CHEMISTRY COMMUNITY

Ximeng Guo 2K

Thank you Dr Lavelle! You are the best!!!

Ximeng Guo 2K

For the Ag(s)|AgBr(s) || Br-(aq)|Br2(g)|Pt(s) notation, should there be a line between Ag and AgBr or a comma? I think they are both solid and single line is used to separate different phases?

Ximeng Guo 2K

I don't know if our numbers are the same or not but I think the general idea should be the same. The Ea of the forward reaction is basically the energy difference between the transition state and the reactants. The Ea of the reverse reaction is the energy difference between the transition state and...

Ximeng Guo 2K

To add on, for zero order reactions, concentration will have no effect on the overall rate of the reaction.

Ximeng Guo 2K

Is the rate constant k also always positive?

Ximeng Guo 2K

To add on the previous answer, the rate equation of formation of IO- doesn't need to involve the conc. of IO-. Since the provided mechanism shows that there is intermediate formed and the slowest reaction is the second one, we do not need to consider the last step of the reaction.

Ximeng Guo 2K

I will try and give an analogy that helps me visualize this. Think about a very wealthy community. Imagine giving each individual $2000. Now think about a much poorer community. Imagine giving each individual $2000. The money will have a much greater impact on the poorer community than the wealthie...

Ximeng Guo 2K

Anusha Chaudhary 1J wrote:
Ximeng Guo 2K wrote:Is energy barrier equivalent to activation energy?

Yeah! In order for a reaction to get past the energy barrier, it requires activation energy either equal to or greater than the energy barrier. Hope this helps!

Thank you!

Ximeng Guo 2K

Thank you guys for the examples provided!

Ximeng Guo 2K

To add on, the unit of k for zero order is M/s.

Ximeng Guo 2K

If a catalyst is added, will k change?

Ximeng Guo 2K

It's possible, but it's not on our syllabus.

Ximeng Guo 2K

Change temp and add a catalyst will alter k.

Ximeng Guo 2K

A decrease in temp. might reflect a positive delta H I think?

Ximeng Guo 2K

I found this clarification on the internet: Thermodynamics describes the overall properties, behavior, and equilibrium composition of a system; kinetics describes the rate at which a particular process will occur and the pathway by which it will occur. So for thermodynamics we are interested in the ...

Ximeng Guo 2K

Is energy barrier equivalent to activation energy?

Ximeng Guo 2K

I interpret in the way that if the activation energy of a reaction is overcomed, then it's the time for a spontaneous process to happen.

Ximeng Guo 2K

If both the reactant and product have same moles in the chemical equation, these two will have the same numerical value.

Ximeng Guo 2K

Generally, delta S of gas > liquid > solid. The IMFs also affect delta S as well.

Ximeng Guo 2K

Since there is a flow of e- in circuit of simple cell, it creates a current and hence generates potential difference between to electrodes.

Ximeng Guo 2K

Salt bridge provided free ions into the solutions to compensate the lost or gain of e-. I don't think we need to include them in tha half equations.

Ximeng Guo 2K

Since G = H - TdeltaS and H, S >0, when T is large enough, G will be negative and the reaction is spontaneous.

Ximeng Guo 2K

Use Kelvin. K=C+273

Ximeng Guo 2K

For temperature difference, C and K are both fine. For others, normally we use K, but it depends on the question.

Ximeng Guo 2K

There are two positions to put N and O, so degeneracy W is not 0. Thus residual entropy is not 0.

Ximeng Guo 2K

Track the units would be fine. Normally we use 8.314.

Ximeng Guo 2K

No I think all we need to know is on the equation sheet.

Ximeng Guo 2K

increase in both volume and temperaturen = increase in entropy

Ximeng Guo 2K

Amount of disorder

Ximeng Guo 2K

I think the questions and equation sheet will specify the units for each quantity we use.

Ximeng Guo 2K

The number of arrangement for diatomic molecules like O2 is 1. If you switch the position of the two O atoms it's still one arrangement. In contrast of CO, which can be CO and OC so 2 arrangements. Therefore entropy of O2 is 0 as ln1=0. But for CO it's not 0.

Ximeng Guo 2K

Constant pressure doesn't mean volume is constant. If more moles of gases are formed in the products, volume needs to expand in order for the pressure to be constant.

Ximeng Guo 2K

If heat is produced from system to surroundings or volume of system increases, the change in work is negative for the system.

Ximeng Guo 2K

Internal energy of a system is always >=0. But the change in internal energy can be both + or -.

Ximeng Guo 2K

When there is no change in volume, there is no delta w, delta U = delta H

Ximeng Guo 2K

No I don't think so

Ximeng Guo 2K

In the equation pv=nRT, if p is in pascal and v is in m^3, use 8.314; if p is in atm and v is in L, use 0.08206.

Ximeng Guo 2K

Kelvin(K)
K=℃+273

Ximeng Guo 2K

Vaccume is an example of isolated system where there's no energy exchange between the system and the surroundings.

Ximeng Guo 2K

Work is dependent on the path taken as W=Fd where d is distance.

Ximeng Guo 2K

For this midterm I think so.

Ximeng Guo 2K

Salt can be neutral, acidic, basic depending on the cations and anions

Ximeng Guo 2K

H is enthalpy
q is heat energy

Ximeng Guo 2K

In addition, temp doesn't change even when heat is supplied/released during phase change.

Ximeng Guo 2K

Exothermic reactions have a negative delta H and endothermic reactions have a positive delta H.

Ximeng Guo 2K

HClO + H2O -> H3O+ + ClO-
ClO- + H2O -> HClO + OH-
Since HClO and ClO- are conjugate acid and base pair, we can use Ka*Kb=Kw=10^(-4) to calculate Kb for ClO-. Then use Kb to calculate the concentration of OH-. Covert it into pOH and then calculate pH.

Ximeng Guo 2K

I found this conclusion from Differencebetween.com quite useful: "1. pH is the reciprocal of the logarithm of H+ concentration. pKa is the logarithm of Ka value. 2. pH gives an idea about the amount of H+ions present in the medium. pKa value gives an idea about to which side the equilibrium is ...

Ximeng Guo 2K

Is there any example where water is considered a reactant and is included in the calculation of Kp and Kc?

Ximeng Guo 2K

When calculating Kp, why should we only use bar/atm as unit for each partial pressure instead of pascal?

Ximeng Guo 2K

To add on, partial pressure and concentration for gases can be inter-transferred using ideal gas law, PV=nRT.

Ximeng Guo 2K

R is an ideal gas constant. T is the temperature which is a variable.

Ximeng Guo 2K

Another one to add is that neon is considered the most "ideal" one in real gases, as it has very small molecular volume and small london force.

Ximeng Guo 2K

I had the same problem and I used the $72 credit to enroll in 14B on sapling.

Ximeng Guo 2K

Can we use Pa in this course?

Ximeng Guo 2K

P = Pressure
V = Volume
n = # of moles
R = gas constant
T = Temperature (in K)

Ximeng Guo 2K

Yes they are out on Saturday

Ximeng Guo 2K

Check the notes before winter and we'll be fine

Ximeng Guo 2K

I am very stressed during the final week also.

Ximeng Guo 2K

Ximeng Guo 2K

I will spend time with my family and travel with friends

Ximeng Guo 2K

No because we assume that strong acids/bases are completely ionized in water

Ximeng Guo 2K

After dissociating H+, the resulting anion for trichloroacetic acid is more stable as Cl is electronegative and withdraws the negative charge. Therefore it's more stronger than the acetic acid.

Ximeng Guo 2K

I think there can be acids of negative pH

Ximeng Guo 2K

I think since the halogens are in the same group and all have 7 valence e-, the effect of electronegativity leading to polarization is not significant as the effect of bond length. Thus, the strength of H-X bond is mainly due to bond length. Longer bond = weaker bond = much easier to dissociate H+ =...

Ximeng Guo 2K

It's said to be cumulative but I am not sure whether there will be specific focus on sections after midterm2

Ximeng Guo 2K

The H-F bond is short and strong, which makes HF harder to give out protons. Thus weak acid.

Ximeng Guo 2K

Consider the shape tetrahedral: there will be no arrangement for the two C-Cl polar bonds to cancel each other out. Therefore it's polar.

Ximeng Guo 2K

Not sure either. 3C can make a ring but it must be highly unstable.

Ximeng Guo 2K

carbon can have 4 bonds maximum
so it shall be CH3OH

Ximeng Guo 2K

To add on, a double bond is stronger than a single bond as bond length is shorter. Not sure whether we can determine a sigma bond is stronger than a single pi bond.

Ximeng Guo 2K

Yes we use molecular shapes to determine bond angles.

Ximeng Guo 2K

Large difference in electronegativity between two atoms -> polar bond
vector of polar bonds do not cancel out (determine from the 3D shape) -> polar compound

Ximeng Guo 2K

formal charge = (# of valence electrons)- (# of bonded electrons/2 + # of lone pair electrons); I think we use them to determine how stable a resonance structure of a covalent molecule is. In other words, oxidation number is more to do with "loss and gain" of electrons when forming a compo...

Ximeng Guo 2K

Hope that everyone can get a spot on second pass.

Ximeng Guo 2K

Thank you Dr. Lavelle! I was so desperate to find there was no spot after first pass):

Ximeng Guo 2K

Atoms in molecule have electrons and positive nuclei, so there will always be some random fluctuation of charges (induced dipole) when molecules come closer and interact with each other. It thus creates London Dispersion Forces.

Ximeng Guo 2K

I think both will be fine for the midterm.

Ximeng Guo 2K

Thank y'all!

Ximeng Guo 2K

The least electronegative atom goes to the center. But sometimes I just simply put atom with the least number in the molecular formula like C in CCl4 and O in H2O in center and it usually works.

Ximeng Guo 2K

I think for some atoms "formal charge = 0" outweighs the need to become an octet. But since they are not octet, they can potentially accept another pair of electrons (a characteristic of lewis acids).

Ximeng Guo 2K

unequally shared of electrons. The atom with greater electronegativity, like O in H2O, will attract the shared electrons towards it and become delta -. Since the shared electrons are closer to O, H will become less negative and thus delta +.

Ximeng Guo 2K

Polar covalent is different from ionic. I think for polar covalent the difference in electronegativity between atoms is not that significant as ionic.

Ximeng Guo 2K

Can we use the concept of pi bond and sigma bond to interpret this question?

Ximeng Guo 2K

Do we need to attend Tuesday's discussion?

Ximeng Guo 2K

Yea H-bonds are intermolecular forces while covalent bonds are intramolecular sharing of e- to achieve octet.

Ximeng Guo 2K

Scores will be available on Thursday this week.

Ximeng Guo 2K

I had the same question and thank you guys for the explanations.

Ximeng Guo 2K

Better to write a complete configuration. But I don't think we will encounter this type of question.

Ximeng Guo 2K

number of e- = number of protons in an atom (atomic number)

Ximeng Guo 2K

px, py, pz are three different types of p-orbitals which are perpendicular to each other. It doesn't matter which one to be filled first.

Ximeng Guo 2K

5 orbitals in each d sub-shell

Ximeng Guo 2K

atoms lose or gain e- to form ions

Ximeng Guo 2K

Thanks for clarification!

Ximeng Guo 2K

Thank you guys for clarifying. I was not sure whether particle in a box will be in midterm and now problem solve.

Ximeng Guo 2K

Yes. E=hv, v increases, E increases.

Ximeng Guo 2K

This will not be on midterm.

Ximeng Guo 2K

Thank you for the explanations!

Ximeng Guo 2K

Hi! I am also not sure about the equation. I think for midterm we just need to know that conceptually the orbitals are mathematical models relating total energy, kinetic energy, and potential energy of an electron.

Ximeng Guo 2K

There will be equations provided and I think we don't need to print them.

Ximeng Guo 2K

The dark regions on the screen are showing destructive interference as the light waves "cancel each other out".

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