CHEMISTRY COMMUNITY

ChathuriGunasekera1D

The anode is the one that gets oxidized (loses electrons) and the cathode is the one that gets reduced (gains electrons)

ChathuriGunasekera1D

I know it's mentioned as one of the hw problems but I wouldn't worry about it. I'm pretty sure he wouldn't put a problem that we wouldn't know how to do entirely.

ChathuriGunasekera1D

15.29 For the first-order reaction A(s)-----> 3B + C, when [A] 0 0.015 mol/L, the concentration of B increases to 0.018 mol/L, in 3.0 min. (a) What is the rate constant for the reaction expressed as the rate of loss of A? (b) How much more time would be needed for the concentration of B to increase ...

ChathuriGunasekera1D

The unique reaction rate tells how how much the reactant is being used in that specific equation (reliant on the coefficients).

ChathuriGunasekera1D

Hi! How would you use a balanced equation to create the rate of formation and use on a graph? For example, given 2A ----> B + 3C, how would you be able to visualize the graph that shows how fast A is used and C and B are made?

ChathuriGunasekera1D

Hi! n is the moles of electrons transferred. Once you find the reduction and oxidation reactions, balance them. The coefficient in front of the electron value should be the same for the reduction and the oxidation reactions, and that is the n value.

ChathuriGunasekera1D

I know that I use the word "unique" to know if I have to use (-1/a)(dR/dt)

ChathuriGunasekera1D

No, but it makes the process easier. Chances are, you have a H+ that can balance out the excess H created by balancing the O, but you won't have an O2 to balance out the excess H.

ChathuriGunasekera1D

Hi! It isn't anything new in particular, it's just a combination of ∆G° = - RT ln K and ∆G° = ∆H° - T∆S°. You don't need to memorize it as long as you can make those equations equal to each other.

ChathuriGunasekera1D

Hi! Faraday's constant has C/mol and E has V which is J/C so you are left with J/m which is how G is written. You don't need to convert because the units work out anyways. The reason why the mole remains even after the moles of electrons are multiplied in is because if represented the energy release...

ChathuriGunasekera1D

Hi! If it's a reversible process, you know that S has to change to something because the process cannot return to its original state.

ChathuriGunasekera1D

Hi! A negative dG of formation means that the G of reactants is greater than the G of products, so if free energy decreases, the end product is more stable. Lower energy means more stability.

ChathuriGunasekera1D

Hi! I had a quick question about determining how you know if the E cell is positive or negative. I know that it's positive if the cathode is right and negative if the cathode is on the left, but the electron flow always goes towards the cathode regardless of what side the cathode is on. I'm wonderin...

ChathuriGunasekera1D

Hi! You use the sign based on whatever your given ΔH is. If you aren't sure about that, you can use q and see if the temperature increased or decreased to see if the reaction is endothermic or exothermic. Endothermic reactions absorb heat, and the temperature increases so q is positive and ΔH is pos...

ChathuriGunasekera1D

Can you post the problem? Thanks!

ChathuriGunasekera1D

So if you're going from gas to liquid, the reaction is H 2 O (g) -----> H 2 O (s) and if you wanted to write out the entropy of the reaction you could say it is the standard entropy of products - standard entropy of reactants. We're given those values: 196.9 J/molK - 86.8 J/molK = 110.1 J/molK and l...

ChathuriGunasekera1D

If you use PV = nRT you get 0.243 mol of N2. Using the standard enthalpy of formation of NO, you can say that the same amount of heat is released per mole of N2.

(0.243 mol)(180.6 KJ/mol) = 43.8 KJ

ChathuriGunasekera1D

Hi! Yeah you're right you have to use q_p = nC_pdT but you also have to use C_p = (5/2) R. This comes from the equation and we didn't really cover it in class but it's what you use for a constant pressure situation. Calling the gas ideal allows us to use this specific heat capacity value.

ChathuriGunasekera1D

Hi! Here is a basic set of guidelines q is positive if system absorbs energy q is negative if system releases energy w is positive if work is being done on the system w is negative if work is being done by the system An example is going from solid to liquid; it requires heat to break the IMFs, so th...

ChathuriGunasekera1D

Hi! There some specifications for q=delH. The pressure has to be constant, so it's really q_p=delH

ChathuriGunasekera1D

Hi! I'm having trouble starting this problem off. Any help is appreciated! An ideal gas in a cylinder was placed in a heater and gained 5.50 kJ of energy as heat. If the cylinder increased in volume from 345 mL to 1846 mL against an atmospheric pressure of 750. Torr during this process, what is the ...

ChathuriGunasekera1D

Hi! Can someone explain what work being positive or negative means, and how you can tell based off of the action that is being performed? This is the problem: 8.3 Air in a bicycle pump is compressed by pushing in the handle. If the inner diameter of the pump is 3.0 cm and the pump is depressed 20. c...

ChathuriGunasekera1D

Yes! The OH comes from the NaOH. If you wanted to write another equation, you could have Na2O+H2O --> OH + Na to show the inevitable dissociation of NaOH into Na and OH, but not Na2O+H2O --> NaOH + OH + Na because it kind of implies that NaOH does not completely dissociate.

ChathuriGunasekera1D

Hi! So using the 17.78 M OH from part b, you can use that and the balanced equation to work backwards. Because NaOH is a strong base, it will completely dissociate, meaning that [OH] = [Na] = [NaOH]. If we have 17.78 M [OH], the we have 17.78 M [NaOH]. The equation is Na2O + H2O ----> 2NaOH If we ha...

ChathuriGunasekera1D

So [C2H5OH] is 2 M and [CH3COOH] is 1 M and K is 4 so the ICE table looks like C2H5OH (aq) + CH3COOH (aq) ⇌ CH3COOC2H5 (aq) + H2O (l) 2 M 1 M 0 M -x -x +x 2-x 1-x x The equation is (x)/((2-x)(1-x)) = 4 and because K isn't that small, you have to do the whole quadratic and all that. x/(2-3x+x^2) = 4 ...

ChathuriGunasekera1D

Hi! I think this goes back to the coordination compounds that we learned about in 14A. When Al forms coordinate covalent bond, it bonds with 6 other waters.

ChathuriGunasekera1D

I think it works for anything as long as it follows that guideline

ChathuriGunasekera1D

Given Kw of 2.1 10^-14, you know that [H3O+] and [OH-] have to be the same because water is neutral so one cannot be greater than the other. So the [H3O+] and [OH-] is the square root of Kw

ChathuriGunasekera1D

Hi! Set up the reaction to solve for Q. Given the equation PCl5(g) <----> PCl3(g) + Cl2(g), Q = ((PPCl3)(PCl2))/(PPCl5)). You're given PPCl5 = 1.18 bar, PCl2 5.43 bar, and K=25. You set Q=K and plug in what you have.

[(PPCl3)(5.43)]/(1.18) = 25

PPCl3 is 5.43 bar

ChathuriGunasekera1D

Left means reactants and right means products. For example in the formation of ammonia 2NH3 <----> N2 + 3H2 If the reaction is left leaning, then it tends to want to make more ammonia. If the reaction is right leaning, it tends to make more nitrogen gas and hydrogen gas. Increasing the concentration...

ChathuriGunasekera1D

Honestly I just do homework problems and the audio visual practice (you find it on his website). His UAs are undergraduate assistants that host small review sessions. There are: Step ups for people that struggle a bit with chem, it's for them to start from the basics and work their way up Drop ins f...

ChathuriGunasekera1D

Hi! As long as you post 3 times a week in any fashion (answering questions, posting, responding in discussion), you get points! The way to check is to revisit a thread that you have posted on and underneath your name, you'll see how many times you've posted. You can also check in the section called ...

ChathuriGunasekera1D

Hi! It can't be tridentate because CO3 2- is trigonal planar, so the molecule can't reorient itself to bond all three oxygens to the central atom. However, one or two oxygens can bind to the central atom because the molecule won't need to readjust to let that happen.

ChathuriGunasekera1D

Hi! They write OH2 to indicate that it's the O that bonds. So I think you have to write it like OH2. I think you write them in alphabetical order?? But I could be wrong about that.

ChathuriGunasekera1D

Hi! Ideally, if NH3 was a Bronsted base, it would accept the H+ from CH3COOH. But because CH3COOH does not donate a proton, it is not a Bronsted acid, so NH3 cannot be a Bronsted base. There isn't a clear cut transfer of protons. If this were a Bronsted problem, the reaction would look like: CH3COOH...

ChathuriGunasekera1D

Hi! It should have the same number of sigma and pi bonds regardless of whether or not the molecule has resonance. Otherwise the structures would not be equal. Because SO3-2 has two single bonds and one double bond, here are three sigma bonds (one per single bond and one in the double bond) and one p...

ChathuriGunasekera1D

So for sp, the bond angle is 180. We can say (sort of) that 50% of this is contributed from the s and 50% is contributed from the p (because sp is one half s and one half p). For sp2, the bond angle is 120. Now about 33% of the angle is from the s and 67% is from the p (sp2 is one third s and two th...

ChathuriGunasekera1D

You only need to look at the trends. I don't think we get access to actual values, so we use trends to estimate which compounds have higher ionic character.

ChathuriGunasekera1D

So you're right, but when you think about its molecular geometry, it makes sense why it's at least a little polar. CH2Cl2 is tetrahedral, and when you arrange the atoms around C in the tetrahedral shape, you can never orient them so that the charges fully cancel. Nothing is directly across from each...

ChathuriGunasekera1D

Be has 4 electrons total, and from that has 2 valence electrons. If of course cannot follow the octet rule because it doesn't have enough electrons. So, it can only form one bond with each carbon which uses up its two valence electrons. The structure will look like 3HC-Be-CH3 but the Hs will be spre...

ChathuriGunasekera1D

If an ion has more polarizing power, that means that it is able to influence another atom's electron cloud shape. For example, Ca2+ has polarizing power, and can alter the electron cloud of Cl- (or other ions). The smaller the size of the ion, and the larger the charge, the more polarizing power a c...

ChathuriGunasekera1D

Hi! Hydrogen bonding is stronger than most intermolecular forces, so it is requires more energy to break those IMFs, and therefore needs a higher temperature for the compound to move from solid to liquid. I thought covalent bonds and ionic bonds were stronger? Or do those not count as IMF? Covalent...

ChathuriGunasekera1D

Hi! I think they're asking for the AXE format, where A is the central atom, X are the atoms that bond to the central, and E is the number of lone pairs on the central atom. For example, SCl4 has one central atom, four surrounding atoms, and one lone pair on the central atom So the VSEPR formula is A...

ChathuriGunasekera1D

Yeah, I think we need to memorize bond angles for the major molecular geometry shapes (180, 120, 109.5, 90/120, 90) as you move from linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral, and then just put ranges for anything with lone pairs. Eg. Trigonal planar: 120, but Bent could...

ChathuriGunasekera1D

I think until you can intuitively figure it out, it's best to draw the Lewis structures. For me, unless I'm 100% sure of its structure (like CH4), I will draw the full Lewis structure.

ChathuriGunasekera1D

Hi! Hydrogen bonding is stronger than most intermolecular forces, so it is requires more energy to break those IMFs, and therefore needs a higher temperature for the compound to move from solid to liquid.

ChathuriGunasekera1D

The stronger the dispersion forces, the stronger the force between the molecules. The stronger the intermolecular forces (IMFs) are, the more solid the compound becomes (except for H20, which has stronger IMF as a liquid vs. solid, which explains why water is denser than ice). IMFs will form between...

ChathuriGunasekera1D

There's not really a difference when drawing, because Lewis structures just outline the lone pairs and what types of bonds exist within a molecule.

ChathuriGunasekera1D

Hi! I have 6th edition but I think you're talking about the baseball problem. You use De Broglie's equation for this problem. m would equal the mass, and you need to convert 5.15 oz to kilograms, which is 145.745 x 10^3 g. If you convert 92 mph to m/s you get 41.128 m/s as the velocity. You are give...

ChathuriGunasekera1D

Hi! A radical is when an atom has one unbonded electron that is not in a pair. For example, in CH3, C has a radical because its fourth electron is not bonded. You draw your Lewis structure normally, and just put one dot instead of two to indicate a radical.

ChathuriGunasekera1D

Hi! You are allowed to have one double bond (between the N and O), but you cannot have two for both N O bonds. Because N is not in the 3rd period, it cannot make an expanded octet using the empty 3d orbital. If it could, then it could have five bonds (two double bonds to O and one single bond to Cl)...

ChathuriGunasekera1D

You could use IE to determine if a bond has more ionic character or covalent character. The greater the difference between IE between the elements, the more ionic character is has. The others have uses too, but I can't think of any right now.

ChathuriGunasekera1D

So F normally is [He] 2s2 2p5 but because you have F-, you add another electron. This electron would go to the p orbital (becuase 2p5 still has space for one more electron), and then you have [He] 2s2 2p6 which is the same as [Ar].

ChathuriGunasekera1D

S has 6 valence electrons so it's allowed to make two double bonds (takes 4 electrons) and two single bonds (takes 2 electrons). You want the central atom to have as few lone pairs as possible for stability purposes, that's why S makes some double bonds too

ChathuriGunasekera1D

Some of the exception are H, He, Li, Be because they don't have enough electrons to even make the octet. Sulfur, chlorine, silicon and phosphorus can all go beyond the octet, so they are also exceptions. There are some more for sure, but Br follows the octet rule (someone please correct me if I'm wr...

ChathuriGunasekera1D

Hi! I'm sure if you talk to your TA, they can let you know if you got credit for it.

ChathuriGunasekera1D

Okay I think you would start off by dividing 1 meter by 1,650,763.73 wavelengths to get the length of one wavelength of Krypton-86. This makes sense because the units become meters/one wavelength and that's what wavelength measures. 1/1,650,763.73 = 6.058 x 10-7 m which is 605.8 nm. That is in the v...

ChathuriGunasekera1D

Hi! Dr. Lavelle posted #55c as an error in the solution's manual for 6th edition, so I'm assuming the same happened for 7th. He says that it should be (n-1)d3 ns2. So you're right, the group 5 transition metals start with Vanadium and so on.

ChathuriGunasekera1D

Yeah I think you're right I was going through lecture notes and he mentioned that today. Thank you!

ChathuriGunasekera1D

Hi! Do we need to include the f orbital when finding the electron configuration of Tungsten (z=74) for example? Would it be

[Xe] 4f14 5d4 6s2 or [Xe] 6s2 5d4

Also if it is the first one, in what order should the orbitals be listed? Thank you!

ChathuriGunasekera1D

I totally agree, I think it should be 4s2, 3d10 just because the periodic table moves in that progression. I think as long as you include everything, it shouldn't matter!

ChathuriGunasekera1D

He also mentioned that he really does not want us using Rhydberg's equation because there's more room for error. He wants us to use E = -hR/(n^2) for each energy level and then go E(final)-E(initial) so that we actually understand the problem and don't get positive and negative signs mixed up. Someo...

ChathuriGunasekera1D

Hi! I think you mean mass of electron. Photons have no mass, so because this question asks "what is the wavelength of the ejected electron" you use De Broglie's equation. The mass of an electron is always the same, regardless of the element. I think it's 9.11 x 10^-31 kg and you have to le...

ChathuriGunasekera1D

I think you use the first equation when finding the energy change from one energy level to another, and the second equation when the question specifically mentions a side length, or something like "box".

ChathuriGunasekera1D

Hi! I know that we should know that 400 nm is about violet and 700 nm is about red, but I don't think we need to be able to match color to wavelength exactly.

ChathuriGunasekera1D

(a) Using the equation c = λ * v (λ is wavelength, v is frequency, c is the speed of light), you can sub in what you know! 3.0 x 10^8 m/s = (7.1 x 10^14 s^-1) x (λ) λ = 4.225 x 10^-7 m and they want it written in nanometers so it become 422.5 nm. And because 422.5 nm falls on the violet light wavele...

ChathuriGunasekera1D

I think it's just like basic compounds, like knowing that CO3 is carbonate so Na2CO3 is sodium carbonate. If you looked up "polyatomic ions", those are probably the only ones that you would need to know. There's only like 6 I think that you would really need to know. Carbonate CO3 (-2) Nit...

ChathuriGunasekera1D

I think that's a typo!

ChathuriGunasekera1D

Thank you!

ChathuriGunasekera1D

Hi! I think the equation wasn't fully balanced. This is what is given: C6H9Cl3 + 3AgNO3 ---> AgCl + C6H9(NO3)3. But there are 3 moles of Ag on the left and only 1 mole on the right. There should be a 3 in front of the AgCl. That means if you multiply your 0.5732 mol by 3, you get 1.72 g and that is ...

ChathuriGunasekera1D

Hi! I don't think it really matters, but I tend to convert to liters if the value exceeds the thousands place. So I would convert 3,000 ml to 3 L just to keep it shorter. I think a TA said that it doesn't matter for the test!

ChathuriGunasekera1D

Hi! I was having some trouble with this problem. I have found the molar mass of XA to be 338 g/mol, but I'm not sure where to go from there. The acid HA (where A stands for an unknown group of atoms) has molar mass 231 g/mol. HA reacts with the base XOH (molar mass 125 g/mol) to produce H2O and the ...

ChathuriGunasekera1D

Nitrate is NO3, and has a charge of -1 Copper is Cu, and is given as +2 (as indicated by the (II)) Sodium is Na, and has a charge of +1 Hydroxide is 0H, and has a charge of -1 Copper (II) nitrate would be Cu(NO3)2 and sodium hydroxide would be NaOH. The products would be NaNO3 and Cu(OH)2. The react...

ChathuriGunasekera1D

You're right, but I think because the aspartame has nitrogen in it, the gas needs to escape as a product somehow. I think that in general, all combustion reactions have CO2 and H2O as products, but might sometimes include other products to make up for any elements in the reactant that is not hydroge...

ChathuriGunasekera1D

The mass of the products should equal the mass of the reactants. There can be more moles of product than reactant or vice versa, but there cannot be different masses.

ChathuriGunasekera1D

Thank you! I think I rounded a little bit too much so my values weren't as close to a whole number.

ChathuriGunasekera1D

From part (a) and (b) we know that the original concentration of the sodium carbonate (2.111 g) is .07966 M. This comes from converting grams of sodium carbonate to moles, and then dividing that by the total volume in liters (2.111 g) / (106 g) = 0.0199 mol Na2CO3 / 0.250 Liters = 0.07966 M Na2CO3 T...

ChathuriGunasekera1D

Hello! I am struggling to finish the last steps of this problem: "The mass percentage composition of cryolite, a compound used in the production of aluminum, is 32.79% Na, 13.02% Al, and 54.19% F." I am asked to find the empirical formula. I am fine up until after I have divided the three ...

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