CHEMISTRY COMMUNITY

Sean Cheah 1E

Molecularity describes the number of reactant molecules that are required to collide in each elementary step of a reaction. Elementary steps that involve only a single reactant molecule are unimolecular, those that involve two are bimolecular, three termolecular, and so on. Note that molecularity de...

Sean Cheah 1E

Use the same concept behind the pre-equilibrium approximation to derive expressions for the intermediates in your rate laws. For example, for the reaction 2\textup{NO} +{\textup O_{2}} \rightarrow 2\textup{N}{\textup O_{2}} with elementary steps 2 \textup{NO} \rightleftharpoons \textup N_{2} \textup...

Sean Cheah 1E

If you are given a reaction mechanism and an overall rate law, sometimes it is possible to intuitively determine the slow step. This is because the overall reaction rate is determined by the rate of the slow step (which is why it is also called the rate-limiting step). For example, the reaction NO2 ...

Sean Cheah 1E

0.44, in this case, is not your k value, but rather the actual observed rate so you cannot put it into a rate law the way that you did. The unique rate refers to the -1/a*(d[R]/dt) or 1/b*(d[P]/dt) for the general reaction aR --> bP. Thus, to get the actual rate at which a specific product is produc...

Sean Cheah 1E

Am I missing something here? I believe they tell you in the very first sentence that the decomposition reaction is first-order.

Sean Cheah 1E

Q is a reaction quotient, which means that for most redox reactions, as long as you have the balanced equation, it should look identical to the equilibrium expression (with appropriate exponents to match the coefficients in the balanced equation). The equilibrium expression always puts products in t...

Sean Cheah 1E

Q is easy enough to find for typical redox reactions. It gets a little weirder for concentration cells, but writing out the half-reactions should immediately make it clear which concentration to use for the products and which to use for the reactants. What I don't understand is how one is supposed t...

Sean Cheah 1E

Since I'm confused as well, the full question is, "What range (in volts) does a voltmeter need to have to measure pH in the range of 1 to 14 at 25C if the voltage is zero when pH is 7?" Can someone point out the mistakes I'm making in the reasoning below? Since the two physical metal elect...

Sean Cheah 1E

You can easily determine the units of k for any order of reaction by looking at a generic rate law of that order. For example, a possible fifth-order rate law is rate = k*[A]^5. Rate always has the units M/s and concentration always has the units M. Therefore k, in this case, must have the units M/s...

Sean Cheah 1E

Can somebody post a full solution to this problem? Like the original poster, I don't understand where the expression for Q comes from. Why is Q not something like [H+ in solution]/[H+ inside pH electrode]?

Sean Cheah 1E

The standard convention is to list standard reduction potentials, meaning the E values for the reduction reactions. If you are dealing with an oxidation half-reaction (the reverse of the reduction half-reaction), you typically have to negate the given E value.

Sean Cheah 1E

You can think of delta G as the amount of energy released by a system that is free (hence the name Gibbs' free energy) to do work. Naturally, the maximum amount of work can be done must then be equal to delta G (i.e. all released free energy is used to do work). Delta G is not the same as delta U (c...

Sean Cheah 1E

The potentials listed in most tables are standard reduction potentials, with higher values indicating that reduction is more likely to occur. The strongest reducing agents (i.e. the species most likely to give up electrons and become oxidized) are the ones that are the least likely to be reduced. Th...

Sean Cheah 1E

Yes, work (max) equals delta G under constant temperature and pressure. What is the exact formula? He basically already wrote out the exact formula. At constant temperature and pressure, w_{max} = \Delta G . You can think of delta G as the amount of energy that is released that is free to do work. ...

Sean Cheah 1E

The standard convention is to put the oxidation reaction (anode) on the left and the reduction reaction (cathode) on the right. In this particular example, the Cl- is being oxidized to Cl2 on the left and the H+ is being reduced to H2 on the right.

Sean Cheah 1E

Yeah, it bothers me as well that their reaction quotient appears to mix partial pressures and molar concentrations. When someone asked this question during my discussion section, the TA said something about how equilibrium constants are ideally calculated using activities that can be estimated using...

Sean Cheah 1E

Your reasoning is correct but, to be precise, a high K indicates a positive E naught and a low K indicates a negative E naught (which is measured at standard conditions of 298.15K, 1M, 1 atm).

Sean Cheah 1E

Strictly speaking, you can use either as long as you are able to calculate the Q for the reaction. It is often more convenient to use log for problems involving pH because pH = -log[H+]. Do note that that you must multiply the logarithmic term by a factor of 1/log(e) = ~2.303 when you use the log eq...

Sean Cheah 1E

I had no clue how to do this one either. I just assumed that the Cl2(g) on the products side was a typo and balanced the equation assuming they meant to put Cl-(aq) there. The question just didn't make any sense to me otherwise.

Sean Cheah 1E

After determining the coefficients for the species being reduced (in this case O3) and the species being oxidized (in this case Br-) by balancing the electron transfer (# of e- lost = # of e- gained), you must ensure that your final equation has balanced charges as well (total charge of product side...

Sean Cheah 1E

Yes, for redox reactions in basic solutions, add OH- to both sides when balancing out the H+. On the side that has H+, the two species will combine to form neutral H2O.

Sean Cheah 1E

Here is a website with some common rules that one should follow when assigning oxidation numbers: https://www.thoughtco.com/rules-for-assigning-oxidation-numbers-607567.

Sean Cheah 1E

Oxidation numbers of many elements are not fixed. Here is a site with more information about some rules that one must follow when assigning oxidation numbers: https://www.thoughtco.com/rules-for-assigning-oxidation-numbers-607567.

Sean Cheah 1E

In the case of an isothermal reversible expansion, n = number of moles of gas = constant, R = gas constant, and T = constant. By the ideal gas law, since nRT is constant, PV is also constant. It does not matter whether you use the initial or final volume or pressure as their product will be constant.

Sean Cheah 1E

For constant pressure the Cv would actually be switched to CP. Is this actually true? I thought that this equation for delta U only works with Cv. For example, in an isobaric process, q = nCp(delta T) and w = -P(delta V) = -nR(delta T). Thus delta U = q + w = n(Cp - R)(delta T). Since Cv = Cp - R, ...

Sean Cheah 1E

The definition of a reversible process requires that the entropy of the universe (total delta S) does not change. This is because the system undergoing this process is always in equilibrium with the surroundings. For an irreversible isothermal free expansion into a vacuum, the pressure is constant a...

Sean Cheah 1E

Looking at the equation delta G = delta H - T*delta S, delta S must simply be greater than delta H/T for delta G to be less than 0 (spontaneous reaction). For spontaneous exothermic reactions (delta H and delta G < 0), this means that delta S can be negative as long as it is greater than delta H/T.

Sean Cheah 1E

One equation for entropy is delta S = nR*ln(P1/P2). From this, it follows that increasing pressure decreases entropy and decreasing pressure increases entropy.

Sean Cheah 1E

Because the process is isothermal, q = -w. Decreasing pressure must mean that volume is increasing, therefore work is negative. Then q is positive and delta S = q/T is also positive.

Sean Cheah 1E

While it does not matter whether or not a process is reversible, delta S = q/T cannot be used to calculate entropy for non-isothermal reactions. In those cases, an integral must be set up using dS = dq/T from T1 to T2. At constant pressure, dq = n(Cp)(dT). At constant volume, dq = n(Cv)(dT).

Sean Cheah 1E

why is the change in entropy sometimes equal to (-deltaH/T)? in other words, what does the - imply? The change in entropy of the surroundings is equal to the negative change in the enthalpy of the system divided by temperature. This makes sense because when the system gives off heat (negative delta...

Sean Cheah 1E

As noted previously, all isothermal processes by definition have delta U = 0. This means that the work done does not necessarily have to be 0. In the case that work is not zero, q will simply be the negative of that so they cancel out.

Sean Cheah 1E

Isothermal means that delta T = 0 and therefore delta U = nc(delta T) = 0. This does not mean that delta Q = 0.

Sean Cheah 1E

405318478 wrote:Hey.
Could someone explain where the equation for work, W=nR(deltaT) comes from? This is for calculating work at constant pressure.

This comes from using the Ideal Gas Law to replace the P(delta V) in the original equation for work with nR(delta T).

Sean Cheah 1E

BeylemZ-1B wrote:They aren’t the same in the answer key though.

That's strange. In my answer key, they are indeed the same (about 28 J).

Sean Cheah 1E

Use the first equation for irreversible expansions/contractions occurring against constant external pressure. Use the second equation for reversible expansions where the external pressure is always equal to the internal pressure (pressure of the expanding/contracting gas).

Sean Cheah 1E

I don't know if there is a best method but I remember Dr. Lavelle specifically saying that using bond enthalpies was the least accurate method because these are calculated as averages across many different molecular configurations.

Sean Cheah 1E

More generally, W =-\int_{V_{i}}^{V_{f}}P\ dV . In the case of an irreversible process against a constant pressure, this gives you W = -P(V_{f} - V_{i}) = -P\Delta V . In the case of a reversible process, the P in the work equation is equal to the P of the gas -- the external pressure on the...

Sean Cheah 1E

Brittney Hun 2C wrote:How do you know the copper releases energy?

Heat always flows from objects of higher temperature to objects of lower temperature.

Sean Cheah 1E

The Van der Waals equation modifies the Ideal Gas Law slightly to better predict the behavior of real gases, taking intermolecular forces and volume taken up by gas molecules into account. It's really interesting to see how this was done but for the purposes of this course it should be safe to assum...

Sean Cheah 1E

Even if the temperature is not 25C, assuming that you are dealing with pure water, [H3O+] = [OH-] so $[H_{3}O^{+}] = \sqrt{K_{w}}$ .

Sean Cheah 1E

To see if something is endothermic r exothermic we look at delta G. If delta G is positive, the reaction is endothermic, but if it is negative the reaction is exothermic. When determining whether a reaction is exothermic/endothermic, one should look at the sign of delta H (change in enthalpy). The ...

Sean Cheah 1E

The names for the phase changes you listed are, in order, condensation, freezing, and deposition. These are indeed simply the reverse of vaporization, melting, and sublimation, respectively, and are therefore exothermic.

Sean Cheah 1E

I don't believe that there is a strict cutoff of k = 10^-3 when determining whether or not a given compound is a weak acid. An acid is considered weak if it does not completely ionize in solution. That means that even acids with a Ka of 1 can technically be considered weak because this would imply t...

Sean Cheah 1E

To be completely honest I have no idea what's going on here either. I was under the assumption that Le Chatelier's Principle, like the equilibrium constant, does not take pure solids and liquids into account. Any further insight would be greatly appreciated! Edit: So I did some digging through sever...

Sean Cheah 1E

The net chemical equation for photosynthesis is deceptively simple and excludes many of the complex intermediate steps that must take place when converting the reactants to products. For this class, those details are not important. Just know that when you sum all the energy expended to break bonds a...

Sean Cheah 1E

The reaction quotient depends on the concentrations/pressures of the reactants and products. For reactions not at equilibrium, these values are constantly changing and thus Q is nonconstant. Even for reactions at equilibrium, changes in the concentrations/pressures (adding/removing more reactants/pr...

Sean Cheah 1E

The Henderson-Hasselbalch equation states that $pH = pK_{a} + log(\frac{[A^{-}]}{[HA]})$ . Use an ICE table to determine the concentrations of the conjugate acid-base pair.

Sean Cheah 1E

As previous posters have already noted, pay careful attention to the units of the given values in each problem when determining which R constant to use. For example, when converting from Keq to delta G, the correct R value would be about 8.314 J*K^{-1}*mol^{-1} , which makes sense seeing as your fin...

Sean Cheah 1E

$K_{p} = K_{c}(RT)^{\Delta n}$ , where $\Delta n$ is the sum of the coefficients of the gaseous products minus the sum of the coefficients of the gaseous reactants (the change in the number of moles of gas), $R \approx 8.314 \frac{J}{K\cdot mol}$ , and T is temperature in Kelvins.

Sean Cheah 1E

I assume you're referring to part b and not part a as the answer to part a should be 1/41. First note that the given information tells you K = \frac{[N_{2}][H_{2}]^{3}}{[NH_{3}]^{2}} = 41 . Then note that the K value for part b is K = \frac{[N_{2}]^{\frac{1}{2}}[H_{2}]^{\frac{3}{2}}}{[NH_{3}]} = \sq...

Sean Cheah 1E

A net-ionic equation is one where the species (ions, solvents, etc.) that appear in both the reactants and the products are omitted, leaving only the ions that actually change in some way during the course of the reaction. An net-ionic equation for the dissociation of a solid salt in water would loo...

Sean Cheah 1E

Kp is calculated using partial pressures and can therefore only be used for reactions that exclusively involve gaseous substances. Kc is calculated using molar concentrations and can be used for all reactions. In those calculations, the concentration of any given gas would simply be the number of mo...

Sean Cheah 1E

Is the value of Kp supposed to change depending on the units used to measure pressure? For part c of 5G.5, the squared term in the numerator of the expression for Kp means that multiplying each pressure measurement by a constant (as one would do when converting between units) would result in a compl...

Sean Cheah 1E

Yes you would look at the same characteristics to determine the strength of a base. Essentially a stronger base, wants to accept a proton easier, and will give off more hydroxide ion. So how would that work though? Cause like for acid, you look at the strength between H and A (the bigger the A, the...

Sean Cheah 1E

The seesaw relationship between the relative strengths of a conjugate acid/base pair states that the conjugate base of a strong acid will be a very weak base and the conjugate acid of a strong base will be a very weak acid. In these cases, the conjugates are so weak that they have basically no effec...

Sean Cheah 1E

One of the factors that affect the strength of an acid is the net electron-withdrawing force acting on the H+ that the acid can donate. In the case of oxoacids, increasing the number of oxygens increases this electron-withdrawing force and further polarizes the compound by shifting the electron dens...

Sean Cheah 1E

Unfortunately, memorizing a table of common ions may be your best bet here. As far as I know, there is no easy way to determine the charge of an ion given its molecular formula alone.

Sean Cheah 1E

I don't actually know the answer to your question but it did remind me of the fact that all amino acids with acidic side chains are negatively charged when deprotonated and all amino acids with basic side chains are positively charged when protonated. Whether or not an acid/base is protonated depend...

Sean Cheah 1E

En is the abbreviation for ethylenediamine (C2H4(NH2)2) and edta is the abbreviation for ethylenediaminetetraacetic acid (C10H16N2O8).

Sean Cheah 1E

Chelating ligands are polydentate ligands that are able to form a ring-like structure of atoms that includes the central metal cation. As an example, each ethylenediamine (molecular formula: C2H4(NH2)2) ligand in the compound pictured below binds the central Cobalt cation to form two such rings. htt...

Sean Cheah 1E

Sometimes it can be difficult to tell whether a given molecule is an acid or a base from its molecular formula alone. For example, NH3 (ammonia) is a weak base despite having several H atoms.

Sean Cheah 1E

Alison Trinh 1A wrote:Can someone explain what exactly a ligand is?

In this context, a ligand refers to an electron-pair donor that forms one or more coordinate covalent bonds with a central metal ion.

Sean Cheah 1E

Neutralization reactions involving weak acids and bases are a lot more complicated than those that only deal with strong acids and bases. This is because the incomplete ionization of the weak acids and bases results in an equilibrium that one would then need to create a so-called ICE table to solve ...

Sean Cheah 1E

In this context, a ligand is a Lewis base (electron pair donor) that can form coordinate covalent bonds with transition metal cations. To make things confusing, ligands take on an entirely different definition in the context of biochemistry (cell signaling, enzymatic reactions, etc.).

Sean Cheah 1E

We haven't yet covered equilibrium yet so you likely don't need to know the specifics, but logically speaking it should make sense that the heme group has a higher chance of binding oxygen when the oxygen is at a higher concentration (which is essentially what a higher partial pressure means). Conve...

Sean Cheah 1E

A strong acid is one that fully dissociates in water while a weak acid is one that only partially dissociates (equilibrium dictated by something called a Ka value).

Sean Cheah 1E

When determining the hybridization scheme of a certain atom, all electron domains should be counted, whether they be bonding or lone pairs. Bonding electrons will essentially singly occupy their respective hybridized orbitals (allowing sigma bonds to form via interactions with other orbitals only co...

Sean Cheah 1E

^^ What is a step by step method? I don't think there is any kind of step-by-step method. All you need to determine the hybridization scheme of a certain central atom is the number of regions of electron density around that atom (which one can very easily determine using the Lewis dot structure). 2...

Sean Cheah 1E

Honestly, I think you just have to approach the problem like any other Lewis structure question and work it out by hand using the number of total valence electrons. Wikipedia says that this ion (the pentazenium cation) has six possible resonance structures given by: [N≡(N+)−(N−)−(N+)≡N]+ ↔ [(N−)=(N+...

Sean Cheah 1E

Not 100% sure if this is correct, but I drew it with the Se in the middle and the O's surrounding it. For 2 of the O's I drew double bonds and 2 lone pairs, and for the other 2 O's I attached H's and 2 lone pairs. Yeah, a quick Google search reveals that this is the correct Lewis structure. Now for...

Sean Cheah 1E

Electronegativity is actually one of the more consistent periodic trends and generally increases across a period and decreases down a group. While a similar trend characterizes the ionization energies, do note (as mentioned above) that there is no decrease in electronegativity from N to O and from B...

Sean Cheah 1E

I found this simple table online that might help you visualize and remember all the various molecular geometries.

Sean Cheah 1E

You can count the number of regions of electron density and the number of lone pairs around a central to quickly determine its corresponding molecular geometry. Trigonal planar geometries occur when there are 3 regions of electron density of which none are lone pairs. Trigonal pyramidal geometries o...

Sean Cheah 1E

Always make sure that the total number of electrons in your final Lewis dot structure (whether they be in lone pairs, bonds, or single dots in the case of the unpaired and nonbonding electrons of radicals) is equal to the sum of the valence electrons of all the atoms in the structure.

Sean Cheah 1E

Remember that electronegativity generally increases going across a period (left to right) and decreases going down a group. You can use this trend to figure out the relative magnitudes of the electronegativity differences.

Sean Cheah 1E

The way I like to remember this is that ions with larger charge/size ratios have higher polarizing power. Thus, as both smallest by ionic radii and one of only two in your list carrying a relatively large 2+ charge, Be2+ has the largest charge/size ratio and therefore also has the highest polarizing...

Sean Cheah 1E

Lattice energy represents the change in energy that occurs when separated gaseous ions are packed together to form the ionic solid, which is always negative due to the process being exothermic. This kinda gets into the realm of physics but essentially the negative derivative of the electrostatic for...

Sean Cheah 1E

Additionally, an electronegativity difference less than 0.5 generally represents a nonpolar covalent bond while an electronegativity difference between 0.5 and around 1.5 represents a polar covalent bond.

Sean Cheah 1E

While it is true that the atoms with the lowest ionization energies generally go in the center, this is not always the case. For example, I challenge anyone who does not already know the structure of HNO2 (nitrous acid) to draw its Lewis structure from scratch and compare that with its real structur...

Sean Cheah 1E

Honestly, while memorizing the equation is fine and good, I like to tally up the number of electrons to subtract by simply counting each bond as one electron as each lone pair as two electrons. For example, if I saw a double bond, a single bond, and two lone pairs around a hypothetical atom, I could...

Sean Cheah 1E

How do you know which atom can hold more of the negative charge? To maximize the stability of your structure, any remaining negative formal charge should be preferably assigned to the more electronegative elements first. I'm sure Lavelle will not expect us to memorize the electronegativities for al...

Sean Cheah 1E

After getting the formal charge on each atom as close to zero as possible, place the remaining negative formal charge(s), if present, on the more electronegative atom(s) and the positive formal charge(s), if present on the less electronegative atom(s) to get the most stable structure. The reason tha...

Sean Cheah 1E

In cases where it is impossible to entirely eliminate the formal charge on each atom, the most stable structure will have the negative formal charges on the more electronegative atoms and the positive formal charges on the less electronegative atoms.

Sean Cheah 1E

Does Hund's rule apply to the Pauli Exclusion Principle? This is a bit of a strange question to ask. Hund's Rule describes the order in which electrons fill a subshell and is composed of two main statements: every orbital in a subshell must be singly occupied before any are doubly occupied, and all...

Sean Cheah 1E

Hi, To my understanding, Z is not one of the four types of quantum numbers [those are principal quantum number (n), the orbital angular momentum quantum number (l), the magnetic quantum number (ml), and the electron spin quantum number (ms)]. Z, in this context, is involved with the quantum number ...

Sean Cheah 1E

All the answers above me are close but just ever so slightly off. While it is true that each shell contains one or more orbitals, it is not entirely correct to say that each angular momentum quantum number l represents an orbital within a given shell with principal quantum number n. This becomes obv...

Sean Cheah 1E

If my understanding is correct, the squared absolute value (well, technically modulus but I won't get into that) of the wave function describes the probability density of observing an electron in the orbital described by the function, essentially how likely you are to find an electron at any given t...

Sean Cheah 1E

If it helps, you can think of the various quantum numbers for electrons as a hierarchy of sorts. Starting from the bottom going up, the spin quantum number ( m_{s} ) represents the spin of a given electron. The magnetic quantum number ( m_{l} ) represents the specific orbital that contains that elec...

Sean Cheah 1E

As mentioned by previous posters, the 4s orbital is filled before the 3d orbitals because it actually has lower energy than the 3d orbitals before they are populated. Note however that this reverses as soon as electrons begin to fill the 3d orbitals, at which point the electrons in the 4s orbital wi...

Sean Cheah 1E

As posted above, the letters s, p, d, and f are refer to subshells, which, as their name implies, are subsets of shells. Each of these subshells contain one or more discrete orbitals that are represented by the various possible orbital angular momentum quantum numbers for a given shell and subshell....

Sean Cheah 1E

also how do you convert the 10^-12 answer into nm As stated above, the answer to your first question is that c in this case simply represents the speed of light, which is a constant that is approximately equal to 3\times 10^{8} m/s. To convert from 10^{-12} m to nm, simply multiply by the conversio...

Sean Cheah 1E

Hi! I haven't done this problem before but I think what it means is that after you calculate the difference of the wavelengths (between protons and neutrons), you need to then calculate how much of the wavelength of neutron does this difference in wavelengths take up. Namely, diff divide by wavelen...

Sean Cheah 1E

I almost got tripped up by this question as well. After thinking it over, I believe that it is asking for the wavelength of the light that would eject an electron that with the given final velocity. In this case, you need to use the equation \frac{1}{2}mv^{2}=h\nu -\Phi , where m is the mass of an e...

Sean Cheah 1E

Drake Choi_1I wrote:When converting from photons to moles, would it be 6.022 x 10^23 photons to 1 mole?

Yes, that is correct. Whenever you are converting to moles you simply divide the amount of whatever you have by Avogadro's number.

Sean Cheah 1E

This contradicts a lot of what's already been posted here, but I always thought that an electron could be excited to any electron level from 1 to a theoretical infinity (e.g. completely removed from the atom) depending on the energy of the photon that it interacts with. I don't think the energy leve...

Sean Cheah 1E

Can someone please tell me which units cancel out when solving the Rydberg equation? The principal quantum numbers (the n's) have no units. The Rydberg constant has the units m^{-1} . Basically, everything on the right side will work out to some answer with the units m^{-1} . This matches the left ...

Sean Cheah 1E

are the final units wirtten in Hz or Js? This depends on what the question is asking for. The traditional form of the Rydberg formula, as posted by MAC 1G above, allows you to plug in constants on the left side to get 1 over the wavelength of the photon that will be emitted/absorbed when an electro...

Sean Cheah 1E

In class, Professor Lavelle wrote the Rydberg equation with a negative sign on the right to show that energy was being lost. Unfortunately when I used the equation that way, my negatives got all messed up and my answer was wrong...Should I just forget that negative? When using the Rydberg formula, ...

Sean Cheah 1E

curry 1E wrote:Can someone explain what Angstroms are? I was confused on the test on this question.

Angstroms are a unit of length equivalent to 10^(-10) m.

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