CHEMISTRY COMMUNITY

Jane Wang 1E

Hi! I think we will be told if it's reversible or not.

Jane Wang 1E

Yes, I got the same!

Jane Wang 1E

I think it's like in order for a bond to form between two atoms within to molecules, the collision needs to happen in the correct orientation and direction. For example, if we want to form a bond between two Br atoms within two HBr molecules, we need two Br to collide together. If it is the H and th...

Jane Wang 1E

Enthalpy is talking about the heat absorbed/release by a formation, but entropy is a measure of randomness of the system, like solid has is much more ordered than a gas.

Jane Wang 1E

The units should be consistent, and we almost always use second for kinetics.

Jane Wang 1E

Yeah, because when we are deriving the equation for half life, the concentration at a time is equal to half of its initial concentration. rate = k[A]=-d[A]/dt. If we integrate both sides, we get [A]=[A0]*e^(-kt), and so (1/2) *[A0]=[A0]*e^(-kt). We take ln for both size, so we can get ln[1/2]=-kt = ...

Jane Wang 1E

Hi! The order of each reactants in the slow step is only depend on how many reactant molecules collide together in the slow step. It doesn't matter what reactants are in the overall reaction, only the slow step is needed to determine the rate equation.

Jane Wang 1E

The slow step is also called the rate determining step, and that is the step that we can decides the order of each reactant in the reaction.

Jane Wang 1E

EMF is the maximum potential difference between two potential difference of two half cells. Ans the potential difference given in the appendix is the reduction potential of those half-cells.

Jane Wang 1E

For second order reaction: rate = k[A]^2 =d[A]/dt, and we integrate both size to get the relationship between concentration at anytime and the initial concentration. When the concentration at a time when the [A] = 1/2 of the initial concentration, the time taken is the half life. So we plug in [A] =...

Jane Wang 1E

I think the glass electrode has an ion-selective membrane that allow specific ion to go through, and inside the membrane, it contains a certain fixed concentration of a solution as reference. Hope this helps!

Jane Wang 1E

The half equation will show the states of each reactants and product, so when there is no conductor in a half cell, we should use platinum as a conductor.

Jane Wang 1E

Yes! Platinum electrode is added when there is no solid metal to act as electron in the half cell. Use platinum as a electrode so it can transfer electrons and it's stable enough that there it won't be a part of the chemical reaction.

Jane Wang 1E

I think you are right that if the deltaH is highly negative, deltaS is slightly negative and when the temperature is low, the deltaG is still going to be negative, so the reaction would still be spontaneous. When dealing about spontaneity, we first look at deltaG, but when we cannot get deltaG, we c...

Jane Wang 1E

Galvanic cell produce the potential difference from the electron-gaining ability of different solutions in the two half cells, but concentration cells produce the potential difference from the concentration difference of the same type of solution in the two half cells.

Jane Wang 1E

Hi! When I was typing in my balanced chemical equation it showed an error message in the box and I could not clear it. I tried other browers but none of them worked to clear the box and let me type in my answer for this question. I also refreshed the page, but it did not work.What should I do? Below...

Jane Wang 1E

Different solutions have different values of Kb, but the product of Ka and Kb of a pair of conjugates is constant, which is 1*10^(-14), Kw.

Jane Wang 1E

An oxidizing agent is being reduced in a chemical reaction, so it gains electrons. Hence, a greater electronegativity makes is a better oxidizing agent.

Jane Wang 1E

I found first balancing number of atoms really helpful, then adding electrons to balance the charge.

Jane Wang 1E

Reducing agents are being oxidized in a chemical reactions, and they lose electrons and the oxidation states increases.

Jane Wang 1E

A more positive standard reduction potential represents a stronger oxidizing agent.

Jane Wang 1E

Hi! Isothermal means the the system is at a constant temperature, and internal energy is constant at a constant temperature(no energy change), so delta U is zero.

Jane Wang 1E

Hi! The general form of the equation calculating delta G is: ΔG=ΔG0+RTlnQ. Under standard conditions, concentration of products and reactants are the same, so Q becomes 1(lnQ = 0) and the equation turns into deltaG = delta G0 = deltaH-T*deltaS. Under equilibrium conditions, Q=K and ΔG=0 so ΔG0=−RTln...

Jane Wang 1E

Any value of a negative delta G means the reaction is spontaneous, and a 0 delta G mean the reaction is in thermodynamic equilibrium. The more negative the G, the less possibility that we can reverse it.

Jane Wang 1E

we can just use deltaG of formation of products minus the deltaG of formation of reactants.

Jane Wang 1E

Yes, delta S is positive when the system is changing from a less ordered state to a more ordered state (Ssolid < Sliquid < Sgas). And the delta H is positive when the reaction is an endothermic reaction(eg. melting).

Jane Wang 1E

I think standard condition means that the substance is at standard temp(298K) and 10^5 Pa. And under different conditions, the values for experimental results will be different(eg.deltaH..)

Jane Wang 1E

temperature stay the same for a reaction that absorbs energy: the melting of ice is at constant temperature at its melting point. The energy absorbed is used to break the bond, so it does work instead of increasing the temperature. Isothermal means the system has no change in temperature, so change ...

Jane Wang 1E

I think work is differentiated by volume change and we are considering the external pressure not the pressure of the internal, which changes suddenly, the external pressure is considered as a constant, and the change in volume is delta V (delta V = finalV-initialV). However, in reversible reactions,...

Jane Wang 1E

No, just outline 1, 2 and 3 will be on the midterm1.

Jane Wang 1E

Yes, in Friday's lecture I think Dr. Lavelle mentioned that.

Jane Wang 1E

subscriptions should not be considered, since n is the mole of gas, not the moles of atoms. So why the change in moles of gas is not 1/2 ?

Jane Wang 1E

If you plot the graphs of pressure against volume, the area under the curve of reversible reaction has a greater area, hence a greater work done. Another aspect might be that there is a greater heat loss in reversible reactions, but I am not so sure why the heat loss is more.

Jane Wang 1E

Hi! I am just wondering that will chem 14C be delivered online in the summer session A? Since I can get a result after searching for the online option, but the location indicates Young Hall. Thank you!

Jane Wang 1E

Hi! What is the difference between the K and Kc here in the table and when should I use which one? And for Exercise 5.39, where does the 6.1*10^(-3) in part a) in the answer come from this table? Here is the exercise: 5.39 In an experiment, 0.020 mol NO was introduced into a flask of volume 1.00 L a...

Jane Wang 1E

Hi! I think because the change in heat is calculated though c*m*deltaT, so you need the mass of water instead of the volume, and the density can give you the mass by using mass = density*volume.

Jane Wang 1E

Hi! It's always true that deltaH is equal to the heat(q)? Also, when we are calculating q, we can use q=c*m*deltaT, but the specific heat capacity used here is the Cp or Cv?
Thank you!

Jane Wang 1E

Hi! For this question, can we use the deltaT at both sides if the equation, and then use 0+deltaT or 45-deltaT to get the final temperature? Why I can only get the correct answer when I use (Tfinal - 0) at left side and (Tfinal - 45) at the right side? All the other terms are the same and I took int...

Jane Wang 1E

Because in benzene, there are delocalized electrons forming instant double bonds that are always changing its position. The dot lines are representing those electrons, as they are not in fixed positions.

Jane Wang 1E

I think it is just the heat capacity, not specific heat capacity or the molar heat capacity. Because the mass and volume of the colorimeter is not changing, so we can directly use the heat capacity.

Jane Wang 1E

When pKa is large, Ka is small, which means the equilibrium favors the backward reaction. Comparing to the environmental pH, if the environmental pH value is small, then there are more H+ / less OH-, which can shift the equilibrium, causing onside has more molecules than the other side before reachi...

Jane Wang 1E

If I remembered correctly:
Decimal places of a logarithm answer = significant figures of the number in log parenthesis.

Jane Wang 1E

For most questions, they will give the change in enthalpy, and a negative enthalpy means an exothermic reaction.

Jane Wang 1E

I think we have pH=-lg[H+], pKa = -lg(Ka), Ka*Kb=Kw=1*10^(-14). But we can derive most of the functions from these basic functions and there is a formula sheet on Dr. Lavelle's website for use.

Hope this helps!

Jane Wang 1E

Hi! I think it is because the original solution is all NH3, which is the B, and B reacts with water forming BH+. So, actually the original concentration before pronation should be the sum of those 2 molecules.

Jane Wang 1E

There are several ways to make a buffer solution, depending on the equilibrium pH you need. For example, a weak acid and its conjugate base can make a buffer solution with a buffer range in the weak acid range.

Jane Wang 1E

Hi! For most acids their second deprotonation has a really small Ka, and we just consider the first deprotonation step. But for sulfuric acid, it is such a strong acid that its second deprotonation Ka is still relatively large, so we need to consider that. But for other acids, unless the question sa...

Jane Wang 1E

Since K values are calculated form the [product] / [reactant], there are only multiplication or division relationships, never add or minus them with one the other.

Jane Wang 1E

pKa = -lgKa (use the log to the base of 10)

Jane Wang 1E

According to Le Chatelier principle, removing the reactant will shift the reaction to the left to balance the equilibrium.

Jane Wang 1E

We can also calculate the partial pressure using the equation PV=nRT, so partial pressure P = nRT/V.

Jane Wang 1E

Hi! First we can look into the list of reactions and see which two reactions can be combined together to form the given one. The first reaction can combine with the reversed reaction of the third one forming the given equation, so we use value Ka and Kc. But the first reaction needs to be multiplied...

Jane Wang 1E

Hi! I think setting up an ICE table will make the question easier to understand. First, just notes down the equilibrium concentration of each reactants and products, then as the concentration of NO is changed to 0.7M, and the system tends to change back to equilibrium, and we assume the change for o...

Jane Wang 1E

If it is at the same/constant temperature, then yes.

Jane Wang 1E

I think we will be given the value for delta H, which is the enthalpy change. When delta H is negative, then the reaction is exothermic, and if delta H is positive, the reaction is endothermic.

Jane Wang 1E

I guess it is related to ionziation energy. Like if it is easier to remove a electron from the metal surface, the ionization energy is gonna be low, and also the work function energy.

Hope this helps!

Jane Wang 1E

Empirical formula is the least whole number ratio of atoms within a molecule.

Jane Wang 1E

Hi! The wave property is that a wave can be diffracted. Just as photoelectric effect is the evidence for particulate nature, the diffraction experiment is the evidence for the wave-like nature.

Jane Wang 1E

Hi!
Chelating: a complex containing one or more ligands that form a ring of atoms that includes the central metal atom. Two lone pair acceptor in the chelating ligand should be in neighbour. If they are not in neighbour, the ligand will become a bridging ligand.

Jane Wang 1E

No, the conjugate base of a weak acid is a strong base, and vice versa.

Jane Wang 1E

Coordination number is the total number of dative covalent bonds formed from the ligands to the central metal ion.

Jane Wang 1E

Generally yes, because normally metals form positive ions and in salts, the anion is negatively charged as they tend to gain electrons forming an anion.

Jane Wang 1E

Since [Co(NH3)4Cl2]Cl as a whole is not charged, and Cl ion is negatively charged, so [Co(NH3)4Cl2] complex should be +1 charged. Ligand need to donate at one pairs of lone pair electron, so Cl in the complex ion must be negatively 1 charged. Because N in NH3 has a lone pair already, so it can be ne...

Jane Wang 1E

Hi!

The lone pair on N replaces the Cl, so N in guanine binds to Pt through a dative covalent bond, blocking the DNA replication.

Hope this helps

Jane Wang 1E

A polydentate can bind to more than one other atoms/ions.

Hope this helps!

Jane Wang 1E

A sigma bond is formed when two p orbitals overlapped head-to-head, like both p-orbitals lie down and their heads overlap. Whereas pi bonds are formed by sideway overlap of two p orbitals, like both p orbitals are standing together, and overlap.

Jane Wang 1E

It depends on which theory you want to use to distinguish them. For Bronsted acid/base, the proton donor is the acid, and the proton acceptor is the base, whereas in Lewis acid/base theory, lone pair acceptor is the acid, and the lone pair donor is the base.

Jane Wang 1E

It's 109.5 degree, because the atoms/groups of atoms are arranged into a least repulsion form.

Jane Wang 1E

We can count the no. of lone pairs electron and no of atoms connected to that molecule, then add together to get the hybridization. For example, if the total is 4, then it's sp3 hybridized, and sp2 for a total of 3.

Jane Wang 1E

I think the question will always be clear when referring to this, like it will tell you to choose which one is a Bronsted acid / base and which one is lewis acid/base.

Jane Wang 1E

Acidity is effected by how well a proton can dissociate with the molecule, so as HBr has longer hence weaker bond, then it's easier to dissociate. Therefore, Hbr is a stronger acid comparing to HCl

Jane Wang 1E

Hi!
When both molecules are non-polar, then the intermolecular force is just the London dispersion force. Dipole-induced dipole force occurs only when a polar molecule encounters a non-polar one.

Jane Wang 1E

Polarizability is a measure of how easy an atom's electron can be distorted, whereas the polarizing power is a measure of the ability of an ato to distort electron clouds of other atoms. So, they are kind of an opposite property.

Jane Wang 1E

The lesser the difference in electronegativity between 2 atoms, the more covalent characteristics the bond will have. Because electrons are more likely to be shared instead of being donated / accepted.

Jane Wang 1E

Hi!
I think we can first draw other bonds to satisfy octet rule for all other atoms, and we use the total number of electrons minus the electrons we have already put in the Lewis structure, and those electrons haven't been used should be put on Xe as lone pairs.

Jane Wang 1E

Hi! CHCl3 would have a higher boiling point because there are both London dispersion forces and dipole-dipole interactions between its molecules. Whereas CCL4 has only London dispersion forces in between molecules. (CHCl3 is a polar molecule as it has polar bonds with dipoles aren't cancelled, but C...

Jane Wang 1E

Hi! The general rule is looking at the group it is in the Periodic Table. Atoms in group 1,2,3,4 will lose 1, 2, 3 ,4 electron(s) respectively, forming positive ions with +1+2... charges. And atoms in group 5,6,7 tends to form anions with +1, +2, +3 charges.

Jane Wang 1E

Yeah, I am having the same problem.

Jane Wang 1E

Hi! When atoms lose their electrons, they will lose electrons from the outermost shell. So, 4s will lose the first, because when 4s orbital is filled, it will have higher potential energy than 3d. Hope this helps!

Jane Wang 1E

Hi! Generally, ionic bonds are formed between atoms with oppositely charged ions, with donating and accepting of electrons. However, covalent bonds are formed between atoms of similar electronegativity, and electrons are shared instead of donated/accepted.

Jane Wang 1E

Hi! It means the energy required to break one mole of that type of bond in its state, when bonds / interactions from, energy is released. Hope this helps!

Jane Wang 1E

Hi! I think my high school teacher mentioned that the resonance structure with the negative charge on the most electronegative atom are the most stable one. Also, all the atoms should have complete full valence shells.

Jane Wang 1E

Hi! Only the covalent bond formed between 2 identical atoms is completely non-polar, other covalent bonds are all polar.

Jane Wang 1E

Hi, covalent bonds are formed by sharing of electrons, whereas ionic bonds are formed between oppositely charged ions through donating and accepting of electrons.

Jane Wang 1E

Hi, when the formal charge equals to zero, it is when the structure of the molecule is the most stable one. So, we need to use the equation to find the molecule with the lowest energy (most stable).

Jane Wang 1E

Hi! Spreading of electrons helps to stabilize the system because there is a spreading of forces due to delocalization of electrons.

Jane Wang 1E

Hi! Formal charges indicates the gain/loss of electrons while forming covalent bonds, and it is calculated by FC= no. of valence electron - (no. of lone pair electron + no.of bonding electron/2). When FC = 0, the bond is in its most stable form. Hence, the molecule is more stable. I think we first n...

Jane Wang 1E

Hi!
Orbitals are described by magnetic quantum number, which has values from +l to -l, and the total number of orbitals equals to 2l+1. For example, in 4p subshell, l=1 because it is a p subshell, so number of orbitals=m(l)=2*1+1=3. These 3 are actually Px, Py and Pz orbitals.

Jane Wang 1E

The second ionisation energy is always higher, because removing electron from a positive ion requires more energy compared removing a electron from a neutral atom.

Jane Wang 1E

Hi! Objects with mass can have wavelike properties when they have motion, and the wavelength they have is called De Broglie wavelength, like they are not originally electromagnetic wave, but they are masses with wavelike properties, hence have wavelength. But normal wavelength describes the waveleng...

Jane Wang 1E

Hi! Basically m(l) is the magnetic quantum number, and it represents the orbital, for example, in p subshell, it has Px, Py and Pz three orbitals. Plus, the value of m(l) are from -l to +l, including 0.

Hope this helps!

Jane Wang 1E

Hi! Effective Nuclear Charge is like the charge from electrons that can really interact with the positive charge of the nucleus. Shielding effect is that inner electron will shield the interaction between outer electrons and the nucleus, and the shielding effect need to be deducted from the total nu...

Jane Wang 1E

The half-filled orbitals are more stable, so yes, other elements tend to follow the same trend.

Jane Wang 1E

Yes, the threshold energy and the work function energy is the same, they are the less amount energy that a metal surface need to emit an electron.

Jane Wang 1E

Hi! The two exceptions are Cu and Cr, because they can form half-filled electron configurations for the 4s and 3p orbitals. The half filled orbital(1 electron in 4s and 5 electrons in 3p for Cr) is more stable than, for example, Cr, having 2 electrons in the 4s orbital and 4 electrons in 3p.

Jane Wang 1E

The electrostatic potential energy of an electron is negative inside an atom is because the forces between the nucleus and the electrons are attractive forces, and I think I have learnt in high school that attractive forces are always negative, so does the energy. We say the energy increases when it...

Jane Wang 1E

Hi! I think there is a email sent said what we will learnt on Wednesday will not appear on the exam.

Jane Wang 1E

Moles are culculated by n=mass/molecular mass, and it is describes the amount of a substance, while molarity=n/volume, which describes the concentration of a solute in a solution measured in mol/dm*3 usually.

Jane Wang 1E

I use Khan Acedemy to find videos and questions. Maybe edX and Coursera can help too!

Jane Wang 1E

When looking for the molecular formula of a given problem, how do you know how much to multiply the empirical formula by? Usually the question give the molucelar mass, so you can divide the molecular mass by the total mass of atoms in empiriacal formula, and the resulting integer(or rounded integer...

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