CHEMISTRY COMMUNITY

Sammy Thatipelli 1B

When you reverse standard cell potentials, do the signs change?

Sammy Thatipelli 1B

The question I am confused about is what has greater residual entropy at 0K: NO or BF3?

For this question, do we look at the masses, or do we draw out the Lewis structures to figure out the answers?

Sammy Thatipelli 1B

Without doing any calculations, how do we know that ethane has a higher heat capacity than ethene? This was one of the questions in practice problems, but I am not sure how we can automatically come to that conclusion.

Sammy Thatipelli 1B

Can someone explain the different Cv values for each of the gases in the solution manual? Because each process is isochoric, I thought originally that the Cv would always equal 3/2R in the containers, but this is incorrect. Can someone explain the reasoning behind why the Cv values have to be differ...

Sammy Thatipelli 1B

How do you draw a proposed activated complex for a reaction? For example, in 15.85, the third part of the question asks us to do this.
Can someone explain the process of drawing out the activated complex?

Thank you

Sammy Thatipelli 1B

To be add on for when we use the Arrhenius Equation, it is clear we need to use it if we are working with the Boltzmann distribution e^-Ea/R, which gives the fractions of collisions wth energy,Ea, or greater temp at T. In this case, A would be the frequency factor or pre-exponential factor (includes...

Sammy Thatipelli 1B

Because a rate constant is "constant" then it will not change, no matter what experiment you pick. This is done with two different experiments because sometimes there may have been an error the first time with calculating k.

Sammy Thatipelli 1B

If a reaction has a low activation energy, does it change much with temperature? Can someone explain the logic behind this question and why the reaction barriers are easier to overcome?

Sammy Thatipelli 1B

For test 3, do we need to know what Lavelle went over in Lecture on Friday before he taught about reaction mechanisms? This would include material like the pseudo-rate law (1st and 2nd order) and the equation k=k'/[B]o^m[C]o^l. These equations/concepts are not in the reading for 15.1-15.6 in the boo...

Sammy Thatipelli 1B

For 15.23, I am confused on why we use the equation ln[A]o/[A]t=kt, because I thought that the equation for first order half life reaction is used when it is written as: [A]t-[A]0=-kt. Can someone explain when we have to use this equation in our homework versus the other equation in our homework? I ...

Sammy Thatipelli 1B

to calculate the order of b, use experiments 2 and 3. increasing the concentration of B by the ratio 3.02/1.25 increases the rates by (3.02/1.25)^2, and this makes B second order.

Sammy Thatipelli 1B

Can someone explain why in the solutions manual 0.055 mol A/L is equal to 0.37 [A]o? I am confused on how this conclusion came through, but I understand where the 0.055 value came from.

Sammy Thatipelli 1B

What does it mean when the question asks you to find the unique rate of the reaction? Can someone explain the process to figuring it out? Thanks!

Sammy Thatipelli 1B

When do we use platinum in our cell diagrams, and is there another solid that we can choose from for our diagram? Can someone list the rules/cases in which we include platinum (or other solids) in our diagrams?

Sammy Thatipelli 1B

This is because the E potential does not change. Its sign never changes, because potential doesn't change even if the reaction is reversed.

Sammy Thatipelli 1B

I do not think order matters as long as your reactants and products are on their respective sides.

Sammy Thatipelli 1B

Are there more rules in regards to when we use platinum in our cell diagrams? Other than the fact that we use it when the reaction has no conducting solids. Can someone list the rules/cases in which we include platinum in our diagrams?

Sammy Thatipelli 1B

you use S=klnW when you are trying to find the entropy when dealing with the fact that molecules can be rearranged in various ways. You will use the Boltzman's constant and then multiply that by the degeneracy, which show the multiple states the molecules can occupy/be in. This gives you the total d...

Sammy Thatipelli 1B

What is the property of enthalpy that allows us to use Hess's Law?

Sammy Thatipelli 1B

From the notes that I took, you can use Van't Hoff when you have two different temperatures and you are trying to calculate the K, accounting for the face that you need to subtract temperatures to find the deltaT.

Sammy Thatipelli 1B

You can balance the half reactions like you would normally when balancing equations, and you can check your answer by making sure the charges on either side of the reaction are the same

Sammy Thatipelli 1B

in G(products)-G(reactants) you are basing the values for G based on standard calculated values, not accounting for non-ideal temperatures and pressures. However, the equation deltaG=deltaH-TdeltaS accounts for the free energy with varying conditions.

Sammy Thatipelli 1B

Yes, that is true. The anode is always the oxidation half reaction and the cathode corresponds to the reduction half reaction.

Sammy Thatipelli 1B

Another questioned was asked about molar entropy, and I found this helpful: the word "molar" is clear to show that we need Avogadro's number in our equation, because it is the number of units in one mole of any substance. And because the molar entropy is the entropy of one mole of a substa...

Sammy Thatipelli 1B

I believe deltaS is referring to the reaction happening, so it would be delta S of the reaction. I guess if the "universe" being referred to is the entire system in our reaction of hot and cold water, where you are adding hot and cold water's entropies, then you would be finding the deltaS...

Sammy Thatipelli 1B

In question 1, the entropy is referring to that of the surroundings, which means that the rate of entropy generation will be a negative number. But in question 3, the entropy we are calculating is referring to the system's, and when we talk about deltaS of the system, the deltaS is positive

Sammy Thatipelli 1B

In combustion, the product water would always be liquid. We know this because the phase of water is based on the temperature by which the reaction is occurring. Reactions occurring at the standard room temperature, 25 degrees C, will produce water as a liquid. This is the case for the combustion rea...

Sammy Thatipelli 1B

Yes, you would because there are different equations for constant pressure or constant volume. If the case were constant pressure you would have to know that deltaH=Qp, so internal energy for example would be deltaU=deltaH. If the case were constant volume, you would use Q=deltaH-PdeltaV. These equa...

Sammy Thatipelli 1B

When using Hess's Law in this example, you actually don't need to end up with N2 in the final equation. You just need to cancel out the two NO2's because those are present in both equations. In the solutions manual, I believe they just show you that N2+5/2O2 gives you N2O5 just to show you a model o...

Sammy Thatipelli 1B

another helpful tip that i found was that in the book, like in number 67, you are asked to "estimate" the enthalpy of formation. I think that the key word "estimate" is a hint that you can use the method of bond enthalpies. If the question asks you for a more specific value and n...

Sammy Thatipelli 1B

This number comes from the fact that standard heat of atomization is the amount of heat energy required to produce one mole of gaseous atoms from the element in its standard state. because it is a single carbon and not part of the bond, we use the value for atomizing Carbon, which is a set value giv...

Sammy Thatipelli 1B

in the problems in 8.67, do we subtract the change in enthalpy based on the physical change of vaporization because we are moving from gas to a liquid product? i know that vaporization is moving from liquid to vapor, and that reaction needs heat so it is endothermic for example, if the problem had t...

Sammy Thatipelli 1B

When a gas or vapor is above its boiling point, it is said to be superheated. Supercooling is the melting of a solid above its freezing point which is very difficult because most times, the solid will almost always melt at the same temperature for a given pressure.

Sammy Thatipelli 1B

the equation for C at constant volume for a monatomic gas is C=3/2R, and R is the set value for this gas, 8.3145 J*K^-1*mol^-1. By multiplying this value by 3/2, you get the value for C as 12.5 *K^-1*mol^-1.

Sammy Thatipelli 1B

you can get the enthalipies for CO2 and H20 by looking at appendix 2A, they are given as -241.82 kJ/mol for water and -393.15 kJ/mol for CO2. These are the delta Hs for these specific compounds, but others are also listed in that section of the chem book. After getting these delta H values, you can ...

Sammy Thatipelli 1B

How do u know the reaction is the reverse reaction though?

Sammy Thatipelli 1B

In this question, My answer for x was -1.5 after doing the ice table and solving for x. However, x is negative and I was wondering what you need to do in this question to get a positive number?

Sammy Thatipelli 1B

Can someone explain what we are supposed to know about acid rain? Do we need to know what acids cause it, like H2SO4, H2CO3 and NO?

Sammy Thatipelli 1B

What happens when expansion increases on a system at equilibrium? Would the reaction shift right or left? I am confused as to how to understand how the system would respond to this change.

Sammy Thatipelli 1B

polarizing power has to do with describing cations and their ability to pull in electrons from ions (it is the electron pulling power of the ion). Smaller cations with a high charge are more likely to have high polarizing power when bonding with large anions because they want to give up their electr...

Sammy Thatipelli 1B

Why is it that for number 1 on test 4, there are no dipole moments for PO4^3- and PCl^5? I thought that there are dipole moments because the arrows point towards the negative oxygens (for PO4^3-) or negative chlorine molecules (PCl^5)? Can someone explain how to know if there is a dipole moment or n...

Sammy Thatipelli 1B

you do plus x if it is a product being formed, and you do minus x for the reactant (because you are losing the reactant as it is being decomposed to form the product)

Sammy Thatipelli 1B

On one of the chemical equilibrium models, there are questions that Dr. Lavelle wrote that ask you to convert concentrations to pressures and vice versa. Yes, you would use the equation PV=nrt, and know that concentration is equal to P/rt. I would definitely study this because it was one the module ...

Sammy Thatipelli 1B

Will LeChatlier's principle be on test 4? for example, do we need to know how equilibria responds to changes in conditions? This is shown in Part 4 Chemical Equilibrium's module

Sammy Thatipelli 1B

are solvents included in equilibrium concentrations? This was on the post assessment part b for the module, and I am not sure how to figure this out.

Sammy Thatipelli 1B

Why does a have the answer 1/PBCl3^2, instead of just writing PBCL3^2? Why is the inverse partial pressure the correct answer in this case? Because in b, the answer is just the two aqueous concentrations multiplied together, without any inverse equation. Can someone explain how to do part a in terms...

Sammy Thatipelli 1B

How do you know which flask represents equilibrium in this question? For equilibrium, does it occur when the number of double molecules equal the number of single molecules in the flask? I am having a hard time understanding how the diagram shows equilibrium.

Sammy Thatipelli 1B

For determining whether a molecule is polar or not, do we have to look at if the molecule has any lone pair electrons? Do lone pair electrons affect the dipole moment in any way?

Sammy Thatipelli 1B

an atom is trigonal planar if it has three regions of electron density and the angles are at 120 degrees evenly, like an equilateral triangle. it is t-shaped if you have three regions of electron density, but one of these regions consists of a lone pair of electrons. the lone pair electrons cause th...

Sammy Thatipelli 1B

another way that I remember it is that electronegative elements like in group 17 really want to have additional electrons to reach the noble gas configuration (reach the noble gas state). this is the opposite of group 1 because they form cations and want to lose electrons to reach the noble gas conf...

Sammy Thatipelli 1B

There is a special rule for the 3d orbital. the lowest energy state for the electron configuration is made when the d orbital has either 5 electrons or 10 electrons. This is why you would remove from the 4s orbital first if you had to on a homework problem. when there are 5 electrons in the d orbita...

Sammy Thatipelli 1B

No, they wouldn't have the notation of S2- or S5+. Dipole moments is the measure of net polarity in a molecule and the plus and minus signs are used to to point towards either the positively charged element or towards the negatively charged element. They occur when there is a separation of charge.

Sammy Thatipelli 1B

Bond order is found by counting the number of bonds. So if it is a triple bond, there are three bonds connecting the two elements together. The bond order of H2 is equal to 1 because to connect two hydrogens, you need a single bond.

Sammy Thatipelli 1B

From what he have been doing in lecture, this week homework in Outline 2 on the 14A website is based on Chapter 2, with the introduction to orbitals and periodic table trends. So if you are looking at Outline 2, it corresponds with Week 4 's lecture notes. Also, Test 3 next week does cover the rest ...

Sammy Thatipelli 1B

To be honest, I think its a rule that is just set and there is not much of a strict reasoning behind it. I think that in the case of sulfate, having the S in the center allows for symmetry for the Lewis structure, as it would be difficult is putting 4 oxygens in the center. So, symmetry is a valid r...

Sammy Thatipelli 1B

In the module, this question is being asked: Can molecular spectroscopy be used to identify molecules? If so why is it possible?

Can someone explain the answer for this question?

Sammy Thatipelli 1B

Energy can be a negative answer-this just means that the electron emits energy as it is going down an energy level. however, wavelength and frequency cannot be negative answers, so if, for example, you need to further in the problem find the wavelength after finding the energy, make the energy posit...

Sammy Thatipelli 1B

The photoelectric effect shows that light (electromagnetic radiation) acts as particles with particle-like properties. Therefore, d would not explain the photoelectric effect because d is an equation for wave-like properties, like wavelength.

Sammy Thatipelli 1B

I was wondering if anyone knew if there was a key available online to check the work for Test 1 Questions that we made mistakes on?

Sammy Thatipelli 1B

For m, the mass would be the mass of an electron. Therefore you would plus in 9.10939*10^-31 kg in the equation 1/2mv^2

Sammy Thatipelli 1B

To solve this problem, you need to use the equation lambda=h/(mv) for both the proton and the neutron. lambha proton=h/mv. lambdaproton=(6.63*10^-34 J*s)/(1.673*10^-27kg*2.75*10^5 m/s) lambda proton=1.44 pm lambda neutron=h/mv lambdaneutron=(6.63*10^-34 J*s)/(1.675*10^-27 kg*2.75*10^5 m/s) lambda ne...

Sammy Thatipelli 1B

It is not correct to write the chemical equation without identifying the states of matter because you need information on the states of matter to understand the equation fully. It is necessary to write the states of matter in parenthesis because it tells you what type of experiment is occurring (ex....

Sammy Thatipelli 1B

A formula unit is the group of ions that match the formula of the smallest unit of an ionic compound. When the question is asking for the number of formula units, the procedure is to first calculate the number of moles by dividing the mass of the sample given by the molar mass of the compound. Then,...

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