Physical Sciences, Part 1

Question 1: Define bond enthalpy and list the number of bonds formed by carbon, oxygen, nitrogen, and hydrogen.

Answer 1: Bond enthalpy refers to the amount of energy it takes to break a chemical bond, and can also be viewed as the amount of energy being stored by a bond. It is usually expressed using kJ/mol. Enthalpy is influenced by the molecular environment, so exact enthalpy changes and listed enthalpies may be averages. In most cases, stronger bonds are created by shorter bond lengths. For example, in a bromine to bromine bond, the enthalpy is 193 kJ/mol and the bond length is 0.228 nm. In a chlorine to chlorine bond, the enthalpy is 243kJ/mol and the bond length is 0.199 nm. Enthalpy is indicated using the symbol H. The formation of bonds is considered exothermic, while the breaking of bonds is endothermic.Carbon forms four bonds, oxygen forms two, nitrogen forms three, and hydrogen forms one.

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Question 2: Compare and contrast liquid and solid crystals.

Answer 2: The two kinds of crystals, or crystalline solids, are liquid and solid. A crystal is considered to be a well-organized, repeating configuration of atoms, ions, or molecules. The strong attractive forces between oppositely-charged ions are responsible for the repeating patterns in solid crystals. These arrangements of repeating patterns can occur as a liquid cools to form a solid, as is seen with cubes of salt or ice. The three-dimensional crystal lattice pattern of a solid is lost when it melts into a liquid. However, this may only be lost in one or two dimensions, so some of the crystalline properties may remain. Crystalline structures occur across all classes of materials and form from all types of bonding. Salt is an example of a crystal formed with ionic bonds. A diamond is formed with covalent bonds. The majority of naturally occurring and artificially prepared solids have crystalline structures. Solids that do not are called amorphous solids because they have random arrangements.

Question 3: Summarize the characteristics of crystals.

Answer 3: The crystalline structure of crystals is also known as a lattice. Crystal particles have symmetrical arrangements. Solidification may result in the formation of a single crystal or a group of crystals, which is known as a polycrystalline structure. The symmetry of a crystal is used to determine its classification. Although they are organized, structured, and symmetrical, crystals are imperfect and can have defects. Some may also have specific electrical properties. Crystal systems are grouped in accordance with their axial systems (there are three axes). The unique systems of crystals are cubic, hexagonal, rhombohedral, orthorhombic, monoclinic, and triclinic. Crystals in the cubic system are the most symmetric. The unit cell is the spatial arrangement of atoms. The spacing between unit cells is known as the lattice parameters. Salts form in cubic crystals. Common table salt (NaCl), for instance, forms a cubic lattice through ionic bonding. Rapid evaporation of a salt water solution results in small crystals, while slower evaporation leads to larger crystals.

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