CHEMISTRY COMMUNITY

Lucian1F

For this question and problems like this where two rate constants at two different temperatures for the same reaction and the activation energy is given as well, use the Arrhenius equation to find the answer.

Lucian1F

The book changed the mmol to mol in this problem I believe.

Lucian1F

I don't remember him saying anything about dropping our lowest test. Maybe you're thinking of another class?

Lucian1F

The reaction the question asks you to calculate the standard potential for is actually Cr(s), not Cr3+. So, you need to use the equation you mentioned and also 2Cr3+ + 6e- -> 2Cr(s). However, the same species (Cr) is being reduced in both equations, and when the same species is involved in both step...

Lucian1F

You look for two reactions where there is a difference in concentrations for just one of the reactants, then use this and the change in the rate to get the order of that reactant. Repeat this to get the orders of the others reactants. Once you have all the orders, you can pick any trial you want, pl...

Lucian1F

Yep, both things you said are right. The standard reduction potential for Br2 is 1.07 V and for I2 it is 0.54V. So Br2 has a greater tendency to get reduced, meaning it has a lower reducing power since it is worse at reducing other species, and it has a stronger oxidizing power since it has a higher...

Lucian1F

Let's say we have a reaction A -> B. For zero order reactants, the rate at which the reaction occurs is not affected by the concentration of that reactant. So if [A] is 1.0M in one reaction but [A] is 2.0M in another reaction, the rate at which the reaction occurs will be the same for both concentra...

Lucian1F

Yep, it's just the sum of the individual orders

Lucian1F

The graph is plotting 1/concentration against time and since as time increases, the concentration of a reactant will decrease, but since the y axis is 1/concentration, the y value will increase as time goes on since the y value will increase as concentration decreases because it is in the denominator

Lucian1F

We don't need to know negative or fractional orders but if you know the orders of all the reactants you should know that the overall order is just the sum of those individual orders

Lucian1F

If temperature isn't stated then you can assume a reaction is at standard temperature (25 Celsius)

Lucian1F

H2 and Cl2 are on different sides of the salt bridge but they are both products. The cell reaction is 2HCl -> H2 +Cl2

Lucian1F

The oxygen that is bonded to the bromine got reduced to -2

Lucian1F

If the equation is flipped then the sign of the voltage is switched too, but the reaction will be the same with reactants and products on opposite sides

Lucian1F

For a constant volume, not pressure, the work is 0 so U=q=H. The example you stated says U=q at constant volume

Lucian1F

You need to add the change in entropy due to the change in volume to the change in entropy due to the change in temperature. So basically, you do two different equations and add them together. I think it is assumed that the pressure is 1 atm, so you can calculate n with PV=nRT

Lucian1F

Nope it doesn't matter because the ratio will be the same so Q/K will be the same regardless of units of pressure

Lucian1F

If you find it where G=0, you can figure out exactly what the temperature needs to be greater than or less than depending on the entropy for the reaction to be spontaneous

Lucian1F

Your method was totally fine, but just be careful of what the question wants you to do. It may ask for you to do the problem a specific way, but if it doesn't the way you did it was perfectly fine and will get the same result as the other way for other questions

Lucian1F

The value of the specific heat itself is intrinsic because it will always be the same for some molecule at some phase. The specific heat does not change as mass does, the heat/energy does

Lucian1F

For ratios the units do not matter as long as they are the same for pressure and volume. With temperature it is different since you are adding a factor of 273 to turn celsius into kelvin but for pressure and volume, converting between units is multiplication or division, so the ratio will always be ...

Lucian1F

You need to set the heat gained by the ice equal to the heat lost by the tea and dont forget to include the heat of fusion for the ice tho.

Lucian1F

If you remember the curve on a PV graph for a reversible pathway, it curves downward and the area under this curve is the work. The curve looks like this: https://upload.wikimedia.org/wikipedia/ ... m_work.png

Lucian1F

The heat of the system doesnt change since the surroundings keep it stable and the heat of the surroundings doesnt change because it is a reservoir

Lucian1F

If you imagine a piston thats controlled by a weight and that weight is slowly removed, then the system will slowly expand and do work

Lucian1F

Can someone help me figure out how to do 9.65? It asks "Which of the following compounds become less stable with respect to the elements as the temperature is raised: a. PCl5 (g) b. HCN (g) c. NO (g) d. SO2 (g)"

Lucian1F

I believe it's saying that there are 1 mol of atoms in a, b, and c, but in b and c those atoms bind together to form diatomic molecules so then there are .5 moles of particles for b and c so a has the most particles.

Lucian1F

If it was not irreversibly, some of the equations we use would not work because irreversible reactions occur at constant temperature but if it was not irreversible there would be a change in temperature

Lucian1F

If the volume is expanding then the system is doing work

Lucian1F

Yes they are the same just with S replacing H

Lucian1F

We haven't gone over how to calculate whether or not a reaction is spontaneous. The examples Dr. Lavelle gave in lecture were obvious examples like how gas would spontaneously expand in its container and it would never spontaneously bunch up in its container, but for more ambiguous processes there a...

Lucian1F

I highly doubt we will need to show the derivations. Dr. Lavelle just showed us the derivations so we understand where the formulas came from so we wouldn't blindly memorize these formulas without knowing where they came from

Lucian1F

Since the system is at a constant volume, the work is 0, so the change in internal energy is equal to heat, so delta U = q.

Lucian1F

We didn't cover the concepts of this question in class so you don't need to know how to do it. 71 isn't listed as one of the homework problems to choose from on the syllabus. If you want to figure it out for your own sake no one's stopping you, just read the section that covers it in the book!

Lucian1F

The question gives you heat capacity, not molar heat capacity. When you are given molar heat capacity, you need to multiply by the number of moles so the units cancel to leave units of energy, but with heat capacity all you need to do is multiply it by the change in temperature to get units of energ...

Lucian1F

Positional entropy is related to how many positions an object, like an atom, can be in.

Lucian1F

It will probably be stated in the question and only one of these will probably be given to you to solve a problem. If both are given, you may need to combine the two methods or choose to do either method if the problem can be solved like that

Lucian1F

The temperature of the water is higher than 0 degrees Celsius so you need to take the amount of energy it takes to melt the given amount of ice first and consider that in your calculations.

Lucian1F

It may not be needed for anything in 14B but for 14BL there are acid/base labs

Lucian1F

Definitely draw out the lewis structures! It will help tremendously and if you dont it is very likely you will miss at least one bond.

Lucian1F

Remember that the periodic table is split into metals and nonmetals and that metals are typically solids at standard conditions and nonmetals are typically gases. Also, remember that mercury and bromine are liquids at standard conditions!

Lucian1F

For me I think about how they sound. The "en" in endothermic sounds like in and the "ex" in exothermic reminds me of exit.

Lucian1F

It may also be helpful to know that fusion is another term that is the same phase change as melting (solid to liquid)

Lucian1F

Their equilibrium constants are basically infinity since there are hardly any reactants, so no

Lucian1F

When the hydrogen is removed from boric acid, the oxygens will experience a more negative charge. Boron has a low electronegativity, so it cannot help stabilize the anion

Lucian1F

The strengths for these acids depend on different factors. Since the H-F bond is shorter than the H-Cl bond, HF is less likely to dissociate than HCl, so it is a weaker acid. However, in the case of C2O2HF3 and C2O2HCl3, the strength of the acid depends on electronegativity. Since F is more electron...

Lucian1F

Did you make sure to omit the solid molecule from the calculations? Solids are not included in equilibrium constant equations

Lucian1F

It solely relies on the amount of areas of electron density, including the lone pairs

Lucian1F

The ICE box is used when the problem involves using both equilibrium and initial concentrations of the products and reactants

Lucian1F

The equilibrium constant does not change with a change in pressure

Lucian1F

The chemical equilibrium constant Kc=[Br]^2/[Br2]. So you take the square root of the product of Kc and Br2 to get the answer, which is C

Lucian1F

If there are both gases and aqueous molecules in a reaction then you will need to use PV=nRT to find the concentration of the gases, then find Kc.

Lucian1F

You need to take into account the molecular shapes. It needs to be possible for multiple electron pairs to bind to the transition metal with the molecular shape of the ligand

Lucian1F

The molecule's shape matters in this case. It will be trigonal planar around each carbon, so only two oxygen atoms will be able to bond to the metal at a time.

Lucian1F

There is no such thing as a double sigma bond. A double bond consists of one sigma bond and one pi bond. A triple bond consists of one sigma bond and two pi bonds.

Lucian1F

The bond angle for the molecule with 2 bonds and 1 lone pair would be greater than the bond angle for the molecule with 2 bonds and 2 lone pairs since more lone pairs means more repulsion

Lucian1F

The superscript is showing how many p-orbitals have been hybridized. So in sp, one p-orbital is hybridized, sp^2 has two hybridized, and sp^3 has three hybridized.

Lucian1F

In the best lewis structure, the formal charge for every atom in the molecule, not just the central atom, will be as close to 0 as possible

Lucian1F

I think what you're getting at is if there is a negative formal charge, then it should go on the most electronegative atom. This happens since the more electronegative atom will draw in electrons stronger, so it would be more stable for the more electronegative atom to have more electrons resulting ...

Lucian1F

For the See-saw vsepr shape is the bond angle between the two equatorial atoms 120 or less than 120?

Lucian1F

If there were 3 double bonds then nitrogen would have more than an octet (it would have 12 electrons in its valence shell) but nitrogen is only in the second period so it cannot have more than 8 valence electrons. Also, this would make the formal charges of nitrogen and oxygen much farther from 0, w...

Lucian1F

Yes, this is correct

Lucian1F

Will we need to know what the Lyman and Balmer series are for the midterm? Thanks!

Lucian1F

For molecules with many carbon atoms, you probably need to make a carbon ring. In this case, make a carbon ring, then put oxygens on carbons opposite from each other, and put the one hydrogen on the other carbons. The carbon-oxygen bonds will be double bonds, and the bonds between carbons that are b...

Lucian1F

We don't need to know how to calculate a dipole moment. We only need to know what it is and determine if there is one

Lucian1F

This molecule, and all molecules with a square planar shape are rather rare. You don't need to know the element as long as you know how to draw the lewis structure. Once you have the lewis structure down, you can see the regions of electron density and see if they are lone pairs or bonds, then this ...

Lucian1F

As you go down the halogen group, they decrease in ionization energy and electron affinity, so they decrease in electronegativity as well. Therefore, the difference in electronegativity between silver and the halide will decrease as you go down the halogen group. So, since an large difference is cha...

Lucian1F

The trends are just general trends. In general, the ionization energies of elements will decrease down the periodic table and increase across. However, there are multiple exceptions. These trends are just in general, they do not apply to every single case. We didn't go over specific exceptions to th...

Lucian1F

I think we will be given electronegativity values of each element, then using these values, we find the differences in electronegativities between the two elements bonded. If the difference is greater than 2, the bond is ionic. If the difference is less than 1.5, it is covalent. There are rough esti...

Lucian1F

When we were doing those graph/chart things with quantum mechanics and exciting electrons to higher energy levels, as we increased the energy level, the differences in energy between them kept decreasing. So as you get higher and higher, the differences between energy levels and subshells become les...

Lucian1F

Yes, any atom that has a principle quantum number of n=4 or greater can have electrons in the f subshell

Lucian1F

For the d sublevel, you need to subtract one from the principle quantum number, so it would be 5d. For the f sublevel, you need to subtract two form the principle quantum number, so it would be 4f.

Lucian1F

The second ionization energy is the energy required to take the next electron after an electron is already taken from an atom. They have numerical values, but I don't think those can be determined with anything we've learned. To determine whether it is higher than lower or something, treat it like y...

Lucian1F

A covalent bond is between nonpolar, or atoms with little charge. An ion is between two ions which have very high charge in comparison. Elements want a full octet, or achieve the noble gas configuration, because this configuration is the most stable. What holds the two atoms together in an ionic bon...

Lucian1F

I'm pretty sure all you have to know is that energy levels n=4 and after have f-orbitals, the f-orbitals are l=3, and ml= -3,-2,-1, 0, 1, 2, 3.

Lucian1F

The work function is typically related to the photoelectric effect where photons hit a solid metal and if the energy of a photon is higher than the work function, then the electrons are ejected from the metal. For ionization energy, the energy is related to the energy required to remove an electron ...

Lucian1F

The values of the lines will be the same since the energy levels for each element are constant. The lines only change as you change elements. The difference is in what is measured. If the wavelengths/frequencies of light that are being released by the atom are measured, it results in an emission spe...

Lucian1F

If you go to the website and you go to lecture outlines, it can be found in outline 2. It is: Read: Ch 1 Do Problems: 3, 5, 7, 9, 11, 13, 15, 21, 23, 25, 27, 33, 37, 39, 41, 43, 55, 57, 59, 65, 67, 69 Read: Ch 2 (Omit Table 2.1) Do Problems: 1, 13, 17, 19, 21, 23, 25, 27, 29, 31, 33, 37, 39, 41, 43,...

Lucian1F

I highly doubt that we need to memorize those series since they aren't in the textbook and I believe they haven't been covered in class either.

Lucian1F

You don't need to memorize the exact values for each type of electromagnetic radiation. However, we do need to know that visible light is around 400-700 nanometers, and having rough estimates of the other electromagnetic waves would probably be a good idea.

Lucian1F

Calculate the wavelength of a 275 kg single-seat electric car traveling at a speed of 125 km.hr-1. Do electric cars have wavelike properties? A. λ = 6.94 x 10-38 m , Yes B. λ = 6.94 x 10-38 m , No C. λ = 1.93 x 10-38 m , Yes D. λ = 1.93 x 10-38 m , No The correct answer is B. The wavelength is so sm...

Lucian1F

It is not required to memorize the exact values for each type of electromagnetic radiation, but you should know that visible light is around 400-700 nanometers. It would probably be a good idea to have some approximation of each type memorized, but Dr. Lavelle said that we did not need to memorize t...

Lucian1F

Alright, thank you!

Lucian1F

"When an electron beam strikes a block of copper, x-rays with a frequency of 2.0 x 10^18 Hz are emitted. What is the wavelength (in picometers) of these x-rays?" I got an answer of 150 pm but the back of the book says 150 nm. I'm pretty sure this is just a typo, especially since the questi...

Lucian1F

In this case it is possible to use the MiVi = MfVf equation. Mi = 9.85M and Vi = .075L and Vf = .125L. So, if you multiply Mi by Vi then divide this product by Vf you end up with 5.91M. Another way you could solve this problem is simply converting the grams into moles, which would be .739 mol, then ...

Lucian1F

You first need to calculate the amount of moles for each compound by dividing the given masses by the molar masses resulting in .00671 mol KCl, .00453 mol K2S, and .00236 mol K3PO4. Therefore, there are .00671 mol K in KCl, .00906 mol K in K2S, and .00472 mol K in K3PO4 for .0205 mol K total and .00...

Lucian1F

There are 2 sig figs in 460. In this case, the 4 and the 6 are significant. All nonzero numbers are significant, so in 123.45, there are five sig figs. Also, all zeros in front of a number without a decimal point in front of them are insignificant, so in 00460, there would still be just two sig figs...

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