CHEMISTRY COMMUNITY

Kelly Tran 1J

I would go with what Dr. Lavelle stated in lecture. If Ka < 10^-4, you can approximate to solve for x. If you are unsure, you can check to see if your approximation is valid by determining whether the value of x you solved for is less than 5% of the initial concentration. If it is, then the approxim...

Kelly Tran 1J

Increasing pressure (by decreasing volume) only applies to gases because for liquids and solids, changing the pressure does not change the volume. Therefore, you would only consider the moles of gases.

Kelly Tran 1J

[H30+] and [OH-] must be the same/equal in a neutral solution at 25C because the equilibrium constant for water, Kw, does not change. Since Kw = 1.0 x 10^-14 and is a constant, it will not change. This means that [H30+] = [OH-], and the only value that works for those concentrations is 1.0 x 10^-7.

Kelly Tran 1J

The concentrations of pure substances (solids and liquids) are not involved in the calculation of the equilibrium constant, K, of chemical reactions because the change in their concentrations is so small, it is negligible. For example, if you have $1 million and you lose $1, you would still say that...

Kelly Tran 1J

Yes, you can use the approximation method when K < 10^-4. In today's lecture (Lecture #4), Dr. Lavelle mentioned that you can use approximations not just for cubic equations, but also for quadratic equations as long as K < 10^-4.

Kelly Tran 1J

Both Q and K are calculated in the same way by calculating the ratio of [P] / [R]. If the chemical reaction is at equilibrium, you are calculating for K, but if the chemical reaction is not at equilibrium, you are calculating for Q.

Kelly Tran 1J

As others have expressed above, changing the pressure by changing the volume will cause the reaction to change because changing the volume changes the concentration of the products and reactants. On the other hand, changing the volume by adding an inert gas will not cause the reaction to change beca...

Kelly Tran 1J

As others have stated above, only the moles of gasses are considered when determining the effect of changing the pressure.

Kelly Tran 1J

The reaction quotient Q is used when you are calculating the ratio of the concentration of products not at equilibrium to the concentration of reactants not at equilibrium. In other words, you are calculating Q when the reaction is not at equilibrium.

Kelly Tran 1J

Homogeneous equilibrium refers to all of the reactants and products being in the same phase. Heterogeneous equilibrium refers to at least one or more of the reactants or products being in different phases. The molar concentration of a pure substance (solid or liquid) does not change in a reaction, s...

Kelly Tran 1J

When Qc > Kc, the reverse reaction is favored (reactants are favored).
When Qc < Kc, the forward reaction is favored (products are favored).
When Qc = Kc, neither reactants nor products are favored.

Kelly Tran 1J

Pure liquids are not involved in the calculation for the equilibrium constant because their change in concentration is so small that the change is negligible. Professor Lavelle used money as an example to illustrate this concept. For example, if you had $1 million and you gained or lost $1, you woul...

Kelly Tran 1J

Q is the reaction quotient and K is the equilibrium constant. Both Q and K are calculated in the same way; however, the difference between the two is that Q is calculated when the reaction is not at equilibrium whereas K is calculated when the reaction is at equilibrium. In other words, Q = [P] / [R...

Kelly Tran 1J

The units will tell you whether you are calculating for Kc or Kp. If the molar concentration is given, you are calculating for Kc. If the units of atm/bar is given, you are calculating for Kp.

Kelly Tran 1J

The first statement means that the concentration of the reactants and the concentrations of the products are the same. For example, [R] = [P] = 1.0 M. This statement would be false because the concentrations of the reactants and products do not have to be the same values. The second statement means ...

Kelly Tran 1J

Arrows indicating a dipole moment point in the direction of the more electronegative atom.

Kelly Tran 1J

pKa is the -log(Ka) and it tells you how acidic the compound is (the strength of the acid). The lower the pKa value, the stronger the acid. The higher the pKa value, the weaker the acid.

Kelly Tran 1J

Yes, you are correct. Bronsted acids are proton donors while bronsted bases are proton acceptors, and lewis acids are lone pair acceptors while lewis bases are lone pair donors.

Kelly Tran 1J

A lewis acid is an electron lone-pair acceptor, and a lewis base is an electron lone-pair donor. A bronsted acid is a proton donor, and a bronsted base is a proton acceptor. The difference between lewis acids/bases and bronsted acids/bases is what you are referring to. If you are describing the tran...

Kelly Tran 1J

The bond angle would be less than 109.5 due to the tetrahedral arrangement. The exact value of the bond angles depends on the atoms being bonded.

Kelly Tran 1J

[OH-] is the concentration of hydroxide ions (OH-) whereas pOH is the -log of [OH-]. In other words, pOH is a value that indicates how basic the solution is.

Kelly Tran 1J

Seesaw and trigonal pyramidal are not the same shape. Seesaw has 5 regions of electron density (1 of which is a lone pair while 4 are bonding pairs). Trigonal pyramidal has 4 regions of electron density (1 of which is a lone pair while 3 are bonding pairs).

Kelly Tran 1J

As others have said above, wedges and dashes are used to help you visualize the shape and structure of the molecule. If you had a 3-d model of the molecule in front of you, a wedge indicates that the bond is facing towards you while a dash indicates that the bond is facing away from you.

Kelly Tran 1J

Yes, lone pairs come into play in hybridization since it is a region of electron density and regions of electron density contribute to hybridization.

Kelly Tran 1J

When calculating the pH, pOH, [H+], and [OH-] of a solution, it is important to keep these formulas/equations in mind:
[H+] [OH-] = 1.0 x 10^-14
pH + pOH = 14
pH = -log[H+]
pOH = -log[OH-]

Kelly Tran 1J

To add on, pH is the -log of [H3O+] (hydronium ion concentration) whereas pKa is the -log of the Ka value or acid dissociation constant. The lower the pKa value, the stronger the acid, and the higher the pKa value, the weaker the acid.

Kelly Tran 1J

Yes, a coordinate covalent bond connects the ligands to the transition metal ion. The ligand is contributing both electrons since it forms a coordinate covalent bond with the transition metal ion.

Kelly Tran 1J

Bronsted bases and lewis bases are essentially the same; the only difference is how you use the term to describe the molecule/compound. When referring to protons, bronsted bases are used to indicate that it is a proton acceptor. When referring to electrons, lewis bases are used to indicate that it i...

Kelly Tran 1J

To reiterate what others have said, a lewis acid is an electron lone pair acceptor while a Bronsted acid is a proton donor.

Kelly Tran 1J

A lewis acid is an electron acceptor. BF3 is a lewis acid since it is electron deficient, which means that it can accept a lone pair of electrons.

Kelly Tran 1J

A bronsted acid is a proton donor whereas a bronsted base is a proton acceptor. This is similar to lewis acids and bases where lewis acids are electron acceptors whereas lewis bases are electron donors.

Kelly Tran 1J

You can determine whether a ligand is neutral by calculating its formal charge or by memorizing which ligands are neutral. Calculating the formal charge allows you to determine the charge of the ligand, which will help you calculate the oxidation number of the transitional metal since the individual...

Kelly Tran 1J

Since the Cl and H20 are outside of the brackets, they do not interact directly with the transition metal and therefore are named as they normally are. The rules for naming coordination compounds only apply to the molecules/ions inside of the brackets.

Kelly Tran 1J

You would use the prefixes bis-, tris-, tetrakis-, or pentakis- if the ligand has the prefixes di-, tri-, or tetra- in its name or if the ligand is polydentate. For your example the name of the coordination compound is bisethylenediammine.

Kelly Tran 1J

Hi! A sigma bond is a bond that is formed when two orbitals, each with 1 electron, interact end-to-end. A pi bond, on the other hand, is a bond that is formed when two orbitals, each with 1 electron, overlap side-by-side. All single bonds are sigma bonds. A double bond has one sigma bond and one pi ...

Kelly Tran 1J

Since bond angles are experimentally determined, we are not able to definitively state what the bond angles between atoms are. However, we can use the VSEPR model to estimate what the bond angle should be. Identifying the electron arrangement of the molecule will help us qualitatively determine the ...

Kelly Tran 1J

A single bond is a sigma bond. A sigma bond is when 2 orbitals, each with 1 electron, interact end-to-end to form a bond. When two atoms are bound by more than one covalent bond, one of those bonds will be a sigma bond, and the additional bonds will be pi bonds.

Kelly Tran 1J

The coordination number is the number of bonds present within the molecule. In [Fe(CN)6]^4-, the coordination number is 6 since there are 6 things bonded to the central atom.

Kelly Tran 1J

When naming the shape of a molecule, you would only consider the position of the bound atoms (bonding pairs). Considering the regions of electron density (bonding pairs and lone pairs) allows you to determine the electron arrangement of the molecule. Having different atoms surrounding the central at...

Kelly Tran 1J

As others have said above, bond order is the number of bonds present between 2 atoms. If there is a single bond between 2 atoms, the bond order is 1. If there is a double bond between 2 atoms, the bond order is 2. If there is a triple bond between 2 atoms, the bond order is 3.

Kelly Tran 1J

To determine if a covalent bond is strong, you can determine how many bonds the molecule has. Triple bonds indicate the strongest bond, double bonds indicate a stronger bond, and single bonds are the weakest bonds. You can also use the atomic radius to determine the bond length as well. For example,...

Kelly Tran 1J

Ionic bonds are typically formed between a metal and a nonmetal while covalent bonds are typically formed between 2 nonmetals. An atom on the far left of the periodic table and an atom on the far right would form an ionic bond since the electronegativity difference is higher while 2 atoms close toge...

Kelly Tran 1J

Hi! Lone pairs occupy a larger volume, and the repulsion strength between a lone pair and a bonding pair is greater than that of a bonding pair and a bonding pair. Due to this, the lone pairs push down on the bonded pairs, distorting (decreasing) the bond angles. Hope this helps!

Kelly Tran 1J

The octet rule is more of a guideline as there are some exceptions to it. For example, an expanded valence shell is an exception to the octet rule. Atoms in period 3 or higher can accommodate more than 8 valence electrons.

Kelly Tran 1J

Sigma bonds are present in every covalent bond (so they are present in single, double, and triple bonds), while pi bonds are present in double and triple bonds. For example, if there is a single bond, it is a sigma bond. In a double bond there is 1 sigma bond and 1 pi bond, and in a triple bond ther...

Kelly Tran 1J

I'm not sure if this is true, but I think they technically can? For example, a molecule can have polar bonds, which describes a polar characteristic. However, the molecule may be nonpolar overall if the dipole moments cancel out, which describes a nonpolar characteristic.

Kelly Tran 1J

The electron repulsion between lone pairs is stronger than the repulsion between bonding pairs since lone pairs occupy a larger volume. In other words, since lone pairs occupy a larger volume, they are able to push down on the bonded pairs, which is why lone-lone pair repulsion is stronger compared ...

Kelly Tran 1J

I agree with the above posts. Radicals are exceptions to the octet rule since they have an unpaired electron or an odd number of electrons. Since, CH3 and OH have 7 electrons, there will be an unpair electron in the lewis structure.

Kelly Tran 1J

Polarizability refers to the ability of the anion's electrons to be distorted or pulled into the shared/bonding region. Large anions are highly polarizable. Polarizing power refers to the cation's ability to pull (distort) the anion's electrons into the shared/bonding region. Small, highly charged c...

Kelly Tran 1J

Hi! Polarizing power is the ability of the cation to pull (distort) the electrons of the anion into the shared/bonding region. Polarizability refers to the ability of the anion's electrons to be distorted and pulled into the shared/bonding region. Hope this helps! Small, highly charged cations have ...

Kelly Tran 1J

Hi! The VSEPR notation would be AX4 as you said. Since, the VSEPR notation is a general formula, it does not matter if the bonded atoms are different; the formula would still be the same.

Kelly Tran 1J

Hi! Electron density refers to the regions of where electrons are present as a lone pair or bonding pair. The lone pairs push down on the bonded atoms since the electron repulsion strength between a lone pair and bonding pair is stronger than that of a bonding pair and a bonding pair. Hope this helps!

Kelly Tran 1J

To determine whether a bond is ionic or covalent, you can calculate the electronegativity difference between 2 atoms. If the difference is >2, it is an ionic bond. If the difference is <1.5, it is a covalent bond. Typically the bond between elements in the far left and far right of the periodic tabl...

Kelly Tran 1J

A main distinction between lewis acids and bases is whether they accept or donate a lone pair of electrons. Lewis acids are lone pair acceptors since they require a lone pair to achieve an octet. Lewis bases are lone pair donors since they have a lone pair available that can be accepted by a lewis b...

Kelly Tran 1J

I don't think the direction of the spin state matters as long as Hund's rule is applied correctly. However, the spin up state is typically drawn first in each orbital and then the spin down state is drawn.

Kelly Tran 1J

Only elements in period 3 or higher can have expanded octets, and since nitrogen is an element in period 2, it can not have an expanded octet. Due to this, nitrogen can not have 5 bonds (in the picture: 1 single bond and 2 double bonds), so there is only 1 double bond (N=O) in the lewis structure of...

Kelly Tran 1J

I agree with the above posts. I think using the formal charge equation is the best way to determine the total formal charge because it will ensure that you calculate the formal charges of each individual atom correctly. It will also ensure that you assign the correct formal charge to each individual...

Kelly Tran 1J

I agree with the above post. I think the important thing to focus on is the trend of bond lengths and strength. Single bonds are longer, and double bonds are shorter. Longer bonds are weaker due to a lesser attractive force between the electrons, and shorter bonds are stronger due to a greater attra...

Kelly Tran 1J

As stated above, elements in period 3 and above have expanded octets since they have d-orbitals. This means that those elements can have more than 8 valence electrons.

Kelly Tran 1J

As others have stated above, the order of the numbers do not matter as long as the magnetic quantum numbers are not assigned to a specific orbital. However, we typically associate the order of the numbers with the order of the orbitals. For example, the magnetic quantum numbers -1, 0, 1 are typicall...

Kelly Tran 1J

As others have stated above, we calculate the formal charge of each atom to determine the most favored/preferred lewis structure. The lewis structure that has more atoms with a formal charge of 0 is the preferred structure since it is the most stable.

Kelly Tran 1J

The most favored resonance structure is the one with the lowest formal charge. In other words, the most favored lewis structure is the one that has more atoms with a formal charge of 0. I believe Dr. Lavelle mentioned this in Monday's lecture as he went over formal charge and how to calculate formal...

Kelly Tran 1J

Like others have said above, I think a good tip would be to write out every step of the problem solving just so you ensure that you are using every value that is required in the calculation. I also recommend writing out all of the units for each value as well because it is a great indication of whet...

Kelly Tran 1J

When two electrons are paired, it means that they are in the same orbital and will have opposite spin (Pauli Exclusion Principle) where one electron will be spin up and the other will be spin down. When two electrons are parallel, it means that they occupy different orbitals and will have the same s...

Kelly Tran 1J

The number in front of the configuration (in your case 1 and 2) is the principal quantum number(n), which is called the shell and tells you the energy and size of the electron. The larger the value of n, the larger the electron, and the more energy it has.

Kelly Tran 1J

You would have to draw out the orbitals and use the Pauli Exclusion Principle and Hund's Rule to fill in the orbitals. Since your possibilities for ml are -1, 0, +1, - if the last electron is in the first orbital, ml = -1 - if the last electron is in the second orbital, ml = 0 - if the last electron...

Kelly Tran 1J

An atom has an octet when it has 8 electrons. To determine if the atom has 8 electrons, you would count up the lone pairs and the bonded electrons. Each lone pair has 2 electrons, and each bond (line) consists of 2 electrons.

Kelly Tran 1J

I agree with Astha! The first 3 quantum numbers (n, l, and ml) can be the same for 2 electrons.

Kelly Tran 1J

Another way to calculate how many electrons could have the specified set of quantum numbers is by determining the number of orbitals and then multiplying by 2. For example, n=3 means that there is a 3s, 3p, and 3d orbitals. Therefore, there is 1 s-orbital, 3 p-orbitals, and 5 d-orbitals for a total ...

Kelly Tran 1J

I agree with the above responses as well. I would input the entire electron configuration. Also, make sure to use the sapling tool to insert exponents instead of using the ^ symbol, and don't forget to remove the comma!

Kelly Tran 1J

When solving the the De Broglie equation, all of the units should be the same, so make sure to convert the mass in grams to kilograms.

Kelly Tran 1J

In v=R(1/(n2)^-1/(n1)^2), n2 is the lower energy level and n1 is the higher energy level. I like to use v=R(1/(nF)^-1/(nI)^2), where nF is the final energy level and nI is the initial energy level. In my opinion, using nF and nI instead of n2 and n1 is "better" since I have seen others swi...

Kelly Tran 1J

Once given the wavelength, you should be able to figure out whether it belongs to the Lyman or Balmer series. For example, if the wavelength belongs to one from the visible light spectrum, then it is the Balmer series, meaning n2 should be 2. If in Lyman's series, n2 is 1 After doing what was menti...

Kelly Tran 1J

An electron being excited means that the electron is moving up to a higher energy level. In order for the electron to move up to a higher energy level, the energy of the incoming photon must be sufficient enough to overcome the threshold energy (the energy required to remove an electron). When the e...

Kelly Tran 1J

Knowing the 4th quantum number is not only important because it tells you the electron's spin direction, but it also specifies the electron. While 2 electrons can have the same first 3 quantum numbers, no 2 electrons can have the same 4th quantum number as the spin states can either be spin up or sp...

Kelly Tran 1J

Yes, you would multiply each coefficient by 2 since the stoichiometric coefficients must be whole numbers. In the notes for this topic, Dr. Lavelle stated how multiplying both sides of the chemical equation by a number does not affect the balance of each atom type.

Kelly Tran 1J

Yes! In this case, intensity is proportional to the number of photons, so increasing the intensity of the light will not increase the energy of the photons. (If each individual photon does not have enough energy, increasing the amount of photons does not change the energy of each individual photon.)...

Kelly Tran 1J

Yes, in order to calculate the wavelength, you would divide 1,650,763.73 by 1.

Kelly Tran 1J

To calculate the kinetic energy of the ejected electron, you would use the equation: Kinetic energy = (1/2)(M)(V)^2, where M = the mass of the electron and V = the velocity of the electron. You would not use the equation: Kinetic energy = E(photon) - work function since there are 2 unknowns; the kin...

Kelly Tran 1J

To calculate the energy of the incident light, use the equation:
E(photon) = work function - kinetic energy

To calculate the wavelength of the incident light, use the equation:
lambda = hc/E

Kelly Tran 1J

The smallest detectable wavelength is 10^-15 m. Anything smaller than this is not detectable, which is why objects with larger masses have particle-like behavior and do not have any detectable wave-like properties.

Kelly Tran 1J

If the energy of the photon is less than the threshold energy, then the photons will pass through the metal. According to the Bohr Frequency Condition, light is absorbed or emitted if the frequency (or energy) of the incoming photon is equal to or greater than the energy difference.

Kelly Tran 1J

The theoretical yield is the maximum amount of product that can be formed and is typically in grams since the experimental yield is typically given as a mass.

Kelly Tran 1J

I'm not sure if we have to know the wavelengths of the regions of the EM spectrum, but if the wavelength is between 700nm - 400nm, it is part of the visible light region.

Kelly Tran 1J

To summarize, the original purpose of the photoelectric effect was to measure how much energy of light was required to remove an electron from a metal atom. This experiment was conducted by shining light on a metal surface. If the light had enough energy, it would interact with the atoms on the meta...

Kelly Tran 1J

I'm not sure if we have to know the wavelengths of each region of the electromagnetic spectrum, but if the wavelength ranges between 400nm and 700nm, it is in the visible region.

Kelly Tran 1J

Another way you can solve this problem is by using the equation: En = -hR/n^2, where n is equal to the energy level. First, you would calculate En for n = 3 and n = 1. Next, you would calculate the change in energy for the electron, Delta E, by plugging those values into the equation: Delta E = E(fi...

Kelly Tran 1J

If a certain frequency of light is not enough to eject an electron, then increasing its intensity will not cause it to eject an electron because the energy of each individual photon is still not sufficient enough to overcome the threshold energy (the energy required to remove an electron from the me...

Kelly Tran 1J

Also, when multiplying and dividing numbers in scientific notation, make sure that your answer has the correct number of significant figures. Your answer should have the same number of sig figs as both values. However, if both values have a different number of sig figs, your answer should have the l...

Kelly Tran 1J

I would say use scientific notation if the problem lists the values in scientific notation of if the answer is a large decimal or has many zeros. As long as your answer is in the correct number of sig figs, it should not matter if it is in scientific notation or not.

Kelly Tran 1J

Empirical formulas show the relative number of atoms whereas molecular formulas show the actual number of atoms. You can think of empirical formulas as a ratio of the atoms and molecular formulas as the total number of atoms.

Kelly Tran 1J

Personally, I balance the elements from left to right. I find that it is easier to do balance chemical equations in this way because you are able to keep track of which elements you have already balanced and which ones you have not. I keep track of the coefficients in my head because I find that wri...

Kelly Tran 1J

I would say that your answer should match the units specified in the problem. For example, if the problems specifies that mL are used, your answer should be in mL. However, if the unit is not specified in the problem, I would use SI units. As long as your answer has the correct number of sig figs, I...

Kelly Tran 1J

Yes, you would need to find the limiting reactant first in order to calculate the theoretical yield since the limiting reactant limits the amount of product.
The actual yield will be given in the problem.

Kelly Tran 1J

To find the uncertainty in speed of the electron using the Heisenberg Uncertainty principle, you first have to find delta P. To find delta P, you would use delta P = h/(4pi*delta x). This means you would have to calculate the position of the electron using the values from the problem. Then, once you...

Kelly Tran 1J

You would use Avogadro's Number when converting from moles to a number of atoms/particles, or from the number of atoms/particles to moles. To convert from moles to atoms/particles, you would multiply the given amount of moles by 6.022 x 10^23 (Avogadro's Number). To convert from number of atoms/part...

Kelly Tran 1J

Empirical formulas can be thought of as the relative number of atoms or the ratios of atoms. Molecular formulas, on the other hand, show the actual or total number of atoms.

Kelly Tran 1J

Typically you would wait until the very end before rounding to the specified amount of sig figs. If I remember correctly, Dr. Lavelle mentioned that everyone would be using the same periodic table for the midterms and final, but I am not entirely sure.

Kelly Tran 1J

Yes, molecular spectroscopy can be used to identify molecules because every molecule has its own spectral pattern similar to how people have their own set of fingerprints.

Kelly Tran 1J

To convert grams to moles you would take the sample mass and divide it by the molar mass. (i.e. sample mass (g) / molar mass (g.mol^-1). To convert millimoles to moles you would move the decimal place over 3 places. (i.e. 100 --> 0.100).

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