CHEMISTRY COMMUNITY

Jamie2002

(Assume the deltas are in the following equations...) G= H -TS G=H when TS=0 When is S=0? S is closest to 0 generally when x moles of solid go to x moles of solid. S is usually the most when x moles of solid/liquid go to kx moles of gas (a phase change occurs that increases entropy, k is some posit...

Jamie2002

When given a set of reactions to choose from, how would we determine which ones would have the most similar delta H naught and delta G naught?

Jamie2002

Because HNO 3 dissociates completely in solution, they can be written as aqueous ions in the chemical formula. In balancing the oxidation half-reaction, it becomes clear that NO 3 - is not one of the products of the reaction- the question is somewhat misleading when it suggests that Cu(NO 3 ) is be...

Jamie2002

I was confused on how to set up problems that require more than just adding H2O, OH-, or H+. An example is self test 6k.1A. How do you come up with the half reactions? Are you supposed to just ignore the "extra" species and count them as spectator ions?

Jamie2002

Thank you for asking this, I was also confused!

Jamie2002

Thank you all!!

Jamie2002

Hi, I was wondering what the general rule of thumb is for including Pt as our electrode. The textbook says we use it if the oxidized and reduced species are in the same solution, but what does this mean exactly? I have seen examples where Pt is used even when one species is a solid and the other is ...

Jamie2002

^This is the work for the problem from the solutions manual but I don't understand how they got this.

Jamie2002

For 6L.3d, we are asked to find the balanced equation from this cell diagram: Pt(s)|O2(g)|H+(aq)∥OH−(aq)|O2(g)|Pt(s). I don't understand how to set up the half reactions since the left hand side has the species O2 and H+, which don't have the same elements.

Jamie2002

E naught is 0 in a concentration cell, where the substances on either side are the same, just in different concentrations.

Jamie2002

Cell potential is an intensive property, so it does not depend on the number of moles, and you wouldn't need to multiply it by the coefficients.

Jamie2002

You need to balance the chemical reaction. Typically this happens with the same process you would use when splitting the reaction into half reactions. n is the number of electrons transferred once the reaction is balanced.

Jamie2002

It is always 0 in a concentration cell.

Jamie2002

They aren't always equal, but when they are, the maximum potential difference has been reached. No energy gets transferred.

Jamie2002

1. Split into 2 half reactions 2. Balance the oxygens by adding H2O to the appropriate side 3. Balance the H by OH- to the appropriate side 4. Balance the charge by adding e- to the appropriate side 5. Multiply the two half reactions by a coefficient so that the e- on either side can be cancelled ou...

Jamie2002

I think we would be provided the necessary values (like we were on midterm 2, when we had to find enthalpy, entropy, or Gibbs free energy)

Jamie2002

They will usually give you the phases in the original reaction, so just stick to those! Any water added is a liquid, ions are usually aqueous.

Jamie2002

1. Determine if it is acidic or basic 2. Split into 2 half reactions 3. Balance the oxygens by adding H2O to the appropriate side 3. Balance the H by adding H+ (for acidic) or OH- (for basic) to the appropriate side 4. Balance the charge by adding e- to the appropriate side 5. Make sure the equation...

Jamie2002

Yes, you would use it to fine the reduction potential for the elements involved in the reaction

Jamie2002

Just convert kPa to atm before proceeding with the problem, using 1 atm = 101.325 kPa.

Jamie2002

C∆T is generally used when calibrating calorimeters. q = nC∆T is used when substances are placed inside the calorimeter.

Jamie2002

only the equation sheet and periodic table that Dr. Lavelle provides is allowed.

Jamie2002

If you are trying to find the mols of gas using PV = nRT, use R= 0.08206 L (atm)/ (mol (K)). The units will cancel out and you will be able to find the number of mols.

Jamie2002

You should use Kelvin for most calculations, including this one!

Jamie2002

Look at the units in the problem you are working on. If Joules are involved, use 8.314 J/(K x mol); if atm is involved use 0.08206 (L x atm)/(mol x K)

Jamie2002

Sometimes we are not provided the mass of the substance so we cannot use specific heat capacity

Jamie2002

Yeah! m stands for mass while n stands for moles

Jamie2002

C is the specific heat capacity, not the heat capacity

Jamie2002

they're interchangeable if you have constant pressure!

Jamie2002

Combustion is exothermic because a water molecule is being formed. Formation of bonds releases energy, so it is exothermic.

Jamie2002

Extensive properties depend on the amount of matter that's being measured but intensive properties don't. Heat capacity would be an extensive property but molar heat capacity is an intensive property.

Jamie2002

That's correct! Just pay attention to the units you are working with to figure out which to use. They should cancel out with the units of the other numbers.

Jamie2002

Energy can't escape isolated systems because there is no interaction with surroundings. So, energy within the system is constant.

Jamie2002

compression

Jamie2002

A lower pKa corresponds to a stronger acid. Since strong acids have low pHs, lower pKa corresponds to a lower pH.

Jamie2002

This is a strong polyprotic acid and it deprotonates twice. You would have to use both Ka1 and Ka2. Since it's a strong acid, H2SO4 fully dissociates in the first deprotonation. However, for the second deprotonation you would have to set up an ICE table.

Jamie2002

It's 273.15 K for temperature, 1 bar (which is 100 kPa) for pressure

Jamie2002

Constant pressure doesn't mean no pressure! It just means that there is no change in final and initial pressure. It could be both 0 or a nonnegative number.

Jamie2002

For this one, it's good to look over all the answer choices first. Think about what is going on in the reaction. Is it combustion? Is something specific being formed?

Jamie2002

This quadratic equation calculator can also be used to check your work: https://www.mathpapa.com/quadratic-formula/

Jamie2002

This quadratic formula calculator can also be used to check your work: https://www.mathpapa.com/quadratic-formula/

Jamie2002

Hess's Law has to be used in the first problem because it involves N2 and O2, and both their standard enthalpies of formation are 0. Hess's Law is the method that works with these elements.

Jamie2002

That's right! It's only when there is a phase change that added heat does not increase the temperature. In these cases, the heat goes towards breaking the bonds between the molecules.

Jamie2002

From the syllabus: 4D: 1, 3, 5, 7, 9, 11,15, 17, 19, 21, 23; 4E: 5, 7, 9

Jamie2002

P = pressure
V = volume
n = number of mols
R = gas constant
T = temperature

Jamie2002

The P is just the notation we use, as to not confuse it with the notation used when using concentrations to find k.

Jamie2002

They're not the same. K is constant at all times except when there is a change in temperature. Q is the momentary ratio of products to reactants right after there is a change in pressure, volume, or concentration.

Jamie2002

The different constants are used depending on which units the problem uses (ie J vs L)

Jamie2002

On the formula sheet, is it 8.206 x 10^-2 (L x atm/ (K x mol)

Jamie2002

I believe it does follow the rule!

Jamie2002

Temperature is really important, especially if doing questions with PV=nRT!

Jamie2002

You can use PV=nRT

Jamie2002

https://i.ytimg.com/vi/54n1XppP-lA/hqdefault.jpg

Jamie2002

Here is a helpful worked example!

Jamie2002

claire ikemiya_2I wrote:Yes! All elements below row 3 can expand their octet.

Thank you!

Jamie2002

Jiwon_Chae_3J wrote:Cl can have an expanded octet but I think that Cl is the central atom here because it is the least numerous element in the compound, though I am not sure why this takes precedence over the electronegativity rule.

Thank you!

Jamie2002

Cl can have an expanded octet because its in the 3rd period on the periodic table, and the 3p elements and lower can have an expanded octet. I believe for this problem Cl would be the central atom for the sake of symmetry where it could have two O on either side that are double bonded, and a lone p...

Jamie2002

When drawing the Lewis Structure for ClO2+, I initially put O as the central atom because it is less electronegative than Cl and I thought that Cl only has one available bond (since it has 7 valence electrons). Does Cl have an expanded octet? Why it is the central atom if it is more electronegative?

Jamie2002

Steph Du 2B wrote:If the bond being broken for the compounds you are comparing is different, you compare bond length (ex. HCl and HBr). If the bond being broken is the same, then you compare electronegativity (ex. CCl3COOH and CH3COOH).

Thank you!

Jamie2002

Rose_Malki_3L wrote:First compare bond length. If bonds are the same in the compounds you are comparing then compare stability of the anions (so compare electronegativity)

Thank you!

Jamie2002

Bond length is mainly considered when comparing binary acids, such as HF, HCl, and HI. In this case, the acids with longer bonds will be stronger acids, as they have weaker bonds and can deprotonate easier. Electronegativity and how well the conjugate base anion dilutes the negative charge is used ...

Jamie2002

When determining relative acidity, do we consider bond length first or electronegativity? Which one is more important?

Jamie2002

It would also be helpful to note the ligands Dr. Lavelle mentions in lectures. Hope this helps!

Jamie2002

In his lecture, Dr. Lavelle said that stronger acids are ones that lose protons easily. This happens when the resulting anion is stabilized by resonance or electronegative atoms that stabilize the - charge by withdrawing electron density. For example, if two different atoms are bonded to an oxygen, ...

Jamie2002

Coordination numbers concern the atoms inside the coordination sphere (inside the brackets []).

Jamie2002

Delocalized bonds occur in resonance structures, where the electrons are equally shared between all the bonds.

Jamie2002

https://www.google.com/url?sa=i&url=htt ... AdAAAAABAD

Jamie2002

remember that the number of hybrid orbitals = the regions of electron density

Jamie2002

I agree, I think pi bonds should be stronger since they are present in double and triple bonds (which are stronger than single bonds) while sigma bonds are only in single bonds.

Jamie2002

here is a helpful image of the possible structures: https://haygot.s3.amazonaws.com/questio ... ff183f.png

Jamie2002

The third one is cyclic

Jamie2002

I believe these occur in resonance structures, where the electrons are delocalized.

Jamie2002

Since atoms bound by a sigma bond are free to rotate about their axis, I believe they can rotate both clockwise and counterclockwise.

Jamie2002

The hydrogen in one molecule is partially positive, and would form a hydrogen bond with an oxygen in another molecule, which is partially negative.

Jamie2002

I think you're right, but it has 3 hydrogen bonds when pairing with cytosine.

Jamie2002

Are boiling point and melting point directly related? Do molecules with higher melting points have stronger intermolecular forces?

Jamie2002

When a question asks for the electron spin, do we always say that it can either be +½ or -½? Or if we know that the electron in question is an unpaired electron, we can say that it has an electron spin of +½?

Jamie2002

Jagveer 2L wrote:We are not expected to know that. The questions on the midterm are going to be modeled after the problems given in the syllabus and 1D#5 was not on the syllabus, so I do not think that we will be quizzed on it.

Okay, thank you!

Jamie2002

Question:
Show that the electron distribution is spherically symetrical for an atom in which an electron occupies each of the three p-orbitals of a given shell.

The solution to this involves lots of equations with sin and cos. Are we expected to know how to do this for the midterm?

Jamie2002

You should know that electronegativity increases going from left to right across a period and decreases going down a group.

Jamie2002

I would draw out perodic tables and draw arrows describing each trend (i.e. for electronegativity, draw an arrow starting in the bottom left hand corner and going to the top right corner, where electronegativity is the highest). This is the most straightforward way to represent each trend.

Jamie2002

Dipole moments occur when there is a difference in electronegativity between two atoms. The larger the difference in electronegativity, the larger the dipole moment. Electrons are more drawn to the more electronegative atom, distorting the electron cloud around the molecule. Dipole moments occur whe...

Jamie2002

A hydrogen bond is a bond between an partially positive hydrogen atom of one molecule and a partially negative N, O, or F atom of a neighboring molecule. The lone pair in that N, O, or F atom is what interacts in the hydrogen bond. It is what gets drawn to the partially positive H atom.

Jamie2002

The more bonds there are, the less lone pairs the atoms will have (i.e. two atoms joined with a triple bond have less lone pairs than two atoms joined in a single bond because more electrons are involved in the bond). Thus, molecules with single bonds have more free electrons, which repel each other...

Jamie2002

It will probably be covered soon; in the meantime here is a helpful chart in case you want to take a look: https://www.google.com/url?sa=i&url=https%3A%2F%2Fchemistryonline.guru%2Foxidation-number%2F&psig=AOvVaw0JmA1MDHorFL6buG72dNmb&ust=1604961159100000&source=images&cd=vfe&...

Jamie2002

Here is a helpful chart: https://www.google.com/url?sa=i&url=htt ... AdAAAAABAD

Jamie2002

This sounds great! Thanks for organizing it

Jamie2002

Specific bond lengths are usually determined experimentally. You should just know that the more bonds there are, the shorter the bond length is (i.e. a triple bond is shorter and stronger than a single bond).

Jamie2002

The number of bonds that can be formed in general depend on the number of valence electrons an atom has. For example, carbon has 4 valence electrons and can therefore form 4 bonds with hydrogen (4 C-H bonds) to reach an octet. Oxygen has 6 valence electrons, so it needs 2 more electrons to reach an ...

Jamie2002

If you missed the workshop, I'd advise asking someone who did attend for the key. I think they provide answer keys at the session.

Jamie2002

I don't know if you can watch lectures offline, but the video modules on his website can be downloaded.

Jamie2002

The code should have been emailed to you soon after you bought the textbook. My email got lost in between all my other ones so I would suggest searching up "ASUCLA" or "UCLA Store" in your email to make sure that you see it. After you get the code, you will have access to both sa...

Jamie2002

I believe we can see scores at the end of Week 5!

Jamie2002

You can click on Quick Links on the upper left hand corner, which will take you to "Your Posts". You can see all the posts you made and make sure you posted 5 times this week.

Jamie2002

Wouldn't the uncertainty be 10 m/s, since it is (5 m/s + 5 m/s) - (5 m/s - 5 m/s)?

Jamie2002

Do we disregard the +/- 5.0 m/s^-1 because the problem calls for the minimum uncertainty?

Jamie2002

A bowling ball of mass 8.00 kg is rolled down a bowling alley lane at 5.00 ± 5.0 m⋅s^−1. What is the minimum uncertainty in its position? The textbook says the uncertainty in velocity is 5.00 m/s, but why? When given a number +/- another number, do we just disregard the second number (in this case t...

Jamie2002

The uncertainty in position should be 10m, not 5m. So you should use the following equation to solve for the uncertainty in momentum first: delta p >= h/(4pi)(delta x) when you plug everything in, you'd get delta p >= (6.626x10^-34 Js)/(4pi)(10m) and then delta p >= 5.2728 x 10^-36kgms^-1 After, yo...

Jamie2002

You are caught in a radar trap and hope to show that the speed measured by the radar gun is in error due to the Heisenberg uncertainty principle. If you assume that the uncertainty in your position was +/- 5 m when your speed was measured, and that the car has a mass of 2150 kg, what is your calcula...

Jamie2002

Before you divide h by v, I think you need to add together the kinetic energy of the ejected electron (E excess ) with the kinetic energy needed to remove that electron (E threshold ) to get the energy of the incident light, then find its frequency Adalia is right; energy must be conserved . Theref...

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