CHEMISTRY COMMUNITY

Ray Guo 4C

For question 14 part (c). What information is needed to determine the order of each step?

Ray Guo 4C

I used $E=-\frac{RT}{NF}lnQ=-\frac{8.314\times 298.15}{1\times 96485}ln\frac{10^{-14}}{10^{-7}}\approx 0.414$ . I thought Q is between 10^-7 and 10^7, but the answer seems to imply that instead it should be between 10^-14 and 10^14.

Ray Guo 4C

Can anyone tell me how to arrive at this answer?

Ray Guo 4C

The more positive the reduction potential, the stronger the oxidizing power and the weaker the reducing power. Just think reduction potential as measuring a compound's tendency to be reduced. A more positive reduction potential means that itself is more easily reduced than oxidized.

Ray Guo 4C

In the book, A is just called the pre-exponential factor. I think it will become clearer in the next lecture as we get into collision theory. Then you see what constants are included in A.

Ray Guo 4C

First, a galvanic cell is not connected to a battery. A galvanic cell is a cell that generates electricity through a spontaneous chemical reaction. A cell that is connected to a battery is an electrolytic cell. An electrolytic cell uses energy from the battery to drive reactions. Then to answer your...

Ray Guo 4C

Use any formula from rate law, integrated rate law, and half-life. Taking a look at the examples in the textbook is pretty helpful.

Ray Guo 4C

Because $t_{1/2}=(ln2)/k$ for first-order reactions.

Ray Guo 4C

Because it has a constant half-life. All first-order reactions have constant half-lives.

Ray Guo 4C

Can you specify what you mean by changing the "stoichiometric coefficient of one reactant changes"? Will the equation still be balanced if you change only one reactant's coefficient?

Ray Guo 4C

From the 7th ed textbook page 433.

Ray Guo 4C

For the same question, how is the right-half cell set up with three solids?

Ray Guo 4C

In some half-cells the solid reactant can be used as the electrode, such as zinc in Zn/ $Zn^{+2}$ . How do I know which solid reactants can be used as electrodes and which ones cannot?

Ray Guo 4C

I have two questions about porous disks. 1. When a porous disk is used, why do electrons flow through electrodes instead of through the solution to the other half-cell? 2. When a porous disk is used, are the reactants mixed? If they are, why don't they react directly without donating electrons to an...

Ray Guo 4C

When it's above 0, it means that the flow of electron from the left half-cell to the right half-cell is spontaneous. That is, oxidation in the left half-cell and reduction in the right half-cell.

Ray Guo 4C

I think platinum is often used as an inert electrode because it does not react with the reactants in the cell. They are only responsible for conduction of current. An example is a hydrogen half-cell with uses platinum as the electrode.

Ray Guo 4C

G=H-TS. Gibbs free energy is the energy available to do work. It's less than the energy of a system because not all of the energy can be used to do work.

Ray Guo 4C

If the external pressure is constant, it's irreversible expansion.

Ray Guo 4C

It depends on the system at hand. If the system is isolated, heat cannot come into the reversible expansion and replace the additional energy loss, so the first law of thermodynamics tells us that in the case that both systems (reversible and irreversible) are isolated, the irreversible one would i...

Ray Guo 4C

Have trouble getting the right answer of Self-test 4B.3A on the 7th ed book. "Estimate the contribution of motion to the molar internal energy of water vapor at 25 Celcius degrees." Why is it not 3.72 kJ.Mol^-1?

Ray Guo 4C

The expansion for a reversible system is done infinitely slowly and less heat is released to the surroundings resulting in more work done. You can better understand this process by viewing the pressure volume graphs for the two types of systems. The area under the curves shows you the work done by ...

Ray Guo 4C

I understand mathematically and graphically why a reversible expansion does more work, but does gas undergoing irreversible expansion ends up with more internal energy because it does less work?

Ray Guo 4C

I think they are the same, except the enthalpy ones are often in J per mol while specific heat capacity can be in J per mole or J per gram. The value of the enthalpy ones can also change when you change the stoichiometry of the reaction of vaporization/sublimation.

Ray Guo 4C

And if concentration is concerned, it would be 1M.

Ray Guo 4C

According to Boltzmann’s equation S=k*ln(W), where W is the degeneracy, what I understand to be the number of possible microstates of the same energy level, more the number of microstates with the same energy level higher the entropy value.

Ray Guo 4C

I think any system changes energy when work is done on it.

Ray Guo 4C

An increase in T means that the surroundings have taken up some energy given out by the reaction. If a reaction system gives off energy then its own energy content has decreased, hence a decrease in enthalpy (H).

Ray Guo 4C

I think a large surface area allows a quicker rise in temperature instead of a larger one. Which means the magnitude of the increase in temperature would still be the same when two objects of different surface areas are given the same amount of thermal energy.

Ray Guo 4C

I think it's always exothermic because the only change in bonds is the formation of water molecules. Since the other ions are dissolved in water, they don't have a change in bonds.

Ray Guo 4C

Heat energy is used to break intermolecular bonds, so kinetic energy stays constant.

Ray Guo 4C

Yes, because breaking a bond is always endothermic.

Ray Guo 4C

If possible, you can compare their X-H bond strength to see which one is more likely to lose protons. That would be the stronger acid.

Ray Guo 4C

Do you mean what not to include in equilibrium constant formula? If so then you should just disregard anything in solid and liquid state.

Ray Guo 4C

Ka2 would always be smaller than Ka1 because it's harder to lose a proton from an already positively charged molecule.

Ray Guo 4C

A base tends to take on a proton, so having an OH group does not make it a base. You can try to memorize the names and formulae of common acids and bases. They are listed somewhere in the Fundamentals part of the book if you are using the 7th edition.

Ray Guo 4C

Don't we need the volume to solve question 5 on worksheet 1?

Ray Guo 4C

lindsey_ammann_4E wrote:
Meigan Wu 2E wrote:On Worksheet 2, why is the concentration of CO3 the same as Ka2 for problem 4b?

^ I have the same question

Because some approximation was used. You can look at the example on page 488 on the 7th edition textbook.

Ray Guo 4C

Is hydrofluoric acid a strong acid?

Ray Guo 4C

Do you know the volume? I think an ICE box can be used for partial pressure.

Ray Guo 4C

I think we should always use Kelvin unless you are just using the temperature difference.

Ray Guo 4C

You can multiply the equilibrium constants of the two reactions to get that of the third reaction.

Ray Guo 4C

Yes, we should only consider the number of moles of gases on each side in case of a change in pressure.

Ray Guo 4C

What defines ideal gases?

Ray Guo 4C

In a reaction that only involves gases, Kp is much more useful than Kc.

Ray Guo 4C

Question 6.5 in 7th edition asks that, in the reaction between hydrogen peroxide H2O2 and sulfur trioxide SO3 to form H2SO5, which reactant is Lewis acid and which is base. Can anyone explain why SO2 is the acid and H2O2 the base?

Ray Guo 4C

when it can donate or accept more than one proton

Ray Guo 4C

Can anyone also explain question (d)? I know that HClO4 is a strong acid and H3PO4 is not, but why is H3PO4 weaker?

Ray Guo 4C

Oxides formed by metalloids in the diagonal band are often amphoteric, such as barium oxides, aluminum oxides, gallium (Ga) oxides, tin (Sn) oxides, antimony (Sb) oxides and lead oxides.

Ray Guo 4C

I guess I wasn't clear enough about my question. I do understand the relationship between steric number and type of hybridization. What I am trying to ask is what (hybrid) orbitals is the sulfur atom using to form the three pi bonds. Normally pi bonds are formed by p orbitals, but in this case, only...

Ray Guo 4C

If you look at the Lewis structure of this molecule, a central S atom is bonded to 3 O atoms. Each S-O bond is a double bond, and S has no lone pairs. Thus, S has 3 regions of electron density (a double bond counts as a single region of electron density, and there are 3 of them, hence 3 regions of ...

Ray Guo 4C

Why is SO3 sp2 hybridized? If two of the three p orbitals are used in hybridization, how are the three pi bonds formed with only one unhybridized p orbital left? Can d orbitals form pi bonds with p orbitals?

Ray Guo 4C

I think the less electronegative one should be in the middle because they more readily share electrons.

Ray Guo 4C

Yes, the chlorine ions are bound to the metal. The metal forms ionic bonds with chlorine ions and coordinate covalent bonds with ligands. You can consider this as adding a few ligands to an ionic compound.

Ray Guo 4C

the shape should look like this

Ray Guo 4C

Shouldn't the molecule be linear? If the chlorine atom has two lone pairs and two bonding pairs. if there are two lone pairs and two bonding pairs it should be bent. I know it sounds weird bc the two bonding and lone "cancel" each other out but if you think about it, we all know the shape...

Ray Guo 4C

Shouldn't the molecule be linear? If the chlorine atom has two lone pairs and two bonding pairs.

Ray Guo 4C

I think they are the same. We always consider lone pairs when determining the shape of a molecule, even if we kind of ignore them when naming the shape. So an angular molecule is never trigonal planar whether we are talking about "shape" or "molecular shape".

Ray Guo 4C

Try to determine whether the molecule is ionic, polar covalent, or non-polar covalent. Ionic compounds have ion-ion interaction. Polar molecules have polar-polar or hydrogen bonding. And, I think, all three have London dispersion forces.

Ray Guo 4C

Lewis structure only considered valence electrons, while VSEPR also takes minimization of electron repulsion into account.

Ray Guo 4C

The newest version of test schedule on the website says "Starting Tuesday November 27", though the version I downloaded earlier says November 20.

Ray Guo 4C

E_p∝−(α_1 α_2)/r^6

Ray Guo 4C

I think this is just an exception. Nitrogen is in the center because that gives the lowest formal charges of all three atoms.

Ray Guo 4C

Valence electrons refer to the outermost electron shell, which also has the highest quantum number (n). So for d-block elements, I think we just need to show the 2 electrons in the outermost s orbital. Don't worry about the outermost d orbital because it's at a lower energy level, with a smaller qua...

Ray Guo 4C

I think a molecule is the most stable when the formal charge of each atom, instead of the whole molecule, is zero. In SO4 we can't organize electrons in the way that every atom has a formal charge of 0, and the SO4 with a 2- charge is the most stable one we can get which has a structure with least f...

Ray Guo 4C

You'd have to check every single atom in the molecule because the molecule is the most stable when each atom in it has a formal charge of 0. You can calculate the formal charge of an atom using the formula [# of valence electrons] - [# of bonded electrons/2] - [# of non-bonding electrons]

Ray Guo 4C

So stronger the electron affinity, larger the value of Eea, because X would be fairly unstable with high energy right?

Ray Guo 4C

Because of λ=h/p, an increase in uncertainty of p increases the uncertainty of λ.

Ray Guo 4C

Thank you so much Karen :)

Ray Guo 4C

• for a photon: E=pc, E=hv
• ∴pc=hc/λ → λ=hp

Ray Guo 4C

You need to divide deltaX by 2 to get the +/- value.

Ray Guo 4C

four events are mentioned in the question, you just need to infer the type of electromagnetic wave in each row using the wavelength you get, and then knowing the type of wave gives you the event

Ray Guo 4C

Ray Guo 4C

the 4 orbitals are 2s, 2px, 2py, and 2pz

Ray Guo 4C

I'm confused, when is mass squared in 1/2 pc? Isn't that for electrons, 1/2 pc=1/2 mv*c=1/2 mv*v?

Ray Guo 4C

No, you don't.

Ray Guo 4C

KE=1/2 mv^2, but why is Energy=pc true instead of Energy=1/2 pc?

Ray Guo 4C

What's a nodal 0?

Ray Guo 4C

Because other units are based on kg, like joule(J) for energy and watt(W) for power. Joule is ‎kg⋅m2⋅s−2, and watt is ‎kg⋅m2⋅s−3. Using kg saves us from changing unit of mass during calculation.

Ray Guo 4C

The energy is still positive. It's just a convention to set the energy of free electrons as 0, which makes electrons in atoms have negative values when the formula you mentioned is applied.

Ray Guo 4C

Do we round the answer in every step of the calculation, or do we just round the final answer to the number of significant figures? Because when I round the answer in every step, my final answers are often different from those given by the textbook.

Ray Guo 4C

maybe the larger number of photons means more energy in total

Ray Guo 4C

My TA also said that we don't need to worry about states for now

Ray Guo 4C

Josephine Chan 1B wrote:Im not sure about 14b but for 14a, dr lavelle sent an email out about the class website with the password. So try checking the emails you got from the class email.

Thanks a lot!

Ray Guo 4C

Does anyone know the password to the 14B class website?

Ray Guo 4C

tonnes?

Ray Guo 4C

I think so. Percent yield is in some questions in Fundamentals.

Ray Guo 4C

Should use significant figures instead of decimal places because finding the mass percent composition uses division. The number of significant figures in your answer should be the same as that of the value with the least significant figures that you use in the calculation.

Ray Guo 4C

You can round the number. When calculating the empirical formula you often get numbers that are really close to whole numbers like 3.997 or 2.003. But if you get something like 6.667, consider multiplying it by a factor that gets you close to a whole number, like 3 in this case.

Ray Guo 4C

hey, where did you find the quizzes, on the class website?

Ray Guo 4C

Does anyone know when the grading for discussion posts starts, week 1?

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