Search found 17639 matches

by Chem_Mod
Wed Aug 24, 2011 12:09 am
Forum: Properties & Structures of Inorganic & Organic Acids
Topic: Why acetic acid is weaker acid than formic acid?
Replies: 1
Views: 397

Why acetic acid is weaker acid than formic acid?

In what reaction does the -CH3 in acetic acid have proton donating capabilities that would make it weaker than formic acid?
by Chem_Mod
Tue Aug 23, 2011 11:33 pm
Forum: Octet Exceptions
Topic: Why expanded octet?
Replies: 1
Views: 297

Re: Why expanded octet ?

Answer: They are in the 3rd period and they do have empty 3d orbitals.
by Chem_Mod
Tue Aug 23, 2011 11:33 pm
Forum: Octet Exceptions
Topic: Why expanded octet?
Replies: 1
Views: 297

Why expanded octet?

Question: Why do S, Cl and P have expanded octets?
by Chem_Mod
Tue Aug 23, 2011 11:32 pm
Forum: *Making Buffers & Calculating Buffer pH (Henderson-Hasselbalch Equation)
Topic: Approximate Ka for ICE
Replies: 1
Views: 206

Re: Approximate Ka for ICE

Answer: Its definitely ok if K < 10-5. It's never ok if K > 10-3. If it's in between, you need to check your answer.
by Chem_Mod
Tue Aug 23, 2011 11:31 pm
Forum: *Making Buffers & Calculating Buffer pH (Henderson-Hasselbalch Equation)
Topic: Approximate Ka for ICE
Replies: 1
Views: 206

Approximate Ka for ICE

Question: When using the ICE chart, how small does K have to be in order to approximate it?
by Chem_Mod
Tue Aug 23, 2011 11:28 pm
Forum: Dipole Moments
Topic: Why is COCl2 polar?
Replies: 1
Views: 542

Re: Why is COCl2 polar?

Answer: Yes, since the dipoles do not cancel, it makes the molecule polar.
by Chem_Mod
Tue Aug 23, 2011 11:27 pm
Forum: Dipole Moments
Topic: Why is COCl2 polar?
Replies: 1
Views: 542

Why is COCl2 polar?

Question: Why is COCl2 polar? Are the electric dipoles of Cl signifigantly larger than O?
by Chem_Mod
Tue Aug 23, 2011 11:24 pm
Forum: *Indicators
Topic: Is Cu2+ acidic or basic? What indicator should be used?
Replies: 1
Views: 2073

Re: Is Cu2+ acidic or basic? What indicator should be used?

Answer: Since the oxidation state is Cu2+ it is expect to be basic. For indicators, the pK of the indicator should be equal to the stoichiometric point.
by Chem_Mod
Tue Aug 23, 2011 11:24 pm
Forum: *Indicators
Topic: Is Cu2+ acidic or basic? What indicator should be used?
Replies: 1
Views: 2073

Is Cu2+ acidic or basic? What indicator should be used?

Question: Transition metals act as Lewis acids and make hydronium ions, but O2- is also a very strong base, making hydroxides, so is CuO acidic or basic? How is an indicator chosen?
by Chem_Mod
Tue Aug 23, 2011 11:20 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: What is the geometry of CO2H2 ?
Replies: 1
Views: 424

Re: What is the geometry of CO2H2 ?

Answer: There are only three regions of electron density around C, so it is trigonal planar.
by Chem_Mod
Tue Aug 23, 2011 11:19 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: What is the geometry of CO2H2 ?
Replies: 1
Views: 424

What is the geometry of CO2H2 ?

Question: What is the geometry of CO2H2 around C? wouldn't it be trigonal planar?
by Chem_Mod
Tue Aug 23, 2011 10:50 pm
Forum: Ideal Gases
Topic: How to find delta n?
Replies: 1
Views: 19196

Re: How to find delta n?

Delta n is the difference in the number of moles of gas between reactants and products.
by Chem_Mod
Tue Aug 23, 2011 10:49 pm
Forum: Ideal Gases
Topic: How to find delta n?
Replies: 1
Views: 19196

How to find delta n?

For the ideal gas law, PV=nRT, sometimes "delta n" is used. How is delta n found?
by Chem_Mod
Tue Aug 23, 2011 10:49 pm
Forum: *Molecular Orbital Theory (Bond Order, Diamagnetism, Paramagnetism)
Topic: Assigning x,y,z when making molecular orbital diagram
Replies: 1
Views: 204

Re: Assigning x,y,z when making molecular orbital diagram

Since the bond axis is the z-axis, then the sigma bond is sigma(z) which makes the pi bonds x and y.
by Chem_Mod
Tue Aug 23, 2011 10:48 pm
Forum: *Molecular Orbital Theory (Bond Order, Diamagnetism, Paramagnetism)
Topic: Assigning x,y,z when making molecular orbital diagram
Replies: 1
Views: 204

Assigning x,y,z when making molecular orbital diagram

When drawing a Molecular Orbital Diagram does it matter which is labeled x, y, and z (as in the sigma or pi bonds)?
by Chem_Mod
Tue Aug 23, 2011 10:46 pm
Forum: *Indicators
Topic: Stoichiometric point explanation [ENDORSED]
Replies: 5
Views: 1550

Re: Stoichiometric point explanation [ENDORSED]

Answer: The stoichiometric point is when # moles H3O+(OH¯) = # moles OH¯(H3O+) in sample.
by Chem_Mod
Tue Aug 23, 2011 10:46 pm
Forum: *Indicators
Topic: Stoichiometric point explanation [ENDORSED]
Replies: 5
Views: 1550

Stoichiometric point explanation [ENDORSED]

Question: What is the stoichiometric point?
by Chem_Mod
Tue Aug 23, 2011 10:42 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Will reactants or products be favored if volume is decreased
Replies: 1
Views: 310

Re: Will reactants or products be favored if volume is decre

C is a solid C(s) and since gases are far more sensitive to pressure one should consider this 1mole gas --> 2mole gas.
by Chem_Mod
Tue Aug 23, 2011 10:42 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Will reactants or products be favored if volume is decreased
Replies: 1
Views: 310

Will reactants or products be favored if volume is decreased

For the equation: H2O + C <> H2 + CO (the 2's are subscripts), will reactants or products be favored if volume is decreased? Is it but isn't this a 2 to 2 mole ratio? (#9.81)
by Chem_Mod
Tue Aug 23, 2011 8:54 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: What is the difference between AX2E and AX2E2?
Replies: 1
Views: 2242

Re: What is the difference between AX2E and AX2E2 ?

Answer: AX2E is bent with bond angle near 120, while AX2E2 is bent with angle near 109.
by Chem_Mod
Tue Aug 23, 2011 8:54 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: What is the difference between AX2E and AX2E2?
Replies: 1
Views: 2242

What is the difference between AX2E and AX2E2?

Question: For VSEPR, are both AX2E and AX2E2 called angular/bent with angles of 120 degrees each, or does AX2E2 actually have a more tetrahedral arrangement, with a bond angle of slightly less than 109.5 degrees?
by Chem_Mod
Tue Aug 23, 2011 8:53 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Range of value of K is considered appropriate
Replies: 1
Views: 326

Re: Range of value of K is considered appropriate

There is no set value of K at which the assumption of neglecting x in the denominator can't be used. The point is that if K is quite large, then significant dissociation may occur at quite low concentrations and then you may wish to use the quadratic formula. The assumption is best for small K and h...
by Chem_Mod
Tue Aug 23, 2011 8:52 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Range of value of K is considered appropriate
Replies: 1
Views: 326

Range of value of K is considered appropriate

What range of value of K is considered appropriate to assume that x = 0? Is there a certain range for K in which we can use the 5% rule? If K < 1 * 10^-5, then can we assume that x = 0, but if K > 1 * 10^-5, then we have to use the quadratic formula to solve for K. In the final, if we know that K is...
by Chem_Mod
Tue Aug 23, 2011 8:50 pm
Forum: Accuracy, Precision, Mole, Other Definitions
Topic: Intensive and extensive property of matter
Replies: 1
Views: 1277

Re: Intensive and extensive property of matter

Answer: Intensive properties DO NOT depend on the amount of substance (density, pressure, temperature, molar energy, chemical potential).

Extensive DO depend on the amount of substance (mass, volume, energy).
by Chem_Mod
Tue Aug 23, 2011 8:50 pm
Forum: Accuracy, Precision, Mole, Other Definitions
Topic: Intensive and extensive property of matter
Replies: 1
Views: 1277

Intensive and extensive property of matter

Question: What are some examples of intensive and extensive properties? I know that volume is an extensive property and density is an intensive property,
by Chem_Mod
Tue Aug 23, 2011 8:44 pm
Forum: Empirical & Molecular Formulas
Topic: Rounding molecular ratios, to write proper molecular formula
Replies: 1
Views: 2019

Re: Rounding molecular ratios, to write proper molecular for

Answer: Yes just round off. No rule, but use common sense. You wouldn't round a number like 3.5 when it is clear that you can multiply it by 2 to get a more accurate answer.
by Chem_Mod
Tue Aug 23, 2011 8:43 pm
Forum: Empirical & Molecular Formulas
Topic: Rounding molecular ratios, to write proper molecular formula
Replies: 1
Views: 2019

Rounding molecular ratios, to write proper molecular formula

Question: After dividing by the smallest mole to find the ratio for the empirical formula, the ratios must be whole number. Can the answer be rounded off if the answer is 5.1 for example? Is there any rule for when to or when not to round off?
by Chem_Mod
Tue Aug 23, 2011 8:38 pm
Forum: Trends in The Periodic Table
Topic: What is ionization energy?
Replies: 1
Views: 405

Re: What is ionization energy?

Answer: Yes, although the general term for ionization energy would be the energy required to remove the outer most electron for an atom (g). In other words, not just any electron. Yes the energy released is related to electron affinity.
by Chem_Mod
Tue Aug 23, 2011 8:37 pm
Forum: Trends in The Periodic Table
Topic: What is ionization energy?
Replies: 1
Views: 405

What is ionization energy?

Question: Is "ionization energy" equal to "Energy(of a free electron) - Energy(of a bound electron in a specific energy level)"? Also, when a free electron becomes a bound electron, is the change in energy equal to "Energy(of bound electron in specific energy level) - Energ...
by Chem_Mod
Tue Aug 23, 2011 8:34 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Why isn't Iodine electron configuration half-filled?
Replies: 1
Views: 566

Re: Why iodine e-configuration doesn't choose to be half-fil

Answer: Only the transition metals gain stability with half filled (d5) and filled (d10) orbitals. Also in the case of iodine the e- would be going to a higher energy (s to p)
by Chem_Mod
Tue Aug 23, 2011 8:34 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Why isn't Iodine electron configuration half-filled?
Replies: 1
Views: 566

Why isn't Iodine electron configuration half-filled?

Question: The e- configuration of iodine is [Kr] 4d10 5s2 5p5. Why doesn't the p orbital take an electron from the s orbital since it is almost full to become [Kr] 4d10 5s1 5p6?
by Chem_Mod
Tue Aug 23, 2011 8:32 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: e- configuration for Cu, esp. 3d and 4s config
Replies: 1
Views: 223

Re: e- configuration for CU, esp. 3d and 4s conf

Answer: Cu is [Ar]3d10 4s1. 4s is higher than than 3d.
by Chem_Mod
Tue Aug 23, 2011 8:32 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: e- configuration for Cu, esp. 3d and 4s config
Replies: 1
Views: 223

e- configuration for Cu, esp. 3d and 4s config

Question: What is the e- configuration for Cu? Specifically, what is the order of the 3d and 4s orbitals?
by Chem_Mod
Tue Aug 23, 2011 8:30 pm
Forum: Formal Charge and Oxidation Numbers
Topic: How we determine formal charge?
Replies: 1
Views: 233

Re: How we determine formal charge?

Answer: The latter. That is, compare the formal charge of each atom as described in the solution.
by Chem_Mod
Tue Aug 23, 2011 8:30 pm
Forum: Formal Charge and Oxidation Numbers
Topic: How we determine formal charge?
Replies: 1
Views: 233

How we determine formal charge?

Question: How is the overall formal charge of the compound determined? Are the structures lower in energy if the sum of the formal charges of all the individual atoms is closer to zero? Or is the overall charge more stable if there are more individual atoms with a Formal Charge of zero in that mole...
by Chem_Mod
Tue Aug 23, 2011 8:15 pm
Forum: Lewis Structures
Topic: Lewis structure of CCl4
Replies: 1
Views: 596

Re: Lewis structure of CCl4

Answer: Yes.

Cl > 7e- x 4 = 28 e-

C > 4 e- = 4e-

28 + 4 = 32 e-
by Chem_Mod
Tue Aug 23, 2011 8:15 pm
Forum: Lewis Structures
Topic: Lewis structure of CCl4
Replies: 1
Views: 596

Lewis structure of CCl4

Question: For drawing a Lewis structure for CCl4, would there would be 32 valence electrons?
by Chem_Mod
Tue Aug 23, 2011 8:14 pm
Forum: Bond Lengths & Energies
Topic: Why the bond length for NN in H2NNH2 is 150pm?
Replies: 1
Views: 782

Re: Why the bond length for NN in H2NNH2 is 150pm?

Answer: It is a N--N single bond 2 x 75 = 150pm.
by Chem_Mod
Tue Aug 23, 2011 8:14 pm
Forum: Bond Lengths & Energies
Topic: Why the bond length for NN in H2NNH2 is 150pm?
Replies: 1
Views: 782

Why the bond length for NN in H2NNH2 is 150pm?

Question: The bond length for NN in H2NNH2 is 150pm. But shouldn't this be 110 (55+55) since the NN bond is triple, which has 55pm length?
by Chem_Mod
Tue Aug 23, 2011 8:11 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: e- configuration of SOCl2 and CO3
Replies: 1
Views: 409

Re: e- configuration of SOCl2 and CO3

Answer: SOCl2: S 6e- O 6e- 2 x Cl 14e- total = 26e- S has the lowest ionization energy, place it in the center. O needs a double bond has it has no charge (4e-) and 2 lone pairs (4e-). Each Cl has a single bond (4e-) and three lone pairs (12e-). So far used 24e-. Now place the two remaining e- as a...
by Chem_Mod
Tue Aug 23, 2011 8:11 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: e- configuration of SOCl2 and CO3
Replies: 1
Views: 409

e- configuration of SOCl2 and CO3

Question: What are the configurations for SOCl2 and CO3?
by Chem_Mod
Tue Aug 23, 2011 8:08 pm
Forum: Octet Exceptions
Topic: For GaI3, is Ga usually an exception to the octet rule?
Replies: 1
Views: 2304

Re: For GaI3, is Ga usually an exception to the octet rule?

Answer: Ga + 3I gives 24e-. One I has a double bond and the others single bonds. GaI is trigonal planar, AX3. Each atom has an octet.
by Chem_Mod
Tue Aug 23, 2011 8:08 pm
Forum: Octet Exceptions
Topic: For GaI3, is Ga usually an exception to the octet rule?
Replies: 1
Views: 2304

For GaI3, is Ga usually an exception to the octet rule?

Question: In drawing the Lewis structure for GaI3, is Ga usually an exception to the octet rule? Or would this molecule be considered electron deficient?
by Chem_Mod
Tue Aug 23, 2011 8:01 pm
Forum: *Molecular Orbital Theory (Bond Order, Diamagnetism, Paramagnetism)
Topic: What is the LUMO, pi2py* or sigma 2p*?
Replies: 1
Views: 218

What is the LUMO, pi2py* or sigma 2p*?

For the MO diagram for 02+ would the LUMO be pi2py* or sigma 2p*?
by Chem_Mod
Tue Aug 23, 2011 7:57 pm
Forum: Properties & Structures of Inorganic & Organic Acids
Topic: Why HClO2 is a stronger acid than HBrO2?
Replies: 6
Views: 6635

Re: Why HClO2 is a stronger acid than HBrO2?

It's the O--H bond that is broken. Also the resulting ClO2- is more stable due to Cl's higher electronegativity.
by Chem_Mod
Tue Aug 23, 2011 7:57 pm
Forum: Properties & Structures of Inorganic & Organic Acids
Topic: Why HClO2 is a stronger acid than HBrO2?
Replies: 6
Views: 6635

Why HClO2 is a stronger acid than HBrO2?

HClO2 is a stronger acid than HBrO2. Doesn't the fact that Cl and Br are in the same group and the Br-O bond is longer and weaker than the Cl-O bond outweigh the greater electronegativity of the Cl bond though?
by Chem_Mod
Tue Aug 23, 2011 7:55 pm
Forum: Bronsted Acids & Bases
Topic: Is NaOH as a whole a Bronsted base and Lewis acid?
Replies: 1
Views: 1051

Re: Is NaOH as a whole a Bronsted base and Lewis acid?

Answer: NaOH is a Bronsted base as it generates -OH. Most oxides are highly basic. e.g., CaO, MgO.
by Chem_Mod
Tue Aug 23, 2011 7:55 pm
Forum: Bronsted Acids & Bases
Topic: Is NaOH as a whole a Bronsted base and Lewis acid?
Replies: 1
Views: 1051

Is NaOH as a whole a Bronsted base and Lewis acid?

Question: Is NaOH as a whole a Bronsted base and Lewis acid? Or is it just the individual OH- ion that comes out of it that is considered the Bronsted base? Are oxides considered basic or acidic?
by Chem_Mod
Tue Aug 23, 2011 7:54 pm
Forum: Shape, Structure, Coordination Number, Ligands
Topic: How can aluminum and copper attach to (H20)6
Replies: 1
Views: 346

Re: How can aluminum and copper attach to (H20)6

Many metals form octahedral complexes with water as ligands. (Square planar and tetrahedral are less common).
by Chem_Mod
Tue Aug 23, 2011 7:53 pm
Forum: Shape, Structure, Coordination Number, Ligands
Topic: How can aluminum and copper attach to (H20)6
Replies: 1
Views: 346

How can aluminum and copper attach to (H20)6

How can aluminum and copper attach to (H20)6. Is there any general rule about this?
by Chem_Mod
Tue Aug 23, 2011 5:56 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Determing heat or entropy loss
Replies: 1
Views: 179

Re: Determing heat or entropy loss

In an isothermal free expansion, we know that dE = 0 since dT = 0 and we know that w = 0 because Pext = 0. Therefore, q = E - w = 0. This means that our system did not gain or lose heat in the process, thus the surrounding didn't gain or lose heat. We conclude that dS(surrounding) = q/T = 0
by Chem_Mod
Tue Aug 23, 2011 5:52 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Determing heat or entropy loss
Replies: 1
Views: 179

Determing heat or entropy loss

How do you know that there is no heat or entropy loss? Is this true for all irreversible reactions?
by Chem_Mod
Tue Aug 23, 2011 5:51 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Difference between reversible and equilibrium for entropy
Replies: 1
Views: 207

Re: Difference between reversible and equilibrium for entrop

dS = 0 is a condition for an equilibrium state, but they are not "the same". In an isothermal free expansion, we know that dE = 0 since dT = 0 and we know that w = 0 because \dpi{100} \fn_jvn p_{ext} = 0. Therefore, q = E - w = 0. This means that our system did not gain or lose heat in the...
by Chem_Mod
Tue Aug 23, 2011 5:51 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Difference between reversible and equilibrium for entropy
Replies: 1
Views: 207

Difference between reversible and equilibrium for entropy

The solution manual says that reversible means dS(total) = 0, which is also true at equilibrium, so that does that mean they are the same? How do you know that there is no heat or entropy loss? Is this true for all irreversible reactions?
by Chem_Mod
Tue Aug 23, 2011 5:50 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Need to use Kelvin for finding entropy?
Replies: 1
Views: 569

Re: Need to use Kelvin for finding entropy?

To be safe, convert all temperature information to Kelvin. Although, if you are only using differences in temperature, it's safe to remain in Celsius.
by Chem_Mod
Tue Aug 23, 2011 5:49 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Need to use Kelvin for finding entropy?
Replies: 1
Views: 569

Need to use Kelvin for finding entropy?

For calculating the entrophy, does the temperature have to convert to Kelvin even the temperature given in the problem is in C instead of K?
by Chem_Mod
Tue Aug 23, 2011 5:48 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Difference between entropy and enthalpy?
Replies: 1
Views: 970

Re: Difference between entropy and enthalpy?

Enthalpy is the heat absorbed/released by the system under constant temperature, wherease entropy is a measure of disorder. They are related, since at constate pressure, the change in enthalpy of a system is propotional to the change in entropy of the system, dH = T * dS, where T is the temperature ...
by Chem_Mod
Tue Aug 23, 2011 5:48 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Difference between entropy and enthalpy?
Replies: 1
Views: 970

Difference between entropy and enthalpy?

What is the main difference between entropy and enthalpy? is that the difference between change in H and change in S? thanks!
by Chem_Mod
Tue Aug 23, 2011 5:47 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Determining second order reaction using elementary steps
Replies: 1
Views: 216

Re: Determining second order reaction using elementary steps

The overall order for a reaction depends on the reaction mechanism. For an elementary step, you are right, the order would be 1 if there were just one reactant A. But a more complex reaction that is the sum of several elementary steps in a mechanism may have a rate law that doesn't exactly reflect t...
by Chem_Mod
Tue Aug 23, 2011 5:47 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Determining second order reaction using elementary steps
Replies: 1
Views: 216

Determining second order reaction using elementary steps

I don't understand why a second order reaction is sometimes based on the fact that there are two reactants, such as 2A or A+B, but can also be if there is just one reactant A.
by Chem_Mod
Tue Aug 23, 2011 5:45 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Finding concentration using half life in second order rxn
Replies: 1
Views: 272

Re: Finding concentration using half life in second order rx

No you can only do that for first order reactions. It is because first order has the only half life reaction that is independent of initial concentration. For second order reactions, you should just calculate k using the half-life and then plug the value into the integrated form of the rate law.
by Chem_Mod
Tue Aug 23, 2011 5:45 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Finding concentration using half life in second order rxn
Replies: 1
Views: 272

Finding concentration using half life in second order rxn

When we are given the half-life time for a first-order rxn, we can calculate the time that 1/4 of the concentration left by multiplying the half-life time by two. But can we use the same method for second-order rxn? Or is there another way?
by Chem_Mod
Tue Aug 23, 2011 5:44 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Why can't we find absolute value of internal energy?
Replies: 1
Views: 3689

Re: Why can't we find absolute value of internal energy?

Sometimes it is easier to measure a change than it is to measure an absolute value. For example. It is easier to measure how much energy is needed to remove an electron from an atom (the energy is called ionization energy). It is much harder to measure all the binding energy between the protons and ...
by Chem_Mod
Tue Aug 23, 2011 5:44 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Why can't we find absolute value of internal energy?
Replies: 1
Views: 3689

Why can't we find absolute value of internal energy?

The book says " we can't measure the absolute value of the internal energy of a system because that value includes the energies of all the atoms, thier electrons, and the components of their nuclei. Therefore the best we can do is measure change in the internal energy." Can you explain thi...
by Chem_Mod
Tue Aug 23, 2011 5:43 pm
Forum: Calculating Work of Expansion
Topic: Do surroundings do work?
Replies: 1
Views: 251

Re: Do surroundings do work?

Yes, you are correct. The external pressure is the force that opposes expansion in the cylinder.
by Chem_Mod
Tue Aug 23, 2011 5:43 pm
Forum: Calculating Work of Expansion
Topic: Do surroundings do work?
Replies: 1
Views: 251

Do surroundings do work?

A system is doing work on the surrounding if it expands against an external force. So is the surrounding constantly doing work as well because it keeps the piston in place?
by Chem_Mod
Tue Aug 23, 2011 5:42 pm
Forum: Calculating Work of Expansion
Topic: Which gas constant, R, to use?
Replies: 1
Views: 15617

Re: Which gas constant, R, to use?

In SI units, the real gas constant, R, is equal to 8.3145 Joules/mol K. However, if we express R in units of L atm/mol K, it's value is 0.08206. It's just a case of unit conversion. If we are calculating using the ideal gas equation PV=nRT then we use the .08206 because we will we will be calculatin...
by Chem_Mod
Tue Aug 23, 2011 5:42 pm
Forum: Calculating Work of Expansion
Topic: Which gas constant, R, to use?
Replies: 1
Views: 15617

Which gas constant, R, to use?

Can you clarify when R is 8.3145 and when R is .08206 ?
by Chem_Mod
Tue Aug 23, 2011 5:42 pm
Forum: Calculating Work of Expansion
Topic: Maximum work and difference between reversible and free
Replies: 1
Views: 223

Re: Maximum work and difference between reversible and free

The maximum amount of work for a given volume change occurs in an isothermal expansion of an ideal gas. In an isothermal free-expansion, no work is done because the gas expands against a vacuum, thus = 0. The latter expansion is also an example of an irreversible process.
by Chem_Mod
Tue Aug 23, 2011 5:41 pm
Forum: Calculating Work of Expansion
Topic: Maximum work and difference between reversible and free
Replies: 1
Views: 223

Maximum work and difference between reversible and free

The text talks about how a process produces maximum work if it takes place reversibley and that the work of a reversible reaction is more negative than the work of a irreversible reaction. Can you please elaborate on this? Also, what is the difference between an isothermal reversible expansion and a...
by Chem_Mod
Tue Aug 23, 2011 5:38 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Treat temperature as a constant during integration ok?
Replies: 1
Views: 238

Treat temperature as a constant during integration ok?

For a reversible, isothermal expansion of gas, can we take the temperature outside of the integral while calculating, considering the temperature should be constant?
by Chem_Mod
Tue Aug 23, 2011 5:38 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: q=0 in adiabatic closed system?
Replies: 1
Views: 264

q=0 in adiabatic closed system?

Is it always true that q=0 in an adiabatic closed system?
by Chem_Mod
Tue Aug 23, 2011 5:37 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Negative sign for calculating heat loss
Replies: 1
Views: 827

Re: Negative sign for calculating heat loss

The species that loses heat has the - sign in front of the q.
by Chem_Mod
Tue Aug 23, 2011 5:37 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Negative sign for calculating heat loss
Replies: 1
Views: 827

Negative sign for calculating heat loss

When you are asked for the specific heat capacity of the metal, you set the heat gained by water equal to the heat lost by the metal. However, in the solution they put the negative sign on the water. Why? Since the metal is losing heat shouldn't that be negative instead?
by Chem_Mod
Tue Aug 23, 2011 5:37 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Does heat capacity of water change with temperature?
Replies: 1
Views: 292

Re: Does heat capacity of water change with temperature?

Heat capacities for a particular substance only change with phase, not temperature.
by Chem_Mod
Tue Aug 23, 2011 5:32 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Does heat capacity of water change with temperature?
Replies: 1
Views: 292

Does heat capacity of water change with temperature?

Why the heat capacities of water at two different temperature are the same?
by Chem_Mod
Tue Aug 23, 2011 5:31 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Heat Capacity/Heating Curve
Replies: 1
Views: 241

Re: Heat Capacity/Heating Curve

A material with a low heat capacity can be heated faster. Therefore the slope of a heating curve is greater.
by Chem_Mod
Tue Aug 23, 2011 5:31 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Heat Capacity/Heating Curve
Replies: 1
Views: 241

Heat Capacity/Heating Curve

What does it mean to say that "the slope of a heating curve is greater for samples that have a low heat capacity than for those with a high heat capacity"?
by Chem_Mod
Tue Aug 23, 2011 5:30 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Use of a bomb calorimeter
Replies: 1
Views: 282

Re: Use of a bomb calorimeter

The purpose of a bomb calorimeter is to determine the enthalpy of combustion for hydrocarbons. Since combustion reactions are usually exothermic (give off heat), the enthalpy value is typically negative. In your text, it defines calorimetry as "an apparatus used to determine the heat released o...
by Chem_Mod
Tue Aug 23, 2011 5:30 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Use of a bomb calorimeter
Replies: 1
Views: 282

Use of a bomb calorimeter

Can you explain what goes on with a bomb calorimeter, and what is important about this type of problem?
by Chem_Mod
Tue Aug 23, 2011 5:29 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: What is enthalphy of neutralization?
Replies: 1
Views: 279

Re: What is enthalphy of neutralization?

Enthalpy of neutralization is the heat released at constant pressure in an acid/base reaction (i.e. neutralization).
by Chem_Mod
Tue Aug 23, 2011 5:28 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: What is Delta H vap= q/mol?
Replies: 1
Views: 739

Re: What is Delta H vap= q/mol?

is defined as the heat required at constant pressure to vaporize 1 mol of a substance. However, this is also equivalent to the heat required to vaporize any finite amount of a subtance divided by that amount of the substance represented in moles.
by Chem_Mod
Tue Aug 23, 2011 5:28 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: What is Delta H vap= q/mol?
Replies: 1
Views: 739

What is Delta H vap= q/mol?

Where does the equation " " come from and can it be applied to fusion, sublimation, etc.?
by Chem_Mod
Tue Aug 23, 2011 5:27 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Need to memorize enthalpy values?
Replies: 1
Views: 440

Re: Need to memorize enthalpy values?

H Vapor is the enthalpy of formation of 1 mole of gaseous water. H Liquid is the enthalpy of formation of 1 mole of liquid water. You do not need to memorize any enthalpy values.
by Chem_Mod
Tue Aug 23, 2011 5:26 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Need to memorize enthalpy values?
Replies: 1
Views: 440

Need to memorize enthalpy values?

When looking at the equations regarding the Enthalpies of Physical Changes, what do we enter for the Enthalpy values of certain phases. For example: Delta H Vapp = H Vapor - H Liquid What do we enter in the H Vapor and H liquid? Will there be a table given with these values? Do we need to memorize t...
by Chem_Mod
Tue Aug 23, 2011 5:22 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Using enthalpy of combustion of H2 for H2 + 1/2O2 -> H2O
Replies: 1
Views: 1107

Re: Using enthalpy of combustion of H2 for H2 + 1/2O2 -> H2O

You can't use the standard enthalphy of combustion of , because that will form liquid.
by Chem_Mod
Tue Aug 23, 2011 5:21 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Using enthalpy of combustion of H2 for H2 + 1/2O2 -> H2O
Replies: 1
Views: 1107

Using enthalpy of combustion of H2 for H2 + 1/2O2 -> H2O

If you have the equation \dpi{100} \fn_jvn H_{2}+\frac{1}{2}O _{2} -> H_{2}O and you want to find the reaction enthalpy I understand that you can just use the standard enthalpy of formation of \dpi{100} \fn_jvn H_{2}O but why can't you use the enthalpy of combustion of \dpi{100} \fn_jvn H_{2} . Isn'...
by Chem_Mod
Tue Aug 23, 2011 5:19 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Do we account for enthalpy of formation of elements
Replies: 1
Views: 324

Re: Do we account for enthalpy of formation of elements

When species are in their "naturally occuring" phases you don't have to form them considering they are already "there". So it follows that their enthalpies of formation are 0.
by Chem_Mod
Tue Aug 23, 2011 5:19 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Do we account for enthalpy of formation of elements
Replies: 1
Views: 324

Do we account for enthalpy of formation of elements

Why don't you have to take the enthalpies of formation of C and Fe into consideration when calculating enthalpy of reaction?
by Chem_Mod
Tue Aug 23, 2011 5:17 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Explaination of "stability"
Replies: 1
Views: 241

Re: Explaination of "stability"

In the limited sense, a substance A is said to be "stable" with respect to forming a substance B, when the process that takes you from A to B is endothermic. In other words, if a reaction requires an input of energy then the reactants are more stable than the products. The converse is true...
by Chem_Mod
Tue Aug 23, 2011 5:17 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Explaination of "stability"
Replies: 1
Views: 241

Explaination of "stability"

Concerning the standard enthalpy of formation, the book lists graphite as carbon in its most "stable" form and that carbon in diamond form requires +1.9 kJ to change the bonding arrangement to graphite. graphite however requires zero amount of heat. When a substance is designated as "...
by Chem_Mod
Tue Aug 23, 2011 5:16 pm
Forum: Phase Changes & Related Calculations
Topic: During phase change, why no temperature change?
Replies: 2
Views: 1335

Re: During phase change, why no temperature change?

Because of the phase change. Remember it takes heat to melt something. So all heat added to the system is used in the phase change, and it does not increase the temperature.
by Chem_Mod
Tue Aug 23, 2011 5:16 pm
Forum: Phase Changes & Related Calculations
Topic: During phase change, why no temperature change?
Replies: 2
Views: 1335

During phase change, why no temperature change?

In the lecture notes, it is stated that heat is required during melting or boiling and the temperature of the sample remains constant even though heat is being supplied. Why doesn't the temperature increase?
by Chem_Mod
Tue Aug 23, 2011 5:01 pm
Forum: *Cyclohexanes (Chair, Boat, Geometric Isomers)
Topic: Axial atoms in cyclohexane
Replies: 1
Views: 325

Re: Axial atoms in cyclohexane

In the chair conformation of cyclohexane, three H's point up and three H's point down. Theses six H's are axial hydrogens.

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