Termochemistry of the chemical substances

thermochemistry of the chemical substances [1936]
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Overview of Thermochemistry Thermochemistry is the branch of chemistry concerned with the heat evolved or absorbed in a chemical process. Conceptual Vocabulary. Thermochemistry is the study of the heat evolved or absorbed in chemical reactions. The enthalpy or heat content is a quotient or description of thermodynamic potential of a system equivalent to the sum of the internal energy of the system plus the product of its volume multiplied by the pressure exerted on it by its surroundings.

Thermodynamics

Heat of combustion. The heat of combustion is the energy released when a compound undergoes complete combustion with oxygen.

source site Exothermic describes a process or reaction that releases energy in the form of heat. Endothermic describes a process or reaction that absorbs energy in the form of heat.

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A calorimeter is a device used for measuring the heat of chemical reactions or physical changes as well as heat capacity. Standard conditions for temperature and pressure. Standard temperature and pressure is a standard set of conditions for experimental measurements, to enable comparisons to be made between sets of data. Hess's law. Developed through conceptualizing cyclic reaction processes in which the return path is different than the forward path, Hess's Law of Heat Summation is used to predict the enthalpy change regardless of the path through which it is to be determined.

Reaction calorimeter. A reaction calorimeter is an instrument that enables the energy being released or absorbed by a reaction to be measured. Calorimetry is the science of measuring the heat of chemical reactions or physical changes. Bond dissociation energy. Bond dissociation energy is defined as the standard enthalpy change when a bond is cleaved by homolysis, with reactants and products of the homolysis reaction at 0K absolute zero. Standard enthalpy of Combustion. The standard enthalpy of combustion is the change in enthalpy of the total reacting system when one mole of a substance completely reacts with oxygen, and is observed at K and 1 atmospheric pressure.

Standard enthalpy of formation. The standard enthalpy of formation of a compound is the change of enthalpy that accompanies forming 1 mole of a substance in its standard state from its constituent elements in their standard states. Because enthalpy change is not path dependent, but dependent on change in thermodynamic state, we can derive the extremely useful principle, Hess's Law of Heat Summation, which states that the heat exchange accompanying a transformation is the same whether the process occurs in one or several steps.

A large part of skill in Thermochemistry involves imagining pathways for chemical change that help you understand the enthalpy change of a process which may or may not occur by that path. Thermochemistry on the MCAT The sequence of thermochemical reasoning is one of the primary conceptual arcs underlying the understanding of chemistry, so while there may only be a couple of questions that lead to an 'enthalpy change' as an answer, your thermochemical understanding will be in operation throughout the exam.

When you learn to see the substances involved in a chemical reaction as a thermodynamic system, you can see any chemical process as a transformation through which internal energy may change, thermodynamic work may be performed, and heat may be evolved or absorbed.

Thermochemistry

Over the next chapters, after we begin making the connections between Thermochemistry and Chemical Thermodynamics, knowledge of Thermochemistry will serve as the basis for a deeper understanding of spontaneity and chemical equilibrium. Conceptual Vocabulary Thermochemistry Thermochemistry is the study of the heat evolved or absorbed in chemical reactions. Enthalpy The enthalpy or heat content is a quotient or description of thermodynamic potential of a system equivalent to the sum of the internal energy of the system plus the product of its volume multiplied by the pressure exerted on it by its surroundings.

Heat of combustion The heat of combustion is the energy released when a compound undergoes complete combustion with oxygen. Exothermic Exothermic describes a process or reaction that releases energy in the form of heat. Endothermic Endothermic describes a process or reaction that absorbs energy in the form of heat.

Calorimeter A calorimeter is a device used for measuring the heat of chemical reactions or physical changes as well as heat capacity. Standard conditions for temperature and pressure Standard temperature and pressure is a standard set of conditions for experimental measurements, to enable comparisons to be made between sets of data. Hess's law Developed through conceptualizing cyclic reaction processes in which the return path is different than the forward path, Hess's Law of Heat Summation is used to predict the enthalpy change regardless of the path through which it is to be determined.

Reaction calorimeter A reaction calorimeter is an instrument that enables the energy being released or absorbed by a reaction to be measured. Calorimetry Calorimetry is the science of measuring the heat of chemical reactions or physical changes. Bond dissociation energy Bond dissociation energy is defined as the standard enthalpy change when a bond is cleaved by homolysis, with reactants and products of the homolysis reaction at 0K absolute zero.

Standard enthalpy of Combustion The standard enthalpy of combustion is the change in enthalpy of the total reacting system when one mole of a substance completely reacts with oxygen, and is observed at K and 1 atmospheric pressure Standard enthalpy of formation The standard enthalpy of formation of a compound is the change of enthalpy that accompanies forming 1 mole of a substance in its standard state from its constituent elements in their standard states Born-Haber cycle The Born-Haber Cycle is an approach to analyzing reaction energies involving the formation of an ionic compound from the reaction of a group I or group II metal with a non-metal.

Standard state The standard state of a material is its state at 1 bar kilopascals exactly. Lattice energy The lattice energy of an ionic solid is a measure of the strength of bonds in that ionic compound, equivalent to the amount of energy required to separate a solid ionic compound into gaseous ions.

Chapter Outline

Thermal energy depends on the quantity of a substance present and is thus known as an extensive property. A chemical system is a closed system that contains only the reactants and products involved in the reaction. Endothermic describes a process or reaction that absorbs energy in the form of heat. Different types of energy include thermal energy, electrical energy, radiant energy, chemical energy, mechanical energy, and nuclear energy. Because 2H 2 O g occurs on both sides of the equation, it can be cancelled. Potential and kinetic energy are interconvertible; that is, one form can change to the other. Heat or light must be absorbed from the surroundings before this type of reaction takes place.

Standard enthalpy change of reaction The standard enthalpy change of reaction is the enthalpy change that occurs in a system when one mole of matter is transformed by a chemical reaction under standard conditions. Enthalpy of atomization The enthalpy of atomization is the enthalpy change that accompanies the total separation of all atoms in a chemical substance Isenthalpic process An isenthalpic process is one that proceeds without any change in enthalpy also known as a throttling process.

Isodesmic reaction An isodesmic reaction is a chemical reaction in which the type of chemical bonds broken in the reactant are the same as the type of bonds formed in the reaction product. Undergraduate level physics, chemistry, organic chemistry and biology are presented by this course as a unified whole within a spiraling curriculum. Contact Us. A boundary separates the system from the surroundings.

The first law of thermodynamics relates the energy change belonging to a system to the amount of work and heat crossing the boundary. A statement of the first law applied to chemical reactions in which only heat and work cross the boundary is given by the expression:. Here U products represents the energy of the products and U reactants represents the energy of the reactants. The heat associated with the reaction is given as q , and w represents work done during the transformation of reactants to products.

If the volume of the system changes during the reaction and the applied pressure remains constant, the work carried out is termed pressure-volume work. For example, reaction 2 converts one mole of gas and two moles of liquid to a total of three moles of gas. The volume of the system increases during the reaction because, under standard conditions, a mole of gas occupies more volume than a mole of liquid.

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The first law of thermodynamics also states that U is a state function. State functions are very important in thermodynamics; they depend only on the present state of a system and not on its past history. Neither q nor w are state functions. An understanding of the concept of state function is furthered by considering the example of one's taking a trip from San Diego, California, to Denver, Colorado. The change in altitude that one experiences during this trip does not depend on the route taken and, thus, is similar to a state function.

In comparison, the distance traveled between the two cities does depend on the route one follows; similarly, q and w are path-dependent quantities. Thus, we have:.

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The Thermochemistry of the Chemical Substances. By P. It. Bichowsky and P. D. itossini. Pp. New York: Reinhold Publishing Corp.; London. Chapman. The Thermochemistry of the Chemical Substances (Bichowsky, F. Russell; Rossini, Frederick D.) Otto Reinmuth. J. Chem. Educ., , 13 (11), p

The symbol q p represents the heat accompanying a chemical change carried out at constant pressure; in our previous example this would be equivalent to our specifying the exact route of travel between the two cities. The enthalpy of a system H is related to the energy of a system by the expression:. Enthalpy, like energy, is a state function. Thus, equation 6 shows that, for a reaction carried out at constant pressure, q p depends only on the reactants consumed and the products formed. The change in enthalpy accompanying the conversion of reactants to products in a chemical reaction determines the amount of heat liberated or absorbed by the reaction.

For a reaction carried out at constant pressure the enthalpy change depends only on the reactants and products.