Processes have a natural tendency to occur in one direction under a given set of conditions. _ processes happen under the indicated conditions; they may occur quickly or slowly, but the process does occur. A nonspontaneous process, on the other hand, _ take place, no matter how long you wait, unless it is “driven” by the continual input of _ from an external source. The spontaneity of a process is not correlated to the _ of the process. A process that is spontaneous in one direction under a particular set of conditions is nonspontaneous in the _ direction. Other processes are spontaneous in only _ direction.
Knowing the _ change alone does not make it possible to predict the conditions under which a process will occur spontaneously. An important factor in determining the spontaneity of a process is the extent to which it changes the dispersal or distribution of matter and/or _. _ is a measure of the degree of disorder or randomness in a system. Entropy is a measure of the dispersal of a system’s energy, _ when there are more energetically equivalent ways to arrange the components of that system. The _ law of thermodynamics states that spontaneous processes always result in an overall _ in entropy, S, of the _. The tendency toward disorder is a statistical probability. The mathematical definition of entropy is given by
S=k lnW
where k is the Boltzmann constant (1.38 × 10−23J/K) and W is the number of energetically equivalent arrangements (microstates) possible for the system. Equivalent microstates form a _; the most probable macrostate is the one containing the highest number of __.
The entropy of a substance is influenced by the structure of the particles (atoms or molecules) that comprise the substance. Heavier atoms possess _ entropy at a given temperature than lighter atoms, For molecules, greater numbers of _ increase the number of ways in which the molecules can vibrate and thus the number of possible microstates and the entropy of the system. Compared to a pure substance, in which all particles are identical, the entropy of a mixture of two or more different particle types is _. This is because of the additional orientations and interactions that are possible in a system comprised of _ components.
A positive value for ΔS of a process, indicating an increase in entropy, results when there is
• a phase change from solid to _,
• a phase change from solid or liquid to _,
• the dissolution of a _ into an aqueous solution,
• an _ in temperature within a phase, or
• an __ in the number of gas particles.
The _ law of thermodynamics states that the entropy of a pure crystalline substance at absolute zero is _. Experimental measurements of entropy can be used to determine absolute values of entropy based on the zero point established by the third law. __ molar entropies for substances are measured at 1 bar and 298 K. They can be used to calculate the standard entropy change for chemical reactions and phase changes.
Entropy is a _ function so its change depends only upon the initial and final states of a system.
To determine if a process is _, you can determine the overall entropy change
ΔSsurr is calculated as . The entropy change of the surroundings is inversely proportional to the _. The effect of the enthalpy change of the system on the entropy of the surroundings is _ at higher temperatures than at lower temperatures. Negative ΔSrxn values can be offset by large increases in ΔSsurr, making ΔSuniv _ overall. For objects at different temperatures, _ flows from the hotter to the cooler object. This is always observed to occur spontaneously.
ΔSuniv is positive for all __ processes.
- Predict the sign of ΔS for these processes:
a. 2 H2(g) + O2(g) → 2 H2O(l)
b. H2O(l) → H2O(s)
c. CaCO3(s) → CaO(s) + CO2(g)
d. CO2(s) → CO2(g)
e. 2 C8H18(l) + 25 O2(g) → 16 CO2(g) + 18 H2O(l)
Calculate ΔS°, ΔSsurr and ΔSuniv 298 K for the reaction: 2 KClO3(s) → 2 KCl(s) + 3 O2(g)
Gibbs free energy provides chemists with a way to determine if a process is spontaneous using only properties of the system. ΔG is meaningful only for changes in which the temperature and pressure remain _. These are the conditions under which most reactions are carried out in the laboratory, wherein the system is usually open to the atmosphere (constant pressure) and the process begins and ends at room temperature (after any heat added or liberated by the reaction has dissipated.) In a spontaneous change, Gibbs energy always decreases and never _. Once the free energy reaches its minimum possible value, all net change comes to a halt (equilibrium).
The free energy change is equal to the maximum amount of _ a system can perform on its surroundings while undergoing a spontaneous change. For a _ process, the free energy change represents the minimum amount of work that must be done on the system to carry out the process.
Free energy is a _ function. The standard free energy of formation is the change in free energy that occurs when 1 mol of a substance in its standard state is formed from the component _ in their standard states.
The standard free energy of a process can be calculated from standard free energies of __ of the reactants and products
or by using the standard enthalpy and entropy values for the process. .
When ΔG < 0, the process is spontaneous as written. When ΔG > 0, the process is nonspontaneous as written, but spontaneous in the reverse direction.
When ΔG = 0, the system is at __.
The sign of the _ change determines whether the reaction becomes more or less spontaneous as the temperature is raised. Because ΔH and ΔS usually do not vary greatly with temperature in the absence of a phase change, we can use tabulated values of ΔH° and ΔS° to calculate ΔG° at various temperatures, as long as no _ change occurs over the temperature range being considered.
ΔH ΔS ΔG = ΔH – TΔS Reaction characteristics
– + – __ at all temperatures
- – + Nonspontaneous at all temperatures
– – + or – Spontaneous at __ temperatures
Nonspontaneous at high temperatures - + + or – Spontaneous at __ temperatures
Nonspontaneous at low temperatures
Setting the Gibbs free-energy equation equal to __ gives an equation for the temperature at which the sign of ΔG changes.
Systems proceed spontaneously until they reach equilibrium (where ΔG = 0) because that is the most stable, _ energy situation for that system. Standard free-energy changes are related to the _ constant by The magnitude of the equilibrium constant is directly influenced by the tendency of a system to move toward __ entropy and seek the lowest energy state possible.
Value of K Value of ΔG Direction to proceed (spontaneously) to equilibrium
K > 1 ΔG < 0 forward direction K = 1 ΔG = 0 at equilibrium, no net change K < 1 ΔG > 0 reverse direction
To determine changes in free energy, ΔG, under __ conditions, scientists use
Sample Solution