Understanding Chemical Bonds, Reactions, and Forces
Classified in Chemistry
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Chemical Bonding and Intermolecular Forces
Polarity
When two atoms with different electronegativities bond, the electrons in the bond are drawn closer to the more electronegative atom. This creates a negative charge density around that atom, which in turn causes the other atom to exhibit a positive charge density. This forms a dipole, and the bond is called polar or heteropolar.
Metallic Bonding
Various theories explain metallic bonding. The most consistent and widely accepted theory suggests that metallic bonding results from the strong attraction between a delocalized "sea" of electrons and the positively charged nuclei of the metal atoms. This explains the characteristic properties of metals, such as hardness, toughness, and malleability.
Hydrogen Bond
A hydrogen bond is an attraction between a hydrogen atom of one molecule (often water) and a lone pair of electrons on an electronegative atom (such as oxygen) of an adjacent molecule. This type of bond explains certain properties of water, such as its relatively high boiling point.
Resonance (Mesomerism)
Some chemical compounds exhibit properties that cannot be explained by a single, specific type of chemical bond. In these cases, the substance behaves as if it has a "hybrid" bond, participating simultaneously in two or more different bonding arrangements without being definitively one or the other. This phenomenon is known as resonance or mesomerism.
Van Der Waals Forces
In addition to intramolecular forces (forces within a molecule), there are also intermolecular forces that hold molecules together. These forces originate from the electrostatic attraction between the nucleus of each atom and the electron clouds of neighboring atoms, and vice versa. These cohesive forces occur between molecules of the same substance, while adhesive forces occur between molecules of different substances.
Chemical Reactions
Acid-Base Reactions
Acid: A substance that produces H+ ions in aqueous solution.
Base: A substance that produces OH- ions in aqueous solution.
Indicators, such as phenolphthalein, are substances that change color depending on the acidity or basicity of a solution.
A neutralization reaction occurs between an acid and a base in aqueous solution, producing salt and water.
Oxidation-Reduction (Redox) Reactions
Oxidation: The loss of electrons by a chemical species, resulting in an increase in its oxidation number.
Reduction: The gain of electrons by a chemical species.
If two substances react, where one is oxidized and the other is reduced, the process is called oxidation-reduction (redox).
Oxidizing agents are substances that cause oxidation. Reducing agents are substances that undergo oxidation.
Combustion Reactions
Combustion is a chemical reaction between a substance and oxygen, typically producing heat and light.
Reaction Kinetics
Reaction Rate
Reaction rate is the change in concentration of the substances involved in a chemical reaction per unit time. For two substances to react, they must first collide, and secondly, the collision must be effective. An effective collision requires the molecules to have sufficient energy to break existing bonds and form new ones.
Factors affecting reaction rate include:
- Concentration of reactants
- Temperature
- Pressure (for gases)
- Catalysts
- Surface area of solid reactants
Complete and Incomplete Reactions
A reaction is considered complete or irreversible if it proceeds until one of the reactants is completely consumed. For example: S(s) + O2(g) → SO2(g) (occurs in open air).
A reversible or incomplete reaction occurs when the products formed can react to regenerate the original reactants. For example: H2(g) + I2(g) ⇌ 2HI(g)