Physical and chemical changes occur
with changes in heat.
Energy, heat and temperature
Several definitions are useful in understanding thermodynamics:
- Energy: The ability to do work or
supply heat.
- System: Particles under-going change.
- Surroundings: Everything surrounding
the system.
- Temperature: proportional to the
average kinetic energy of the particles.
- Heat (q): Flow of energy from a hotter
object to a cooler object.
- Enthalpy (H): Takes into account
internal energy, pressure and volume. In an open-air
environment, it�s the same as heat.
Physical changes
The specific heat capacity is the amount of energy that can be
absorbed before temperature begins to change.
where
m = mass; Cp = specific heat capacity and DT = T2 � T1. During
changes in state, the heat of fusion and heat of vaporization are
used. Melting:
Hfus
= enthalpy of fusion. Boiling:
Hvap
= enthalpy of vaporization. Heating curves show the combination of
the processes�changing temperatures and changing states.
Chemical changes
The heat of formation is the energy change when a compound is formed
from its elements. Hess�s law says that since energy is a state
function, the path doesn�t matter�only where you began and ended,
then the heat of a reaction can be found by adding up stepwise
reactions that add up to the overall chemical reaction. This allows
the heat of reaction to be found from formation values:
Calorimetry
When the system loses energy, the surroundings gain it and vice
versa. Therefore, the energy change in the surrounding can be
measured and used to determine information about the system.
Calorimetry can be used for physical or chemical processes.
Enthalpy, Entropy and Free energy
Entropy is disorder or randomness. All spontaneous processes result
in a net increase in entropy for the universe. The spontaneity of a
process (shown with a negative free energy value) is found by
relating enthalpy, entropy and temperature to find free energy.
| Enthalpy
|
Entropy |
Spontaneous at |
| + |
+ |
High temps |
| + |
- |
Never |
| - |
- |
Low temps |
| - |
+ |
All temps |
