## Search found 156 matches

Sat Mar 17, 2018 6:18 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: enthalpy of vap
Replies: 1
Views: 283

### enthalpy of vap

On formula sheet, I am given that deltaHvap = 40.7kj/mol

But when I find that using the numbers given, in his equation:
H20(l) --> H20(g)
the deltaHrxn = 44.01 kj/mol

Why are they different?
Sat Mar 17, 2018 6:10 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: delta g equilibrium
Replies: 2
Views: 271

### delta g equilibrium

does delta g = 0 mean equilibrium?
Sat Mar 17, 2018 6:10 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: delta G = 0 phase change
Replies: 1
Views: 1424

### delta G = 0 phase change

Why should the value of delta G be zero at 100 Celsius for water? CONCEPTUALLY. I am referencing 9.91. I get how experimental data can skew numbers, but why should delta G be zero in the first place. Is this saying a phase change is at equilibrium?
Sat Mar 17, 2018 10:44 am
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: Entropy of vaporization? 9.37
Replies: 1
Views: 273

### Entropy of vaporization? 9.37

Why wouldn' you subtract the entropy of vaporization in this problem for part a?

H2(g) + 1/2O2(g) --> H2O(l)
Delta S = Sproducts - S reactants
There is a phase change. We did something like this for enthalpy, so why not for entropy.
Sat Mar 17, 2018 12:25 am
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: 9.37 and subtracting vaporization
Replies: 1
Views: 206

### 9.37 and subtracting vaporization

Why wouldn' you subtract the entropy of vaporization in this problem for part a?

H2(g) + 1/2O2(g) --> H2O(l)
Delta S = Sproducts - S reactants
There is a phase change. We did something like this for enthalpy, so why not for entropy.
Fri Mar 16, 2018 10:54 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Meaning behind E
Replies: 1
Views: 156

### Meaning behind E

What does cell potential mean? What does it mean when I have to choose something with the highest reducing power?

When I am doing a galvanic cell, I want to choose the reaction with the cell potential that is more positive? Does more positive mean less reducing power?
Fri Mar 16, 2018 10:50 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Rule when writing cell diagrams?
Replies: 5
Views: 341

### Rule when writing cell diagrams?

What's the rule behind writing a cell diagram equation when I have H+ or OH- present in my overall redox reaction? Do I include H+/OH-? How do I include them?
Mon Mar 12, 2018 4:10 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Constant pressure and delta U
Replies: 1
Views: 179

### Constant pressure and delta U

In Example 8.6 in the in-chapter problems, I don't understand their reasoning for using Cv in the constant pressure problem.

Why does delta U = nCvdeltaT for the delta U of a reaction at constant pressure?
Mon Mar 12, 2018 3:02 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Why is entropy extensive?
Replies: 5
Views: 1735

### Why is entropy extensive?

If q=mCT

and delta S = q/T, doesn't that involve mass (intensive)
Sun Mar 11, 2018 5:37 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Slow reaction
Replies: 2
Views: 237

### Slow reaction

How do I know what to make the approximation to?

In my slow reaction, I have:

Fast 1st step:
forward O3 rate decomp= k1[O3]
reaverse form rate= kprime1[O2][O]

Slow 2nd Step:
forward O3 consume rate = k2[O][O3]

Why is it k2[O][O3] << kprime1[O2][O]

and not

k2[O][O3] << k1[O3]
Sun Mar 11, 2018 5:07 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Page 634 Elementary Reactions Confusion
Replies: 1
Views: 171

### Page 634 Elementary Reactions Confusion

For finding the rate of formation of NO2 from 2NO(g) + O2(g) --> 2NO(g), Rate of Formation of NO2 = k[NO] 2 [O2] Why do you find reverse of Step 1 but not Step 2. I am aware that Step 2 is a slow reaction and therefore would not affect rate, but the book included it in net rate of formation. You wil...
Tue Mar 06, 2018 6:26 pm
Forum: General Rate Laws
Topic: 15.3
Replies: 3
Views: 235

### 15.3

Why don't we divide the average rate by the number of moles.

In this problem, when I found reaction rate of NO2, I got 0.0065 mole/Ls

Why wouldn't I divide or multiply that number by 2 based off the equation given of 2NO2 --> 2NO + O2?
Tue Mar 06, 2018 3:21 pm
Forum: General Rate Laws
Topic: Products and Rate Law
Replies: 3
Views: 229

### Products and Rate Law

Let's say you were given a table of concentrations (reactants and products) at some point in a reaction. When product is produced, it inhibits the reactionn. The equation is A + B --> C A & B are each first order, and C is negative one order. Would the rate law be: Rate=k[A] 1 [B] 1 [C] -1 Is th...
Tue Mar 06, 2018 2:59 pm
Forum: First Order Reactions
Topic: pseudo rxn
Replies: 4
Views: 355

### Re: pseudo rxn

So, this is a theorhetically application and not really applicable to real life reactions of multiple reactions in terms of integrated rate laws?
Tue Mar 06, 2018 2:54 pm
Forum: General Rate Laws
Topic: Conceptual meaning behind rate law
Replies: 1
Views: 115

### Conceptual meaning behind rate law

What does the rate law mean?

Rate=k[A]

It is just dependent on the concencration of [A]? Not time?
Tue Mar 06, 2018 2:52 pm
Forum: General Rate Laws
Topic: Products having an effect on rate law
Replies: 1
Views: 148

### Products having an effect on rate law

Whta if the creation of products inhibits the rate of the forward reaction? How would this be expressed in the rate law equation
Tue Mar 06, 2018 2:49 pm
Forum: First Order Reactions
Topic: pseudo rxn
Replies: 4
Views: 355

### pseudo rxn

Can someone explain the conceptual meaning and purpose behind a pseudo reaction
Tue Feb 27, 2018 12:05 pm
Forum: Calculating Work of Expansion
Topic: Midterm Q4A [ENDORSED]
Replies: 4
Views: 314

### Re: Midterm Q4A[ENDORSED]

You can find the internal pressure of the system using the constants you were given. I did that on the test, and I found that the internal pressure of was not the same as the external pressure
Tue Feb 27, 2018 11:57 am
Forum: General Rate Laws
Topic: Writing a rate equation
Replies: 3
Views: 225

### Writing a rate equation

Why do I only include reactants in my rate law

rate=k[Reactants]

How would I express it in terms of products?
Tue Feb 27, 2018 11:55 am
Forum: Zero Order Reactions
Topic: K0 vs K1
Replies: 2
Views: 225

### K0 vs K1

What is the difference between k0 and k1?

Why is k0 for zero order, but k1 for first, second, third, etc order???? Why not k2 for second order?
Mon Feb 19, 2018 11:50 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 14.47 Mistake in Notation?
Replies: 1
Views: 152

### 14.47 Mistake in Notation?

In 14.47, the question provides the cell diagram notation: Pt(s) / Sn4+(aq), Sn2+(aq) // Pb4+(aq), Pb2+(aq) / C(gr) Shouldn't it be reactants / products? Because when I went to do the problem, the manual (and I) did the half reactions and the anode was: Sn2+ --> Sn4+ + 2e- Shouldn't it be written Sn...
Mon Feb 19, 2018 11:44 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Rule of Thumb for Cell Diagrams
Replies: 3
Views: 193

### Rule of Thumb for Cell Diagrams

How should I go about writing the notation for a cell diagram. Can someone break it down for me because some of the things I see in solution manual confuses me
Mon Feb 19, 2018 11:41 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: 14.91 Electrolyte
Replies: 2
Views: 224

### 14.91 Electrolyte

What does it mean conceptually when it says "A negatively charged electrolyte flows from the cathode to the anode," which completes the circuit. Is this the salt bridge?
Mon Feb 19, 2018 8:40 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Cell Potential - Intensive Property
Replies: 2
Views: 193

### Re: Cell Potential - Intensive Property

If you multiply an equation by a coefficient in order to perform "Hess' Law" on it, you do not multiple the cell potential by the coefficient as you would enthalpy/entropy
Mon Feb 19, 2018 8:36 pm
Forum: Balancing Redox Reactions
Topic: 14.23 part b
Replies: 1
Views: 414

### 14.23 part b

What exactly is the cathode with cell potential +0.92V in this reaction, 2Hg2+ + 2Br- --> Hg2 2+ + Br2?
Explain
Mon Feb 19, 2018 7:15 pm
Forum: Balancing Redox Reactions
Topic: 14.15 part b
Replies: 2
Views: 222

### 14.15 part b

I am having trouble understanding how to write a cell diagram notation. In part b, the answer is Pt/O2/OH-//H+/O2/Pt from the equation H+ + OH- --> H2O

How do I know what element/part of equation goes where in the cell diagram notation
Mon Feb 19, 2018 5:25 pm
Forum: Balancing Redox Reactions
Topic: 14.11
Replies: 2
Views: 228

### 14.11

can someone do step by step of part d. I don't understand why, in the solutions manual, it says:

O2 +2H2O + 4e- --> 4OH- for the cathode (positive E value). OH- is listed before O2, so shouldn't the O2 be on the right side and OH on the left with a positive E value
Mon Feb 19, 2018 5:06 pm
Forum: Balancing Redox Reactions
Topic: 14.9
Replies: 2
Views: 207

### 14.9

When calculating the n (moles) value in the gibbs free energy equation using cell potential, how do I get that.

How does one get 2 moles from the equation:

2Ce4+ + 3I- --> 2Ce3+ + I3
Mon Feb 19, 2018 5:00 pm
Forum: Balancing Redox Reactions
Topic: 14.5 part d
Replies: 7
Views: 394

### 14.5 part d

For the reduction reaction of P4 --> 4PH3

Why do I not add an H2O on right side (the side that is reduced state) just like it did in all the previous problems
Mon Feb 19, 2018 4:20 pm
Forum: Balancing Redox Reactions
Topic: 14.5 part a
Replies: 2
Views: 212

### 14.5 part a

How is O3 --> O2 being reduced, according to this question?

If the oxidation charge on the left is -6 and it is going to -4, isn't that oxidation? Am I doing this wrong?
Mon Feb 19, 2018 3:21 pm
Forum: Balancing Redox Reactions
Topic: 14.1 chromium
Replies: 1
Views: 118

### 14.1 chromium

How am I supposed to know that in Cr2O7 2-, Cr has a +6 charge? How would I go about figuring it out?
Tue Feb 13, 2018 8:39 pm
Forum: Calculating Work of Expansion
Topic: W=-P(deltaV) sign
Replies: 3
Views: 219

### Re: W=-P(deltaT) sign

p delta V?
Tue Feb 13, 2018 8:38 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: change in pressure
Replies: 1
Views: 139

### Re: change in pressure

pv=nrt
Tue Feb 13, 2018 8:35 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Equilibrium Equation
Replies: 1
Views: 109

### Re: Equilibrium Equation

Q is when the concentrations/partial pressures change in a system. You use Q to compare to K to see which way the reaction shifts
Tue Feb 13, 2018 6:08 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: 9.63
Replies: 1
Views: 159

### 9.63

Te questions asks to examine whether compounds are stable with respect to decomp. Th delta G of formation of PCl5 is a negative number (-305kJ), therefore stable. HOWEVER, I am confused because it says with respect to decomposition (PCl5 ---> products) So wouldn't the gibbs free energy be delta G = ...
Tue Feb 13, 2018 5:25 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: 9.47 [ENDORSED]
Replies: 1
Views: 176

### 9.47[ENDORSED]

In an isothermal, irreversible free expansion for this question, how do we maintain the temperature if there is no heat input or work despite a volume change since delta S surr = 0?
Mon Feb 12, 2018 3:47 pm
Forum: Calculating Work of Expansion
Topic: q and temp increase
Replies: 2
Views: 177

### q and temp increase

If the q of a reaction is negative, does that mean that the temperature is decreasing.

If delta U is negative, does that indicate a temperature decrease
Mon Feb 12, 2018 3:25 pm
Forum: Calculating Work of Expansion
Topic: equilbrium = reversible
Replies: 3
Views: 237

### equilbrium = reversible

Is something at equilibrium considered reversible? Is is only reversible if at equilibrium?
Mon Feb 12, 2018 3:00 pm
Forum: Calculating Work of Expansion
Topic: Free Expansion
Replies: 2
Views: 210

### Free Expansion

Is

delta U=0
q=0
w=0
delta H=0

in a free expansion?

Can free expansion only occur in an isolated system? Are they synonymous?
Mon Feb 12, 2018 2:57 pm
Forum: Calculating Work of Expansion
Topic: Isolated System
Replies: 5
Views: 365

### Isolated System

Is

delta U=0
q=0
w=0
delta H=0

in an isolated system?
Mon Feb 12, 2018 2:45 pm
Forum: Calculating Work of Expansion
Topic: Free Expansion
Replies: 4
Views: 303

### Free Expansion

Are the values of Free Expansion the same as the values for an Isolated System in terms of deltaU, w, q, and deltaH?

All would be equal to zero?

Are free expansion and isolated system synonymous?
Mon Feb 12, 2018 2:38 pm
Forum: Calculating Work of Expansion
Topic: 8.16 Constant Volume
Replies: 1
Views: 146

### 8.16 Constant Volume

Closed System with Constant Volume.
Always True, Always False, or True Under Certain Circumstances:

1. delta u=0
2. w=0
3. w<0
4. delta U=q
5. delta U=w

I am confused on this and do not know the answers
Mon Feb 12, 2018 2:27 pm
Forum: Calculating Work of Expansion
Topic: Delta P
Replies: 2
Views: 213

### Re: Delta P

I know that at constant pressure, delta h = qp.

If not at constant pressure then what does delta H equal?

If w=-PdeltaV, would a changing pressure make it w=-deltaPV?
Mon Feb 12, 2018 1:03 pm
Forum: Calculating Work of Expansion
Topic: Delta P
Replies: 2
Views: 213

### Delta P

How does delta P come into play in all of this (whether at constant volume or delta V)? How will the equations/values change for delta H, delta U, q, and w?
Mon Feb 12, 2018 1:01 pm
Forum: Calculating Work of Expansion
Topic: constant pressure impact
Replies: 1
Views: 96

### Re: constant pressure impact

Sorry I mean CHANGING PRESSURE
Mon Feb 12, 2018 1:00 pm
Forum: Calculating Work of Expansion
Topic: constant pressure impact
Replies: 1
Views: 96

### constant pressure impact

Can someone help me understand what delta H and delta U will be like when PRESSURE is constant in terms of their formulas? What about work and heat?
Sun Feb 11, 2018 10:04 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: delta S total
Replies: 2
Views: 186

### delta S total

Based on 2nd law of thermodynamics,

Is delta S total of universe = 0?

Or is it constantly increasing? Delta S total of universe = - limit to infinity?
Sun Feb 11, 2018 10:03 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: Reversible/equilibrium
Replies: 1
Views: 174

### Reversible/equilibrium

Are all reversible reactions at equilibrium since delta S total=0
Sun Feb 11, 2018 10:01 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Water effect 11.115
Replies: 2
Views: 363

### Water effect 11.115

Why is there little change to the reaction when I add water to this reaction.

Wouldn't increasing the amount of water cause favoring of the forward reaction?
Sun Feb 11, 2018 7:13 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: H2(g) --> H2(g) Entropy
Replies: 1
Views: 149

### H2(g) --> H2(g) Entropy

Let's say we have H2(g) in some larger equation and we are attempting to determine the delta G for this rxn. Exluding the other products and reactants, I want to look at just changes in the values associated with H2. Would the standard molar entropy be 0 (or 130.6 as I found in the Appendix) Would d...
Sun Feb 11, 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: H2(g) --> H2(g)
Replies: 2
Views: 227

### H2(g) --> H2(g)

Let's say we have H2(g) in some larger equation and we are attempting to determine the delta G for this rxn. Exluding the other products and reactants, I want to look at just changes in the values associated with H2. Would the standard enthalpy of formation be zero? Would the standard molar entropy ...
Sun Feb 11, 2018 6:15 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: Equilibrium and entropy
Replies: 1
Views: 177

### Equilibrium and entropy

Can/does delta S total=0 when delta G=0? Explain

Does delta G=0 mean its reversible?
Sun Feb 11, 2018 6:14 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: delta Stotal
Replies: 1
Views: 162

### delta Stotal

Why does deltaS total = 0 in a reversible reaction?
Sun Feb 11, 2018 6:13 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: REsidual vs postional [ENDORSED]
Replies: 1
Views: 128

### REsidual vs postional[ENDORSED]

What's the difference between residual and positional entropy?
Sun Feb 11, 2018 6:12 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: Finding entropy of vaporization for water at room temp
Replies: 2
Views: 242

### Finding entropy of vaporization for water at room temp

Why does it make sense that I can calculate the entropy of vaporization of water at 25 degrees Celsius (there is a problem in the book that deals with 85 degrees celcius)? Would I be asked to find the enthalpy of vaporization of water at 25 degrees? And the answer would be different than water's del...
Sun Feb 11, 2018 6:06 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: Delta S total
Replies: 2
Views: 215

### Delta S total

Under what conditions does delta Ssys = -delta Ssurr?

In other words, when does delta Stotal = 0
Thu Feb 08, 2018 9:54 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: delta G=0
Replies: 6
Views: 386

### delta G=0

When delta G=0, does that mean delta H and delta S are both zero too? Or they have numerical values that cancel?
Thu Feb 08, 2018 7:15 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: 11.19
Replies: 1
Views: 99

### Re: 11.19

ln(K) = 184.42

To isolate K, use e as a base as according to log rules. e will cancel out the natural log.

e^ln(K) = e^184.42

K=1.2 x 10^80
Thu Feb 08, 2018 7:09 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Circumstances of delta G
Replies: 3
Views: 202

### Circumstances of delta G

In what circumstance will delta G be zero?

When will delta S be zero?

(Please do not speak in terms of the values of enthalpy and entropy. I want a conceptual explanation, not a numerical one)
Thu Feb 08, 2018 7:07 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: delta G formation
Replies: 2
Views: 220

### delta G formation

What was stated in class about H2(g) --> H2(g) and its delta G of formation/entropy?

I know for this reaction, delta H of formation would be zero.
Is delta G of formation zero?
Does that mean delta S is zero too?
Thu Feb 01, 2018 11:35 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: bond enthalpy [ENDORSED]
Replies: 4
Views: 271

### bond enthalpy[ENDORSED]

does polarity of a molecule have anything to do with bond enthalpy?
Thu Feb 01, 2018 11:30 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: entropy to surroundings
Replies: 3
Views: 209

### entropy to surroundings

The second law states that disorder is always increasing, thus if I have a reaction with decreasing entropy, then the surroundings would have increases entropy. How would I calculate this
Thu Feb 01, 2018 11:29 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Endergonic vs Endothermic
Replies: 2
Views: 177

### Endergonic vs Endothermic

Why would a reaction be endergonic if the enthalpy is exothermic. Doesn't that contradict itself?
Sat Jan 27, 2018 5:13 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: 9.7 and 9.11 formula differences
Replies: 4
Views: 310

### 9.7 and 9.11 formula differences

9.7 uses the formula, deltaS = n C ln(T2/T1)
9.11 uses the formula, deltaS = n R ln (T2/T1)
What's the difference? Why
Tue Jan 23, 2018 6:48 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.65
Replies: 6
Views: 323

### 8.65

Where does the third equation come from? The third equation being 2NO + 3/2O2 --> N2O5?
Tue Jan 23, 2018 5:10 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Enthalpy Change in a Closed System
Replies: 1
Views: 895

### Re: Enthalpy Change in a Closed System

Depends on the system. In this class, we usually deal with systems at constant pressure or constant volume, usually constant temperature too. So if you have a net production of gas (change in moles) than the pressure would increase in response to more frequent collisions. However, in this class, I w...
Tue Jan 23, 2018 5:04 pm
Forum: Phase Changes & Related Calculations
Topic: Steam
Replies: 7
Views: 753

### Re: Steam

Steam causes burns because the amount of heat that 100*C of gas has compared to 100*C of liquid is much higher because the heat of vaporization requires a lot of heat input before phase change (liquid to gas)
Tue Jan 23, 2018 5:02 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Hess' Law fractions
Replies: 3
Views: 625

### Hess' Law fractions

Why is it okay/normal to put fractions in these equations to perform Hess' Law? For example, H2 + 1/2O2 --> H20
Tue Jan 23, 2018 5:00 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: delta H = delta U
Replies: 1
Views: 177

### delta H = delta U

When does delta H equal delta U?
Tue Jan 23, 2018 5:00 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Isothermal expansion
Replies: 3
Views: 197

### Isothermal expansion

Why does delta U = 0 in an isothermal expansion. q=-w? Explain
Sat Jan 20, 2018 5:26 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.117
Replies: 2
Views: 208

### 8.117

I don't understand how the net moles produced was 2/3.

Also, where did the 1/3 applied to the delta H come from at the end of the solution?
Sat Jan 20, 2018 5:18 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.51 [ENDORSED]
Replies: 2
Views: 152

### Re: 8.51[ENDORSED]

Enthalpy of formation is a given, experimental value discovered in a lab. We can use these values to find enthalpy of reaction.
Sat Jan 20, 2018 5:16 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Pressure's Relationship with Volume
Replies: 2
Views: 127

### Pressure's Relationship with Volume

In class, I questioned, "If volume is decreasing in a system, wouldn't the pressure increase as well?" Lavelle answered that the pressure would remain constant because the temperature would decrease, which would relive the tension of confined space. But, why is this? Why and how would the ...
Sat Jan 20, 2018 5:12 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.115
Replies: 2
Views: 185

### 8.115

Why is it not final minus initial in this problem for p1v/rt - p2v/rt?

n1 - n2 instead of n2 - n1
Sat Jan 20, 2018 3:08 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Pressure
Replies: 2
Views: 104

### Re: Pressure

I meant to say "decreasing volume" after "In reality" ^
Sat Jan 20, 2018 3:05 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.85 part b
Replies: 1
Views: 120

### 8.85 part b

Why can I not us the conversion ml to grams in this situation? It makes me use PV=nRT
Sat Jan 20, 2018 2:46 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Pressure
Replies: 2
Views: 104

### Pressure

Why does a system cool when the volume is decreased? I am aware that we are dealing with constant pressure so it would have to cool to maintain constant pressure but how exactly does that happen? In reality, wouldn't decreasing the pressure increase atom collisions which then causes increase in ener...
Sat Jan 20, 2018 2:41 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.73a
Replies: 1
Views: 144

### Re: 8.73a

Think of a resonance structure, the C-H are bounded and when you have 3 moles of c2h2, you are not breaking those c-h bonds or forming. You are putting the c-c bonds together. Look up c6h6 on internet.
Thu Jan 18, 2018 11:23 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.67 Enthalpy of Formation
Replies: 2
Views: 126

### 8.67 Enthalpy of Formation

In this question, it asks to find the enthalpy of formation of various substances, like H20. Why are the answers different than the enthalpy of formation found in Appendix A2. The delta H formation of H20 in this problem is -242kj In Appendix A2, the delta H formation of H20 is -285kj Both are in li...
Wed Jan 17, 2018 12:05 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.53
Replies: 1
Views: 101

### 8.53

Does Delta U = q in this problem since there is no mention of work? Is there an assumption no work is done?
Wed Jan 17, 2018 12:03 am
Forum: Administrative Questions and Class Announcements
Topic: When is HW due?
Replies: 2
Views: 308

### Re: When is HW due?

During discussion!
Wed Jan 17, 2018 12:00 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.51
Replies: 2
Views: 150

### Re: 8.51

The heat of formation for N2 and O2 is zero because that is their most stable form
Tue Jan 16, 2018 11:56 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.65 and 8.67
Replies: 2
Views: 198

### 8.65 and 8.67

In both of these homework problems, the solution manual provides necessary equations out of thin air: for 8.65, N2(g) + (5/2)O2(g) --> N2O5(g) for 8.67, H2(g) + (1/2)O2(g) --> H2O(l) Where do these equations come from? For question 8.65, I found the correct answer without using that equation. I wasn...
Tue Jan 09, 2018 9:35 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: J vs. KJ [ENDORSED]
Replies: 2
Views: 157

### Re: J vs. KJ[ENDORSED]

It's safe to use joules throughout the math process in this section since R=8.314J/Kmol and since 1atmL = 101.325J.

For sig figs, I think it would be smart to always convert final answer to kJ
Tue Jan 09, 2018 9:32 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Potential Energy
Replies: 1
Views: 167

### Potential Energy

Since potential energy is related to position, why, in an isothermal expansion, is there no potential energy change as molecules move further apart?
Tue Jan 09, 2018 9:26 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Delta U
Replies: 3
Views: 325

### Delta U

Delta U means change in internal energy, but what does this mean in simple terms? When is delta U = 0?
Sun Dec 10, 2017 12:09 am
Forum: Lewis Acids & Bases
Topic: CONJUGATE
Replies: 3
Views: 252

### Re: CONJUGATE

Explain
Sat Dec 09, 2017 11:47 pm
Forum: Lewis Structures
Topic: Same Formal Charge and Different Stability? [ENDORSED]
Replies: 2
Views: 247

### Re: Same Formal Charge and Different Stability?[ENDORSED]

Avoid putting a charge on the central atom. N should have a formal charge of zero. Also, are you sure N is in the middle?
Sat Dec 09, 2017 11:37 pm
Forum: Lewis Acids & Bases
Topic: CONJUGATE
Replies: 3
Views: 252

### CONJUGATE

I know an acid produces a conjugate base, but what does this mean. Is the conjugate base, basic?
Sat Dec 09, 2017 11:36 pm
Forum: Lewis Acids & Bases
Topic: Conjugate Acids and Bases
Replies: 1
Views: 174

### Conjugate Acids and Bases

In a rxn involving a strong acid, is the conjugate base weak?

In a rxn with weak acid,is conjugate base strong?
Sat Dec 09, 2017 11:32 pm
Forum: Bronsted Acids & Bases
Topic: 12.25?
Replies: 3
Views: 568

### Re: 12.25?

Memorize :(
Sat Dec 09, 2017 8:43 pm
Forum: Shape, Structure, Coordination Number, Ligands
Topic: What part of ligand attached
Replies: 2
Views: 230

### What part of ligand attached

In the case of the CO ligand, would I attach C or O to the central atom.
Sat Dec 09, 2017 8:18 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Adding a compound to reaction
Replies: 2
Views: 1879

### Re: Adding a compound to reaction

Question 37
Sat Dec 09, 2017 8:08 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Adding a compound to reaction
Replies: 2
Views: 1879

### Adding a compound to reaction

NH4HS(s) <---> NH3(g) + H2S(g)

What would happen in PbCl2(s) was added to products side.

This is from Lyndon's practice test. He said would shift to the left. I thought added a solid would have no effect, even if it has nothing to do with compounds involved. He claimed Q>K
Sat Dec 09, 2017 7:40 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: if Kc is Halved
Replies: 1
Views: 469

### if Kc is Halved

If Kc is x at 400K, what happens when all the concentrations are halved.

On the practice test, Lyndon put square root x, but when I used numbers, I got 2x
Sat Dec 09, 2017 7:33 pm
Forum: Significant Figures
Topic: Logarithms
Replies: 3
Views: 290

### Re: Logarithms

If he didn't mention it in class, I wouldn't worry about it.
Sat Dec 09, 2017 7:27 pm
Forum: Hybridization
Topic: Paramagnetic vs diamagnetic
Replies: 2
Views: 206

### Re: Paramagnetic vs diamagnetic

An unpaired electron acts like a magnet. So fluorine with 5 e in p subshell, because of its unpaired electron, is weakly attracted into a magnetic field, and is said to be paramagnetic. All of calcium's electrons are in filled subshells. It has no unpaired elecectrons and is, therefore, not attracte...
Sat Dec 09, 2017 6:07 pm
Forum: Photoelectric Effect
Topic: Photoelectric Experiment Question
Replies: 1
Views: 183

### Re: Photoelectric Experiment Question

v=h/m*wavelength is deBroigle, for everything other than photons

E=hv only applies to PHOTONS because photons travel at speed of light (c=wavelength*v)
Sat Dec 09, 2017 6:02 pm
Forum: Ionic & Covalent Bonds
Topic: C2+ C2-
Replies: 4
Views: 1016

### Re: C2+ C2-

c2- has two extra electrons, therefore, it has a higher pulling power since more negative charge
Sat Dec 09, 2017 6:00 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: SiO2 Polar or Nonpolar
Replies: 2
Views: 5875

### Re: SiO2 Polar or Nonpolar

Hi Ally ;)

This question is talking about the bond polarity not the compound polarity. The dipole moments between Si and O are not equal. As a compound , yes, it is nonpolar