CHEMISTRY COMMUNITY

Brian Tangsombatvisit 1C

Once you figure out the chemical equation for the reaction, solving for the equilibrium constant is pretty straightforward. From the picture, you can see the equilibrium concentrations of the reactant and products based on where the number is when the line flattens out, because this means that the c...

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A reaction is zero order when the rate of reaction is independent of the concentration of R, often when the catalyst and enzyme are saturated and increasing the concentration of R does not increase the rate. The rate is limited by k only. It can be determined by looking at experimental data. For exa...

Brian Tangsombatvisit 1C

A zero order reaction's rate is independent of the concentration of R, often when the catalyst or enzyme is saturated and increasing the concentration of R does not increase the overall rate. Since the rate is dependent solely on k, the presence of a catalyst would increase the reaction rate since i...

Brian Tangsombatvisit 1C

This information should be given to you by the problem as there is no way to determine whether one reaction step is faster or slower than another. Just know that the slow step determines the rate law and if there is an intermediate in the slow step, treat the fast step above it as an equilibrium and...

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The activation energy of the catalyzed reaction is 75 kJ/mol. The activation energy of the uncatalyzed reaction is 125 kJ/mol. Therefore, the activation energy of the catalyzed reaction is 75/125 times the activation energy of the uncatalyzed reaction, which is 125 kJ/mol. Therefore, (75/125) times ...

Brian Tangsombatvisit 1C

You can probably assume both of the electrodes are exposed to air since they are not normally closed cells. Therefore, oxygen plays a role in both the anode and cathode half reactions.

Brian Tangsombatvisit 1C

After balancing the half reactions at the anode and cathode, and multiplying by whatever number to make the number of electrons the same in the cathode and anode half reactions, you would use the number of electrons in either the anode or cathode half reaction as n (it should be the same number for ...

Brian Tangsombatvisit 1C

The KOH is just another way of representing the presence of OH- ions at the anode. KOH will fully dissociate into K+ and OH- ions, so the K+ is a spectator ion and does not affect the reaction at the anode. If you were doing this experiment in a lab you would probably add OH- ions to the cell in the...

Brian Tangsombatvisit 1C

The left side of the cell diagram corresponds to the anode, so you know oxidation will occur here. Since the overall molecule of HCl is neutral, this must mean that the H has a charge of +1 and the Cl has a charge of -1. Since HCl is a strong acid and completely dissociates, we can assume that H2 is...

Brian Tangsombatvisit 1C

You can use either forms of the equation with either log or ln because you will get the same answer either way. It just might be sometimes more convenient to use the log form of the Nernst equation if you're dealing with pH since pH = -logH+. In most of the problems however either equation will work...

Brian Tangsombatvisit 1C

You need to use platinum as a metal electrode for both sides since none of the products or reactants are conducting metal solids. I2 is a solid at the anode, but since it is a metal it doesn't count. At the cathode, there are only aqueous ions so you would also need the platinum electrode here to al...

Brian Tangsombatvisit 1C

What exactly is the cathode and anode in the pH meter? How does it work? I know it measures Ecell and then uses that to calculate H+ concentration and thus pH.

Brian Tangsombatvisit 1C

The system in this case is the potassium nitrate and water. Enthalpy for the system depends on the entropy of the surroundings because the heat going into the system comes from the surroundings (it doesn't just appear out of nowhere). If the surroundings loses heat as a result of heat gain from the ...

Brian Tangsombatvisit 1C

You're right, if the Cu half reaction is written on the right side of the equation, it should be the cathode. This is probably a typo in the textbook as the solutions manual writes it on the left side of the cell diagram, making it the anode.

Brian Tangsombatvisit 1C

For cells that involve gases, such as hydrogen gas or chlorine gas, the pressure of these species matters. In these cases, the partial pressures of gases would probably be used over concentrations. Depending on the partial pressures, E would differ because of the nernst equation, E = Enot - (RT/NF)lnQ

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It can be any positive and negative ions that are soluble in solution. They don't have to include the ions in either anode or cathode solution as long as they balance the charge of the 2 solutions after the reaction takes place. Usually the salt bridges contains a salt such as sodium or potassium ch...

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To balance a half reaction we would normally add H20 first to balance the number of oxygen atoms on both sides of the half reaction. Since we added H20, there is now an imbalance of H's, so we would add the number of H20 molecules on the opposite side per H atom needed. Since there are twice as many...

Brian Tangsombatvisit 1C

You can tell a compound's charge by looking at its net charge, which is usually 0 if there is no superscript shown. The superscript of the compound (ex: -2 or +3) shows its overall charge. From this information you can calculate the individual charge of each atom within the compound since they all h...

Brian Tangsombatvisit 1C

You use the Van't Hoff equation to find a different K at a different temperature if the enthalpy of a reaction is known. Using the equation, you plug in all the values you are given (K1, H, T1 and T2) to find K2. Or you can be given K2 instead of T2 and use the equation to find T2.

Brian Tangsombatvisit 1C

for redox reactions, it's not always going to be metals that are oxidized and reduced. In this example, the Cr is being reduced, but the C, a nonmetal, is actually being oxidized. If you calculate the charge of C in C2H5OH you will see that it is -2 in this molecule, but in the products (C2H4O) it i...

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there has to be a conservation of charge on the reactant and product side of the equation. If you think of an electron as an element, there has to be the same number of both the reactant and product side.

Brian Tangsombatvisit 1C

More complex and larger molecules have higher residual entropies, since they have more vibrational/rotational modes and possible states of arrangements. Also, all molecules in the gas phase have more residual entropy than all molecules in the liquid phase, which all have more entropy than molecules ...

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for a system at constant volume, there is no work being done since this cancels out the term. therefore, the internal energy equals heat, which is q (U=q). The equation for enthalpy is H = U + PV, so in order to find enthalpy, you would plug in q for U and nRT for PV if you are given moles and tempe...

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For an ideal gas, the only type of energy that contributes to its internal energy is the average kinetic energy (since supposedly for an ideal gas there are no interactions between the molecules, so the gas's energy only comes from its motion, KE). The equation for internal energy (avg KE) of an ide...

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the 2 main types are residual/positional/statistical and thermal entropy. Residual entropy is related to the number of microstates a system can have, which increases with greater volume or greater number of states or greater number of atoms (calculated by S = kblnW or deltaS = nrln(v2/v1)). Thermal ...

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Reversible work = integral of Pdv, which simplifies to -nRTln(v2/v1). External pressure is not constant. During reversible processes, the system is always at equilibrium and therefore the entropy for these systems is 0. These processes are usually isothermal, meaning that energy lost by the system t...

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q is heat and is in units of joules. It is really heat transfer from one object to another. Heat capacity is the amount of heat needed to raise an object by 1 degree Celsius, in units of J/C. It is an extensive property since it depends on the amount of material present (larger object = larger heat ...

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would the enthalpies for the reactants and products in steps 1 and 3 be both positive? or would reactants be positive and products be negative?

Brian Tangsombatvisit 1C

Suppose a researcher finds that delta Hrxn = -2756 kJ for the reaction at 200. C. Assuming all heat capacities are constant, calculate delta Hrxn at the temperature of the human body, 37 C. Hint: since enthalpy is a state function, the process can be divided into three steps. Can somebody help me on...

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Right now, the only instance where we would have to use calculus is calculating the work done by reversible expansion, which is given by w = -(integral) Pdv. However, in many cases this can simplify to w= -nRTln(V2/V1), which is a simplified version of the integral that we can use if are given the n...

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Some extra things to remember: when heating an endothermic reaction, the K of the reaction permanently increases; conversely when heating an exothermic reaction, the K of the rxn permanently decreases. Also, when determining reaction shifts according to changes in pressure (and the rxn involves gase...

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To get a number for W, you would have to determine the total number of possible states that your molecule or atoms could be in. For a 2-state system, the total number of possible outcomes your system could have is 2^n, where n= the number of atoms. You would typically use this equation to solve for ...

Brian Tangsombatvisit 1C

For reversible expansion, if we know the number of moles, temperature, and final/initial volume of the sample, we can just use the simplified version of the integral, which is w = -nRTln(V2/V1) instead of integrating. If we were to integrate, I believe we would need a function for pressure with volu...

Brian Tangsombatvisit 1C

When using the equation -nRTln(V2/V1) for reversible reactions, does n stand for the change in moles after the reaction takes place or does it stand for the total number of moles?

Brian Tangsombatvisit 1C

I guess we would actually only have to integrate if we were given some sort of equation for pressure that is a function of volume, without the number of moles or temperature for the problem. Otherwise, if we know the final and initial volume, the number of moles, and temperature at which the reactio...

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I've seen kJ and kJ/mol being used interchangeably for both reaction enthalpy and enthalpy of formation, so are either units okay? Or does enthalpy of formation typically have units of kJ/mol and does reaction enthalpy typically have units of just kJ?

Brian Tangsombatvisit 1C

The units of specific heat capacity and molar heat capacity can vary in terms of temperature units, but they are distinct based on whether there are grams or moles in the denominator. Joules or kilojoules can be in either numerator, and units of kelvin or celsius are also interchangeable in both. I ...

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There is not really a singular equation you would use to solve this part b) of this question. Once you have calculated the heat necessary to raise the temperature of water and the heat needed to raise the temperature of the entire solution, you can just divide the heat transferred to water by the to...

Brian Tangsombatvisit 1C

When water in liquid form at 100 C comes into contact with skin, energy is transferred from the water to your skin since it is at a higher temperature. However, with water in vapor form at 100 C, when it comes into contact with your skin, it condenses, transforming from a vapor to a liquid, releasin...

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If you were doing a calorimeter experiment in a lab, then you would have to account for the heat absorbed by the calorimeter itself and the surroundings. For the purposes of this class, however, I believe you can consider this fact to be negligible when doing homework problems and calculations.

Brian Tangsombatvisit 1C

My explanation would be that the resulting solution is not 100% pure water (it has salt in it), but the total volume of the products is still 100 mL; when we are finding the volume of water we are assuming that it is the total volume of the solution after the neutralization reaction. I guess Dr. Lav...

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I doubt we will need to use much calculus for Chem 14B, but if we do it will probably only be basic integration and nothing very complicated. If there are special integration or calculus skills that we need to use for problems I am sure Dr. Lavelle will go through it thoroughly in lecture.

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Acids donate their protons and are usually written as HA, where A is the rest of the molecule. Bases accept protons and usually have an OH somewhere within the molecule (an exception to this is the weak base NH3). Really long molecules with an amine group (NH2) are usually bases as the nitrogen atom...

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You would set up an ice table. Initial concentrations: 1 for H2, 2 for Cl2, and 0 for HCl. We don't know how much the change is, so we represent this with -x, -x, and +2x, respectively. Therefore, the expressions for the equilibrium are: H2 = 1-x, Cl2 = 2-x, and HCl = 4x^2. Write the expression for ...

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Since Ka1 is much larger than Ka2, the second dissociation reaction can be ignored. We can just focus on the first dissociation reaction, where (COOH)2 donates one of its protons to water (like any other acid), forming H3o+ and (COOH)COO-. You would set up your ICE table like normal and solve for x ...

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You would choose the gas constant to use based on what unit what you want to convert your concentration value into. Generally, you would use R when solving for P given the concentration using P = (n/V)RT. If you wanted your partial pressure to be in units of atmospheres, then you would use the gas c...

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Yeah. Even if you add a lot of reactant or product, give the reaction enough time, and it will eventually settle back to equilibrium with the same ratios of product to reactant. There will be more of both product and reactant in this scenario, but the ratio of the 2 will remain the same.

Brian Tangsombatvisit 1C

The explicit way to determine the response to a change in volume or pressure is to calculate the new concentrations of the each reactant and product. For example, since concentration is represented by n/V, increasing V would decrease concentration. Similarly, since pressure and volume are inversely ...

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Keep in mind that this method of determining the direction of the reaction can only be used if the pressure is increased via compression. If the pressure were to be increased by the addition of inert, noble gases, then this would have no effect of the reaction. The real reason why compression forces...

Brian Tangsombatvisit 1C

a) Set up a quick ice table. We estimate the initial concentrations to be A=27, B=C=0. The change will be: A goes from 28 to 18, B goes from 0 to 5, and C goes from 0 to 10. We can see that the differences between initial and final concentration are in multiples of 5 (A = -10, B = +5, and C = +10). ...

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Typically, if you are given a reaction with all reactants and products in the gas phase (and their units are given in partial pressures, not concentration) then it's assumed you're supposed to use Kp. If you do, just denote it. You could technically still calculate Kc in this scenario (where all the...

Brian Tangsombatvisit 1C

a) convert Cl2 to mol/L (for concentration) first. Then, set up your ice table (start with 0.0010 M Cl2 and 0 M Cl). You don't know how much of Cl2 was used up, so you represent the change as -x. Plug in the expressions at equilibrium into the expression for K, which is in terms of x. Use algebra to...

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H2O is a solvent, and we omit all solvents from the K expression since they will be in large excess compared to the concentrations of the other aqueous reactants and products. Its concentration will not change after equilibrium is met, so it is insignificant and can be omitted from the K expression.

Brian Tangsombatvisit 1C

In this case, since all the reactants and products for both reactions are in the gas phase, it is easier to derive the Kp if given the units for the gases. This is because Kp is another way to write the equilibrium constant, but for a reaction with all gases since each reactant and product will have...

Brian Tangsombatvisit 1C

If a molecule that is more stable than the previous molecule can form, then it will readily form this new molecule, since nature favors stability. Therefore, if a product is more stable than the reactants, as the reaction proceeds, more product will form, resulting in a higher equilibrium constant s...

Brian Tangsombatvisit 1C

The equilibrium constant is written as Kc when you use the concentrations of the reactants and products in the formula, in mol/L. The equilibrium constant is written as Kp when you use the partial pressures instead of the concentrations for the equation. You cannot mix concentrations and partial pre...

Brian Tangsombatvisit 1C

You can use partial pressures when all the reactants and products of a reaction are in the gas phase and when the units are in the gas phase (bar, atm, mmHg, Torr, etc). You normally use concentrations when other phases other than the gas phase are involved in the reaction, and you can use PV=nRT to...

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You would calculate the reactant quotient when you want to determine whether the reaction will proceed in the forward or the backward direction, assuming that the reaction is currently not at equilibrium. If the reactant quotient is greater than the equilibrium constant, then the reaction will proce...

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When naming coordination compounds given their formula, just to make sure, for NC (isocyano) and NCS (isothiocyanato), when alphabetizing the ligands you would could the first letter of each of these ligands as "i," correct? So for [Fe(NC)3F3]6- it would be trifluorotriisocyanoferrate(VI)?...

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Lyndon sees his crush and panics, accidentally dropping 382.7 mg of HCl and 147.1 mg CaO into a 1L flask. After filling that flask with water up to the 1L mark, what is the pH of the resulting solution?

Brian Tangsombatvisit 1C

Is the porphyrin ligand bound to Fe, which forms a heme complex, chelating? Do we also need to know the shape of myooglobin, which is octahedral? But the shape of the heme complex is square planar..

Brian Tangsombatvisit 1C

One example Dr. Lavelle did in lecture was [FeCl2(H2O)4], where the charge on iron was +3, so the charge on the overall molecule was +1. Because Fe was a highly charged, small cation, it weakened the O-H bond on one of the H2O's, releasing a proton and therefore acting as a Bronsted acid. It did thi...

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Is hydrogen bonding a form of dipole-dipole force, and if not, do we have to list both in a molecule like NH3?

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Explain why salts of weak bases produce acidic solutions and salts of weak acids produce basic solutions. Also, how do we determine whether the salt produced during a titration makes the solution acidic or basic?

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When writing the chemical formula for a coordination compound given its name, is there a particular order we write the anionic ligands or neutral ligands in if there are multiple? I know that you write the metal cation first, then anion ligands if any, then neutral ligands. For example, if there are...

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When naming complex ions I've noticed that some people use amine to represent (NH3), while some use ammine. Is there a difference? Does it matter which we use? Also, I've noticed that dien and en use "amine" instead of ammine when writing out the full names of these structures: diethylenet...

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I tried using the relationships pH= -log[H3O+], [H3O+][OH-]= 1.0x10^-14, etc, but I can't seem to get the right values for filling out the table.

Brian Tangsombatvisit 1C

when the transition metal is part of a complex ion that is negatively charged, you would name it using its latin prefix and adding -ate at the end instead of just using its regular name. I would say you should probably know a few of the common latin names besides iron like copper (which is cuprate) ...

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In both HBrO2 and HClO2, the H+ atom is bonded to an O, not Cl and Br. If the H+ were bonded to the Cl or Br, then you would be correct in that the H-Br is a weaker bond than H-Cl, making it a stronger acid. However, this is not the case. To determine the acid that is stronger, we would have to look...

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To show a chemical reaction of an acid reacting with water, the acid should donate one of its protons to water to form hydronium. However, since AlCl3 doesn't have any protons, it must first be hydrated with water. This makes sense since Al is a transition metal, and transition metals react with wat...

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If an anion from an acid after losing its H+ has resonance, then this resulting anion is more stable because the negative charge of the overall anion is delocalized. Since nature favors stability, the acid would continue to give away more of its H+ if the anion is stable compared to other acids with...

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An oxoacid is a compound that contains hydrogen, oxygen, and at least one other element, with at least one hydrogen atom bond to oxygen that can dissociate to produce the H⁺ cation and the anion of the acid. Oxoacids are stronger if they have more oxygen atoms, have very electronegative anions, or h...

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For HI, Iodine is a much larger and less electronegative atom than Fluorine, so it exerts a much weaker pull on H+. As a result, HI has a weaker bond, allowing it to dissociate to a much greater extent than HF. Since the strength of an acid is directly proportional to how much it is able to dissocia...

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Since Al and Cu are both transition metals, when AlCl3 and Cu(NO3)2 react with water, they become hydrated and form coordination compounds or complex ions. Al and Cu in particular like to form octahedral complexes and bind to 6 water molecules. They are both aqueous since they are dissolved in water.

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Transition metals can have multiple oxidation states because of their many electrons in the d subshell. The transition metals have several electrons in this shell with similar energies, so different numbers of them can be oxidized, depending the circumstances, resulting in different oxidation states...

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The atoms in Group 12 are transition metals, so they would likely either form ionic compounds with nonmetals or coordination compounds with ligands. There is no real way to determine a molecule's shape just looking at the central atom. You would have to determine which molecule you want to look at f...

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Dr. Lavelle mentioned many examples of biological applications in lecture, but I wouldn't say any one of them is more important than the other. Some examples he talked about was Cisplatin (cis-diamine-dichloro-platinum(II), a chemo drug which forms a coordination compound w/ DNA to stop cell divisio...

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A ligand is an ion or molecule that binds to a central metal atom to form a coordination complex, and the nature of the bonding with the metal ion can range from covalent to ionic. Ligands can be neutral or anions. Some examples of neutral ligands are H20, NH3, NO, and CO. Some anion ligands are any...

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I would think that since a radical is only 1 electron, it would cause less repulsion than a lone pair since it has 2. Therefore, I would conclude that a radical results in larger bond angles than if it was a lone pair because it repulses the other bonds less.

Brian Tangsombatvisit 1C

Yep, electron arrangement is only determined using the # of regions of electron density, while the molecule's shape is determined using the actual number of lone pairs and bond pairs. This means that molecules with different shapes (and different formulas, ex: AX4E for SF4) can have the same electro...

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Yes, if you tried to rotate a molecule along its double bond, the pi bond would break. This would only occur during a chemical reaction. You can only rotate molecules along their sigma bonds since doing this would not break the bond.

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CO2 only has London dispersion forces, or induced dipole - induced dipole interactions. Even though CO2 has polar bonds, the molecule is nonpolar because the dipoole moments cancel out. A nonpolar molecule cannot have dipole-dipole interactions. Only polar molecules can.

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Lone pairs have a stronger repulsion than bonding pairs because bonding pairs are farther away from the central atom since it needs to be connected to the sharing atom. Also, lone pairs are delocalized, meaning they are more spread out than bonding pairs and therefore fill more volume. They are also...

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The easiest way is to find the # of regions of electron density first by drawing the correct Lewis structure. Once you have found this number, you can determine the number of hybrid orbitals. Make sure the total number of orbitals and electrons remain the same before and after hybridization and that...

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All of the different Lewis structures contribute to the resonance model since it is a blend of these structures. Because of this, all of the bond lengths in this type of atom are the same, but are in between a single and double bond's length. However, resonance does not affect a molecule's bond angl...

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There are only dipole - dipole interactions when the overall molecule has a dipole. This is the case for a) and c) since the dipoles within the molecule do not cancel each other out and result in an overall dipole moment for the entire molecule. However, for d), even though the s-o bond is polar, th...

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Intermolecular forces occur between two molecules, not between two atoms. Examples of intermolecular forces include induced dipole - induced dipole (LDFs), induced dipole - dipole, dipole - dipole, ion - dipole, and ion - ion (in order of increasing strength). Induced dipole - induced dipole forces ...

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Van der waals is just another way of saying London forces, which are also known as induced dipole - induced dipole forces. All molecules have these forces, whether they are polar or not. When two molecules get close to each other, their electron distributions fluctuate, resulting in induced dipoles ...

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You should also remember that hydrogen bonding is the actually the intermolecular attraction between the hydrogen atom covalently bonded to O, N, of F and another O, N, or F with an available lone pair.

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Highly distorted electrons (from a large anion with weaker pull, for example) are described as being highly polarizable, which results in an ionic bond with more covalent character. Ions which cause large distortions have high polarizing power (cation). Anions usually have strong pulls, while cation...

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The structure that you have drawn has formal charges of 0 on the Cl and O, however the C has a formal charge of +2 since it only has 2 bonds. It also shows 26 valence electrons instead of 24. In order for the formal charges on all the elements to be 0, Carbon must be placed in the middle and have a ...

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The ionic bond behaves more covalent when you have a large anion and small cation, since the anion has a lower electronegativity and the cation has a higher electronegativity. This results in covalent-like behaviors since the cation and anion are more likely to share the electrons between them, rath...

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To determine the most stable Lewis structure, you would see which one has the most formal charges that are closest to 0. Molecules that have a net positive or net negative charge will inevitably have one or two elements that do not have a formal charge of 0, since a molecule's formal charges are sup...

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Professor Lavelle said in lecture previously that we would not be focusing on elements including the f-block. However, we did do that example in lecture today with an element that included the f-block. For studying, I would focus on elements including the d-block, but know how to do an electron conf...

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I'm not sure how to exactly calculate the effective nuclear charge felt by an electron when there's electron shielding. Professor Lavelle said in lecture that the outer electrons feel a reduced electrostatic attraction to the nucleus, resulting in an effective nuclear charge. It's not quite clear ho...

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The 3d shell is a continuation of the n=3 shell, so it does not play any role in affecting the number of valence electrons. The number of valence electrons is determined by the number of electrons in the outermost shell. The outermost shell for period 4 elements is the n=4 shell, therefore these tra...

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Bismuth is one of the weird elements that doesn't always follow the typical rules or have the usual characteristics of one of the first 18 atoms. When you see the roman numeral "III" just know that it is a cation with a charge of +3. I don't think we would have to memorize this cation of B...

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The En=hR/n^2 formula represents the energy an electron has when it is in the nth shell. The Rydberg equation, 1/wavelength=Rh((1/n1^2)-(1/n2^2), is derived from the change in energy formula, which is final energy - initial energy, meaning that it is delta E = Ef=hR/n^2 - Ei=hR/n^2. To get the Rydbe...

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Usually the atom with the lowest ionization energy or lowest electronegativity goes in the center of the atom. This is because this atom is less likely to hold onto its valence electrons tightly and more likely to share its electrons with other atoms. In the case with ONF, Nitrogen should go in the ...

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If you use Heisenberg's Indeterminacy Equation, you will find that the change in position and change in momentum variables are inversely related. Meaning, if you design your experiment so that you minimize the error found in its position, you will have a large error in momentum (velocity since mass ...

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