CHEMISTRY COMMUNITY

Luyan Zhang - 2D

For number 7, when you flip the second one around, the E value becomes negative. Do you use this negative E value to calculate G? I thought you would still use the standard reduction potential.

Luyan Zhang - 2D

Ok. I am just very confused about everything (currently sipping tea at home wondering how everything will unfold)

But will there an online review session this Friday led by UAs. Or at least, will the work for ENDGAME be posted on Chemistry Community?

Luyan Zhang - 2D

Is kinetics always controlling the reaction at the lower temperature since it is the reaction with the lower activation energy?

Luyan Zhang - 2D

You should rearrange the equation so that rate of NH3 formed is in term of rate of consumption of H2.

Luyan Zhang - 2D

I understand that the review sessions may be cancelled, but is there a UA made worksheet that I can do for extra practice for the final?

Luyan Zhang - 2D

Does anyone know if the Review session will also be canceled?

Luyan Zhang - 2D

What is the difference if the rate determining step is the first step in a series of elementary steps versus if it is a later step? I remember Dr. Lavelle said something about pre-equilibrium approximation where the steps before the slow step is at equilibrium because the slow step will cause a bott...

Luyan Zhang - 2D

A is a constant: it is the fraction of molecules that make collisions which are significant to the chemical equation.

Luyan Zhang - 2D

The one that matches the experimentally derived rate law is the slow step.

Luyan Zhang - 2D

On the final, will the integrated rate laws and half-life equation for zero-order, 1st order, and 2nd order reactions be given on the equation sheet?

Luyan Zhang - 2D

Shouldn't pH + pOH be 14?

at pH=7: K= [H+]/[OH-]= 1.0 x 10^-7 mol.L^-1/1.0 x 10^-7 mol.L^-1= 1 and E = 0

at pH=1: RT/nF (lnK)= .025693 V ln(1.0 mol.L^-1/ 1.0 x 10^-13 mol.L^-1)= +.71 V

at pH=14: RT/nF (lnK)= .025693 V ln(1.0 x 10^-14 mol.L^-1/1.0 mol.L^-1)= -.83 V

Luyan Zhang - 2D

When a pH meter was calibrated with a boric acid–borate buffer with a pH of 9.40, the cell potential was + 0.060 V. When the buffer was replaced with a solution of unknown hydronium ion concentration, the cell potential was + 0.22 V. What is the pH of the solution? How do you know n = 1? Which redox...

Luyan Zhang - 2D

In the book, there are two reactions for the reduction of water

2H2O + 2e- ––> H2 + 2OH- E = -0.42V
O2 + 4H+ + 4e- --> 2H2O E = +0.82V

Can someone tell me when to use each one? And how do we know what is reduced or oxidized?

Luyan Zhang - 2D

In a WORKING concentration cell, is the concentration at cathode always going to be the product in K?

Luyan Zhang - 2D

Kevin Antony 2B wrote:We need a solid to be the electrode. As there is no solid on the right hand side, we use Platinum.

Well, there are no solids on the left either, so wouldn't we need to add Pt on the left too?

Luyan Zhang - 2D

for part (c), why is KOH not part of the cell diagram on the right side (cathode)?

Luyan Zhang - 2D

For the anode side, the I2 is in solid state, but the solution manual included Platinum on the anode side of the cell diagram too. Why is that?

Luyan Zhang - 2D

TWO moles of electrons are being transferred. You can see this in both the oxidation and reduction half reactions.

Luyan Zhang - 2D

Where in the textbook can I find standard reduction potential values?

Luyan Zhang - 2D

Why can't we balance O3(aq) and O2(g) by simply putting coefficient in front of them? Why do we have to balance by putting H2O and OH- on either side?

Luyan Zhang - 2D

If the E cell value is positive it means that oxidation-reduction reaction is spontaneous. If the E cell value is negative it means that the reverse reaction is spontaneous, which is reduction at anode and oxidation at cathode. I thought the anode always participate in oxidation and cathode always ...

Luyan Zhang - 2D

The anode is always oxidized, so it loses electrons. The cathode is reduced so it gains electrons. Electrons will always move from anode to cathode. The cell potential only switches signs when you are balancing and equation and you flip one of the equations. Thanks! I understood that when you flip ...

Luyan Zhang - 2D

Does electron always move from anode to cathode? I'm not sure if I understood this right: if the positions of the the two electrodes are switched between the solutions, the cell potential would alter in sign?

Luyan Zhang - 2D

Cl- is a common salt used in a salt bridge. What type of salts would you NOT want to use in a salt bridge? How do you decide what salt to use?

Luyan Zhang - 2D

Can someone explain how salt bridge functions? What would happen without the salt bridge?

Luyan Zhang - 2D

For part a) I got 1.31 * 10^-9?

Did you guys get delta H = 176.01 kJ?

Luyan Zhang - 2D

Can someone find where my error is? The question is: (a) Calculate the reaction Gibbs free energy of I2(g) -> 2I(g) at 1200. K (K 5 6.8) when the partial pressures of I2 and I are 0.13 bar and 0.98 bar, respectively. (b) Indicate whether this reaction mixture is likely to form reactants, is likely ...

Luyan Zhang - 2D

(a) Calculate the reaction Gibbs free energy of I2(g) -->2I(g) at 1200. K (K = 6.8) when the partial pressures of I2 and I are 0.13 bar and 0.98 bar, respectively. (b) Indicate whether this reaction mixture is likely to form reactants, is likely to form products, or is at equilibrium. Can someone go...

Luyan Zhang - 2D

Can someone explain #6 to me? I dont really understand how to calculate each one of them. And is delta U = 0?

Luyan Zhang - 2D

Initially an ideal gas at 323 K occupies 1.67 L at 4.95 atm. The gas is allowed to expand to 7.33 L by two pathways: (a) isothermal, reversible expansion; (b) isothermal, irreversible free expansion. Calculate delta S(tot), delta S(system), and delta S(surrounding) for each pathway. I understood par...

Luyan Zhang - 2D

delta S = q/T

delta S = k*ln(W)

According to the Boltzmann formula, if the temperature of the system increases, W also increases, and there fore S increase. But according to the first one, temperature is inversely proportional. How does that work?

Luyan Zhang - 2D

Why does entropy decrease with an increase in temperature? If the temperature increases, shouldn't the system be more disordered?

Luyan Zhang - 2D

Delta S = q/T is used for reversible process. How do we calculate change in entropy for irreversible processes?

Luyan Zhang - 2D

Hydrochloric acid oxidizes zinc metal in a reaction that produces hydrogen gas and chloride ions. A piece of zinc metal of mass 8.5 g is dropped into an apparatus containing 800.0 mL of 0.500 m HCl(aq). If the initial temperature of the hydrochloric acid solution is 25 degrees C, what is the final t...

Luyan Zhang - 2D

For combustion reaction, is the H2O made in liquid form or gas form? One of the HW question asks me to calculate the reaction enthalpy, so I would need the standard enthalpy of formation of H2O. Since it's at 25 degrees Celsius, does that always mean it would be in liquid form?

Luyan Zhang - 2D

Katherine Wu 1H wrote:For the benzene ring, you are supposed to use the single/double resonance bond enthalpy (518 kJ) for all six of the bonds.

OH ok thank you!

Luyan Zhang - 2D

Can someone explain what those "dog bones" Dr.Lavelle drew on the slides? Are they just representative of the two states?

Luyan Zhang - 2D

Can anyone help me on a)? I thought of it like this: Breaking 3 mol C-C triple bond: 3(837) Breaking 6 mol C-H single bond: 6(412) Forming 3 mol C-C double bond: 3(-612) Forming 3 mol C-C single bond: 3(-348) Forming 6 mol C-H single bond: 6(-412) When I added all of them up, I got -369 kJ. I think ...

Luyan Zhang - 2D

For part c., I got -38.72kJ for some reason, but the answer is 15.28kJ? Can someone explain please?

Luyan Zhang - 2D

Do we need to know equipartition theorem for this class?

Luyan Zhang - 2D

A system's internal energy is increased by 982 J when 492 J of heat was supplied. Because change in energy is equal to the sum of work and heat, it makes sense that work must have been involved for the the change in energy to be that high. Using that equation, it makes sense that 982 J = 492 J + w,...

Luyan Zhang - 2D

Why is Internal energy is a state function, but since delta U = heat + work, and neither heat nor work are state functions? Can state functions have non-state function components?

Luyan Zhang - 2D

For this question, we need to take into account both the mass of the water and copper and calculate the heat required (q) for both of them and add it together. Your equation should be set up as: (400g)(4.18 J/cg)(100-22C)+ (500g) (0.38 J/Cg)(100-22C)= 1.4 x 10^2 KJ. In the solutions manual, they co...

Luyan Zhang - 2D

a) Calculate the heat that must be supplied to a copper kettle of mass 400.0 g containing 300.0 g of water to raise its temperature from 20.0 degrees C to the boiling point of water, 100.0 degrees C. (b) What percentage of the heat is used to raise the temperature of the water? I got a different ans...

Luyan Zhang - 2D

For problem 6D.15 (b), how do we know what Ka for the chemical reaction is?

Luyan Zhang - 2D

Me too! I think the solution manual is wrong. Because taking the negative log of a number greater than one is negative. But the solution has a positive pH

Luyan Zhang - 2D

On the solution manual, the Kc is 6.1*10^-3, but on table 5G.2, the constant is 6.1*10^23.

Is this a type on the textbook?

Luyan Zhang - 2D

Do we have to use bar as the unit to calculate equilibrium constant?

Luyan Zhang - 2D

A gaseous mixture consisting of 2.23 mmol N2 and 6.69 mmol H2 in a 500.-mL container was heated to 600. K and allowed to reach equilibrium. Will more ammonia be formed if that equilibrium mixture is then heated to 700. K? How would we do this problem if we didn't know that this reaction is exothermic?

Luyan Zhang - 2D

do we set the ICE box with concentration or mols? Or does it not matter?

Luyan Zhang - 2D

In the answer for part b, it says that the concentration of F2 is 8*10^-4 M. But since 2.0 mmol of F2 was added into the vessel containing 2.0L, shouldn't the concentration b e 0.001M?

And can somebody also explain why Cl2 would be more stable? Is it because it disassociated less?

Luyan Zhang - 2D

1.1 * 10^-5 M

Luyan Zhang - 2D

K describes the reaction once it reaches equilibrium. Q is calculated the same way as K, but it is for the reaction at any point in time that is not equilibrium. You can find Q and compare it with K to determine which way the reaction will proceed. Hope this helps.

Luyan Zhang - 2D

For Kc and Kp, would the constant be the same no matter whether you plug in the concentration or the partial pressure?

Luyan Zhang - 2D

We went over comparing which substance is more acidic, but this question asks which one is a stronger base. How would you approach this problem?

Luyan Zhang - 2D

Andrew F 3B wrote:Yes, Just look up "Marshmallow" in Chemistry Community's search bar and you can find it for the review session

Thank you so much!

Luyan Zhang - 2D

Will there be another practice packet for the final review session given like the midterm?

Luyan Zhang - 2D

HI is stronger because Iodine is a larger atom, and therefore the atomic radius is larger. Because the radius is larger, the bond between them is longer and weaker, and H+ is easily lost to surrounding solution.

Luyan Zhang - 2D

Ok that makes more sense now, thank you!

Luyan Zhang - 2D

I wasn't able to make it to the lecture on Wednesday, and I was kind of confused by the slides. Why is Cl––O––H a stronger acid than Br––O––H? I thought Br has a longer atomic radius, which means a longer and weaker bond, and therefore it would be easier to strip proton from it. Or is it different f...

Luyan Zhang - 2D

Look at the charges on individual ligands. Make sure everything in the bracket adds up to the overall charge of the complex.

Luyan Zhang - 2D

CN- has a charge of -1, and since there are six of them, it would be -6. BUT the total charge of the complex is a -4, which must mean that Fe has a charge of +2; -6 + 2 = -4.

Hope this helps.

Luyan Zhang - 2D

How many of these common ligands should we commit to memory? Or should we just memorize all of it?

Luyan Zhang - 2D

Are all ligands basically Lewis bases? And do molecules that donate only one electron count as a Lewis base? Or does "Lewis base" only refer to an atom/molecule that donates a pair of electrons?

Luyan Zhang - 2D

Cynthia Gong 1L wrote:I think that's because the textbook used the resonance structure where there is one single-bonded O and one double-bonded O, and so as a result there is only one pi bond.

But the formal charges would not be zero...

Luyan Zhang - 2D

for 2F.3 a), why does SO2 have two sigma bonds but only one pi-bond? I thought there are two pi-bonds

Luyan Zhang - 2D

Lone pair-lone pair > Lone pair-bonding pair > bonding pair-bonding pair

I think you will just have to remember this.

Luyan Zhang - 2D

I thought there should be a double bond between Sb and O, since the formal charge would be zero for both that way. But the answer key depicted it as a single bond? I know it doesn't really matter for the VSEPR shape, but I'm still confused on whether it is the correct lewis structure or not.

Luyan Zhang - 2D

The bond angle is linear (180 degrees) because the central atom (Be) is bonded with two atoms (both Carbon) with no extra lone pairs to potentially distort the shape of this bond angle. I would assume that the bond angle would be 120 degrees if the central atom was a Boron atom bonded to three Carb...

Luyan Zhang - 2D

For C–Be-C angle, why is it 180 degrees rather than 120 degrees. Is it because the two valence electrons on Be are lone half pairs?

Luyan Zhang - 2D

Both VSEPR shape with the formula AX2E and AX2E2 have bent shape. What are the differences in their bond angles?

Luyan Zhang - 2D

Do we have to have this figure memorized? Or should we just try to figure out the shape based on the lone pair and bonding atoms ourselves?

Luyan Zhang - 2D

What are the differences in trigonal bipyramidal and square pyramidal? They both have five atoms attached to the central atom. My thinking is that in square pyramidal, there could be a lone pair on the central atom, so it would have an electron arrangement of a octahedral, but have a shape of square...

Luyan Zhang - 2D

Polar molecules have a permanent dipole moment. Polar just means that there is an unequal sharing of electrons.

Luyan Zhang - 2D

I think this is what happens. When alpha increases, that means the polarizability increases, and since polarizability depends on the size of the atom (and how many e- there are), r increases too so the denominator "grows" faster than the alphas in the numerator so even though the numerato...

Luyan Zhang - 2D

It is how strongly two atoms/molecules are held together. It is proportional to their polarizability and inversely proportional to the distance between them.

Luyan Zhang - 2D

Since there's a negative sign in front of the equation, that means that when r increases, the denominator increases meaning it gets closer to 0, which means that Ep actually increases (from more negative to less negative). If that is the case, with higher alpha (polarizability), the interaction wou...

Luyan Zhang - 2D

It is always negative because there is always an attractive force pulling two atoms or molecules together

Luyan Zhang - 2D

Professor Lavelle said that larger atoms/molecules have stronger interactions due to their polarizability. But since larger atoms have a larger distance, wouldn't the interaction potential energy be less? I thought that the distance would contribute more to the equation because it is r to the sixth ...

Luyan Zhang - 2D

Pd and Ni are in the same group, but electron configuration for Nickel is [ ]3d8 4s2, and electron configuration for Pd is [ ]4d10. Why does the electron from the s-orbitals from Pd move to d-orbitals to fill the full-shell, but the electrons in Ni don't?

Luyan Zhang - 2D

Lattice energy is the energy stored in the bonds of an ionic solid, such as NaCl. This energy is released when Na and Cl come together. This energy is much greater than the energy needed to strip electron off of Na, when shows why NaCl has a lower energy than Na or Cl.

Luyan Zhang - 2D

Usually, a bond is composed of two electrons, and one coming from each of the bonded atom. A coordinate covalent bond refers to a bond in which BOTH electrons come from ONE atom, as in the case of BF4, when both electrons came from the fluoride ion.

Luyan Zhang - 2D

A resonance refers to one structure of many different but equivalent lewis structures of a molecule. The term "delocalization" refers to an electron that doesn't belong to just one single bond. A delocalized electron is spread out over the molecule. Professor Lavelle mentioned that a true ...

Luyan Zhang - 2D

Yes they do, except for the exceptions Lavelle talked in class. Remember that resonance structures have the same position of atoms, but just different arrangement of electrons and bonds.

Luyan Zhang - 2D

I think he said in class that we don't need to know how to calculate it, and just understand the relationship between the variables.

Luyan Zhang - 2D

Does anyone know when Test 2 is?

Luyan Zhang - 2D

The effective nuclear charge is just the attraction the electrons feels from the nucleus. Since in a multi-electron atoms, there will be electron-electron repulsion from different shells, the effective nuclear charge will always be less than the actual nuclear charge. As the shells increase, the eff...

Luyan Zhang - 2D

What does the infinity-like symbol next to the equation (q1)(q2)/r mean? Also, would we need this equation to do calculations or is it just the concept about like attraction and potential energy that we need to understand? That symbol means that the potential energy is DIRECTLY PROPORTIONAL to (q1)...

Luyan Zhang - 2D

Yes, this answer is unreasonable, and you should definitely recheck your work. Perhaps you've made a calculator error.

Luyan Zhang - 2D

On the large scale, light acts like a wave (diffraction). On the small scale, light is composed of photons, or particles (photoelectric effect).

Luyan Zhang - 2D

Increasing intensity means you increase the NUMBER of photons (not the energy each particular photon carries), and to emit e- from metal surface, you need EACH photon to carry a certain amount of energy (threshold energy). In order to do that, you must increase the frequency of the light source

Luyan Zhang - 2D

The series refer to the different states electrons start at and return to. For instance, for all of n(final) = 2, the line is in the Balmer series (visible light)

Luyan Zhang - 2D

If the wavelength of something with a large mass cannot be detected by lab equipments, then how did de Broglie come up with this relation? Or is it all theoretical?

Luyan Zhang - 2D

LNgo 14A wrote:I believe the uncertainty would be 5.0. The plus and minus signs just mean that the variation can go either direction.

But in the worked example in the textbook, the velocity is plus or minus 1m/s, but in the solution for it, it used 2m/s as the uncertainty for speed

Luyan Zhang - 2D

In the practice problem, 1B.27, it states that the velocity of the bowling ball is 5.00 +/- 5.0ms-1,
does this mean that the uncertainty of the velocity is 10m/s because of the plus or minus sign?

Luyan Zhang - 2D

The >= sign just means that the uncertainty can be infinitely greater (which makes sense). Usually, I think you would just use it as an equal sign because it is the minimum uncertainty

Luyan Zhang - 2D

If the question asks for "mass", you should put grams

Luyan Zhang - 2D

You use the the conversion:

1 mole = 6.022 * 10^23 molecules/atoms

Luyan Zhang - 2D

1. Leading zeroes are not significant eg. 0.00007 only has one sf 2. Trailing zeros in decimals are significant eg. 0.0000700 has 3 sig figs 3. Trailing zeroes in whole numbers are not significant eg 7000 only has one sf (If there is a decimal points after 7000, like 7000., then there are 4 sf) 4. T...

Luyan Zhang - 2D

What should the threshold be for decimals when solving for empirical formulas? I think that if the results are 0.1 away from the nearest whole number, then you round to that whole number. Otherwise, I think you multiply to the nearest whole number. However, I see in some example problems, like one ...

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