Search found 26 matches

by Michael_Johanis_2K
Sat Mar 18, 2017 8:12 pm
Forum: *Nucleophilic Substitution
Topic: Do we need to draw the Wedges for this class?
Replies: 1
Views: 470

Do we need to draw the Wedges for this class?

Do we need to draw the wedges and the dashed lines for SN2 mechanisms (When I say "dashed," I am not referring to what is being broken/formed)? Is it fine if we draw all bonds as lines (except for the broken/formed ones, which in the TS I will draw as dashed)?
by Michael_Johanis_2K
Sat Mar 18, 2017 8:04 pm
Forum: *Electrophilic Addition
Topic: Major Product vs Minor Product
Replies: 2
Views: 509

Major Product vs Minor Product

For electrophilic addition, 2-methyl-1-butene + HBr -----> 2-bromo-2-methylbutane OR 1-bromo-2-methylbutane. Does it matter which one we write? Should we write both? Do we have to know the distinction between minor product and major product? I think I remember hearing that Chem 14C covers where we a...
by Michael_Johanis_2K
Fri Mar 17, 2017 10:42 pm
Forum: *Organic Reaction Mechanisms in General
Topic: Partial Charge [ENDORSED]
Replies: 1
Views: 495

Partial Charge [ENDORSED]

On the final, if we need to draw a mechanism with the transition step(s), do I have to put the electronegativity of each atom? For example, in electrophilic addition, propene + HBr ---> 2-bromopropane. For the first step, do I need to put partial positive next to H, and partial negative next to Br? ...
by Michael_Johanis_2K
Mon Mar 13, 2017 1:51 pm
Forum: *Constitutional and Geometric Isomers (cis, Z and trans, E)
Topic: When to Use Z and E?
Replies: 4
Views: 421

When to Use Z and E?

Is there a rule (at least for Chem 14B) that tells me when I need to put Z or E in front of a compound? I know how to determine whether I should use Z or E, but I am not quite sure when I should even have one of these letters in front of my compound's name.
by Michael_Johanis_2K
Tue Mar 07, 2017 8:51 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Why is K = kfor/krev?
Replies: 3
Views: 507

Why is K = kfor/krev?

Can someone explain why K = k/krev? In lecture, did Dr. Lavelle cover how to derive or infer this?
by Michael_Johanis_2K
Fri Mar 03, 2017 10:47 pm
Forum: *Electrophilic Addition
Topic: Always 2 Transition States for Electrophilic Addition?
Replies: 1
Views: 371

Always 2 Transition States for Electrophilic Addition?

Is it true that every electrophilic addition reaction will have two steps and therefore two transition states?
by Michael_Johanis_2K
Mon Feb 27, 2017 1:02 pm
Forum: *Nucleophiles
Topic: Nucleophiles that are Neutral?
Replies: 2
Views: 357

Nucleophiles that are Neutral?

Page 144 of "Introduction to Organic Chemistry" states that nucleophiles are "often negatively charged, although they can also be neutral." The text then states that H2O, NH3, and NH2CH3 are examples of nucleophiles. What makes H2O, NH3, and NH2CH3, by definition "electron-r...
by Michael_Johanis_2K
Fri Feb 24, 2017 4:48 pm
Forum: General Rate Laws
Topic: 15.5 Positive vs Negative
Replies: 2
Views: 308

15.5 Positive vs Negative

C2H2+3O2--->2CO2+2H2O The unique rate of combustion is .44 M/s. Find the rate at which oxygen reacts. I solved this by using: -1/3 * d[O2]/dt = .44 M/s d[O2]/dt = -1.32 M/s However, the answer is (positive) 1.32 M/s. Why is my answer wrong? Am I incorrectly using the unique reaction rate rule, which...
by Michael_Johanis_2K
Tue Feb 14, 2017 5:52 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Changes in both volume and temp [ENDORSED]
Replies: 5
Views: 616

Re: Changes in both volume and temp [ENDORSED]

I am confused by Chem_Mod's answer. I have been under the impression that we need to use BOTH DeltaS = nClnT2/T1 AND DeltaS = nRlnV2/V1 if both the temperature and volume change. For example, if initial temperature = 10 K, initial volume = 5 L, final temperature = 20 K, and final volume = 40 L, then...
by Michael_Johanis_2K
Tue Feb 14, 2017 4:46 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: 9.43 - Where do we get Cp of H2O (l) = 75.3 J/Kmol?
Replies: 1
Views: 539

9.43 - Where do we get Cp of H2O (l) = 75.3 J/Kmol?

Suppose that 50 g of H2O(l) at 20 degrees C is mixed with 65 g of H2O(l) at 50 degrees C at a constant pressure in a thermally insulated vessel. Calculate delta S and delta S total for the process. I understand that we find the sum of two entropies, one for each water sample. We use ChangeEntropy = ...
by Michael_Johanis_2K
Mon Feb 13, 2017 1:49 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Why Pt(s) when we already have I2 (s)? HW 14.13
Replies: 2
Views: 279

Why Pt(s) when we already have I2 (s)? HW 14.13

2I- (aq) + 2Ce4+ (aq) ----> I2 (s) + 2Ce3+ (aq)

I thought the cell diagram should be:
I2(s)| I- (aq) || Ce4+ (aq), Ce3+ (aq) | Pt(s)

However, it is:
Pt(s)| I- (aq) | I2 (s) || Ce4+ (aq), Ce3+ (aq) | Pt(s).

Why is Pt the solid on the left side even though we already have I2 (s)?
by Michael_Johanis_2K
Sat Feb 11, 2017 9:15 pm
Forum: Phase Changes & Related Calculations
Topic: Specific Heat Capacity vs Cv(Vap) vs Cp(vap) [ENDORSED]
Replies: 1
Views: 443

Specific Heat Capacity vs Cv(Vap) vs Cp(vap) [ENDORSED]

"A pressure cooker containing 100.0 g of liquid water at 25 C is placed on the stove until all water becomes superheated steam at 121 C. Assume no steam escapes the pressure cooker. a) Find q from 25C to 100C b) Find q during boiling c) Find any other heat gained or lot by the water as it goes ...
by Michael_Johanis_2K
Thu Feb 09, 2017 4:38 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: equations
Replies: 1
Views: 243

Re: equations

I believe that we will be given w = -(Pex)(DeltaV), though this is a very easy equation to memorize.

We can easily find the other equations by simply substituting some values.

We know that PV = nRT. Therefore, by applying a change in one value to each side, P(DeltaV) = (DeltaN)RT = nR(DeltaT).
by Michael_Johanis_2K
Wed Feb 01, 2017 5:56 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Significant Figures
Replies: 1
Views: 285

Significant Figures

Hi. This question is for TA Jack, who read my email and said he would answer this: When we use bond enthalpy, reactant - product, and Hess's Law, assuming that we have exact numbers of moles (from the equation), the significant figures in the answer are ONLY limited by addition, right? For instance:...
by Michael_Johanis_2K
Tue Jan 31, 2017 4:17 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: Rule that Guarantees all Re-Arranged Lewis Structures?
Replies: 1
Views: 230

Rule that Guarantees all Re-Arranged Lewis Structures?

For HW 9.25, to calculate the residual entropy of SO2F2, we need to draw out all the arrangements of the Lewis structure. I found that there are 6 different arrangements. After find the degeneracy, I used to S = Kb lnW. Is there a method to be sure that I have drawn out every possible Lewis structur...
by Michael_Johanis_2K
Thu Jan 26, 2017 9:06 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: HW 9.5
Replies: 1
Views: 261

Re: HW 9.5

One reservoir loses heat (q is negative), while the other reservoir gains the heat (q is positive).
by Michael_Johanis_2K
Tue Jan 24, 2017 8:56 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: If T = 0, can residual entropy still exist?
Replies: 1
Views: 225

If T = 0, can residual entropy still exist?

If T = 0, there is no thermal entropy, correct?

If T = 0, is there residual (positional) entropy? I think that if W > 1, then there is residual entropy, even if T = 0, correct?
by Michael_Johanis_2K
Sun Jan 22, 2017 12:52 am
Forum: Calculating Work of Expansion
Topic: Combustion Reaction - Writing and Balancing
Replies: 2
Views: 313

Combustion Reaction - Writing and Balancing

For 8.93, we have to write the combustion reaction for C6H6 (l). The solutions manual says: C6H6 (l) + 7.5 O2 (l) ----> 6CO2 (g) + 3H2O (l) 1) Why is 3H2O a liquid? Shouldn't all the products in combustion reactions be gases? 2) Why is 7.5 the coefficient for O2? Shouldn't everything be in whole num...
by Michael_Johanis_2K
Sun Jan 22, 2017 12:39 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Given q and w, will a gas's pressure increase or decrease? [ENDORSED]
Replies: 4
Views: 410

Re: Given q and w, will a gas's pressure increase or decrease? [ENDORSED]

Thank you! But what if q = -501 kJ and w = 500 kJ (compression)?

Delta U = -1 kJ

PV = nRT
The temperature decreases since DeltaU = negative. The volume decreased since this is compression.
If T goes down, V goes down, n is constant, and R is constant, what happens to P?
by Michael_Johanis_2K
Sun Jan 22, 2017 12:33 am
Forum: Calculating Work of Expansion
Topic: Combustion Products - Physical States
Replies: 1
Views: 190

Combustion Products - Physical States

In 8.93, we need to write the combustion reaction for C6H6 (l).

Are the products for all combustion reactions always in the gas phase (H2O gas + CO2 gas)?
by Michael_Johanis_2K
Thu Jan 19, 2017 11:06 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.51 Sig Figs, Sign [ENDORSED]
Replies: 3
Views: 322

8.51 Sig Figs, Sign [ENDORSED]

The enthalpy of formation of trinitrotoluene (TNT) is -67 kJ·mol^-1, and the density of TNT is 1.65 g·cm^-3. In principle, it could be used as a rocket fuel, with the gases resulting from its decomposition streaming out of the rocket to give the required thrust. In practice, of course, it would b...
by Michael_Johanis_2K
Tue Jan 17, 2017 1:46 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Values Given on Exams?
Replies: 2
Views: 307

Values Given on Exams?

Do we need to memorize the specific heats of H2O in the solid, liquid, and vapor states?

For example, in the equation q = m * C * DeltaT for H2O (l), do I have to memorize that 4.184 J/(g C) is the specific heat capacity?
by Michael_Johanis_2K
Tue Jan 17, 2017 1:43 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Given q and w, will a gas's pressure increase or decrease? [ENDORSED]
Replies: 4
Views: 410

Re: Given q and w, will a gas's pressure increase or decrease? [ENDORSED]

Are DeltaU and q not important? Does it only matter that w is negative (gas expands)?
by Michael_Johanis_2K
Tue Jan 17, 2017 1:40 pm
Forum: Calculating Work of Expansion
Topic: Significant Figures Question [ENDORSED]
Replies: 1
Views: 291

Significant Figures Question [ENDORSED]

I have a question regarding significant figures. "Air in a bicycle pump is compressed by pushing in the handle. If the inner diameter of the pump is 3.0 cm and the pump is depressed 20. cm with a pressure of 2.00 atm, (a) how much work is done in the compression? (b) Is the work positive or neg...
by Michael_Johanis_2K
Tue Jan 17, 2017 9:57 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Given q and w, will a gas's pressure increase or decrease? [ENDORSED]
Replies: 4
Views: 410

Given q and w, will a gas's pressure increase or decrease? [ENDORSED]

From 2007 Midterm: Given that a a gas sample expands, doing 536 kJ of work, while 214 kJ of heat is added to the gas: a) Calculate DeltaU b) Will the pressure of the gas be higher or lower when these changes are completed? I understand Part A: a) DeltaU = q + w. q is +214 kJ since the heat is ADDED ...
by Michael_Johanis_2K
Mon Jan 09, 2017 11:46 pm
Forum: Calculating Work of Expansion
Topic: Quiz 1 Preparation: Question #3 - Conflicting Information? [ENDORSED]
Replies: 4
Views: 450

Quiz 1 Preparation: Question #3 - Conflicting Information? [ENDORSED]

#3. "If 2.00 mol of an ideal gas at 300. K and 3.00 atm expands isothermally and reversibly from 6.00 L to 18.00 L and has a final pressure of 1.20 atm, what is q,w, and DeltaU?" Can anyone clarify what I am doing wrong? w = -nRT ln (V2/V1) OR w = -nRT ln (P1/P2) ln(V2/V1) and ln(P1/P2) ha...

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