CHEMISTRY COMMUNITY

Shrinidhy Srinivas 3L

Hi! When you break the bond, you need energy to do it because when a compound is bonded, it becomes more stable. To break that stability, it will require some energy. On the contrary, when you form a bond, you're causing stability which is what atoms "want" and as such, energy is released....

Shrinidhy Srinivas 3L

Hi! The R values differ based on the units so you need to evaluate what units are given in the problem and go off of that. It's usually the pressure unit that varies, but most of the time, you'll either be given bar or atm (since these are the SI units). I hope this helps!

Shrinidhy Srinivas 3L

Hi! It largely depends what information you are given to work with. If you are given some other reactions, then you should use Hess's Law. However, if you're specifically given heat of formations, then you should use the third method. I hope this helps!

Shrinidhy Srinivas 3L

Hi! So, I think that you just need to consider the most stable resonance structure, since that's where the compound will "spend most of its time." I'm pretty sure that for the exam, Dr. Lavelle will give us quite simple molecules so you shouldn't need to worry too much about this. I hope t...

Shrinidhy Srinivas 3L

So, the first method, which is Hess's method, does not necessarily use the elements in their most stable form. The goal of Hess's method is to use the delta H from known reactions to find the delta H of an unknown reactions. However, when considering standard enthalpy of formation, it's specifically...

Shrinidhy Srinivas 3L

Hi! I think the only thing you need to know about carbon (in its graphite form) is that this is the most stable form of carbon, it is a solid, and its standard heat of formation would be 0 kJ. I hope this helps!

Shrinidhy Srinivas 3L

Hi! I've attached my work below, but basically you use the pH to work backwards for the solution to the problem. I hope this helps!

Shrinidhy Srinivas 3L

Hi! So the set-up is weird for this problem because of the nature of the equation. The equation in this problem is PCl3 + Cl2 --> PCl5. Since PCl5 is a product and you're starting off with pressure of a product, you can only subtract from the product and add to the reactant side. Another way to thin...

Shrinidhy Srinivas 3L

Hi! If you remember from previous lectures, we were told that we could approximate and ignore -x or +x if K was less than 10^-4. In this part of the lecture, Dr. Lavelle was just referring to the fact that this is another way to know if your approximation is okay (but specifically in regards to acid...

Shrinidhy Srinivas 3L

Hi! For this problem, you would set up an ICE box the way you normally do and solve for the equilibrium concentrations. Make sure that before you start the problem that you solve for Q to know which way the reaction will proceed. Also, make sure that when you set up your K and Q equations that you d...

Shrinidhy Srinivas 3L

Hi! Percent ionization just refers to how much of the weak acid actually ionized in solution. The formula for this would be (conj base concentration/initial acid concentration) * 100%. One thing to note is that when the problem has acids, it's usually referred to as percent ionization, but when the ...

Shrinidhy Srinivas 3L

Hi! So, when you have a strong acid or base, you don't need to make an ICE table because the original molarity of the acid or base will match the molarity of the H+ or OH- (depending on the compound). It's only when you're dealing with weak acids or bases that you need to actually make an ICE table....

Shrinidhy Srinivas 3L

As stated above, Q is the reaction quotient at a given period of time (that is not necessarily equilibrium). K is the reaction proportion at equilibrium specifically. When Q < K, the reaction will shift towards the products. When Q > K, the reaction will shift towards the reactants. When Q = K, the ...

Shrinidhy Srinivas 3L

For now, there's not a way to calculate K using only the temperature. We will learn about how to do this in the thermodynamic unit using the van't Hoff equation. For now, you just need to know how an increase or decrease in temperature would affect the shift in equilibrium. I hope this helps!

Shrinidhy Srinivas 3L

Endothermic is referring to heat while endergonic is referring to entropy. However, I think, for now, we only need to know about endothermic reactions and we do not need to know about endergonic. I hope this helps!

Shrinidhy Srinivas 3L

Hi! I find it helpful to know what phases would appear in Qc versus Qp. In Qc, you can include gaseous and aqueous molecules. In Qp, you can only include gaseous molecules. I hope this helps!

Shrinidhy Srinivas 3L

You can only ignore the subtraction or addition of x (the change component) when K < 10^-4. Otherwise, you will have to use the quadratic equation or solve for x because the change is actually significant. I hope this helps!

Shrinidhy Srinivas 3L

Hi! In order to differentiate when you would use partial pressure versus concentration, you would need to look at what information the problem provides. If the problem provides you with concentrations or mols or if it mentions Kc, you should use concentration. If it mentions pressures or uses Kp, th...

Shrinidhy Srinivas 3L

The best way to determine it in my opinion is to play a game of mix and match as the person above said. If you have LiCl, you would make each ion an acid/base. LiOH is a strong base and HCl is a strong acid. As such, the salt would cancel out and the solution would have a pH of 7. I hope this helps!

Shrinidhy Srinivas 3L

Acid rain largely exists because of the effect of fossil fuels and other air pollutants. When we release these air pollutants, they react with water to create acids, which lowers the pH of the rain. Anything that reduces the production of CO2, SO2, or NO2 (which are the main pollutants) would reduce...

Shrinidhy Srinivas 3L

I believe that we don't need to know about all the different points on a titration diagram. The main thing is that you should be able to predict whether the stoichiometric point of a titration would be greater than, less than, or equal to 7 based on the acids and bases that interacted. For reference...

Shrinidhy Srinivas 3L

The main polyprotic acids that we need to know for the test are carbonic acid, sulfuric acid, and phosphoric acid. It's also important to remember that the intermediate states for any of these compounds can result in that intermediate compound acting amphoteric (having acidic and basic properties). ...

Shrinidhy Srinivas 3L

The way I remember this is by thinking about bases as being the opposite of acids. For acids, when pH > pKa, the acid will deprotonate. However, for bases, when pH > pKa, the base usually stays the way it is because that is the way that pH stays greater. The same principle applies for the other way ...

Shrinidhy Srinivas 3L

Hi! Just as a tip, it helps a lot to remember the number 14 when doing anything relating to pH. pH + pOH = 14 and [H+] * [OH-] is equal to 10^-14. I hope this helps a little!

Shrinidhy Srinivas 3L

HClO3 is a strong acid, which means that it will dissociate easily in water. I hope this helps!

Shrinidhy Srinivas 3L

The equilibrium constant essentially represents the ratio of the rates at which the forwards and backwards reaction are happening. It also represents the concentrations of the products and reactants. Hope this helps!

Shrinidhy Srinivas 3L

Hi! Just keep in mind that H+ is more of a shorthand for what is actually happening. H3O+ represents the ion in the reaction that is actually happens, which symbolizes that H2O gets an H+ ion. Hope this helps!

Shrinidhy Srinivas 3L

When remembering the difference between amphoteric and amphiprotic, think of the roots in each word. Amphiprotic contains the root "prot" which is similar to proton. As such, an amphiprotic compound can donate and accept protons. An amphoteric compound can act as a base or an acid.

Shrinidhy Srinivas 3L

The best way to find the answer to this is to look at the periodic table that Dr. Lavelle mentioned today in class. Metal oxides will form strong bases and nonmetal oxides will form strong acids. Anything that is not a metal oxide or a nonmetal oxide is likely an amphoteric compound.

Shrinidhy Srinivas 3L

As stated previously, you have to keep in mind the easiness of the acid to break in water. HCl is a stronger acid than HF, so it breaks easier in water. It could theoretically still form a coordinate covalent bond, but it would not form as easily. Hope this helps!

Shrinidhy Srinivas 3L

Hi! The overall charge for this molecule is -2. As such, all the oxidations states need to add up to -2. The chlorine in this case has an oxidation state of -1. As such, -1 * 6 is -6. Through the addition, we can find that Platinum has an oxidation state of +4.

Shrinidhy Srinivas 3L

I find that drawing the Lewis structure helps me find out if a molecule is a polydentate. If more than one of the atoms on the molecule has a lone pair of electrons, then it is likely a polydentate. I hope this helps!

Shrinidhy Srinivas 3L

You can identify the number of ligands by thinking about the coordination sphere and the coordination number of the compound (which is the amount of bonds to the central atom). Hope this helps!

Shrinidhy Srinivas 3L

A bonding area and a region with a lone pair are considered regions of electron density. Moreover, keep in mind that having a single bond versus having a triple bond doesn't change whether an area is a region of electron density. Hope this helps!

Shrinidhy Srinivas 3L

Hybridization helps provide low energy levels and it also is a more realistic explanation of the probability of actually finding an electron in a certain orbital. Hope this helps!

Shrinidhy Srinivas 3L

When you compare molecules with sp3 hybridized orbitals with sp2 hybridized orbitals, it is apparent that the sp2 orbitals have more s characteristics because as the amount of p characteristic decreases, the amount of s characteristic increases. We can also see that as the s characteristic increases...

Shrinidhy Srinivas 3L

The formula for iodine trichloride is ICl(3). As such, the total amount of electrons would be 7 + 3(7) = 28 electrons. The Lewis structure would involve the three chlorines being bonded to the iodine and the iodine having two lone pairs, making it a T-shape geometry. Hope this helps!

Shrinidhy Srinivas 3L

The shape of this molecule is AX(2)E. This is the Bent geometry. When a molecule is Bent, it has an angle of 120 degrees. As such, the smallest bond angle would be 120 degrees. Hope this helps!

Shrinidhy Srinivas 3L

I think that for trigonal pyramidal, you just need to remember that the bond angle will be slightly less than 109.5 degrees but the exact measurement varies depending on the molecule. I don't think Lavelle expects us to know the exact bond angles. Hope this helps!

Shrinidhy Srinivas 3L

When you think of axial atoms, associate that with the "axes", meaning that those atoms are around a pole that goes through the central atom. The equatorial atoms are ones that "surround" the central atom in a sense. Hope this helps!

Shrinidhy Srinivas 3L

The lone pair on the equatorial plane is closer to the atoms so it will have a greater repulsion from the atoms. Hope this helps!

Shrinidhy Srinivas 3L

To whoever asked, typically when you have two lone pairs, you do try to place them far away from each other as lone pairs repel each other. Hope this helps!

Shrinidhy Srinivas 3L

Part A of the problem features a central atom with two atoms bonded to it at 120 degree angle. Because of the angle, we know that there must be at least one lone pair of electrons here. For part B, there still could be two lone pairs on the central atom which would balance each other out because the...

Shrinidhy Srinivas 3L

Hi! When assigning shape, the main things that you should take into consideration are the bonding pairs and the lone pairs that surround the central atom. Think of the bond angles as a "result" of the shape, not something that causes the shape. Hope this helps!

Shrinidhy Srinivas 3L

Bonding pairs are really stable because two molecules are being connected. However, when you have lone pairs, imagine two electrons floating around and being repulsed from one another. Have two of those pairs and the repulsion becomes even stronger. Hope this helps!

Shrinidhy Srinivas 3L

Yes! The VSEPR model helps show the shape and structure and by doing so, shows you the polarity of the model. One of the examples was the octahedral model that Lavelle talked about in lecture today!

Shrinidhy Srinivas 3L

For this problem, I would recommend drawing out the Lewis structures for each molecule and seeing how everything balances out. If the molecule is polar, it is likely that there will be dipole-dipole interactions. If the molecule is nonpolar, there probably will be no significant dipole-dipole intera...

Shrinidhy Srinivas 3L

The fluctuations in a nonpolar molecule are caused by van der Waal forces, rather than dipole-dipole forces. Hope this helps!

Shrinidhy Srinivas 3L

You're right when you say that hydrogen bonding is involved here. However, I believe that Dr. Lavelle was just referring to the fact that H-F has both dipole-dipole interactions and hydrogen bonding. Both have separate energies. Hope this helps!

Shrinidhy Srinivas 3L

To put it concisely, Coulomb's equation gives you the relationship between electrostatic charge and radius (or distance) between ions. Hope this helps!

Shrinidhy Srinivas 3L

Just to clarify what has been said above, single bonds consist of one sigma bond. Double bonds consist of one sigma bond and one pi bond. And triple bonds consist of one sigma bond and two pi bonds. Hope this helps!

Shrinidhy Srinivas 3L

The way that I remember this is that atomic radius is the only one that decreases across a period and increases down a group. The rest of the trends (electronegativity, electron affinity, etc.) all increase across a period and decrease down a group. Hope this helps!

Shrinidhy Srinivas 3L

This was actually really helpful for me, because I didn't quite understand what a radical was. My follow up question is whether any element can be a radical or if only a certain group are radicals.

Shrinidhy Srinivas 3L

Typically, elements in the d block form ionic bonds, because they are metals. So, they don't usually have expanded octets. The main elements that have expanded octets are phosphorus, sulfur, and chlorine. Hope this helps!

Shrinidhy Srinivas 3L

L refers to the number of lone pair electrons that an atom has. Make sure to count the electrons and not just the number of pairs. For instance, if oxygen had 3 lone pairs, L would be 6. Hope this helps!

Shrinidhy Srinivas 3L

Hi! So, the reason for this is because when you try to remove an electron forcibly from an atom, you're removing it from its valence shell and it's difficult to remove it. When you look at the work function, you're typically referring to metals that have a lattice electron structure which is easier ...

Shrinidhy Srinivas 3L

The nodal plane is the place where there is the zero probability density. To put it another way, the nodal plane is the place where you cannot find any electrons. Knowing where the nodal planes are helps us zone in on the location of electrons. Hope this helps!

Shrinidhy Srinivas 3L

HI! So, the reason that this doesn't happen is because then the s orbital would be empty and the d orbital would be full. Although this sounds good, it doesn't make sense for the s orbital to be empty as this breaks the rule of filling up lower energy shells first. Hope this helps!

Shrinidhy Srinivas 3L

Keep in mind that the valence electrons are the ones on the outermost shell. This helps differentiate between the valence shell configuration and electron configuration. Hope this helps!

Shrinidhy Srinivas 3L

Hi! So, you're correct when you say that the value of l will determine the possible values of ml. However, keep in mind that the value of l determines the orbitals, whereas the value(s) of ml determines the orbitals of the subshell (which is the orientation). You don't actually need to know the shap...

Shrinidhy Srinivas 3L

The 4th quantum number shows us the spin of the electron. Using the 4th quantum number, we can determine the magnetic field of the electron and it shows us which "way" the electron is going, to put it simply.

Shrinidhy Srinivas 3L

You should generally use an inequality symbol because the uncertainty of the position, momentum, or velocity is always equal to or greater than whatever you calculate it to be. Hope this helps!

Shrinidhy Srinivas 3L

It largely depends on what the question is asking. In the lecture, Dr. Lavelle found the uncertainty of the velocity to prove a point and show why that model was incorrect. In some instances, the problem may just ask for uncertainty of position and in other instances, it may ask for uncertainty of v...

Shrinidhy Srinivas 3L

Dr. Lavelle just talked about it in his lecture! You don't need to know the actual equation, but you just need to understand it conceptually. You also need to understand how Schrodinger's equation links to orbitals. Hope this helps!

Shrinidhy Srinivas 3L

It's also important to note that JJ Thompson came up with the plum pudding model, whereas Rutherford came up with the idea that most of the mass in an atom was in the center of the atom and concentrated on what became the nucleus. Hope this helps!

Shrinidhy Srinivas 3L

You got the wavelength right, so what you should do is take the frequency that you are given and calculate the wavelength from the frequency using c = (wavelength)(frequency). From there, you compare the two values for wavelengths and if they are not equal, you can determine that it's not due to the...

Shrinidhy Srinivas 3L

Keep in mind that if something is stationary, then it's really easy to find its momentum and position, even if it is small. As such, the particles need to be moving, so the answer is D. Hope this helps!

Shrinidhy Srinivas 3L

Hi! When a photon hits a metal, it interacts with exactly one electron and if the energy of the photon is high enough, it will eject an electron from the metal. As more photons come in, those photons can also interact with electrons. Essentially, one photon will put all its energy into one electron,...

Shrinidhy Srinivas 3L

I think the most important thing that you need to remember about the work function is that it represents the threshold energy. The amount of energy that the photon comes in with minus the threshold energy gives you the kinetic energy of the ejected electron. Hope this helps!

Shrinidhy Srinivas 3L

Hi! Think of it like this. If you have water that is pouring into a bucket on a scale, that stream of water that is pouring will look constant and the scale will increase steadily. If you slow down the stream of water, the stream of water will still be constant and the scale will still increase stea...

Shrinidhy Srinivas 3L

It's also important to note that each element has its own unique "fingerprint" of lines. For instance, Hydrogen has the Balmer and Lyman series, but only Hydrogen has these. Carbon, Oxygen, and other elements will have their own lines so keep that in mind. Hope this helps!

Shrinidhy Srinivas 3L

I've attached my work, but in essence, once you find the limiting reactant, you can translate the amount of limiting reactant that is used into how much product would be produced from that limiting reactant. Once you find the amount of product, you can translate it back into moles of BrF3 (the exces...

Shrinidhy Srinivas 3L

Heat doesn't factor into the aspect of mass. Mass can only be found when a compound is in one of the four states of matter (solid, liquid, gas or aqueous). Involving heat as a part of mass starts to go into the realm of nuclear reactions, which we're not doing right now. Hope that helps!

Shrinidhy Srinivas 3L

When you balance an equation, you do sometimes end up having coefficients that are all divisible by a common number. If you are able to divide it, you should because it's really only considered proper if all of the coefficients are properly reduced.

Shrinidhy Srinivas 3L

The empirical formula helps identify a starting point and it can help you find the molecular formula from the mass percentage composition. It can also help you see how different molecules may be related to each other.

Shrinidhy Srinivas 3L

This formula is basically used for every problem relating to dilution. If you have three of those components, you can find out the last one. Hope this helps!

Shrinidhy Srinivas 3L

Its also important to note that the molecular formula can sometimes be the same as the empirical formula. However, the molecular formula must always be either the same as the empirical formula or a multiple of the empirical formula.

Shrinidhy Srinivas 3L

Measuring tools usually have the aspect of precision because it represents how close to the actual answer the measurement will be. It shows how close together the results will be, which makes sense according to the definition of precision.

Shrinidhy Srinivas 3L

They're the same thing! Mol is just an abbreviation of mole.

Shrinidhy Srinivas 3L

This is super helpful! Thank you!

Shrinidhy Srinivas 3L

You should assume that the molar mass is 110 grams/mol. The + and -10 doesn't really matter too much!

Shrinidhy Srinivas 3L

When doing a dilution, you usually pour more water in or pour a more concentrated solution in. You don't usually pour more solute in. For instance, if I had a bowl with 10 blueberries and 10 strawberries, I could reduce the ratio of blueberries by putting more strawberries. I didn't change the amoun...

Shrinidhy Srinivas 3L

To solve this problem, you should use the equation M1V1 = M2V2. So, (0.0380 M)(x mL) = (0.00150 M)(250 mL) and from there, you solve for x. The moles of the solute doesn't change, because when you are diluting or concentrating, you change the volume of the solution by pouring more solution or pourin...

Shrinidhy Srinivas 3L

I attached my work for the problem. But, you need to be able to find the total moles of the ion you're looking for and divide it by the total volume to find the answer for each part. For instance, since each molecule has potassium in it, you would convert the grams of each product into moles of pota...

Shrinidhy Srinivas 3L

Make sure that you acknowledge exactly what the "7" is listed as in the problem. If it just says 7, then that's one significant figure, but if it says 7.00 that would be three significant figures. Hope this helps!

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