CHEMISTRY COMMUNITY

Lily Guo 1D

Would you ever include H2O in a cell diagram if it were part of a half reaction? For example, how would you write this reduction half reaction in a cell diagram: NO3- + 2H + e - ==> NO2 + H2O?

Lily Guo 1D

When do you need an inert electrode? If you have an inert electrode, do you put it on both sides of a cell diagram or just one?

Lily Guo 1D

"If SO2F2 adopts a positionally disordered arrangement in its crystal form, what might its residual molar entropy be?" I understand how to do the math for this, but whenever I input the numbers in my calculator: 1.38 x 10^-23 ln(6^6.02 x 10^23), I get an "overflow error". Is this...

Lily Guo 1D

The higher the speed, the more likely particles are to collide and break/form bonds. Activation energy is the energy required to break/form bonds, so the higher the activation energy, the more difficult it will be to break/form bonds because the reaction requires a large input of energy before the p...

Lily Guo 1D

Yes, just plug your given values into the equations and solve!

Lily Guo 1D

K = k/k' where k' is the rate constant for a reaction's reverse form.

Lily Guo 1D

How do we know if something is a pseudo-first-order reaction? Or would that be specified?

Lily Guo 1D

"15.99 Which of the following plots will be linear? (a) [A] against time for a reaction that is first order in A; (b) [A] against time for a reaction that is zero order in A; (c) ln[A] against time for a reaction that is first order in A; (d) 1/[A] against time for a reaction that is second ord...

Lily Guo 1D · Homework for Test 3 What homework problems cover 15.1-15.6 for Test 3?

What homework problems cover 15.1-15.6 for Test 3?

Lily Guo 1D

I think that usually cell diagrams are written in the order of oxidation | reduction, although the solutions manual sometimes does have it in a different order. I would stick with oxidation | reduction because that's what we've mostly seen in lecture.

Lily Guo 1D

The equation that the book gives for the integrated rate law of first-order reactions is [A] t = [A] 0 e -kt , but the equation for the integrated rate law of first-order reactions given in lecture is ln[A] = -kt + ln[A] 0 . Does it matter which equation we use since both are essentially the same? I...

Lily Guo 1D

"A 1.0 m NiSO4(aq) solution was electrolyzed by using inert electrodes. Write (a) the cathode reaction; (b) the anode reaction. (c) With no overpotential or passivity at the electrodes, what is the minimum potential that must be supplied to the cell for the onset of electrolysis?" The solu...

Lily Guo 1D

Does it matter what order the items in the anode side and cathode side of a cell diagram are written? For example, if you have Cl2 (g) and HCl (aq) on the anode side, would it make a difference if you wrote Cl2 (g) | HCl (aq) or if you wrote HCl (aq) | Cl2 (g) for the anode?

Lily Guo 1D

I'm a little confused about how to calculate Q. I thought that the expression for Q was [product A]^a/[reactant B]^b using either the concentrations of the reactant and product or the partial pressures of the reactant and product. However, when doing 14.37a and c, the solutions manual combines the c...

Lily Guo 1D

Is it possible to find delta H for an isothermal reversible expansion?

Lily Guo 1D

The equation for an ideal gas is U=3/2*nRT. When you're calculating the change in internal energy, the number of moles and the gas constant are constant values and that means the change in internal energy is dependent upon temperature change. But in an isothermal expansion temperature doesn't chang...

Lily Guo 1D

Does delta G = 0 for phase changes at the proper melting/boiling points? For example, homework problem 9.91b says that the standard Gibbs free energy for the vaporization of water at 100°C is 0. Why is this? Does this apply for the fusion/vaporization of other substances at their melting/boiling poi...

Lily Guo 1D

"Which is the thermodynamically more stable iron oxide in air, Fe3O4(s) or Fe2O3(s)? Justify your selection." The solutions manual says to look at the reaction converting Fe3O4 to Fe2O3, calculate the delta G for that, and then based on the delta G/spontaneity of the reaction, you'll know ...

Lily Guo 1D

Why does delta U = 0 for the isothermal expansion of an ideal gas?

Lily Guo 1D

There is no temperature change because energy from heat transfer is equal to energy from work. If they weren't equal, then the energy change from the heat transfer would result in a temperature change. However, because they are equal, the energy supplied by work makes up for that would-be temperatur...

Lily Guo 1D

The rule that I usually follow is to use kJ for standard delta G of a whole reaction because the products aren't necessarily only present in 1 mole. Therefore, you can't say that standard delta G of a reaction is necessarily going to be kJ/mole; all that "standard delta G" implies is that ...

Lily Guo 1D

Trouton's law says that the standard entropy of vaporization are commonly 85 J/(K*mol). It talks about entropy, not enthalpy. The question, on the other hand, asks you to solve for the enthalpy of vaporization of bromine liquid. The question asks you to apply the assumption that the entropy of vapo...

Lily Guo 1D

Example 9.10 says, "Calculate the change in entropy of the surroundings when mercury freezes at 49 C; use Hfus(Hg) 2.292 kJ mol 1 at 49 C." It then goes on to solve the problem and sets T as (-49 + 273) K = 224 K. Are you just supposed to assume that the initial temperature is 0 C aka 273 ...

Lily Guo 1D

I'm a little confused about how to apply Trouton's Law. For example, self-test 9.7A says "Use Trouton’s rule to estimate the standard enthalpy of vaporization of liquid bromine, which boils at 59 C." I thought that Trouton's Law said that most liquids have a standard enthalpy of vaporizati...

Lily Guo 1D

I think the problems would say if they involve reversible or irreversible expansions.

Lily Guo 1D

Reversible expansion is expansion that occurs as a result of infinitesimally small steps that can push the expansion either forward or backwards with the smallest of changes. This results in a maximum of work being done. In contrast, irreversible expansion is expansion that occurs as result of finit...

Lily Guo 1D

If the universe is constantly expanding, then entropy is also increasing with it.

Lily Guo 1D

Not necessarily, but delta G = 0 does mean that the reaction is at equilibrium.

Lily Guo 1D

delta H (fusion) of ice would have been given

Lily Guo 1D

I think that positional disorder has to do with how much space particles have to move around in. The greater the space, the greater the positional entropy because the more particles can move around. Thermal disorder has to do with temperature, so the higher the temperature, the greater the particle ...

Lily Guo 1D

If an equation is isothermal, that means there is no heat transfer and therefore no change in temperature.

Lily Guo 1D

The units are J/K (Joules per Kelvin)!

Lily Guo 1D

It might also be easier to think about this in terms of PV = PV. Pressure and volume are inversely related. Therefore, the bigger the pressure (in this case, 2 atm instead of 1 atm), the smaller the volume (the volume for 2 atm would be half that at 1 atm).

Lily Guo 1D

My TA once said to assume that you're under standard conditions (298K and 1 atm) if the textbook doesn't say what temperature/pressure you're at, so I think you'd just assume that you're at standard state.

Lily Guo 1D

The heat capacities for ice and water are different. Therefore, you would use the heat capacity of ice for ice and the heat capacity of water for water.

Lily Guo 1D

If there is no change in the number of moles (no delta n), then that means that there is no change in volume (no delta V). Because w = -P(delta)V, if there is no delta V then there is no work done!

Lily Guo 1D

But that ultimately give you a negative work, whereas work done ON a system should be positive??? I just realized my mistake. Work done ON a system is positive, but EXPANSION work means that work is done on the surroundings/BY the system because the system is expanding. Therefore, work would be neg...

Lily Guo 1D

For 8.47, why does the solutions manual use the equation (delta)H = U + P(delta)V instead of just U = q+ w? I'm also confused as to why work would be positive instead of negative because I thought that expansion work done ON a system is negative.

Lily Guo 1D

Does the equation (change in)U = q + w assume that there is constant pressure? Or does the pressure not have to be constant?

Lily Guo 1D

Is the only difference between reversible and irreversible processes the fact that reversible processes are isothermal?

Lily Guo 1D

I'm not entirely sure, but I think that the 44.0 kJ/mol is just a value that was experimentally determined. I think that we would be given that in a table of H values or something if we needed it on a test (don't quote me on this though). The values given -- Hvap 40.7 kJ/mol at 100 degrees Celsius a...

Lily Guo 1D

I know that the standard enthalpy of formation of an element in its most stable form is zero, but does that mean that the element begins in an unstable form and goes to its stable form, ex: N2 (l) ==> N2 (g)? Or would the element begin in its most stable form and then stay in its most stable form, e...

Lily Guo 1D

Is Ka the same thing as [H3O+]?

Lily Guo 1D

You use the equilibrium arrow for weak acids and bases because they don't fully dissociate. Strong acids and bases completely dissociate, so you would use a regular arrow for equations with strong acids/bases.

Lily Guo 1D

I'm not sure if you would get full credit, but try to round as little as possible in the course of solving the problem until you get to the very end so that your answer is as correct as possible.

Lily Guo 1D

OH2 is written with the O first because the O is the atom with which bonds are formed.

Lily Guo 1D

Q is the reaction quotient, while K is the equilibrium quotient. They both follow the same expression -- the product of the concentrations or partial pressures of each product raised to the power of its stoichiometric coefficient over the product of the concentrations or partial pressures of each re...

Lily Guo 1D

Partial pressure units should usually be atm. However, sometimes other units may be used, so be careful to convert then!

Lily Guo 1D

NH3 is polar because it has 3 dipoles that do not cancel out. Each N-H bond is polar because N is more electronegative than H. NH3 is overall asymmetrical in its VSEPR shape, so the dipoles don't cancel out and it is therefore polar.

Lily Guo 1D

Yes, it does!

Lily Guo 1D

Px, Py, and Pz are simply the three orbitals in the P subshell (the three horizontal lines we draw). You don't necessarily need to write them out; usually we would just write P2, P3, etc. P(however many electrons are in the P subshell).

Lily Guo 1D

When you're calculating the frequency emitted from a jump in principal energy levels (i.e. n=1 to n=3), how would you determine the significant figures needed for the answer?

Lily Guo 1D

Would we be expected to know how to draw the Lewis structure for a lot of organic molecules on the midterm? Or would we mainly be drawing Lewis structures for inorganic compounds?

Lily Guo 1D

A metal and a non-metal will typically form an ionic bond, whereas non-metals bonding together will typically form a covalent bond!

Lily Guo 1D

When the textbook refers to "Group 5 Transition Metals", does that mean Group 5 where Group 1 is the alkali metals? Or does it mean Group 5 where Group 1 is the first column of transition metals (Sc, Y, etc.)?

Lily Guo 1D

Did you know that you can cool yourself to -273.15˚C and still be 0k?

Lily Guo 1D

If the electron is going from n=4 to n=2, then n=2 would be the final energy state and n=4 would be the initial energy state.

Lily Guo 1D

In class, Professor Lavelle mentioned that the De Broglie equation can only be used for particles with rest mass, like electrons, and therefore can't be used for light. Are there any other equations that can only be used for particles with rest mass or that can only be used for light?

Lily Guo 1D

Thompson W 3B wrote:I believe the conversion is given on a sheet of constants and equations, which is attached to the tests. Here is the sheet: https://lavelle.chem.ucla.edu/wp-conten ... ations.pdf

Thank you so much! I didn't see that earlier.

Lily Guo 1D

Problem 1.23 gives the energy of gamma ray photons in kiloelectronvolts. Do we have to know the conversion between KeV and joules or do you guys think that energy will be given to us in J?

Lily Guo 1D

2.998 x 10^8 m/s is more accurate, but Professor Lavelle uses 3.00 x 10^8 in his lecture slides and example problems, so I don't think using 3.00 x 10^8 would be an issue.

Lily Guo 1D

I think your answer is like correct, meaning approx. close to the answers given b/c when you round the answer you got and change it to scientific notation, you should get 1.00 * 10^5 m/s and since it's way less than speed of light, which if the fastest speed known so far, answer should be reasonabl...

Lily Guo 1D

No, but sometimes starting with the least-occurring element when balancing a chemical reaction gives you an easier starting place for balancing a reaction.

Lily Guo 1D

Keep the molar mass as precise as possible while calculating. You should try to round as little as possible until you reach your final answer! Then you should be fine.

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