CHEMISTRY COMMUNITY

Nicole Elhosni 2I

We use the equation lnk= -Ea/RT + lnA, and we are solving for A.
You get k from the question above, so ln(50.45 M⁻²s⁻¹)=-(23 x 10³ J/mol)/(8.314 J/Kmol x 298K) +lnA
lnA=ln(50.45 M⁻²s⁻¹) + (23 x 10³ J/mol)/(8.314 J/Kmol x 298K)
So A=5.43x10⁵ M⁻²s⁻¹.

Nicole Elhosni 2I

Yes that is the answer.

Nicole Elhosni 2I

One thing I like to do is make sure I know how to use every equation and the concepts behind them. They kind of summarize the big points of what we learn in class.

Nicole Elhosni 2I

When you take the integral of the rate equation, -d[A]/dt=k[A]², as it is a second order reaction and you are assuming all the coefficient a =1. You can use separation of variables and rewrite the equation as (-1/[A]²)(d[A]) = kdt. Integrate both sides and you will get 1/[A]=kt+C. The antiderivative...

Nicole Elhosni 2I

In class we discussed the half life of second order equations, so we do need to know it. The equation is t₁/₂= 1/k[A]ₒ

Nicole Elhosni 2I

Does the frequency factor change if the reaction is multiplied by 2? N2O --> N2 + O2 vs 2N2O --> 2N2 + 2O2?

Nicole Elhosni 2I

And question 4J.5, it was asking for the formation of NH3 for (a), which is given by N2(g) + 3H2 (g) --> 2NH3 (g). You still standard enthalpy of formation of products - standard enthalpy of formation of reactants, but the standard enthalpy of the reactants is 0.

Nicole Elhosni 2I

The question is asking for the standard enthalpy of the reaction, which is found by the sum of the standard enthalpy of formation of the products - the standard enthalpy of formation of the reactants.

Nicole Elhosni 2I

You only put the elements or compounds in the cell diagram that are being oxidized or reduced. If H+ was reduced to H2, then you would put it at the cathode. But when using H+, H20, and OH- to balance redox reactions, they are not placed into the cell diagram notation.

Nicole Elhosni 2I

From the example Dr. Lavelle went over in class, diamond to graphite with a ΔG of -3, we know that graphite is thermodynamically stable compared to diamond. Because he told us the reaction occurred slowly, that meant the activation energy was too high to be kinetically spontaneous.

Nicole Elhosni 2I

I don't think there has to be a certain order in the total redox reaction, but I know if you are writing the whole process of the galvanic cell with the notation, then the oxidation reaction (anode) has to be on the left of the II (representing the salt bridge) and the reduction reaction (cathode) h...

Nicole Elhosni 2I

You also need to take whether it is in an acidic or basic solution into consideration (this will be told). If it is occurring in an acidic solution, you will balance the O with H2O and the H with H+. If it is occurring in a basic solution, you will balance the O with H2O and the H with H2O, and add ...

Nicole Elhosni 2I

NAD is reduced to NADH, NAD + 2e + (H+) - --> NADH
NAD is reduced, so it is the oxidizing agent as it allows for there to be a form that results as a loss of electrons.

Nicole Elhosni 2I

It is similar to work and how work affects the system and surroundings. If work is put into the system, it is because the surroundings have released it. And if the surroundings are worked on, it is because the system has released it. So work of the surroundings = work of the system. I think it is th...

Nicole Elhosni 2I

You can assume pressure is constant because the question says the system expands irreversibly, which is an expansion process with constant pressure.

Nicole Elhosni 2I

found this gem

Nicole Elhosni 2I

Since for an ideal gas the internal energy is proportional to temperature, it follows that there is no change in the internal energy of the gas during an isothermal process.

Nicole Elhosni 2I

I think temperature is not included in w=-pΔV, because it is a closed system and the pressure is constant, so when you change the temperature by adding or removing heat, the only thing that it affects is the volume. So the temperature change is manifested in the volume.

Nicole Elhosni 2I

When pressure is constant (in an irreversible expansion), you will use the first equation w=-pΔV.
When pressure is not constant (in a reversible expansion), you will use the second equation w=-nRTln(V2/V1)

Nicole Elhosni 2I

Q is equal to enthalpy when the pressure is kept constant, so Qp is always equal to enthalpy.

Nicole Elhosni 2I

Volume is equal to the area of the base times the height, and the height is equal to the distance!

Nicole Elhosni 2I

The modules on the class website only covered equilibrium, which finished week 1.

Nicole Elhosni 2I

q(p) is equal to the change in enthalpy, so the final enthalpy (H of products) - the initial enthalpy (H of reactants). So when heat is absorbed, the enthalpy of the the products will be higher than the enthalpy of the reactants, and then ΔH will be positive, and thus q(p) will be positive. When hea...

Nicole Elhosni 2I

If you have trouble visualizing it, it is kind of like the transitive property in math. So if A-> B, and B->C, then A->C and you can add each reaction's change in enthalpy, A->B and B->C, to get the entire reactions change in enthalpy, so from A->C!

Nicole Elhosni 2I

Helium is an inert gas so it will have no effect on the reaction. But if it was a gas that did have an effect, in the case of this reaction, it would not lean right or left since the moles of gases are the same for the reactants and products.

Nicole Elhosni 2I

Pressure and volume and concentration do not affect K, but temperature does. If it is an endothermic rxn and you increase the temperature, more products will form and K will increase. So K will change from a change in temperature.

Nicole Elhosni 2I

If we were given a reaction of a weak acid dissociating, then the reverse reaction would be the conjugate base. So if we are given Ka, can we find Kb by taking the inverse? Or should we use that Ka x Kb = Kw every time?

Nicole Elhosni 2I

If Q < K and the desire is to get the ratio to equal K as the reaction is in the process of reaching equilibrium, then we have to think about how we can increase Q. Both are calculated by dividing the concentration of products raised to their stoichiometric coefficients by the concentration of react...

Nicole Elhosni 2I

When we are using quadratics to find the equilibrium concentration, is it okay to assume that x in our ICE tables for the reactants is very small and thus the initial concentration - x = initial concentration? Or can we only assume this when it is a cubic function?

Nicole Elhosni 2I

How come water is included in the equilibrium constant when it is in the gaseous state? Is it because all gasses are included in the calculation when finding k?

Nicole Elhosni 2I

In class, Dr. Lavelle pointed out though that both SO4 2- and CO3 2- have resonance, which means their electrons are delocalized and the anion will be stable. Doesn't this make the acid a strong acid then?

Nicole Elhosni 2I

In the outline at the bottom, it only says to know how to find pH of strong acids and bases.

Nicole Elhosni 2I

We do not need to know how to calculate pH of weak acids/bases. The pH scale only deals with strong acids and bases.

Nicole Elhosni 2I

I think you have to look at the lewis structure and what kind of molecular geometry it forms. When I was doing this question and I looked up how many ligands CO3 2- could form, and it could either be monodentate or bidentate depending on the bond angle. I still don't really understand it but here is...

Nicole Elhosni 2I

Also one thing I always mix up between charges and oxidation states is how to denote them. For an oxidation state, you write + or - #; for charges you write # + or -.

Nicole Elhosni 2I

Does resonance structure have any effect on whether a molecule can be polydentate?
For example, CO3 2-. The book said it could be either monodentate or bidentate, but I thought it might be tridentate since the electron density is the same between all three so each oxygen has electrons to donate.

Nicole Elhosni 2I

I usually focus on the lectures and homework and I read if I am still confused.

Nicole Elhosni 2I

It is asking about the regions of electron density, as the # of hybridized orbitals is equal to the # of regions of electron density. So for sp, there are 2 regions, so it is linear. sp2 has 3 regions, trigonal planar. sp3 has 4 regions, tetrahedral. sp3d has 5 regions, trigonal bipyramidal. sp3d2 h...

Nicole Elhosni 2I

Why does BeCl2 even have a lewis structure with bonds in the first place? I thought it was an ionic compound but then I just figured we could draw it with bonds because no ionic compound is 100% ionic... is that the reason?

Nicole Elhosni 2I

On the chem portal, it says "Test 3 covers: End of Bonding from 3.12 (6 Ed.) and from 2D (7 Ed.); and all of Molecular Shape and Structure (see Syllabus and Outlines for details)."

Nicole Elhosni 2I

Is it that the greater the distortion between the two bonding atoms of elements, the more covalent character there is? Because the electron density regions are pulled into the area between the two atoms?

Nicole Elhosni 2I

The smaller the ionic radius of the cation, the greater its ability to distort the electrons of the pairing anion. So if it is between Na+ and Mg2+, Mg2+ will have a greater polarizing power because it is holding its remaining electrons super close, and is smaller as a result.

Nicole Elhosni 2I

Why is that in the CH2Cl2 molecule, the dipole moments of the chlorine atoms do not cancel out and result in a nonpolar molecule?

Nicole Elhosni 2I

Bond length is dependent on what atoms are involved, but the trend follows that a triple bond will be the shortest and strongest and a single bond will be the longest and weakest.

Nicole Elhosni 2I

H2O has a higher melting and boiling point because the oxygen on one water molecule forms a hydrogen bond to the hydrogen onto another water molecule. These hydrogen bonds are the reason water has a higher melting and boiling point. A solid has many bonds between molecules so when changing phases to...

Nicole Elhosni 2I

Both copper and chromium have 1 electron in their valence shell, because both have 4s^1 as their final orbital in their electron configuration, Cr [Ar] 3d^5 4s^1, Cu [Ar] 3d^10 4^1. I don't think knowing their valence electrons is as important as elements in group 1,2, 13-17 anyway because Cr and Cu...

Nicole Elhosni 2I

K3P is an ionic compound, so it is going to be a transfer of electrons. K has 1 valence electron and P has 5 valence electrons. K wants to lose its 1 valence electron so it will have a full, stable electron configuration that resembles [Ar]. P wants to gain 3 electrons in order to have a full, stabl...

Nicole Elhosni 2I

When you are calculating the formal charge with the equation: FC= (Valence electrons) - (lone pair electrons + shared electrons/2) then the lone pair electrons will be counted as individuals, since it is asking for the number of electrons in the lone pairs and not the number of lone pairs. (for exam...

Nicole Elhosni 2I

Chromium is an exception, because it will be more stable to have half of the d block filled and only one electron in the s block, [Ar]3d5 4s1 instead of [Ar]3d4 4s2. Copper is also an exception, and its electron configuration will be [Ar]3d10 4s1. From its placement on the periodic table, you would ...

Nicole Elhosni 2I

Ionization energy is the energy required for an element to lose an electron in a gaseous state; electron affinity is the amount of energy released when an element gains an electron in a gaseous state.

Nicole Elhosni 2I

You don't really have to memorize the octet exceptions because with the electron configuration of the elements and their ions, it sort of does it for you! For hydrogen to be stable (and to be stable is to have a full valence shell), it only needs to gain one electron because the valence shell is 1s....

Nicole Elhosni 2I

NH4+ and SO42- are both polyatomic ions, ammonium and sulfate. In my highschool chemistry, we had to memorize a list of polyatomic ions, but I am not sure for this class. But you can figure out how the entire molecule, (NH4)2SO4 breaks down into ions without knowing the polyatomic ions before! (NH4)...

Nicole Elhosni 2I

In the photoelectric effect, light with a long wavelength (and high intensity) in turn has a low frequency and therefore low energy. Because a photon shown onto a metal only has the capability of removing one electron, it will not matter how many photons there are if the energy of the light does not...

Nicole Elhosni 2I

I think will be posted on MyUCLA but they just haven't been updated yet. So far, I have been getting back homework and tests in discussion after they have been scored.

Nicole Elhosni 2I

Electrons in the Lyman Series are part of the UV spectrum because they are all dropping to n=1, which is the farthest distance and therefore requires the most energy. The energy levels of a hydrogen atom converge as they increase, so the energy difference between higher levels decrease (in compariso...

Nicole Elhosni 2I

I am going to try and remember the ranges they are in. For example, that x-rays are 10^-9 m. There was a question on the homework that asked to identify what spectrum the light was in based on its wavelength.

Nicole Elhosni 2I

When an electron absorbs energy, it will increase its energy level and as a result, have a higher energy(conserving the energy). When the electron loses energy by going from a high energy level to a lower one, the energy lost between the two energy levels will be emitted as light (therefore conservi...

Nicole Elhosni 2I

Okay, because it is the hydrogen atom and it is known to be in the UV region (aka Lyman Series), we know that the final energy level will be 1. Given the 102.6 x 10^-9 m, we can use wavelength x frequency = speed to find the frequency. From this we can use the frequency and multiply it by Plank's Co...

Nicole Elhosni 2I

The equation is ΔE= -(h x R)/n² When you are calculating the energy difference between an electron that has gone from n=4 to n=2 (emitting light), you find ΔE for n=4, n=2 and then you subtract the difference. In this case, it would be ΔE for when n=2 - ΔE for when n=4, as it is final - initial, and...

Nicole Elhosni 2I

I got stuck on that problem as well! I searched online because I had been trying to balance the equation for some time, and when I searched it, it said that the equation they asked us to balance was not possible.

Nicole Elhosni 2I

For that question, you have to convert 25.2 kg of UF6 (subscript) to grams to moles. And then because it is asking for the amount of moles of F- ions, you multiple the amount you just calculated by 6 because there are 6 F- ions in this compound. Since it is asking for moles of ions, we do not need t...

Nicole Elhosni 2I

The modules are recommended but they are not officially due. When you take the pre and post assessments, it says that it is not for a grade and they do not know which students are taking the assessments. I did them and they helped me a lot.

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