## Search found 59 matches

Fri Mar 16, 2018 2:42 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: State Functions
Replies: 4
Views: 459

### Re: State Functions

q and w are NOT state functions.

delta U, delta H, q, etc. are state functions
Fri Mar 16, 2018 2:41 am
Forum: Administrative Questions and Class Announcements
Replies: 5
Views: 614

### Re: Class Study Google Doc

Fri Mar 16, 2018 2:38 am
Forum: Balancing Redox Reactions
Topic: Balancing in a basic solution
Replies: 4
Views: 2653

### Re: Balancing in a basic solution

For basic soln's you use the same method as acidic, except you balance the H with H2O and OH-, because you cant add in H+ since that would make it acidic
Thu Mar 15, 2018 1:36 pm
Forum: *Alkanes
Topic: Multiple Functional Groups [ENDORSED]
Replies: 2
Views: 778

### Re: Multiple Functional Groups[ENDORSED]

Just want to confirm - we don't need to learn Newman diagrams for the exam right? I don't remember it being covered in lecture... just functional groups + line diagrams.
Wed Mar 14, 2018 10:54 pm
Forum: Administrative Questions and Class Announcements
Topic: Lavelle: Professor by day, wizard by night
Replies: 6
Views: 1069

### Lavelle: Professor by day, wizard by night

Found this gem last night when I was looking through old Chemistry Community posts and it's actually super helpful. Skip to 1:01 to see Lavelle's acting debut :D

Mon Mar 12, 2018 2:13 am
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: 15.71
Replies: 1
Views: 162

### 15.71

How do you know OH- is a catalyst?
Mon Mar 12, 2018 1:41 am
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: Kinetic Energy vs Potential Energy
Replies: 2
Views: 225

### Kinetic Energy vs Potential Energy

In explaining the reaction profile graph for an exothermic reaction (Figure 15.30), the book states: "A reaction profile shows the potential energy of the reactants and products, their total energy arising from their relative location, not their relative speed. Now consider what happens when re...
Mon Mar 12, 2018 1:10 am
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: 65.b
Replies: 2
Views: 228

### 65.b

Could we know the reaction is endothermic without plotting a graph?
Mon Mar 12, 2018 12:28 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Example 15.7
Replies: 2
Views: 196

### Example 15.7

There were no HW problems assigned that followed the same procedure for setting up an overall rate law from a proposed mechanism (as in Example 15.7) with steady-state approximation. Will we need to know this method for the final?
Sun Mar 11, 2018 11:49 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Pre-equilibrium Condition
Replies: 3
Views: 353

### Pre-equilibrium Condition

Can someone elaborate on this concept? I don't really understand the way that they described in in the textbook. "The assumption that the rate of consumption of the intermediate in the slow step is insignificant relative to its rates of formation and decomposition in the first step is called a ...
Sat Feb 24, 2018 10:25 pm
Forum: Second Order Reactions
Topic: Question 51 in the homework [ENDORSED]
Replies: 4
Views: 1179

### Re: Question 51 in the homework[ENDORSED]

Why is the speed of the rxn relevant in this problem? (slow vs fast)
Sat Feb 24, 2018 10:21 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: 15.47
Replies: 1
Views: 163

### 15.47

Is there a specific method of going about this problem? I keep getting confused keeping track of each of the molecules because they all look so similar
Sat Feb 24, 2018 9:47 pm
Forum: Second Order Reactions
Topic: 15.35
Replies: 2
Views: 229

### 15.35

How would you find k in this equation? I don't understand where the solution manual got the equation they used
Tue Feb 20, 2018 12:14 am
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Question 23 part a
Replies: 1
Views: 184

### Question 23 part a

How did they get n = 6 moles for the delta G calculation?
Mon Feb 19, 2018 6:59 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Choosing a salt for a salt bridge
Replies: 1
Views: 462

### Choosing a salt for a salt bridge

Why is KCl often used for salt bridges? Are there any salts that you definitely would not want to use for cells and why
Mon Feb 19, 2018 6:32 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Galvanic Cells vs Electrochemical Cells
Replies: 4
Views: 460

### Galvanic Cells vs Electrochemical Cells

Is there a difference between Galvanic cells and electrochemical cells?
Tue Feb 13, 2018 8:16 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Relationship between free energy and work
Replies: 2
Views: 244

### Relationship between free energy and work

What is the relationship between free energy and work?

Also, what does Gibb's free energy mean exactly? Is it just the amount of energy you have free to drive a reaction?
Tue Feb 13, 2018 2:45 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Cp values
Replies: 1
Views: 153

### Cp values

Hi

Was just wondering why, conceptually, the Cp value for a substance as a liquid is greater than the Cp value for that same substance as a gas.

Don't gasses have higher entropy values since they are more disordered and can occupy larger volumes?

Thanks!
Fri Feb 09, 2018 11:54 am
Forum: Biological Examples (*DNA Structural Transitions, etc.)
Topic: 9.73
Replies: 2
Views: 534

### 9.73

I understand a,b,and c.

But why for d is it delta S total > 0 for a spontaneous reaction? What equation tells us this?

Stotal = Ssurr+Ssytem
Stotal = Ssystem-(deltaH/T)

???
Fri Feb 09, 2018 11:47 am
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: Which molecule has higher Standard Molar Entropy
Replies: 1
Views: 2219

### Which molecule has higher Standard Molar Entropy

I noticed there were many questions in the homework (#27,29,31) in which we would be given two molecules and were asked which of them was predicted to have a higher molar entropy. From my interpretation, the trends are as follows. Molecules have a higher molar entropy IF: -they are more massive -the...
Fri Feb 09, 2018 11:28 am
Forum: Biological Examples (*DNA Structural Transitions, etc.)
Topic: 9.69
Replies: 2
Views: 838

### Re: 9.69

Is it because the second reaction, when using 3 mol NADH, produces 6 e-'s? Therefore the third reaction must be multiplied by 3 to also get 6 electrons?
Fri Feb 09, 2018 11:24 am
Forum: Biological Examples (*DNA Structural Transitions, etc.)
Topic: 9.69
Replies: 2
Views: 838

### 9.69

In the textbook, this problem encompasses the formation of ATP and how much Gibbs free energy can be released based on 3.00 mols of NADH, which is appears in a separate reaction. I am trying to follow along in the solutions manual. I understand why they multiply the second reaction by 3, since the q...
Tue Jan 30, 2018 1:12 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: 9.25
Replies: 3
Views: 289

### 9.25

Is there a general rule to figuring out how many possible arrangements an atom can have based on its lewis structure? In 25 we are given the molecule SO2F2. The lewis structure is tetrahedral but has 6 possible orientations. I am having trouble visualizing the orientations and figuring out the numbe...
Tue Jan 30, 2018 12:34 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: 9.11
Replies: 2
Views: 202

### 9.11

This homework problem says to calculate the change in entropy when the pressure of 1.50 mol Ne(g) is decreased isothermally from 15.0 atm to 0.500 atm. Assume ideal behavior. I believe we uses the equation delta S = n*R*ln(P2/P1) But what R value do we use? The 8.314 J/K*mol value wouldnt cancel wit...
Sat Jan 27, 2018 4:56 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: 9.12 Textbook Example : Calculating the total entropy change for the expansion of an ideal gas
Replies: 1
Views: 100

### 9.12 Textbook Example : Calculating the total entropy change for the expansion of an ideal gas

Could someone help break down this example conceptually?

It says to calculate change in S, change in S of surroundings, and change in S total for

a) the isothermal, reversible expansion

b) the isothermal, free expansion of 1.00 mol of ideal gas molecules from 8 to 20 L at 292 K.
Thu Jan 25, 2018 1:46 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: Molar Entropy vs Residual molar entropy
Replies: 2
Views: 245

### Molar Entropy vs Residual molar entropy

Could someone explain the difference conceptually and mathematically to me?

I thought they were both calculated the same using the Boltzmann equation???
Mon Jan 22, 2018 4:45 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: HW #113
Replies: 1
Views: 125

### HW #113

I am confused about part b of this problem. It asks us to calculate the enthalpy change for the production of 200 L of hydrogen at 500 Torr and 65C by the reaction: C(s) + H2O(g) --> CO(g) + H2(g) The solution manual says to first calculate the moles using the ideal gas law pv=nrt (also convert to c...
Mon Jan 22, 2018 4:37 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Exothermic Reactions
Replies: 2
Views: 156

### Exothermic Reactions

Question 111 describes an exothermic reaction.

Part c asks "would the reactor need to be cooled or heated to maintain a constant temperature?"

The solution manual says "highly exothermic reaction, so the reactor would need to be cooled."

Can someone elaborate on this?
Mon Jan 22, 2018 4:24 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: 109
Replies: 2
Views: 448

### 109

Coal fired steam engines get their power by using the heat from burning coal to boil water. Suppose that coal of density 1.5 g/cm^3 is carbon. The combustion of carbon is described by the equation C(s) + O2(g) --> CO2(g) and delta H = -394 kJ a) calculate the heat produced when a lump of coal of siz...
Mon Jan 22, 2018 12:45 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: 8.99 Negative Specific Heat Capacity
Replies: 3
Views: 279

### Re: 8.99 Negative Specific Heat Capacity

When you say "it" lost heat, what exactly do you mean? The system lost heat to the surroundings? Since our delta H value for 1 mol of zinc was -20kJ, this means the reaction is exothermic and gives off heat to the surroundings. So does this mean that in an exothermic reaction, the Cs value...
Fri Jan 19, 2018 1:02 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: (g) vs (l) state of H2O in combustion rxn. Homework 93
Replies: 2
Views: 220

### (g) vs (l) state of H2O in combustion rxn. Homework 93

#93b asks to calculate the standard enthalpy of the reaction in the combustion of 1.00 C6H6(l) at 25 C and 1 bar. They do not give the combustion reaction, but the solutions manual states the reaction as C6H6(l) + 15/2O2(g) --> 6CO2(g) + 3H2O(l). How do we know the states of the reactants and produc...
Fri Jan 19, 2018 12:51 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: q and delta H interchangeable in calculations?
Replies: 4
Views: 788

### q and delta H interchangeable in calculations?

If heat and enthalpy are by definition different, then why in some problems (like 93), we use them interchangeably? For example, we calculate the work in part a, then the delta H in part b, then in part c it asks us for delta U (which is q+w). But the solutions manual simply added part a (w) and par...
Fri Jan 19, 2018 12:31 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Homework 87
Replies: 2
Views: 120

### Homework 87

I was able to follow the math for the 5 steps all the way through the problem, I just had a question about the final signs of the delta H values you add at the end. Why are they all positive?

Are phase changes and heat capacity enthalpies always positive?
Thu Jan 18, 2018 7:46 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 67.a
Replies: 1
Views: 91

### 67.a

How do we know that the reaction of H2(g) + 1/2O2(g) --> H2O(g) is exothermic? When I calculated the bond enthalpies and did products - reactants, I got +242 KJ, but the solutions manual says -242 KJ.
Wed Jan 17, 2018 10:27 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.73a
Replies: 3
Views: 266

### 8.73a

Could someone explain the negative signs in this reaction? I was able to get up to the point where you have the enthalpy values to break the bonds, but from there, I don't understand why in the solutions manual it is -6*518, and +3*837. Do you add the two numbers after that?
Mon Jan 15, 2018 7:28 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Enthalpy vs Heat
Replies: 2
Views: 201

### Enthalpy vs Heat

Could someone explain the difference between enthalpy (H) and heat (q)?
Sat Dec 09, 2017 1:39 pm
Forum: Photoelectric Effect
Topic: Photoelectric Experiment Question
Replies: 1
Views: 197

### Photoelectric Experiment Question

In one photoelectric experiment the wavelength of an electron emitted from a metallic surface is 4.00e-10 m. What is the velocity of the ejected electron in this experiment? So I immediately thought to use KE = 1/2(mv^2), then since we know E = hc/wavelength, we could combine the equations to get hc...
Sat Dec 09, 2017 12:34 pm
Forum: Hybridization
Topic: CH3NNCH3
Replies: 1
Views: 846

### CH3NNCH3

Q3A. Identify the hybrid orbitals used by the atom in boldface and underlined in the following molecules: I attached a picture of the question. I drew the lewis structures to find out the hybridization, and I got the first one right but I'm having trouble figuring out how to draw the lewis structure...
Sat Dec 09, 2017 10:45 am
Forum: Lewis Acids & Bases
Topic: Lewis Acids/Bases vs Bronsted Acids/Bases
Replies: 3
Views: 297

### Lewis Acids/Bases vs Bronsted Acids/Bases

Does anyone have any tricks to memorize the definition of lewis acids/bases vs bronsted acids/bases? I always confuse the definitions
Sun Nov 26, 2017 11:26 pm
Forum: Non-Equilibrium Conditions & The Reaction Quotient
Topic: 11.37
Replies: 3
Views: 327

### 11.37

Can someone explain the logic behind the solution to this question? For the reaction N (g) + 3 H (g) <--> 2 NH (g) at 400. K, K = 41. Find the value of K for each of the following reactions at the same temperature: (a) 2 NH3(g) <--> N2(g) + 3 H2(g) (b) 1/2 NH3(g) + 3/2N2(g) <--> NH3(g) (c) 2 N2(g) +...
Sun Nov 26, 2017 11:21 pm
Forum: Naming
Topic: Polydentate
Replies: 3
Views: 301

### Re: Polydentate

How do you tell if a ligand is polydendate? Do you have to draw it out or can you tell just by the name? And when you draw it out, what are you looking for?
Wed Nov 22, 2017 10:54 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: 11.43
Replies: 1
Views: 223

### 11.43

Consider the reaction 2 NO(g) <--> N2(g) O2(g). If the initial partial pressure of NO(g) is 1.0 bar, and x is the equilibrium concentration of N2(g), what is the correct equilibrium relation? (a) K = x2/(1.0 - x) (b) K = x2 (c) K = x2/(1.0 - 2x)2 (d) K = 4x3/(1.0 - 2x)2 (e) K = 2x/(1.0 - x)2. I thou...
Sun Nov 19, 2017 6:16 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Why lone pairs occupy equatorial plane
Replies: 2
Views: 1004

### Why lone pairs occupy equatorial plane

Can someone explain why lone pairs occupy the equatorial plane as opposed to the axial plane? I read the section in the textbook about it, it was talking about achieving lowest energy and the experimental lengths of the bonds in the axial and equatorial plane in a PCL5 molecule, but I just don't con...
Sun Nov 19, 2017 5:56 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: AX2E2
Replies: 2
Views: 563

### AX2E2

Why is a molecule such as H2S not linear? Since there are two pairs of lone electron pairs, wouldnt it make sense that they arrange themselves to give the molecule a linear shape because the angle would then be 180 degrees, which is farther away then a bent shape. Isn't the whole point of the VSEPR ...
Sun Nov 19, 2017 5:51 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Octahedral vs Trigonal Bypyramidal family
Replies: 3
Views: 352

### Octahedral vs Trigonal Bypyramidal family

In the trigonal bypyramidal family, I know there are always 5 regions of electron density around the central atom, and there is the axial and equatorial plane. When you start to account for lone pairs, they occupy the regions of electron density in the equatorial plane (see-saw, T-shaped). But for s...
Sun Nov 12, 2017 5:19 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Thiosulfate ion shape
Replies: 2
Views: 711

### Thiosulfate ion shape

Why is the (S2O3)2- ion tetrahedral and not see saw shaped?

What is the method for deciding what the shape is after you determine the VSEPR formula (i.e. AX4E1 for this ion)? I never know which side to put the lone pair
Sun Nov 12, 2017 3:51 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Textbook Problem Chp 4 #9
Replies: 3
Views: 256

### Textbook Problem Chp 4 #9

4.9 (a) What is the shape of an ICl3 molecule (iodine is the central atom) I drew the Lewis structure and made sure the formal charge was equal to zero for all atoms. Iodine had 2 lone pairs of e- and 1 single bond to each Cl. Therefore, since there were 5 regions of electron density, the electron a...
Thu Nov 02, 2017 7:38 pm
Forum: Lewis Structures
Topic: Expanded rules
Replies: 4
Views: 198

### Re: Expanded rules

If you are in the 3rd period, your n = 3. Therefore, your l value can be 0, 1, or 2. 0 = s, 1 = p, 2 = d, so any element in period 3 or below has a d-block and can therefore have an expanded octet
Mon Oct 30, 2017 11:53 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Removing an Electron [ENDORSED]
Replies: 5
Views: 502

### Re: Removing an Electron[ENDORSED]

Is this the same case for adding in electrons in the case of anion electron configurations? We always add to the outermost electron shell?
Mon Oct 30, 2017 11:52 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: test 3
Replies: 2
Views: 291

### Re: test 3

Lavelle posted an announcement on his website for the content on test 3 -- just 1.6 and all of chapter 2. It's also helpful to look at the outlines on his website which give you homework problems that match the concepts we need to know for the test.
Mon Oct 30, 2017 11:49 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Anion vs Cation electron configuration
Replies: 2
Views: 1299

### Anion vs Cation electron configuration

I know to determine the electron configuration for cations, you would remove electrons from the outermost shell. But to write the electron configurations for anions, do you add electrons to the outermost shell too? So in the order p, then s, then d? Same as the order you'd take electrons away in cat...
Mon Oct 30, 2017 11:33 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: 2.47 [ENDORSED]
Replies: 2
Views: 341

### Re: 2.47[ENDORSED]

You have to consider the electron configurations for each of the atoms, and then look at the outer most energy level, which would be where an electron would be removed to form a +1 ion. For example, Ge = [Ar]3d104s24p2. Therefore, to pluck off an e- to make a +1 ion, you would take it from the 4p or...
Tue Oct 24, 2017 10:40 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: #55 d
Replies: 3
Views: 253

### #55 d

For Cu, Ag, and Au, why is the electron configuration ns1(n-1)d^10? Instead of ns^2(n-1)d^9?
Tue Oct 24, 2017 10:39 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: #55 part c
Replies: 1
Views: 148

### #55 part c

Can someone explain why the notation for the valence shell configuration of the group 5 transition metals are (n-1)d^5ns^2? I thought it would be ns2np5(n-1)d^3. Where did the p shell go?
Wed Oct 18, 2017 12:05 pm
Forum: Heisenberg Indeterminacy (Uncertainty) Equation
Topic: Module Question 18
Replies: 3
Views: 444

### Re: Module Question 18

Would you plug in the mass of a hydrogen atom? So the atomic mass 1.01?
Wed Oct 18, 2017 11:42 am
Forum: Heisenberg Indeterminacy (Uncertainty) Equation
Topic: Post Module Question #16
Replies: 2
Views: 243

### Post Module Question #16

16. Which one of the following statements is correct in describing Heisenberg's uncertainty (indeterminacy) equation? A) The less precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa B) The more precisely the position is determined, the more ...
Thu Oct 12, 2017 1:38 pm
Forum: Accuracy, Precision, Mole, Other Definitions
Topic: Formula units E.9
Replies: 2
Views: 482

### Re: Formula units E.9

The formula would be MgSO4 * 7H2O. The molar mass would then be 246.48 g/mol. a) First, convert the 5.15 g MgSO4 * 7H2O to moles by dividing by the MM. = 2.09*10-2 moles. Then convert moles of MgSo4 * 7H20 to atoms of O. 2.09*10-2 moles MgSo4 * 7H20 * (11 mol O atoms/ 1 mol MgSo4 * 7H20 ) * (6.02*10...
Mon Oct 09, 2017 12:34 am
Forum: Molarity, Solutions, Dilutions
Topic: L39 [ENDORSED]
Replies: 1
Views: 216

### Re: L39[ENDORSED]

28.25-26.45=1.90g

a) convert 1.50 g of Sn to moles --> 1.264*10-2 mol Sn
b) convert .40 g of O to moles --> 0.025 mol O

Divide both moles of O and Sn by 1.264*10-2 to get empirical formula

SnO2 (Tin Oxide)
Mon Oct 09, 2017 12:26 am
Forum: SI Units, Unit Conversions
Topic: Significant Figures [ENDORSED]
Replies: 2
Views: 281

### Re: Significant Figures[ENDORSED]

4.465 with 3 sf's would be 4.46. If the last digit is 5, you round to the nearest even number (2.35 with 2 sf would be 2.4, and 2.65 would be 2.6). If the last digit is above 5, you round up (2 sf for 2.36 would be 2.4). If the last digit is below 5, you round down (1 sf for 2.36 is 2).