CHEMISTRY COMMUNITY

Nha Dang 2I

I believe they said that due to the wording, the point was given to both A & B, but A was the correct answer.

Nha Dang 2I

This asks to derive the rate law implied by the mechanism, and confirm that it matches the observed rate law for the rate of decomposition of O3. I understand up to the substitution of [O], but can someone explain how they got the 2? -- Edit: I understand how that they got 2 by multiplying by the co...

Nha Dang 2I

You would use ln([A]t/[A]0)=-kt with [A]t=.719[A]0

Then, you would can cancel out [A]0 in ln(.719[A]0/[A]0)=-k(10 minutes) and solve for k from there.

Nha Dang 2I

Because liquids have more entropy, detS for the reaction is positive as it goes from a lower entropy (solids) to higher entropy (liquids).

Nha Dang 2I

Yes, K doesn't change when a catalyst is added.

Nha Dang 2I

Wayland Leung wrote:So we would need to know all the functional groups and their line diagrams and how to draw them?

Yes. He introduced it to us in Monday lecture and will continue to go over more examples on Wednesday.

Nha Dang 2I

I don't remember him saying that either! Plus, Lavelle usually doesn't drop tests.

Nha Dang 2I

I had written the equation, w=-pdetV, and then noted that there was a change in volume at a constant pressure.

Nha Dang 2I

(If reduction potentials are given--) A higher reduction potential means a higher oxidizing power (as it's more likely to reduce), and a lower reduction potential means a higher reducing power (as its more likely to oxidize).

Nha Dang 2I

You're trying to find the standard potential for the whole reaction using the reduction potentials given. You can set it up by saying detG(3)=detG(1)+detG(2) with 3 being the whole reaction, and 1 and 2 being the parts of the reaction with the reduction potential given. You then plug in -nFE for eac...

Nha Dang 2I

Why would you multiply [B] by 2 and not divide?

Nha Dang 2I

Will we have to know exactly how to integrate the rate laws or can we just memorize the equations?

Nha Dang 2I

What concentration of N2O5 remains 10.0min (600 s) after the start of its decomposition at 65 C (in reaction A) when its initial concentration was .040 mol/L? See Table 15.1 for the rate law.

Where did they find k which equals 5.2 x 10^-3? Why isn't it 3.7x10^-5?

Nha Dang 2I

If we were to forget the species to which the rate refers for average reaction rate, would the answer be incorrect?

Nha Dang 2I

But why would you use argon when the question is asking about oxygen?

Nha Dang 2I

In one experiment, 1.00 mol O2(g) was compressed suddenly (and irreversibly) from 5.00 L to 1.00 L by driving in piston, and in the process its temperature was increased from 20.0 C to 25.2 C. What is the change in entropy of the gas? In Step 2, why did they use the heat capacity of argon in deltaS=...

Nha Dang 2I

Self-Test 9.3A Calculate the change in molar entropy of an ideal gas when it is compressed isothermally to one-third its initial volume.

So far, I can see that you use the equation deltaS=nRln(V2/V1), but what would you use for n in this case?

Nha Dang 2I

The Van't Hoff equation isn't on the equation sheet, so that derivation would probably be nice to remember.

Nha Dang 2I

w = -nRT ln (Vfinal/Vinitial) is also used when the pressure is not kept at a constant.

Nha Dang 2I

If you go to "Administrative Questions and Class Announcements," it's the second post!

Nha Dang 2I

How do you know to add the standard enthalpy of sublimation of carbon rather than subtracting it?

Nha Dang 2I

For bonds broken, it's C=C and H-H. For bonds formed, it's C-C, C-H, and C-H.

Nha Dang 2I

You set it up as q=mcdeltaT + deltaH(fusion)(1/2m), which becomes deltaH(fusion)=((2)(q-mcdeltaT))/(m). Then you just plug in the numbers and you end up getting 3.7 kJ/g.

Nha Dang 2I

I solved this by changing the reaction so that I would have 4 moles of C instead. I multiplied the entire reaction by 4/3 to get 4 C (graphite) + 16/3 H2 (g) --> 4/3 C3H8 (g). And then I did Hess's Law according to the new equation with the 4 moles of C.

Nha Dang 2I

The answer given at review session was 16 degrees.

Nha Dang 2I

deltaG=0 when the reaction is at equilibrium, so for that problem, it's because it's at a phase change.

Nha Dang 2I

For #7, I understand that we have to include deltaH(fusion) into the calculations, but can someone explain where and how? And when calculating q(ice), would we be using the specific heat for ice in this case?

Nha Dang 2I

I feel like it should be 2 sig figs as well given the 25 degrees.

Nha Dang 2I

In terms of calculating things such as the iced tea problem on the previous test, can someone explain conceptually why you have to add delta H fusion/vaporization on top of calculating the Q=-Q ?

Nha Dang 2I

I believe you use 75.3 J/(molxK) which is the molar heat capacity.

Nha Dang 2I

In an exothermic reaction at constant pressure, 50 kJ of energy left the system as heat and 20 kJ of energy left the system as expansion work. What are the values of (a) deltaH and (b) delta U for this process? Answer: (a) 50 kJ (b) 70 kJ Why don't you include the energy that left the system as expa...

Nha Dang 2I

how similar are the homework questions to lavelle's test questions? On the syllabus, it says, "Homework problems will be included in tests and exams"! And I believe I saw somewhere that said at least 1 homework problem will be on the test? Either way, doing the homework always helps for L...

Nha Dang 2I

Self-Test 8.1 Water expands when it freezes. How much work does 100g of water do when it freezes at 0.0 C and pushes back the metal wwall of a pipe that exerts an opposing pressure of 1070 atm? The densities of water and ice at 0.0 C are 1.00gcm3 and .92 gcm3, respectively. [Answer: w=.86kJ] How wou...

Nha Dang 2I

I found this on the bottom of page 464: "In Fundamentals, Section J, we saw that an Arrhenius acid is a compound that produces hydronium ions in water and an Arrhenius base is a compound that produces hydroxide ions in water. The Brønsted definition is more general because it includes the possi...

Nha Dang 2I

For strong acids, it's best to memorize the seven (HCl, HBr, HI, HNO₃, H₂SO₄, HClO3, HClO₄), and I believe it's usually assumed all other acids are weak acids.

Nha Dang 2I

Usually just seeing if K < 10-3 is sufficient, but if you want to be extra certain, you can use the 5% rule. I think either works.

Nha Dang 2I

Yeah, you don't include water because it is a liquid. Liquids and solids aren't included in the constants because they don't affect the reactant amount at equilibrium.

Nha Dang 2I

Is there every an instance where there's both a strong acid and a weak base reacting together? And if so, would you still use the equilibrium arrow or would you use a regular chemical reaction arrow in that case?

Nha Dang 2I

No, Ka is [H3O+][A-]/[HA-], so the products divided by the reactants for K!

Nha Dang 2I

When using an ICE chart, do you only you use it for weak acids/bases where there's an equilibrium reaction? Why?

Nha Dang 2I

Here's a thread with the explanation of the question: https://lavelle.chem.ucla.edu/forum/viewtopic.php?t=8795 In terms of memorization of strong acids/bases, I believe we just have to (or it would be helpful to) memorize the common ones, like the "strong six" acids (HCl, HBr, HI, HNO₃, H₂...

Nha Dang 2I

The equation is

Ba(OH)2 ---> Ba2+ + 2 OH-

So for every mole of Ba(OH)2, you have 2 moles of OH-, so you multiply the concentration by 2.

Nha Dang 2I

I think it would probably be helpful to know the "strong six" (HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄) for acids, and the rules for strong bases (Group 1 Hydroxides, Group 1&2 Oxides).

Nha Dang 2I

Not sure if we'll have the pKb/pKa provided but based on the tables in the text, the pKb of the hypobromite ion is 5.31 and the pKb for C17H19O3N is 5.79. Since the pKb of hypobromite is smaller, it's the stronger base.

Nha Dang 2I

During review session yesterday, they mentioned that strong acids/bases don't have conjugate bases/acids? Can someone explain this?

Nha Dang 2I

For the final, will we have to memorize the whole table of ligands? or just the more common ones?

Nha Dang 2I

How do we know when to use the equilibrium arrow when writing reactions with acids and bases? Do you do it whenever there's weak acids and bases? I was reading through Fundamentals J, and I noticed on F84 that they wrote a net ionic equation for the neutralization of the weak acid/base but they used...

Nha Dang 2I

4.9 (a) What is the shape of an ICl3 molecule (iodine is the central atom)? (b) What value is expected for the ClICl bond angle?

When we finish filling up the octet, we can just put the extra electrons on the central atom? How does this work?

Nha Dang 2I

4.7 (a) What is the shape of the thionyl chloride molecule, SOCl2? Sulfur is the central atom. (b) How many different OSCl bond angles are there in this molecule? (c) What values are expected for the OSCl and ClSCl bond angles? For A, I'm not sure if I drew it correctly. Is it supposed to be single ...

Nha Dang 2I

Does anyone know why cyanido is written before cobalt even though that's not alphabetical?

Nha Dang 2I

17.29 Name each of the following complex ions and determine the oxidation number of the metal: (b) [Co(NH3)6]3+

Can someone explain why cobalt has a +3 oxidation number? or how we know the oxidation number of NH3?

Nha Dang 2I

What functions/constants/equations will we be given for the quiz?

Nha Dang 2I

For the homework this week, will it be including Chapter 2? Or is it still just Chapter 1 #3-41?

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