Your roommate is having trouble understanding why solids form. He asks, “Why would atoms bond into solids rather than just floating freely with respect to each other?” To help him understand at least one type of bonding in solids, you decide to embark on an energy explanation. You show him a drawing of a primitive cell of a sodium chloride crystal, NaCl, or simple table salt. The drawing is shown in the picture below where the orange spheres and Na ions and blue spheres are Cl ions. Each ion has a charge of magnitude equal to the elementary charge e. The ions lie on the corners of a cube of side ‘d’. You explain to your roommate that the electrical potential energy is defined as zero when all eight charges are infinitely far apart from each other. Then you bring them together to form the crystal structure shown.
Thermochemistry
Thermochemistry can be considered as a branch of thermodynamics that deals with the connections between warmth, work, and various types of energy, formed because of different synthetic and actual cycles. Thermochemistry describes the energy changes that occur as a result of reactions or chemical changes in a substance.
Exergonic Reaction
The term exergonic is derived from the Greek word in which ‘ergon’ means work and exergonic means ‘work outside’. Exergonic reactions releases work energy. Exergonic reactions are different from exothermic reactions, the one that releases only heat energy during the course of the reaction. So, exothermic reaction is one type of exergonic reaction. Exergonic reaction releases work energy in different forms like heat, light or sound. For example, a glow stick releases light making that an exergonic reaction and not an exothermic reaction since no heat is released. Even endothermic reactions at very high temperature are exergonic.
Your roommate is having trouble understanding why solids form. He asks, “Why would atoms bond into solids rather than just floating freely with respect to each other?” To help him understand at least one type of bonding in solids, you decide to embark on an energy explanation. You show him a drawing of a primitive cell of a sodium chloride crystal, NaCl, or simple table salt. The drawing is shown in the picture below where the orange spheres and Na ions and blue spheres are Cl ions. Each ion has a charge of magnitude equal to the elementary charge e. The ions lie on the corners of a cube of side ‘d’. You explain to your roommate that the electrical potential energy is defined as zero when all eight charges are infinitely far apart from each other. Then you bring them together to form the crystal structure shown.
(a) Evaluate the electric potential energy of the crystal as shown and
(b) show that it is energetically favorable for such crystals to form.
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