CHEMISTRY COMMUNITY

Kassidy Ford 1I

Post all thoughtful poetry here!

Kassidy Ford 1I

On the chemical kinetics outline, it does not mention anything specifically about enzymes. But I would know how enzymes work as catalysts.

Kassidy Ford 1I

Also, cell potential only refers to electrochemistry, more specifically redox reactions. In kinetics, elementary steps are used to find the reaction mechanism that matches the rate law... they usually wont have a given cell potential as these are usually not reduction or oxidation reactions.

Kassidy Ford 1I

^^^ does this mean that it is literally Gibbs free energy??

Kassidy Ford 1I

^^^ like they said, most heterogenous catalysts work by simply giving the reactants more surface area to react on. I believe that is all you will need to know about adsorption!

Kassidy Ford 1I

Since A has to be solved for experimentally, we should be given it if it is needed in the equation. I think we will mostly be using the Arrhenius equation to solve for different values of k

Kassidy Ford 1I

Since A must be solved for experimentally, I don't think we will have to know how to find it.

Kassidy Ford 1I

You can also tell by looking at the graphs of the reactions concentration over time. For a zero order reaction, the graph will be linear if you plot concentration over time. For first order, the graph will be linear if you plot ln(concentration) over time, implying an exponential relationship. For 2...

Kassidy Ford 1I

As for the slow part in your notes, Lavelle was trying to emphasize that the rate is only dependent on the slowest step in the reaction mechanism. Think of his brownie analogy, if a group is making brownies and it takes 5 minutes to make the batter but 25 minutes to bake the brownies, then it no mat...

Kassidy Ford 1I

^^what they said, and I also think Lavelle mentioned that the rate of products formed is the same as the rate for reactants but with the opposite sign. Can someone confirm that I am remembering correctly?

Kassidy Ford 1I

if it helps to visualize, if the reaction is expressed by k[A}^2[B]^3, then the overall reaction order would be 2+3, so 5.

Kassidy Ford 1I

It is good to measure at close to t=0 because there will only be reactants present and no products formed. If there are products present then there is a high chance that the reverse reaction is occurring simultaneously so it will be hard to determine the rate.

Kassidy Ford 1I

^^ like they said, and how you determine the m and n is the coefficients of the reactants in the overall reaction

Kassidy Ford 1I

You can relate Gibbs free energy to K using the following equation: ∆Gº = -RTlnK As you can see, this is not dependent on pressure or temperature. However, the max work that can be done is given by ∆G at constant pressure and temperature. Hope this helped! You said that this isnt dependent on tempe...

Kassidy Ford 1I

^^ because this reaction is in acidic solution, for each half reaction balance excess oxygen with H2O and then add H+ to the other side to balance excess H+. Then use electrons to make sure the charges are balanced.

Kassidy Ford 1I

deltaG and E can be related by the equation: deltaG = -nFE

Kassidy Ford 1I

I believe we use a porous disk so that ions can move through to either side until their concentrations reach equilibrium. I don't think this would happen if a salt bridge was used

Kassidy Ford 1I

I have written down that standard energy potential is an intensive property and so it doesn't depend on how many times the reaction occurs, it is always the same. However, reaction potentials are only given for reduction reactions so if your half reaction is an oxidation then you must flip the sign ...

Kassidy Ford 1I

The question will usually state if the reaction is taking place in a basic or acidic conditions. Usually you will need to use H2O to balance excess oxygen, when you do this you will then add either H+ or OH- to the other side of the reaction to then balance the excess hydrogen from the H2O. If it is...

Kassidy Ford 1I

Galvanic cells are related to a positive standard cell potential, which means the process will occur naturally without work having to be put in (spontaneously). Voltaic is another term for Galvanic. On the other hand, an electrolytic cell is related to a negative standard cell potential, so the proc...

Kassidy Ford 1I

^^To go off what they said, you add H+ to balance the hydrogens and then add the same amount of OH- to both sides. The H+ and OH- that is on the same side will combine to form H2O

Kassidy Ford 1I

There are several molecules with known oxidation numbers, for example O is known to have an oxidation number of -2. Using the known oxidation numbers you can solve for oxidation numbers of other molecules, for example if you have the molecule MnO4 with an overall charge of -1, then you know that Mn ...

Kassidy Ford 1I

Yes, for example if Cr^6+ changes to Cr^3+, then you know it was reduced because the half reaction would be Cr^6+ + 5e- --> Cr^3+

Kassidy Ford 1I

It is helpful to know all the oxidation numbers and that is one way to do it, but there are several other ways as well. We will usually be given a table of reduction reactions with there E values, so you can tell from that as well. It also helps to look at oxygen, because that is where the root of t...

Kassidy Ford 1I

^^ as they said above, in order for a spontaneous reaction to occur, deltaG must be negative. This means that the value for T*deltaS must be greater than the change in enthalpy. For an endothermic reaction where enthalpy is a positive number this is often not true except at very high temperatures.

Kassidy Ford 1I

The ideal gas heat capacity at constant pressure (Cv) is represented as 3/2R for a monoatomic gas. This is different from a substance like water, where the heat capacity is 4.186 J/g*C, where J represents joules supplied in the form of heat. As you can see, the heat capacity of a monoatomic gas does...

Kassidy Ford 1I

^^ While what they said is true, positional entropy is only one kind of entropy. For example there is also entropy when there is a volume, pressure, and temperature change in a system. There is nothing wrong with thinking of it as disorder, I think lavelle just doesn't want us to think that way beca...

Kassidy Ford 1I

deltaS(surr)=-deltaS(sys) only under reversible conditions. So this means isothermal.

Kassidy Ford 1I

Larger molecules have more particles and therefore have a greater positional entropy value. For example an element with more particles (electrons, protons, and neutrons) (such as Pb vs C) will have a larger degeneracy number, where W=#microstates^(#particles). Then use the equation S=kb(lnW) to solv...

Kassidy Ford 1I

the question will specify, for example if it says the volume increased from 20M to 25M then u know the volume changed... use the context clues in the question

Kassidy Ford 1I

Also notice that this equation can't be used if there is change in V, P, or T, because you have to use one other equations. This also implies reversibility.

Kassidy Ford 1I

You should know (delta)U=q+w, (delta)H=(delta)U+p(delta)V, which can also be written as (delta)H=(delta)U+nR(delta)T. Also know when change in V is 0, (delta)U=q, and when q=0, (delta)U=w. And when there is no change in temperature there is no change in U so that means q=-w.

Kassidy Ford 1I

Yes. Ideal gas Monoatomic molecules have a heat capacity of 3/2R while linear molecules will have a heat capacity of 5/2R because they can have more degrees of motion.

Kassidy Ford 1I

I think it's because it is very difficult to change the volume of solids and liquids by a significant amount.

Kassidy Ford 1I

This also makes sense if you think about the law of conservation of energy, because energy can never be created or destroyed, only transferred. Therefore if a system loses energy that means that its surroundings gain that energy and vice versa.

Kassidy Ford 1I

In lecture, Lavelle said that a system at equilibrium is a reversible system. I am kind of confused on what kind of system would be irreversible?

Kassidy Ford 1I

^^^ like they said, pay attention to the specific heat given. If you are given molar heat capacity (Cm), you use moles (n). If you are given specific heat capacity (Csp), then use grams. Make sure the units match!

Kassidy Ford 1I

just think about the law of conservation of energy. energy cannot be created or destroyed, only transferred. So the positive q(reactants) must have come from a -q(calorimetry).

Kassidy Ford 1I

heat capacity is a constant depending on the substance. The substance will have different heat capacities depending on what phase it is in.

Kassidy Ford 1I

When you are calculating enthalpy, I'm pretty sure the pressure has to be constant. Otherwise the changes in pressure would change the enthalpies as well.

Kassidy Ford 1I

A state function, like the ones mentioned above, describe the current state of the system regardless of how the system arrived at that state. So temperature is a state function while heat is not.

Kassidy Ford 1I

if something is a state property, that means be addition and subtraction can be used on it. So Hess's law relates to this because you can add enthalpies of reactions together to find the overall enthalpy of reaction

Kassidy Ford 1I

Work is a positive value when more work is done by the system than on it. So if the value for work is negative, then work was done on the system. If the value for work is positive, then work was done by the system

Kassidy Ford 1I

The total heat added to or lost from a system is the enthalpy. Enthalpy can only be calculated if the system is at a constant pressure. Heat is just the difference in temperature

Kassidy Ford 1I

But in this course often a question will ask for a specific K, like Kc or Kp. In this case, make sure your units match with what the question is asking for. But K does not necessarily have to have this specificity

Kassidy Ford 1I

The ideal gas law assumes that the gases are their ideal state when it is used. This might not be true, but especially for this course we always assume that the gases are at their ideal state so that we can use the equation.

Kassidy Ford 1I

If the questions asks for just K, and not Kc or Kp, then you need to look at the equation. If the reactants and products are gases then you should use Kp, because it is implied. If the reactants and products of the equation are aqueous then you should use Kc because it is implied. However if it ask...

Kassidy Ford 1I

Adding an inert gas never changes the equilibrium constant or pressures of the gases reacting in the equation so I think that explanation is sufficient

Kassidy Ford 1I

How do you tell which gases are inert gases? is it any gas that doesnt react in the chemical equation?

Kassidy Ford 1I

Since the reaction goes both ways you can think of products being a reactant for the reverse reaction. This helps me think about it because you can just write the reaction with both

Kassidy Ford 1I

Each reaction has a different K's that correspond with different temperatures. This is why the temperature is often given in the question

Kassidy Ford 1I

^^^ to go off what they said, if Q=K then the reaction is at equilibrium, so the concentrations would be the same as well

Kassidy Ford 1I

When asked the equilibrium constant of a chemical reaction involving gases, do we solve for Kc or Kp? The question will probably ask what to solve for; either Kc or Kp, because for gases you can solve for either. if they give you partial pressures in the question than solve for Kp and if they give ...

Kassidy Ford 1I

^^^what they said is true, but ALSO Q is useful to use when you are trying to see where the reaction is in comparison to equilibrium, so like when Q is less than K then you know that more products need to be formed to reach equilibrium, when Q > K you know that more reactants need to be formed to re...

Kassidy Ford 1I

Also the radius of an anion will be bigger than its parent's atom because the added electron(s) will lead to more repulsion. The radius of a cation will have a smaller radius than it's parent atom because the remaining electrons will be pulled closer to the nucleus

Kassidy Ford 1I

Also if you are making a mixture of a strong acid and a strong base with the same concentration, the resulting salt and solution will be neutral; you can think of the acid and base cancelling each other out. If you mix a strong acid with a weak base, the resulting solution will be acidic (although w...

Kassidy Ford 1I

A bronsted acid is a proton donor, and a bronsted base is a proton acceptor. This is easy to see in chemical reactions, because you can usually see the transfer of H+ going on. For example, HCl + H2O --> H3O + Cl, HCl is the bronsted base because it donated an H+ to H20, in this case the bronsted ac...

Kassidy Ford 1I

You can also think of it in terms of bond length; since the bond between H-I is the longest it is the strongest acid because that bond will be easier to break, so it will dissociate the easiest. HI>HBr>HCl>HF. HF is a weak acid because F's high electronegativity and small size makes that bond short ...

Kassidy Ford 1I

Strong acids dissociate the easiest, so a longer bond length between H and an atom will mean a stronger acid. This is why HI is a stronger acid than HCl. When comparing weak acids, look at the stability of the conjugate base that is left over after the acid is deprotonated. A more stable conjugate b...

Kassidy Ford 1I

^^^ I am not sure if we have to draw them but it we definitely have to label them and know that pi bonds restrict rotation

Kassidy Ford 1I

if a paramagnetic electron means it is unpaired, then does that mean the molecule is radical? okay I thought about this more and I am going to answer my own question, I realized that a paramagnetic electron from one atom could bond with a paramagnetic electron from another atom so it is not always ...

Kassidy Ford 1I

if a paramagnetic electron means it is unpaired, then does that mean the molecule is radical?

Kassidy Ford 1I

Nathan Rothschild_3D wrote:So is solid stronger or are they both the same? or does it depend on the molecule, not the state

I think it is safe to say that the intermolecular forces in a solid compound are generally stronger than in a gas phase

Kassidy Ford 1I

The diagonal band they are referring to includes Be, Al, Ga, Sn, Sb, and Pb. The examples Lavelle used in class were all bonded with oxygen, so BeO, Al2O3, Ga2O3, SnO2, SbO5, and PbO2

Kassidy Ford 1I

if it is nonpolar then it will only be affected by dispersion forces. If it is non-polar it will be affected by dipole-dipole forces. if there are H bonded to N,O,F then it will experience H bonding.

Kassidy Ford 1I

Since in tetrahedral molecules there are no molecules directly opposite one another, the only way it will be nonpolar is if all of the surrounding atoms are the same

Kassidy Ford 1I

I understand that CH2Cl2 is polar because a tetrahedral shape isn't completely symmetrical. However if a shape is symmetrical like a square planar would it be considered nonpolar? For example if it was a square planar with two Cl and two H atoms around a central atom, would it be considered non pol...

Kassidy Ford 1I

If you have a molecule like CHF3, it will be polar because not all of the atoms surrounding the C are the same, but will that affect the bond angles or do you still assume that it is 109.5

Kassidy Ford 1I

are we going to have to know the d hybridizations or are we only supposed to know the p hybridizations?

Kassidy Ford 1I

my TA said there are no calculations you will really have to do on the test, but you do need to know the concepts behind the equations

Kassidy Ford 1I

also don't bond lengths depend on the atoms, ie a bond between two very electronegative atoms would be shorter? also has to do with atomic radius

Kassidy Ford 1I

are dipole-induced dipole forces the ones that are always present in everything?

Kassidy Ford 1I

^^^ what they said, van der waals forces are constant but also constantly changing, that is why it is useful to think of them as induced-diple induced-dipole forces

Kassidy Ford 1I

u can think of lone pairs as another region of electron density, so it will effect the shape as if there was another bonding pair

Kassidy Ford 1I

if you have lone pairs it will also affect your dipole moment, so lone pairs are important

Kassidy Ford 1I

^^^ not all bonds between different elements produce a dipole because you have to take molecular shape into account. For example, in a symmetric molecule like CCl4 there would be dipoles between the C-Cl bonds, but they would all cancel each other out because of the symmetrical shape of the molecule

Kassidy Ford 1I

what is the most electrons an expanded valence shell can have? 10? 14? is it different for different atoms?

Kassidy Ford 1I

does the d shell have to be completely empty? I thought it could be partially full also

Kassidy Ford 1I

*pair

Kassidy Ford 1I

What they said above, and the reason they don't stay very long is because the unpaired electron will make it very reactive as it will attempt to par that electron

Kassidy Ford 1I

do resonance structures depend only on the FC of central atom or on all of the atoms

Kassidy Ford 1I

Will bond length be given to us on tests or are we supposed to know bond lengths/assume resonance? Do you just base your lewis structure on the lowest formal charge?

Kassidy Ford 1I

Just to confirm, we would draw all the up arrows first for a specific subshell and then start filling in all the down arrows right? Yes, this is because you would fill all unpaired electrons first, and when electrons are unpaired they have parallel spin. then, when you go to fill in the pairs you w...

Kassidy Ford 1I

It is kind of confusing because the electrons closest to the nucleus take more energy to remove from the atom. so how come electrons in shells that are further away have more energy?

Kassidy Ford 1I

705198479 wrote:is there a rule that restricts a sub shell ? is it important to remember for this class?

you should know how many electrons each shell can hold; the s shell can only hold 2, p holds 6, d holds 10 and f holds 14

Kassidy Ford 1I

It is also good to know that when electrons are paired, they have opposite spins. when electrons are unpaired but in the same orbital, they have parallel spins

Kassidy Ford 1I

considering we get a list of equations and constants, i dont think you need to derive any equations. rearranging equations perhaps, but i wouldnt worry about it

Kassidy Ford 1I

the fourth number represents the spin state of that same electron, so it can't be used to describe other electrons. The fourth number only indicates if the electron is spinning up or down.

Kassidy Ford 1I

as the person above said, de broglies equation can only be used for particles with a resting mass. Since light does not have a resting mass, the equations are not interchangeable.

Kassidy Ford 1I

while nanometers is common for wavelength, so is picometers, or leaving your answers in meters. honestly you could leave it in any unit of meters you want, lol

Kassidy Ford 1I

Light has no resting mass, but electrons do. That's why de Broglies equation can be used for electrons, and it is also why electrons don't move at the speed of light

Kassidy Ford 1I

I think different equations are in terms of kg, while some you can use grams. For molar mass, obviously use grams

Kassidy Ford 1I

In lecture Prof Lavelle urges not to use the Rydberg equation if it confuses you but instead to use the H-atom spectrum equation, which is E(n)= -hR/n^2. you do that for both energy states, n=2 and n=1, and then subtract the initial energy from the final energy to get the change in energy. you can t...

Kassidy Ford 1I

As the person above said, a smaller particle with tiny masses will actually lead to a larger wavelength detected, because of their negligible mass. This is why large objects that have velocity, such as cars, do not act like waves, because their large masses lead to undetectably small wavelengths acc...

Kassidy Ford 1I

The photon will be absorbed, exciting the electron to a higher energy state, and then as the electron drops back down to the ground state the photon will be emitted again. The photon will have the same amount of energy as it is absorbed and as it leaves because of the law of conservation of energy

Kassidy Ford 1I

They are essentially the same equation. The Rydberg equation is simply derived from using the H-atom equation twice, with the final and initial n values. You can use either and get the same answer, however Prof Lavelle suggested using the H-atom equation because it is easier to understand than tryin...

Kassidy Ford 1I

The work function is the energy needed to remove an electron from the surface of a metal. There is an equation that in words goes like this "E(photon)-E(energy needed to remove e-)=kinetic energy of e-). So the work function is needed to solve for the kinetic energy of an electron, or excess en...

Kassidy Ford 1I

This problem confuses me too, why wouldn't you use the equation M1V1=M2V2? Is it because you are trying to find the mass?

Kassidy Ford 1I

To elaborate on the above comment, usually you round sig figs to the least amount of sig figs given in the problem. This is to make the answer as accurate as possible.

Kassidy Ford 1I

What is the purpose of having the states of matter in the chemical equation? What does it imply? Does it only matter in a lab/experiment setting?

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