EnergyCommon forms of energy include kinetic energy and potential energy. The SI unit of energy is the joule (J). When a force causes an object to move, work is done:

 Thermochemistry: Some Basic Terms?Thermochemistry is the study of energy changes in physical processes or chemical reactions. Basic thermochemical ideas include the notion of a system and its surroundings and of closed, open, and isolated systems; the concepts of kinetic energy, potential energy, and internal energy; and the distinction between two types of energy exchanges, heat (q) and work (). Work involves an energy transfer resulting from the movement of an object, and heat involves an energy transfer resulting from a difference in temperature between a system and its surroundings.

 Internal Energy (U), State Functions, and the First Law of Thermodynamics?Internal energy (U) is a state function, which means it has a unique value once the state, or condition, of a system is defined. The change in the internal energy of a system is equal to the difference between the internal energy of the final and initial states. According to the law of conservation of energy, energy can be neither created nor destroyed, but it can be transferred to or from the surroundings. The first law of thermodynamics??relates the heat and work exchanged between a system and its surroundings to changes in the internal energy of a system. This law is the basis for many of the calculations of thermochemistry.

 Heats of Reaction and Enthalpy Change ?For any reaction occurring at constant temperature, the heat of reaction is the quantity of heat exchanged between the reaction system and its surroundings. If a reaction is carried out at constant volume, the heat of reaction at constant volume is equal to the change in internal energy For constant-pressure processes, enthalpy (H) is a more useful property than internal energy, to which it is related. At a constant pressure and with work limited to pressure?volume work, the heat of reaction is equal to the enthalpy change

Like internal energy, enthalpy is a function of state. Enthalpy changes can be written into chemical equations and incorporated into conversion factors that relate amounts of substances to quantities of heat released or absorbed in chemical reactions. In an exothermic reaction, enthalpy decreases and heat is given off to the surroundings. In an endothermic reaction, enthalpy increases and heat is absorbed from the surroundings. These changes can be represented graphically in the form of an enthalpy diagram.

 Calorimetry: Measuring Quantities of Heat?Calorimetry is the process of measuring quantities of heat, and a calorimeter is the device used for the procedure. The actual measurements are of masses and temperature changes. Other data required are heat capacities and/or specific heats. The heat capacity (C) of a system is the quantity of heat needed to change the temperature of the system by 1 °C (or by 1 K):

The molar heat capacity is the heat capacity of one mole of substance. The specific heat is the heat capacity of a 1-g sample of the substance:

Many reactions can be carried out under the constant pressure of the atmosphere in a plastic-foam calorimeter. The results obtained are or values. Combustion reactions are generally carried out under constant-volume conditions in a bomb calorimeter. The results are or values; values can be calculated from values when necessary.

 Hess's Law of Constant Heat Summation?Because enthalpy is a state function, the value of is the same whether a reaction is carried out in one step or several. We can write the equation for a reaction as several steps, and use Hess?s law to evaluate by summing the enthalpy changes of the steps.

 Standard Enthalpies of Formation?At the temperature of interest, the standard state of a pure solid or liquid is the substance at 1 atm; for a gas, it is the pure gas behaving ideally at 1 atm. The standard enthalpy of reaction, , is the enthalpy change for a reaction in which both the reactants and the products are in their standard states. The standard enthalpy of formation, is equal to zero for an element in its reference form, which is usually the most stable form at 1 atm and the temperature of interest. The standard enthalpy of formation of a compound is the standard enthalpy change of a reaction in which the compound is formed from the reference forms of its elements in their standard states. Standard enthalpies of formation (usually listed at 25.00 °C) can be used to calculate standard enthalpies of reaction through the following equation (where the symbols and are stoichiometric coefficients):

 Combustion and Respiration: Fuels and Foods?In this section, two aspects of life in which thermochemical ideas play key roles are explored: the combustion of fossil fuels?coal, natural gas, and petroleum?and the metabolism of foods?carbohydrates, fats, and proteins

 

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