CHEMISTRY COMMUNITY

Lorraine Jiang 2C

How would we solve for the frequency factor? I understand what it is but what is the information we need to solve for it? Hi! we can solve this by plugging in the values of activation energy, the rate constant, and temperature into the Arrhenius equation. We can usually find the activation energy, ...

Lorraine Jiang 2C

I believe it is the number of collisions with the correct orientation of molecules and energy needed under certain conditions.

Lorraine Jiang 2C

Hi! Catalysts are the substances that are first consumed (not a reactant in the overall reaction) in a reaction, then produced in another reaction (not a product in the overall reaction). Recall that enzymes are not used up in the reaction.

Hope it helps!

Lorraine Jiang 2C

Hi! Basically, intermediates are the substances that are produced first, then consumed by another reaction. However, catalysts are consumed first, then produced later in the reaction.

Hope it helps!

Lorraine Jiang 2C

Hi!

For the answer to this question, we can substitute 3 into [B] since the concentration tripled. However, since reactant B is in second order, we need to do 3^2 = 9. So the final answer should be the original rate divide by 2 times 9.

Hope it helps!

Lorraine Jiang 2C

The order of the reactants are the powers are the reactants in a rate law equation of a reaction. The order of the reaction is the sum of all the order of the reactants in the reaction. The order of reactants show us how each reactant play a role in the kinetics of the reaction, for example, if the ...

Lorraine Jiang 2C

There are a few ways that you can calculate K depending on the question. One of the ways is to plot the graph and find the gradient of the line to be K (the shape of the graph depends on the order of the reaction). Another way is to calculate K from t1/2 using the formula according to the specific o...

Lorraine Jiang 2C

Hi! Basically, pseudo rate laws are used when we are trying to determine the rate law for a complicated reaction (one that contains a few reactants). We try to make one reactant concentration small and the others very large to calculate the K' and the order of reaction of each of the reactants. Then...

Lorraine Jiang 2C

Yep, the slowest step is considered the rate-determining step. This is because no matter how fast the fast steps are, the rate will still be determined by the slowest step.

Lorraine Jiang 2C

Mari Williams 1K wrote:Is the order of the reaction referring to the total sum of the order of the reactants?

Yep! The overall order of the reaction will be the sum of the orders of the reactants.

Lorraine Jiang 2C

Hi! Just to add to the previous answer, don't forget to put a minus sign in front of your answer is the formula is Enot - RT/nFlnQ and Enot is 0V here.

Hope it helps!

Lorraine Jiang 2C

Yep! STP is at 1atm,and 273.15K.

Lorraine Jiang 2C

Hi! I had the same issue but I fixed it by putting H2O as the last item on one side of the equation.

Hope it helps!

Lorraine Jiang 2C

Hi! I think you need to know the basic oxidation states like O is -2 and K is +1. But for transition metals, we usually need to figure it out when we have the question.

Hope it helps!

Lorraine Jiang 2C

I am also stuck on this problem, the hint says to balance the half reactions but I am confused how to do that with compounds, how do you know what goes into each half reaction? You would first need to identify the things getting oxidized and reduced. From the problem I had, Au was being oxidized to...

Lorraine Jiang 2C

I think he said entropy is always positive for a continuous(spontaneous) reaction?

Hope it helps!

Lorraine Jiang 2C

Hi! I think the graph is showing that as the reaction proceeds, the pressure of reactants decreases, and the pressure of products increases.

Hope it helps!

Lorraine Jiang 2C

Hi! I would say that delS sys = -delS surr is because the system is at equilibrium and delS total = 0.

Hope it helps!

Lorraine Jiang 2C

Hi! Stotal = Ssystem + Ssurroundings. Hope it helps!

Lorraine Jiang 2C

Thank you so much!

Lorraine Jiang 2C

Hi! Does anyone know what formula we should use for this question? 4.19 Calculate the molar kinetic energy (in joules per mole) of Kr(g) at (a) 55.85 8C and (b) 54.85 8C. (c) The difference between the answers to parts (a) and (b) is the energy per mole that it takes to raise the temperature of Kr(g...

Lorraine Jiang 2C

Hi! I think it is fine to assume standard conditions unless the question states the specific conditions.

Lorraine Jiang 2C

Hi! I think it is best to memorize the fraction coefficients as they are not provided on the constant equation sheet. However, there is a chance that in our midterms and finals the coefficients will be provided. But just in case if they aren't, it is best to memorize them.

Lorraine Jiang 2C

Hi! I got the exact same answer as you! Also, Dr. Lavelle has updated the errors in the textbook solution pdf on his website you can go check your answers!

Lorraine Jiang 2C

Hi! The sign for work here is negative because the system is "doing work". Similarly, the system is giving off the heat so the sign for "heat" for the combustion system should also be negative. It is only positive for the "cooling system" because the cooling system abso...

Lorraine Jiang 2C

Hi! I think you need to convert torr to atm first and convert ml to L. Then you will have work in terms of Latm, then you can convert it to J, then KJ.

Hope it helps!

Lorraine Jiang 2C

Hi! Can someone explain when do we use the formula q=nCdeltaT and when do we use the formula q=Cdelta T? For this question, I used the first one by multiplying Cand deltaT by mass of compound A and I got the wrong answer. Can someone explain this to me please? At constant volume, the heat of combust...

Lorraine Jiang 2C

I think we hold either volume or pressure to be constant in experiments or calculations because these two things are important in chemistry and biology. When we want to test the effect of changing pressure, we must hold volume constant as a control variable. When we want to see the effect of changin...

Lorraine Jiang 2C

Entropy will be the same because it is a state function.

Hope it helps!

Lorraine Jiang 2C

Gibbs free energy determines whether a reaction is spontaneous or not. It is dependent on both delta H and delta S.

Hope it helps!

Lorraine Jiang 2C

Hi! I think under constant pressure, qp equals delta H.

Hope it helps!

Lorraine Jiang 2C

Hi! Yea I think so too because resonance structures have more possible states (delocalized pi bonds)

Hope it helps!

Lorraine Jiang 2C

Hi!
Yes, I believe you are correct! When we multiply the equation by 2, the delta H is just multiplied by 2. However, the K value is squared.

Hope it helps!

Lorraine Jiang 2C

Hi! I think you are right; calculating delta H is always more preferable to use Hess's law and not bond enthalpies. This is because bond enthalpies are averaged values of a lot of bond lengths in many molecules, and it is measured when the molecules are in the gas phase. Since not all molecules invo...

Lorraine Jiang 2C

what's the best way to type the values into the calculator? that's where I'm messing up and its the easiest part. Hi! I think the best way is to use the exact values all the way until your answer. (So you only round up your answer). If we round at very early stages, then the answer will be off some...

Lorraine Jiang 2C

Hi! You can flip the reactions they give in order to reach the final reaction. But be careful when you flip the reaction, you also need to change the sign of the delta H of reactions. So if it was positive originally, after you flip it you should change it to negative. A tip for this type of questio...

Lorraine Jiang 2C

Hi! You can use the formula as the others have mentioned. In order to explain the concept of bond enthalpy, we are basically finding the total amount of energy required when bonds are broken minus the total amount of energy released when bonds are formed to find the delta H of the reaction. This met...

Lorraine Jiang 2C

Hi! I agree with what Alison said about part a), you need to use Hf to find the delta H of reaction, which is +131.29KJ/mol (endothermic). The next step requires us to use the gas constant equation, which is PV=nRT. The pressure is 500 torr, the volume is 200L, the gas constant is 62.364 torr L K^-1...

Lorraine Jiang 2C

Okay I got it! Thank you!

Lorraine Jiang 2C

Hi! Did anyone get pH = 1.11 for the textbook problem 6E.3 b)? Calculate the pH of each of the following diprotic acids at 25C, ignoring second deprotonations only when the approximation is justified: (a) 0.010 m H2CO3(aq); (b) 0.10 m (COOH)2(aq); (c) 0.20 m H2S(aq). I used Ka1 value for (COOH)2, wh...

Lorraine Jiang 2C

Hi! My own way to look at it is that heat is needed for the phase change as bonds are being broken (endothermic reactions). Temperature (of the surrounding) remains constant because the energy is being used in bond-breaking. After the phase change has been completed, the temperature started to incre...

Lorraine Jiang 2C

Hi! I think steam causes more severe damage because steam already used 40KJ/mol of energy to overcome the phase change. So when our skin contacts with the steam, it will immediately release energy (which causes the burn) and condense to liquid form. So that is an additional 40KJ of energy being adde...

Lorraine Jiang 2C

Hi! I agree with what the others have said. Breaking bonds will require energy since they are endothermic reactions. Forming bonds will release energy since they are exothermic reactions. When bonds are formed, the potential energy in the form of chemical energy is lowered, thus energy is being rele...

Lorraine Jiang 2C

Yes, that is correct! Exothermic reactions always have a negative delta H, and an endothermic reaction will always have a positive delta H.

Hope it helps!

Lorraine Jiang 2C

Hi! I think vapor and gas are the same things. But for this class, we will use vapor instead of gas because Dr.Lavelle used this term in his lectures.

Hope it helps!

Lorraine Jiang 2C

Hi! So basically the equilibrium constant of a reaction with gases only shifts when there is a change of volume. Adding a gas (inert gas specifically) will not change the volume of the reaction environment as the inert gas will not react with the reactants nor the products. So the equilibrium consta...

Lorraine Jiang 2C

Hi! The reaction quotient basically represents the concentration of production to reactant at any given time in the reaction. However, K specifically implies the ratio at equilibrium.

Hope it helps!

Lorraine Jiang 2C

Hi! So only gases are affected when there is a change in pressure so I think we don’t look at solids and liquids and we only look at gases in the reaction.

Hope it helps!

Lorraine Jiang 2C

Hi! An endothermic reaction means that it requires energy. By increasing the temperature of the reaction environment, we are helping the reactants to overcome the activation energy so the equilibrium will shift to the product side.

Hope it helps!

Lorraine Jiang 2C

Hi! I believe increasing/decreasing pressure in a reaction system only has an effect on gases.

Hope it helps!

Lorraine Jiang 2C

Hi! I think the double arrow just highlights that value A is definitely smaller/larger than value B.

Hope it helps!

Lorraine Jiang 2C

I think for this class we can use 8.314 J·K^-1·mol^-1.

Hope it helps!

Lorraine Jiang 2C

Yep! I think we always use molarity to calculate Kc. For partial pressure, I think the same logic applies to Kp although I didn't see much examples on this topic.

Hope it helps!

Lorraine Jiang 2C

Hi! I think the equilibrium constant is only temperature-dependent. I have linked a website that helped me when I was studying equilibrium constants. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Equ...

Lorraine Jiang 2C

Hi! Basically, both Kc and Kp are values for chemical dynamic equilibrium. However, we use Kp when the reaction is in gas phase, and we use Kc when the reaction is in the aqueous phase.

Lorraine Jiang 2C

Hi! For this question, you only need to consider the pKa of the species and the pH of the condition. For example, if pH is higher than the pKa, then acid will deprotonate and it will become charged.

Hope it helps!

Lorraine Jiang 2C

Hi! I think professor Lavelle mentioned in the lecture that the amphoteric compounds mostly line up with the metalloids in the periodic table. Some examples are H2O and Al2O3.

Hope it helps!

Lorraine Jiang 2C

Hi! The formula for Ka will be [A-][H3O+]/[HA] so that is the concentration of the anion multiplies the concentration of the H+ ion divided by the concentration of the acid.

Hope it helps!

Lorraine Jiang 2C

Hi! I agree with the posts above, so basically you should consider the general formula of the salt first. If the salt contains a cation that is favorable to lose H+ when dissolved, then we know that the salt is acidic. If the cation is from group1 or 2, and the anion is capable of getting another H+...

Lorraine Jiang 2C

Hi! The formula to calculate pH is -log[H+]

Hope it helps!

Lorraine Jiang 2C

Bronsted acids/bases and Lewis acids/bases are two sets of definitions of acids and bases. Bronsted focus on the transfer of protons and Lewis focus on the transfer of electron. For example, Bronsted acid is a proton donor, and lewis acid is an electron acceptor. However, there are cases of acids an...

Lorraine Jiang 2C

Hi! So from the question, we know that the total charge of the coordination compound is neutral, we know that the complex ion has a total charge of 1+ (since Cl is 1-). From that, we know that the ligand Cl has 1- charge, and there is one Cl ligand within the complex ion. Since the NH3 ligand is nat...

Lorraine Jiang 2C

Hi! Amphoteric compounds can act as either acids or bases. And there is a diagonal line of elements in the periodic table that forms amphoteric oxides, for example, BeO, Al2O3, Ga2O3... etc.) It is similar to the metalloid diagonal line but not exactly the same.

Hope it helps!

Lorraine Jiang 2C

NH3 is neutral because all of its atom's formal charges add up to 0.

Hope it helps!

Lorraine Jiang 2C

Hi! So coordination number is basically the number of ligands bonded to the central transition metal in a complex ion.

Hope it helps!

Lorraine Jiang 2C

I think you just need to know that there is a sigma bond and two pi bonds in a triple bond, but I think it is most common to say sigma bond first before pi bonds.

Hope it helps!

Lorraine Jiang 2C

Hi! I think if you consider the region of electron density in this molecule it will be easier for you to determine the hybridization of the orbitals. For example, this molecule has 4 electron density regions so it will be sp^3 (1+3 = 4)

Hope it helps!

Lorraine Jiang 2C

Hi! I think you can think of hybridized orbitals a combination of s and p orbitals, and for 2sp^2 orbitals, it is closer towards the 2p orbital because it has slightly higher energy. And because of this, we will not have a paired electron spin at the 2sp^2 but a single electron at the 2p orbital bec...

Lorraine Jiang 2C

Yep! I think regions of electron density accounts for both bonded regions and lone pairs.

Hope it helps!

Lorraine Jiang 2C

Hi! I would remember it like this: Every single bond consists of a sigma bond, every double bond has a sigma bond and a pi bond, every triple bond has a sigma bond and two pi bonds...etc.

Hope it helps!

Lorraine Jiang 2C

Hi! I think molecular geometry considers all-electron densities in a molecule, but a molecular shape only considers the bonded atoms. For example, if you have three atoms bonded to the central atom and you have a lone-pair, the molecular geometry is tetrahedral, but the molecular shape is Trigonal P...

Lorraine Jiang 2C

I think we can predict the bond angles using the VSEPR theory, so for example how long-pairs repel each other more than lone-pairs repel a bonding pair. So if we have a lone-pair of electrons on the central atom we will know that the other atoms bonded to the central atom will be pushed down and the...

Lorraine Jiang 2C

I think one lone pair of e- is counted as one electron density. So for example, like H2O, on the oxygen atom, there are two lone pairs of e- and we say there are four electron density clouds, 2 bonding and 2 lone-pairs.

Hope it helps!

Lorraine Jiang 2C

Yes, the shape of H2O can either be bent or angular.

Lorraine Jiang 2C

Hi! I think by "electron density" Dr. Lavelle is referring to different regions that have electrons. And he also mentioned that lone-pair vs lone-pair repulsion > long-pair vs. bonding pair repulsion > bonding pair vs. bonding pair repulsion. To answer your last question, the lone-pairs pu...

Lorraine Jiang 2C

I think electronegativity and electron affinity are two different things. Electronegativity measures the tendency of an atom to attract a shared pair of electron. Electron affinity is the amount of energy released when an electron is added to an atom. The tendency to attract a pair of electron is di...

Lorraine Jiang 2C

Radicals have the key feature of having one unpaired electron next to it. It is usually represented like this: .CH3
Hope it helps!

Lorraine Jiang 2C

Only polar molecules can have dipole-dipole intermolecular force. A polar molecule means there is a difference between the electronegativity between the two bonded atoms so that one of them is delta positive and the other is delta negative. Induced dipole-induced-dipole occurs when two non-polar mol...

Lorraine Jiang 2C

I think it is better to use the formal charge equation as well because it is the most straight forward way to solve the formal charge problems.

Lorraine Jiang 2C

Hi! So covalent bonds are known as intramolecular bonds, and dipole-dipole (and other examples professor Lavelle gave in the lecture e.g. hydrogen, dipole-induced dipole... etc.) are intermolecular bonds. The difference between intramolecular bonds and intermolecular bonds is that intramolecular mea...

Lorraine Jiang 2C

OH yes now I understand! Thank you so much Bella, Selena, and Will :))

Lorraine Jiang 2C

Hi! I think professor Lavelle mentioned that p orbitals have 3 parallel sub-orbitals: px, py, and pz, and each orbital can hold at most 2 electrons with opposite spins (ms). According to Hund's rule, we will fill one electron in all px, py, and pz sub-orbitals first before we started filling in the ...

Lorraine Jiang 2C

Hi, can someone explain the concept of the effective nuclear charge? Is it the same as the shielding effect, so when electrons are further away from the nucleus they are more shielded by the inner electrons and they have a higher effective nuclear charge?

Thank you!

Lorraine Jiang 2C

If you are considering Na+ and F-, an easy way to think is to imagine that sodium has lost an outermost electron on its outermost shell, so it loses one shell (3s) and because of this, the electrostatic attraction is stronger and the nucleus will pull the other shells closer to it. so the radius dec...

Lorraine Jiang 2C

The inert-pair effect has to do with the tendency of the outermost two electrons in the s orbitals to remain unshared in post-transitional metals, and it explains why heavier atoms like Sb and Pb have a difference in the oxidation state of 2. But since professor Lavelle mentioned we only need to kno...

Lorraine Jiang 2C

I think oxygen has a lower ionization energy than Nitrogen is because an electron is added to an already half-filled p orbital. According to Hund's rule, half-filled orbitals are more stable, so it is fairly easy to remove the 'extra' electron in the p orbital in oxygen and that is why its first ion...

Lorraine Jiang 2C

Hi! I think the nuclear charge doesn't really have much to do with the orbitals since they are just math functions that predict how the electrons will move. And I think you are right about electrons will get attracted to the nucleus (because of the nuclear charge) and that is the reason behind the s...

Lorraine Jiang 2C

Hi! In general terms, the electrons have to fill up the 3d orbital first before they move on to the 4s orbital because it is at a lower energy state. But there are two exceptions, Potassium and Calcium, which fills the 4s orbital first.

Hope it helps!

Lorraine Jiang 2C

Hi! When ml is given we can basically just look at that and only 2 electrons can occupy an orbital so the answer will be 2. Hope it helps!

Lorraine Jiang 2C

Ok I got it! Thank you so much!

Lorraine Jiang 2C

Hi! The spin state (ms) is a fourth quantum number discovered after Schrodinger's wave function (which consists of 3 quantum numbers: n, l, ml). The spin state can either be (+1/2) or (-1/2). +1/2 means the electron is spinning upwards, and -1/2 means it is spinning downwards. And professor Lavelle ...

Lorraine Jiang 2C

Hello! I know these two questions are not included in homework but I was doing them for extra practice and I couldn't get the correct answer.

For E31, I got the mass of water which is 1.6kg, but I couldn't find the cost of it.
If anyone can help with these two questions I would really appreciate it!

Lorraine Jiang 2C

Hi! Whenever we are given "+ -" the delta x is always 2 times the number after "+-". I believe there is a mistake in the solution and here is the link to all the mistakes that professor Lavelle found. https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14A/Solution_Manu...

Lorraine Jiang 2C

Hi! I believe you will have to use the DeBroglie Equation for this question. To find the wavelength, take the Plank's constant and divide it by the momentum of the oxygen atom. Since the question gave us a hint about oxygen is a diatomic molecule, we need to use 16g/mol x 2 = 32g/mol, then convert i...

Lorraine Jiang 2C

Yep, I accidentally entered the wrong number in my calculator, and I got the same answer as you now! Thank you!

Lorraine Jiang 2C

Hi! The infrared region is n=3 and it is called the Paschen series. I think in our tests we only need to perform calculations related to the Lyman series and the Balmer series.

Hope it helps!

Lorraine Jiang 2C

I think you should use DeBroglie's equation here. So first convert the 5.15 ounces to kg, then use Plank's constant divide the momentum of the baseball (mass times velocity) to get the wavelength. Hope it helps!

Lorraine Jiang 2C

Hi, does anyone know how to do sapling question 17? I know I need to use the DeBroglie equation, but when I am converting mass, I used (35.45 x 2)/6.02 x 10^23 to find the mass of one chlorine atom. And then the question hints says that the mass needs to be in kg so I divided the number by 1000. May...

Lorraine Jiang 2C

Hello! The answer should be 6.7 x 10^-37 m since there is a mistake in the textbook answer. Professor Lavelle has a list of mistakes in the textbook answers on his website and this is the link to the document: https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14A/Solution_Manual_Errors_7...

Lorraine Jiang 2C

Nope, they will not. This is because their sizes are too big for them to have diffraction patterns. On the other hand, electrons, or photons, are small enough so that they will have diffraction patterns. I think professor Lavelle mentioned this before, that whether a diffraction pattern will occur i...

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