Chapter 19 - Chemical Thermodynamics Flashcards
19.1
What is a spontaneous process?
- One that proceeds on its one without any outside assistance.
- Occurs in a definite direction (ex. you don’t see bricks rising; you see them falling).
- Processes that are spontaneous in one direction are nonspontaneous in the opposite direction.
- Temperature & pressure are often important to consider when deeming a process spontaneous or not.
19.1
Who was Marcellin Bertolet?
A chemist during the 1870’s who suggested that the direction of spontaneous changes in chemical systems was also determined by the loss of energy: all spontaneous chemical & physical changes were exothermic.
There are exceptions; for example, the dissolving of NH4NO3 is endothermic, but is also spontaneous.
19.1 - Give It Some Thought p. 804
If a process is nonspontaneous, does that mean the process cannot occur under any circumstances?
No, it just means that the surroundings need to help make the process happen; it can’t happen on its own.
19.1
Who was Sadi Carnot?
A French engineer who published an analysis of the factors that determine how efficiently a steam engine can convert heat to work. Observed that it was impossible to convert the energy content of a fuel completely to work b/c a significant amount of heat is always lost to surroundings.
19.1
Who was Rudolph Clausius?
A German physicist who concluded that a special significance could be ascribed to the ratio of the heat delivered to an ideal engine and the temperature at which it is delivered, q/T, or entropy. Believed that the importance of entropy was comparable to that of energy.
19.1
What is a reversible process?
- A system is changed in such a way that the system and surroundings can be restored to their original state by exactly reversing the change… we can completely restore the system to its original condition with no net change to either the system or its surroundings.
- Produces the maximum amount of work that can be achieved by the system on the surroundings (w rev = w max)
- Reverse direction whenever an infinitesimal change is made in some property of the system
19.1
What is an irreversible process?
One that cannot simply be reversed to restore the system and its surroundings to their original states.
19.1 - Give It Some Thought p. 805
If you evaporate water and then condense it, have you necessarily performed a reversible process?
No. Just because the system is restored to its original conditions doesn’t mean that the environment is.
19.1
What does isothermal mean?
A process that occurs at constant temperature; for example, the expansion of an ideal gas at constant temperature.
19.1
Why is any spontaneous process irreversible?
After a spontaneous process happens, the reverse process is nonspontaneous, and a nonspontaneous process can occur only if the surroundings do work on the system,
19.2
What is entropy (S)?
- The extent of randomness in a system or with the extent to which energy is distributed or dispersed among the various motions of the molecules of the system.
- State function
19.2
What does the change in entropy in a system depend on?
Entropy final - entropy initial
19.2
What does change in entropy depend on in an isothermal process?
Qrev / T (constant T)
Qrev = heat that would be transferred if the process were reversible
…can calculate this because S is a state function; can be used for irreversible isothermal reactions.
19.2 - Give It Some Thought p. 807
How do we reconcile the fact that S is a state function but that delta S depends on q, which is not a state function?
delta S depends on q reversible. Although there are many possible paths that could take a system from its initial to final state, there’s always only one reversible isothermal path between two states. Thus, delta S has only one particular value regardless of the path taken between states.
19.2
How do we calculate delta S fusion for melting a substance?
delta S = q rev / T = delta H fusion / T
19.2
What is the second law of thermodynamics?
Any irreversible process results in an overall increase in entropy, whereas a reversible process results in no overall change in entropy. In other words,
delta universe = delta S system + delta S surroundings = 0 for reversible and > 0 for irreversible
19.2 - Give It Some Thought p. 809
The rusting of iron is accompanied by a decrease in the entropy of the system (the Fe and O2). What can we conclude about the energy change of the surroundings?
Rusting is spontaneous, so delta S universe has to be positive, meaning delta S surroundings must increase and be positive and larger than the delta S system decrease.
19.3
What is translational motion?
The entire molecule can move in 1 direction.
Ex. Baseball being thrown around a baseball field, motions of the particles of an ideal gas
19.3
What is vibrational motion?
Atoms in the molecule move periodically toward toward and away from one another
Ex. Tuning fork vibrates about its equilibrium shape
19.3
What is rotational motion?
Molecules spinning like tops and tumbling.
19.3
What is the motional energy of a molecule?
The different ways in which a molecule can store energy.
19.3 - Give It Some Thought p. 810
What kinds of motion can a molecule undergo that a single atom cannot?
Vibrational and rotational
19.3
What is a microstate when it comes to thermodynamic systems?
The state of a system at a particular instant; one of many possible energetically equivalent ways to arrange the components of a system to achieve a particular state.
19.3
What is Boltzmann’s equation?
S = k ln W
k = 1.38 x 10^-23 J/K S = entropy W = # of microstates of a system
19.3 - Give It Some Thought p. 811
What is the entropy of the system that has only one microstate?
0
19.3
What is the entropy change accompanying any process?
delta S = k ln W final - k ln W initial = k ln (W final/W initial)
19.3
Why does the entropy of a system increase with increasing temperature?
More microstates = more randomness.
19.3
Generally, when does the number of microstates available to a system increase?
- Increase in volume
- Increase in temperature
- Increase in # of molecules
19.3
Why do real molecules have a greater number of microstates available than the same number of ideal gas molecules?
With real molecules, one must consider the different amounts of vibrational and rotational energies in addition to their kinetic energies. Ideal gas particles have no volume and no bonds,, so they can’t have vibrational or rotational motion.
19.3
Why does the dissolving of salts with highly charged ions sometimes result in a net decrease in entropy?
The highly charged ions form ion-dipole attractions with the water, so the water molecules have less motional energy than before.
19.3
Explain why the reaction 2 NO (g) + O2 (g) —> 2 NO2 (g) results in a decrease in entropy.
The newly formed N-O bonds restrict the number of degrees of freedom, or forms of motion, available to the atoms. This results in fewer microstates, along with the 3 mol to 2 mol decrease, in turn resulting in lower entropy.
19.3
What is the 3rd Law of Thermodynamics?
The entropy of a pure crystalline substance at absolute zero is zero: S(O K) = 0. Why? It has 1 microstate, and k ln 1 = 0.
19.3
When do we see sharp, straight-up increases in entropy?
At melting/freezing & condensing/vaporizing points.
19.3
Why does entropy generally increase with increasing temperature?
There are more ways for it to store its energy because there is more energy.
19.3 - Give It Some Thought p. 817
If you’re told that the entropy of a certain system is zero, what do you know about the system?
It’s at absolute zero (it has 1 micro state).
19.4
Why do standard molar entropies generally increase with an increasing number of atoms in the formula of a substance?
Generally, the # of degrees of freedom for a molecule increases w/ increasing # of atoms, and thus the number of accessible microstates also increases.
19.4
How do you find the entropy change in a chemical reaction?
delta S standard = n delta S (products) - m delta S (reactants)
19.4
For an isothermal process, the entropy change of the surroundings is given by:
delta S surroundings = -q system / temperature
19.4
How do you calculate the entropy change for a reaction occurring at constant pressure?
delta S surroundings = - delta H system/Temperature
Use delta H products - reactants to find delta H sys (delta H rxn)
19.4
What is the equation for delta S universe?
delta S surroundings + delta S system
19.4
What does it mean when delta S universe is positive?
The reaction system will move spontaneously in one direction, and is thus an irreversible reaction.
19.5
When does free energy always decrease?
In any spontaneous process at constant temperature and pressure
19.5
What does it mean if Q < K, Q > K, Q = K?
1) Spontaneously in forward to reach equilibrium (there are excess reactants)
2) Spontaneously in reverse to reach equilibrium (there are excess products)
3) At equilibrium
19.5
How do you calculate the standard free-energy change for a chemical process?
delta G products - delta G reactants
19.6
When is a reaction spontaneous at all temperatures?
+ ΔS
- ΔH; -TΔS; ΔG = ΔH - TΔS
19.6
When is a reaction nonspontaneous at all temperatures?
+ ΔG, -TΔS, ΔH
- ΔS
19.6
When is a reaction spontaneous at low temperatures?
- ΔH, ΔS
+ -TΔS
+/- ΔG
19.6
When is a reaction spontaneous at high temperatures?
+ ΔH, ΔS
- -TΔS
+/- ΔG
19.7
How do you calculate the free-energy change under any nonstandard conditions?
ΔG = ΔG* + RT ln Q
19.7
What is an equation that relates ΔG* and K?
ΔG* = -RT ln K
K = e^(-ΔG*/RT)
19.7
What is the relationship between ΔG* and K?
If ΔG* is negative, then ln K must be positive, making K > 1. If ΔG* is positive, then ln K must be negative, making K < 1.
