CHEMISTRY COMMUNITY

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Hi! Can someone explain what standard reaction enthalpies and standard enthalpy of formation mean theoretically? I know how to solve questions that involve them but I struggle with the actual theory of them.

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Correct me if I am wrong, but I believe this happens in isolated systems when the temperature of an internal system cannot change as the system is isolated. I think in open and closed systems, there are ways to change the energies.

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At first, I solved this question using ln, but now I realized that it is not needed as seen through the formula. This thread was helpful!

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I think the final will be consistent to prior midterms, but just in case textbook problems usually have us solve for most things and usually there is only one unknown variable which we have to solve.

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We do not include H2O when H2O is in the solvent. We know this is if it H2O is on both sides of the chemical reaction.

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As long as the correct R-value is used, there isn't a preferred unit of pressure.

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On the formula sheet, I believe it is specified as well in regards to which units you are supposed to use and the corresponding value.

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Delta G needs to be negative in order for it to be spontaneous.

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Usually, there would need to be a catalyst but they do not need to be at the start of a reaction mechanism because catalysis lower the activation energy for the reactions which makes the reaction rate go up.

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Usually, there would need to be a catalyst but they do not need to be at the start of a reaction mechanism because catalysis lower the activation energy for the reactions which makes the reaction rate go up.

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A reaction might be in zero-order when the rate is independent of the [r](concentration of reactants). The rate law would be k[A]^0.

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k' is the rate constant for a reverse reaction whereas k1 and k2 represent multi-step reactions.

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The place where the oxidation half-reaction takes place is where the anode is whereas the place where the reduction half-reaction takes place is where the cathode is.

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Q gives you the ratio of products to reactants at any time during a chemical reaction which can be compared to K and can give us information whether Q is larger or smaller than K (like which direction the system is moving in on its way to equilibrium.

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When the standard cell potential of an oxidation or reduction is negative, it basically means that the reverse reaction is spontaneous

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I think something that affects the ozone layer is greenhouse gases which consist of several chemical compounds. This depletes the ozone layer and reduces protection from the sun.

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I think the most important thing to note is that temperature is the only variable that can change the equilibrium of a chemical reaction, but other factors like volume change, and adding moles of reactants or products does not affect the ratio of K. This principle talks about the adaptability of che...

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Just to add, Dr. Lavelle said that heat capacity is usually not used unless it is a calorimeter example.

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In an open system, there can be an energy change by adding any materials, changing the temperature, and adding more material. In a closed system, there can be a change of energy through temperature change.

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Ka and Kb is used with weak acids and bases as they do not dissociate completely and do not have high values.

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I don't really have a quote, but I do know that grades are not everything and what we get on the final does not determine our future :)

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Based on the information Dr. Lavelle has given us, it represents the concentration of Hydronium ions and there is a scale that represents acids, neutral, and bases.

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Just to add, Qp represents enthalpy ( at a constant pressure).

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There are a lot of rules under the assumptions of ideal gases and it is when gases are do not take up any space/volume and do not attract/repel each other.

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Under standard conditions, it is 25 degrees!

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It is important to note that Hess's law can only be done because enthalpy is a state function so the enthalpies can be added or subtracted.

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Reversible is related to equilibrium and the processes happening infinitesimally whereas irreversible reactions happen suddenly and cannot be reversed.

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Yes this is right, an enzyme catalyzes a reaction by lowering the activation energy.

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What is the difference between integral and differential equations for first-order reactions?

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The A values relate to the number of collisions in the proper orientation and the larger molecules have smaller A values and vice versa.

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The math is similar but for Kp, you need to think about the gas and their partial pressures but for Kc, you need to think about concentrations.

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N= 6.02214 x 10^23 mol-1 but this does not need to be memorized because it is on the constant sheet!

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It decreases the rate constant because of the initial concentrations for the reactants. With a high activation energy, there would be a lower rate constant because there is high activation energy and vice versa.

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When the volume or pressures change (physical parameters), there is no change in K. There is a reaction but the ratio of K does to change. When volume decreases and there are more moles of gas on the left side, the reaction proceeds towards the reactants but when V decreases and more moles of gas is...

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log is base 10 and denoted for base e. Ln is a natural logarithm and is log subscript e.

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It is false because it is a reversible reaction and the rate increases but then goes back to equilibrium. In the long term, it is at equilibrium still. We let it have some time before we measure the concentrations. However, we will see more collisions when the rate of reactants increases.

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The slope is k and on a graph, t is on the x axis and 1/[](concentration) is on the y axis. K is also positive.

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An extensive property depends on the amount of a substance as seen through molar heat capacities whereas a state property in the mass and volumes. A state property is final - initial and the pathway is not looked at. Common examples are enthalpy, temperature, pressure, etc.

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The homework on Achieve is due sunday night of week 10. If achieve is not up by then, I'm sure we would get an extension.

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Delta H represents the loss and gain of heat; negative sign means it is loosing heat and vise versa.

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The main difference is that an open system has constant pressure whereas a closed system does not. This relates to crucial differences when work is being done and when there are calorimeters being set up.

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A pre-exponential factor and frequency factor both relate to A. They both describe different details of A as there is both collisions and it comes before exponents. Hope this helps!

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In simple terms, it has a lower activation energy because there is not that much energy needed to go back to reactants. If it was going to products, then it would be higher. The products have higher energies than the reactants so it will take less energy to go back to reactants so the activation ene...

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It would be positive since work is being done on the system. It is in relation to energy and when work is being on the system, there is energy being lost from the surroundings but would increase the energy on the system; therefore it would be positive.

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Spectator ions don't take place in the chemical reaction and a common example of this is is a conjugate base or an acid as the conjugate would not be reactive enough.

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For this question, it is definitely important to think about Le Chatlier's principle. If a reaction requires heat (endothermic) while forming a product, then heating will favor product formation because additional heat is used. Whereas if a reaction gives off heat (exothermic) while forming products...

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A lower case k is in relation to kinetics whereas an upper case K is in relation to chemical equilibrium! k is dependent on activation energy, orientation, and temperature whereas K is the ratio of concentrations of products to reactants.

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Inert gas is added without the need to react with any of the reactants. If there is something added for a reaction to take place, a buffer would be added. But just for inert gas, it is already stable enough and does not affect the equilibrium in any way.

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For me, I feel like it helps just to remember the COOH group and a nitrogen lone pair. Whenever I get stuck, I usually draw a lewis structure the way learned from 14A.

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Conceptually it makes sense because it would go down as the reaction goes on but also, the concentration of the the biological system goes down at the same time which makes it negative.

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The coefficient just marks that the hybridized orbitals are in between the 2s and 2p unhybridized orbitals.

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Also just to add on, in a lewis structure we started out by figuring out the number of electrons and then equally placing the atoms around the central atom ( lowest ionization energy ). But for Molecular geometry, it includes the VESPR model and bond theory.

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Lower ionization energy means not as much energy compared to other atoms is required to remove an electron. Therefore, the central atom can form more bonds.

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I believe it will be on the exam. Knowing the number of regions of electron density along with the number of bonded atoms is crucial to figure out the bond angle.

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The s orbital can only hold two electrons. If there are more electrons it will fill the p orbital and then the d orbital.

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Also just to add on, from most of the examples I did, a lewis structure is more stable when its central atom has a lower formal charge. We can achieve this by adding double bonds and just toggling around with our original lewis structure.

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Lone pairs contributes too electron density and therefore the arrangement of the lewis structure.

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Usually, the weaker bases are composed of Nitrogen atoms with lone pairs. On the other hand, strong bases are usually formed by group one and group two elements. Also, I believe from achieve, you can pull up a list of strong acids and strong bases and everything not listed on that list is either a w...

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It represents the equilibrium constants for acids and bases.

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Quantum mechanics speaks a lot towards properties of light and is proven by multiple experiments so I believe it is best to know it. An easy way to remember what quantum is by understanding that light acts as various packets and is not continuous. He gave an example in class where there was when you...

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Fluorine only needs one more bond or one more electron to be stable. Two more electrons or a double bond will make it unstable.

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I also think it will be content heavy on hybridization, coordinate covalent bonds, and acids and bases.

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In theory, it is also one atom favoring electrons causing it to have a partial negative charge. Since one atom is favoring electrons, there is a slight positive charge on the other side of the molecule.

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To interpret this, it is easier to think of 1 mole of things is equal to Avogadro's constant. I always use the 1 dozen = 12 things while analyzing what Avogadro's constant means.

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Also, lone pairs contribute to the bond being weaker, just something I thought I'd say in terms of recognizing stronger and weaker bonds.

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Electron affinity is the amount of energy released from an atom and ionization energy is the amount of energy needed to remove an electron. You can also think about electronegativity (pulling power of electrons) which is needed for measuring polarity.

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Another thing to note is that energy is proportional to its frequency. This is helpful in atomic spectra.

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Pi bonds are weaker and the first to usually break in a reaction. Therefore the sigma bond is stronger.

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Pi bonds are weaker and the first to usually break in a reaction. Therefore the sigma bond is stronger.

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Also for naming these, I believe we just refer to electron density spaces.

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Also keep in mind that only atoms bonded by sigma bonds can be a ligand. Also there has to be space (planars) between two atoms so it is in the same axis.

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This stems from Molarity is equal to moles (n) over Volume (L). The amount of moles in a solution is always the same no matter the amount of solution. That is where you get your equation from.

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Also, make sure you look at the vectors in the direction. If they cancel out then a molecule is nonpolar.

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Also, these are usually just the metalloids.

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The 100g is just needed when transitioning the percentages to grams of each atom in the molecule.

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Figuring out the charge of the TM atom is crucial to figure this out. Usually, when the molecular formula is long, it's hard to figure out. Does anyone have any tips on this?

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Another thing to note is that we always end with the same number of orbitals from where we started. Also in hybridization, the energy difference between two shells is smaller than when it is not hybridized.

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In a reaction, pi bonds would be the first to break. Also when you have a larger lewis structure and two central atoms are connected with a sigma and pi bond, and each side has the same format, each side (either side of the central atom) would be trigonal planar or anything else depending on the str...

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Another thing to add is that two lone pairs on the same electron cannot act as a polydentate ( I got confused on this at first so thought I'd mention it). Also, we can only count sigma bonds, not pi bonds.

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I heard that it starts off as a continuation of 14A. That being said, there won't be as much review so it moves a bit faster ( this quarter, we reviewed a lot of high school chem but I don't think 14B will be like that). This is just what I heard. Good luck!

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I usually, just mark the sig.figs. after each step of the question as well and then I round it at the end.

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Also, the theoretical yield is the best-case scenario. There are usually other factors that contribute to the actual yield being lower.

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I think for some it can be quite arbitrary. For example, a trigonal pyramid is just less than 109.5 because the lone pairs push the rest of the bonding pairs a bit lower because of electron-electron repulsion.

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Whenever the bonding is the same, we also look at electronegativity. When the electronegativity of one element is greater, it acts more like an acid.

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They are the same thing and also just to add on the bond angles would be less than 120 degrees.

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I think in class I only remember him talking about the metalloids being able to act as either a base or an acid depending on whether it bonds with an acid or a base.

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Hi! Yes, we are during their office hours. Our TA told us to just email a time that works for us and he'd accommodate us into his schedule.

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I think it would be tough to take three stem classes all at once and I would space them out just to avoid too much stress :) But that's just what I would do.

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It depends on the formal charges!

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I think we are only allowed a periodic table and the equations sheet.

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I like the plan! I definitely want to try and finish the textbook problems more in advance so I have a good amount of time to get clarifications on anything.

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Also, Before you multiply, remember to divide the molar mass by the atomic mass from the empirical formula -- sometimes I forget to do this last step!

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Hi! I am still trying to figure out a study plan too but what has been working for me so far is watching the module videos from the CHEM 14a website before the lecture. After the lecture or the day after, I try to go through the post-assessment questions and then move on to the textbook questions/ac...

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Also, a quick thing that I read that I previously did not know is that any standard factor or conversion does not contribute to the lowest s.f. when solving for something.

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Also, in the wave model, having a higher intensity increases the amount of photons present (Intensity is proportional to the number of photons). I also saw somewhere that the amplitude^2 is equivalent to the intensity.

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I think it is going to be 15 MCQ's and some will be from the textbook.

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I would memorize it. My TA said that just knowing the units of something can help you figure out other parts of the problems when you are stuck. I realized this is a helpful tip when I was solving my own homework problems as well.

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His past recorded lectures help me a lot. Watching them before our lectures helps a lot because I know what to look for.

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Just to clarify, this represents the kinetic energy, right?

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It has a direct relationship to light as well and as the electron returns back to its ground state, it releases light.

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