Thermochemistry Essay

817 words - 4 pages

Thermochemistry is the application of thermodynamics to chemistry.Thermochemistry is used to predict the energy exchanges that occur in the following processes:1. chemical reactions2. phase changes such as boiling and melting3. the formation of solutionsThermochemistry is primarily concerned with the following state functions defined in thermodynamics:Internal energy (U)Enthalpy (H).Entropy (S)Gibbs free energy (G)Most identities in Thermochemistry arise from application of the first law of thermodynamics, the Law of Conservation of Energy, to these state function.In chemical reactions, bonds are broken and reformed, and heat may be given off or taken up in such processes. Since reactions are often carried out at constant pressure, the change in enthalpy is the appropriate measure of heat given off or absorbed.Laws of Thermochemistry1. The magnitude of H is directly proportional to the amount of reactant or product.-Thus H can be used as a ...view middle of the document...

Heat is not conserved, it can be either created or destroyed. In the metric system, heat is measured in units of calories, which are defined as the amount of heat required to raise the temperature of one gram of water from 14.5oC to 15.5oC.In the SI system, the unit of heat is the joule.Heat CapacityThe heat capacity of a substance is the amount of heat required to raise the temperature of a defined amount of pure substances by one degree (Celsius or Kelvin). The calorie was defined so that the heat capacity of water was equal to one.The specific heat of a substance is the number of calories needed to raise the temperature of one gram by 1oC. Because one degree on the Celsius scale is equal to one Kelvin, specific heats in the metric system can be reported in units of either cal/g-oC or cal/g-K. The units of specific heat in the SI system are J/g-K. Because there are 4.184 joules in a calorie, the specific heat of water is 4.184 J/g-K.The ease with which a substance gains or loses heat can also be described in terms of its molar heat capacity, which is the heat required to raise the temperature of one mole of the substance by either 1oC or 1 K. In the metric system, the units of molar heat capacities are therefore either cal/mol-oC or cal/mol-K. In the SI system the units of molar heat capacities are J/mol-K.Heat and The Kinetic Molecular TheoryThe system is the small portion of the universe in which we are interested, such as the water in a beaker or a gas trapped in a piston and cylinder, as shown in the figures below. The surroundings are everything else in other words, the rest of the universe.The system and its surroundings are separated by a boundary. Heat is transferred across the boundary between a system and its surroundings.One of the fundamental principles of the kinetic theory is the assumption that the average kinetic energy of a collection of gas particles depends on the temperature of the gas and nothing else. A gas becomes warmer if and only if the average kinetic energy of the gas particles increases. Heat, when it raises the temperature of a system, produces an increase in the speed with which the particles of the system move, as shown in the figure below.

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