Search found 31 matches

by Xuan Kuang 2L
Sat Mar 16, 2019 8:16 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: 4c.3
Replies: 1
Views: 182

Re: 4c.3

There is no change in enthalpy for part b since it is under constant volume, not pressure. You would use 765 J= qv
by Xuan Kuang 2L
Sat Mar 16, 2019 8:09 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Question 14.51
Replies: 1
Views: 168

Re: Question 14.51

For this problem, you solve for Ecell, using this formula for the Nernst equation: Ecell=Eocell-(0.025693V/n)lnQ. Then substitute that value in for deltaG= -nFEcell.
by Xuan Kuang 2L
Sat Mar 16, 2019 8:00 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Determining n
Replies: 6
Views: 217

Re: Determining n

The easiest way to figure out n is to split the the reaction into its reducing and oxidizing half-reactions, and then balance out those reactions os they both have the same number of e- in each rxn. For part b), we know to put the 14H+ for Cr because we see that its product has an O in it, meaning t...
by Xuan Kuang 2L
Sun Mar 10, 2019 7:21 pm
Forum: First Order Reactions
Topic: 7B.9a
Replies: 1
Views: 30

7B.9a

Could someone please help me on part a? Thanks!

For the first-order reaction A-->3B + C when [A]0=0.015 mol.L^-1, the concentration of B increases to 0.018 mol.L^-1 in 3.0 min
a) What is the rate constant for the reaction expressed at the rate of loss of A?
by Xuan Kuang 2L
Sun Mar 10, 2019 7:11 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Final Exam
Replies: 23
Views: 292

Re: Final Exam

Yes, I believe the final is cumulative, so chemical equilibria will probably be on the exam.
It is under the file titled "Important Final Exam Information" on the 14B website I think :)
by Xuan Kuang 2L
Sun Mar 10, 2019 7:03 pm
Forum: Second Order Reactions
Topic: 7B.13 Help
Replies: 4
Views: 92

7B.13 Help

Could someone help to explain this question to me?

Q:The half-life of A in a second-order reaction is 50.5s when [A]0= 0.84 mol. L^-1. Calculate the time needed for the concentration of A to decrease to
a) one-sixteenth
b) one-fourth
c) one-fifth

Thank you!
by Xuan Kuang 2L
Thu Feb 28, 2019 10:49 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 6M.1
Replies: 1
Views: 71

6M.1

A student was given a standard Cu(s)|Cu2+ (aq) half-cell and another half-cell containing an unknown metal M in 1.00 M M(NO3)2(aq) and formed the cell M(s)|M+(aq)||Cu2+ (aq)|Cu(s). The cell potential was found to be -0.689 V. What is the value of E(M2+/M)? From the cell diagram, I though Cu/Cu2+ was...
by Xuan Kuang 2L
Thu Feb 28, 2019 10:39 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 6L.9
Replies: 1
Views: 65

6L.9

a) Write balanced half-reactions for the redox reaction of an acidified solution of potassium permanganate and iron(II) chloride. b) Write the balanced equation for the cell reaction and devise a galvanic cell to study the reaction (write its cell diagram) For this reaction, how do you determine whi...
by Xuan Kuang 2L
Thu Feb 28, 2019 10:13 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 6L.7a Help
Replies: 1
Views: 65

6L.7a Help

Write the half-reactions and devise a galvanic cell (write a cell diagram) to study each of the following reactions:
a) AgBr(s)--> Ag+ (aq) + Br-(aq), a solubility equilibrium

Could someone explain to me how to approach this problem? Thanks!
by Xuan Kuang 2L
Mon Feb 25, 2019 12:56 pm
Forum: Balancing Redox Reactions
Topic: Balancing Redox Reactions in Basic Solutions
Replies: 1
Views: 39

Re: Balancing Redox Reactions in Basic Solutions

Essentially, the methods are the same as the acidic reaction up until the part when you have to cancel out electrons. From that point, you add an additional step where you add the same amount of OH- ions on both sides so that the number of H+ ions will completely react with the OH- ions to create H2...
by Xuan Kuang 2L
Mon Feb 25, 2019 12:51 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Platinum in Cell Diagrams
Replies: 1
Views: 38

Platinum in Cell Diagrams

If one side of the diagram yields no solid metal, do we automatically know that we should write Pt(s) on that side? Is there a way to know which alternative metal we use, if we don't use Pt(s)?
by Xuan Kuang 2L
Mon Feb 25, 2019 12:44 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Cell Potential Value
Replies: 1
Views: 33

Cell Potential Value

Just for clarification from Friday's lecture, why does the E of a reduction/oxidation reaction not change whenever you alter a reaction by multiplying the coefficients? Thanks in advance!
by Xuan Kuang 2L
Sun Feb 17, 2019 10:09 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Problem 115 ch 11 6th edition
Replies: 2
Views: 50

Re: Problem 115 ch 11 6th edition

In this case, we consider Le Chatelier's Principle. If the partial pressure of O2 were to increase, there would be more O2 molecules present, which results in the equilibrium ratio having a higher partial pressure of O2 than initially (Qp>Kp). To minimize this effect, the reaction will shift to the ...
by Xuan Kuang 2L
Sun Feb 17, 2019 9:43 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Elements with Gibbs Free Energy
Replies: 1
Views: 42

Elements with Gibbs Free Energy

Do all elements in their most stable form have standard Gibbs Free Energy of formation of zero? If so, could someone conceptually explain to me why this is so?
by Xuan Kuang 2L
Sun Feb 17, 2019 9:36 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Entropy with Temperature and Volume
Replies: 2
Views: 76

Re: Entropy with Temperature and Volume

For those particular problems, you need to solve for a total change in entropy caused by both temperature and volume. I believe in those cases we are generally considering a reversible reaction, so we use two separate equations, one for temperature change (S=nCln(T2/T1)) and another for volume (S=nR...
by Xuan Kuang 2L
Sun Feb 10, 2019 4:07 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: Entropy at Equilibrium
Replies: 1
Views: 54

Entropy at Equilibrium

Could someone re-explain for me why entropy is a maximum at equilibrium? More specifically, could someone explain why at equilibrium, all possible states have been reached?
by Xuan Kuang 2L
Sun Feb 10, 2019 4:03 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: 11.89 6th edition
Replies: 1
Views: 54

Re: 11.89 6th edition

Yes, I believe that is the case because they're converting it to atm I think.
by Xuan Kuang 2L
Sun Feb 10, 2019 4:02 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: HW Prolem (4G.4) 7th Edition
Replies: 2
Views: 42

Re: HW Prolem (4G.4) 7th Edition

COF2 would have a higher molar entropy than BF3. When you take a look at the molecular structures, BF3 is a trigonal planar with the same B-F bond on each end. This means that is it very ordered, compared to COF2, which has two F's and an O at the ends. This means that for some molecules, it could p...
by Xuan Kuang 2L
Sun Feb 03, 2019 1:48 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: 8.101b 6th edition
Replies: 1
Views: 41

Re: 8.101b 6th edition

SO2 would be the limiting reactant because if you had 0.030 mol of O2, then by the molar ratio given to us by the coefficients of the reaction (2 mol SO2 for every 1 mol of O2), then you'd need 0.060 mol of SO2 to fully react. However, in the question, we are given only 0.030 mol of SO2, half the nu...
by Xuan Kuang 2L
Sun Feb 03, 2019 1:44 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: 103 6th edition
Replies: 2
Views: 62

Re: 103 6th edition

I believe average kinetic energy is calculated using the formula 3/2RT
by Xuan Kuang 2L
Sun Feb 03, 2019 1:30 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Why Bond Enthalpies are the least accurate
Replies: 3
Views: 92

Re: Why Bond Enthalpies are the least accurate

Bond enthalpy is the least accurate because it represents the average of a series of different bonds from different molecules, so it represents an approximation of a bond rather than a more precise measurement like formation.

Hope that helps!
by Xuan Kuang 2L
Sun Feb 03, 2019 1:27 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Seventh Edition ? 4C.3
Replies: 1
Views: 61

Re: Seventh Edition ? 4C.3

To approach this problem, you'd first have to recognize that you're using the formula qp=nCp,mdeltaT. You would then substitute given values, with 765 J being qp, 0.820 mol for n, and 298 K for initial temperature. However, in this problem, you'd also have to recognize the rotational contribution of...
by Xuan Kuang 2L
Sat Jan 26, 2019 10:54 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Method 2 Example
Replies: 3
Views: 51

Method 2 Example

On Friday's Lecture, there was an example for Method 2 which considered the bond enthalpies regarding CH2=CH2 + H-Br --> CH3-CH2Br Could someone reiterate why we utilized the value of the C=C bond on the reactants' side, and why we considered using the bond enthalpy of C-C on the products' side? I g...
by Xuan Kuang 2L
Sat Jan 26, 2019 10:30 pm
Forum: Phase Changes & Related Calculations
Topic: 6th edition 8.15
Replies: 1
Views: 72

Re: 6th edition 8.15

Hello, I hope these answers help! a) The change in internal energy would only be zero if q+w=0. Since we've established no energy is transferred as heat (q=0) we can then say that this statement is only true when w=0, or if no work is done b) This is always true because the question stated that ther...
by Xuan Kuang 2L
Thu Jan 24, 2019 2:30 pm
Forum: Calculating pH or pOH for Strong & Weak Acids & Bases
Topic: 6B.9
Replies: 1
Views: 201

6B.9

6B.9 a) Complete the following table [H3O+] [OH-] pH pOH i) 1.50 mol.L-1 ii) 1.50 mol.L-1 iii) 0.75 iv) 0.75 b) Rank the four solutions in order of increasing acidity (I understand iii) and iv), but I am not sure why I keep getting i) and ii) wrong. When I calculate for i), I get [OH-]= 6.67x10^-15 ...
by Xuan Kuang 2L
Mon Jan 21, 2019 10:08 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Chemical Equilibrium part 4 post module assessment question #12
Replies: 2
Views: 48

Re: Chemical Equilibrium part 4 post module assessment question #12

Both sides of the reaction contain the same number of moles (2:2), so the reaction doesn't favor any particular side.
by Xuan Kuang 2L
Mon Jan 21, 2019 9:49 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Approximating for Equilibrium Constants
Replies: 1
Views: 39

Re: Approximating for Equilibrium Constants

Hey there! I believe we could use this method with Kc as well, if we needed it.
by Xuan Kuang 2L
Mon Jan 21, 2019 9:47 pm
Forum: Identifying Acidic & Basic Salts
Topic: Effect of pH
Replies: 2
Views: 241

Effect of pH

Hello there, not sure if anyone asked this already, but could someone explain to me how we know that Group I/II and some of the halogens (Cl-, Br-. I-) have no effect on the pH when it dissociates in water?
by Xuan Kuang 2L
Fri Jan 11, 2019 7:46 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Changes in Systems
Replies: 3
Views: 74

Re: Changes in Systems

I believe that this is always the case. There is an equation relating the temperature to K that may help you understand why, and you can see the full derivation of it in the textbook (For me, it is in 5H, pg 413, not sure about the 6th edition)

Hop this helps
by Xuan Kuang 2L
Fri Jan 11, 2019 7:36 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Q 11.49
Replies: 1
Views: 41

Re: Q 11.49

Hey there! I believe it has to do with the coefficient of the reactant, which has a 2 on it, I assume (Sorry, I'm not too sure about this because I don't think I have the same edition as you) If there's a 2 as a coefficient, we have to consider this when forming the ICE Box. In this case, if the pro...
by Xuan Kuang 2L
Fri Jan 11, 2019 7:28 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: 6th edition 11.71
Replies: 1
Views: 60

Re: 6th edition 11.71

Not sure if you wanted all of the answers, but I provided them all down below with some explanations. Hope it helps! a) When you add NO, Q>K, so more reactants are formed. This means more products are used up, so the amount of H2O, which we assume is constant and hasn't changed in concentration, dec...

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