CHEMISTRY COMMUNITY

LBacker_2E

Will Ecell increase if the size of the anode metal is increased? Does the metal have anything to do with it? And will Ecell increase if a substance is added to the cathode side? I do not think that changing the size of the anode metal (if you mean the actual electrode) will have an affect on Ecell....

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When looking at multiple mechanisms and asked to find the rate law and k that matches the desired overall reaction, how do you figure out if the rate law of the proposed mechanism is the same as the actual rate law? I think you have to be given the actual rate law, or see which one makes the most s...

LBacker_2E

Since you know how much the concentration of B rises, you can figure out that the concentration of A will decrease by this much, 0.034M and make sure to multiply it by 2 (because there is a coefficient of 2 in front of A). From there you subtract that amount from the original concentration of A to f...

LBacker_2E

Since the problem tells you that the system is at constant pressure, you use Pex, which is for an irreversible system.

LBacker_2E

You always use w=Pex x (delta)V for an irreversible expansion because the change in volume is measurable. For a reversible process you use the other equation because the change in volume is infinitestimally small. For the reversible expansion, pressure falls as the volume increases which is reflecte...

LBacker_2E

Actually MnO4- is reduced, but it is the oxidizing agent. The reduction reaction serves as the oxidizing agent. In contrast, the Ce3+ to Ce4+ is the oxidation reaction, but Ce3+ is the reducing agent. I hope this helps!

LBacker_2E

Usually you use platinum, but graphite or any other non-reactive metal would suffice.

LBacker_2E

Since you are at equilibrium, you are solving for E knot.

LBacker_2E

I think its just a biological application that we should be aware of, but didn't go over in class. Basically to form rust, iron is oxidized by O2 and H2O.

LBacker_2E

deltaG= -nFE: You only take into account the moles of electrons (the stoichiometric coefficient in front of the electron), because the coefficients in front of the compounds do not affect Ecell.

LBacker_2E

However, I think you would also say a reaction is third order, if it is first order with respect to one of the products, and second order with respect to another.

LBacker_2E

I am very confused how the book got a cell diagram that contained KOH as the aqueous compound on the anode side instead of just OH-. Also why is there Ni(s) on the cathode side and no OH- present?

LBacker_2E

Usually you are given the reduction half reactions, so you have to reverse the anode in order to get the oxidation half reaction, as you already have the reduction half reaction for the cathode. If you do not know which is the anode and the cathode, then you pick the less positive/more negative reac...

LBacker_2E

This question asked you to write out the half reactions and cell diagram for the reaction: H+ + OH- --> H2O. I use the half reactions that involved H+ and H2(g) cancelled out. However the book's work showed a use of OH- and O2(g) canceling out. It would have gotten me back to the correct reaction, b...

LBacker_2E

I am getting a different answer than the solution manual when it asks you to predict the potential of each cell. I am using the E = E*-(0.0592/n x lnQ), but the book uses 0.025693/n. Which constant is correct, because I thought Dr. Lavelle told us the first one in lecture, but all the work in the bo...

LBacker_2E

I think you should be able to understand why the derivations work concept wise, but you probably don't have to memorize them. Most of the equations needed should be given on the equation sheet.

LBacker_2E

I am confused on why the components that make up the delta G*r are different for a, b, and c. For a only the product is considered (H2O, g), for b the product (CO2, g) is subtracted from one of the reactants (CO, g), and for part C, the reactant (CaCO3, s) is subtracted from the sum of the products ...

LBacker_2E

I am confused on why the answer in the book said that the I was the reactant and the I2 was the product in the graph, when the equation we were given said the reverse. Is this just a typo, or can someone explain why it should be this way?

LBacker_2E

Since the electrons are being taken from the solution with an anode (the oxidation reaction) and being transferred to the solution with a cathode (the reduction reaction), the cathode side is going to be more negatively charged, and the anode side more positively charged. However negatively charge i...

LBacker_2E

I think the standard just means that the system is at equilibrium, where as when it is not standard, it is the potential that is required to reduce or oxidize the ions.

LBacker_2E

My TA said it is better to use kJ for ∆H, and J for the other values. Do you think this is really important, or is it just a preference?

LBacker_2E

When you are calculating the work and then applying that to the internal energy equation, is internal energy always equal to 0 for reversible/isothermal expansions, but only sometimes equal to 0 for irreversible reactions? How do you know the delta U for an irreversible reaction?

LBacker_2E

Is this related to the relationship between ∆G and the direction of a spontaneous reaction? Does a spontaneous reaction have a -∆G and proceed in the forward direction?

LBacker_2E

When you are calculating the change in entropy in response to a change in volume, would you use the nR value for constant pressure denoted by a P (5/2 x R)?

LBacker_2E

On the outline when it says "explain how how ∆S is related to ∆H for a change at constant temperature and pressure and explain the relationship" I am a little confused. Like I know the general equation to relate the two, but is there a trend that when you increase ∆S, ∆H will either increa...

LBacker_2E

You said you can make assumptions about delta H when pressure is constant, so can you make assumptions about internal energy when volume is constant? If so, what would they be?

LBacker_2E

When you are calculating the specific heat capacity this will be in grams, but when you are calculating the molar heat capacity this will be in moles. As far as degrees C goes, what every your constant is given in (kelvin or celcius) is what you will have to work with. It does not seem to matter tha...

LBacker_2E

I think I understand when to use the ratios for R in terms of when to plug it into my calculations, but what is the point of them and what do they mean?

LBacker_2E

When do you use standard enthalpy of formation vs just enthalpy? I understand the difference is that standard enthalpy means all products and reactants are in their standard states, but why/when would it be better to use one or the other?

LBacker_2E

When you are determining the change in enthalpy, I know you have to take into account any phase changes. I know with bond enthalpies you add the delta H of the phase change(s) (to/from gas form) to the delta H of bonds broken to find the delta H of the reaction. But, when you are trying to determine...

LBacker_2E

Don't forget to add the enthalpy of any state changes when you are calculating the enthalpy for a reaction. This phase change from liquid to gas is crucial in determining how much energy will be left or used after the reaction.

LBacker_2E

Reversible reactions usually take place when weak acids and bases are involved, as opposed to strong acids and bases.

LBacker_2E

When something is insulate that means there is not a relationship with the surroundings, and therefore it would be an isolated system. An example of an isolated system is something that occurs in a bomb calorimeter.

LBacker_2E

In 4C #3 you are given 765J of energy transferred as heat to 0.820mol Kr(g) at 298K and 1.00atm. When given constant pressure you plug in n((5/2)R)deltaT and constant volume is the same, but the ratio is 3/2. Where do these ratios come from?

LBacker_2E

I recall that in the book, it said this applied specifically to benzene. However, I think that any example where there is resonance (meaning half the C-C bonds are single and half are double, but in reality they are all a hybrid of single/double bonds) would have that bond enthalpy.

LBacker_2E

Yes, you are correct, a pH<7 is acidic, pH=7 is neutral, and pH>7 is basic.

LBacker_2E

Was anyone else getting different concentrations of H3O+ for ii when they put it in their calculator? I was getting something to the 1.50 x 10^-15, but that was different than what the book had.

LBacker_2E

Is there a special way to calculate sig figs for pH and pOH? I feel like this might have been mentioned in class, but now I am getting confused if this is for my chem lab instead? Do you determine sig figs based off of the decimal places of the numbers you are given or vice versa?

LBacker_2E

How do we know if the acid/base equations are reversible? Are strong acid/base equations never reversible, and weak ones always are?

LBacker_2E

Does anyone understand how we determine the Kb for AlCl3 on 6D 15b? I thought you would multiply the Kb of Cl- by 3 and that Al would count as a spectator ion, but the chemical equation was very complicated and confusing. Would this first way be correct, and if not why does it not work?

LBacker_2E

I think this question is referring to problems like 5H #1. In this example you are given an equilibrium equation, and in one instance the coefficients are halved, in which case you take the square root (or to the 1/2 power) of K. In the other case, the coefficients are multiplied by 2, in which case...

LBacker_2E

I think this is necessary when you are trying to find the equilibrium constant K. For example, if you are trying to solve for Kc and you are given the partial pressure, you can then plug that into PV=nRT to solve for n/V which is the molarity or concentration. Once you have the problem in terms of c...

LBacker_2E

No, when you are given bar as units that means you solve for Kp not Kc. In this case you create your ICE table in terms of partial pressure instead of concentration, so the volume is not part of the equation.

LBacker_2E

Yes, volume, pressure, and changes in concentration have no effect the value of K, only temperature can change the value of the K constant.

LBacker_2E

We relate everything to Kw because it is equal to the concentration of H3O+ times the concentration of OH-. These can then be used to determine the pH and pOH, or the Ka/Kb and pKa/pKa using logarithmic functions. So it is the other side of the equation that makes Kw so useful.

LBacker_2E

When I did this problem, I used:
H2 + Cl2 <--> 2HCl
2BrCl <--> Br2 + Cl2
The Cl2's cancel out to get the complete reaction, and both are listed at 300K in the book.

LBacker_2E

I think it is kind of like thinking of it in terms of the reverse reaction (so in this case the products are acting like reactants). But, I also don't think the limiting reactant concept really applies because you are trying to reach equilibrium (instead of trying to complete a reaction to completio...

LBacker_2E

If the question just asks for K, you can choose between Kc and Kp by looking at other information in the problem. If dealing with aqueous solutions you should probably find Kc, but if dealing with gaseous solutions, it might be better to calculate Kp. This seems to be the general trend, but may not ...

LBacker_2E

"A mixture of 2.5 moles H2O and 100 g of C are placed in a 50 L container and allowed to come to equilibrium subject to the following reaction: C(s) + H2O (g) ⇌ CO (g) + H2 (g). The equilibrium concentration of hydrogen is found to be [H2] = 0.040 M. Calculate the equilibrium constant Kc of thi...

LBacker_2E

I am confused how you solve this problem from the post-assessment: "A researcher fills a 1.00 L reaction vessel with 1.84 x 10-4 mol of BrCl gas and heats it to 500 K. At equilibrium, only 18.3 % of the BrCl gas remains. Calculate the equilibrium constant, assuming the following reaction is tak...

LBacker_2E

Thank you, your explanation was really helpful!

LBacker_2E

Can someone give an explanation of why formic acid (HCOOH) is a stronger acid than acetic acid (CH3COOH) because the one in the textbook was a little confusing and unclear to me?

LBacker_2E

I had the same questions for parts d and f, and also why do you not consider different amount of H's in part d?

LBacker_2E

I am confused when you use which rule to identify which compound is the stronger acid. In lecture we mainly discussed bond length and anion stability, but the textbook solutions only justified their reasoning with bond length to say that HCl was stronger than HF (in part a). I thought you would use ...

LBacker_2E

For HClO and HClO2 (part b) and HNO3 and HNO2 (part e) the book claimed that the compound with more O's would be a stronger acid because more O's meant more electronegativity helping to pull e- of H atom out of H-O bond, which I think makes sense, so do you always go with the atom with more O's if t...

LBacker_2E

For part b, is there a formula that tells us we can plug in the (200 x 0.025)/250 to the -log, or are we just doing the conversion to molarity using M1V1=M2V2 inside the pH=-log[H3O+] formula?

LBacker_2E

When you are supposed to identify the Bronsted acid and Bronsted base in a chemical equation, will they both always be on the left side of the equation (like are they always going to be the reactants)? If not, can you give an example where the Bronsted acid and/or base is on the right side of the eq...

LBacker_2E

If you count the number of valence electrons, you can determine the atoms hybridization. For example if you have a Carbon atom, which has 2 electrons in the 2s and 2 electrons in the 2p orbitals, then these orbitals would combine to form a hybridization of sp3. This means that now the Carbon atom ca...

LBacker_2E

I'm confused, is this question asking you to write out the hybridizations for each sigma and pi bond like in 2.47 or is it asking for the hybridizations of the central atoms?

LBacker_2E

I have a question on this problem. I understand how to determine hybridization when it is a simple compound such as BF3, but I do not understand how we find the multiple hybrids from a big compound like this. Do the two hybridizations in the parentheses represent the hybridizations of the molecules ...

LBacker_2E

I was confused on the problem where you are supposed to write out the formula of the compound sodium bisoxalato (diaqua) ferrate (III). My formula had the oxolato before the water, but the answer from the solutions said the water should be first (and written as OH2 instead of H2O. How do you know wh...

LBacker_2E

I think the final exam is cumulative, so everything we have learned this quarter.

LBacker_2E

In instantaneous dipoles, when we learned that two rod-shaped molecules are closer, and thus the intermolecular forces will be stronger, than two spherically-shaped molecules. But is there a way of knowing which shape the molecule is, rod or spherical?

LBacker_2E

I think the main thing you need to know/most of what we learned is that one sigma bond occurs in any type of bond, single, double, or triple. For pi bonds there will be 0 in a single bond, 1 in a double bond, and 2 in a triple bond.

LBacker_2E

I think you are also supposed to know the VSEPR Model, dipole moments and how that is related to polarity, the strength of different intermolecular forces. Anything that is on Homework 3F and 2E may be on the test.

LBacker_2E

Yes I think the question is asking for the orientation of the orbitals. It is asking if they oriented towards the corners or opposite of each other.

LBacker_2E

When we are determining the molecular shape of a compound that has a central atom with two bonded pairs and two lone pairs, is the name of this shape "angular" or "bent"? I think they are the same thing, if they are not what is the difference between the two? If they are the same...

LBacker_2E

Yes, difference in bond angles in a tetrahedral may occur if the atoms surrounding the central atom differ. For example the bigger atom will cause bigger bond angles (than 109.5 degrees) between it and the other atoms (because the electrons will repel each other more) and the other bond angles will ...

LBacker_2E

For part d, there is no opportunity for Hydrogen bonding in C[2]H[4], and Hydrogen bonds are stronger so that is why CH[3]OH would have a higher melting point (hydrogen bonds can occur between the H's and O's). For part b, I am unsure though since there are the same amount of H and O molecules allow...

LBacker_2E

Are the lone pairs always going to cause a bent shape, or will there be times when the lone pairs are opposite of each other?

LBacker_2E

In our notes it says bonds between two ions have -250kJ of energy and are most favorable. Is the next most favorable type of bond, a Hydrogen bond?

LBacker_2E

What is the difference in characteristics of molecules with dipole-dipole bonds vs. molecules with dipole-induced dipole bonds? I know that virtually all molecules have London dispersion forces and that polar molecules will have dipole-dipole forces, but is it possible for a molecule to have both di...

LBacker_2E

Since SiCl has a greater difference in electronegativity than SiBr (increases going to the right, but then decreases to go down the group to get to Br), SiCl will be more electronegative than SiBr.

LBacker_2E

Overall, if an expanded octet can be formed (is past 2p orbitals) to yield lower formal charges on the central atom, will this always be a more stable structure?

LBacker_2E

I understand that Lewis acids and bases form a coordinate covalent bond, but is there a difference between the characteristics of a coordinate covalent bond and an ordinary covalent bond?

LBacker_2E

Bond length and bond strength have an inverse relationship. A longer bond such as a single bond is weaker, while a shorter bond such as a triple bond will be stronger.

LBacker_2E

I don't think we need to focus on electron affinity when determining which atom to put a negative formal charge on. The main thing is if you have to choose between two, give the atom with the lower electronegativity a negative charge.

LBacker_2E

Is there a way to determine which atom the unpaired electron will go on when doing problems involving radicals?

LBacker_2E

I asked Professor Lavelle about this question during office hours and he said he did not realize that this was included on the homework, and that he is taking it off for next year because it is more complicated than we need to know. I think he also did an explanation somewhere on Chemistry Community...

LBacker_2E

It is lower energy to add electrons to the s orbital than to the d orbitals (for example you add 4s before 3d). However, when you are removing electrons from subshells, you will remove from the s subshell before d. The exceptions comes to play when you have placed two electrons in the 4s subshell an...

LBacker_2E

If most bonds exhibit both ionic and covalent properties to varying degrees, do we ever draw ionic compounds as Lewis Structures instead of two ions next to each other? Is there an example of this? I can't think of anytime when this would be the case. If you have an ionic compound then you are going...

LBacker_2E

I think the answer to your second question would be yes, structures that have resonance with one another should have the same numbers of single/double/triple bonds. These bonds would be moved around however, causing the formal charges for the individual elements in the diagram to change. These are n...

LBacker_2E

I doubt that we will be given the electronegativity table, I think we will just have to base it off of our knowledge of the general trend. Also, the electronegativity rule is not true 100% of the time, especially when the values are very close. We did an example in my discussion class and when we tr...

LBacker_2E

When there are delocalized electrons you get a resonant structure, meaning there is an average Lewis structure that best represents the actual formation of electrons in a compound.

LBacker_2E

If a triple bond yields a formal charge closer to 0 than one or multiple double bonds, then yes triple bonds would be more stable than double bonds (so long as the octet rule is satisfied).

LBacker_2E

In response to the question about knowing what the formal charge should add up to, a compound should have a formal charge of 0, if it does not have a positive or negative charge. However if it is an ionic compound with a charge, then it should not add up to 0. When there are multiple possible struct...

LBacker_2E

Sometimes there will not be other possible ways to draw a structure for example, if the central atom is bonded to 4 elements and cannot form an expanded octet, then no double bonds will occur. However if there are other possibilities, such that the central atom is not bonded to the maximum amount of...

LBacker_2E

Also if you are unsure if there should be a lone pair or a double bond, which would cause resonance in the Lewis structure, you should calculate the formal charges of each and which ever one is lower will be the better/more accurate structure.

LBacker_2E

I think any of the possible Lewis dot structures are acceptable. If you note that there is resonance, and maybe explain why, then it should be okay to only include one of the possible structures.

LBacker_2E

To the person who asked if you should always convert kJ to J, when you are using other values with units that involve J, yes you should 100% convert kJ to J. However, if it is your final answer, you would probably be fine leaving it either way.

LBacker_2E

Yes, it will probably be important to know (on exams) the different types of wavelengths both in terms of the order from long to short wavelengths and from low to high energy.

LBacker_2E

As mentioned above Hz is equivalent to s^-1, which is why you can often cancel out Hz when you have m/s in your calculations.

LBacker_2E

I was wondering how you are supposed to solve part a of number 13, because I thought the question was just asking for how many possible values of l there could be with a principle quantum number of n=4. However, the answer says there are 7 possible values not 4, how do you arrive at this answer?

LBacker_2E

I was wondering the same thing because I tried to use those equations, but I did not get the correct answer. The worked out solution manual used the equation wavelength=h(mv)^-1, and I don't remember learning this equation in lecture, so how are we supposed to solve this problem?

LBacker_2E

Finding a limiting reactant allows you to calculate the amount of product that is going to be produced. Thus the next step in a limiting reactant problem is often to find the amount of a certain product produced, but it may also be to find the amount of excess reactant that is left over after the re...

LBacker_2E

Yes, you use Avogadro's constant to find both, but the units may be different. Obviously molecules are the units when calculating the number of molecules, but when calculating formula units, your answer can be in terms of molecules or formula units. At least that is how the answers were to the homew...

LBacker_2E

Yes the constant is the speed of light in a vacuum, but I believe you can still use it even if the problem does not specify that it takes place inside a vacuum.

LBacker_2E

I have a question regarding when we are solving for an empirical or molecular formula: some of the problems, then have you name the compound. For example in L39, the formula ends up being SnO2, and its name is tin (IV) oxide. But how are we supposed to know that it is tin (IV)? Do we need to know co...

LBacker_2E

In general, when you are writing a net ionic equation (like in this problem), how do you know which ions are spectator ions? Since some of the ions are going to be spectator ions, do those ions still need to be balanced on both sides of the equation?

LBacker_2E

When there are 0's without a decimal point, such as the number 200, they do not count toward the sig figs- this number has 1 sig fig. But if the 0's are after the decimal point, such as the number 2.00, they do count toward the sig figs- this number has 3 sig figs. Finally if there are 0's before a ...

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