CHEMISTRY COMMUNITY

Nishma Chakraborty 1J

What's a way to tell a chemist and a coal miner apart?

ask them to pronounce "unionized"

Nishma Chakraborty 1J

What's the chemical formula for a banana?

BaNa2

Nishma Chakraborty 1J

Yes, when a system expands reversibly the total entropy of a system and its surroundings is unchanged.

I found this page helpful: http://www.chem.ucla.edu/~vinh/disc7.html

Hope this helps! :)

Nishma Chakraborty 1J

I'm pretty sure that the rate of a chemical reaction can only be positive.

Nishma Chakraborty 1J

On my test we had to rank Zn, Mn, and Cr in order of increasing reducing power - I put Cr, Zn, Mn because the values would be -.74, -.76, and -1.18. But, I only got 1 point... Does anyone know what I might have lost a point on? I just checked--that's the order I had too, and I got the points. I'd c...

Nishma Chakraborty 1J

I had to kind of break it down and think of it like this: reduction potentials (given to us): show how much potential a species has to be reduced, or gain electrons, the more positive the potential, the more it wants to gain electrons --the opposite is true: the more negative a reduction potential i...

Nishma Chakraborty 1J

Reducing power is the ability of a species to reduce something else. If it's reducing something else, it is being oxidized itself. Therefore, based on reduction potentials, the more negative a potential is, the more likely it will be oxidized (and therefore has a greater reducing power as it is dona...

Nishma Chakraborty 1J

I think it's because of its derivation from the original formula: ln[A]=-kt +ln[Ainitial]. When [A]=1/2[Ainitial], ln(1/2[Ainitial])= -kt +ln [Ainitial] ln(1/2[Ainitial])-ln[Ainitial]= -kt ln{(1/2[Ainitial])/[Ainitial]= -kt Through this, the initial concentration of A cancels out, and t=(ln2)/k. Hop...

Nishma Chakraborty 1J

My TA did mention that derivations were technically fair game (at least for the final)

Nishma Chakraborty 1J

Why did the bear dissolve in water?

-because it was a polar bear

Nishma Chakraborty 1J

Hey Sammy! The unique rate of the reaction is the rate of appearance (products) and disappearance (reactants) divided by the stoichiometric coefficient of a particular species. The question asking for NO2's unique rate of reaction would be (change in [NO2])/(change in time)*(1/2) (because there are ...

Nishma Chakraborty 1J

ChemistNate's channel on YouTube also has a series of neat vids and the derivations for the formulas!
https://youtube.com/playlist?list=PLBVb ... IpvUTzEuUE

Nishma Chakraborty 1J

Hi! The anode (oxidation reaction) is always written on the left side, and the cathode (reduction reaction) is always written on the right side, with a salt bridge separating the two. For instance, for a redox reaction like: 2Ag+(aq) + Cu(s) --> Cu+2(aq) + Ag(s), Cu is being oxidized, while silver i...

Nishma Chakraborty 1J

I think it's because I2, even though it's in its solid state, is a nonmetal. A metal is needed to transfer electrons from the anode to cathode, so Pt(s) has to be used. Hope this helps!

Nishma Chakraborty 1J

Hey! My go-to is usually Khan Academy if I need some concepts clarified, because they have videos and sometimes have practice problems! Here's the link for their electrochem stuff: https://www.khanacademy.org/science/chemistry/oxidation-reduction Bozeman Science and CrashCourse are good for clarifyi...

Nishma Chakraborty 1J

I don't know for sure, but I've used 8.314 J/(molK) for the most part this quarter. However, some practice problems have used the 0.08206 (L*atm)/(molK), so honestly I'd check to see if my units cancel just to be safe. Hope this helps!

Nishma Chakraborty 1J

Hey! So typically, larger, more complex molecules have a higher molar entropy than simpler ones, because there are more possible arrangements of atoms within a larger molecule. Also, gases have a greater molar entropy than liquids, and liquids have a greater molar entropy than solids. All else equal...

Nishma Chakraborty 1J

Hey! We use W=-pdeltaV for expansion work questions, typically regarding gases! I don't think temperature has to be constant in order to use this equation. Hope this helps!

Nishma Chakraborty 1J

Hey! So, the question is asking which of the four compounds are stable compared to their individual elements. If a value of free energy is positive, that means that the compound is unstable with regards to its individual elements. Basically, that reaction is endothermic-- you have to put in heat/ene...

Nishma Chakraborty 1J

Residual entropy relates to the entropy of a substance as T≈0 kelvin. There are fewer degenerative states at that temperature. I think residual entropy is calculated using the absolute entropy equation, S=kb*ln(W). Regular (or change in) entropy is calculated using the change in pressure, change in ...

Nishma Chakraborty 1J

Hey!

Statistical entropy (S) is defined by: S= kln(W), where W shows the number of different states where a system can exist in. Residual entropy calculates the number of states/entropy of a system that has near zero energy. The entropy at ≈zero Kelvin is residual entropy.

Nishma Chakraborty 1J

Molar heat capacity can be either J/(mol∙C) or J/ (mol∙K)

Nishma Chakraborty 1J

Yeah, the NO in the first reaction cancels with the NO in the second reaction! :)

Nishma Chakraborty 1J

When we're using bond enthalpies, I follow "the sum of the bonds broken - the sum of the bonds formed." The bonds that are broken are on the reactants side, and the bonds that are formed are on the products side. Forming bonds releases energy, while breaking bonds requires energy. This is ...

Nishma Chakraborty 1J

Hi! Compression of a gas means that work is being done on the system. When we look at deltaU= q + w, we are looking at it from the perspective of the system. So because the system is not losing any energy by doing work (its surroundings are instead), the sign of work will be positive. So, compressio...

Nishma Chakraborty 1J

Hi!

Common state properties are internal energy, pressure, volume, density, and temperature.

Work and heat are not state properties.

Hope this helps! :)

Nishma Chakraborty 1J

This problem reads: Use the bond enthalpies in Tables 8.6 and 8.7 to estimate
the reaction enthalpy for (a) 3 C2H2(g) --> C6H6(g).

When we're calculating bonds broken vs. the bonds formed, why does the solutions manual not account for the C-H bonds?

Thank you!

Nishma Chakraborty 1J

I don't think pressure is necessarily constant

Nishma Chakraborty 1J

Hey! So, I think it's -947 kJ because the cylinder itself isn't absorbing the heat (then it would be +947 kJ), but rather the cooling system around it is absorbing the heat. Basically, the cylinder is giving off the heat (thus -947 kJ) that the cooling system absorbs. The wording is tricky! Hope thi...

Nishma Chakraborty 1J

Hey! For part a) of this problem, I used the equation used the equation q=m*Cs*change in T. I converted the mass into kilograms and used Table 8.2 to find the specific heat capacity (Cs) of both water and copper. So, for water I had: 0.4kg*4.184J*78=130.5kJ, and for copper I had: 0.5kg*0.38J*78=14.8...

Nishma Chakraborty 1J

Hey! It depends on the direction of the reaction (which you can determine from the equilibrium constant). For instance, if Kc is 2*10^-5, there are more reactants than products, so the system will shift towards the products in order to balance out/reach equilibrium. In that case, you would subtract ...

Nishma Chakraborty 1J

Also I think another difference is that -cis molecules are polar, while -trans molecules are non-polar

Nishma Chakraborty 1J

I'm pretty sure we can use the abbreviations

Nishma Chakraborty 1J

Hey!

For strong acids and strong bases, I'm pretty sure we use the forward arrows only because strong acids and strong bases dissociate completely.

Hope this helps! :)

Nishma Chakraborty 1J

Hey! So to clarify, the coordination number of a coordination compound refers to the number of points that the ligands (Lewis bases) are attached to the central metal atom. For example, the coordination number of Ni(CO)4, is 4 because the Lewis bases in CO are able to bind to Nickel four times. hope...

Nishma Chakraborty 1J

Hey! (CO3)-2 can either be a bidentate or monodentate due to its Lewis structure and resonance. When we draw the lewis structures for (CO3)2-, there are two possibilities, with one structure having a C=O bond and two C-O^-1 bonds. With this structure, the two O^-1's can bond to a metal at two differ...

Nishma Chakraborty 1J

Hey! Yes, I think that there are some more cases where there can be VSEPR structures with lone pairs that are non-polar. For example, a shape with five electron densities (trigonal bipyramidal) is considered linear when it has 3 lone pairs. However, these lone pairs lie on the equatorial axis, so th...

Nishma Chakraborty 1J

Hey!

The shape of the polydentate is a set structure determined by drawing Lewis structures/analyzing formal charges/etc. Given that structure, it has ions that are available to bind to a transition metal multiple times.

Hope this helps!

Nishma Chakraborty 1J

Hey!

I'm not sure if I interpreted the drawing of the Lewis structures correctly because of formatting, but if you were to draw double bonds on the chlorine (as in the second structure shown), it would have a formal charge of +3 which is undesirable.

Nishma Chakraborty 1J

Hi! So, lone pair - bond pair repulsions are by nature stronger than bond pair - bond pair repulsions. This is because lone pairs of electrons are closer to the positively-charged nucleus, so they repel each other more. In order for the atom to stay as stable as possible, it places the more repulsiv...

Nishma Chakraborty 1J

Hi! First, the central atom of a Lewis structure can have an expanded octet if it is capable of having a d-orbital or greater (so it has to be in the third energy level or higher/third period or below on the periodic table.) Non-metals (groups 13-17 on the periodic table) are the ones that tend to h...

Nishma Chakraborty 1J

Hey! A more negative electron affinity corresponds with greater attraction for an electron. Electron affinity tends to increase (as in more energy is released when an electron is added to an atom) as you move up a group on the periodic table. Nobles gases have an electron affinity of approximately z...

Nishma Chakraborty 1J

Hey, so I think my test asked about an energy emitted as an electron moved from n=4 to n=1, but I solved for the version you had - did you make sure to calculate for frequency? I got -1.94*10^18 J for Energy (the negative due to the fact that the electron is emitting energy) as well. You set 1.94*10...

Nishma Chakraborty 1J

Hi! Ionization refers to when an atom or molecule gains or loses electrons and becomes an ion (called a cation if it loses electrons, or an anion if it gains electrons). Ionization energy is the minimum amount of energy necessary in order to remove an electron (the most loosely-bound one, aka the va...

Nishma Chakraborty 1J

Hey, so for part b, I didn’t use the de Broglie equation. Carrying on from part a. (which referred to electron spectroscopy and the equation: (Energy of the Photon - Work function = Kinetic Energy), I used the definition of kinetic energy (1/2m*v^2) in order to find the velocity of the ejected elect...

Nishma Chakraborty 1J

Hey! To follow up, the direction of the first electron is always represented by an arrow drawn facing upwards (which signifies that the first electron has the +1/2 spin. The electron following is represented by an arrow pointing downwards because it has a negative +1/2 spin. This aforementioned is i...

Nishma Chakraborty 1J

Hi!
The electron spin quantum number (ms) can have values of +1/2 or -1/2. Hope this helps!

Nishma Chakraborty 1J

Hey!

For 2) I'm pretty sure that when an electron emits a photon, the value of energy will be a positive number. Honestly, I'm not too sure about 1) myself, so if anyone else could clarify, that'd be great! Thanks!

Nishma Chakraborty 1J

Hey! So, the atoms of each element all consist of electrons with unique arrangements. Their unique arrangement coincides with each individual electron having a different pattern of energy (which is released as a photon when an electron is excited and goes from a higher energy to a lower one). This d...

Nishma Chakraborty 1J

Hey! From by best understanding (and from what my teachers have done in the past), molarity has always been in the standard mol/L. Whenever a volume is given in ml, I convert it to L. Hope that helps!

Nishma Chakraborty 1J

Hey! So based on the diagram, we interpret the molecular formula to be C10H16O (which has a molar mass of 152.228 g/mol). The question is asking for the mass percentage composition of each element in the molecular formula. That can be found by using this equation: (mass of element/total mass of comp...

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