CHEMISTRY COMMUNITY

Dhwani Krishnan 1G

During the test of an internal combustion engine, 3.00 L of nitrogen gas at 18.5 C was compressed suddenly (and irreversibly) to 0.500 L by driving in a piston. In the process, the temperature of the gas increased to 28.1 C. Assume ideal behavior. What is the change in entropy of the gas? How can yo...

Dhwani Krishnan 1G

it is the frequency factor, and basically gives the frequency with which reactants will collide in the correct orientation in order to form product.

Dhwani Krishnan 1G

I don't think so, but on previous exams/midterm sig figs weren't counted for every problem, only for 1-2
you always need the correct units though!

Dhwani Krishnan 1G

you can use it to determine how the overall rate will change with a change in reactant concentration

Dhwani Krishnan 1G

They don't affect the rate law itself, but do effect the value of k as you need to know the ratio of moles produced/used during the reaction to find k.

Dhwani Krishnan 1G

For the most part, yes. The questions should say "assume ideal behavior."

Dhwani Krishnan 1G

the units on the rate constant k must ensure that the overall units for the rate is (g/mol)/s.

Dhwani Krishnan 1G

do both yield the same results?

Dhwani Krishnan 1G

also, log is used when dealing with pH

Dhwani Krishnan 1G

No. Ecell = Ecath - Eanode, where the E values are for the reduction reaction (so you don't make anything negative), which is the method used in the book. Ecell = Ecath + Ean when you add the actual potentials for each half-reaction. Either method will yield the same answer.

Dhwani Krishnan 1G

Enthalpy doesn't really show spontaneity, except that exothermic reactions are generally spontaneous because delta S is usually positive for these reactions.

A positive entropy generally indicates a spontaneous process, but only with a negative delta G can you be sure.

Dhwani Krishnan 1G

Hydrogen has only 1 valence electron. When combined with a less-electronegative element (eg. a metal), it gains another to complete its p-orbital, giving it an oxidation state of -1. In other cases, it loses its electron, giving it an oxidation state of +1. Since there is only 1 electron to begin wi...

Dhwani Krishnan 1G

First, write out the half-reactions and balance each as you normally would. Then, balance it so the charges on both sides add up to the same number.

Dhwani Krishnan 1G

Can a nonspontaneous process with a positive deltaS become spontaneous if the temperature is increased (assuming that deltaH and deltaS are both independent of temperature)?

Dhwani Krishnan 1G

what is the difference between a salt bridge and porous disk?

Dhwani Krishnan 1G

what is the difference between an anode and cathode?

Dhwani Krishnan 1G

Why would you separate the equation? Like how would this make it easier?

Dhwani Krishnan 1G

enthalpies of formation are always given /mol. As for reaction enthalpies, I believe that they're generally given in just kJ because it's for the entire reaction (so there are multiple moles of substances used).

Dhwani Krishnan 1G

More and more energy (heat) is required to attain higher energy phases because you have to separate the molecules more. in terms of energy, solids < liquids < gases. Therefore, the enthalpy of fusion is less than that of vaporization.

Dhwani Krishnan 1G

The standard enthalpy of formation for N2 is 0! All elements in their standard states (N2, O2, etc.) have a standard enthalpy of formation of 0

Dhwani Krishnan 1G

why does work being done correlate to heat being added?

Dhwani Krishnan 1G

What is the difference between $\Delta H$ and q? How are they related?

Dhwani Krishnan 1G

Delta U is the change in internal energy of a system. The equation states that Delta U is equal to the heat lost/gained (q) plus the work done on or by the system (w). This is because the only way for a system to lose internal energy is through heat or work.

Dhwani Krishnan 1G

You could use Pka and an ICE table if necessary to find [H3O+].

Dhwani Krishnan 1G

The energy to boil water is (energy to melt) + (energy to boil).
The energy to have steam is (energy to boil) + (energy to vaporize)

Therefore, more energy (and thus, more heat) is required to form steam, which is why the burns are more severe.

Dhwani Krishnan 1G

Dr. Lavelle said in class that those values will be given.

Dhwani Krishnan 1G

What exactly do we need to know about them?

Dhwani Krishnan 1G

If all the reactants/products are gases, it doesn't matter which you use. However, Kc and Kp are not always equal! This is due to concentration and Pressure being related via the Ideal Gas Law (PV = nRT) This link explains the technicalities of it well: https://chem.libretexts.org/Bookshelves/Physic...

Dhwani Krishnan 1G

Buffer solutions are used to keep the pH constant. To my understanding, you don't "ignore" the salt, but because it is a buffer, it doesn't affect pH / affect [H3O+] and factor into the ICE table.

Dhwani Krishnan 1G

For A\rightleftharpoons B + C To make an ICE table, first enter all the numbers you are given into the table, and add in 0 in the "Initial" column if they're not given and there's no way to calculate it (you would be able to calculate it if you have Q, for example). If you're given an equi...

Dhwani Krishnan 1G

What are the conditions for an Ideal Gas in terms of Temperature/Pressure/Volume etc?

Dhwani Krishnan 1G

According to the Ideal Gas Law, P = nRT/V. Therefore, P and T are directly proportional. My question is, conceptually, how/why does an increase in Pressure cause and increase in Temperature and vice-versa?

Dhwani Krishnan 1G

By that same logic, would HClO2 be weaker than HClO3?

Dhwani Krishnan 1G

It can be assumed that given a strong acid, it completely dissociates in solution, and therefore the pH is -log[H3O+] (we don't have to worry about the Ka values or anything like that).

Dhwani Krishnan 1G

Yes -an amphoteric substance like water would act like a base or an acid depending on what it is reacting with.

Dhwani Krishnan 1G

Is there a way to tell denticity of a ligand from its lewis structure or VSEPR shape? How do you find denticity otherwise?

Dhwani Krishnan 1G

Do all bases neutralize acids and vice-versa? Or does the strength of the acid/base affect how they react?

Dhwani Krishnan 1G

Does water have a neutral pH because it can act as an acid or base depending on the situation?

Dhwani Krishnan 1G

But both Bronsted and Lewis acids dissociate into Hydronium ions in water, correct?

Dhwani Krishnan 1G

Are all Lewis acids bronsted acids and vice-versa? Is the same true for Bases?

Dhwani Krishnan 1G

it wouldn't make sense for a Nitrogen lone pair to act as an acid, as acids are e- pair acceptors by definition, not donors.

Dhwani Krishnan 1G

Probably the ones mentioned in class -- Carboxylic as the most common Organic acid, HBr/HCl/etc as strong acids, and CH3COOH (acetic) as an example of a weak acid
For bases, you should probably know the Alkali/Alkaline Earth metals as strong bases and NH3/Ammines as weak bases

Dhwani Krishnan 1G

Because of electron repulsion, to maximize distance the orbitals end up "pointing to the corners."

Dhwani Krishnan 1G

there are 3 p-orbitals, each holding up to 2e-, for a total of 6 electrons, not 6 orbitals.

Dhwani Krishnan 1G

The question reads, "Noting that the bond angle of an sp3 hybridized atom is 109.5 and that of an sp2 hybridized atom is 120, do you expect the bond angle between two hybrid orbitals to increase or decrease as the s-character of the hybrids is increased?" What is the s-character of a hybri...

Dhwani Krishnan 1G

I don't think so. By definition, single bonds are generally longer than double bonds, especially between the same elements.

Dhwani Krishnan 1G

What is the difference between these two terms, and what they have to do with electronegativity?

Dhwani Krishnan 1G

Why isn't H2O a linear molecule, but CO2 (for example) is? How do we know the bond angle isn't 180?

Dhwani Krishnan 1G

I'm still a little confused on how to find resonance structures. How can you tell how many resonance structures a molecule has / which structure is the most stable?

Dhwani Krishnan 1G

When atoms covalently bonded unequally share electrons (occurs in any bond with atoms of different elements), the more electronegative atom "pulls" the shared e- towards them, giving that side of the molecule a slight negative charge. Similarly, the less electronegative atom ends up with a...

Dhwani Krishnan 1G

I understand why Chromium and Copper don't follow the electron configuration pattern, but my question is, why do Vanadium and Nickel? Their electron configurations are [Ar]3d3 4s2 and [Ar]3d8 4s2 respectively, but wouldn't [Ar]3d5 and [Ar]3d10 be more stable? Or is it because they need electrons in ...

Dhwani Krishnan 1G

In Class, Dr. Lavelle mentioned that you should not draw 2 cations next to each other when drawing the lewis dot structure of an ionic compound. Why is that?

Dhwani Krishnan 1G

Why do they have a tendency to form multiple ions? Also, are elements that form ions w/ variable valence all transition metals?

Dhwani Krishnan 1G

I'm still confused as to why atomic radius decreases across a period. Shouldn't more electrons being added to new orbitals increase the size?

Dhwani Krishnan 1G

How was the fact that long wavelengths, regardless of intensity, could not eject an e-, but shorter wavelengths could used to determine that light can be quantized as photons? What is the reasoning behind this conclusion?

Dhwani Krishnan 1G

So to calculate deltaE from, for example, the second to the first energy level, it would be E(subscript)2 - E1?

Dhwani Krishnan 1G

Do emission and absorption spectra look the same? Or are they "opposites" of each other?

Dhwani Krishnan 1G

the speed of light changes with the medium, in air it is 3.0x10^8m/s.

Dhwani Krishnan 1G

For the Balmer series, the lowest energy level (the final energy level) is n=2. The electron is falling from a higher energy level to n=2, but since it hasn't reached ground state, it is still excited. Energy is still given off.

Dhwani Krishnan 1G

Do all dark bands correlate to destructive interference/light bands correlate to constructive interference? Why does this happen?

Dhwani Krishnan 1G

I also have a question: when you have a reaction in the form of MgSO4.7H2O where the 7 is in front of the H2O, would you have to do 7x(molar mass of H2) + the molar mass of O or 7x(the molar mass of H20)? Because 2 hydrogens are Bonded to 1 oxygen (to form water), the "7" is the coefficie...

Dhwani Krishnan 1G

I don't think it would be continuous. Because only 1 molecule is entering at a time, the amount of water is so small that the scale (as Leela said) would have "jumps" as it recalibrates. Another analogy would be when you don't close the sink tap fully, and one droplet at a time comes out -...

Dhwani Krishnan 1G

Are salts the only compounds for which you use Formula Units as opposed to Molecule? Or is any compound with 2 different elements a Formula Unit?

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