module 12 chemistry
Name: __________ _________________________
CHEM 1212 Module 12 Reading Guide
Spontaneity
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.
Entropy
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.
Practice Problems
4. 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
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
Practice Problems
Calculate the Gibbs free-energy change at 298 K for 2 KClO3(s) → 2 KCl(s) + 3 O2(g).
Determine the temperature range in which the reaction is spontaneous.
Sulfur dioxide reacts with oxygen gas to produce sulfur trioxide gas according to the following reaction: 2 SO2(g) + O2(g) ⇌ 2 SO3(g)
a. Calculate the standard free-energy change for this process
b. Calculate the free-energy change at 298 K when the partial pressure of SO2 is 0.250 atm, the partial pressure of O2 is 0.500 atm, and the partial pressure of SO3 is 0.400 atm.
c. Calculate the equilibrium constant for the reaction at 298 K.
d. Use the Le Châtelier principle to explain the difference in value between ΔG and ΔG°.
Text segments from OpenStax, Chemistry. OpenStax CNX. Sep 15, 2020 http://cnx.org/contents/85abf193-2bd2-4908-8563-90b8a7ac8df6@13.1.; White, J. et al. Interactive General Chemistry. MacMillan, NY. 2019.; Gibbs Energy Change and Equilibrium. (2020, August 25). Retrieved April 3, 2021, from https://chem.libretexts.org/@go/page/24319 Reading guide style Adapted from "Ionic equations Study Guide" by Montgomery College is licensed under CC BY 4.0 Document not to be reposted on the internet without express permission.