CHEMISTRY COMMUNITY

Mulin_Li_2J

Depends on what you want. By default, all the half-rxns should be presented in reduction formats. If you are trying to calculate E, flip the one with lower E and then simply add two cell potential together. Or instead of flipping any of the two, you can minus the lower half-rxn cell potential from t...

Mulin_Li_2J

Depending on the scenario you suggest, when measuring standard cell potential of other substances, then yes SHE is always on the anode side by definition. Otherwise, it can be on either the anode side or the cathode side.

Mulin_Li_2J

Standard cell potential is an intensive property so it's independent of concentrations of reactants and products. As for cell potentials, apply Nernst equation and you should be good!

Mulin_Li_2J

Standard cell potential is an intensive property because it is measured under standard conditions(1M solution, 1 atm, 298K, etc). I don't think cell potential is an intensive property because it's concentration-dependent according to the Nernst equation we derived in class. The way I understand this...

Mulin_Li_2J

Relationship between temperature of the reaction and its rate constant k is described in Arrhenius equation. k = Ae^(-Ea/R*T). Ea, R, and A are all constants for a specific reaction. We can see when T increases, e^(-Ea/R*T) increases, k increases as well.

Mulin_Li_2J

Yes, I believe if you represent rate of the reaction using rate law, it should still be k no matter what stochiometric coefficient is in front of the reactant.

Mulin_Li_2J

Can someone explain and give an example on microscopic reversibility?

Mulin_Li_2J

Remember in the lecture, Dr. Lavelle said that in a multi-step reaction, the step before the rate-limiting step would be in equilibrium. So can we still apply the method of initial rates on the step in equilibrium to calculate reaction rates?

Mulin_Li_2J

Ecell = E*cell - (RT/nF)lnQ.
The equation reveals the relationship between concentrations and Ecell.

Mulin_Li_2J

The concentration causes electron flow because Ecell is concentration-dependent, whose value can be calculated with the Nernst equation, Ecell = E*cell - (RT/nF)lnQ. In a concentration cell, E*cell equals 0 because anode and cathode have the same redox couples, and therefore same E* value under stan...

Mulin_Li_2J

Is electromotive force of a cell decreasing over time as the redox reaction approaches its equilibrium? Or electromotive is like the largest possible cell potential of a cell?

Mulin_Li_2J

What is the difference among delta Phi(Galvani potential difference), electromotive force, and cell potential of a cell?

Mulin_Li_2J

Delta Phi is the denotation for Galvani potential difference. i stands for electric current. So basically, the equation in your notes says that when the electric current equals to 0 which happens at precisely the instant at which a cell is connected, the value of Galvani potential difference of the ...

Mulin_Li_2J

It depends on whether H2O is serving as a reducing or an oxidizing agent or an electricity conductor in the reaction. Basically, all parts of an electrode either conduct electricity or participate in redox reaction.

Mulin_Li_2J

Delta Phi is a denotation for the Galvani potential difference. The value of delta Phi is very similar to the value of cell potential for the same cell. I think for the purpose of this course, you can just equal dealt Phi to cell potential(denoted as E). Use E(cell) = E(Cathode) - E(Anode). Hope thi...

Mulin_Li_2J

Check your ln(Q). You might accidentally flip reactant and product upside down. The concentrations of products are the numerator and the concentrations of reactants are the denominator.

Mulin_Li_2J

In acidic solution, you would expect high concentration of hydronium ions and thus use H+ to balance hydrogen on both sides of the equation. In basic solution, you would expect high concentration of hydroxide ions and thus use OH- to balance hydrogen. A quick way to balance redox reaction in basic s...

Mulin_Li_2J

In the lecture on Tuesday, Dr. Lavelle demonstrated two different ways of calculating Eo of a cell. Both of them use Eo of cathodes and anodes, but one adds them together while the other one substrate E(anodes) from E(cathodes). How should I distinguish the two?

Mulin_Li_2J

Why do you need constant pressure and constant temperature to calculate Eo? Isn't electricity doing non-expansion work so pressure and temperature do not affect the value of Eo?

Mulin_Li_2J

Can anyone tell me what is the relation between Gibbs Free Energy and work? Is ΔG = work(max)?

Mulin_Li_2J

Three different system implies different conditions under which the reaction takes place. For an open system, pressure is constant. q = ΔH. Cp,m should be used. ΔU might not be 0 as matters and energy can be added to or removed from the system at any point. For a closed system, only energy can flow ...

Mulin_Li_2J

I think when Dr. Lavelle said bomb calorimeter is an isolated system, he means that the calorimeter is separated from the outer space. But still within the calorimeter, the combustion reaction is the system and the rest of the calorimeter is the surrounding. There is still internal energy because in...

Mulin_Li_2J

You probably see the equation ΔS = ΔH / T and ask this question. To explain this, first remember that we have two different forms of entropy: thermal entropy and residual entropy. For an exothermic reaction, its thermal entropy does decrease according to the equation. However, for some chemical reac...

Mulin_Li_2J

Another two heat capacity you might want to understand is Cv,m, molar heat capacity of gas under constant volume, and Cp,m, molar heat capacity of gas under constant pressure. Using C = q/T and q(constant pressure) = ΔH and ΔH = ΔU + PV, we can derive the equation that Cp,m = Cv,m + R. Then based on...

Mulin_Li_2J

When it's a isothermal irreversible process with a constant external pressure, which is the condition required to use the formula w = -Pex*ΔV.

If you are talking about when ΔU = -Pex*ΔV, it would be an adiabatic system with a constant external pressure.

Mulin_Li_2J

In this problem, you want to consider the residual entropy of each gas molecule by applying Boltzmann Formula instead. For one, the two gases look pretty much the same macroscopically because they all follow ideal gas law. But they differ quiet a lot in microscopic state because of different atomic ...

Mulin_Li_2J

Briefly, there are two types of entropy we've learnt so far. The first one is statistical, or residual, entropy, calculated using Boltzmann Formula: S = k*ln(W). It describes entropy as a result of positional disorder, namely the number of microstates a system can occupy while achieving the same ove...

Mulin_Li_2J

Isothermal, adiabatic, isobaric, isochoric, reversible, and irreversible are all important terms you need to know. Isothermal means ΔT = 0. This also implies that ΔU = 0, and because ΔU = q + w, q = -w. You can also use the formula for reversible expansion w = -nRTln(V2/V1), and the second law of th...

Mulin_Li_2J

Yes and I can give you some explanation on this. You might refer to "constant pressure" mentioned when we try to calculate the amount of work done by a system. First, let's take a look at the definition of work: w = F(o)*D. F(o) here refers to an opposing force. Basically, this formula imp...

Mulin_Li_2J

Hi, I know it's a lot to ask, but can anyone tell me the difference among internal energy, thermal energy, potential energy, kinetic energy, rotational energy, vibrational energy, transitional energy, enthalpy, q(under constant pressure), and q(under constant volume)? You don't have to answer them a...

Mulin_Li_2J

Can anyone explain to me how does degeneracy of a system relate to the amount of entropy a system can own?

Mulin_Li_2J

Recommend the sapling learning website if you have them. There are practice problems for every topic we've learned, all with very detailed explanation provided for each answer.

Mulin_Li_2J

Based on the textbook, an isolated system can exchange neither matter or energy with the surroundings. I am wondering that does this definition include work? Can the surroundings do work on an isolated system, or the reverse way?

Mulin_Li_2J

1, Add or Remove the amount of substances from a system 2, Heat or cool the system 3, Do work on the system, or let system do work on the surroundings. For a open system, all three work. For a closed system, only 2 and 3 work. Note that the energy of the system refers to the internal energy denoted ...

Mulin_Li_2J

Expansion work is the work arsing from the changes in volume of the system, such as a gas pump pumping air into a basketball. Non-expansion work is the work that does not involve volume changes, such as the electric current in a wire powering a computer or a TV. You might also want to know the diffe...

Mulin_Li_2J

1, Add or Remove the amount of substances from a system
2, Heat or cool the system
3, Do work on the system, or let system do work on the surroundings.

Note that the energy of the system refers to the internal energy denoted as U.

Hope this can help!

Mulin_Li_2J

Why the bond enthalpy for C= C is higher than that for C-C ? Does it mean that all two bonds in C=C are broken during the measurement? But why can't C=C just break one Pi bond?

Mulin_Li_2J

Basically what Hess's Law says is that enthalpy changes can be additive, mainly because enthalpy is a state property. For a reaction at given temperature and pressure, the enthalpy change of the reaction remains the same no matter how many times you conduct the reaction. So if you know the enthalpy ...

Mulin_Li_2J

Can anyone tell me what is the difference between a state property and a state function?

Mulin_Li_2J

So definitely check out the heating curve of H2O. Basically, what happens when a stream of steam hits you is that the steam releases a huge load of heat on you and reduces its temperature to 100 Celsius, assuming 1 atm. Because there is still a big difference between the steam and your body temperat...

Mulin_Li_2J

Standard Reaction Enthalpy is not necessarily measured in gaseous state. It is defined to be from its elements in their most stable form at 1atm and the temperature of interest. For example, elements for CO2 are C and O; the most stable state for C is graphite which is in solid state, and the most s...

Mulin_Li_2J

So first let's make it clear that under the same temperature, the equilibrium constant stays the same no matter how many reactants or products there are at the beginning, and in the long-run the reaction will reach the equilibrium. To determine which way the reaction is going to reach equilibrium, w...

Mulin_Li_2J

Since the equilibrium constant for weak acids and bases are very small, only a very small amount of them dissociate. A normal percent ionization would be less than 5%. The calculated percent ionization after approximation should be less than 5% to ensure that the approximation is not too way off the...

Mulin_Li_2J

After we dissolve KNO2 in water, KNO2 dissociates into K+ and NO2-. While K+ is a weak conjugate acid of a strong base KOH with a very high K(Base), NO2- is a strong conjugate base of a weak acid HNO2 with a low K(Acid). Thus, sudden change in [H3O+] won't affect KOH too much but might change the di...

Mulin_Li_2J

You should be able to employ ICE table on reverse rxns the same way as you do it on rxns. Since products formation and reactants formation take at the same time as the reaction goes on, under the same temperature, you should get the same concentration result, whether you use reaction or its reverse ...

Mulin_Li_2J

Since the chemical equilibrium constant we use assumes that reactions take place in very dilute solutions, even if water does participate in the reaction, change in concentration of water before and after the reaction can basically be ignored because the solution is so dilute. Instead, we only focus...

Mulin_Li_2J

Since we use concentrations to approximate chemical activities of reactants and products, we can also use chemical activities directly instead. And because chemical activities of solvent and solid both equal to one so they exert no effect on the value of K. Thus, we can just simply ignore solid and ...

Mulin_Li_2J

The value of K for a reaction is deeply related to the value of the energy gap between reactants and products shown in a typical potential energy diagram. As long as the energy gap stays the same, the value of K will not change. This explains why catalysts cannot change K because catalysts work by l...

Mulin_Li_2J

Consider the equation we learned in lecture for calculating K (units are not specified to simply my explanation): K = [Products]/[Reactants] When K = 1, which is a very rare situation, the denominator equals to the numerator of the equation so at its equilibrium the reaction is said to favor neither...

Mulin_Li_2J

I think if the question does not specify the unit being used, you can freely choose the most logical one: molarity if most of the reactants and products are in aqueous state, or partial pressure (in atms, or in bars, not in other units) if most of the reactants and products are in gaseous state.

Mulin_Li_2J

Can there be a hydrogen bond between water and oxygen gas? Though oxygen atoms in oxygen gas do not have a partial charge, they do have lone pairs which a hydrogen atom can get closed to.

Mulin_Li_2J

I still don't understand why solvent does not count for equilibrium constant. In the example of dissolution of HCl, some water molecules would be added one proton and become a hydronium ion, and both the concentration of water and the concentration of hydronium ions change during the process. Why do...

Mulin_Li_2J

Hybridization is created to explain how bonds are formed and oriented into experimentally detected geometric shapes. For example, if hybridization does not exist, single bonds in NH3 will be perpendicular to each other with a H-N-H angle of 90 because each p-orbital is perpendicular to each other. I...

Mulin_Li_2J

This is a heme group, with Fe ion at the center and surrounded by four coordinated N atoms. Typically, Fe ion in the heme group will bind to six ligands; in a hemoglobin, the other two ligands usually are an oxygen molecule and an amino acid histidine.

Mulin_Li_2J

Amphoteric and amphiprotic are two terms easily confused together. Amphoteric describes any molecules with both acidic and basic properties. As a very general definition, acidic and basic properties here can refer to any acids and bases definitions, including Lewis, Bronsted, etc. On the other hand,...

Mulin_Li_2J

Pi is the Greek letter for "p". The nomenclature is defined in this way because it has similar properties as p orbitals, including two electron dense regions seperated by a nodal plane. Pi bond is not shown in the p orbital; it is formed by overlapping two p orbitals from two atoms side-by...

Mulin_Li_2J

So when calculating the pH of 0.09 M HCl solution in a 1L flask, do we simply take -log([HCl]), or do we need to consider the amount of hydronium ions naturally dissociation in water and take -log([HCl] + 10^-7)?

Mulin_Li_2J

Can I say H2O molecule is a weak acid or a weak base which dissociates partly with an equilibrium constant of 1e-14?

Mulin_Li_2J

I don't know if you can know it from the lewis structure, but there are some trends discussed in lecture about properties of oxides I am sure you can use: metal oxides react with water to form strong base, non-metal oxides react with water to form acids, and diagonal band of amphoteric oxides form a...

Mulin_Li_2J

For electronegativity of atoms, there is a general trend in the periodic table that the upper right corner has higher electronegativity. So when comparing electronegativity of two atoms, simply check their relative positions to Florine which has the highest electronegativity. The closer the atom is,...

Mulin_Li_2J

I think for weak acids, we will be given the equilibrium constant under standard conditions. And from there, we should be able to calculate the percent of ions and molecules if molarity of protons and of dissociated ions are given.

Mulin_Li_2J

Bond strength is described as the energy required to break the bond. As the bond length gets longer, the attraction the two bonding atoms impose on the bonding electrons becomes weaker and lowers the energy needed to break the bond. Thus, longer bonds are easier to break than shorter bonds are.

Mulin_Li_2J

Can anyone give me more examples on sp3d2 hybridization orbitals?

Mulin_Li_2J

It can be non-polar if dipole moments cancel each other, which basically requires all the atoms surrounding the central atom to be the same element. Or the dipole moments won't match each other.

Mulin_Li_2J

Is electron repulsion generated by free radicals smaller or larger than that generated by a lone pair, and how does that affect bond angle?

Mulin_Li_2J

When we calculate the charge of the central atom in a coordination compound, we add up charges of ligands first and subtract it with the total charge of the complex. However, why don't we calculate the central atom's charge using formal charge, like what we did in previous modules, since formal char...

Mulin_Li_2J

We've learnt from lecture how coordination compound is formed and how each ligand donate their lone pair electrons to the central atom. But do coordination complex really satisfy octet rule? For example, in Cu(OH2)6+, does (Cu)2+ have an octet structure? Or is it that octet structure doesn't really ...

Mulin_Li_2J

T-Shapes you mention here might refer to trigonal bipyramidal without two equilateral lone pairs. Trigonal bipyramidal and octehedra are the two shapes with 90 degree bond angles, and T-shape can never happen to octehedra because it would be very unstable. Thus, Trigonal bipyramidal without two equi...

Mulin_Li_2J

Since VSEPR model only take into account the number of electron density regions surrounding the central atom, whether single bond, double bond, triple bond, or combined resonance bond is formed between two atoms, they are still considered as only a single electron density region, and thus VSEPR rule...

Mulin_Li_2J

The basic idea of a dipole-dipole interaction between two polar molecules is that dipole moments surrounding the central atom do not cancel each other. In this case, both CH3Cl and CHCl3 have uncancelled dipole moments because first C-H and C-Cl have different dipole moments due to different electro...

Mulin_Li_2J

Electron repulsion between electron density regions is directly proportional to the size of electron clouds. Since lone paired electrons sense attraction only from one nucleus, they typically take up a larger space and thus exert a larger repulsion force. On the other hand, bonding paired electrons ...

Mulin_Li_2J

A molecule itself does not have a dipole-dipole or ion-dipole force. Those two forces generally describe intermolecular forces, namely the attraction between two molecules. And from their names, we can know that dipole-dipole forces typically form when uncancelled dipole moments of two polar molecul...

Mulin_Li_2J

Yes, I think you are right about the CH3Cl bond angles that due to a bigger electronegativity difference between C atom and Cl atom, the electron clouds of the bonding region between the two atoms shift towards Cl atom, polarizing the bond and thus reducing the electron repulsion between H-C-H bond ...

Mulin_Li_2J

The derivation of De Broglie Equation takes three other equations. The first one is E = mc²; the second one is E = hv; the third one is c = vλ. First, we take c = vλ, transform it to be v = c/λ, and replace it with v in E = hv. Then we get E = h*c/λ . Second, we transform E = mc² to E = m*c*c. Since...

Mulin_Li_2J

The electronegativity scale we use is Pauling Electronegativity scale, and in this scale electronegativity of a neon atom has no data. Of course there are other electronegativity scales which includes electronegativity of a neon atom. For example, Sanderson Electronegativity scale assigns a neon ato...

Mulin_Li_2J

Well, I think dipole-dipole forces typically appear between polar molecules, meaning that they do not have an evenly distributed electron cloud. Such forces explain why water has a high freezing point and high boiling point compared to non-polar molecules like CO2 gas. On the other hand, induced-dip...

Mulin_Li_2J

Basically, as long as the two atoms bonded together are not the same element, there will most likely be a dipole moment between them. In terms of calculating the dipole moment between two atoms, I think you might want to do it when determining the structure of a complex molecule where various dipole...

Mulin_Li_2J

Electronegativity is the not the same concept as electron affinity. The two differs from each other in both definition and measurement process. Electron affinity is defined as the energy it takes to add one electron to an atom, the equation being E(Neutral atom)-E(Neutral atom + 1 electron). It is o...

Mulin_Li_2J

Can anyone tell me what is the difference between electric dipole moment and magnetic dipole moment as well as how to represent those two on a polar bond respectively?

Mulin_Li_2J

A general definition of valence electrons is number of electrons existed in the out-most shell of the atom. But does this definition really apply to transitional metals? Because if it does, Cu will technically have 11 valence electrons.

Mulin_Li_2J

Electronegativity of an element is derived by combing both its ionization energy and electron affinity mathematically with the equation (Ionization Energy + Electron Affinity) / 2. When going down a column in the periodic table, ionization energy and electron affinity both decreases because as the a...

Mulin_Li_2J

Electronegativity of an element is derived by combining both its ionization energy and electron affinity mathematically. In today's review session, the instructor said that Electronegativity = (Ionization Energy + Electron Affinity) / 2. Hope this helps!

Mulin_Li_2J

I think Al is capable of gaining 5 electrons to gain Octet, but it is also capable of forming only three bonds and thus has only 6 valence electrons, which is very similar to the bonding property of Boron.

Mulin_Li_2J

Dr. Lavelle discussed this briefly and I didn't quite understand it. How exactly is Heisenberg Indeterminacy related to Resonance Structure of a molecule?

Mulin_Li_2J

From what I understand of resonance, those alternative structures Dr. Lavelle shew us in his lecture are contributing resonance structures of the molecule because all of these structures are plausible states of the molecule but with different formal charges and electron distributions. However, exper...

Mulin_Li_2J

Generally speaking, Octet Rule exception has something to deal with the special structure of the atom. For example, H and He only have 1s orbitals at their ground state. Thus, 2 electrons is their Octet. Another example would be Al and B. Those two atoms can maintain a stable structure with only 6 e...

Mulin_Li_2J

I have a general explanation for this weird pattern of 5th row transitional metals. Since ground state of an atom is its most stable structure or owns the lowest energy, the reason for those weird electron configurations is that experimental data show that those configurations are their lowest energ...

Mulin_Li_2J

Can anyone tell me why would a double bond be shorter than a single bond? I know double bond has higher electron density than single bond, but doesn't high electron density means high repulsion forces among electrons and thus a longer bond length?

Mulin_Li_2J

I still don't understand why electrons will fill up empty orbitals first before getting into pairs. If Aufbau principle applies to this case, does it suggest that electrons in empty orbitals have lower energy than that in paired electrons?

Mulin_Li_2J

So p has three orbitals p(x), p(y), p(z). When filling up the electrons orbitals, does electron necessarily fill up in the order of p(x), p(y), p(z)? or just in random?

Mulin_Li_2J

You probably won't do any actual calculations with this equation but do make sure you know how to qualitatively understand what it means. Wave function^2 = probability density, which ranges from 0 to 1. High probability density means higher chance of finding electrons in this region and vice versa. ...

Mulin_Li_2J

Yes, extra electrons do affect radius of an atom because compulsion forces among electrons push them away from each other. However, this information is not relevant to this question. In this case, S^2-, Cl^1-, P^3- all have the same electron configuration of [Ar], meaning that they have the same num...

Mulin_Li_2J

The energy of a configuration is the sum of the energy of each electron. The ground state means the total energy should be its lowest value. To achieve the lowest energy, simply put electrons one by one into energy shells from lowest to the highest --- in order of 1s, 2s, 2p, 3s, 3p, etc. In another...

Mulin_Li_2J

Hi,

I am a bit confused with plus/minus signs relative to a energy level. Does the negative sign on each energy level mean that the electron has the tendency to lose energy or gain energy?

Mulin_Li_2J

Hi,

Is there any videos or websites illustrating details of those experiments conducted to examine properties of atomic spectra of a hydrogen atoms? I am so interested in all the experiment details.

Mulin_Li_2J

So here is the question, The energy gap between two consecutive energy level decreases as n goes up. As n approaches infinity, the energy gap approaches 0, but it will never equal to zero. Does it mean that an electron could never get away from influence of hydrogen nucleus because mathematically it...

Mulin_Li_2J

The definition of intensity of light here is usually confused with the Energy Equation E=hv because if energy of light is determined by frequency, what does amplitude do, and why is it related to intensity of light. Well, do remember two facts. First, E=hv describe the energy of one photon; if you w...

Mulin_Li_2J

Hi, I want to add one more point. The Lymen series and Balmer series are categorized based on frequency of emitted and absorbed EM waves, Lymen series for EM waves in UV region and Balmer series for EM waves in visible region. Remember that for each energy level, the gap between them gets smaller sm...

Mulin_Li_2J

But by definition, is it a SI Unit?

Mulin_Li_2J

I am a bit confused with the definition of Molar Ratio. Does it describe the ratio of a single element in a certain compound with respect to other elements(like C in CO2), or is it like stoichiometric numbers indicating relative amount of reactants and products in a chemical reaction(Like CO2 in a c...

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