CHEMISTRY COMMUNITY

Courtney Quan 1C

For a specific reaction at a specific temperature, there is only one rate constant k. However, if this same reaction took place at a different temperature, k would change. Note that there are different rate constant k's for different reactions.

Courtney Quan 1C

A as the pre-exponential factor/measure of the rate at which molecules collide is one of the Arrhenius parameters (the other being activation energy), which means it can only be found through experiments. I'd assume that we would be given the value of A if needed to solve a problem involving the Arr...

Courtney Quan 1C

As said above, molecularity is the number of reactant atoms/molecules/ions taking part in a specified elementary reaction. So yes, you would only look at the reactants to determine molecularity.

Courtney Quan 1C

An average rate is taken over an interval of time. An instantaneous rate is taken at one specific instance/moment in time.

Courtney Quan 1C

K is a constant and does not change because k is characteristic for a specific reaction at a specific temperature. It follows that a different reaction, or even that same reaction at a different temperature, would result in a different value for k.

Courtney Quan 1C

Note that a higher order also means that the reaction is less likely to occur/favorable. A second-order reaction depends on the interaction of two molecules at the same instant to form product, whereas a third-order reaction relies on the interaction among three molecules at the same time and would ...

Courtney Quan 1C

In both acidic and basic solutions for redox reactions, you start with balancing the core element, and then move on to balancing the number of oxygens by adding H2O. Balancing the hydrogens after this is where the two differ: in acidic solutions, you simply add H+ to the side that needs more hydroge...

Courtney Quan 1C

Specifically looking at expansion work, the system does work when expanding (-w, b/c it costs energy to do work and this lowers the internal energy of a system ΔU), but in the case of compression, the system is having work done on by its surroundings (+w).

Courtney Quan 1C

For constant pressure, ΔU = q + w, where q=heat and w=work. But we also know that q=ΔH, the change in enthalpy (for constant pressure and expansion work). (On another note, w can also be replaced with PΔV). So, since ΔU= q(aka ΔH) + w, when there is no work being done, ΔU=ΔH. Conceptually, for there...

Courtney Quan 1C

My TA mentioned that Test 2 covers all topics on Gibbs free energy and electrochemistry up to but not including the Nernst equation, so any material from weeks 6 and 7 is fair game. I'm not sure if that corresponds exactly to those sections in the textbook, but just make sure they're covered!

Courtney Quan 1C

Note that the syllabus says, "Each test and exam has a total score but is not assigned a grade. Only at the end of the class when the class average score (out of 500 points) is known are final grades assigned. This class does not use a curve." Your final grade will be determined at the end...

Courtney Quan 1C

On that note, Gibbs free energy is important because it is used for looking at changes in a system with constant pressure and temperature. Most biomedical reactions occur at constant temperature and pressure, and \Delta G relates back to the spontaneity of a reaction. This is more convenient than lo...

Courtney Quan 1C

You would only use it for substances in the gas phase since PV=nRT is the ideal gas law.

Courtney Quan 1C

Cp,m is the molar heat capacity (heat capacity divided by number of moles) of an ideal gas at constant pressure. Cv,m is the molar heat capacity of an ideal gas at constant volume.

Courtney Quan 1C

An entropy of zero indicates there is no disorder. An example of this would be in a perfectly ordered material. If a system can only be prepared one way (W=1), using S=k b ln(W), we have S=0, which indicates that the system could be a perfect crystal. This leads us to the third law of thermodynamics...

Courtney Quan 1C

This equation only applies for a constant temperature. We would expect disorder to increase when a system is heated, since the supply of energy increases the thermal motion of the molecules. To calculate the change in entropy when the temperature of a system is changed, we use delta S = C*ln(T 2 /T ...

Courtney Quan 1C

A negative q signifies that heat is being lost/released from the system into the surroundings.

Courtney Quan 1C

When an ideal gas isothermally expands or is compressed, it does so at the same (constant) temperature. Since there is no change in temperature, the gas molecules will continue to move at the same speed and there is no energy change, which is why the internal energy change delta U is equal to zero.

Courtney Quan 1C

The system + the surroundings make up the universe. If the system is a reaction, then everything else is the surroundings. Simply put, if something is not the system, it is a part of the surroundings.

Courtney Quan 1C

For the 7th edition, this is still part of Chapter 4 since we are still covering outline 2. Professor Lavelle mentioned that we started with the latter half of outline 2 first, and now we're going back to the first half, which would be chronologically first in the textbook.

Courtney Quan 1C

Enthalpy is defined as the amount of heat released or absorbed at a constant pressure. You are correct in speculating that it is related to exothermic (releasing energy) and endothermic (requires heat) reactions, and that relates to phase changes. In a phase change, energy is required to break the c...

Courtney Quan 1C

The homework we turn in each week should reflect the material that we are currently covering in class. So yes, next week's homework should be questions from Outline 3.

Courtney Quan 1C

You can ignore the change in acid concentration when K_a < 10^-3, or when the change is 10^-4 and lower. We assume that the change in acid concentration is negligible because it is a weak acid and will not completely ionize as a strong acid would.

Courtney Quan 1C

An amphiprotic substance, such as water, can both donate and receive protons while an amphoteric substance can act as either an acid or a base and can thus react with both acids and bases.

Courtney Quan 1C

K itself is the equilibrium constant, but K_c specifies usage of concentrations in mols/L rather than K_p, which specifies usage of partial pressures (applicable to gases only).

Courtney Quan 1C

As mentioned above, K as the equilibrium constant is calculated when the reaction is in equilibrium. Q, the reaction quotient, is found when the reaction is at any point other than equilibrium- in other words, when the reaction is not at equilibrium. While K and Q are calculated the same way ([P]/[R...

Courtney Quan 1C

Molar ratios will affect the way in which you proceed with your ICE table since, for example, not all reactions use up all of the reactants to form a product. If, for instance, a balanced chemical reaction produces 1 mole of product for every 2 moles of reactant, the concentration of the reactant wi...

Courtney Quan 1C

As mentioned above, whether you need to find Q_c or Q_p will usually be indicated by the units of the given quantities. Moles/liter specifies Q_c whereas bars, atm, ex. indicates Q_p. I've found that some of the questions will specifically ask for you to find Q_p (rather than Q_c) if needed.

Courtney Quan 1C

As far as I know, you can use the same account as the one you used from 14A! I just made sure to update my profile so that it would reflect the right lecture number and discussion section for this quarter.

Courtney Quan 1C

As mentioned above, when K=1, we can derive that the amounts of concentration of the products and reactants are equal at equilibrium. Such an occurrence indicates that there is equal stability in both the reactants and products and the reaction favors neither reactants nor products at equilibrium.

Courtney Quan 1C

Polarizability is the ease with which an electron cloud can be distorted. Polarizing power, on the other hand, is the ability of a cation to distort an anion's electrons. Highly distorted electrons are described as being highly polarizable.

Courtney Quan 1C

By definition, Lewis bases only donate lone pairs of electrons (which are accepted by Lewis acids). A coordinate covalent bond is formed when two electrons are contributed by a single atom, so to the extent of my knowledge, you are correct.

Courtney Quan 1C

Look at the intermolecular forces present: NH 3 has the capacity to form hydrogen bonds because of its H-N bond. Hydrogen bonds, which are the strongest intermolecular forces, also have dipole-dipole and London forces (in descending order of strength of the intermolecular forces). CH 4 , on the othe...

Courtney Quan 1C

No, it is not necessary to label the atoms numerically in resonance structures. It was just used in the manual to emphasis the fact that resonance spreads multi-bond character across the overall compound.

Courtney Quan 1C

An electronegativity difference > 2 between two atoms indicates an ionic bond, while an electronegativity difference < 1.5 indicates a covalent bond. Generally, a greater electronegativity difference will result in greater ionic character.

Courtney Quan 1C

By definition, ligands are and will always be Lewis bases that supply lone pairs of electrons (in other words, contribute all the electrons needed for a bond) in order to form coordinate covalent bonds with a central metal atom/ion that will be the Lewis acid. The central metal atom/ion bonded to li...

Courtney Quan 1C

Hybridization is the concept that atomic orbitals can be mixed to form new hybridized orbitals that helps us explain experimentally determined molecular structures. Another way to think of it is that hybridization acts as an explanation as to why structures undertake the shape that they do, for inst...

Courtney Quan 1C

Lone pairs weaken the strength of a bond because of electron repulsion - the lone pair electrons of an atom will repel the bonded electrons and such electron-electron repulsion will reduce the overall bond's strength.

Courtney Quan 1C

Generally, formal charge is used to predict the most stable and therefore most correct structure, so it would be correct to assume that the most stable Lewis structure would be used to determine molecular shape via VSEPR.

Courtney Quan 1C

As mentioned above, all elements in period 3 have access to their d-orbitals because their principal quantum number n=3. With n=3, the angular quantum number l can be 0, 1, and 2, which corresponds to s, p, and d-orbitals respectively. The d-orbital of sulfur simply isn't in use during its ground st...

Courtney Quan 1C

Iodine is a larger atom and therefore has more electrons than fluorine does. Because of iodine's greater number of electrons, there is a better likelihood/tendency of electron fluctuations to occur and create a dipole (greater polarizability). This greater likelihood of electron fluctuations/shifts ...

Courtney Quan 1C

It should be posted sometime soon, especially since we have all gotten our midterms back. It takes a little while for the grade to be posted on MyUCLA, maybe a few days after we've gotten our tests back, but it's never anything too long.

Courtney Quan 1C

As mentioned above, larger atoms and molecules have more electrons that are in higher energy levels and therefore further away from the nucleus, making them less tightly bound. In the context of molecules, the electrons are able to be more easily distorted and stronger intermolecular forces take pla...

Courtney Quan 1C

As far as the content we have covered goes, I don't believe so. Just know that multiple bonds are stronger and shorter than single bonds, increasing in strength but decreasing in length from single, to double, to triple bonds. Also recall that lone pairs on neighboring atoms will repel one another a...

Courtney Quan 1C

Dipole-dipole interactions occur between molecules that have permanent dipoles. Induced dipole interactions occur when a molecule with a permanent dipole causes another molecule to have a dipole moment by distorting its electron cloud and thus giving it an uneven charge distribution, with one area b...

Courtney Quan 1C

As said above, formal charge is only assigned to an atom and is most stable with it is equal to zero. Make sure that you check the formal charge of each individual atom per each structure to determine which is the most stable. The total charge of a molecule is always one indicated charge and is foun...

Courtney Quan 1C

In theory, it could be possible but extremely unlikely. What matters is the stability of the structure as a whole rather than how many bonds an atom could form by accessing its d and f orbitals.

Courtney Quan 1C

Hi! If by rate you mean the number of electrons emitted per a given time, the light's intensity would affect that rather than the energy. Intensity is linked to the number of photons, so a higher intensity would result in more photons. As long as the energy of each photon is greater than or equal to...

Courtney Quan 1C

Hi! 1.41x10^20 actually refers to the number of photons (emitted by the light in 2 seconds) instead of the amount of moles per 2 seconds. Because you already have the number of photons, you just need to convert it to moles- you already calculated the number of photons emitted in 2 seconds so you don...

Courtney Quan 1C

Hi! Professor Lavelle addressed during lecture that we won't be doing any calculations with Schrodinger's wave function equation. Conceptually, all you need to know is that the Schrodinger equation uses a wave function (psi)/orbital to describe an electron and its position in an atom. It uses the co...

Courtney Quan 1C

Hi! I wouldn't worry about knowing the polar coordinates for the test. Professor Lavelle didn't address them during lecture either so I wouldn't be too worried. As long as you know the concepts behind Schrodinger's equation (which is what the spherical polar coordinates refer to), you'll be more tha...

Courtney Quan 1C

Hi! A nodal plane is basically a two-dimensional plane that indicates the electron density distribution is zero, owing to the shape of the orbital. In other words, there is zero chance of finding an electron in a nodal plane. For instance, an s-orbital is spherically shaped and has a uniform electro...

Courtney Quan 1C

As mentioned above, a nodal plane essentially indicates an area where the electron probability density is zero. In other words, there is zero probability of finding an electron in a nodal plane. Nodal planes also depend on the shape of the orbital - for instance, an s-orbital is spherically shaped w...

Courtney Quan 1C

I agree, I think the experiment needs to be performed underneath a vacuum so that the electrons can be detected. Without one, electrons displaced with a lower amount of kinetic energy might not necessarily be detected otherwise.

Courtney Quan 1C

I'm not sure either, but I refer back to the given information of the problem when determining the number of significant figures in my answer. You shouldn't have more significant figures in your answer than was present in the information with the least number of significant figures in the question. ...

Courtney Quan 1C

You will use only the subscripts in the chemical formula to calculate molar mass rather than using the stoichiometric coefficients that are present when a chemical reaction is balanced. Stoichiometric coefficients are looked at to get the molar ratios of the reagents and products of a chemical react...

Courtney Quan 1C

The energy of an individual photon (calculated by E=hv) must be greater than or equal to the work function (energy required to remove an electron from the metal) in order to displace an electron. E photon -E work function/energy to remove an electron =E kinetic energy of electron , so a photon with ...

Courtney Quan 1C

I've noticed that a lot of the solutions in the back of book have different numbers of significant figures than given in the info in the problem. I would go back and use the number of sig figs in the info given to you. In this case, your answer would be rounded to 3 sig figs instead of 2 as shown in...

Courtney Quan 1C

I typically go back to using the number of significant figures as provided by the info given in the problem. It's possible that using some conversion factors in dimensional analysis might give you additional significant figures during calculations, but I would ultimately refer back to the original i...

Courtney Quan 1C

Yes, you need to divide by Avogadro's constant in order to convert from atoms to moles. And yes, you can submit questions from the sixth edition, just make sure that it's clearly written somewhere on your homework (alongside the chapter and homework number) before you turn it in :)

Search found 60 matches