔG < 0, ΔS > 0, and ΔH < 0.
– State function: Depends only on the current state of the system (e.g., temperature, pressure).
– Path function: Depends on the path taken to reach a particular state (e.g., work, heat).
Classification of Elements
Mendeleev’s Periodic Table
Organization and Merits
Mendeleev organized elements by atomic weight, proposing periodic repetition of properties. Merits include being the first comprehensive classification, correcting atomic weights, predicting properties of undiscovered elements, and grouping elements with similar properties.
Drawbacks
Drawbacks include placing dissimilar elements in the same group, uncertain positions for hydrogen and lanthanides/actinides, and not strictly following atomic weight order.
Modern Periodic Law and Trends
Modern Periodic Law and Transition Elements
Modern Periodic Law, proposed by Moseley, states that properties depend on atomic numbers. Transition elements exhibit colored compounds, variable oxidation states, paramagnetism, and catalytic properties.
Atomic Radius and Isoelectronic Species
Atomic radius decreases across periods and increases down groups. Isoelectronic species are atoms/ions with the same number of electrons.
Ionization Enthalpy and Electron Gain Enthalpy
Ionization enthalpy is the energy to remove an electron. It increases across periods and decreases down groups. Electron gain enthalpy (∆egH) is the heat change when adding an electron. It becomes more negative across periods and less negative down groups.
Equilibrium
Equilibrium Constant and Le Chatelier’s Principle
Characteristics of Equilibrium Constant
Indicates the extent of a chemical reaction at equilibrium. Remains constant at a given temperature. Changes with temperature, but not with concentration or pressure.
Le Chatelier’s Principle
A system at equilibrium will respond to any stress by shifting the equilibrium position to counteract the stress. Stress can be changes in concentration, temperature, or pressure.
Factors Affecting Reaction Rate
Concentration, Temperature, and Pressure
– Concentration: Increasing reactant concentration generally increases reaction rate.
– Temperature: Increasing temperature generally increases reaction rate.
– Pressure: Affects gas-phase reactions involving changes in moles of gas.
Acids and Bases
Arrhenius, Bronsted, and Lewis Concepts
– Arrhenius: Acids produce H⁺ ions, bases produce OH⁻ ions.
– Bronsted: Acids donate protons, bases accept protons.
– Lewis: Acids accept electron pairs, bases donate electron pairs.
pH, Common Ion Effect, and Buffer Solutions
pH and Common Ion Effect
– pH: Measure of acidity or basicity (pH = -log[H⁺]).
– Common ion effect: Reduced solubility of an ionic compound due to a common ion.
Buffer Solution and Solubility Product
– Buffer solution: Resists pH changes. Consists of a weak acid and its conjugate base, or vice versa.
– Solubility product: Constant for a sparingly soluble salt.