CHEMISTRY COMMUNITY

Jiapeng Han 1C

Burning gasoline releases heat so it is an exothermic reaction!

Jiapeng Han 1C

The standard enthalpy of formation for N2 and O2 is 0! Because they are in their naturally occurring state! That is why we use Hess's Law to calculate the delta H.

Jiapeng Han 1C

I think at least according to the definition, endothermic reaction has positive H while exothermic reaction has negative H.

Jiapeng Han 1C

I think q typically denotes a change in heat energy while H denotes a change in enthalpy.

Jiapeng Han 1C

I think for the sake of this course, vapor and gas are the same thing. However, at least for me, I think gas typically refers to something that is naturally in gas phase, such as nitrogen and hydrogen, while vapor refers to something that is "vaporized" into gas, such as water vapor and io...

Jiapeng Han 1C

We need to first determine whether the forward/reverse reaction is endothermic/exothermic. If the forward reaction is endothermic, then increasing the temperature will shift the reaction in the forward direction, hence K will increase; if the forward reaction is exothermic, then K will decrease.

Jiapeng Han 1C

We should only count the moles of gas molecules because they are the ones that will be affected by changes in pressure. By the way, solid and liquid play no role in the calculation of the equilibrium constant.

Jiapeng Han 1C

Percent ionization is just the ratio of concentration of H+ ions ionized to the initial concentration of the acid.
Percent ionization=[H+]/[HA]*100%.

Jiapeng Han 1C

Aqueous(aq) is when you dissolve something into water to form a solution. For example, if you dissolve NaCl into water, you will get NaCl(aq). In contrast, liquid(l) simply means that a substance is in a purely liquid state. For example, H20(l) and C2H5OH(aq), which correspond to liquid water and li...

Jiapeng Han 1C

This is actually a very classical mistake when doing the ICE table! It should be (2x)^2, that is 4x^2, instead of 2x^2!

Jiapeng Han 1C

Q could be used at anytime of a reaction that is happening, it is just the ratio of products to reactants. However, K shall only be used when the reaction reaches equilibrium.

Jiapeng Han 1C

This one is actually important, we use Kelvin for the temperature in the Ideal Gas Equation.

Jiapeng Han 1C

We actually use Kelvin for the temperature in Ideal Gas Equation.

Jiapeng Han 1C

For the reverse reaction, k of forward reaction will become 1/k in the reverse reaction.

Jiapeng Han 1C

P=pressure
V=volume
n=number of moles
R=constant(8.314J/(mol*K))
T=temperature

Jiapeng Han 1C

NH3 is indeed a weak base as it could donate lone pair of electron. However, NaCl is a salt because when it dissolves in water, both ions will form ion-dipole attraction with water molecules. It is a mere physical process as nothing is really changed.

Jiapeng Han 1C

With regard to exam, you should really remember some of the most widely-known strong acid. These include: HClO4, HI, HBr, HCl, H2SO4, HNO3. To classify whether an acid is strong or weak, you should look at how easily the H atom would be lost. Weak A-H bond will result in a strong acid since the H at...

Jiapeng Han 1C

Like AlCl3, B in B(OH)3 also has empty orbital which means that it could accept electron pair from the OH- group, meaning it is a Lewis acid.

Jiapeng Han 1C

The carboxyl group could donate a H atom to form COO- and H+, so it is an acid, a weak acid though.

Jiapeng Han 1C

Water can act as either an acid or a base, depending on the reactants.

Jiapeng Han 1C

The main distinction would be that a Lewis acid has empty orbitals while a Lewis base has lone pairs of electrons. So that's why we say Lewis acid is electron acceptor while Lewis base is electron donor.

Jiapeng Han 1C

If a species has an empty orbital, then it is a Lewis acid since it can accept lone pairs. If a species has lone pairs of electrons, then it is a Lewis base since it could donate electrons.

Jiapeng Han 1C

Higher stability means a species is more easily to be formed. If the resulting anion isn't stable enough, then the dissociation won't be complete, which means that the acid is by definition, weak. The same logic applies to base as well.

Jiapeng Han 1C

HBr is the stronger acid because it has longer H-A bond, so weaker attraction between H and Br, thus proton is more easily dissociated.

Jiapeng Han 1C

For a structure of H-A, the longer the bond, the weaker the attraction, thus the stronger the acidity. In this case, HBr is more acidic than HCl.

Jiapeng Han 1C

Not necessarily so, however, lone pair could indeed disrupt the symmetry of a molecule. For a molecule with lone pairs to be non-polar, the lone pairs must cancel out. For example, consider XeF4 where the lone pairs are opposite each other(one on top and one on bottom) and the covalent bonds form a ...

Jiapeng Han 1C

Delocalized pi bond is formed when 2 or more pi bonds are too close together so that they overlap. As a result, the electrons are free to move in the overlapped region. An example would be benzene where 3 pi bonds overlap, causing the formation of 6 delocalized electrons.

Jiapeng Han 1C

My best guess is that cisplatin can bind to the N7 of the guanine molecule.
Check out the link below!
https://images.app.goo.gl/ZGgwc2LkrLssVM3j7

Jiapeng Han 1C

A triple bond contains 1 sigma bond and 2 pi bonds.

Jiapeng Han 1C

Typically, for a chelate, the ligand must not be a monodentate. This means the ligand must have multiple lone pairs so that it can form two or more coordinate bonds with the central atom.

Jiapeng Han 1C

Although CO bond is polar, CO2 molecule is linear in shape so that the dipoles cancel out. For SO2, it has two SO double bond and a lone pair on S atom. As a result, the shape is bent so that the dipoles can't cancel out.

Jiapeng Han 1C

The double bond in this case matters because this way the C atoms can't rotate. Cis means that both of the chlorine atoms are on the same side of the compound. C-Cl bond is polar while C-H bond is non-polar, so the overall dipole moment will point to the chlorine side.

Jiapeng Han 1C

C-Cl bond is polar while C-H bond is non-polar. As a result, the dipoles can't cancel out.

Jiapeng Han 1C

Actually we combine 1 s orbital and 3 p orbitals to form 4 sp3 orbitals. Both of the electrons in the s orbitals are now in the sp3 orbitals.

Jiapeng Han 1C

Order of intermolecular force from strongest to weakest: Hydrogen bond(strongest form of dipole-dipole interaction) dipole-dipole dipole-induced dipole interaction Van der Waal's forces Order of intramolecular forces: Ionic bond(assume solid ionic compound) covalent bond This answer is given by Khan...

Jiapeng Han 1C

Molecules that are non-polar only experiences London Dispersion Forces. For example, hydrogen gas H2 and methane CH4.

Jiapeng Han 1C

From what I remembered from high school, bent and angular are the same thing where you have two bond pairs and two lone pairs on an atom.

Jiapeng Han 1C

Typically, symmetrical molecule is non-polar while asymmetrical molecule is polar because the dipoles can't cancel out.

Jiapeng Han 1C

Actually molecular shape is one very effective way to determine polarity. Typically, symmetrical molecule is non-polar while non-symmetrical molecule is polar because the dipoles can't cancel out.

Jiapeng Han 1C

Cation with greater charge density--greater charge/size ratio--has greater polarizing power. So polarizing power: Rb+<Sr2+<Be2+.

Jiapeng Han 1C

I would say the charge density matters the most--that is the charge/size(surface area) ratio. Cation which has a high polarizing power will have a high charge density so that its charges are very concentrated. Typically, cation with a high charge and a small size has greater polarizing power.

Jiapeng Han 1C

A coordinate bond is formed when one atom donates a lone pair of electrons to an empty orbital of another atom. For example, an ammonia molecule has a lone pair of electron on the N atom, and a hydrogen ion has an empty orbital. As a result, the N atom donates the lone pair of electron to the hydrog...

Jiapeng Han 1C

A coordinate bond is formed when one atom donates a lone pair of electrons to an empty orbital of another atom, so both electrons in the bond come from the same atom. However, a polar covalent bond just means that there is a big difference in electronegativity of the two atom, electrons are still sh...

Jiapeng Han 1C

Myoglobins don't necessarily form hemoglobin, they are just structurally very similar to the sub-unit of hemoglobin. Also, each myoglobin can only bind to one oxygen molecule.

Jiapeng Han 1C

A regular covalent bond involves the sharing of electrons between 2 species. However, a coordinate bond is formed when one species donates a lone pair of electrons to an empty orbital of another species, which means that both electrons are supplied by the same species. We usually use an arrow to ind...

Jiapeng Han 1C

Ligand is a species that have one or more lone pairs of electrons available to donate to a central metal ion, forming coordinate bonds.

Jiapeng Han 1C

Firstly, you need to check if some of the atoms are more electronegative than others. Secondly, you need to see if the dipole cancels out. For example, CO2 has polar bonds, but the molecular is non-polar.

Jiapeng Han 1C

Intermolecular bonding is the bond formed between molecules, such as hydrogen bond and Van der Waal's forces. Intramolecular bonding refers to the bond formed within a molecule, such as covalent bond.

Jiapeng Han 1C

For hydrogen bond, H atom must be bonded to a very electronegative atom such as N, O, or F so that the only electron of hydrogen atom is nearly completely pulled over to the lone pair electronegative atom.

Jiapeng Han 1C

Firstly, there is attraction between the positively charged nucleus and the electrons. Secondly, there is repulsion forces between electrons in the same orbital because same charge repels each other. Thirdly, there is the shielding effect caused by the repulsion from the inner electron shells. Elect...

Jiapeng Han 1C

For example, if speed is 10 $\pm$ 3m/s, you could either multiply 3 by 2 or use maximum value to minus the minimum value, that is 13-7=6m/s. Both way is correct.

Jiapeng Han 1C

To calculate the uncertainty in position, you firstly have to determine the uncertainty in momentum. The speed is 5 \pm 5m/s, so the uncertainty in speed is 10-0=10m/s. We then calculate the uncertainty in momentum using p=mv. Finally, we plug the value of momentum into Heisenberg's equation, and th...

Jiapeng Han 1C

Firstly, you need to calculate the mass of a single fluorine molecule. To do that, you could divide the molecular mass(this is in g) of fluorine molecule by the Avogadro number. Please note that you need to convert g to kg! Once you have the mass of a single fluorine molecule, you could use the form...

Jiapeng Han 1C

So for calculating energy of photons, you could use the equation E=h*f where frequency of light could be determined via c=f*wavelenth.
For calculating energy of electrons, you could firstly calculate the momentum of electron via p=h/wavelength, and then apply the value of p to the equation p^2/2m=Ek.

Jiapeng Han 1C

According to De Broglie's equation: wavelength=planck constant/momentum.
Momentum=mass*velocity
As a result, wavelength=planck constant/(mass*velocity), so velocity=planck constant/(mass*wavelength).

Jiapeng Han 1C

According to De Broglie equation,any object with a mass and a velocity(that is to say, any object with momentum)can be considered as a wave. However, since baseball has a relatively large mass, the wavelength will be too small to be detectable. Let's say the baseball weighs 145g and moves at 40m/s. ...

Jiapeng Han 1C

For part A: kinetic energy is just Ek=1/2 m v^2
For part B: note the unit here--Kj/mol, so you need to divide the work function by the Avogadro's constant, and the unit should be Jk
For part C: firstly you add work function and kinetic energy together. Then you divide the sum by the Planck constant.

Jiapeng Han 1C

According to the equation E=h*f, higher frequency means each photon has higher energy. Intensity is essentially number of photons, so if you increase the intensity of the light, the light beam will get brighter. To increase the amplitude, or intensity of a light wave, in reality you just need to inc...

Jiapeng Han 1C

Fields are essentially vectors which have directions. Direction of electric field and direction of magnetic field are perpendicular to each other--they are oscillating at 90 degrees to each other.

Jiapeng Han 1C

Work function is the threshold energy--the minimum amount of energy needed to release electrons from a metal surface. Too calculate it, you need to know exactly at which frequency of light does the electrons get released. Then simply use the equation: E=h*f. If the energy of light is big enough to r...

Jiapeng Han 1C

You must have 2 units of B and 1 unit of A in order for the reaction to proceed. So if you have fewer moles of B than A, A is then clearly in excess. What you really need to do is doubling the mole of A to see if it equals number of moles of B. If it is more than moles of B, than B is the limiting r...

Jiapeng Han 1C

Combustion is essentially a oxidation reaction because you use "oxygen" to burn something--to oxidize something. However, with regard to burning, you don't necessarily have to use oxygen. You just need to make something burn--a lot of chemicals can burn things.

Jiapeng Han 1C

Electrons and protons are charged so if they are not conserved, the number of charges of a reaction would change which clearly violates the "conservation of charge" law.

Jiapeng Han 1C

Accuracy is how close you are to the real/true value. Precision is how close your experimental data is to each other. For example, let's say you want to measure the length of your pencil which is 15cm long and you measure 3 times. If you get 15.1cm, 15.0cm, 14.9cm, then you are both accurate and pre...

Jiapeng Han 1C

Just 3! A quick method to count number of significant figures would be you start counting from the first NON-ZERO digit--that is 7! Ignore any decimal point.

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