Search found 40 matches
- Sat Mar 17, 2018 7:12 pm
- Forum: Concepts & Calculations Using Second Law of Thermodynamics
- Topic: Irreversible reactions
- Replies: 1
- Views: 265
Irreversible reactions
All spontaneous reactions are irreversible but are all irreversible processes spontaneous?
- Wed Mar 14, 2018 12:13 am
- Forum: Reaction Mechanisms, Reaction Profiles
- Topic: H2O in rate law/15.101
- Replies: 1
- Views: 152
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...
- Tue Mar 13, 2018 11:12 pm
- Forum: Reaction Mechanisms, Reaction Profiles
- Topic: 15.101b)
- Replies: 1
- Views: 146
15.101b)
OH- is an intermediate, so why is it allowed to be included in the rate law?
- Tue Mar 13, 2018 9:03 pm
- Forum: Reaction Mechanisms, Reaction Profiles
- Topic: Steady state mechanism
- Replies: 1
- Views: 177
Steady state mechanism
The syllabus does not list it, but just to confirm, will we be tested on it during the final?
- Sun Mar 11, 2018 10:45 pm
- Forum: Arrhenius Equation, Activation Energies, Catalysts
- Topic: 15.79
- Replies: 2
- Views: 219
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...
- Sun Mar 11, 2018 10:19 pm
- Forum: Arrhenius Equation, Activation Energies, Catalysts
- Topic: 15.67b
- Replies: 1
- Views: 163
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...
- Sun Mar 11, 2018 10:11 pm
- Forum: Arrhenius Equation, Activation Energies, Catalysts
- Topic: SN2
- Replies: 2
- Views: 186
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...
- Sun Mar 11, 2018 10:02 pm
- Forum: Arrhenius Equation, Activation Energies, Catalysts
- Topic: 15.71
- Replies: 2
- Views: 168
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...
- Sun Mar 11, 2018 9:49 pm
- Forum: Arrhenius Equation, Activation Energies, Catalysts
- Topic: 15.71
- Replies: 2
- Views: 168
15.71
How do you know H2O is not an intermediate, considering how it is not in the first step of the reaction?
- Sun Mar 11, 2018 9:47 pm
- Forum: *Nucleophiles
- Topic: What is a nucleophile? [ENDORSED]
- Replies: 6
- Views: 730
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.
- Wed Mar 07, 2018 11:58 pm
- Forum: First Order Reactions
- Topic: Simpler way to calculate change in concentrations
- Replies: 2
- Views: 195
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...
- Wed Mar 07, 2018 11:49 pm
- Forum: General Rate Laws
- Topic: Coefficients
- Replies: 5
- Views: 307
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...
- Wed Mar 07, 2018 11:17 pm
- Forum: General Rate Laws
- Topic: Coefficients
- Replies: 5
- Views: 307
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...
- 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: 221
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.
- Wed Feb 28, 2018 9:03 pm
- Forum: Balancing Redox Reactions
- Topic: 14.115
- Replies: 2
- Views: 298
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...
- 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: 221
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)?
- 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: 188
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)?
- Wed Feb 28, 2018 8:46 pm
- Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
- Topic: 14.57 [ENDORSED]
- Replies: 1
- Views: 167
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?
- Sun Feb 25, 2018 7:21 pm
- Forum: Interesting Applications: Rechargeable Batteries (Cell Phones, Notebooks, Cars), Fuel Cells (Space Shuttle), Photovoltaic Cells (Solar Panels), Electrolysis, Rust
- Topic: 14.55
- Replies: 3
- Views: 469
Re: 14.55
Ammar Amjad 1L wrote:Here is a link to a post that answers your question.
viewtopic.php?f=143&t=11404&p=28252&hilit=
Thank you.
- Sat Feb 24, 2018 9:52 pm
- Forum: Interesting Applications: Rechargeable Batteries (Cell Phones, Notebooks, Cars), Fuel Cells (Space Shuttle), Photovoltaic Cells (Solar Panels), Electrolysis, Rust
- Topic: 14.55
- Replies: 3
- Views: 469
14.55
Why is SO4 2- not reduced? E°(SO4/H2SO3)>E°(Ni2+/Ni)?
- 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: 170
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...
- Wed Feb 21, 2018 10:23 pm
- Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
- Topic: When writing cell diagrams, how do you know when to include an additional element at the electrode?
- Replies: 5
- Views: 341
Re: When writing cell diagrams, how do you know when to include an additional element at the electrode?
Adrian Lim 1G wrote:Isn't I2 a solid though?
You need a conducting solid.
- 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: 306
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.
- 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: 306
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.
- Wed Feb 14, 2018 10:23 am
- Forum: Concepts & Calculations Using First Law of Thermodynamics
- Topic: ∆U for isothermal reversible
- Replies: 3
- Views: 287
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...
- Wed Feb 14, 2018 1:57 am
- Forum: Concepts & Calculations Using First Law of Thermodynamics
- Topic: ∆U for isothermal reversible
- Replies: 3
- Views: 287
∆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?
- Tue Feb 13, 2018 9:59 pm
- Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
- Topic: Super basic question about systems
- Replies: 1
- Views: 131
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...
- 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: 179
∆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?
- 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: 204
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...
- 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: 204
9.35
Can someone explain to me why the C values to calculate Box A, B and C are different?
- 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: 264
Re: 9.27a
Xihui Yin 1I wrote:So what does elementary particles mean?
I got it, nvm.
- Sat Feb 10, 2018 5:52 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: 264
Re: 9.27a
So what does elementary particles mean?
- 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: 264
9.27a
Can someone explain to me the solution? What does elementary particles mean?
- Mon Feb 05, 2018 12:06 am
- Forum: Concepts & Calculations Using Second Law of Thermodynamics
- Topic: positional residual entropy
- Replies: 4
- Views: 273
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...
- Sun Feb 04, 2018 11:59 pm
- Forum: Concepts & Calculations Using Second Law of Thermodynamics
- Topic: positional residual entropy
- Replies: 4
- Views: 273
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...
- 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: 185
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?
- Sun Feb 04, 2018 11:22 pm
- Forum: Phase Changes & Related Calculations
- Topic: Determining if reversible or irreversible
- Replies: 2
- Views: 198
Determining if reversible or irreversible
How do we determine if a reaction is reversible or irreversible, or will all related questions explicitly state it?
- Sun Jan 21, 2018 11:15 pm
- Forum: Phase Changes & Related Calculations
- Topic: ΔU vs. ΔH
- Replies: 10
- Views: 1543
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.
- Sun Jan 21, 2018 10:48 pm
- Forum: Phase Changes & Related Calculations
- Topic: Carbon as graphite [ENDORSED]
- Replies: 7
- Views: 533
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...
- 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: 108
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...