CHEMISTRY COMMUNITY

sofiakavanaugh

Potassium permanganate and iron (II) chloride in acidic solution

I am confused on how to write the reaction for this. How do you know which ions to keep in your equation when they disassociate in water and which ones not to include?

Thanks

sofiakavanaugh

For this one you had to write the 1/2 rxn, balanced equation, calculate Eo cell and write the cell diagram for the unbalanced rxn: Cl2(g) +H2(g) --> HCl(aq) For the cell diagram the answer is Pt(s) | H2(g) | H+(aq) || Cl2(g) | Cl-(aq) | Pt (s) , but I got it the other way around. Why is that wrong? ...

sofiakavanaugh

Hi,

When writing cell diagrams, do we need to include the coefficients we assigned when balancing the chemical rxn?

For example if you had Pt(s) | H2(g) | 2H+(aq) || Cl2(g) | 2Cl-(aq) | Pt (s) , do you include the 2 in front of the H+ and the Cl-? Why or why not?

Thanks

sofiakavanaugh

Write the 1/2 rxn, balanced equation, calculate E cell and write the cell diagram for the chemical rxn: Au+(aq) --> Au(s) + Au3+ I got this for the cell diagram: Pt(s)| Au+(aq), Au 3+(aq) || 2Au+(aq) | 2Au(s) Can someone please explain to me why this is wrong and how to get the correct diagram? Than...

sofiakavanaugh

Hi, In this problem you are suppose to find the 1/2 rxns, balance, write the full chemical equation, calculate the Eo of the cell and write the cell diagram. For the reaction Ce4+(aq) + I-(aq)--> I2(s) +Ce3+(aq), I got the correct cell diagram 2I-(aq)|I2(s)||Ce4+, Ce3+(aq)|Pt(s) except I didn't have...

sofiakavanaugh

Hi,

There are some homework problems where we have to write combustion reactions, and I know the products are always CO2 and H2O, but is the water in the liquid or gas phase?

Thanks

sofiakavanaugh

8.27 Calculate the work for each of the following processes beginning with a gas sample in a piston assembly with T " 305 K, P " 1.79 atm, and V " 4.29 L: (a) irreversible expansion against a constant external pressure of 1.00 atm to a final volume of 6.52 L; (b) isothermal, reversibl...

sofiakavanaugh

Hi, I was reviewing my thermo homework and was taking a look at 8.11 where it asks "A piston confines 0.200 mol Ne(g) in 1.20 L at 25 'C. Two experiments are performed. (a) The gas is allowed to expand through an additional 1.20 L against a constant pressure of 1.00 atm. (b) The gas is allowed ...

sofiakavanaugh

Hi friend!

So the 0.37 is actually for part a, it just is lined up weirdly in the solutions manual so it looks like its part of part b.

Hope this helps

sofiakavanaugh

I think this is one were you first have to find [A]t. To do this you first write the increase in [B] in terms of [A], which means you need to multiply mol B/L x (1 mol A/3 mol B). Then you take your [A]o and subtract the number you just calculated from it. Since its first order, you use the integrat...

sofiakavanaugh

Hi,

I have tried to look at other people's answers for this question but for some reason I still don't understand what is happening conceptually: to find [A]t given an increase in [B], why do you have to do [A]o-[B]xmolA?

Thanks

sofiakavanaugh

Hi,

for question 27 I got the wrong number of sig figs for each one, i got 3 sig figs since the time has 3 sig fogs and therefore so did k, but in the solution manual it keeps using two?

Thanks

sofiakavanaugh

14.0 g of SO2Cl2 in 2500. L vessel is heated to a specific temp, what mass will remain after 1.5 hours for a first order reaction and for a k value of 2.81 x10^-3 1/min I keep doing this problem over and over again, and for some reason I am ending up with more grams than the initial, which is obviou...

sofiakavanaugh

For this problem, there is a big table full of experimental values and you are asked for the rate law. I was confused on how to calculate the order of B, sorry I cant type the table but if someone could explain to me how they did it that would be amazing.

Thanks

sofiakavanaugh

Hi,

I was just wondering if we have to worry about third order reactions at all or if we will only be examining 0-2nd order reactions.

Thanks

sofiakavanaugh

Hi, So this is a really simple question but I am just looking for a little clarification, on 15.5 it gives you the unique rate of the rxn: 0.44 m/Ls and it gives you the reaction C2H4(g) + 3O2(g) ----> 2CO2(g) + 2H20(g) and asks for the rate at which the O2 reacts. So in the answers you multiply the...

sofiakavanaugh

I am also confused on this: if this is true then what exactly is the difference between rate of reaction and unique rate of reaction and why are they calculated differently?

sofiakavanaugh

When balancing redox reactions, if they dont tell you whether or not it is in acidic or basic solution, can you choose to add OH- or H+ to balance as you please? Or do you always have to assume it will be acidic and use H+ to balance the hydrogens?

sofiakavanaugh

Hi, For part a of question 14, the reaction is O3(aq) + Br-(aq) --> O2(g) + BrO3-(aq) The bromine is going from -1 to +5 so its losing electrons, therefore being oxidized, but the oxygen appears not to change oxidation numbers ( 0 to 0). I understand if one is being oxidized its logical that the oth...

sofiakavanaugh

Hi, For problems where we are doing deltaG calculations for phase changes, I understand we need to calculate deltaH and deltaS values from appendix 2A first and then calculate delta G. My question is, when we use to calculate deltaH individually for phase changes sometimes we would have to add a q=m...

sofiakavanaugh

Hi,

I was wondering in what specific situations we would need to use w=-pdeltaV versus other work equations

Does the temperature need to be constant?

Thanks!

sofiakavanaugh

Hi,

I was wondering in what specific situations we would need to use w=-pdeltaV versus other work equations

Does the temperature need to be constant?

Thanks!

sofiakavanaugh

Why is deltaG(formation) zero for diatomic molecules again? and same with enthalpy values? (But not for entropy values?)

sofiakavanaugh

Hi, I was wondering if it always will get you the same answer if you calculate deltaG by calculating deltaH(formation) and delta S(formation) and then using deltaG=deltaH-TdeltaS or by using the deltaG based on the deltaG of formation values in the table. Will you always get the same answer? Does it...

sofiakavanaugh

Will there ever be a case where a reversible process is not isothermal? And how would calculating entropy for that be different?

sofiakavanaugh

Hi, The question asks: assuming the heat capacity of an ideal gas is independent of temperature, calculate deltaS associated w/ raising temp of 1.00 mol of ideal gas atoms reversibly from 37.6 to 157.9 degrees C. a) at constant pressure b) at constant volume My question is that for example in part a...

sofiakavanaugh

Hello,

This is a really simple question but 9.1 and 9.3 appear to use the same equation to calculate entropy but one added a negative sign to the equation before doing the calculation and the other didn't? I was just wondering why.

Thanks!

sofiakavanaugh

N2(g) + O2(g)---> 2NO (g) deltaH°=180.6 KJ c) bomb calorimeter; 492 J absorbed. Mass of N2 oxidized? How I did this problem was (492J/1 mol rxn)(1 mol rxn/1 mol N2)(1 mol/28.02g)=17.6g Obviously this is the wrong answer, but can someone please explain to me why this logic is incorrect? Also, in the ...

sofiakavanaugh

In 8.67 part a, it asks you to calculate the enthalpy of formation in the liquid state of water. I get that we need to use the equation H2 + 1/2O2--> H20 (l) for this using the bond enthalpies. What I don't understand is why in the solutions manual it says that when we calculate the enthalpy for thi...

sofiakavanaugh

The question is 4C(s) + S8(s) ---> 4CS2 (l) deltaH=358.8 KJ, and its asking how much heat is absorbed at constant P with 1.25 mol S8 My understanding of this is that the 358.8 KJ implies that it is for 1 mol of rxn even though it doesn't say KJ/mol, and so you have to use the 1.25 mol of S8 to find ...

sofiakavanaugh

“Benzene is more stable and less reactive than would be predicted from its Kekulé structures. Use mean bond enthalpies to calculate the lowering in molar energy when resonance is allowed between the Kekulé structures of benzene.” I don’t really know what they are talking about in this problem (molar...

sofiakavanaugh

Hi, For 8.65, you are given the rxns 2NO(g)+ O2(g) --> 2NO2(g) with deltaH=-114.1 KJ and 4NO2(g) +O2(g) ---> 2N2O5(g) with delta H=-110.2 KJ. you are asked to use this and deltaH formation of NO (90.25 KJ/mol) to find the delta H formation of N2O5. Then in the solutions manual, it says that the reac...

sofiakavanaugh

Hi,

for 8.49 we have to use the equation deltaU= deltaH-delta(n)RT and it doesn't give us a temperature to use, but in the solutions manual it uses 298K (25°C). Is it always safe to assume a reaction is at 25 degrees C if it is not stated in the problem?

Thanks

sofiakavanaugh

The standard state of something is the state it is in at 1 atm and 25 degrees C. So if the temp is 25 degrees and the pressure is 1 atm and the substance is a liquid, than liquid would be its standard state, for example.

sofiakavanaugh

Heat capacity is the amount of heat energy needed to raise the temp of something by 1 degree Specific heat capacity is the amount of heat energy needed to raise the temp of something by 1 degree per unit of mass So basically I think one just includes the mass of the substance while the other does not.

sofiakavanaugh

I think a thermochemical equation is just a chemical equation that includes the change in enthalpy for the reaction (delta H)

sofiakavanaugh

Hi,

Does anyone have any suggestions for which homework questions would be good ones to practice based on what we have learned so far, since its kind of out of order?

Thanks

sofiakavanaugh

Just make sure you know your periodic trends and exceptions to those! (E.i. N>O for ionization energy, N<C for electron affinity)

sofiakavanaugh

You first write your composition for the sigma bond part of it like normal and then 2x pi(C2p, N2p) underneath, that's how they did it in one of the review sessions

sofiakavanaugh

That means that not all the states of the atoms/molecules are the same in the rxn, for example there might be gases and solids where in a homogenous rxn they would all be gases (for example).

sofiakavanaugh

I think the Ligand always acts as a Lewis base in the presence of a transition metal in order to create the complex

sofiakavanaugh

Hi, I was wondering if someone could please clarify/confirm my reasoning for why CaO(s) in water will form Ca2+ and OH-. I feel like I might be over complicating it. I understand that it dissociates which creates the Ca2+. Does the OH- come from he Oxygen from the CaO breaking off, essentially takin...

sofiakavanaugh

Just realized I posted this in the wrong section but I am not sure how to move it... sorry!!

sofiakavanaugh

"State if the oxides are acidic, basic, or amphoteric:" a) BaO b) SO3 c) As2O3 d) Bi2O3 I am getting a little lost in the logic here... Usually when I look at a rxn to identify which will be the acid or base I look to see if one has a Hydrogen out in front, like HCl or H2SO4, and that look...

sofiakavanaugh

Unless I am mistaken, you would have to be given some sort of other information. The enthalpies, for example, or be told if the reaction requires energy (endothermic) or releases energy (exothermic). Also you know that combustion reactions are always exothermic.

sofiakavanaugh

So first what you need to do is identify your acid and your base. Your acid is the one that donates the proton, for example HNO3, to the other molecule in the reaction, such as water. An acid's conjugate base is whatever is leftover after it gives away the proton, in this case NO3-. The base in this...

sofiakavanaugh

Hybridization occurs to lower the overall energy and to create more availability for bonds to occur, which often is necessary in cases like carbon where we know that four bonds will often form but the energy diagram tells us differently

sofiakavanaugh

Is it correct to say that pi bonds will only occur when there is a double or triple bond in the molecule?

Thanks

sofiakavanaugh

To determine bond strength, you must draw the lewis structure so that you can find out which bonds are triple, double or single (triple is the strongest type of bond, single is the weakest). Also atoms with a greater electronegative difference will have shorter bonds, which makes them stronger.

sofiakavanaugh

Can someone please explain what the structure for 8a would look like and why? (draw the lowest energy lewis structure for (NH2)2CO2 )

sofiakavanaugh

This could be wrong, but I think that a formal charge of zero is the most stable, and the further away you get from zero the more unstable it is

sofiakavanaugh

I'm not sure about the equation, but hybridization isn't something we have learned yet. I think we will probably get to it soon because its related to what we have been talking about with expanded octets, electron configurations, and orbitals but we haven't covered it in lecture yet.

sofiakavanaugh

How many electrons can have the following quantum numbers:
b) n=4, l=2, ml=-2
d) n=3, l=2, ml=+1

Can someone please explain why the answer is 2 for both of them? The part I don't understand really is M(l).

Thanks!

sofiakavanaugh

Lines in the Balmer series of the hydrogen spectrum are observed at 656.3, 486.1, 434.0, and 410.2 nm. What is the wavelength of the next line in the series? Could someone please explain to me how they used the values given in the problem to determine which values to plug in the Rydberg Equation? Th...

sofiakavanaugh

Hi,

Is the DeBroglie equation the only time that we use kg for a calculation? In any other equation we would want to use g (or J, m, or any other base unit without prefixes like kilo), right?

Thanks,
Sofia Kavanaugh

sofiakavanaugh

Hi, I was going through homework problems and I was a little confused when it comes to sig figs when you use the equation E=-hR/n^2, because there doesn't seem to be much to go off of. For example, 1.13 asks for the wavelength of radiation generated from the n=4 to n=2 transition. I understand how t...

sofiakavanaugh

In class someone asked the question about what to do about sig figs if you have a 5; e.i. 4.465- and you only have 3 sig figs. Normally I was taught you round up but Professor Lavelle mentioned that that is not always the case and I'm not sure I fully understand the concept. Can someone please expla...

sofiakavanaugh

A 1.50 g sample of metallic tin was placed in a 26.45 g crucible and heated until all the tin had reacted with the oxygen in air to form an oxide. The crucible and product together were found to weigh 28.35g. What is the empirical formula of the oxide? Write the name of the oxide. Could someone plea...

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