CHEMISTRY COMMUNITY

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Last post for me!

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Thank you for all that you've done! You've truly made chemistry more enjoyable!

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As others have mentioned, the pass cutoff is the same and grades will be assigned based on performance. The average midterm grade was quite similar to last quarter's, so I'd assume it will be pretty similar despite the very different final we will take

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Will the answer format be multiple choice or fill in the blank?

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Subashni Rajiv 1K wrote:Lyndon just posted practice problems for Electrochemistry and Kinetics that can be found if you search up ENDGAME.

Thank you!

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Okay, thank you so much!

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For a conceptual explanation for zero order reactions, this is what I found online: "There are two general conditions that can give rise to zero-order rates: 1. Only a small fraction of the reactant molecules are in a location or state in which they are able to react, and this fraction is conti...

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The coefficient does not automatically translate to the exponent in the rate law. As for how to determine the rate law, it is always based on experimental data

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I believe we will not always be given the order of the reaction, but you can tell by looking at graphs of ln[A] vs time, 1/[A] vs time, and [A] vs time. The graph that is a linear line indicates the order of that reaction

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Would we ever be asked to propose a reaction mechanism?

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Here is a link to the constants and equations page that we will be given on exams. Hope this helps!

https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14B/Constants_Equations.pdf

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I think that using both reactants and products factors in the rate of the reverse reaction occurring at the same time as the forward reaction. For this class, I believe we are generally focused on just the reactants because that defines the initial reaction rate

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They can be in any unit as long as you make sure you use the corresponding R value

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I read over my notes after each lecture, do all the homework problems, and attend review sessions. Having access to past exam questions is also incredibly useful!

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Intermediates are easily recognizable in a reaction mechanism, which is the sequence of elementary reactions to go from reactants to products. The molecules that are formed and consumed in these elementary reactions are intermediates

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Also, they have different standard potentials, so be careful when doing calculations!

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The affect of concentration on cell potential can be seen through the Nerst Equation, which includes Q. The reaction quotient is determined by the concentrations of the products and reactants, so concentrations will affect the cell potential

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Especially considering that the class average for this midterm was a bit higher than last quarter's, I think it is currently highly unlikely for a significant curve

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It is the table that lists reductions and its potentials. To find the potential for an oxidation, you flip the sign of the potential on the electrochemical series

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Also, how is the n value determined here?

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To be honest, some of the questions on the midterm were not what I expected in terms of the types of questions and way you were to answer them. I did all the homework questions, went to review sessions, and paid attention in class. Did anyone else also feel this way? Does anyone have advice on how t...

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If I remember currently from the beginning of last quarter, Dr. Lavelle mentioned that there is the possibility of a curve at the end of the quarter if the grade distribution deems it necessary. Also, if there were to be a curve, it would only help your grade, not hurt it.

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Maya Beal Dis 1D wrote:When are the posts due every week? Is it Friday or Sunday?

Posts are due every Sunday

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In particular, redox reactions hold a great importance in biological systems. Cellular respiration, for example, greatly relies on redox reactions in order to efficiently and usefully harvest energy from glucose.

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This means that an error in W will have a small effect on S, helping S be a more accurate measurement of entropy

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What is the units for change in entropy? When you take the ln of V2/V1 or T2/T2, does the unit of the volume or temperature remain?

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Entropy depends on the number of molecules and the number of states the molecules can be in. Therefore, the greater the number of molecules, the greater the amount of entropy. Also, the more complex the molecule, the greater the amount of entropy

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Could someone please summarize which equations (relating to delta U, q, w, and delta S) should be used for which type of reaction? I've been a bit confused and some clarification would really help!

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How do you calculate delta U in an open system?

I'm especially confused on how to calculate work if the reactants and products are gases and the system is open. How do you know if there is a change in volume? Do you compare the sum of the coefficients of the products and of the reactants?

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I was wondering the same thing! Should we never use the specific heat unless the problem uses a calorimeter?

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Yes, the equation of work as an integral applies only to reversible processes in which the system is at equilibrium

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How will we be able to differentiate between a reversible and irreversible process on an exam? Will it be given to us or do we have to make the distinction?

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For the purposes of this class, I'm pretty sure we are supposed to assume that insulated calorimeters are an isolated system

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Dr. Lavelle announced on his website that the midterm will cover equilibrium, acid & bases, thermochemistry, and thermodynamics

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On the midterm last quarter, I lost all points on a calculation question following a step in which I forgot to divide by Avogadro's number. I talked to my TA about the partial credit policy, and he said that partial credit can only be given if it is specifically allotted on the rubric. So, please be...

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From my experience, the method you use will be determined by the information given to you (bond enthalpies, delta H for steps of the reaction, or standard enthalpies of formation)

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Endergonic and endothermic are not the same thing! Endergonic means that the reaction requires energy and is not spontaneous. Endothermic means that the reaction requires heat. Also, keep in mind that an endothermic reaction is not always endergonic and vice versa. Delta H, the temperature, and delt...

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Here is a graph that I think helps illustrate the point Dr. Lavelle made during lecture https://www.google.com/url?sa=i&source=images&cd=&ved=2ahUKEwigq6Gdzp_nAhXjOn0KHWllDhMQjRx6BAgBEAQ&url=http%3A%2F%2Fhyperphysics.phy-astr.gsu.edu%2Fhbase%2Fthermo%2Fphase.html&psig=AOvVaw1vI32...

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I checked the learning outcomes for Acids & Bases and all we need to know about buffers is how to calculate their pH

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Does anyone know if we also receive a key to compare our answers to?

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When multiplying the stoichiometric coefficients of a reaction by x, the K value is raised to the power of x. This is why K=(41)^(1/2)

Also, keep in mind that the K value of the reverse reaction is K^(-1)

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Do you have to change the volume in order to change the pressure? Or would adding a non-inert gas affect pressure as well? There are many ways to change the pressure. If you change the volume, the partial pressures can change. If you add a non-inert gas, then the partial pressures can change depend...

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Okay, thank you!

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Do the concentrations used in the calculation of Q have to be in moles per liter? For example, homework problem 5I.11 give the gases with the unit mmol. Should I convert this to mol?

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When Q<K, [R]>[P] because [R] is the denominator in determining Q and a larger denominator will make Q smaller than K. The same logic works the other way around. However, it is important to keep in mind that when referring to [R] and [P], the value is the concentrations raised to the power of their ...

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It would be the inverse of the concentration because the value for the products would be 1

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It is a value you can compare to K to determine which direction the reaction is favored in to reach equilibrium

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I believe you can convert Kc to Kp and vice versa. Here is the link to a video working through this concept: https://www.brightstorm.com/science/chemistry/chemical-equilibrium/tips-for-converting-to-kp-from-kc/

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When determining K, only reactants and products that are aqueous or a gas are considered.

Also, a reaction's considered values will either all be pressures or all be concentrations.

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Can NO2 also react with water to form HNO2 and HNO3?

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Does anyone know which transition metals need to be named using their latin name in coordination compound naming?

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I believe that EDTA holds industrial applications when it bonds with metal ions and forms a coordination complex

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The porphyrin ligand in polydentate (tetradentate, to be specific), making myoglobin a chelating complex. As to the shape of myoglobin, I believe we do not need to know its shape. However, the "marshmallow" practice problems included a question on the shape of the heme complex. In the revi...

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There were multiple examples that Dr. Lavelle touched on during lecture, but I would say that the hemoglobin example is particularly as he spend a good chunk of time on it and since it serves such a vital role.

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What do the asterisks next to certain topics on Chemistry Community mean? Are the topics that will be covered on examinations?

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In a reaction with a polyprotic acid, does it lose one of it protons and then the others in separate reactions or does it donate multiple protons at once?

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When selecting the acid and base, my thought process would be to add H to the anion and OH to the cation. The compound with the H would be your acid and the compound with OH would be your base. In a neutralization reaction, the H and OH would form H2O and the remaining anion and cation would form th...

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In addition to the diagonal band of amphoteric compounds, don't forget that H2O is amphoteric too!

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If a ligand is polydentate, then it is a chelating ligand. To determine if a ligand is polydentate, look at the number of electron pairs it can donate. If it is more than 1 electron pair, then it is polydentate

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Does anyone know when review sessions for the final will begin?

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Are HCl, HBr, HNO3, and H2SO4 the only strong acids we are required to know? What about HI and HClO4?

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When are we supposed to use dashes in the name of coordination compounds?

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From my understanding, Lewis acids and Bronsted acids are the same. However, the concept used to identify them are different. Based on the Lewis definition, an acid receives and electron pair. Based on the Bronsted definition, an acid donates a proton

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Knowing either column of naming will result in the correct name for this class, but Dr. Lavelle did mention that the column titled "Name" will be the conventionally used naming system

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sarahsalama1G wrote:did we cover sigma and pi bonds? Are we going to cover them this week?

We have not covered sigma and pi bonds yet, but Dr. Lavelle will be covering them during the beginning portion of Monday's lecture. FYI, they will be on the Test 2 occurring this week

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Does anyone know what type of questions we will be asked regarding sigma and pi bonds? For example, will we just have to identify how many of each type is present in a bond? Will we have to be able to draw it?

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I've also looked at finals from the previous years to have an idea of how the final will be different from the midterm. I think that'd be helpful for you too!

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Essentially, 465/500 will guarantee you a solid A in this class, but having a lower score does not eliminate the potential of getting an A at the end of the quarter.

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Now that we have learned the VSEPR model for determining molecular shapes, does this mean a correct lewis structure (for the purposes of assessments) needs to be drawn in a manner similar to its molecular geometry? Examples: -Should the lewis structure of BF3 be drawn with the F's equidistant from e...

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In addition, the intermolecular forces produced by dipole moments would be ion-dipole, dipole-dipole, or dipole-induced dipole interactions.

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As long as a molecule has electrons, it will have Van Der Waals interactions. Therefore, all molecules do

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During lecture, Dr. Lavelle mentioned that they are the same for the purposes of this class

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The more ionic the nature of the salt is, the more soluble the salt will be in water

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I believe it is necessary to know the bond angles of the most standard shapes. However, for molecular shapes in which lone pairs are present, it is only necessary to know the bond angle in relation to the bond angle in the standard shape

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In the case of ClO4-, the lowest energy Lewis structure must have one atom with a formal charge of -1. Because O is more electronegative than Cl, it is more stable when the O has the negative formal charge. Therefore, 3 Cl-O double bonds and 1 Cl-O single bond is the best lewis structure.

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The other exception to the octet rule is atoms that do not need 8 electrons to be stable. H and Li only want 2 electrons, and B can be stable with 6 electrons.

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You may need to recheck your work in 8b where you find the frequency of the light. The value I calculated was 6.78x10^14 Hz.

However, to directly answer your question, the value you calculated does not fall within the range of visible light.

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I have checked. The lectures are not bruincasted

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The equation applies to electromagnetic radiation, which a photon behaves like

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To clarify, do I need to bracket the Lewis Structure of an ion and note the ion's charge in the top right corner outside of the bracket?

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Individual atoms within resonance structures will have different formal charges than other versions. This is because it can have a double bond in one structure and a single bond in another structure which would change the formal charge value of each. Does this mean that all resonance structures are...

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Why is the atom with the lowest ionization energy the central atom?

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I'm a bit confused on the relationship between resonance structures and formal charge. Will the formal charges of different resonance structures always be the same?

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During lecture, Dr. Lavelle explained that P, S, and Cl can accommodate more than 8 valence electrons. Does this mean they can have 10 valence electrons and only 10? In other words, do they only use one d-orbital when bonding with other atoms?

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If you look closely at Tungsten's location on the periodic table, you'll notice that it follows a f-block of elements (Lanthanum-Lutetium) that is footnoted below the rest of the periodic table. So, tungsten's electron configuration will include the 4f orbital.

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I've found that sometimes I want to post in image as part of a question or response. How do I do this?

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The electron-electron repulsion within the 3d orbital cause it to have a higher energy than the 4s orbital

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When an atom becomes an ion, it either loses its valence electrons or gains electrons to fill its outermost shell. In relation to the electron configuration, this means that the ion either has the same electron configuration as the atom but without the outermost subshell (the last segment of the ele...

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The pull of the nucleus decreases at higher energy levels because of the distance and because of the shielding provided by the electrons in lower energy levels.

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Maybe the second question is asking if the calculated energies can be experimentally detected?

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When an excited election falls back to its original state, is the fall a gradual one (e.g. electron from n=3 to n=2 then to n=1) or does it happen in one step? If its a gradual one, is a photon released for each fall or is only one photon released for the entire process?

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I completely agree with what the previous person said. However, I want to add that wavelength and frequency are inversely related because c= λ ν. Since c is a constant, wavelength increases when frequency decreases and vice versa.

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Professor Lavelle mentioned that UV light is when a high energy e- drops to the n=1 state, visible light is for the n=2 state, and infrared is for the n=3 state. How does this apply to gamma rays, x-rays, microwaves, and radio waves?

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I'm interested too! What do I have to do to join?

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Does anybody know if we are required to know the common polyatomic ions (their names, formula, and charges) for the quiz on Week 2?

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What I do is I use one more digit that the number of significant digits in the given values during all my calculations. At the very, I round my answer to the appropriate number of significant digits.

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My TA said that we currently aren't required to include the states of matter in the chemical equations, but we will have to later in the course

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If the units of a given value don't match with the units of a conversion factor (e.g. molar mass) or are different from the units of the answer you are looking for, definitely convert your units

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