Search found 60 matches
- Wed Feb 27, 2019 8:36 pm
- Forum: Gibbs Free Energy Concepts and Calculations
- Topic: Predicting Gibbs Free Energy Sign
- Replies: 3
- Views: 685
Re: Predicting Gibbs Free Energy Sign
Gibbs Free Energy is dependent on temperature, entropy, and enthalpy. A high temperature can still be multiplied by a positive or negative value of change in entropy, so just knowing the temperature does not give the sign of the Gibbs Free Energy value.
- Wed Feb 27, 2019 8:31 pm
- Forum: Gibbs Free Energy Concepts and Calculations
- Topic: Determining Spontaneity
- Replies: 7
- Views: 990
Re: Determining Spontaneity
While exothermic reactions release energy, they can't be assumed to be spontaneous because a low entropy value could counter a negative enthalpy value ad make delta G positive, making it non-spontaneous.
- Wed Feb 27, 2019 8:24 pm
- Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
- Topic: le chatelier's principle and continuing rxns
- Replies: 2
- Views: 364
Re: le chatelier's principle and continuing rxns
Yes, this is more of a temporary fix in the presence of a lot of reactant. Eventually once it is all consumed the reaction cannot procees.
- Wed Feb 27, 2019 8:16 pm
- Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
- Topic: determining n
- Replies: 4
- Views: 528
Re: determining n
It depends on the the oxidation and reduction half reactions and how many electrons are transferred. It is necessary to multiply by the correct coefficients to find the least common multiple between the two half reactions to determine the correct number of electrons that are actually transferred.
- Wed Feb 27, 2019 8:02 pm
- Forum: Van't Hoff Equation
- Topic: Van't Hoff use
- Replies: 13
- Views: 2156
Re: Van't Hoff use
Usually a Van't Hoff question will give you the reaction enthalpy and ask you to either find a K value from an original K and two temperatures, or a T value from and original T and 2 equilibrium constants.
- Wed Feb 27, 2019 7:52 pm
- Forum: Gibbs Free Energy Concepts and Calculations
- Topic: constant N
- Replies: 2
- Views: 298
Re: constant N
Looks like you answered but in addition, make sure to write out both half reactions and multiply by coefficients as needed to find the true number of electrons transferred.
- Wed Feb 27, 2019 7:51 pm
- Forum: Gibbs Free Energy Concepts and Calculations
- Topic: Delta S Fusion
- Replies: 2
- Views: 1502
Re: Delta S Fusion
At constant temperature and pressure (fusion for example), the standard entropy of fusion is the standard deltaH over the temperature at which the fusion takes place.
- Wed Feb 27, 2019 7:48 pm
- Forum: Gibbs Free Energy Concepts and Calculations
- Topic: Thermodynamic Stability
- Replies: 2
- Views: 337
Re: Thermodynamic Stability
Just know that a negative Gibbs Free Energy corresponds to a thermodynamically stable reaction and vice versa. Labile and nonlabile refer to reaction rates.
- Wed Feb 27, 2019 7:36 pm
- Forum: Balancing Redox Reactions
- Topic: Balancing redox reactions
- Replies: 2
- Views: 321
Re: Balancing redox reactions
When it is necessary to balance hydrogens, H+ is added when the reaction is specified as acidic, and OH- and H2O are added when it is basic.
- Wed Feb 27, 2019 6:11 pm
- Forum: Balancing Redox Reactions
- Topic: Balancing Redox Reactions
- Replies: 3
- Views: 392
Re: Balancing Redox Reactions
The behavior of redox reactions is similar to a normal reaction in that molecules can cancel on either side of the arrow. So if a basic half reaction has 4 H2O on the left and 2 on the right, it can be simplified to just 2 H2O on the left.
- Wed Feb 27, 2019 5:54 pm
- Forum: Balancing Redox Reactions
- Topic: oxidation number
- Replies: 2
- Views: 333
Re: oxidation number
For our purposes in redox problems, the only relevant oxidation numbers are O (-2) and H (+1). These are the only elements coupled with metals, and the metals have an unspecified oxidation number that can be deduced from adding up the oxidation numbers of the O and H ions and then subtracting back t...
- Wed Feb 27, 2019 5:49 pm
- Forum: Balancing Redox Reactions
- Topic: Basic redox reactions
- Replies: 3
- Views: 443
Re: Basic redox reactions
A basic redox problem will not have H+ ions present. Balancing hydrogens is instead done with H2O and OH-. If the left side of the equation needs, for example, two more hydrogens to be balanced, instead add 2 H2O to the left side and 2 OH- on the right side, resulting in a net gain of only two hydro...
- Sat Feb 16, 2019 10:33 pm
- Forum: Calculating Work of Expansion
- Topic: Reversible vs irreversible
- Replies: 6
- Views: 911
Re: Reversible vs irreversible
In addition, a reversible reaction will instead have the equation w=-nrTln(V2/V1).
- Sat Feb 16, 2019 10:30 pm
- Forum: Calculating Work of Expansion
- Topic: delta U = 3/2nR(delta T)
- Replies: 2
- Views: 2561
Re: delta U = 3/2nR(delta T)
This equation is more used in this class to explain the heat capacity at constant volume/pressure to be 3/2R and 5/2R respectively. The equation itself is unlikely to appear on any exam.
- Sat Feb 16, 2019 10:16 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: Degeneracy
- Replies: 3
- Views: 614
Re: Degeneracy
Michelle Nwufo 2G wrote:Also, it’s kind of confusing why work and degeneracy have the same constant. Are they somehow related?
Work has a lowercase w, while degeneracy is an uppercase W. They are not related.
- Sat Feb 16, 2019 10:05 pm
- Forum: Concepts & Calculations Using First Law of Thermodynamics
- Topic: Delta U for Reversible and Irreversible
- Replies: 4
- Views: 1458
Re: Delta U for Reversible and Irreversible
In our class, (delta)U=0 for all reversible reactions. Irreversible ones will not equal zero if there is any expansion work. So if deltaV is not zero, there is energy transferred in the system.
- Sat Feb 16, 2019 9:52 pm
- Forum: Concepts & Calculations Using First Law of Thermodynamics
- Topic: Delta H
- Replies: 3
- Views: 483
Re: Delta H
Enthalpy by definition is heat at constant pressure, making it a state variable. So (delta)U= (delta)H + w at constant pressure
- Sat Feb 16, 2019 9:49 pm
- Forum: Concepts & Calculations Using First Law of Thermodynamics
- Topic: ΔU = 0?
- Replies: 4
- Views: 1018
Re: ΔU = 0?
In addition, reversible and isothermal for our purposes in 14B can be considered synonyms. So any reaction that is reversible is therefore isothermal, and vice versa. These reactions have a (delta)U of 0.
- Sat Feb 16, 2019 9:48 pm
- Forum: Concepts & Calculations Using First Law of Thermodynamics
- Topic: 3/2R vs 5/2R
- Replies: 8
- Views: 2104
Re: 3/2R vs 5/2R
In addition to the other answers, the heat capacity at constant volume and the heat capacity at constant temperature are always separated by a factor of R. (C)v,m = R + (C)p,m. So, for single atoms 3/2R is the (C)v,m and 5/2R is the (C)P,m, while linear atoms have a (C)v,m of 5/2R, and more complica...
- Sat Feb 16, 2019 9:37 pm
- Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
- Topic: Enthalpy of phase change
- Replies: 5
- Views: 821
Re: Enthalpy of phase change
When a phase change occurs in the reaction, there is an additional enthalpy change that must be accounted for on top of the normal reaction enthalpy. The enthalpies of molecules are different in different states.
- Sat Feb 16, 2019 9:16 pm
- Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
- Topic: Is there a significance with 298 degrees Kelvin?
- Replies: 5
- Views: 7626
Re: Is there a significance with 298 degrees Kelvin?
298 K is the standard temperature in an ideal gas equation and it is derived from 25 C + the conversion factor of 273. 298 seems like a random number but is in fact the Kelvin value in standard conditions.
- Sat Feb 16, 2019 9:11 pm
- Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
- Topic: methods
- Replies: 8
- Views: 974
Re: methods
There are three main methods. 1. Standard Enthalpy of Formation, given by the enthalpy of formation of each reactant and product 2. Hess's Law, given by several equations that need to be manipulated to add up to the given equation 3. Bond Enthalpy, given by the enthalpies of each bond in the reactan...
- Sat Feb 16, 2019 9:02 pm
- Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
- Topic: equations for q
- Replies: 9
- Views: 894
Re: equations for q
It depends on what the equation gives you. Usually the C(delta)T equation is only used in calorimeter problems. If you feel more comfortable with one or the other, you can easily convert between grams and moles with molecular weight.
- Sat Feb 16, 2019 8:59 pm
- Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
- Topic: Calculating deltaH of a reaction at a different temperature
- Replies: 2
- Views: 388
Re: Calculating deltaH of a reaction at a different temperature
In this type of problem, the total enthalpy is split into 3 parts: the enthalpy of bringing the reactants from their original temperature to the reaction temperature (modeled by mcdeltaT), the reaction enthalpy at the temperature given, and the enthalpy of bringing the products back to the original ...
- Sat Feb 16, 2019 8:47 pm
- Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
- Topic: enthalpy
- Replies: 2
- Views: 380
Re: enthalpy
The chemistry definition of enthalpy is the amount of energy held in chemical bonds. Bond enthalpy problems prove this fact as the breaking and forming of bonds sum up the change in enthalpy.
- Sat Feb 16, 2019 8:42 pm
- Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
- Topic: Temperature
- Replies: 3
- Views: 381
Re: Temperature
Significant figures are not emphasized in 14B. Step sizes between C and K are the same so a change in temperature can be expressed in either unit.
- Sat Feb 16, 2019 8:39 pm
- Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
- Topic: Bond Enthalpies
- Replies: 4
- Views: 468
Re: Bond Enthalpies
You could draw a simple Lewis structure, but most of the time the structure is given or is relatively easy to figure out so that you may focus strictly on adding and subtracting the bond enthalpies.
- Sat Feb 16, 2019 4:48 pm
- Forum: Thermodynamic Systems (Open, Closed, Isolated)
- Topic: Constant Pressure Calorimeter
- Replies: 5
- Views: 967
Re: Constant Pressure Calorimeter
Calorimeter problems are designed to evaluate reactions in an isolated system so it is understood that there is no interaction with the surroundings. This makes the heat gained/released of the elements inside equal to the heat gained/released by the calorimeter.
- Sat Feb 16, 2019 4:40 pm
- Forum: Thermodynamic Systems (Open, Closed, Isolated)
- Topic: Constant Pressure/Volume
- Replies: 6
- Views: 1012
Re: Constant Pressure/Volume
"Open beaker" implies constant pressure of 1 atm.
- Sat Feb 16, 2019 4:38 pm
- Forum: Thermodynamic Systems (Open, Closed, Isolated)
- Topic: systems [ENDORSED]
- Replies: 2
- Views: 622
Re: systems [ENDORSED]
The terms open/closed/isolated describe a system, not a reaction. So it has nothing to do with whether a reaction is exo/endothermic. Instead, it describes the system's ability to transfer energy and matter to its surroundings.
- Fri Dec 07, 2018 5:24 pm
- Forum: Determining Molecular Shape (VSEPR)
- Topic: Central atom
- Replies: 5
- Views: 533
Re: Central atom
The atom with the lowest ionization energy is in the center. I usually look for which atom is furthest to the bottom left of the periodic table. If there is only a single atom bonded to multiple of a different atom, then the single atom will be central. ex: NH3 has nitrogen in the center
- Fri Dec 07, 2018 5:01 pm
- Forum: Determining Molecular Shape (VSEPR)
- Topic: Correct Bond Angles
- Replies: 6
- Views: 684
Re: Correct Bond Angles
Lone pairs will always have a stronger repulsion force than atoms, so it is unlikely that the bond angles would be exactly 90 or 120. The lone pair makes each angle slightly smaller.
- Fri Dec 07, 2018 4:57 pm
- Forum: Determining Molecular Shape (VSEPR)
- Topic: Lone pairs on octahedral electron density
- Replies: 3
- Views: 403
Re: Lone pairs on octahedral electron density
The axes are not relevant for an octahedral shaped molecule because the shape is the same no matter how the molecule is rotated. So any of the planes can have the four atoms with the two lone pairs above and below.
- Fri Dec 07, 2018 4:52 pm
- Forum: Determining Molecular Shape (VSEPR)
- Topic: Determining polar and non polar molecules from a lewis structure
- Replies: 8
- Views: 2936
Re: Determining polar and non polar molecules from a lewis structure
Anjali_Kumar3E wrote:How do you know HCN is polar or non polar?
HCN is polar. Although the molecule is linear, the electronegativity difference makes the hydrogen side slightly positive and the nitrogen side slightly negative.
- Fri Dec 07, 2018 2:44 pm
- Forum: Determining Molecular Shape (VSEPR)
- Topic: radical
- Replies: 5
- Views: 663
Re: radical
Most of our practice problems feature molecules with an even number of total valence electrons so that every electron is paired. But, radicals are different in that there can be a single unpaired electron in special cases. Usually in the real world, a radical is an incomplete molecule that finds an ...
- Fri Dec 07, 2018 2:42 pm
- Forum: Determining Molecular Shape (VSEPR)
- Topic: VSEPR in equal angles
- Replies: 4
- Views: 789
Re: VSEPR in equal angles
If there are no lone pairs present, then the atoms will be symmetrically arranged around the center atom. If they are all the same element, then yes, the molecule will be non-polar. But if there is a large electronegativity difference, then the molecule could be polar.
- Fri Dec 07, 2018 2:40 pm
- Forum: Determining Molecular Shape (VSEPR)
- Topic: H-bonds
- Replies: 4
- Views: 473
Re: H-bonds
Bonds themselves are not polar or non-polar, only entire molecules have polarity. Hydrogen bonding is a dispersion force that causes molecules to be slightly attracted to each other if they have N, O, or F. But this force between separate molecules has no effect on the polarity of an individual mole...
- Fri Dec 07, 2018 2:37 pm
- Forum: Determining Molecular Shape (VSEPR)
- Topic: Trigonal Bipyramidal
- Replies: 6
- Views: 748
Re: Trigonal Bipyramidal
Maybe I'm just reading this wrong but are you referring to see saw? I don't think it's see saw, I think it is T-shape because I believe the structure being described has 3 bonds and 2 lone pairs, but maybe I also read it wrong. Yes, trigonal bipyramidal has five bonds and no lone pairs, while T-sha...
- Fri Dec 07, 2018 2:32 pm
- Forum: Determining Molecular Shape (VSEPR)
- Topic: Bent vs angular?
- Replies: 15
- Views: 1215
Re: Bent vs angular?
As everyone else has said, they are synonyms. But, the book says angular and that seems to be the more technical term.
- Fri Dec 07, 2018 2:30 pm
- Forum: Determining Molecular Shape (VSEPR)
- Topic: Polarity
- Replies: 2
- Views: 539
Re: Polarity
The VSEPR format allows you to be able to tell what the molecular geometry is. If the geometry is asymmetrical or the atoms have a large difference in electronegativity, the molecule is polar. Generally, any lone pairs will cause areas of asymmetry.
- Tue Nov 27, 2018 10:59 am
- Forum: Dipole Moments
- Topic: Polarity
- Replies: 9
- Views: 987
Re: Polarity
The molecule is linear, but since nitrogen is the least electronegative, it is the central atom. So the molecules are lined up N-N-O. Oxygen is much more electronegative than nitrogen making the molecule negatively charged on the oxygen side and therefore polar. Why are the molecules lined up N-N-O...
- Tue Nov 27, 2018 10:58 am
- Forum: Dipole Moments
- Topic: Polarity
- Replies: 9
- Views: 987
Re: Polarity
The molecule is linear, but since nitrogen is the least electronegative, it is the central atom. So the molecules are lined up N-N-O. Oxygen is much more electronegative than nitrogen making the molecule negatively charged on the oxygen side and therefore polar. Why is it lined up that way though. ...
- Mon Nov 26, 2018 2:30 pm
- Forum: Hybridization
- Topic: 2F.7
- Replies: 1
- Views: 276
2F.7
I am confused on how to convert the normal orbitals into hybrid orbitals on parts b, c, and d? How do you incorporate the electrons in the d block?
- Fri Nov 23, 2018 2:46 pm
- Forum: Dipole Moments
- Topic: Polar vs. Nonpolar
- Replies: 5
- Views: 496
Re: Polar vs. Nonpolar
It is based upon electronegativity, which is highest on the upper right of the periodic table. Non-polar molecules typically have a difference of 1.5 or less between the two atom's electronegativity values. Polar molecules typically have a 2.0 or greater difference. Symmetry and shape also play a pa...
- Fri Nov 23, 2018 2:37 pm
- Forum: Dipole Moments
- Topic: Bond Angles
- Replies: 8
- Views: 894
Re: Bond Angles
Tetrahedral is typically the angle that trips people up because it is assumed that it would be 90 degrees since there are 4 molecules, when in fact it is 109.5 degrees. All the others are fairly straightforward (ex: trigonal planar, 120, or linear, 180) or are estimated instead of exact values (ex: ...
- Fri Nov 23, 2018 2:33 pm
- Forum: Dipole Moments
- Topic: London forces and dipole moments
- Replies: 4
- Views: 469
Re: London forces and dipole moments
Bigger molecules and more polarizable molecules both have stronger of London forces. A bigger molecule has more electrons and therefore a higher chance of causing an instantaneous dipole moment (like you asked), so with more of these moments, the force exuded will be stronger.
- Fri Nov 23, 2018 2:27 pm
- Forum: Dipole Moments
- Topic: Polarity
- Replies: 9
- Views: 987
Re: Polarity
The molecule is linear, but since nitrogen is the least electronegative, it is the central atom. So the molecules are lined up N-N-O. Oxygen is much more electronegative than nitrogen making the molecule negatively charged on the oxygen side and therefore polar.
- Fri Nov 23, 2018 2:18 pm
- Forum: Dipole Moments
- Topic: Difference between dipole and london forces?
- Replies: 5
- Views: 529
Re: Difference between dipole and london forces?
Dipole-dipole forces are specifically between two polar molecules. The negatively charged side of one molecule will be attracted to the positive side of another. The force is strictly between two separate molecules. London forces, like dipole-dipole forces, are between two separate molecules. But Lo...
- Wed Nov 14, 2018 1:19 am
- Forum: Lewis Structures
- Topic: Expanded Octets
- Replies: 9
- Views: 2847
Re: Expanded Octets
Elements that have a d orbital can have expanded octets. However, a few elements on the right side of period 3 can expand with the extra electrons. S and Cl are common examples of central atoms with more than 8 electrons.
- Tue Nov 13, 2018 9:26 pm
- Forum: Bond Lengths & Energies
- Topic: 7th edition 2D. 13 [ENDORSED]
- Replies: 2
- Views: 465
Re: 7th edition 2D. 13 [ENDORSED]
You must draw out each Lewis structure to determine how many bonds are between the molecules in each specific example.
- Tue Nov 13, 2018 9:24 pm
- Forum: Bond Lengths & Energies
- Topic: bond length such as the midterm question
- Replies: 5
- Views: 631
Re: bond length such as the midterm question
Yes, the range of 120-140 would be correct. Most likely, any number between would be counted correct also.
- Sat Nov 10, 2018 6:35 pm
- Forum: Quantum Numbers and The H-Atom
- Topic: Degeneracy
- Replies: 8
- Views: 1405
Re: Degeneracy
Degeneracy is orbitals with the same energy.
- Tue Nov 06, 2018 11:45 am
- Forum: Formal Charge and Oxidation Numbers
- Topic: Stable Condition
- Replies: 8
- Views: 2580
Re: Stable Condition
The charge of individual atoms that make up the ion should always be zero or as close to zero as possible. Atoms are most stable at that point and they always tend towards their lowest energy state. Even in an ion with 2- charge, the formal charge of each atom should be close to 0, not -2.
- Tue Nov 06, 2018 11:37 am
- Forum: Octet Exceptions
- Topic: Central atoms with more than 8 electrons
- Replies: 3
- Views: 1775
Central atoms with more than 8 electrons
How can you tell when a central atom is allowed to have more electrons than the octet? I know the extra electrons go to the d-orbital, but it seems like some of the given atoms like P and S are in period three and have no d-orbital? It just seems a bit random as to when to put 5 or even 6 bonds.
- Sun Oct 14, 2018 7:46 pm
- Forum: Photoelectric Effect
- Topic: 1B.9: Trouble with exponents and units
- Replies: 3
- Views: 997
Re: 1B.9: Trouble with exponents and units
I started the same way you did and found the frequency using v=c/wavelength. I then used this frequency in the E=hv equation, which showed the energy per photon to be 4.7x10^-19. The lamp gives off 32 J/s so in 2 seconds, it released 64 J of energy. Dividing the 64 J of total energy by the 4.7x10^-1...
- Sun Oct 14, 2018 7:28 pm
- Forum: Properties of Light
- Topic: C=Wavelength*Frequency
- Replies: 11
- Views: 3273
Re: C=Wavelength*Frequency
SydBenedict3H wrote:Does it matter if you use 2.99x10^8 or 3.00x10^8? or are they close enough that the difference is indiscernible?
3.00x10^8 should be accurate enough. The constant will most likely be given on a test on the front page equation sheet. But either number works.
- Sun Oct 14, 2018 7:25 pm
- Forum: Properties of Light
- Topic: Colors and Frequency
- Replies: 14
- Views: 1682
Re: Colors and Frequency
Knowing the general range of the visible spectrum is a good idea. But we will not need to identify colors by their wavelengths, as most questions on the topic give both pieces of information.
- Sun Oct 07, 2018 10:19 pm
- Forum: Empirical & Molecular Formulas
- Topic: Question F9
- Replies: 5
- Views: 500
Re: Question F9
The question states in the form of a ratio, so in this case, I wrote it in C:H:O form. However if it doesn't specify, I would stick with the empirical formula.
- Sun Oct 07, 2018 10:15 pm
- Forum: General Science Questions
- Topic: Rusty on High School Chem [ENDORSED]
- Replies: 347
- Views: 465500
Re: Rusty on High School Chem [ENDORSED]
I'm in a similar place having not taken Chemistry in 4 years. I worried about this as well but I have come to realize that this class is lower division and therefore Lavelle is accustomed to teaching rusty students. He gave many useful resources including study sessions both on the Hill and on campu...
- Sun Oct 07, 2018 10:07 pm
- Forum: SI Units, Unit Conversions
- Topic: SI Units on tests
- Replies: 5
- Views: 566
Re: SI Units on tests
The standard SI units should be fine (m, kg, s, as well as L) and the prefixes should be converted back to their bases.