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Chapter 5: Thermochemistry

Chapter 5: Thermochemistry. Energy First Law of Thermodynamics Enthalpy Enthalpies of Reaction Calorimetry Hess' Law Enthalpies of Formation Fuel Value. Energy.

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Chapter 5: Thermochemistry

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  1. Chapter 5: Thermochemistry • Energy • First Law of Thermodynamics • Enthalpy • Enthalpies of Reaction • Calorimetry • Hess' Law • Enthalpies of Formation • Fuel Value

  2. Energy • Energy can be defined as the capacity to do work or to transfer heat. Matter possesses energy both by virtue of its motion (kinetic energy) and by virtue of its position (potential energy). • The SI unit of energy is the joule. One joule is the amount of kinetic energy possessed by a 2 kilogram object moving at a speed of one meter per second:

  3. First Law of Thermodynamics • The total amount of energy possessed by the system and surroundings remains constant. System q > 0, or q is positive Surroundings q < 0, or q is negative System w > 0, or w is positive Surroundings w < 0, or w is negative

  4. Enthalpy • Enthalpy – heat transfer measured at constant pressure • Enthalpy is an extensive property, meaning that the units of enthalpy are kiloJoules per mole, and the amount of heat transferred depends on the amount of substances that react • The enthalpy change of a reaction is equal in magnitude but opposite in sign to the enthalpy change of the reverse reaction • Enthalpy is a state function, meaning that it only depends on the current state of the reactants and products

  5. Calorimetry • Calorimetry – measurement of heat flow • Heat capacity is defined as the amount of heat necessary to raise the temperature of an object by one degree (Celsius or Kelvin)‏ • Specific heat capacity is defined as the amount of heat necessary to raise the temperature of 1 g of a substance by one degree Constant Pressure Equation Constant Volume Equation q = m x C x ΔT q = -Ccal x ΔT

  6. Enthalpy Changes • Hess' Law – The ΔH for a reaction or process that is carried out in a series of steps is equal to the sum of the ΔH for each step • ΔHrxn = ΣΔHproducts – ΣΔHreactants • ΔHprocess = ΣΔHending – ΣΔHbeginning • By definition, ΔH°f (standard enthalpy of formation) of any element in its standard state (the form that is most stable at 298 K and atmospheric pressure) is zero

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