6: Fundamentals of Thermochemistry (Heat and Enthalpy)
This chapter introduces you to thermochemistry , a branch of chemistry that describes the energy changes that occur during chemical reactions. In some situations, the energy produced by chemical reactions is actually of greater interest to chemists than the material products of the reaction. For example, the controlled combustion of organic molecules, primarily sugars and fats, within our cells provides the energy for physical activity, thought, and other complex chemical transformations that occur in our bodies. Similarly, our energy-intensive society extracts energy from the combustion of fossil fuels, such as coal, petroleum, and natural gas, to manufacture clothing and furniture, heat your home in winter and cool it in summer, and power the car or bus that gets you to class and to the movies.
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- 6.4: The First Law of Thermodynamics
- The first law of thermodynamics states that the energy of the universe is constant. The change in the internal energy of a system is the sum of the heat transferred and the work done. The heat flow is equal to the change in the internal energy of the system plus the PV work done. When the volume of a system is constant, changes in its internal energy can be calculated by substituting the ideal gas law into the equation for ΔU.
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- 6.5: Heat Capacity and Calorimetry
- Thermal energy itself cannot be measured easily, but the temperature change caused by the flow of thermal energy between objects or substances can be measured. Calorimetry describes a set of techniques employed to measure enthalpy changes in chemical processes using devices called calorimeters. To have any meaning, the quantity that is actually measured in a calorimetric experiment, the change in the temperature of the device, must be related to the heat evolved or consumed in a chemical reactio
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- 6.6: Molecules as Energy Carriers and Converters
- All molecules at temperatures above absolue zero possess thermal energy— the randomized kinetic energy associated with the various motions the molecules as a whole, and also the atoms within them, can undergo. Polyatomic molecules also possess potential energy in the form of chemical bonds. Molecules are thus both vehicles for storing and transporting energy, and the means of converting it from one form to another when the formation, breaking, or rearrangement of interactions and bonding.
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- 6.7: Heats of Reactions - ΔU and ΔH
- Enthalpy is a state function used to measure the heat transferred from a system to its surroundings or vice versa at constant pressure. Only the change in enthalpy (ΔH) can be measured. A negative ΔH means that heat flows from a system to its surroundings; a positive ΔH means that heat flows into a system from its surroundings. For a chemical reaction, the enthalpy of reaction (ΔHrxn) is the difference in enthalpy between products and reactants; the units of ΔHrxn are kilojoules per mole. Revers
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- 6.8: Indirect Determination of ΔH - Hess's Law
- Hess's law is that the overall enthalpy change for a series of reactions is the sum of the enthalpy changes for the individual reactions. For a chemical reaction, the enthalpy of reaction (ΔHrxn) is the difference in enthalpy between products and reactants; the units of ΔHrxn are kilojoules per mole. Reversing a chemical reaction reverses the sign of ΔHrxn.
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- 6.9: Standard Enthalpies of Formation
- The enthalpy of formation is the enthalpy change that accompanies the formation of a compound from its elements. Standard enthalpies of formation are determined under standard conditions: a pressure of 1 atm for gases and a concentration of 1 M for species in solution, with all pure substances present in their standard states (their most stable forms at 1 atm pressure and the temperature of the measurement). The standard heat of formation of any element in its most stable form.
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Outside Links
- Barrow, Gordon M. "Thermodynamics should be built on energy-not on heat and work." J. Chem. Educ. 1988 : 65, 122.
- Brouwer, Henry. "Small-scale thermochemistry experiment (ML)." J. Chem. Educ. 1991 :68, A178.
- King, Edward L. "Thermodynamics of the thermal decomposition of water by closed chemical cycles." J. Chem. Educ. 1981 , 58 , 975.
- Peckham, G. D., McNaught, Ian J. 'Heat and work are not "forms of energy".' J. Chem. Educ. 1993 : 70, 103.