CHEMISTRY COMMUNITY

Megan Cao 1I

If I could like this post, I would. Thank you!! And thank you for your enthusiasm and excellence as a professor! You’ve help create a strong foundation of chemistry for my future studies, which I am beyond thankful for.

Megan Cao 1I

k' is the rate constant of the reverse reaction.

lowercase k is for the rate constant of the rate law
UPPERCASE K is for the equilibrium constant of reactions

Megan Cao 1I

yes; as the reaction occurs, the concentrations of the reactant(s) will decrease as the concentration of the products increase

Megan Cao 1I

to calculate the overall order of a reaction you would add the exponents of the reactants.

for example, if the rate law =k[A]²[B]¹, then the overall order would be 3 (2+1)

Megan Cao 1I

you can tell by seeing that a change in the concentration of the reactant has no affect on the rate law. this means the reactant is to the zeroth order and is independent of the rate law.

Megan Cao 1I

Usually the UA's have a midterm/final review, but there seem to not have been signs of so. Not that I should entirely rely on the UA's support, but does anyone know if there will be a final review session?

Megan Cao 1I

you would use the Arrhenius equation when you're looking for the equilibrium constant, K (not to be confuse with the rate constant (lowercase) k). The equation demonstrates the relationship of the equilibrium constant (K), temperature (T), activation energy (E_a) and frequency factor (A).

Megan Cao 1I

i would say to consider the oxidation number of the elements and from the difference in their oxidation numbers you can determine which ones are reduced and which are oxidized. the numbers that decrease are gaining electrons (bc electrons are negative), so they're reduced. and vice versa for those o...

Megan Cao 1I

if there is no information given to determine the rate order, then the orders should be given. However it is possible for us to be asked to determine the order of a reaction/reactant based on some data or graphs. For example we could be given an plot of lnK v time and asked to determine the order of...

Megan Cao 1I

In class today, we were calculating the rate law for [NO_s]. the chemical equation was NO₂ + CO --> NO + CO₂ (all are gases). for the overall rate law we got K₁[NO₂]². What happened to the CO? since we're supposed to include all reactants in the rate law, is it the zeroth order?

Megan Cao 1I

here's one candid photo (-: Congrats Rachana Jayaraman!!!

Megan Cao 1I

In addition to that ^^ H2O and OH- are used to balance oxygens and hydrogens of basic solutions

Megan Cao 1I

since cl2 is the only reactant, it's both the oxidizing and reducing agent.
for the half reactions you would get:
Cl2 + 2e- --> 2Cl-
Cl2 + 2H2O --> 2HClO + 2H+ + 2e-
from then you'd simplify and get
Cl2 + H2O --> Cl- + HClO + H+

Megan Cao 1I

using the E cell = E cathode - E anode equation, since E cell = -0.689 and the cell diagram M|M2+||Cu2+|Cu, where Cu|Cu2+ is +0.34, shows that the copper is the anode side: -0.689 = E cathode -(0.34) (all in volts, V) so you get E cathode = -0.349 V. based off the diagram you have to see that the ce...

Megan Cao 1I

I think you can use either, since they're synonymous. H+ just seems easier honestly, since you dont have to consider any oxygens

Megan Cao 1I

the oxidizing agent is the one being reduced and the reducing agent is the one being oxidized.
since cl2 is gaining electrons it's being reduced and is the oxidizing agent
s2o3 is losing elections and being oxidized, so it's the reducing agent

Megan Cao 1I

C changes from solid to liquid and A changes from liquid to gas. A gas has the highest entropy of all three phases. therefore, the change of a substance to it's gas phase would have the most change in entropy

Megan Cao 1I

The cathode is where the reduction is happening, and the anode is where the oxidation is happening. (A small way to remember this is that Cathode and Reduction start with constants (C&R), whereas Anode and Oxidation start with vowels (A&O). for the reaction, since there's only one reactant, ...

Megan Cao 1I

ΔG^° is for the standard conditions.
ΔG is for the given conditions.

analogous to this is Q and K for equilibrium.
K is for the reaction at equilibrium and Q is the "temporary K" for the given conditions of the reaction.

Megan Cao 1I

having the Pt is because it's an inert conductor, so it doesn't affect the reaction. in the diagram it helps transfer electrons from anode to cathode. in the diagram copper is the conducting solid, whereas iron isnt a conducting solid, so the Pt would be with the iron.

Megan Cao 1I

n is the moles of electrons, just be sure not to confuse it with the moles of the agents. you can figure out the number of moles of electrons by writing both the half reactions and seeing the number of electrons transferred.

Megan Cao 1I

S is just entropy itself
delta S is the change in entropy
delta S total is the total change in entropy within a universe ( S_sys + S_surr)
delta S surroundings is the change in entropy within the surroundings

Hope this helps in some way!

Megan Cao 1I

you would use dS= n* C * ln (T2/T1) when there is more or less than 1 mole of the substance. you would use dS= C * ln (T2/T1) if they dont specify of it it's 1 mole. you use this equations for thermal disorder, where the temperature is changing. be sure to note that C can be Cv or Cp depending on if...

Megan Cao 1I

yes, the temperature is constant for that equation. if the temperature were to change you would use S = C x ln (T2/T1). for C you would use either Cv or Cp depending on which is constant.

Megan Cao 1I

What does the term "state function" entail? All I've seem to understand from it is that it's only dependent of its current calculated state and that you can add it's values. Other than that, what does "state function" mean?

Megan Cao 1I

For pKa the greater the value the smaller the Ka value, and the weaker the acid. I understand how the smaller Ka value shows how it's a weak acid, however I can't understand how a greater pKa means a smaller Ka. Could someone possibly explain this?

Megan Cao 1I

Does anyone know when the midterm review answers will be posted?

Megan Cao 1I

it's flat because it's the substance changing phases. the slope is zero so there's no temperature change, but there's a physical state change for the substance

Megan Cao 1I

depending on the C you use for q, you can use either Kelvin or Celsius

Megan Cao 1I

If we were to be asked about bond enthalpies would we be given the lewis structure or would we be expected to know them?

Megan Cao 1I

Are broken bonds considered negative or do they still remain positive?

Megan Cao 1I

for the question it asks for the reaction enthalpy by using the bond enthalpies.

a asks " 3c2h2 (g) --> c6h6 (g)"
do i assume all three bond of c--c to be broken or do i assume 2 bonds are broken of the three?

Megan Cao 1I

yes, but since the midterm is coming up you can do questions from other sections as a form of review

Megan Cao 1I

delta H is the enthalpy of the reaction, which is the total heat of the system. q is the heat that is transferred. when pressure's constant, delta h = q_p.

Megan Cao 1I

the equilibrium constant is calculated as products/reactants. since weak acids/bases only slightly dissociate in water, they'd only form a bit of products. Whereas for strong acids/bases they would completely dissociate, where products >> reactants. from the Kb, the greater the value, the lower the ...

Megan Cao 1I

I think you would use the heat capacity of the calorimeter, since it's what's absorbing the heat of the reaction.

Megan Cao 1I

Can someone differentiate the characteristics of reversible and irreversible reactions? For example, I know that reversible reactions are done with infinitesimally changes. But I honestly can't think of anything else that distinguishes the two.

Megan Cao 1I

the sum of pka and pkb is equivalent to the pkw which is 14. the product of k a and k b is k w , which is 10 -14 . k a is the equilibrium constant for an acid. k b is the equilibrium constant for a base. it's good to know that the lower the pKa, the higher the Ka, and the stronger the acid; and vice...

Megan Cao 1I

it'd be best to explicitly state whether the value is + or - . It wouldn't be wrong if you left our the + sign, since we'd assume you mean a positive value, however it'd be in everyone's best interest to define the value in order to leave any ambiguity.

Megan Cao 1I

you can approximate shortcut when K is less than 10^-3.

Megan Cao 1I

k<10^-3 helps us know it's a weak acid because in the equalibrium constant, in order for it to be such a small number the amount of reactants must be greater than the products. thus telling us that the acid barely dissociates, making it weak by definition

Megan Cao 1I

kp is used for gases since they deal with Pressure. kc is used for questions involving molarity, generally solutions, however you can find the concentration of gases too. you would have to use the ideal gas law, pv=nrt

Megan Cao 1I

in order to convert pressure into moles, you would use the ideal gas law (PV=nRT)

Megan Cao 1I

Someone said they went to the room to check and they said by Week 3. I don't think it'd be possible to get your test prior to your test a day or so in advance. If anything, I'd suggest going to pick it up an hour or so before your test if they're open. Good luck!

Megan Cao 1I

we would cover the topics of outline one, which would be chemical equilibrium and acids and bases.

Megan Cao 1I

generally, yes. they would give you the value of k and initial concentrations, which would make you work for the equilibrium concentrations. They can also give you some equilibrium values, but there are other ways to solve those problems.

Megan Cao 1I

I believe that since it's a holiday, there won't be any UA sessions, since office hours aren't held during the holiday either.

Megan Cao 1I

Question 5.35 isn't on the list of questions to do for the homework, but as practice, from the answers at the end of the book, the balanced chemical equation is 2A --> b + 2C and k=1.54 x10^-2.

Megan Cao 1I

Are all equilibrium equations told that they are able to reach an equilibrium? or are there characteristics that help identify that the reaction is able to reach an equilibrium? I know that the special arrow (⇌) helps indicate it, but are there any other ways to recognize that the reaction has a rev...

Megan Cao 1I

Kc is generally used when you're given molar concentrations of solutions and Kp is generally used when you're given partial pressures.

Megan Cao 1I

Do we use the ICE table for reactions with only gases and aqueous reagents? Can we use them with liquid and solid reagents?

Megan Cao 1I

when a question gives the pressures of gases, but asks for the molar concentration do we just convert the equilibrium constant or do we convert all the pressures to molarity?

Megan Cao 1I

Why is pKa only given to weak acids/bases?

also I know that pKa is an equilibrium constant, but what exactly does that mean? for example if the pKa of an acid was 4.5 x 10^-8, what does that mean?

Megan Cao 1I

In class today, professor Lavelle was talking about how CH₃COOH_(aq) is basic in water? can someone explain this? all I see is acetic acid.

the equation:
CH₃COO^-_(aq) + H₂O <--> OH^-+ CH₃COOH_(aq)

Megan Cao 1I

I believe 2 places after the decimal point should be sufficient enough, unless information is given with more or less

Megan Cao 1I

Transition metals can be a bit tricker... I would say to do the electron configuration in order to figure out how many electrons the element would have for the lewis structure, but transition metals deal w the d-orbital. For Ag, the configuration would be [Kr] 4d 10 5s 1 ; for Ag+ it would be [Kr] 4...

Megan Cao 1I

SO 4 2- in water is considered a lewis acid because it is derived from H 2 SO 4 . The dissociation of H 2 SO 4 is: H 2 SO 4 + H 2 O <--> HSO 4 - + H 3 O which then dissociates to: HSO 4 - + H 2 O <--> SO 4 2- + H 3 O the product of SO 4 2- in the second reaction shows that it's a conjugate base of H...

Megan Cao 1I

would we need to know the uncertainty names for naming ligands/complexes on the final?

ex uncertainty within the atom of CN- for a ligand which the name is either cyanido-kC or isocyano;cyanido-kN

Megan Cao 1I

a polydentate is where ligands can occupy more than 1 binding site simutaneously

also the polydentates would be dien (the first molecule listed) and oxalate

dien would be a tridentate
oxalate would be a bidentate

Megan Cao 1I

to find the oxidation number of an element you would need to know the charge of the molecule. for example: C 2 O 4 2- the oxidation of Oxygen is always -2 unless it's in a peroxide, then it's -1 here the oxidation is 2- since there are 4 oxygens then that means there is a charge of (-2)*(4)=-8 since...

Megan Cao 1I

the roman numerals refer to the oxidation number of the metal atom/ion

Megan Cao 1I

yes! anionic ligands end in -o

so -ide --> -ido
-ate --> -ato
-ite --> ito

Megan Cao 1I

the oxidation state of a free element (H+, F2) is always zero the oxidation number of a group 1 element is +1 the oxidation number of a group 2 element is +2 the oxidation number of a group 17 element is -1 in a binary compound the oxidation number of a monotomic ion is equal to the charge of the io...

Megan Cao 1I

they're both commonly used and impose the same ideas. lewis acid: species that accepts the electron lewis base: species that donated the electron bronsted acid: species that donates proton bonsted base: species that accepts proton here's a picture I found: https://www.chemistrysteps.com/wp-content/u...

Megan Cao 1I

A hydrogen atom will pick up the H+ of the salt, thus creating H3O+

Megan Cao 1I

yes there are positively charged coordination compounds! [Co(NH3)6]3+ aka hexamminecobalt(III)

Megan Cao 1I

bronsted acids are a way acids are classified according to the bronsted acid-base theory. bronsted acids are classified as the species that donates the protons.

Megan Cao 1I

sigma bonds have a greater overlapping area than pi bond, therefore their bonds are greater than pi bonds.

Megan Cao 1I

generally, if the molecule has no lone pairs it should be nonpolar. the only cases where the molecule has no lone pairs and is polar is when the surrounding elements of the central atom are not the same (CCl4 v CCl3F). Also if there are lone pairs involved, but they can cancel it's polarity, the mol...

Megan Cao 1I

I don't think we need to know the exact numerical electronegative values of the elements. however, I think we should know that Fluorine is the most electronegative and that on the periodic table electronegativity increases as you move from left to right and from the bottom up.

Megan Cao 1I

you could write the subscript if you prefer, but in most cases it'll be written as just E. this is because in chemistry it's usually implied that that abbreviation would have a subscript of 1.

ex: H2O, even though oxygen doesn't have subscript of 1, you know there's one oxygen atom

Megan Cao 1I

for CCl4 the polarity of the Cl's for the electrons cancel each other out, which makes the molecule nonpolar and the repulsion of electrons from each atom equivalents. for the CCl3F molecule, the bond angle will no longer be 109 degrees, but instead a bit less. this is because the electronegativity ...

Megan Cao 1I

yes! they're the same shape just a different name. just like how seesaw is also referred to as "sawhorse".

Megan Cao 1I

I think they'll be given but it's good to know these ions (https://i.pinimg.com/originals/f7/00/c1 ... 5ef421.png) I don't think you need to memorize them though.

Megan Cao 1I

convert g CO2 to grams of C and convert g H2O to grams of H. add the masses together and subtract the masses of C+H from 0.255g to find how many g of O you have. Then convert all the grams you have to moles and then divide by the smallest amount of moles you have. from then you should be able to fin...

Megan Cao 1I

yes just convert KJ/mol to either J/mol or J/electron.. usually the question will specify if you need it per electron but I believe it's generally J/mol.

Megan Cao 1I

I don't believe we do. we brushed over them in class but it's more of a concept to help us understand how we look at electrons.

Megan Cao 1I

he said there's going to be 8 questions for the two hours of the midterm.

Megan Cao 1I

radicals are molecules with an atom that doesn't follow the octet rule, with having one unpaired electron. this makes them highly reactive. they're significant because they're an exception to the octet rule.

Megan Cao 1I

yes the order of increasing ionization energy is from bottom to top and then left to right, so Fluorine being the most electronegative. if you're reading it from right to left then the energies will decrease.

Megan Cao 1I

the central atom should be the least electronegative atom

Megan Cao 1I

I always count the number of electrons the molecule is supposed to have. be sure to consider if the molecule has a charge (1+, 1-, 2-, etc) then, I use the most electronegative atom as the central atom. this may seem extra, but I draw the dots of each atom around it's symbol because once I'm done I ...

Megan Cao 1I

yeah a b & c represents excited states because they don't follow the filling rules (pauli's and hund's) and so they're unstable.

Megan Cao 1I

sure!
IE: Ionization Energy
EA: Electron Affinity
EN: Electronegativity
AR: Atomic Radius

Megan Cao 1I

the structure with lower energy is the one with the most atoms that have a formal charge of 0. for SO4- the structure with the lowest energy is the one with 2 double bonds rather than the one with no double bonds because there are more atoms with a formal charge of 0.

Megan Cao 1I

for IE, EA, EN: as you move to the right --> these increase; also as you move up the periodic table they increase
for AR: as you move to the left <-- the AR increases; as you move down the periodic table the AR inrcreases

Megan Cao 1I

they're a part of the d block, so they have extra orbitals to hold the electrons

Megan Cao 1I

delocalized electrons are electrons in a molecule that aren't associated with a single atom or a covalent bond. this means they can switch between bonds/atoms like when we were looking at the resonance structures and he said that the molecule could be either or versions.

Megan Cao 1I

momentum, p, is defined as mass (kg) multiplied by velocity (m/s): p=mv
velocity is used to define momentum

Megan Cao 1I

Hund's rule basically says electrons will fill different orbitals separately than before pairing up. since they are of the same charge they want to repel each other. the electrons are going to do this because they want to be of the lowest energy

Megan Cao 1I

the unit for wavelength is in meters, or more commonly nanometers, nm (10^-9 m) you can see this in the equation c=frequency x wavelength. if you rearrange the equation for wavelength you get c/frequency. the units for speed of light are m/s and the unit of frequency is s^-1. so the seconds cancel o...

Megan Cao 1I

electrons like to be parallel before they are paired--think of them as introverts when you're drawing the spins. drawing them out helps you understand wether they're paired or not

Megan Cao 1I

So in the post module assessment number 28 asks "The meter was defined in 1963 as 1,650,763.73 wavelengths of radiation emitted by krypton-86 (it has since been redefined). What is the wavelength of this krypton-86 radiation?" as far as the information given, I can't seem to understand how...

Megan Cao 1I

it'll probably apply more to labs and data. so you always want accurate data, but precise data will also help show some relationships even if you're not too accurate. as of quantum, I would just say you want accurate answers, just make sure your answers and work make sense

Megan Cao 1I

He said they are due at midnight on Sunday every week! so by week 2 you should have 10 posts which you can check in your profile (: (there's probably a simpler way but that's how I check)

Megan Cao 1I

hello me again so question 34 of the assessment asked "If molybdenum is irradiated with 194 nm light, what is the maximum possible kinetic energy of the emitted electrons?" with the given information of "Molybdenum metal must absorb radiation with a minimum frequency of 1.09 x 1015 s-...

Megan Cao 1I

in the post video assessment question 29 asked How much energy is required to remove an electron from one sodium atom? for when "Light hits a sodium metal surface and the velocity of the ejected electron is 6.61 x 105 m.s-1. The work function for sodium is 150.6 kJ.mol-1." could someone he...

Megan Cao 1I

from what I've learned, quantum is where the values are discrete. so the numbers can only be 1, 2, 3, etc., not 1.2 nor 5.4893. the way I think of it is that electrons can only accept one value (discrete) and nothing else. photons are energy particles of light. so when he talks about the photoelectr...

Megan Cao 1I

yes! all combustion reactions will always have O2 as a reactant and the products of CO2 and H2O (:

Megan Cao 1I

using dimensional analysis: 25.2 kg UF6*(1000g/1kg)*(1mol/352.03g UF6)(6molF-/1mol UF6)=429.51 moles F- =430 moles of F- ions (3 sig figs) first I wrote what I was given, then multiplied by a conversion factor to convert kg to g. then I multiplied by the molar mass of UF6 and then multiplied by the ...

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