8 Thermodynamics Flashcards
(36 cards)
The First Law of Thermodynamics
Energy can be
neither created nor destroyed
The Second Law of Thermodynamics
If a process i spontaneous in one direction, then it can’t be spontaneous in
the reverse direction, and that entropy of the universe always increases during spontaneous reactions.
Enthalpy change {Delta}H, entropy change {Delta}S, and free-energy change {Delta}G are state functions. This means that they all depend on the change that occurs
between the initial and the final states of a system, not on the process by which the change occurs.
Standard State Conditions(6)
Denoted by the º superscript as in {Delta}Hº
- All gases are at 1 atm pressure
- All liquids are pure
- All solids are pure
- All solutions are at 1-molar (1 M) concentration
- The energy of formation of an element in its normal state is defined as zero
- The temperature used for standard state values is almost invariably room temperatuer: 25º C (298 K)
The Basic Rules of Enthalpy
When bonds are formed, energy is
released.
When bonds are broken energy is absorbed
Enthalpy Change {Delta}H =
Hproducts - Hreactants
If the products have stronger bonds than the reactants, then
energy is released by the reaction, or the reaction is exothermic.
If the products have weaker bonds than the reactants, then energy is
absorbed by the reaction, or the reaction is endothermic
If {Delta}Hºf for a compound is negative, energy is released when the compound is formed from pure elements, and the product is more stable than its constituent elements.
That is the process is exothermic.
If {Delta}Hºf for a compound is positive, energy is absorbed when the compound is formed from pure elements, and the product is less stable than its constituent elements.
That is the process is endothermic.
{Delta}Hº =
(Sum) Bond energies of bonds broken - (Sum) bond energies of bonds formed.
Hess’s Law
If a reaction can be described as a series of steps, then
{Delta}H for the overall reaction is siimply the sum of the {Delta}H values for all the steps.
Cp =
{Delta}H / {Delta} T
Cp = Heat capacity
{Delta}H = heat added (J or Cal)
{Delta}T = temperature change (K or ºC)
Specific heat is
the amount of heat required to raise the temperature of one gram of a substance one degree Celsius
Specific Heat
q =
mc{Delta}T
q = heat added (J or cal)
m = mass of the substance (g or Kg)
c = specific heat
{Delta}T = temperature Change (K or Degrees C)
The entropy, S, is
the measure of the disorder of the system; the greater the disorder the greater the entropy.
Liquids have higher entropy values than
solids
Gases have higher entropy values than
liquids
Particles in solution have higher entropy values than
solids
Two moles of a substance have higher entropy value than
one mole
{Delta}Sº =
(Sum) {Delta}Sºproducts - (Sum){Delta}Sºreactants
The Gibbs Free Energy, or simply free energy, G, of a process is
a measure of the spontaneity of the process.
If {Delta}G is negative
the reaction is spontaneous.
If {Delta}G is positive,
the reaction is not spontaneous
