Search found 40 matches

by Xihui Yin 1I
Sat Mar 17, 2018 7:12 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Irreversible reactions
Replies: 1
Views: 186

Irreversible reactions

All spontaneous reactions are irreversible but are all irreversible processes spontaneous?
by Xihui Yin 1I
Wed Mar 14, 2018 12:13 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: H2O in rate law/15.101
Replies: 1
Views: 84

H2O in rate law/15.101

Should H2O be omitted from the rate law if it acts as a solvent? One example would be 15.101, where the solution stated to omit it from the chemical equilibrium: ClO- + H2O <=> HClO + OH-. Since K=[HClO][OH-]/[ClO-], and reverse reaction is k'[HClO][OH-], would the forward reaction be k[ClO-] instea...
by Xihui Yin 1I
Tue Mar 13, 2018 11:12 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: 15.101b)
Replies: 1
Views: 89

15.101b)

OH- is an intermediate, so why is it allowed to be included in the rate law?
by Xihui Yin 1I
Tue Mar 13, 2018 9:03 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Steady state mechanism
Replies: 1
Views: 88

Steady state mechanism

The syllabus does not list it, but just to confirm, will we be tested on it during the final?
by Xihui Yin 1I
Sun Mar 11, 2018 10:45 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: 15.79
Replies: 2
Views: 142

15.79

For a), I don't understand why CH3CHBrCH=CH2 would have a lower Ea when it essentially has a different structure from that of the intermediate, bromine aside. For b), I don't understand the solution at all. Do they mean that lower Ea -> faster reaction -> kinetic control predominate, so a reaction p...
by Xihui Yin 1I
Sun Mar 11, 2018 10:19 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: 15.67b
Replies: 1
Views: 99

15.67b

I used the method of substitution value of Ea and T into the formula k=Ae^(-Ea/RT). For a, I managed to get the correct answer. For b), I used the same method and substituted T=298 and Ea=75000 for k1 and T=350 and Ea=75000 for k2 and divided k2 over k1. However, my answer was wrong and I can't seem...
by Xihui Yin 1I
Sun Mar 11, 2018 10:11 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: SN2
Replies: 2
Views: 119

Re: SN2

Nucleophiles have an extra pair of electrons (either lone pair or in a pi bond) and can attack an electrophile (an electron-deficient species, e.g. positively charged or partially positively charged) to form a bond. In a sense this means that something with the capacity to form a bond due to its ext...
by Xihui Yin 1I
Sun Mar 11, 2018 10:02 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: 15.71
Replies: 2
Views: 96

Re: 15.71

H2O cannot be an intermediate because it is one of the reactants in the overall reaction. Yes, I know, but overall reaction was not given. Considering how you fomulate the overall reaction based on the steps, how would you then know that H2O is not an intermediate when it is in the second step? For...
by Xihui Yin 1I
Sun Mar 11, 2018 9:49 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: 15.71
Replies: 2
Views: 96

15.71

How do you know H2O is not an intermediate, considering how it is not in the first step of the reaction?
by Xihui Yin 1I
Sun Mar 11, 2018 9:47 pm
Forum: *Nucleophiles
Topic: What is a nucleophile? [ENDORSED]
Replies: 6
Views: 471

Re: What is a nucleophile? [ENDORSED]

Not necessarily. What matters is that that have a pair of electrons that can be used to form a bond with an electron-deficient species. This means they either have lone pairs (H2O, NH3, OH-), a pi bond (ethene) or a partial negative charge.
by Xihui Yin 1I
Wed Mar 07, 2018 11:58 pm
Forum: First Order Reactions
Topic: Simpler way to calculate change in concentrations
Replies: 2
Views: 96

Simpler way to calculate change in concentrations

For a first order reaction involving half-lives, will I be penalised for using (1/2)^n=fraction of final concentration/original concentration to find the number of half-lives? I know this does not apply to second order and zero order reactions, but I was wondering if I will be penalised for not usin...
by Xihui Yin 1I
Wed Mar 07, 2018 11:49 pm
Forum: General Rate Laws
Topic: Coefficients
Replies: 5
Views: 188

Re: Coefficients

I had the same question because in the page in Lavelle's website (https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14B/Kinetics_Integrated_Rate_Laws_Examples.pdf) it says "*In the following derivations of the three integrated rate laws, we assume the stoichiometric coefficient &qu...
by Xihui Yin 1I
Wed Mar 07, 2018 11:17 pm
Forum: General Rate Laws
Topic: Coefficients
Replies: 5
Views: 188

Coefficients

Given that the formulas for the various zero first second order reactions assume that the coefficient of the reactant is 1, for cases where the coefficient is not 1, do you have to adjust the formula by the coefficient accordingly? E.g. a A->bB for a first order reaction. Rate= a •k[A], so different...
by Xihui Yin 1I
Wed Feb 28, 2018 11:14 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: 14.107
Replies: 2
Views: 136

Re: 14.107

Alyssa Parry Disc 1H wrote:E naught is standard reduction potential and E is the potential of a reaction

Yes, I know, but the textbook uses E.
by Xihui Yin 1I
Wed Feb 28, 2018 9:03 pm
Forum: Balancing Redox Reactions
Topic: 14.115
Replies: 2
Views: 194

Re: 14.115

Hi, James, So, as the problem states, aluminum should react with oxygen in basic solution, which means that water, OH- and oxygen should all be involved in your redox reaction. Whenever the metal will be placed in the basic solution, it will form a hydroxide and therefore this should be one of your...
by Xihui Yin 1I
Wed Feb 28, 2018 9:00 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: 14.107
Replies: 2
Views: 136

14.107

Why is E° used instead of E? How is this reaction different from the textbook example of a ion-selective electrodes since the textbook uses E? Do we use the textbook formula for ion-selective electrodes (E=E'-0.05916*pH)?
by Xihui Yin 1I
Wed Feb 28, 2018 8:56 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: 14.93b [ENDORSED]
Replies: 1
Views: 120

14.93b [ENDORSED]

Shouldn't E increase as [CrCl3] decreases? RT/nF*ln[anode]/[cathode] should become larger as lnQ increases since the reaction would favour the products as concentration in anode is decreased (Le Chatelier)?
by Xihui Yin 1I
Wed Feb 28, 2018 8:46 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 14.57 [ENDORSED]
Replies: 1
Views: 102

14.57 [ENDORSED]

Why is the E° value used here -0.42V (basic) and not +0.82V (acidic)? Don't the transition metals form ligands which hydrolyse to give acidic solutions?
by Xihui Yin 1I
Wed Feb 21, 2018 10:41 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 14.15a cell drawing
Replies: 1
Views: 105

Re: 14.15a cell drawing

For the anode, why is it written in the order Ag(s)|AgBr(s)|Br-(aq)? Is this because Br- is an ion/aqueous so it's written closest to the salt bridge? If solids are written on the outside, why is Ag(s) written before AgBr? Is this because it's written from reactants to products? But if it's written...
by Xihui Yin 1I
Wed Feb 14, 2018 11:50 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Standard ∆G for a system at equilibrium
Replies: 3
Views: 178

Re: Standard ∆G for a system at equilibrium

Nvm, I figured out that ∆G=0 for equilibirum but ∆G° varies for different magnitudes of K.
by Xihui Yin 1I
Wed Feb 14, 2018 11:32 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Standard ∆G for a system at equilibrium
Replies: 3
Views: 178

Standard ∆G for a system at equilibrium

It says that standard ∆G for a system at equilibrium=0, but why for varying values of K, representing a system at equilibrium as well, not equal to ∆G=0 itself? I.e. K>1, ∆G<0 as opposed to ∆G=0.
by Xihui Yin 1I
Wed Feb 14, 2018 10:23 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: ∆U for isothermal reversible
Replies: 3
Views: 186

Re: ∆U for isothermal reversible

dU=0 for an isothermal expansion or compression of an ideal gas only. This is a direct consequence of U = 3/2 nRT for an ideal gas. Therefore, for ideal gases, no temperature change=no internal energy change. Additionally, it's not really possible to expand/compress liquids or solids in the same wa...
by Xihui Yin 1I
Wed Feb 14, 2018 1:57 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: ∆U for isothermal reversible
Replies: 3
Views: 186

∆U for isothermal reversible

Do we assume that ∆U=0 for all isothermal reversible reactions, or do we have the make sure that the question explicitly mentions the gas as ideal?
by Xihui Yin 1I
Tue Feb 13, 2018 9:59 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Super basic question about systems
Replies: 1
Views: 100

Super basic question about systems

I'm really overthinking this unimportant question but what system is a simple calorimeter? I recall it being a closed system because the cup is covered, but it is at constant pressure and isn't sealed, so shouldn't it be open? I've tried searching past posts but the answers all seem to be a mix of e...
by Xihui Yin 1I
Sat Feb 10, 2018 8:45 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: ∆S=q/T and irreversible reactions
Replies: 2
Views: 114

∆S=q/T and irreversible reactions

Just to confirm, we can use q(rev)/T with the q value derived from an irreversible reaction since entropy is a state property right? I.e. use ∆H from an irreversible reaction to calculate q(rev). Are there any exceptions or stuff I have to take note?
by Xihui Yin 1I
Sat Feb 10, 2018 8:06 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: 9.35
Replies: 2
Views: 131

Re: 9.35

The problem states that in box A, there is a monoatomic gas, meaning that there are single atoms of gas. The Cv value for atoms is 3/2R. In box B, there are diatomic molecules that are not vibrationally active, meaning these are linear molecules. For linear molecules, the Cv value is 5/2R. In box C...
by Xihui Yin 1I
Sat Feb 10, 2018 6:23 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: 9.35
Replies: 2
Views: 131

9.35

Can someone explain to me why the C values to calculate Box A, B and C are different?
by Xihui Yin 1I
Sat Feb 10, 2018 6:22 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: 9.27a
Replies: 3
Views: 142

Re: 9.27a

Xihui Yin 1I wrote:So what does elementary particles mean?

I got it, nvm.
by Xihui Yin 1I
Sat Feb 10, 2018 5:45 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: 9.27a
Replies: 3
Views: 142

9.27a

Can someone explain to me the solution? What does elementary particles mean?
by Xihui Yin 1I
Mon Feb 05, 2018 12:06 am
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: positional residual entropy
Replies: 4
Views: 151

Re: positional residual entropy

Recall the third law - entropy in a perfect crystal at 0K is 0. This means that at 0K there is no thermal entropy, hence only entropy remaining is positional residual entropy, which is based off the "randomness" of the positions of the molecules/atoms arranged. For example a non-polar mol...
by Xihui Yin 1I
Sun Feb 04, 2018 11:59 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: positional residual entropy
Replies: 4
Views: 151

Re: positional residual entropy

Recall the third law - entropy in a perfect crystal at 0K is 0. This means that at 0K there is no thermal entropy, hence only entropy remaining is positional residual entropy, which is based off the "randomness" of the positions of the molecules/atoms arranged. For example a non-polar mole...
by Xihui Yin 1I
Sun Feb 04, 2018 11:50 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.115b) [ENDORSED]
Replies: 1
Views: 165

8.115b) [ENDORSED]

The question states that the the pressure changes from 16atm to 4atm. Since ∆H=∆U+P∆V, and ∆H=q only under constant pressure, why are we using enthalpy of combustion to calculate heat change when pressure is not constant?
by Xihui Yin 1I
Sun Feb 04, 2018 11:22 pm
Forum: Phase Changes & Related Calculations
Topic: Determining if reversible or irreversible
Replies: 2
Views: 115

Determining if reversible or irreversible

How do we determine if a reaction is reversible or irreversible, or will all related questions explicitly state it?
by Xihui Yin 1I
Sun Jan 21, 2018 11:15 pm
Forum: Phase Changes & Related Calculations
Topic: ΔU vs. ΔH
Replies: 10
Views: 475

Re: ΔU vs. ΔH

ΔH is the thermodynamic potential of the system, and thus includes work done and ΔU, ΔH=ΔU + PΔV. ΔU is the summation of energies in a system. When work done=0, ΔH=ΔU.
by Xihui Yin 1I
Sun Jan 21, 2018 10:48 pm
Forum: Phase Changes & Related Calculations
Topic: Carbon as graphite [ENDORSED]
Replies: 7
Views: 337

Re: Carbon as graphite [ENDORSED]

In the graphite form, carbon is sp2 bonded to 3 other carbon atoms to form a hexagonal ring. However, each C still has a spare 2p orbital containing an electron. The 2p orbitals overlap to form a delocalised pi bonding system and as such, rings of carbons exist as individual sheets. Dispersion force...
by Xihui Yin 1I
Sun Jan 21, 2018 10:39 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.77
Replies: 1
Views: 71

Re: 8.77

I'm assuming molar energy refers to the overall summation of all the individual bond energies for one mole of benzene. In the context of the question, benzene has resonance stability. Individually, the summation of separate C-C and C=C bonds without taking into account benzene resonance would result...

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