## Search found 30 matches

Thu Mar 15, 2018 10:05 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: PV Diagrams
Replies: 1
Views: 262

### Re: PV Diagrams

I think you can distinguish between reversible and irreversible on a PV diagram, for the most part. Reversible processes should be entirely differentiable (i guess a good way to put it is "curved", such that you can only integrate it to find the area under) while irreversible processes sho...
Thu Mar 15, 2018 9:53 pm
Forum: Phase Changes & Related Calculations
Topic: Origin of Internal Energy
Replies: 2
Views: 319

### Re: Origin of Internal Energy

I think you're talking about the equipartition theorem. Someone else asked about it not too long ago and I gave a short explanation here: https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=75&t=29894&sid=698fe4496d8474f226e4f407bf8fd120 The syllabus doesn't explicitly say we need to know, a...
Thu Mar 15, 2018 9:35 pm
Forum: Phase Changes & Related Calculations
Topic: Equipartition theorem
Replies: 1
Views: 212

### Re: Equipartition theorem

The equipartition theorem is just a way for us to PREDICT heat capacities of something by factoring in its rotational, translational, and vibrational energy. When you heat something up, some of the energy is stored in these three categories rather than actually heating it up. Therefore, its heat cap...
Fri Mar 09, 2018 12:39 am
Forum: General Rate Laws
Topic: Deriving the Rate Laws
Replies: 3
Views: 243

### Re: Deriving the Rate Laws

In a general sense, it doesn't particularly matter if you use definite or indefinite integrals. All a definite integral does is specify the bounds of integration. You should be able to come to the same equations using both--I think Dr. Lavelle did it in class with indefinite, and the textbook uses d...
Fri Mar 09, 2018 12:13 am
Forum: Method of Initial Rates (To Determine n and k)
Topic: Stoichiometric Coefficients
Replies: 2
Views: 312

### Re: Stoichiometric Coefficients

It sorta depends on the problem, I guess, depending on what information you're given from the start. As long as you adjust your reactant concentrations accordingly, you should be fine. For example, if A -> 2B and they ask you to find k, given initial [A] and final [A], you don't really need to mind ...
Fri Mar 09, 2018 12:04 am
Forum: First Order Reactions
Topic: Log vs ln
Replies: 9
Views: 644

### Re: Log vs ln

I don't really see a real need to convert to log, unless that makes the problem easier/more convenient. You should get the same answer either way if you're doing it right! Whatever's more convenient for you and your calculator/head, I suppose.
Fri Mar 02, 2018 9:05 pm
Forum: First Order Reactions
Topic: First Order Reactions [ENDORSED]
Replies: 2
Views: 1874

### Re: First Order Reactions[ENDORSED]

If a reaction is "first order", it simply means that the rate is proportional to the concentrations of one of the reactants. If a reaction is "second order", it simply means that the rate is proportional to the SQUARE of the concentrations of that reactant. A reaction being first...
Fri Mar 02, 2018 8:52 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Intermediates [ENDORSED]
Replies: 2
Views: 197

### Re: Intermediates[ENDORSED]

Yeah, Eli's right! Unless, of course, the intermediate IS part of the rate-determining step. It kind of depends on the reaction, really--for a coupled reaction, just look for the slowest one. Unless they themselves are part of the rate-determining step, intermediates have little function on the reac...
Fri Mar 02, 2018 6:52 pm
Forum: Zero Order Reactions
Topic: Zero Order Reactions [ENDORSED]
Replies: 3
Views: 237

### Re: Zero Order Reactions[ENDORSED]

Zero order just means the rate of the reaction doesn't depend on the concentration of the reactant(s), so no, not necessarily.
Thu Feb 22, 2018 6:53 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Inert Electrodes
Replies: 4
Views: 294

### Re: Inert Electrodes

I'm not entirely sure about all the scenarios, but I suppose you would need an inert electrode whenever you don't want the electrode messing up the reaction, as in, you only want it to transfer electrons instead of exchanging ions. That's typically the situation for gas/ion electrode reactions. As f...
Thu Feb 22, 2018 6:47 pm
Forum: Balancing Redox Reactions
Topic: Cell Diagram [ENDORSED]
Replies: 7
Views: 390

### Re: Cell Diagram[ENDORSED]

Not entirely sure I understand your question, but it shows that I- ions are oxidized (lose electrons) to become I2.
Thu Feb 22, 2018 6:44 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: n
Replies: 3
Views: 258

### Re: n

I believe n is the # of electrons involved in the balanced redox reaction. I don't particularly know if you necessarily HAVE to find the balanced equation to figure out n, but it's certainly one of the easiest ways to. Just write out the half reactions!
Wed Feb 14, 2018 2:51 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Cp and Cv
Replies: 2
Views: 209

### Re: Cp and Cv

I think the confusion oftentimes comes from a multistep process. Say you want to calculate the entropy change of an ideal gas that expands by a certain amount while it is simultaneously heated to a certain amount. You'd think the volume isn't constant, since the problem says the gas expands. But, if...
Wed Feb 14, 2018 2:32 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: ΔS = q/t vs. ΔS = nCln(T2/T1) [ENDORSED]
Replies: 2
Views: 395

### ΔS = q/t vs. ΔS = nCln(T2/T1)[ENDORSED]

Is the difference between using ΔS = q(rev)/T and ΔS = nCln(T2/T1) that you can only use the former in isothermal conditions, while the latter can be used in any other reversible scenario where the temperature isn't constant?
Wed Feb 14, 2018 2:23 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Delta U as 0
Replies: 3
Views: 295

### Re: Delta U as 0

ΔU = 3/2 nRΔT for monoatomic ideal gases, so anytime ΔT = 0, that's true. Even if it weren't monoatomic, only the 3 changes to some other number, so the ΔT = 0 condition still holds true. ΔU also stays the same if a multi-step process (the PV diagrams!) returns to its original starting point, if tha...
Sun Feb 11, 2018 9:29 pm
Forum: Third Law of Thermodynamics (For a Unique Ground State (W=1): S -> 0 as T -> 0) and Calculations Using Boltzmann Equation for Entropy
Topic: Degeneracy (W)
Replies: 4
Views: 256

### Re: Degeneracy (W)

Everybody above said it well!

If you haven't already, I recommend trying problems 9.21, 9.23, and especially 9.25 in the textbook; those really helped me learn the concept when I was doing them last week :)
Sun Feb 11, 2018 9:23 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Why is deltaG of formation 0 for diatomic molecules?
Replies: 3
Views: 4518

### Re: Why is deltaG of formation 0 for diatomic molecules?

Heat of formation of a certain element is defined to be 0 for that element in its standard state (which is usually the state it is found in naturally, the diatomic one).

I suppose ΔG has a similar definition and entropy doesn't, but I'm not too entirely sure about those.
Sun Feb 11, 2018 9:17 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Isothermal expanision and deltaU [ENDORSED]
Replies: 2
Views: 191

### Re: Isothermal expanision and deltaU[ENDORSED]

I think it's b/c internal energy is directly related to change in temp.

Forgot where exactly, but there's a derivation somewhere in the book that uses the equipartition theorem and says that molar internal energy is U = 3/2 RT.
Therefore, ΔU = 3/2 RΔT. If ΔT is 0, then ΔU is also 0.
Wed Jan 31, 2018 11:37 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Heat Capacity
Replies: 3
Views: 237

### Re: Heat Capacity

Yep, Liz's right. The formal definition of thermal equilibrium between two objects is that there is no net heat transfer between them. Since heat flows from stuff with higher temperature to stuff with lower temperature, if there is no heat flow, then they must be at the same temperature. In fact, th...
Wed Jan 31, 2018 11:18 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: q(REV)
Replies: 3
Views: 177

### Re: q(REV)

Nicole's right. To add on to significance: Remember that during last friday's lecture Dr. Lavelle showed us two different processes--reversible and irreversible. In reversible expansion, the temperature is constant, the volume increases, and the pressure decreases. The work done is the area under th...
Wed Jan 31, 2018 11:00 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Define Entropy
Replies: 4
Views: 221

### Re: Define Entropy

Entropy is a thermodynamic quantity denoted by ΔS, and refers to the "disorder" (an potentially confusing term, if you remember from lecture) of something. In essence, most reactions tend to favor going from low entropy to high entropy (you won't need to put in work). A physical example wo...
Thu Jan 25, 2018 1:30 pm
Forum: Calculating Work of Expansion
Topic: Integrals [ENDORSED]
Replies: 3
Views: 254

### Re: Integrals[ENDORSED]

Not sure I entirely understand your question, but here's my take: Derivatives are super small parts of something. Like a "small piece of X". It's helpful to use derivatives in the sciences to refer to an infinitesimally small part of something that we don't know much about, or something th...
Thu Jan 25, 2018 1:17 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Combustion Equations [ENDORSED]
Replies: 4
Views: 278

### Re: Combustion Equations[ENDORSED]

I think it's liquid water. At least, that's what I've mostly seen in the problems I've done. Have you found a problem that uses gaseous water for combustion?
Thu Jan 25, 2018 1:09 pm
Forum: Phase Changes & Related Calculations
Topic: Change in internal energy as a state function
Replies: 2
Views: 764

### Re: Change in internal energy as a state function

As the delta in front of U suggests, U is a measure of the change of energy within the system; it's simply a measure of what it started as vs. what it ended as. q and w, tho, depends on the path taken within the process itself. They are NOT measures of initial and final states; that's why there's no...
Thu Jan 18, 2018 7:01 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Heat vs. Enthalpy
Replies: 5
Views: 236

### Re: Heat vs. Enthalpy

To add on/consider: Enthalpy is a state function in which when you know the beginning and end states, you'll know the change in enthalpy; whatever happens in the "middle" doesn't matter. On the other hand, heat does depend on the "path" taken between those beginning and end state...
Thu Jan 18, 2018 6:48 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Calorimeter
Replies: 5
Views: 314

### Re: Calorimeter

Um, what problem is this specifically?

I guess for the most part, Q = mCΔT is a good start. You'll want to look at what the "system" is, and then what the "surrounding" is, and note any energy changes that occur.
Thu Jan 18, 2018 6:45 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Constant pressure/volume
Replies: 2
Views: 146

### Re: Constant pressure/volume

Cv = 3/2 * R is the relationship that you derive for monoatomic gases. Cv is the heat capacity at a constant volume. The derivation involves using ΔE = (3/2)nRΔT (for monoatomic gases) and the first law of thermodynamics, Q = ΔE + W. W is 0 at constant volume, so you plug in ΔE on the right, the sub...
Sun Jan 14, 2018 12:03 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Bond Enthalpies
Replies: 3
Views: 226

### Re: Bond Enthalpies

I mean, the general trend of bond enthalpy exists and resonance shouldn’t "affect" the listed enthalpy since that one is found experimentally (whatever you see in those tables). I'm not sure if this is entirely true, but theoretically the enthalpies listed should be the MEAN bond enthalpy ...
Sat Jan 13, 2018 11:47 pm
Forum: Phase Changes & Related Calculations
Topic: Steam Burning
Replies: 7
Views: 484

### Re: Steam Burning

Steam "burns" a lot more b/c it typically carries way more energy than boiling water does, and that's due to phase changes, mostly. You might want to view a water phase diagram as a reference, but here goes: So liquid water at say, room temp, first needs to heat up to 100 degrees C. It nee...
Sat Jan 13, 2018 11:39 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Enthalpy and Internal Energy
Replies: 2
Views: 152

### Re: Enthalpy and Internal Energy

Not entirely sure, but my take: Internal energy (U) is basically all the energy in the system. Enthalpy (H) is the thermodynamic potential, which is internal energy and the work done on/from its surroundings. That latter part can be represented by PV. There's the first law of thermodynamics (#7 on t...

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