CHEMISTRY COMMUNITY

marisaimbroane1J

H+ is added because the H+ shows that the environment is acidic and would need to be present in order for the Galvanic cell to function. H2O is not added because it is a solvent just like in an equilibrium expression.

marisaimbroane1J

If there is not a fast step, you do not need to assume the prior step is at equilibrium to generate the overall rate law from the mechanism. The pre-equilibrium assumption will be used in examples like the one from lecture where there is a fast step preceding the slow step.

marisaimbroane1J

The rate of formation CAN be a unique rate if you apply the 1/(stoichiometric coefficient) term in front of it. There can also be an instantaneous or average rate of formation which are not necessarily the unique rate. (They are if the stoichiometric coefficient of the product being formed is 1.)

marisaimbroane1J

It will end up depending on which half reaction requires the H+. You need to balance both half reactions and know that you should end up with (in that example) 8H+ on the left side of the equation.

marisaimbroane1J

If a catalyst is added to the reaction, the activation energy is lowered and the reaction can proceed faster than without the catalyst. In biology, a catalyst is usually something like an enzyme.

marisaimbroane1J

You integrate the differential rate law to get the integrated rate law. They result in different graphs (rate v concentration and rate v time).

marisaimbroane1J

Your value for n will be the number of moles of e- in the balanced half reactions. So write out the two half reactions and then make sure the number of e- is the same for both of them. This number will be n in the equation.

marisaimbroane1J

In a Galvanic cell, you will always have a reduction and oxidation reaction. The Ecell values in the table are only for reduction reactions, so you need to reverse the reaction for oxidation. This will allow you to balance the equation as well as get the correct sign of the oxidation reaction E value.

marisaimbroane1J

Do we have to add in platinum anytime there is not a solid in either the cathode or anode (or both)?

marisaimbroane1J

Gibbs relates to the enthalpy, entropy, and temperature of a reaction in the equation for Gibbs free energy given in lecture. Conceptually, Gibbs free energy tells you if a reaction is spontaneous if the value you get for delta G is negative. The delta G value can also be used to solve for other ele...

marisaimbroane1J

The spontaneity of a reaction is determined from delta G. Even though a negative delta H is favorable, it does not always mean the reaction will be spontaneous. Any time delta G is negative, the reaction is spontaneous.

marisaimbroane1J

When molecules are more complex, their degeneracy is higher because there are more possible microstates the components of the molecule can fill. Based on the equation we learned in lecture, a higher degeneracy will result in a higher entropy (or S value).

marisaimbroane1J

The microstates are relevant for the equation ΔS=kblnW—they are used to calculate W (the degeneracy). This equation is mainly used to find residual entropy (so like when the system is at 0K and there is no movement; there can still be entropy based on the number of microstates).

marisaimbroane1J

If there is no change in volume but the number of moles of gas change then is work still done in a reaction? I thought that if delta v is zero there is never any work done, but is that just expansion work? Do we then / can we use the ΔnRT (ideal gas law) equation?

marisaimbroane1J

Is E a state function? I wrote it down in my notes as being a state function but is that actually ΔU that is a state function to do with energy? Also, if there is a list of the functions we should know that are state or non-state that someone has and wants to share, I would be very grateful.

marisaimbroane1J

There is a conversion to go from L*atm to Joules on the equation sheet, so when calculating work, you should try and get to atm in order to make the conversion to Joules. The constants/equations sheet is super helpful to look at before the test to know what you will be working with.

marisaimbroane1J

Also, in most real-life examples (like ones that have to do with biochem), the process will be irreversible like Dr. Lavelle mentioned today in lecture. Test questions should specify between the two, though, but you would need to know which work equation to used based on if either the pressure or th...

marisaimbroane1J

Dr. Lavelle usually puts one question from the homework on his midterm and final, but it can be any question from any of the outlines.

marisaimbroane1J

You may want to memorize the diatomic elements (Hydrogen, Nitrogen, Fluorine, Oxygen, Iodine, Chlorine, Bromine). In a standard state, these elements will always have 2 atoms-- O2, N2, F2, etc. I remember for a test question in 14A, you had to know nitrogen was usually N2 to use the correct molar ma...

marisaimbroane1J

The heating curve also showed that there is much more energy required to convert water from a liquid to vapor than from a solid to liquid; this was used to explain why someone with a burn from 100 degree water vapor would have a more serious injury than someone burned with 100 degree water.

marisaimbroane1J

The equation is based off of the fact that if the system gains heat, it has to take it from the surroundings-- resulting in 0 (k)J of heat "lost". The same is true if the system gives off heat (-); the heat goes to the system (+), again the heats "cancel" out, resulting in the ze...

marisaimbroane1J

For the purpose of this class, I think the reactions we will be given will all be at 25 degrees Celsius if we are expected to use Kw. As long as the temperature is 25 Celsius, Kw=10^-14.

marisaimbroane1J

You can think of heat as a reactant in an endothermic rxn and a product in an exothermic rxn. With this in mind, you can use Le Chatelier's Principle in the same was as if you were increasing or decreasing the amount of product or reactant. (Like how if you decrease heat in an endothermic rxn, it is...

marisaimbroane1J

You will use an ICE chart when given the initial concentration of something (especially a weak acid). You then use the ICE chart to find the concentrations at equilibrium--- these concentrations are used to solve for K.

marisaimbroane1J

If the coefficient isn't 1, then you would have something such as -2x (if the coefficient is 2). Then make sure to still square the value when you put it in the equilibrium constant equation (it would be (2x)^2-- so 4x^2).

marisaimbroane1J

@804994652 , K can refer to either one and does not refer to one "more often". Either Kc or Kp can be written as just a general K as well; it is just important to use the information given in the problem to find the one asked for or that is easier to find using the given information.

marisaimbroane1J

Koverall of two reactions combined is equal to K1 times K2. I think this rule may be in the textbook, but my TA went over it in discussion. Cl2 is an intermediate in this case (product in one reaction and a reactant in the next), so it is not seen in the overall reaction.

marisaimbroane1J

I just wanted to get clarification on this in case it came up on an exam-- If we got a K value like 160 as an answer and the question asked if products or reactants are favored, would we say neither since it is not greater than 10^3? Since the K is >1, technically there is a higher concentration of ...

marisaimbroane1J

Concentration is measured in moles/L whereas partial pressure is in a unit such as atm (I know units are not included in the K expression, but that is how you can identify each of them). Partial pressure is basically how much of the total pressure the product/reactant is and can only be used when al...

marisaimbroane1J

This will most likely depend on the information given in the problem and what is asked for. For reactions with all gases, it is possible to use either concentrations or partial pressures.

marisaimbroane1J

Since K is the equilibrium expression, you use the concentrations at equilibrium. The initial concentrations can be used to do other calculations like with weak acids and bases.

marisaimbroane1J

Although the acid may not be 100% dissociated, in calculations with acids and bases, we act like it is. The dissociation is close enough to 100% to assume that the concentrations of the products are equal to the concentration of the reactant.

marisaimbroane1J

The bonds from chelates are also usually in a ring structure and surrounding the ligand. This goes along with the fact that bonds with chelates are stronger.

marisaimbroane1J

You probably need to know that a higher Ka means that the acid is stronger than a lower Ka. This is because the products of a reaction are in the numerator of the Ka equation, so an acid that completely dissociates (strong acid) will have a greater concentration of products than an acid that does no...

marisaimbroane1J

Does it matter on the test if we name AX2E and AXE2E2 bent or angular? Or will either be accepted?

marisaimbroane1J

N has a lower ionization energy than O and is therefore the central atom.

marisaimbroane1J

I don't think we will be asked to distinguish between the two, maybe just say which is more covalent or ionic in character. In that case, just used the periodic table to see which set of atoms have the greater difference in electronegativity-- this bond will be more ionic in character.

marisaimbroane1J

It's important to know that when there is a pi bond, the atoms cannot rotate and are left in a single plane. If the atoms were to rotate and change the shape, the pi bond would break and it would be a chemical reaction.

marisaimbroane1J

VSEPR notation will give you the number of paired and lone pairs of electrons whereas hybridization is based off of the electron density. For hybridization, you really only need the Lewis structure since it gives you all of the regions of electron density. In VSEPR, the Lewis structure is a great st...

marisaimbroane1J

In ethene, there are only 3 regions of electron density-- the double bond counts as one region. Therefore the hybridization is sp2 (hybridization is based solely off of number of electron density regions). Since carbon does have another bond, you have the un-hybridized 2p orbital.

marisaimbroane1J

Yes, the names are interchangeable, but the angles will be different-- <109.5 and <120.

marisaimbroane1J

Yes, the lone pairs do affect the molecular geometry (not the electron geometry). There are specific names for electron geometries with a certain number of lone pairs. Dr. Lavelle has not yet gone through alll of the molecular shapes, but they are in the book at the beginning of the chapter.

marisaimbroane1J

Light is diffracted and is a wave. When electrons are diffracted, they are acting in a wavelike way--- like light.

marisaimbroane1J

If it is not possible for all atoms to have a zero formal charge, make sure that atoms (like oxygen) that are more electronegative have a negative charge. An atom like oxygen should never have a positive formal charge; it would never exist in that state.

marisaimbroane1J

Additionally, when atoms are closer together, they are able to interact with each other more using forces like the Dispersion forces discussed today in class. Therefore, they are more attracted to each other and more difficult to separate, creating a solid structure.

marisaimbroane1J

Adding on, when you have two atoms bonded together, a larger radius means that the center of each atom is further apart. When the bond is shorter, the atoms are closer together and the attractive force between the protons of one and electrons of the other are stronger.

marisaimbroane1J

I think that it is because when using this equation, you really only care about the magnitude of the charge and not the positive or negative value of the charge, so you use the absolute value to determine the magnitude.

marisaimbroane1J

If the difference in electronegativity is greater than or equal to 0.4, a polar covalent bond is formed. If the difference in electronegativity is less than 0.4, a non-polar covalent bond is formed. An ionic bond is formed when the difference in electronegativity is greater than or equal to 1.8.

marisaimbroane1J

Cerium is the first element in the f-block. Lanthanum, which comes before it, is the d1. The f-block is only the two rows separated from the periodic table.

marisaimbroane1J

Why does carbon have a higher electron affinity than nitrogen? Does this pattern stay the same down groups 14 and 15?

marisaimbroane1J

I do not think we will be required to determine bond length; they have to be experimentally determined. Dr. Lavelle just wanted to show us that in resonance structures, there is not one shorter and one longer bond.

marisaimbroane1J

Make sure you set the final and initial calculations equal to a negative energy. The E is emitted and therefore negative. I'm not sure if that will give you the right answer, but try starting with that and see if it works.

marisaimbroane1J

The spin values can also help determine electron configurations because of the rules discussed in class. It's important to always fill the subshell with one spin first then add the other spin when you fill the shell.

marisaimbroane1J

I was wondering if anyone can explain to me if there is a relationship between the energy of electrons in certain states and the effective nuclear charge. On the first homework problem in chapter 2 (6th ed), energy increases because the quantum number goes up, but I though we wrote in our notes that...

marisaimbroane1J

If you read the way Dr. Lavelle described the different orbitals, it will either say along an axis or in a plane. There were just slight differences in the phrasing, but if you wrote down what was projected during the lecture, the differences are in there.

marisaimbroane1J

As the frequency decreases, the amplitude of the wave also decreases. Therefore, the slope (extent of change) decreases as the waves are broadened/more spread out. Think of the extent of change as a derivative (slope) and compare a wave with high frequency to a wave with low frequency.

marisaimbroane1J

Is there a way to do this problem using the Bohr Frequency condition? I tried to solve it this way but ended up using the equation given in the book that Dr. Lavelle did not go over in lecture. If there is another way to solve it using what we learned in lecture, I would really appreciate some insig...

marisaimbroane1J

I tried to use the photoelectric effect equation and convert to wavelength using the found energy, and I did not get the correct answer. There may be another way, but I think for this class we should stick to the De Broglie equation.

marisaimbroane1J

Although I keep getting the correct answers for homework problems using this equation, I cannot seem to figure out how the units cancel in order to get the correct ones (eg meters or meters/sec). Can anyone explain how it works?

marisaimbroane1J

For part A of this question, why do you have to use the DeBrogile Equation instead of using the kinetic energy equation to then solve for wavelength?

marisaimbroane1J

Answer C has to do with the amplitude of the wave. When the extent of change (the slope of the wave--think derivative) decreases, that means the amplitude is decreasing and the wave is being broadened. If you decrease the frequency, the waves become more spread out and therefore the slope of the wav...

marisaimbroane1J

The displaced electrons flow through a connection from the detector back into the metal. During the lecture Dr. Lavelle showed the detector therefore maintaining a slight positive charge.

marisaimbroane1J

Add up the stoichiometric coefficients for the products and reactants then subtract the sum of the coefficients of the reactants from the products. Although it may seem like this goes against the law of conservation, there are still an equal number of atoms on both sides of the equation.

marisaimbroane1J

The standard units are kg, so I think your mass should be in kg. The mass of an electron is 9.11x10^-31 kg; so I think that is the number that should be used in the equation.

marisaimbroane1J

I am not 100% sure, but I think showing units could help you receive partial credit if your final answer is incorrect. If there are units in your work, your TA will be able to see that you knew what you were doing, and it could only benefit you to have them.

marisaimbroane1J

My best guess for why D is four significant figures is that you are dividing two molar masses. In high school chemistry, I was taught to always write molar masses with two digits after the decimal in calculations. When using this strategy to write out the work for part D, the molar mass of oxygen (1...

marisaimbroane1J

I was able to correctly determine the empirical and molecular formula for this problem, but is there a way to know the formula is written beginning with Os? From previous problems, carbon usually begins the formula, so I am wondering if there is a rule to know when this does not happen.

marisaimbroane1J

If the question does not specifically ask for either then you can just choose either one then? (Assuming it is not a single element)

marisaimbroane1J

There are several homework problems that require using Avogadro's Number to convert moles to formula units. I know that whether you write the answer in terms of molecules or atoms it will be the same, but are there certain cases where your final answer should be in terms of one or the other?

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