CHEMISTRY COMMUNITY

Justin An 2L

This is a bit tough because moles of gas remains the same, but it does go from a solid + gas to only a gaseous.... I don't think we'll be given a test question like this without standard entropy values because it's too ambiguous without knowing experimental data I looked up the entropy values and ca...

Justin An 2L

Activation energy for BOTH the forward and the reverse reaction decrease! And since it takes less energy to kickstart the reaction for both forward and reverse pathways, the rates for both are going to be faster! I hope this diagram also helps!

Justin An 2L

Also, by convention I believe it's ln(k2/k1).... (1/T1 - 1/T2)!

Justin An 2L

If it makes it easier, use k = Ae^-Ea/RT to get rid of the natural logs and simplify it!

Justin An 2L

Free energy of activation is also known as Gibbs free energy, and is the difference in Gibbs free energy between the products and the reactant; the activation energy is just the energy required to initiate the reaction

I hope the diagram helps!

Justin An 2L

while OH- is indeed an intermediate between the two fast equilibrium steps, we're more concerned/focused on the reactants and products of the slow step to determine rate, which is why when we write the rate law for this mechanism (rate = k[HClO][I-]), we need to get rid of the [HClO] by using the id...

Justin An 2L

in terms of the "intermediate" that we want to analyze, it would be HClO because it's the intermediate that occurs in the slow step, which is the rate determining step!

Justin An 2L

Best bet is to draw out the ethalpy diagram! When you look at this idea, you can see that in the reverse, the reaction requires the change in enthalpy (which is enthalpy of products - enthalpy of reactants) + the activation energy of the FORWARD to find the activation energy of the reverse

Justin An 2L

in terms of the equilibrium constant, temperature will affect whether it increases/decreases depending on whether the reaction is endothermic (temp is considered a "reactant") or exothermic (temp is considered a "product)

Justin An 2L

pKa = -logKa
Ka = 10^-pKa

Justin An 2L

in the nerst equation, n refers to the number of moles of e-/electrons, which you can determine by balancing the half reactions!

Justin An 2L

can someone quickly summarize the Arrhenius Equation and it's relationship to rate constant, k? As well as how rate constant relates to the equilibrium constant, K (forward and reverse rates relationship to eq. constant)?

Justin An 2L

Temperature is the only main factor that we are focusing on I believe - when temperature increases, the rate constant increases based on the Arrhenius equation!

Justin An 2L

Melting is an endothermic reaction because heat is being absorbed, therefore the change in enthalpy is positive

Justin An 2L

Bimolecular reactions involve two molecules/species interacting whereas unimolecular reactions only involve on molecule/species!

Justin An 2L

given an equation A + B <--> C + D, the rate law (assuming for the forward reaction) would be rate = [A][B]

Justin An 2L

Usually zero order reactions are very rare because they depend solely on the rate constant, k, meaning the rate is independent of the concentration of reactants, which isn't typically

Justin An 2L

When it comes to T, we can utilize Le Chatelier's principle to determine how the equilibrium constant, K (product/reactant), is affected For exothermic reactions: A + B <--> C + D + heat - increasing heat = "increase in products" = favor increase in reactant & decrease in product = K w...

Justin An 2L

k is rate constant!!

Justin An 2L

The slope for a first order reaction is -k, which is determined by analyzing the natural log (ln) vs. time graph

Justin An 2L

n is just number of moles!

Justin An 2L

Kinetics analyzes the pathway from product to reactant (speed/rate of reaction and the steps taken to get from reactant to product) while thermodynamics analyzes the change in heat/energy, which in turn influences concepts like work, Gibbs free energy and spontaneity

Justin An 2L

The difference between a catalyst and an intermediate is that intermediates are consumed/used up in the reaction mechanism while catalysts remain unconsumed! What this means is that you'll see catalysts on the reactant side first and then product side whereas intermediates will be formed (on the pro...

Justin An 2L

q=-q is referring specifically to the transfer in heat/energy usually between a system and its surroundings. In a situation where - heat goes from the system --> surroundings, +q refers to surroundings (since heat/energy is gained) and -q refers to the system (since heat/energy is lost) - heat goes ...

Justin An 2L

Yes, both the forward and reverse reactions are sped up because the activation energy is lowered

Justin An 2L

Personally, I've never heard of a catalyst that slows a reaction - I think what you're thinking of is an inhibitor, which is a substance that decreases the rate of a chemical reaction! https://byjus.com/jee-questions/wha-is-opposite-of-catalyst/#:~:text=The%20opposite%20of%20catalyst%20is,reactions%...

Justin An 2L

I've never heard of a reaction order being a fraction, but according to google, the order of reaction can potentially be fractional

https://byjus.com/chemistry/order-of-re ... 0of%20zero.

Justin An 2L

Furthermore, zero order reactions typically occur when the material required for the reaction to proceed (ie. catalyst) is saturated with reactant. If we assume that there is a >> large amount of reactant, then we can say that changes in reactant concentration are negligible almost, meaning the rate...

Justin An 2L

With a zero order reaction, the rate law would be rate = k[A]⁰, and anything to the 0 power is 1, meaning that rate = k

Justin An 2L

for instance, given an equation A + B <--> C + D
- rate_forward = k[A][B]
- rate_reverse = k'[C][D]

Justin An 2L

while k is the rate constant for the forward reaction, k' represents the rate constant for the reverse reaction

Justin An 2L

From lecture, molecularity refers to the number of molecules colliding in an elementary/rate determining step **ex. mechanism with two reactants to form a product of the rate determining step is bimolecular

Justin An 2L

I noticed this too - based on the units of k, we can tell if it's
- first order: 1/M*s
- second order: 1/M²s
- third order: 1/M³s

is this a correct assumption?

Justin An 2L

I'm going through and doing the textbook/syllabus problems and I'm noticing that a lot of the questions in 6D assume that we know the ka or kb value when solving for concentrations or pH/pOH; on the midterm, will we always be given the ka or kb value or are we expected to memorize ka and kb values?

Justin An 2L

I just went back and checked and it's actually the conjugate base over the weak acid! ex. CH3COOH <--> CH3COO- + H+ and percent ionization is [CH3COO-]/[CH3COOH]

Justin An 2L

When you add an inert gas (I'm assuming we are increasing pressure without increasing volume), although you increase pressure, you don't change the volume of your system! Remember that concentration is moles/L, so if you aren't adding more of your product or reactant gas and you're not changing the ...

Justin An 2L

How do you know which R gas constant to use when solving either PV=nRT or the integral equation? I got that mixed up on the last homework set (I know you can look at the units and see which cancel out, but I just wanna see if there's a different way)

Justin An 2L

Just wanna add! When describing/comparing K as it's own value (K < 10 -3 or K > 10 3 ), saying products/reactants are favored means that's where the equilibrium is more likely to exist - if K < 10 -3 , then there are more REACTANTS at equilibrium, which means that reactants are favored at equilibriu...

Justin An 2L

Real quick! A faster way to calculate the concentrations is to us kw = ka*kb, which is 10 -14 = [H+]*[OH-] Also, pH can be less than 0 or even greater than 14!! Most of the time in chem, we choose to accept 1-14 as the fundamental range, but pH can definitely extend beyond so don't worry about the n...

Justin An 2L

In an open system, both energy and matter can freely flow between the system and the surroundings!

Justin An 2L

I think either is fine because both values should be equal to X from your ice table; I can't think of a weak acid scenario where you would ionize more than one H+ in one reaction

Justin An 2L

Almost always use molarity!! The only instance I can think of to use moles instead would be for things like titrations and buffers where you add a strong acid/base to a weak base/acid and need to find pH in a buffer region or in excess, etc.

Justin An 2L

You would almost always be given the concentration of one or the other or other context clues like moles and liters of a strong/weak acid/base!! And from there you can apply the equation kw = [H+]*[OH-] = 10^-14 to solve for one or the other

Justin An 2L

When the amount of product increases, we are no longer at equilibrium!! You can almost think of equilibrium like a balanced scale - adding more products ("mass" to the right side of the scale) will tip the whole system towards the products and in order to achieve equilibrium again (re-bala...

Justin An 2L

Typically equilibrium is always referred to as DYNAMIC because it's always going back and forth (reactants are still turning into products and vice versa) BUT the defining characteristic of equilibrium is that the forward and reverse reaction rates are equal so no NET change is observed!

Justin An 2L

At what value of k are we allowed to make the assumption that the concentration - x is about equivalent to the concentration: ie. ka = x/(0.010 - x) can be assumed to be ka = x/0.010

I've heard it to be either when k < 10^-4 but also k < 10^-5

Justin An 2L

I think I'm just getting confused by the terminology of work done on vs. work done by... are they the same thing?

Justin An 2L

When work is negative, does that mean work is done on the system (and therefore energy is added to the system) or is it the other way around?

Justin An 2L

Like many people have noted, work done ON the system means energy is ADDED to the system while work done BY the system means energy LEAVES the system. You can almost think of it like are you being pushed ON by someone else (feel the force/"energy" of someone pushing you) or is someone else...

Justin An 2L

What everyone else is saying is absolutely correct! You can think of it almost like a balance (where both sides being perfectly balanced represents equilibrium) - if you add weight to one side, you're going to want to balance it out by adding more to the other side (if you add more product to a syst...

Justin An 2L

If it helps, check out "ICE tables equilibrium chemistry" on google for help! There are plenty of videos that go through the ICE table and work through examples! :D

Justin An 2L

Hi there! So to really quickly sum of the two differences between the first and second scenario: in the first one, we DECREASE VOLUME, which changes concentration (because concentration equals moles of gas/volume) and will subsequently cause a shift in equilibrium. In the second case, we ADD GAS, bu...

Justin An 2L

Hi!! This isn't a question or anything, but just a reminder that I hope helps everyone :D When describing pH... PLEASE PLESAE PLEASE DON'T FORGET HIGHER/INCREASING PH MEANS MORE BASIC NOT ACIDIC (and subsequently decreasing/lower pH means acidic NOT BASIC) I've made this mistake dozens on times back...

Justin An 2L

Honestly, don't worry!! Plenty of my friends decided to take 14B stand-alone and then 14BL the next quarter with 14C for example. Furthermore, people will definitely drop during the first week so be on the lookout for swaps and enrollment!! I use coursicle on the app store to keep tabs and get notif...

Justin An 2L

I hate to break it to you, but I believe the 5 points per week come from doing 5 posts per week, so you would need a total of 50 posts to get the 50 chemistry community points :33

Justin An 2L

Hi there! So based on the rules, we can first determine the charge of the cation (ex sodium has a charge of +1 since it's in the group 1 elements) and then look at the anion charge to help determine how many cation atoms we need to balance our molecule's charge. So in the sodium tetracyanocuprate (I...

Justin An 2L

So when acids/bases are produced, the conjugate is essentially the other ion/compound that is produced. Conjugate acids are the compounds/ions formed when bases gain a proton (H+) and conjugate bases are the compounds/ions acids when acids lose a proton. Attached below are some examples! https://stu...

Justin An 2L

I think there's a little bit of confusion! Strong bases and strong acids (unlike weak acids/bases) fully dissociate when put in an aqueous solution (typically just when you put an acid in water). For example, HCL will completely dissociate into H+ atoms that join with water molecules to become H3O+ ...

Justin An 2L

Formal charge of an atoms shows whether it's missing an electron or has an extra electron, which is why ideally atoms want to have a formal charge of 0 (balanced and as stable as possible). For a molecule, the formal charges of all the individual atoms contribute to the overall charge of the molecul...

Justin An 2L

As many have noted before, this is where the importance of knowing shape/VSEPR theory comes into play; although a lewis structure may look like it's atoms cancel/balance and are directly opposite of each other, it may be a square pyramid shape instead of a tetrahedral for example. Overall, once you ...

Justin An 2L

A good way to remember electronegativity is to think about regions/periodic trends; for electronegativity, it increases as you move to the up and right of the periodic table!

Justin An 2L

Hi there! To my knowledge, sigma bonds bind at end-to-end sites compares to the small overlap of parallel p orbitals for pi bonds. Basically, there is more "contact"/overlap at the end-to-end sites vs. the parallel p orbital overlap which is why sigma bonds are both 1. stronger than pi bon...

Justin An 2L

Conceptually, if we establish that 388nm is the minimum wavelength at which electrons are ejected, we can say that electrons will be ejected since the wavelength shortens to 200nm (and shorter wavelengths have more energy). Mathematically, we can use the E photon = kinetic energy + work, set work = ...

Justin An 2L

The coordination number IS the number of ligands around a central atom! The coordination number can help you predict the shape of the compound/molecule based on the foundations of VSEPR theory!

Justin An 2L

To determine the oxidation number of the metal, you want to consider the overall charge of the compound, the oxidation numbers of the elements you know, and then use almost like a (oxidation number of metal) + (other oxidation number total) = overall oxidation number of compound "equation"...

Justin An 2L

Like others have mentioned, it is the presence of two lone pairs of electrons in addition to the 4 ligands that results in a square planar arrangement. This is because each lone pair causes some repulsion on the ligands (lone pair - bond interactions) on opposite ends that force the ligands from a t...

Justin An 2L

Ionic and covalent thresholds depend on different differences in electronegativity (greater electronegativity differences = ionic while smaller differences in electronegativity = covalent). I'm sure professor won't expect us to know exactly the threshold values and rely more on periodic trends (exam...

Justin An 2L

Sigma and Pi bonds can exist at the same time because sigma bonds rely on end-to-end interactions, whereas pi bonds rely on side-by-side overlap of p orbitals (can remember this like P orbital for Pi bonds.

Justin An 2L

More of a biological description, but myoglobin (like hemoglobin) holds oxygen! The difference between the two is that myoglobin is found in muscle cells and has a higher affinity for oxygen (Fe group oxidizes from 2+ to 3+ when oxygen binds). As a result, myoglobin serves as an oxygen-storage prote...

Justin An 2L

This chart really helps me visualize everything when it comes to shape!!

Justin An 2L

Sigma bonds form from end to end interactions and are free to rotate!! On the other hand, pi bonds form from overlap of parallel p orbitals and are rigid/cannot rotate. On a stand-alone basis, sigma bonds are stronger than pi bonds because the end to end interactions of sigma bonds involve greater o...

Justin An 2L

Once you've drawn your BASE lewis structure (starting with atoms around central atom, single bond with each then lone pairs to meet octet rule, then matching total valence e's by adding more bonds), you want to find the formal charge of each atom. The BEST overall structure is the one with either a ...

Justin An 2L

Like everyone has mentioned before, lone pairs are vital to determining hybridization, shape, polarity and more when it comes to the VSEPR model because although not bonded, lone pair electrons are still considered electron dense regions that repel other electron dense regions!!

Justin An 2L

^^^ technically yes! Again like other people have mentioned, the only difference between Lewis and Bronsted relates to whether you're talking about electrons (Lewis) or protons (Bronsted) https://leah4sci.com/wp-content/uploads/2014/10/Arrhenius-Bronsted-Lowry-Lewis-Acids-and-Bases.png

Justin An 2L

The best way to determine if a molecule is polar or non-polar is to just draw out the lewis diagram and then analyze the molecule's shape/arrangement from there. You should ask yourself if the lewis diagram looks even, if the same element is bonded around the central atom, are there lone pairs, what...

Justin An 2L

Like others have mentioned, strong bases are compounds that easily dissociate into their counterpart ions and OH- groups! An example of a strong base would be NaOH. On the other hand, weak bases struggle to dissociate easily in water or only partially dissociate in water to form OH- ions. An example...

Justin An 2L

Like everyone else has noted, noble gases all have full valence sells, which make them very satisfied in their current state - it wouldn't make sense to try and add or remove an electron from noble gas elements.

Justin An 2L

From what I learned back in the LS series, hemoglobin is found in blood cells and is directly involved in gas exchange between lungs and cells whereas myoglobin is primarily found in muscle cells. Furthermore, a key difference between the two is the myoglobin has a higher affinity for oxygen than he...

Justin An 2L

Hi! Unless there's an open waitlist, you probably can't switch into the other section. However, there are three things you can do: 1. You can try to find someone in the discussion you want to switch into and ask if they would be willing to switch sections with you 2. Contact the professor/TA and ask...

Justin An 2L

Have you tried contacting their tutoring chairs on the website for more information?

Justin An 2L

Hi! Could you give an example of a compound with more than two different transition metals? I don't think I've personally ever seen a compound with more than two different transition metals, but overall, I would just follow the same oxidation number rules to help isolate the remaining oxidation numb...

Justin An 2L

1 sigma and 1 pi bond would correspond to a double bond (double = 2 bonds). When it come to a triple bond (3 bonds between atoms), you would have a sigma bond and the two remaining bonds are pi! Like others have said, the easiest way to predict the number of pi bonds is to always consider 1 sigma bo...

Justin An 2L

Like many other people have noted before me, there are plenty of useful charts to help get VSEPR theory down! I've attached one that I use below that covers shape, angle, an example of a molecule and hybridization! https://data.templateroller.com/pdf_docs_html/86/862/86292/vsepr-theory-molecular-sha...

Justin An 2L

Yep! That's it: Pauli Exclusion is
- 2 electrons per orbital
- must be in opposite spin

*Hund's rule is the one that goes single occupy first before double occupy!

Justin An 2L

The number of valence electrons is essentially the ones digit of the group number!!

Justin An 2L

Like others have said prior, the octet rule follows the idea that atoms want to have 8 valence electrons, and so will tend to form compounds that give them 8 valence e-s; the reason for 8 is that it completes a full shell, which is the most stable configuration (noble gas config).

Justin An 2L

From Friday's lecture, when it comes to writing the ground state electron configuration for an element like Scandium, I get the reasoning why it should be [Ar] 3d 1 4s 2 , but doesn't that inherently break Aufbau's principle, which states that the order should be 4s before 3d? This is just conflicti...

Justin An 2L

Hi there! The reason why the 4s orbitals are filled before the 3d orbitals is because the 4s orbitals are lower in energy than the 3d orbitals (according to the Aufbau principle). Orbitals with lower values of (n+l) have lower energy and are filled first before moving to those with a higher value of...

Justin An 2L

An example of an excited state configuration is 1s 2 2s 2 2p 4 3s 1 and we know this because we would expect an atom with 7 electrons (know this by counting the powers) to have a normal/ground state configuration of 1s 2 2s 2 2p 5 since electrons occupy orbitals individually first before being occup...

Justin An 2L

I don't know if it's proper to think of a nodal plane like this, but to get a better idea of what a nodal plane embodies, I like to think of a tangent plane almost, where the probability around that plane has some value (there are various probabilities of finding electrons as you move away from the ...

Justin An 2L

Like everyone else has mentioned, there is an inverse relationship between wavelength and frequency. A good way to remember this is to think of the equation e = hv which is also equal to hc/λ (where h is Planck's constant, v is frequency, λ is wavelength, and c is the speed of light). Setting hv = h...

Justin An 2L

A good mnemonic that I've heard for memorizing the EM Spectrum is "Ryan May I Visit Ur X-Girlfriend?" for Radio, Micro, Infrared, Visible, Ultraviolet, X-rays, Gamma. However, I think conceptually the only important part to know of the EM spectrum is that this goes in INCREASING frequency/...

Justin An 2L

One that I've heard for memorizing the EM Spectrum is "Ryan May I Visit Ur X-Girlfriend?" for Radio, Micro, Infrared, Visible, Ultraviolet, X-rays, Gamma. Remember this goes in INCREASING frequency or DECREASING wavelength!!

Justin An 2L

Whenever it comes to questions, it's important to evaluate the context of each question in order to determine what units you want to use! Like the previous person said, some fundamentals are grams for moles/molar mass, mL or L for volume, and seconds for time. I don't think I've ever had to use km f...

Justin An 2L

I haven't done this question yet, however, just by looking at your answer, 4.9 x10^-7 molecules is equivalent to 0.00000049 molecules, which could effectively be rounded to 0 molecules left.. Again, I haven't done this problem! However, I noticed you are solving for molecules not moles.. did you use...

Justin An 2L

I feel like this depends on the context of the question! I'm sure Dr. Lavelle will provide us with constants as well as equations for any exam, but again, depending on the context of the question, you can evaluate the units of the other variables in order to choose which constant you want (see if Hz...

Justin An 2L

Like we learned, the main equation we would use to calculate wavelength is E = hv, which is subsequently equivalent to hc/\lambda where v is the frequency of the EM radiation, and lambda is the wavelength (so you set the equation equal to lambda to find the wavelength at an energy level, n). If you ...

Justin An 2L

Overall, it's always best to get to grams since molar mass conversions are denoted as moles per grams! :D

Justin An 2L

Hi there! Theoretical yield is the actual calculated product yield based on the conditions that are outlined in the question (ie. specific mass (g) of reactant/limiting reactant that you've identified). Actual yield is the amount of product that is formed in real life, as it takes into account tons ...

Justin An 2L

So for this question, you want to start off as always by identifying how many moles of white phosphorous (P4) and oxygen (O2) you have for the reactants. From there, determine which is the limiting reacting in the production of P4O6 and then find out how many moles of O2 are going to be consumed by ...

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