## Search found 31 matches

Sat Mar 17, 2018 8:37 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Midterm 2b: relationship between delta H and q
Replies: 1
Views: 240

### Midterm 2b: relationship between delta H and q

In question 2A we get that the delta H for the overall reaction is negative. And in 2B it asks that if the entropy would be positive or negative.
I still don't get that if we say qp = delta H, then S is defined as qrev./T
If the delta H is negative, then why the entropy is not negative too?
Sun Mar 11, 2018 9:19 pm
Forum: First Order Reactions
Topic: Pseudo First Order Reaction
Replies: 7
Views: 820

### Re: Pseudo First Order Reaction

For example, if we have a equation telling us that rate of consumption of I- = k [S2O8 2-][I-]. Because the rate equation involves more than one species, it would be hard to analyze the change of rates. Then we can just set persulfate ions concentration really high, turning an actual second order re...
Sun Mar 11, 2018 9:14 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Replies: 1
Views: 105

For the Michalis-Menten mechanism: Rate of formation of product = k2 [E]0 [S] / (KM+[S])
The equation given would be E + S --> ES

It says kM = (k1'+k2)/k1, and I have no idea why it's like that. Can anyone explain?

Thanks!
Sun Mar 11, 2018 9:08 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: About the diagram of the transition state
Replies: 1
Views: 111

### About the diagram of the transition state

To draw a diagram for the mechanism of 2BrNO (g) --> Br2 + 2NO Why do we draw it like [NO....Br...Br....NO] not like [N...O....Br...Br...O...N]? Is it because NO comes out as the product and the bond between them never breaks? And does the distance between elements in the diagram matter? Like is the...
Sun Mar 04, 2018 9:37 pm
Forum: Second Order Reactions
Topic: Half-lives
Replies: 2
Views: 214

### Half-lives

When can we use logarithm to calculate the time required to reach a certain concentration when we know the half-life of the reaction, and when are we supposed to use the original rate laws? For example, hw question 15.83: The first-order decomposition of compound X, a gas, is carried out and the dat...
Sun Mar 04, 2018 9:19 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: HW15.101 Regarding OH
Replies: 2
Views: 141

### Re: HW15.101 Regarding OH

But my question is that if OH- is formed and then consumed in a later step, why wouldn't we count OH- as an intermediate?
Sun Mar 04, 2018 9:16 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: When to use pre-equilibrium approach?
Replies: 2
Views: 126

### When to use pre-equilibrium approach?

There are basically two approaches that the textbook talked about: pre-equilibrium and steady state approximation. My understanding right now is that if no one tells me which step is the slowest, then I use the steady state; if there is a rate determining step, I use the pre-equilibrium. I just want...
Fri Mar 02, 2018 10:20 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: HW15.101 Regarding OH
Replies: 2
Views: 141

### HW15.101 Regarding OH

The reaction in this problem has three steps: Step 1: ClO-+H2O--->HClO+OH- (Fast equilibrium) Step 2: HClO+I---->HIO+Cl- (Slow step) Step 3: HIO+OH- --->IO-+H2O (Fast equilibrium) The rate law proposed contains OH-. Isn't OH- an intermediate? Since it's produced and then consumed in a later step, wh...
Sun Feb 25, 2018 8:16 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: How to write the rate laws for homogeneous catalyst
Replies: 1
Views: 120

### How to write the rate laws for homogeneous catalyst

If there is a homogeneous catalyst in the reaction, do we write the rate law just as if the catalyst is an intermediate?
Sun Feb 25, 2018 8:06 pm
Forum: Second Order Reactions
Replies: 1
Views: 121

What is the difference between steady state approximation and pre-equilibrium approach? Which one should we choose and based on what conditions? Is pre-equilibirum approach only used when there is a slow step following a fast step?
Sun Feb 25, 2018 4:13 pm
Forum: Second Order Reactions
Topic: Question about half-life regarding hw 15.35
Replies: 1
Views: 133

### Question about half-life regarding hw 15.35

For hw 15.35, we need to use the integrated rate law to calculate how long does it take for the reactant concentration to fall to one sixteenth of its original value. Why we cannot just use half-life times 4 to get the answer but instead we have to do the whole rate law? Full question: The half-life...
Sun Feb 18, 2018 8:51 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: About the standard cell potentials for aqueous solutions
Replies: 2
Views: 131

### About the standard cell potentials for aqueous solutions

There are questions asking about when the two compartments are in aqueous solutions, we need to compare them to the reduction potential of water and choose the process with the least negative value. But how do we do that? And how do we know which equation to use, since there are several equations fo...
Fri Feb 16, 2018 10:35 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: HW 14.107
Replies: 1
Views: 114

### HW 14.107

For the question 107: What range (in volts) does a voltmeter need to have to measure pH in the range of 1 to 14 at 25 C if the voltage is zero when pH 7? In the solution it used the equation of E°=RT/nF ln(K) However, for K it used the ratio: [H]/[OH], and I wonder why K is expressed like that. And ...
Thu Feb 15, 2018 10:44 pm
Forum: Interesting Applications: Rechargeable Batteries (Cell Phones, Notebooks, Cars), Fuel Cells (Space Shuttle), Photovoltaic Cells (Solar Panels), Electrolysis, Rust
Topic: HW 14.55
Replies: 4
Views: 694

### Re: HW 14.55

In the solution manual it says, because SO4 2- will not oxidize thus we have to use the water one. However, I'm so confused that how do you decide whether an ion will oxidize or not? There is an equation for the reduction of SO4 2- in appendix, why it cannot be oxidized? Thanks for anyone who's will...
Tue Feb 13, 2018 7:46 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: About total entropy change for reversible processes
Replies: 1
Views: 108

### About total entropy change for reversible processes

I'm wondering if deltaS total equals to zero for reversible processes, since the guy at the review session said so. So if that's true, and dG=-TdStot, then the change in gibbs free energy is always zero for any reversible process?

Can somebody explain it to me? Thanks!
Sun Feb 11, 2018 9:56 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: About the unit for standard gibbs free energy of a reaction
Replies: 2
Views: 149

### About the unit for standard gibbs free energy of a reaction

The unit for Standard Gibbs free energy of a reaction is kJ/mol, so does that mean, whenever we see something like N2+2O2-->2NO2, we always need to divide the value that we calculated directly by two to get the standard gibbs free energy of the reaction? what if there are two products and the coeffi...
Fri Feb 09, 2018 3:32 pm
Forum: Balancing Redox Reactions
Topic: 14.13(d)
Replies: 2
Views: 107

### Re: 14.13(d)

The way you wrote the half reactions implies that the same species, Au + (aq) , is going to undergo two different types of reactions in the same equation. The way the book writes them has Au + turn into Au(s), and that Au(s) can be oxidized further. So you mean that a reactant can never under go tw...
Thu Feb 08, 2018 10:02 pm
Forum: Balancing Redox Reactions
Topic: 14.13(d)
Replies: 2
Views: 107

### 14.13(d)

We need to balance the reaction Au+(aq)--->Au(s)+Au3+(aq) In the solution manual, it says the two half reactions are: Au+(aq)+e---->Au(s) Au3+(aq)+3e- --->Au(s) I have no idea why the they all contain Au(s)? why are they arranged that way? When I was doing the problem I wrote down these two half rea...
Thu Feb 08, 2018 9:40 pm
Forum: Balancing Redox Reactions
Topic: hw14.13(c) balancing equation with HCl (aq)
Replies: 1
Views: 63

### hw14.13(c) balancing equation with HCl (aq)

In the (c) part of 14.13, we need to balance a redox equation Cl2(g)+H2(g)--->HCl(aq) There is only one product present while there are two reactants, and I'm confused that do we need to balance the half equation first? In the solution manual, they simply split up HCl to H+ ion and Cl- ion, can we a...
Sun Feb 04, 2018 8:04 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: HW 11.99
Replies: 1
Views: 139

### HW 11.99

When the solution manual explains homework problem 11.99, it talks about three different form of energy: bond dissociation energy, free energies of formation, free energy of dissociation. I am kind of confused between the three concepts, can someone explain them to me? What's the difference between ...
Wed Jan 31, 2018 10:31 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: HW 9.35 Vibrational degree of freedom
Replies: 1
Views: 118

### HW 9.35 Vibrational degree of freedom

About the question 9.35, I went through the corrections for the solution manual error but I'm kind of confused with the concept of vibrational degrees of freedom and how that changes the values of Cv when we calculate entropy for the gases. The only thing I was told about Cv for ideal gases was in t...
Tue Jan 30, 2018 3:47 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Regarding the equation deltaS=nCln(Tf/Ti)
Replies: 3
Views: 310

### Regarding the equation deltaS=nCln(Tf/Ti)

For the formula nCln(Tf/Ti), if we were to calculate entropy change for a liquid or solid state substance, do we plug in the heat capacity for that substance in liquid/solid state? I thought Cv or Cp is only used for gases.

Thanks!
Sun Jan 28, 2018 9:33 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Boltzmann equation: positional entropy or thermal entropy?
Replies: 3
Views: 132

### Boltzmann equation: positional entropy or thermal entropy?

I get that entropy has two parts: thermal entropy and positional entropy
I'm just confused that for the one with Boltzmann equation, is that positional or thermal?
Sun Jan 28, 2018 9:25 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: About entropy and gibbs free energy
Replies: 2
Views: 145

### About entropy and gibbs free energy

It says: A reaction that produces a lot of entropy can drive a nonspontaneous reaction forward. (in the textbook section 9.16) However, we know that what actually determines if the reaction is spontaneous or not is the Gibbs free energy. So what does the sentence above mean? If the reaction is itsel...
Sun Jan 28, 2018 9:19 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Replies: 1
Views: 120

It says in the book section 9.7, S(T)=S(0)+deltaS(heating from 0 to T)=deltaS(heating from 0 to T)
Why it can be equaled that way? Since S(0) is only 0 for perfect crystals at T=0?
If it's not a perfect zero, wouldn't S(0) be some number due to the positional entropy?
Sun Jan 21, 2018 8:12 pm
Forum: Phase Changes & Related Calculations
Topic: About phase change for methane in the combustion reaction
Replies: 2
Views: 193

### About phase change for methane in the combustion reaction

Hi! If we are calculating the enthalpy for combustion for methane under room temperature, the product include water in liquid state. However, if we are using the enthalpy of formation of water in gas phase to calculate the enthalpy of the reaction, do we need to subtract the enthalpy of vaporization...
Sat Jan 20, 2018 7:38 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.99
Replies: 1
Views: 103

### 8.99

Hi! For homework question 8.99, the reaction is given as 2HCl(aq) + Zn(s)-->H2(g)+ZnCl2(aq) The question asks about the enthalpy of reaction, which in the solution manual is suggested to be calculated through enthalpies of formation However, I don't understand how the solution manual gets an equatio...
Thu Jan 18, 2018 11:05 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: hw Q8.61
Replies: 1
Views: 707

### hw Q8.61

I have trouble solving question 8.61 Calculate the reaction enthalpy for the synthesis of hydrogen bromide gas, H2(g)+Br2(l)->2HBr(g), NH3(g)+HBr(g)->NH4Br(s) dH=-188.32kJ N2(g)+3H2(g)->2NH3(g) dH=-92.22kJ N2(g)+4H2(g)+Br2(l) dH=-541.66kJ I understand you have to arrange them in some way that the ov...
Sun Jan 14, 2018 9:41 pm
Forum: Calculating Work of Expansion
Topic: Equations for calculating work for irreversible and reversible reactions
Replies: 1
Views: 124

### Equations for calculating work for irreversible and reversible reactions

Hi, I'm so confused about the differences between irreversible and reversible reactions. I know that for irreversible it would be a quick work and it's like rectangles on the graph; and for reversible one it's corresponding to the curve on the graph which has the equation W = -nRT x ln (Vin/Vf) So w...
Sun Jan 14, 2018 9:33 pm
Forum: Phase Changes & Related Calculations
Topic: Question 8.39
Replies: 2
Views: 203

### Re: Question 8.39

So there are two parts of the question: the first part would be to convert ice into water, using the enthalpy of fusion for water; the second part would be to raise that liquid water up 20 degrees celsius, using the specific heat of water. For the first part, we know that the enthalpy of fusion woul...
Fri Jan 12, 2018 10:38 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: dU=0 for expansion and compression of an ideal gas
Replies: 1
Views: 105

### dU=0 for expansion and compression of an ideal gas

In the book, it is explained that the expansion or the compression of an ideal gas results in deltaU=0 because the kinetic energies and the potential energies of the ideal gas is unchanged. I'm just wondering how to explain this by knowing dU=dQ+dW? Because either compression or expansion would cost...