Thermochemistry

Systems

• In thermodynamics we must define the part of the universe where the energy changes occur
• System: part of the universe that we are studying
• Surroundings: everything outside of the system
• Open system: matter & energy exchange with surroundings
• Closed system: exchanges energy as work and/or heat (but not matter)
• Isolated system: neither matter nor energy exchanged

First Law of Thermodynamics

• First law of thermodynamics: energy is conserved
• Internal energy, U, of a system: the sum of ALL potential and kinetic energies of the components of the system
• A precise, numerical value cannot be determined
• We consider only the change in internal energy
  • delta U = Ufinal – Uinitial
• delta U = q + w

State Functions

• Depend only on the present state of the system and NOT on the PATH by which the system arrived at that state
• The internal energy of the system is the same regardless of which PATH was used

Enthalpy

• “PV-work” is work involved in expansion or compression of gases
• At constant pressure: w = -P(delta V)
• If delta V is + → expansion of gas; system does work on surroundings and w is negative
• If delta V is - → compression of gas; work is done on the system and w is positive
• Like internal energy E, both P and V are state functions
• We can combine these state functions to define “ENTHALPY”, H
  • H = U + PV
• At constant pressure: delta H = delta E + P(delta V)
  • delta H = delta U + P(delta V) = (qpw) –w = qp
  • delta H = qp
  • This equation is important because qp is easily measured
• If delta H is positive, the reaction is endothermic
• If delta H is negative, the reaction is exothermic
• Reversing the direction of a reaction changes the sign of delta H
• If the coefficients of a chemical reaction are multiplied by some factor, the enthalpy change must also be multiplied by that factor
  • (delta H is an extensive property).
• Enthalpy change depends on the physical states of reactants and products and these states must be specified.

Constant Pressure Calorimetry

• The reaction is carried out in aqueous solution
• System = reactants and products Surroundings includes the water
• Heat released or absorbed by the reaction changes the temperature of the solution
• delta H can be determined experimentally by measuring heat flow for a reaction at constant pressure

Heat Capacity and Specific Heat

• Heat capacity: the amount of heat required to raise the temperature of an object by one degree C or K
• Specific heat: the heat capacity of one gram of a substance; Cs

Enthalpies of Formation

• Enthalpy of formation: the enthalpy change for the reaction in which one mole of a substance is made from its constituent elements in their elemental (most stable) forms.
• Standard enthalpies of formation: measured under standard conditions