CHEMISTRY COMMUNITY

Joey_Okumura_1E

Thank you! That makes so much more sense. Another way to think about it is that we are combining the combustion reactions of C2H6, C2H2, and H2. Since we need C2H6 as a product (when it is a reactant in the combustion reaction), we use the reverse reaction and therefore multiply -1560 kJ/mol by -1.

Joey_Okumura_1E

What is the difference between a standard state and the most stable state for a compound?

Joey_Okumura_1E

Have we learned how to do problems 4D.3, 4D.9, and 4D.11, yet?

Joey_Okumura_1E

For this problem, why is the answer negative?

Joey_Okumura_1E

This question applies the concept that the standard enthalpy of a reaction is equal to the standard enthalpy of formation of the products minus the standard enthalpy of formation of the reactants.

Joey_Okumura_1E

When there is an increase in pressure due to compression, the system will shift towards the side of the reaction that exerts less pressure on its surroundings. Gases exert pressure on their surroundings while solids, liquids, and solutions exert a negligible amount of pressure. Therefore, in this ca...

Joey_Okumura_1E

The bond enthalpies for diatomic molecules are accurate because, in a diatomic molecule, there are no other atomic charges influencing the bond length. However, in molecules with more than two atoms, the atomic charges of atoms not attached to the bond in question affect the length of the bond. This...

Joey_Okumura_1E

When calculating pH, how do you know when it is acceptable to ignore the second deprotonation for a polyprotic acid? Can you always assume it is negligible?

Joey_Okumura_1E

For 6D.13, I got the following pH values.
a. 5
b. 5.6
c. 2.7
d. 5.03
However, according to the answer key, d should be larger than b. Which value is incorrect?

Joey_Okumura_1E

I think it is just a sig fig error, if anything. I got the same answers and was wondering the same thing.

Joey_Okumura_1E

Kb = [HClO][OH-]/[ClO-] and [HClO]=[OH-] therefore you can solve for [OH-]
pOH + pH = 14 --> pOH = -log[OH-] = 14 - pH

Joey_Okumura_1E

Calculate the pH, pOH, and percentage protonation of solute in each of the following aqueous solutions: (a) 0.057 M NH3(aq); (b) 0.162 M NH2OH(aq); (c) 0.35 M (CH3)3N(aq); (d) 0.0073 M codeine, given that the pKa of its conjugate acid is 8.21. For the above problem, I have done the following: NH3 + ...

Joey_Okumura_1E

I think some questions can be posted under the Acids and Bases page under chemistry 14A. While it is under c14A, the Acids and Base page includes sections that cover topics such as determining the pH of a solution with weak acids and bases.

Joey_Okumura_1E

I find it helpful to think about heat as a reactant or product. In an endothermic reaction , heat is being absorbed and is a reactant. Therefore, adding heat will cause the equilibrium to shift towards the products. In an exothermic reaction , heat is being released and is a product. Therefore, addi...

Joey_Okumura_1E

In an experiment, 0.020 mol NO2 was introduced into a flask of volume 1.00 L and the reaction 2NO2(g) ⇌ N2O4(g) was allowed to come to equilibrium at 298 K. (a) Using information in Table 5G.2, calculate the equilibrium concentrations of the two gases. (b) The volume of the flask is reduced to half ...

Joey_Okumura_1E

1. What effect does adding a catalyst have on a reaction? (Does the reaction shift?)
2. Do catalysts not affect K because they speed up both the forward and reverse reactions equally?

Joey_Okumura_1E

Changing the pressure will only change the concentration if it also changes the volume since C=n/V. If the container is compressed, the volume will decrease and therefore concentration will increase. Adding an inert gas increases pressure because the added gas will cause more force will be exerted o...

Joey_Okumura_1E

Can someone explain why N2 and O2 would be .1 in the ICE table and why we have to calculate for the reverse reaction? O2 and N2 are 0.1 in the ICE table because the problem specifies that, at equilibrium, the concentrations of N2 and O2 are 0.1M. I don't think you need to calculate for the reverse ...

Joey_Okumura_1E

P = pressure in atm
V = volume in L
n = amount of substance in moles
R = gas constant (0.08206 J*K[*]-1 M[*]-1)
T = temperature in Kelvin

Joey_Okumura_1E

The temperature will always be in Kelvin. Kelvin does not have any negative values so this eliminates any sign confusion.

Joey_Okumura_1E

Kc uses the equilibrium molar concentrations while Kp uses the equilibrium partial pressures. Their values will be different. Kp = Kc(RT)^(delta n)

Joey_Okumura_1E

Can someone please explain how to do this problem?: A newly designed laser pointer with a certain frequency is pointed at a sodium metal surface. An electron is ejected from the metal surface with wavelength 1.10 nm. What is the frequency of the light from the laser pointer? The work function of sod...

Joey_Okumura_1E

Please confirm: The approximate bond angles for a bent/angular molecule will be <120 or <109.5 depending on the number of lone pairs. (AX₂E₂ is <109.5 and AX₂E is <120)

Joey_Okumura_1E

For 41D, which 6 atoms can form a hydrogen bond? I thought a hydrogen bond was when H is bonded to two highly electronegative atoms. Therefore, H2O could only form hydrogen bonds with 4 atoms (the two N atoms and the two H atoms bonded to N).

Joey_Okumura_1E

Please confirm: The denticity of a molecule refers to the amount of bonds a ligand can form with a single central atom. Oxalate has four oxygen atoms with lone pairs and therefore has four binding points. However, due to its geometry, it can only form two bonds with a single central atom. Therefore,...

Joey_Okumura_1E

The coordination number is the number of points at which a ligand is attached to the central atom. You need to consider the amount of ligands and the denticity of the ligands. For example, in [NiCl4]2−, four Cl ligands are attached to central atom Ni. Cl is monodentate (which means Cl only binds to ...

Joey_Okumura_1E

Yes (poly = many, protic = protons). Polyprotic acids explain why some anions, despite their negative charge, are still considered acids (ex: HSO4- and H2PO4- are weak acids).

Joey_Okumura_1E

[OH-] is the concentration of OH- ions while pOH is a measure of the basicity of a solution. (It is similar to how [H+] is the concentration of H+ ions while pH is a measure of the acidity of a solution.) pOH = -log([OH-] )

Joey_Okumura_1E

Are we suppose to memorize polyatomic ion names and charges to assist us in naming coordination compounds? (Ex: NH4+, en, CN-) Is there a way to determine this without memorization?

Joey_Okumura_1E

How do we know the compound is square planar and not tetrahedral? (Does Fe have two lone pairs that cause the compound to be square planar? If so, how would we know this?)
How do we know only one O2 molecule can bind?

Joey_Okumura_1E

I try to complete the textbook problems and sapling problems throughout the quarter. Before the exam, I make a one page review sheet (typically one for each chapter) that summarizes all of the concepts that helped me solve the problems. The act of making the review sheet forces me to restudy everyth...

Joey_Okumura_1E

Neither takes precedence over the other. Bond length is considered to determine acidity for binary acids (acids composed of a hydrogen atom and another element). Electronegativity of the central atom is considered to determine acidity for oxoacids (acids that contain O-H bonds) and carboxyl acids (a...

Joey_Okumura_1E

Transition metals are located in the active site of enzymes and help the enzymes bond to their substrate. Substrates fit into an active site of an enzyme like a lock and key. Enzymes are very specific and only bond to a specific enzyme to catalyze a specific reaction. Therefore, it is very important...

Joey_Okumura_1E

Why is CO3[*](2-) a mono or bidentate ligand?

Joey_Okumura_1E

My TA told me that we are allowed to visit any TA's office hours to review the midterm.

Joey_Okumura_1E

Because the Cl2 is outside the brackets, it is named separately from the molecule in the brackets. Therefore, the name is pentaamminechlorocobalt (III) chloride. If it were [Co(NH3)5Cl3], then you would be correct.

Joey_Okumura_1E

What is the VSEPR shape for C2H4? It appears to be two trigonal planar shapes bonded together.

Joey_Okumura_1E

Is every molecule that is arranged in a T-shape polar?

Joey_Okumura_1E

If a molecule has a see-saw shape, will it always be polar?

Joey_Okumura_1E

Dr. Lavelle's models were very helpful because they allowed me to visualize the VSEPR shapes. However, without them, I have trouble visualizing the shapes. Does anyone have any tips?

Joey_Okumura_1E

Recall that for intramolecular forces, multiple bonds are stronger than single bonds. The same concept applies to intermolecular bonds. The strength and quantity are directly related.

Joey_Okumura_1E

Which shapes will always be polar?

Joey_Okumura_1E

There are four outer atoms and one lone pair.

Joey_Okumura_1E

In atoms that contain both 3d and 4s subshells... 1. Are electrons are added to the 4s subshell before they are added to the 3d subshell? 2. If a cation were being formed, would the electron be taken from the 4s orbital? If the answer to both questions is yes, don't they contradict each other? I tho...

Joey_Okumura_1E

Bond lengths and oxidation numbers do not provide insight on how to identify the best resonance structure. Oxidation numbers will be used when we discuss redox reactions. Typically, knowing the resonance structure can help to determine bond lengths and not the other way around.

Joey_Okumura_1E

Are dipole-dipole interactions typically similar in strength to induce dipole-dipole interactions? Intuitively, I would think they are stronger but I believe Dr. Lavelle said they were similar in strength.

Joey_Okumura_1E

Why are lone pairs given to I instead of double bonds forming between Cl and I? I is less electronegative so wouldn't it make more sense for it to not have a negative formal charge?

Joey_Okumura_1E

Please confirm: The formal charge on an ionic compound will not be equal to 0; it will equal the charge of the ionic compound. However, the most stable structure is the structure where the individual atoms within the ionic compound have the lowest formal charges.

Joey_Okumura_1E

The answer key says that OH is not a radical. Why? After drawing the Lewis structure, I think OH is a radical. OH- is not a radical so maybe it is a typo...

Joey_Okumura_1E

The more favorable structure has the lowest formal charge. The central atom is the least electronegative so it should not have a negative formal charge. Often times, the more favorable structure is also the one where the individual atoms have low formal charges.

Joey_Okumura_1E

Yes, Lewis bases donate electrons (and Lewis acids accept electrons). However, Lewis bases can also form ionic bonds.

Joey_Okumura_1E

The direction of the spins are assigned arbitrarily. Therefore, it does not matter which direction the spins are in first. Just make sure the spins of electrons are parallel if they are in different orbitals and antiparallel if they are in the same orbital.

Joey_Okumura_1E

Do Lewis bases tend to have lone pair electrons on their central atoms or lone pair electrons on any of the atoms?

Joey_Okumura_1E

How do double bonds in a molecule attract electrons and therefore cause the molecule to be a Lewis acid?

Joey_Okumura_1E

Is a Lewis structure of a molecule more accurate if all of the atoms in a molecule have a formal charge of 0 (vs one atom having a formal charge of +1 and another atom having a formal charge of -1)?

Joey_Okumura_1E

What are the exceptions to the electron configuration rules? I believe there was one concerning Cr and another concerning Cu.

Joey_Okumura_1E

It does not matter which spin is first because the spin number is assigned arbitrarily. The only thing that matters is that the unpaired electrons in different orbitals have parallel spins and paired electrons have opposite spins.

Joey_Okumura_1E

n represents the shell. l represents the subshell. m_l represents the orbital. Therefore, for 7p⁵, n=7; l=1 indicates it is a p orbital; and m_l would indicate if it was a p_x orbital, p_y orbital, etc.

Joey_Okumura_1E

Yes, this is correct. In order to determine the element type, add the electrons from the element in brackets to the electrons in the rest of the electron configuration. However, this process will not work if the electron configuration represents an ion.

Joey_Okumura_1E

Yes, recall that there are rules for which shells and subshells will be filled first based on their energies (the order is 1s 2s 2p 3s, etc.) These rules will almost always apply. (There are some exceptions to the rules due to the fact that, in some cases, paired electrons are more stable than unpai...

Joey_Okumura_1E

You have to consider the nuclear charges. Cs- and Ba are isoelectronic but Ba has a higher nuclear charge. As a result, Ba will have a smaller radius than Cs- because the nuclear charge is stronger. Cs- and Cs have the same nuclear charge but Cs- has more electrons. Because Cs- has more electron rep...

Joey_Okumura_1E

Light with a wavelength of 190 nm is incident on a metal surface. The most energetic electrons emitted from the surface are measured to have 4.0 eV of kinetic energy. Which of the metals in the table is the surface most likely to be made of - Aluminium (4.08 eV), Calcium (2.9 eV), Potassium (2.3 eV)...

Joey_Okumura_1E

What experiments and equations reflect the wave vs particle properties of electrons and light? My understanding is that... The double slit experiment reflects wave particle duality. -light photons travel through one slit and are absorbed at discrete points --> particle -light photons create a diffra...

Joey_Okumura_1E

My TA said we won’t need to know imperial to metric conversions (despite the fact that some of the homework questions required it).

Joey_Okumura_1E

Typically, +1/2 is used for electrons with an upward spin and -1/2 is used for electrons with a downwards spin. However, the sign on the spin states are arbitrarily assigned. They are correct as long as the two electrons in the same orbital have opposite spin signs.

Joey_Okumura_1E

E = hv gives the total energy that contributed to the ejection of the electrons. E = hv = KE + work required to eject electron. Thus, KE is a part of E=hv but is not equal to it.

Joey_Okumura_1E

This is the notation for electron configurations. The number (5 in this case) reflects the shell number/energy level, the letter (s in this case) reflects the subshell type, and the exponent on the letter (1 in this case) indicates the amount of electrons in the shell.

Joey_Okumura_1E

In this case, you need to compare the nuclear charges of the ions. The ion with highest nuclear charge will have the highest electronegativity. (All of these ions have the same number of electrons so they have similar electron repulsions. Thus, the higher the positive change of the nucleus, the more...

Joey_Okumura_1E

Most of the equations we use will give wavelength in meters. For example, in lambda = c/v, the units for c are m/s and the unit for v is 1/sec. Therefore, the units for wavelength will be meters.

Joey_Okumura_1E

Mai V 4L wrote:Does anyone know if this is going to be on the final?

I do not think we will be tested on this since Dr. Lavelle told us we do not need to worry about it.

Joey_Okumura_1E

A black body is a hypothetical object that absorbs all frequencies of light.

Joey_Okumura_1E

An electron needs a specific amount of energy to be ejected. If the amount of energy applied to the electron is higher than the amount needed to eject it, the extra energy does not just disappear since energy is conserved. The extra energy will become kinetic energy and influence the speed of the el...

Joey_Okumura_1E

Intensity describes the amount of photons. However, the amount of photons does not matter because only one photon can be used to eject an electron. Therefore, an electron will not be ejected unless a photon has sufficient energy.

Joey_Okumura_1E

In a chemistry setting, ionic bonds are stronger due to Coulombic attraction. However, in a biology setting, covalent bonds are stronger. (Biologists think about the relative strength of bonds when present in a solution. Ionic bonds are easily broken in a solution because they form hydrogen bonds wi...

Joey_Okumura_1E

Are we expected to memorize the history of chemistry? For example, do we need to know that Samuel Goudsmit and George Uhlenbeck proposed that electrons behaved like spinning spheres or do we just need to understand the concept of electron spin?

Joey_Okumura_1E

I believe J/mol can be used, but the numerical value would vary depending on which compound the moles correlates to.

Joey_Okumura_1E

When multiplying or dividing, the sig figs will be equal to the number of sig figs in the least precise measurement. When subtracting or adding, the the result should have the same number of decimal places as the least precise measurement.

Joey_Okumura_1E

You can use an arbitrary amount of grams to find mass percent. Using 100 grams just makes it easier since each 1% will be equal to 1 gram. However, you can't completely disregard the 8 grams because it will probably be needed to find the molecular formula, amount of grams of a specific element in a ...

Joey_Okumura_1E

I just wanted to remind you that the chemistry community rules ask that we explain the question in our post and not just reference specific book problems. This allows future students to utilize this site even if the books change. I don't mean to be nit-picky but I don't want you to lose points!

Joey_Okumura_1E

Another thing to note is that M does not stand for mole. M is used for molarity and stands for mol/L.

Joey_Okumura_1E

You may be thinking of c1*V1 = c2*V2. c is another constant that is used to represent molarity.

Joey_Okumura_1E

The amount of moles of each individual element is conserved, but the amount of moles of the compounds aren't necessarily reserved. 2Na+ 2H2O -> 2NaOH +H2 does not exactly translate to 2 moles + 2 moles --> 2 moles + 1 moles. It translates to 2 moles Na + 4 moles H + 2 moles O--> 2 moles Na + 2 moles...

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