CHEMISTRY COMMUNITY

Aijun Zhang 1D

https://www.dropbox.com/s/l2i660irqroxn6k/IMG_6189.JPG?dl=0 You draw a graph like this. P1 refers to product 1, P2 refers to product 2. A is the reactant. The two high peaks are two transition states. You can see that P1 is thermodynamically more stable than P2, so P1 is more favorable in thermodyn...

Aijun Zhang 1D

Ka is the acidic constant, which means it is for acids dissociation reaction. in acid dissociation reaction, H+ is dissociated. Therefore, H+ will always be on the product side. In this way, you write down the whole reaction happens. For example, in practice exam, HF\Leftrightarrow H^{+}+F^{-} . Ka ...

Aijun Zhang 1D

I think if we are given the resonance bond enthalpy for benzene, we should use that value since it is the real true value for benzene.

Aijun Zhang 1D

When you use equation $E = E^{o}-\frac{RT}{nF}lnQ$ , you use lnQ.
When you use equation $E = E^{o}-\frac{0.05916}{n}logQ$ , which is the simpler equation for condition under 298K, you use logQ.

Aijun Zhang 1D

The second step is the slow step, so rate is determined by this step: rate = k[NO3][CO].
And NO3 is formed from step 1, so the rate = k[NO2]^2[CO].
CO cannot be eliminated from the expression since it appears in the slow step.
I do not get why you [NO3] = k1[NO2]^2 / k2[CO].

Aijun Zhang 1D

Rate cannot be negative. It is defined as positive. Similarly, we usually consider speed as positive.

Aijun Zhang 1D

I think both slow step and fast step are not the overall step in the equilibrium.
The step in the equilibrium should be a combined step.

Aijun Zhang 1D

Yes we add the two steps together to get 2AC+B\rightarrow A_{2}B +2C And you can see AB appears twice but is canceled in the two steps. So AB is the intermediate substance. I think you can mark either one as the longest step for the rate determining step since the only restriction is the rate determ...

Aijun Zhang 1D

zero order reaction means that the rate of the reaction is independent of the reactant concentration. So whenever the concentration of reactants changes, the rate will not change. Therefore, the rate differential law equation is Rate = k. [A] = [A]0 - kt Usually, a catalyst is present in zero order ...

Aijun Zhang 1D

I believe we relate kinetics and thermodynamics in entropy part where we can use enthalpy change and entropy to calculate equilibrium constant K.
However, in chapter 15, we do not talk about thermodynamics but only focus on kinetics.

Aijun Zhang 1D

I think you can find the intact solution on the course website.
https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14B/Midterm_exam_ans.pdf

Aijun Zhang 1D

I believe you do not need to put O2 in it since it is just a comment to specify the substance calculated.

Aijun Zhang 1D

Slow step is the rate-determining step. The fast step does not affect the rate of a reaction. So when express the rate reaction of two connecting reactions, always express the reactants in the slow step.

Aijun Zhang 1D

A quick question:
Dr. Lavelle mentioned in class that we considered only forward reaction. Why would we not consider the backward reaction although some of the products will return to the reactant?
Thanks!

Aijun Zhang 1D

Just a quick question.
When we calculate cell potential for a reaction composed of two half reactions. Do we always subtract the smaller standard reduction potential from the larger one? Therefore the final result is always positive?

Thank you!

Aijun Zhang 1D

I think the order of substances does not matter in cell diagram. You just list the substances that participate in reaction on anode and cathode.

Aijun Zhang 1D

Pts are just inert electrodes. The oxygen and H+ ions are on the anode side. Oxidation occurs anode side. So oxygen in the water will be oxidized into oxygen. 2H2O -->4H+ +O2+4e- On the cathode side, reduction occurs. So O2 is reduced to OH-. O2+2H2O+4e-->4OH- Then we need to balance the number of e...

Aijun Zhang 1D

Pts are just inert electrodes. The oxygen and H+ ions are on the anode side. Oxidation occurs anode side. So oxygen in the water will be oxidized into oxygen. 2H_{2}O \rightarrow 4H^{+}+O_{2}+4e^{-} On the cathode side, reduction occurs. So O2 is reduced to OH-. O_{2}+2H_{2}O+4e^{-}\rightarrow 4OH^{...

Aijun Zhang 1D

When the pressure is constant, we use w = -Pext $\Delta V$

When temperature is constant (isothermal), we use w = -nRT ln (Vfinal/Vinitial)

Aijun Zhang 1D

I think the delta G of formation values in the table is just the delta G knot value. It is just the value of a substance in the standard state. However, delta G can be a value at any state, for example, not 298K or 1 atm. The result will be the same if the reaction is under the standard state. Also,...

Aijun Zhang 1D

So you can first write down all the equations related to entropy. Since entropy is a state function, it can be added and substracted. \Delta S = nRln\frac{V2}{V1} \Delta S = nCln\frac{T2}{T1} \Delta S =\frac{q_{w}}{T} \Delta S = k_{b}\ln W Helium gas and Krypton are in a sealed apparatus, so the vol...

Aijun Zhang 1D

Yeah, basically you need delta G to determine spontaneity.
Or you can use K to indirectly find the spontaneity at equilibrium.

Aijun Zhang 1D

$\Delta S = \frac{q_{reversible}}{T}$
When T (in Kelvin) is constant, then q = -w since U = q + v.
So $\Delta S = \frac{-w}{T}$ (T constant)

Also, any energy lost by doing work is compensated by heat going into the system.Then another equation about w is $W = -nRTln\frac{V_{2}}{V_{1}}$

Aijun Zhang 1D

Endothermic reaction means that \Delta H > 0 since it absorbs energy. The Gibb's free energy equation is \Delta G = \Delta H - T\Delta S and we know that this equation determines the spontaneity of a reaction. We want \Delta G to be negative. Now it comes to algebra. There is actually a chart to org...

Aijun Zhang 1D

Gibb's free energy is just "G" which is a state function, but standard free energy change is \Delta G°, indicating substance in a standard state. \Delta G is the G at equilibrium since G is a quadratic function. Its tangent line at local minimum is \Delta G. And at that point, \Delta G = 0...

Aijun Zhang 1D

I think the equation delta S = q/T is only for the reversible question. (for only isothermal process)

Aijun Zhang 1D

Every state has a finite value for the state function entropy S(A). The difference of entropy between two states is delta S = S(B) - S(A) There are many different reversible pathways between two different states. But state function does not depend on the paths it goes from one state to another. It j...

Aijun Zhang 1D

This is set in the two-state system scenario.
When 2 particles are both in one of two states, then W = 4 = 2^2
If there is only 1 particle, then W = 2^1
So for N particles, a particle can be in one of two states, then W = 2^N

Aijun Zhang 1D

If nothing is specified, the standard temperature for gas reaction is 298K.

Aijun Zhang 1D

Reversible process: can change the reaction by changing a variable in subtle amount. The variable is usually pressure.
Also the reversible work is always greater than irreversible work.

Aijun Zhang 1D

I think we should add it. There is an example from my discussion. 20g of Cu at 45 degrees celsius is placed into 10g of water at 90 degree celsius. Find the final temperature. If the question is set in calorimeter, then q(cu)+q(cal) = -q(H2O) Because copper and calorimeter absorb heat, so the total ...

Aijun Zhang 1D

You can just simply look at exercise 8.1 to help understand it. A closed system is when heat is exchangeable but not mass, for example, the mercury in a thermometer. Mercury is sealed in the thermometer, but the temperature will change with the heat supplied. So it is a closed system but not an isol...

Aijun Zhang 1D

Can someone explain how to do the constant pressure and volume question? 8.31 Calculate the heat released by 5.025 g of Kr(g) at 0.400 atm as it cools from 97.6 C to 25.0 C at (a) constant pressure and (b) constant volume. Assume that krypton behaves as an ideal gas. I'm really confused with these c...

Aijun Zhang 1D

I think most of the formulas given are between two to four in tests since the time to finish exam is needed to be counted into consideration. However, I believe you can write infinite forms for Hess's law, especially in organic chemistry. Just consider the reactants and products and try to link them...

Aijun Zhang 1D

Reyna Alonso 1E wrote:bar is the atmospheric pressure at sea level so it can also be translated as as 100 kilopascals which is about 1 atm.

Thank you!

Aijun Zhang 1D

24. The equilibrium constant, KP, for the reaction SO2 (g) + O2 (g) ⇌ SO3 (g) at 700 K is 3 x 104. A mixture of SO2, O2, and SO3, each at 65 bars was introduced into a container at 700 K. Is the reaction at equilibrium? If not, does SO3 tend to form or decompose? A. At equilibrium. B. Not at equilib...

Aijun Zhang 1D

Basically when you calculate the Kc or write Kc expression, you do not include solid and liquids as well as solvents.
And Ka, Kb and Kw also only includes aqueous solution.
Therefore the change in concentration of solids and liquids will not affect the Kc, Ka and Kb.

Aijun Zhang 1D

The point is that the uncertainty of velocity is 0.55 x2 = 1.1 m/s since it can be plus or minus 0.55. The range is actually larger. momentum(p) = m * v. uncertainty in momentum in this case = m * Δv. so Δp = 0.55 * 2 * 1.5 * 10^-3 (you need to convert grams into standard unit kilograms) The Heisenb...

Aijun Zhang 1D

I think CH3NH3+ is the best among the three you gave. Because when we want a conjugated acid of CH3NH2, you need to add one proton, in other words, one H+ to the molecule. Carbon is already connected to 3 hydrogen and then one N atom with two hydrogen asides. It is more normal to add one on N. CH3NH...

Aijun Zhang 1D

Actually the for acid strength: HF < HCl < HBr < HI. HCl is a stronger acid since it is completely ionized in water while HF isn't. The bond strength of HCl is weaker than HF due to the increased radius of Cl, leading to larger distance between H and Cl than H and F. So H-Cl bond is easier to break ...

Aijun Zhang 1D

26. Are solvents included in equilibrium constant expressions?

A. No

B. Yes

C. Sometimes

D. Not always

E. None of the above

I think solvents are always aqueous? but the answer is not B.
Please help me! Thanks!:)

Aijun Zhang 1D

I think the answer is B. N2(g) + 3H2(g) ⇌ 2NH3(g). I 0.250 0.500 0 C -0.075 -0.225 +0.15 E 0.175 0.275 0.15 Basically, the change of concentration of NH3 indicates the change of N2. If concentration of NH3 is increased by 0.15mol/L, then the N2 is decreased by 0.075mol/L, calculated by the coefficie...

Aijun Zhang 1D

"Lie to the right" is used when describing K at the equilibrium. It is a state. From the note, if K is small (K<10^-3), more reactants are at equilibrium. So the equilibrium sits to the left. Vice versa. This basically describes which side is favored. While "equilibrium shifts to the ...

Aijun Zhang 1D

The K expression for original reaction is [NH3]^2/([H2]^3*[n2]), and the K expression for b is [NH3]/([N2]^1/2*[H2]^2/3).
Say the K expression for (b) is K2.
K is the square of K2. So if you want to get the value of K2, you need to take the square root, which is square root of 41. The answer is 6.4.

Aijun Zhang 1D

The Kc for 2BrCl + H2 = Br2 + 2HCl (you wrote Br2 instead of H2 which is wrong) is expressed as [Br2]*[HCl]^2/([H2][BrCl]^2). The Kc for 2BrCl=Br2 + Cl2 is expressed as [Br2][Cl2]/[BrCl]^2; the Kc for H2 + Cl2 = 2HCl is expressed as [HCl]^2/([H2][Cl2]). As the Kcs of later two times, you can get the...

Aijun Zhang 1D

For symmetrical molecules, the polar effect can be canceled. So they are non polar molecules.
Though some molecules will have polar bonds, but if the whole molecule is symmetrical, the polar effect can be canceled out. The polar molecule such as water has an uneven distribution of electron density.

Aijun Zhang 1D

I think K is the constant representing the ratio of concentration of products and reactants at equilibrium, but now initial concentrations.
For any other circumstances, we use Q to represent the ratio.

Aijun Zhang 1D

You should choose the answer that is smaller than the initial concentration of the reactants.
Because x is just an amount of change during the reaction, so the concentration change must be smaller than the initial concentration of reactants. If it is larger, then the answer should be rejected.

Aijun Zhang 1D

For 17.29 b and c, I noticed that the answer for b just contains "cobalt(III) ion"; however, answer for a and c is "ferrate(II) ion" and "cobaltate(III) ion". Why is cobalt in b is written as only the metal name, but cobalt in c is written as -ate form?
Thanks!

Aijun Zhang 1D

Benzene is nonpolar since it has a symmetrical structure and all the bond dipoles cancel. But for pyridine, the C-H group is replaced by a nitrogen. And there is a lone pair on nitrogen. The molecule then becomes not symmetrical since C-N bond has different dipole then C-C bond. Also C-H bond dipole...

Aijun Zhang 1D

the formation of sigma and pi bonds are due to orbitals interaction. Sigma bonds are formed by two orbitals end to end interact and pi bonds are formed by two p orbitals overlap side by side. For an atom in a molecule, we can determine the hybridization and number of sigma bonds and pi bonds in it. ...

Aijun Zhang 1D

How do you know the charges for the elements? memorization? It's basically based on electron configuration, or say, how it would be most stable to form a compound. For example, for group 1 and 2, it is most possible that they are tend to lose electrons when forming a compound. Then the number of el...

Aijun Zhang 1D

For a the answer is HCl since the electronegativity difference is bigger between H and Cl than H and I. Cl is more electronegative than I. Also for H, you can consider it the same as Phosphorus which shares the same electronegativity. For b the answer is CF4 since F is much more electronegative than...

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