CHEMISTRY COMMUNITY

Matthew Mar 1J

For our purposes, I think microscopic reversibility is most important when thinking about the pre-equilibrium approach to determining rate laws. Microscopic reversibility refers to the fact that, in a multi-step reaction, the fast steps are occurring at about equilibrium if followed by a slow step. ...

Matthew Mar 1J

The activated structure is also known as the transition state and it's the energy peak on a reaction graph. Once the sufficient activation energy is input into the reactants, they enter the transition state and the reaction can progress to the products. This is important when thinking about catalyst...

Matthew Mar 1J

Also, don't get tripped up if they mention adsorption alongside heterogeneous catalysts. It's probably not a typo, it's just a fancy way to describe the relationship between reactants and catalysts in different states (specifically reactants on a solid catalyst). I think heterogeneous might also app...

Matthew Mar 1J

I think a negative order indicates that a reactant is involved in a reaction, but actually slows the reaction rate as its concentration increases. I think one example of a reactant is H+, because in some reactions it's involved in, a higher H+ concentration makes the solution more acidic which could...

Matthew Mar 1J

So if it's a pseudo first order reaction, treat it as if its just a normal first order reaction to find the units and so on.

Matthew Mar 1J

Also, unique rate can be found in terms of the rates of formation or consumption by dividing by the coefficients in the equation (the rate of consumption has to have its sign flipped to make it positive).

Matthew Mar 1J

There's also the equation G=H-TS which says free energy relies on enthalphy, entropy, and temperature. Since we know enthalphy refers to the heat of a system under constant pressure, that could be the pressure connection?

Matthew Mar 1J

From a numerical standpoint, you can look at the standard reduction potentials to determine relative reducing/oxidising strength. A lower, more negative potential means something is a stronger reducing agent and a more positive value means a stronger oxidising agent (stuff like this showed up on tes...

Matthew Mar 1J

You should be careful when reading the question though, because sometimes the requested redox reaction will be the reverse of what is listed in the table so you have to flip the sign before ordering them.

Matthew Mar 1J

Generally the reactants that can be used as electrodes will be metals in their solid form because they are conductive and can be involved in the reaction without being dissolved into the solution (there are exceptions like mercury which is a liquid metal and graphite which is made of carbon).

Matthew Mar 1J

If the reaction were to occur without a salt bridge, the electrons would move from one side to another then promptly stop because of the charge difference. The salt bridge equalises the charge by allowing ions to flow between the two sides without allowing any of the solution to transfer.

Matthew Mar 1J

Also, in a more practical sense, if we think of the cell as a battery we know we want the work done by the battery to be at a maximum and the Wmax that a cell can do will be equal to the max potential difference.

Matthew Mar 1J

Although all the necessary equations are there, you also need to be able to rearrange equations. For example, the formula for heat capacity can be found by rearranging q=nC/\T to get nC = q/T. Some problems, like the one Dr. Lavelle did in lecture, require you to switch around the ideal gas law PV =...

Matthew Mar 1J

There are also problems where it asks for the change in entropy for a phase change at a temperature other than its boiling/freezing point where you have to use the entropy of vaporisation or fusion at the boiling or freezing point as the middle step.

Matthew Mar 1J

There are also some tricky ways that constant pressure or volume can be hinted at in the problem. For example, the question might not say constant pressure, but give you a value for enthalpy which in itself implies constant pressure. There are also problems where you have to split the entropy change...

Matthew Mar 1J

Plus, pretty much all of the equations are on the formula sheet (https://lavelle.chem.ucla.edu/wp-conten ... ations.pdf), so I think application is the main point.

Matthew Mar 1J

That would be a megajoule (MJ), millijoule (mJ) uses a lower case m. Although we're unlikely to run into that unit on the test.

Matthew Mar 1J

I don't know if this is directly related, but 3/2R (without the T) is the specific heat capacity of a monatomic ideal gas at constant volume (Cv) which is used in entropy problems (/\S = n Cv ln(T1/T2)) which is a more practical application.

Matthew Mar 1J

Maybe once a substance goes through a phase change, it just has a different set volume for that specific state. Like water as a liquid has a set volume which is different from its set volume as a solid, but their volumes remain constant within the state that they are in at the time.

Matthew Mar 1J

The formula for internal energy is U = q+w, but when pressure is constant w = -P/\V, which gives us /\U = q - P/\V. If there's a gas involved, we can use the ideal gas law PV = nRT to get /\U = q - /\nRT, where /\n is the change in moles. According to this equation, when moles increase, internal pre...

Matthew Mar 1J

Also, exergonic reactions release energy while endergonic reactions absorb energy, so you can think of the negative G value as the reaction losing energy and the positive G value as the reaction gaining energy.

Matthew Mar 1J

However, the equilibrium constant K doesn't change. The pressure shift only causes a change in the amount of products or reactants, not the ratio.

Matthew Mar 1J

Solids and liquids can't be compressed because they have set volumes, while gasses are able to spread out to fill whatever space they are in. Because the change in the reaction is actually a result of a change in volume, solids and liquids, whose volumes are unable to change, aren't affected by pres...

Matthew Mar 1J

A pressure change will only have an effect on a reaction if it is caused by a change in volume. If you increase pressure by reducing the volume, then the molecules within the reaction are compressed and because they want to be stable with the lowest energy, the reaction will favor whichever side has...

Matthew Mar 1J

Autoprotolysis refers to reactions where an amphiprotic molecule reacts with itself. The most common example is water. Water is amphiprotic because it can donate or accept a proton, this gives us the equation 2H2O <-> H3O+ + OH-. One water molecule donates a proton in the form of a hydrogen (forming...

Matthew Mar 1J

Generally, the conjugate acid or base will just be the product other than H3O+ or OH-. For example, in the equation NH4+ + H2O <-> NH3 + H3O+ NH4+ is the acid with NH3 as its conjugate base.

Matthew Mar 1J

Then the equation for K should turn out to be a quadratic and you'd have to use the quadratic formula. The 5% approximation rule is just to make the calculations easier if the change in concentration is small enough to be negligible.

Matthew Mar 1J

We also have no reason to believe there is already product at the start unless it is explicitly stated. If these were actual reactions we were doing in a lab, we could probably measure the initial conditions, but since it's a hypothetical problem, I don't think we can actually determine the initial ...

Matthew Mar 1J

The ratio [O2]^3/[O3]^2 and the ratio [O3]^2/[O2]^3 factor in the coefficients from the equation which represent the actual ratio of products to reactants involved in the reaction. These ratios are also the equilibrium constants for the forward and reverse reactions and equilibrium constants are alw...

Matthew Mar 1J

I think the fact that a reaction is in equilibrium implies that the reaction is stable, because the forward and reverse reactions are happening constantly and keep the overall reaction at a certain ratio of products to reactants. So I guess they're all equally stable? Although I'm not really sure wh...

Matthew Mar 1J

The book might be referring to a sulfite (SO3 -2) ion, where having one double bond and two single bonds would account for the -2 charge on the molecule. I'm not really sure about the SO2 because I'm pretty sure sulfur dioxide is supposed to be neutral.

Matthew Mar 1J

Since iron is past row two, it can have an expanded octet, so it's able to accommodate the extra electrons. Additionally, as a transition metal, iron is capable of taking on a couple different charges while still remaining stable.

Matthew Mar 1J

Myoglobin actually includes more than just the tetradentate ligand from the HEME complex, there's also an amino acid (histamine) bound to it. The tetradentate ligand occupies four bonding sites around the iron while the histamine takes one of the two remaining spots above or below, leaving just one ...

Matthew Mar 1J

Another instance where you'd use the bis-, tris-, etc... prefixes is if the molecule you're naming already has prefixes like di-, tri-, etc... (e.g. (O2)2 -> bisdioxygen not didioxygen or (ea)3 -> trisethylenediamine).

Matthew Mar 1J

Also, the potassium comes first because it is the cation. Think of the coordination compound as just a really big, really complicated salt. You could consider NaCl where Na is the cation which always comes before Cl, the anion. In this instance (K2)+2 is the cation and [Ni(CN)4]-2 is the anion (henc...

Matthew Mar 1J

I think some other important ones are ethylenediamine (ea) (NH2CH3CH2NH2) and oxalate (C2O4). They're both bidentate and show up fairly frequently. As for structure, they're fairly easy to figure out if you just break the atoms into the groups as they're named (i.e. NH2 + CH2 + ...). Just make sure ...

Matthew Mar 1J

Maybe It's because SO3 has the option to include either single or double bonds? If all three oxygens are double bonded to the sulfur, the molecule is stable, but if it acts as an acid, some of the double bonds can switch to single bonds to create room to accept electrons. H2O2 on the other hand is s...

Matthew Mar 1J

By equilibrium symbol, are you referring to the notation used in class with two arrows stacked on top of each other? What exactly does this symbol mean, is it just there to indicate that the reaction is prone to go either way?

Matthew Mar 1J

Basically, the molecule is a carbon ring, but one of the atoms is a nitrogen that has five valence electrons, not four. Because of this, the nitrogen has a lone pair unlike the carbons which each have a single bond to a hydrogen. This lone pair has high electron density which makes the side with the...

Matthew Mar 1J

The T-shape is a planar shape, so you can think of it almost like a trigonal planar shape that just has two lone pairs which push the atoms closer together to a 90 degree angle instead of the 120 degree angle.

Matthew Mar 1J

You do have to be careful when looking at dipole moments because some nonpolar molecules can have dipole moments, they just cancel out against other dipole moments. For a molecule to be polar it must have polar bonds with dipoles that don't cancel. Professor Lavelle also said it would be helpful to ...

Matthew Mar 1J

You can also tell because HCN only has two regions of electron density around the central atom and the shape to position two regions the farthest apart is linear.

Matthew Mar 1J

Since AX2E has 3 regions of electron density, it's form comes from the trigonal planar shape and the angle will be slightly less than 120, but AX2E2 has 4 regions of electron density so it's form comes from a tetrahedral shape where two of the substituents are replaced with lone pairs. Coming from a...

Matthew Mar 1J

I think another way to think of it is to count the regions of electron density there are around the central atom and see which regions correspond to atoms that are bonded to the center and which regions are lone pairs. If you use the notation professor Lavelle used in class (AXE, A=central atom, X=b...

Matthew Mar 1J

Also, bound atoms often take on formations that fill up their orbitals through the octet guideline. This pairs up electrons and keeps them localised around the atom or molecule which keeps them at lower energy.

Matthew Mar 1J

Ionic and covalent character also influence the physical characteristics of molecules. For example, compounds with higher ionic character generally exhibit higher melting points and are more conductive while compounds with more covalent character are the reverse.

Matthew Mar 1J

In class, professor Lavelle indicated the presence of resonance structures by drawing them next to each other and putting a double ended arrow (<-->) between to show that the different Lewis structures are interchangeable. Beyond that, I don't think there's a more sophisticated way to show resonance...

Matthew Mar 1J

Each type of bond has a different length (single bonds are longer than double bonds, etc...), so when we draw a Lewis structure that has both single and double bonds that can be switched around or interchanged (take the classroom example of NO3-, where there are two single bonds and one double bond,...

Matthew Mar 1J

I'm pretty sure the only structures considered to be resonance structures are those that have a neutral charge (or if it's an ion, at least the same charge). Resonance structures blend together into one hybrid structure that represents the actual molecule with averaged bond lengths. All the differen...

Matthew Mar 1J

You may also need to know the distinction between covalent and ionic bonds with regard to their electronegativity difference (difference>2 is ionic, difference<1.5 is covalent, the area in between can go either way depending on which type of character is displayed more).

Matthew Mar 1J

We might be required to identify orbitals given the pictures or name the orbitals according to whatever axis they lie on, but then again, I'm not sure.

Matthew Mar 1J

I think it's actually the d orbital that gets the electron, not the p orbital, since Cr: [Ar] 3d5 4s1 where putting the electron into the d orbital is actually more stable than putting it in to fill the s orbital like we'd expect ([Ar] 3d4 4s2). The same goes with copper (Cu: [Ar] 3d10 4s1 rather th...

Matthew Mar 1J

In most of the problems I've seen, they don't usually give a number as the work function outright. Generally it's referred to as "the energy required to remove an electron from _____ surface" or whatever. I've also seen problems where the frequency or wavelength is given rather than the en...

Matthew Mar 1J

We should probably at least know the order of waves i.e. which has a shorter wavelength vs which has a longer wavelength, etc... since the photoelectric effect, which was an important part of this unit, relies on waves with short wavelength and high frequency (like UV). Longest wavelength-----radio-...

Matthew Mar 1J

How do we rule out the possibility of a and b? Can we use the formula c=λv and say that wavelength and frequency are inversely related, or does that only apply to light?

Matthew Mar 1J

For the question on labeling states of matter, I'm pretty sure they will usually be given unless it's something like a combustion reaction where we should know that the products will be CO2 and H2O in gas form.

Matthew Mar 1J

I think that's the right ordering most of the time, but you have to be careful with ionic compounds since there could be polyatomic ions which don't always follow that kind of ordering (e.g. things like sulfates (SO4) where the oxygen goes at the end or hydroxide (OH) with oxygen before hydrogen).

Matthew Mar 1J

Could actual yield ever be greater than theoretical yield? Maybe there are impurities in the reagents or the scale isn't calibrated correctly? I remember doing labs in high school where we ran into issues of actual yield vs theoretical yield when we got impossible amounts of product during experimen...

Matthew Mar 1J

Also, the amount of product C, which I believe was the next question in that problem set, would only be 1.5 mol because conservation of mass ensures that only the 1 mol A and 0.5 mol B consumed go into making the product.

Matthew Mar 1J

I got stuck on the same problem. I'm pretty sure that there is a typo in the 7th edition of the textbook because the question includes a string of chemical equations, each with a product that is used as a reactant in the next reaction. The second equation FeBr2 + Br2 ---> Fe3Br8 has Fe3Br8 as it's p...

Matthew Mar 1J

It might be important to consider how the units fit with the answer itself too. If you have a minuscule amount, it's probably better to write the answer in mL to avoid dealing with scientific notation and all that. Plus, it's more practical for a real life lab situation. You generally wouldn't tell ...

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