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