CHEMISTRY COMMUNITY

Shutong Hou_1F

Kinetics describes the pathway of reactions, with an emphasis on the activation energy that signifies the energy difference between the free energy of reactants and the transition state. Kinetic analysis is about the speed or rate of reaction, using experimental techniques to observe reactant & ...

Shutong Hou_1F

In addition, if there are more than two reactants, you could make the initial concentration of all reactants except one very large so that as the reaction proceeds, the concentration of only one reactant actually changes.

Shutong Hou_1F

The units of k:
first-order: s^-1, second-order: (s^-1)*(mol^-1)*(L), zero-order: (mol)(L^-1)*(s^-1);

Shutong Hou_1F

I think that whether a reaction is endothermic or exothermic depends only on the final Gibbs free energy and initial Gibbs free energy. If the reactants have higher free energy, then the reaction is exothermic; if the products have higher free energy, then the reaction is endothermic.

Shutong Hou_1F

Just to add, the activated complex is also named transition state. Also, at a higher temperature, there is a higher probability that a portion of the particles has gained enough energy needed to take part in the reaction; however, higher temperature makes faster reaction but temperature doesn't itse...

Shutong Hou_1F

Exactly, and this relates back to the fact that the slowest elementary step is the rate determining step and it determines the rate of the overall reaction. In other words, the overall reaction rate is not determined by any faster step. Therefore, the peak in the reaction profile just implies the ac...

Shutong Hou_1F

Yes, it's termolecular for the elementary step. In addition, I think it's worth notifying that molecularity means the number of species in an elementary step (rather than the overall reaction), which is followed by each elementary step's rate law directly.

Shutong Hou_1F

Many reactions actually have many reactants, but determining the order of each together is very hard and complex, so we make the concentration of one reactant very small while the others very large (concentrations essentially constant) so that the reaction rate depends only on the small concentratio...

Shutong Hou_1F

Exactly. So when there is no product, the forward reaction rate would not be disrupted/counteract by reverse reaction, because no product is present and thus no product can return to reactant.

Shutong Hou_1F

And I think that differential rate law is actually the source of integrated rate law (obtained by integrating the differential rate law on both sides). Also, differential rate law could give the plot of rate versus time.

Shutong Hou_1F

I generally attend TA's review sessions talking about outlines that contain concepts not sound familiar to me to get a big picture of what I need to know. And Lyndon's review session is outstandingly helpful, in which he would help us go through potential tricky questions. Professor Lavelle's lectur...

Shutong Hou_1F

how do you determine it based on the cell potentials? especially if the values are similar? I believe that we should then check the formula sheet or appendix, and choose the substance with more positive reduction potential as the cathode. I think there should always be a difference that is not too ...

Shutong Hou_1F

We set rate 1/rate 2 equal to 2 in the example because the ratio of the initial rates between experiment 1 and experiment 2 is 2 and the units cancel out. And the initial rates of [NH4+] doesn't change, so, after applying equation: rate = k[NH4+]^n*[NO2-]^m, we are left with the ratio between initia...

Shutong Hou_1F

In addition, to be an electrolyte, a substance doesn't have to completely dissolve in a solvent. Strong electrolytes would completely dissolve, whereas weak electrolytes would only partially dissolve.

Shutong Hou_1F

And remember that because standard potential energy is an intensive property, we don't need to multiply by coefficients.

Shutong Hou_1F

I think that to decide which metal to be the cathode, we need to look into the standard reduction potential table, which is provided in the textbook's section that talks about standard reduction potential and its appendix, and will also be provided during tests and exams. The metal with higher posit...

Shutong Hou_1F

When the oxidation number of an element in a reactant increases when it becomes a product, it is oxidized; when the oxidation number of an element in a reactant decreases when it becomes a product, it is reduced. The previous sentence holds because being reduced means gaining electrons while being o...

Shutong Hou_1F

Exactly. And I believe that we don't need to remember this, because the meaning of STP would be given and clearly explained when it appears on the test, just like what happened in the midterm.

Shutong Hou_1F

I think that Nernst Equation and Le Chaterlier's Principle support each other. According to Le Chatelier's Principle, the cell potential should change in corresponding ways when concentrations change in specific ways. Supportively, by manipulating variables in known equation, the Nernst Equation is ...

Shutong Hou_1F

It allows redox reaction to take place continuously in a galvanic cell. Without the salt bridge, as electrons go from anode to cathode, the redox reaction would stop due to charge build up.

Shutong Hou_1F

Oxidation: loss of electrons, increase in oxidation number;
Reduction: gain of electrons, decrease in oxidation number;

Shutong Hou_1F

What exactly is the standard reduction potential and why does it have to be positive for the reaction to be spontaneous? The standard reduction potential is tabulated for compounds with respect to standard hydrogen electrode. It is a measure of the electron-pulling power of the reaction occurring a...

Shutong Hou_1F

The oxidation number of oxygen and hydrogen generally remain the same, but mostly not in the situation when they have an oxidation number of 0. When balancing the half reactions, you could add water to balance oxygen. Then, if the reaction takes place in acidic solution, you could add H+ to balance ...

Shutong Hou_1F

Just to add something. In free expansion, there is no external force present, and the surrounding is actually vacuum, and the process is irreversible because the external pressure is constant and equals 0. Moreover, in a reversible pathway, the external pressure could actually either infinitesimally...

Shutong Hou_1F

I think it would be under the situation that there is an ideal gas that is monoatomic, linear, or non-linear, and molar heat capacity is not given.

Shutong Hou_1F

S just means entropy, rather than change in entropy that should be denoted as ΔS. Entropy is almost never 0 (the entropies of all perfect crystals approach zero as the absolute temperature approaches zero; "perfect crystal" refers to a substance in which all the atoms are in a perfectly or...

Shutong Hou_1F

I agree with the peer above's explanation. In addition, the initial & final states are the same because the process is isothermal, so, according to ΔS = q(rev)/T, ΔS should be the same.

Shutong Hou_1F

For Part A, you have to add up three different delta S values to find the total change of entropy in the system. This includes deltaS for H2 container expansion, deltaS of Krypton gas for container expansion, and deltaS based on temperature change. Since entropy is a state function, this allows us ...

Shutong Hou_1F

I think that the substitution is correct but unnecessary, because the substitution needs even more information. The unit of entropy is always J/k, and you can remember this by the second law of thermodynamics, that is delta S = q/T, in which q has a unit of J and T has a unit of K.

Shutong Hou_1F

I think that the unit of deltaS standard of reaction is J/K, because delta S = q/T; the unit of deltaH standard of reaction is kJ/mol. Since G = H -TS, G should have the same unit as H, so I think that the unit of standard free energy of formation is also kJ/mol.

Shutong Hou_1F

I think that the unit should just be kJ/mol, because I think that adding or removing something from a unit would make things weird and even the unit invalid. If you want specify that you are calculating enthalpy of formation of a particular substance, you could specify that at the beginning of your ...

Shutong Hou_1F

I think that delta H = qp, because "qp" means heat transferred under constant pressure. And by definition, change in enthalpy equals the heat absorbed or released at constant pressure.

Shutong Hou_1F

Also, by global warming, the ecological environment would be significantly changed, leading to the extinguish of many species because they cannot adapt the new ecological environment anymore. The destruction on the ecosystem might be so severe that even ecosystems that cover large areas and sustain ...

Shutong Hou_1F

Yes, and remember to pick ideal gas constant with correct units or convert 750 torr to 750/760 atm; also remember to be consistent with either J or kJ when adding energies.

Shutong Hou_1F

The change of a substance from a phase of a lower energy to a phase of a higher energy requires heat to break intermolecular interactions between molecules, thereby making the substance have higher potential energy between molecules (usually, the volume of a substance: solid state < liquid state < g...

Shutong Hou_1F

In addition, ATP is less stable than ADP, because ATP has an additional phosphate group that is connected to the other part of the ATP by a chemical bond with very high energy.

Shutong Hou_1F

Yes. In addition, bomb calorimeter is an isolated system, so neither the matter nor the energy could be exchanged between the system (inside of bomb calorimeter) and the surrounding (outside of the bomb calorimeter). Therefore, the internal energy of bomb calorimeter is constant.

Shutong Hou_1F

In lecture, the definition of enthalpy is given: the amount of heat released or absorbed at a constant pressure.

Shutong Hou_1F

The change in enthalpy is equal to heat released or absorbed only when the reaction happens at a constant pressure. So the value of q(heat) calculated by Cp is equal to change in enthalpy, but not by Cv.

Shutong Hou_1F

Vaporization, melting (fusion), and sublimation are all exothermic reactions, so the change of enthralpy of each of these three reactions has a positive value. delta H(vap) = H(vapor) - H(liquid); delta H(fus) = H(liquid) - H(solid); delta H(sub) = H(vapor) - H(solid); So delta H(sub) = delta H(fus)...

Shutong Hou_1F

I think that your reasoning is exactly correct. And I believe that it doesn't matter which side you would like to put the negative sign on. The only thing matter is that one substance is losing heat whereas the other one is gaining heat, so if we set the change in heat of one substance negative, the...

Shutong Hou_1F

Is it because the bond enthalpies are measured in the gas phase? We could actually still make good use of bond enthalpies in gas phase by adding enthalpy of vaporization when calculating bond enthalpies in liquid phase.

Shutong Hou_1F

For diatomic molecules, they are accurate, because they are directly measured for those molecules. For all others are averages from many different molecules containing the bond, and thus inaccurate.

Shutong Hou_1F

Temperature remain constant during a phase change, because the increase in internal energy would be used to break intermolecular forces and to increase the distance between molecules. For example, the density of gas is less than that of liquid, which is less than that of solid.

Shutong Hou_1F

Pressure and volume are two totally different measurements, because they measure different parameters of substances and have different units. However, change in pressure, under many circumstances, could imply change in volume, and vice versa, according to the ideal gas law, PV = nRT.

Shutong Hou_1F

Celsius is always 273 units larger than Kelvin. However, they luckily have the same length of each unit. Consequently, since 0 Kelvin is defined by the temperature of absolute zero but 0 Celsius is defined by the temperature of ice-water mixture, they would never have the same numerical value for an...

Shutong Hou_1F

The standard state for a solution is actually 1M at 1 atm. And the reason why it's 1 atm may be that the atmospheric pressure at sea level is 1 atm, so it's convenient to define 1 atm as standard state for gases (and liquids).

Shutong Hou_1F

According to the ideal gas law, PV = nRT, P = nRT/V = concentration (=n/V) * RT. So [partial pressure = concentration * RT].

Shutong Hou_1F

I believe that increasing the partial pressure of O2 is achieved by means of adding more O2 gas particles. And remember that P (partial pressure) = concentration * R * T (temperature).

Shutong Hou_1F

If the partial pressure of CO2 is increased, then the concentration of the product is increased. According to Le Chatelier's principle, the reaction would then shift to the left and favor the reactants. As a result, H2 would be consumed along with CO2 to form more reactants, thereby leading to a dec...

Shutong Hou_1F

Exactly! And also if the percentage ionization of a substance is less than 5%, then the percent of the substance that is ionized is relatively small compared to the initial concentration of the substance, so we could disregard the decrease in the amount of the substance when mentioning the substance...

Shutong Hou_1F

If Ka or Kb of a substance is given, then it's a weak acid/base. The conjugate base of a weak acid is basic, whereas of a strong acid is not, the conjugate acid of a weak base is acidic, whereas of a strong base is not.

Shutong Hou_1F

The example Dr. Lavelle gave in class was if you added Helium to a reaction occurring within a sealed container. While the pressure of the reaction is increasing, it has no effect on the equilibrium because adding gas doesn't have any effect on the actual concentration of either the products or rea...

Shutong Hou_1F

I think that the other gas laws could actually all be easily derived from the ideal gas law, PV = nRT.

Shutong Hou_1F

Actually, the equilibrium constant K has unit, but to include the unit would make the calculation much more complex (and the numerical value of K would be a little bit different--so little that it would be much more convenient to not include a unit for K. To have K have no unit, we would write molar...

Shutong Hou_1F

P(O2)/P(O3) isn't the equilibrium constant of this reaction, but P(O2)^3/P(O3)^2 is. Remember that a^2/b^3 = 5 doesn't mean that a/b = 5. Because the equilibrium concentration is different in these two cases, only equilibrium constant would be the same when comparing these two conditions.

Shutong Hou_1F

I don't think that the equilibrium constant K is the rate at which these reactions occur. K is the ratio of products and reactants at equilibrium. We could speed up the rate by adding reactants, and in other ways according to Le Chatelier's principle.

Shutong Hou_1F

All the function and examples of catalysts are exact and useful. In addition, catalysts decreases activation energy by stabilizing the transition state (having lower energy). In biological systems, without catalysts, mostly enzymes, many reactions important for metabolism and maintaining homeostasis...

Shutong Hou_1F

I think that when the reactants and products, except pure liquids and solids, are all gases, we should use Kp, whereas using Kc is also okay. But when they are not. we can only use Kc.

Shutong Hou_1F

Sulfuric dioxide comes from burning petroleum products, coal, etc., and reacts with water vapor to form acidic rain. Clean coal contains less percentage of sulfur, whereas dirty coal contains more percentage of sulfur.

Shutong Hou_1F

Michelle Song 3I wrote:Boron and Aluminum are also exceptions in that they can form only 3 bonds.

Exactly. But to be more specific, without any additional lone pairs, the number of valence electrons around boron and aluminum is then only 6, not reaching octet.

Shutong Hou_1F

Hydrogen bonding occurs between a hydrogen atom that is bonded to N/O/F and a N/O/F atom with at least one lone pair.

Shutong Hou_1F

nickianel_4b wrote:^^I know that it becomes CH3COO- just from practice, but can someone remind me again why it donates the H+ at the end and not one of the H+ ions attached to the C?

-COOH, or carboxyl group, is the organic acidic group, so when losing a proton, the proton would come from the carboxyl group.

Shutong Hou_1F

But doesn't losing or gaining of electrons (amphiprotic) mean acting as an acid or base (amphoteric)?

Shutong Hou_1F

No, because they are unreactive, therefore unwilling to accept or donate electrons because they have already reached their octet. By the way, molecules don't have to be classified as either acid or base. And also remember that there are amphoteric molecules that could act as either acid or base depe...

Shutong Hou_1F

H2SO4 is more readily to donate H atoms than H2CO3 because S is more electronegative by having more protons (in 4th period, while C is in 3rd period) and thus have more electron density withdrawing power.

Shutong Hou_1F

If an acid is polyprotic is it stronger? A polyprotic acid isn't necessarily stronger than a monoprotic acid. Whether an acid is strong depends on the equilibrium constant for acids, Ka = ([A-][H+])/[AH]. Having more number of H atoms in a molecule doesn't necessarily mean that the molecule is more...

Shutong Hou_1F

If there's more than one H in front of the anion, let's say, H2SO4, or H3PO4 It'll be polyprotic because there is more than 1 hydrogen in front. Exactly! We should count the number of H atoms in the front of the anion, rather than count all H atoms, because not all H atoms would play a role in dona...

Shutong Hou_1F

Yes, and essentially, this rule works because it means the same thing as determining water acts as an acid or a base according to whether water donates/accepts protons (Bronsted's definition).

Shutong Hou_1F

An amphoteric compound can be identified as both a acid and a base in different reactions.

Shutong Hou_1F

Besides all the important, useful rules and hints mentioned by previous peers, when trying to decide the oxidation state of TM, we could solve the problem by knowing the charge on the molecule overall and figuring out oxidation states of all atoms other than the TM.

Shutong Hou_1F

I think that in the cases that acids dissolve in water, the actual reason why acids lower the pH is that they easily release a hydrogen atom that is then taken by water to form a hydronium ion, lowering the pH of the solution.

Shutong Hou_1F

Exactly, and also remember that whether the oxygen is absorbed or de-absorbed depends on the local partial pressure of oxygen. In addition, the reason why transition metals locate as the central atom is that TM 1) have many oxidation states --> good for electron transfer 2) often bound to a cage-lik...

Shutong Hou_1F

Hi, Could someone reiterate the significance of Fe as the central atom in the Heme complex, and how that relates to its function? Thank you! Hi there, According to professor Lavelle's lecture and his slides: The Fe atom plays a role in binding one oxygen molecule, and therefore could perform the fu...

Shutong Hou_1F

Personally, I would prefer to draw the Lewis structure first to see whether the molecule is ionic (ionic-ionic, ionic-dipole) -->whether polar (ionic-dipole, dipole dipole, dipole-induced dipole), or nonpolar (dipole-induced dipole, induced dipole-induced dipole), with the help of either net dipole ...

Shutong Hou_1F

I think that hydrogen bonding could form between a hydrogen atom bound to an N/O/F atom and an N/O/F atom with lone pair electrons.

Shutong Hou_1F

The electron geometry considers all regions of electron density, regardless of bonding pairs electrons or lone pairs electrons; the molecular shape considers only bonding regions. Consequently, when the electron geometry is the same, depending on the number of lone pairs electrons, the molecular sha...

Shutong Hou_1F

How do you know that the two bonds with hydrogen are next to each other, versus across the molecule from each other? I think that it does't matter, and there are 2 reasons. 1) they are all single bonds, so arranging the hydrogen atoms next to each other or across from each other doesn't make differ...

Shutong Hou_1F

Yes, because we are only trying to name the shape of molecules.

Shutong Hou_1F

There are several general rules to help determine, and thus memorization is required. To determine the molecular shape, both lone pair electrons and bonding pair electrons need to be considered; professor Lavelle hasn't discussed this in lecture yet. However, to name the shape of the molecule, only ...

Shutong Hou_1F

In order for the amount of repulsion to be minimized, the distance between bonding pair should be maximized, that is, keeping the bonding regions of electrons as far apart from each other as possible. Unlike Lewis structure that is 2-dimensional, VSEPR model is actually 3-D, allowing bonding regions...

Shutong Hou_1F

De Broglie's wavelength equation applies to any moving particle with momentum so that the particle has wavelike properties, such as electrons, neutrons, and protons. However, only when the particle has extremely small mass would the wavelike property be noticeable.

Shutong Hou_1F

We are actually totally allowed to use the Rydberg equation, whereas using the (final - initial) form of the the Rydberg equation could give us a better understanding of the concept and therefore what we are doing. And I believe that using the form of equation recommended by professor Lavelle elimin...

Shutong Hou_1F

The angular momentum quantum number (l) describes the shape of the atomic orbital.
Allowed values: l = 0 (s-orbital), 1(p-orbital), 2(d-orbital), ..., n-1; each l is called a subshell.

Shutong Hou_1F

According to the information I collected online, the first ionization energy of oxygen is 1313.94 kJ/mol and that of chlorine is 1251.19 kJ/mol. So the ionization energy of Cl is lower than that of oxygen, making Cl reasonable to be put as the central atom.

Shutong Hou_1F

Lewis structure clearly shows number of atoms, valence electrons, and types of bonds in a simple way. However, unlike the VSEPR model, Lewis structure doesn't represent the shape of the molecule or bond angles.

Shutong Hou_1F

When a compound melts, it goes from solid to liquid, increasing the distance between molecules. Hydrogen bonding is a strong intermolecular force, so with hydrogen bonding, increasing the distance between molecules within a compound is harder, requiring more energy, thereby resulting in a higher mel...

Shutong Hou_1F

The electronegativity of O is greater than that of S, because they have the same number of valence electrons, but O has more effective nuclear charge, because O has more protons, therefore more positive charges, in the nucleus.

Shutong Hou_1F

The most favorable structure should be the one with 0 formal charges on each atom. If it is impossible to have each atom with 0 formal charge, then the atoms with negative formal charge should be the ones with greatest electronegativity.

Shutong Hou_1F

Since in forming covalent bonds, electrons are shared between neighboring atoms, an increase in covalent character means that even though in forming ionic bonds, electrons from the less electronegative atom are taken by the more electronegative atom, the less electronegative atom would actually attr...

Shutong Hou_1F

The difference in electronegativity between neighboring atoms leads to difference in charge, which generates an electric field and could be measured.
dipole moment = charge * distance between atoms (bond length)

Shutong Hou_1F

Yes, it works, because it is actually equivalent to the formal formula, numerically. As long as not written formally in exams, I think that using this equation instead is much faster in calculation and harder to make mistakes.

Shutong Hou_1F

Indeed, F has greater electronegativity than Cl. I think it is because of greater electronegativity, electrons are pulled to F by a greater electrostatic force than to Cl.

Shutong Hou_1F

Being isoelectronic elements means that they have the same number of valence electrons. However, this doesn't mean that they have similar chemical properties, partially because they have different nuclear charges. I think that when mentioning isoelectronic, the elements aren't in the state of atom. ...

Shutong Hou_1F

Yes, it matters. The most ideal situation is to have 0 formal charges on each atom. If it cannot be 0, then it is best to have the least absolute sum value of formal charges with the most negative on the most electronegative element.

Shutong Hou_1F

There are many exceptions to the octet rule. The octet rule works mainly well for elements in the first three periods, because they don't have d-orbitals. Since the 4th period, the electron cloud become relatively big (having d-orbitals), therefore allowing electrons more than eight as valence elect...

Shutong Hou_1F

I think that remembering that s is on the left, p is on the right, d is in the middle, and f is at the bottom would be enough for this course. And remember that f-block is actually inserted in the 6th and 7th period, between s-block and d-block.

Shutong Hou_1F

I believe that we don't need to know how to calculate bond length throughout chem 14A. However, I think it is necessary to know that the bond length of triple bond is shorted than double bond, which is shorter than single bond. There are three pairs of shared electrons in triple bond, so the forces ...

Shutong Hou_1F

Because the inner electrons shield outer electrons from electrostatic attraction of the positive nucleus, the outer electrons feel a reduced electrostatic attraction, resulting in an effective nuclear charge. In my opinion, the larger the effective nuclear charge, the higher the first ionization ene...

Shutong Hou_1F

If an element has a high electronegativity, does that mean it has higher electron affinity? I believe so because for one of our practice questions, the answer was something along the lines "If an element has a high ionization energy and a higher electron affinity, it is also highly electronega...

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