CHEMISTRY COMMUNITY

Wendy 1E

Dr. Lavelle, Thank you for being a great chemistry professor and for always supporting your students. I used to hate chemistry in high school because my teacher wasn't so great. But, thanks to you, I found a new interest in chemistry. It was a pleasure to be your student for two quarters. I hope you...

Wendy 1E

Yes, the units can be used to determine the order of the reaction. The way I think about it is, you start out with the units M/s for a zero-order reaction and then you keep dividing by M for each reaction after. For instance, the units for a first-order reaction are 1/s and the units for a second-or...

Wendy 1E

The exam will be submitted electronically. Dr. Lavelle will definitely email us to let us know how to turn it in. In his past emails, he said that he will post the final exam on his class website.

Wendy 1E

I suggest looking at the email that Dr. Lavelle sent us through email titled "Week 10 Review." It has the graphs for each order. Hope this helps!

Wendy 1E

An intermediate is always produced and then consumed. Meanwhile, a catalyst is consumed then reproduced.

Wendy 1E

Will we have to know how to derive the zero-, first-, and second-order equations, along with the half-life equations on the final?

Wendy 1E

Adding to this, the oxidation number of ozone (O3) is 0.

Wendy 1E

You add Pt(s) to the side of the cell diagram (either to the anode or cathode side) that lacks a solid metal conductor.

Wendy 1E

It is not. You should know how to derive it. If you don't know, I suggest just memorizing it.

Wendy 1E

I suggest always using platinum because using granite is not common. If we have to use granite, I feel like we would be specifically told to do so.

Wendy 1E

It doesn't really matter whether you use the ln Q or log Q equation. Some people prefer the log Q equation because on the equations sheet, you are given 0.05916V/n and you do not have to calculate R*T/nF, which you have to calculate with the ln Q equation.

Wendy 1E

If you are observing a galvanic cell, you can also tell if a reaction is spontaneous if E°cell is positive.

Wendy 1E

When E°cell=0, the reaction is at equilibrium and delta G=0. This occurs when a battery is dead.

Wendy 1E

You would apply Pt into cell diagrams when there is not another solid metal electrode.

Wendy 1E

You do have to consider the coefficient when determining the overall charge of a molecule. The charge of 5Cu2+ (charge is 10+) is not the same as the charge of Cu2+ (charge is 2+).

Wendy 1E

The oxidation number of oxygen is usually 2-, except in peroxides where it is 1- and ozone where it is 0.

Wendy 1E

It is possible to have more than two galvanic cells. I believe that they would work the same way in which the electrons move from the anode to the cathode. I am not sure about the specific details.

Wendy 1E

As mentioned, Pt stands for platinum. It is used as an inert conductor to transfer electrons when a half-reaction has no conducting solids. Platinum is written on the outermost part of the cathode side because it is a solid.

Wendy 1E

Can someone explain when a battery is dead in terms of what we discussed in lecture today? Thank you.

Wendy 1E

Most enthalpies of formation are negative because energy is released when bonds are formed, thus, the reaction is exothermic.

Wendy 1E

For this question, you have to look at the equation ∆G= ∆H-T∆S. ∆G= ∆H when ∆S is approximately 0. In answer a., the change in entropy is approximately zero because there are no phase changes. The reactants and the products are both all solids.

Wendy 1E

Enthalpy (ΔH) is the measure of heat released or absorbed at a constant pressure. q is just heat under any conditions. ΔH would be equal to q only when the heat is released or absorbed at a constant pressure.

Wendy 1E

If the ΔG is negative, then the reaction is spontaneous and favors the forward process.

Wendy 1E

If work is done on the system by the surroundings, then work is positive. If work is done by the system onto the surroundings, then work is negative. This is because when work is done by the system, the system loses energy.

Wendy 1E

In lecture today we learned that ΔG is zero at a reaction's boiling point.

Wendy 1E

Yes, using the equation ΔU=(3/2)nRΔT, you can see that if ΔT is zero, so is ΔU.

Wendy 1E

When heat is transferred from the system and work is done by the system, both heat and work are negative. This is because when heat is transferred from the system, the system loses heat (hence the negative q). When work is done by the system, the system is losing energy (hence work is negative).

Wendy 1E

When I am doing calculations, I have a bit of trouble determining when to apply the "-qsystem=qsurroundings." How can you determine when to use this in a problem? Does anyone have any tips?

Wendy 1E

Most likely not. We don't get distribution curves for tests.

Wendy 1E

It is negative because the reaction is exothermic. The reaction is exothermic because heat is negative (reaction releases heat).

Wendy 1E

Unless asked to derive the equation, you would just use the -nRTln(v2/v1).

Wendy 1E

As discussed today in lecture, you can increase the energy of an open system by doing one of the following:
- adding or removing a substance
- heating or cooling the system
- doing work on the system or letting the system do work on its surroundings

Wendy 1E

I would memorize the heat capacity of water, but other capacities most likely will be given in the equations sheet.

Wendy 1E

Can someone explain the difference between heat, temperature, and enthalpy? Thank you!

Wendy 1E

This is correct. A negative value of q indicates that the system lost heat. Thus, the reaction is exothermic. A positive value of q indicates that the system gained heat. Thus, the reaction is endothermic.

Wendy 1E

Think of a closed system as a cup of coffee with a lid. It can exchange energy with its surroundings in the form of heat but cannot exchange matter because the lid is closed. Think of an isolated system, or something close to it, as a sealed hydro flask. You cannot exchange energy (hydro flasks esse...

Wendy 1E

Usually, we do subtract x from the reactants because the reactants are being used up. But, if the problem explicitly states that the products decreased in concentration or pressure, then you would subtract x from the products.

Wendy 1E

Strong acids and bases dissociate completely. Thus, if you know the concentration of a strong acid or base, you can find the hydronium and hydroxide concentration with molar ratios. You can then use this to find the pH or the pOH. For instance, if a problem says that you have 0.138 M HCl, then you k...

Wendy 1E

The reaction is endothermic because heat is required in order to break the hydrogen bonds between two water molecules.

Wendy 1E

I am not sure if they count, but I would suggest knowing how they work. If pH=5.1 and you have to find the hydronium concentration, there are two significant figures total in the pH. Thus, you would put two numbers after the decimal point in your hydronium concentration answer. If you are asked to f...

Wendy 1E

It is possible. All I suggest knowing is that when delta H is positive, the reaction is endothermic and when delta H is negative, the reaction is exothermic.

Wendy 1E

It will be given on tests. If you look at the "constants and equations" section on his website, you will see that the quadratic equation is given.

Wendy 1E

I also came across this and was confused. I am sure it is just a typo. I would suggest doing the calculation with the value given in the textbook for extra practice.

Wendy 1E

Strong acids do have a Ka value. They have large Ka values because the higher the Ka, the more the acid dissociates. They have low pKa values.

Wendy 1E

Adding to this conversation, what does amphiprotic mean again?

Wendy 1E

Since we spent almost an entire lecture learning about ice charts, it is safe to say that we will have to include them on exams as part of showing our work.

Wendy 1E

It does not make sense for solids to have a concentration since the units for concentration are mol/L. For liquids, the change in concentration is insignificant because the change is very small.

Wendy 1E

Instead of looking at K as Kc look at it as Kp. Kp= (partial pressure of products) / (partial pressure of reactants) according to this equation, an increase in the partial pressure of reactants would lead to a decrease in the equilibrium constant instead of an increase in the equilibrium constant, ...

Wendy 1E

Your reasoning that "if there is higher pressure of reactant, it will shift towards product" is correct. However, this does not mean that K will change. Here, only Q is changing because initially Q<K. K is not changing. This is because the temperature is constant. Only when the temperature...

Wendy 1E

In an experiment, 2.0 mmol Cl2(g) was sealed into a reaction vessel of volume 2.0 L and heated to 1000. K to study its dissociation into Cl atoms. Use the information in Table 5G.2 to calculate the equilibrium composition of the mixture. (b) If 2.0 mmol F2 was placed into the reaction vessel instead...

Wendy 1E

The reverse reaction occurs when Q>K. This occurs when there is an abundance of products and not enough reactants.

Wendy 1E

Also relevant to this discussion is how Kc is different than Kp. Kp is the equilibrium constant found using partial pressures. The method of calculating Kp is essentially the same as Kc, except Kp is calculated by the partial pressure of products/the partial pressure of the reactants.

Wendy 1E

I would use the variable x for the change concentration. Putting the variable x for the change concentration makes it clearer and easier to find the value of x as opposed to putting numbers for the change concentration.

Wendy 1E

I don't believe that we have to know how to draw coordination compounds or chelating coordination compounds because it was not thoroughly discussed in the lecture. However, make sure you thoroughly understand the concepts of each and make sure you know how to draw cisplatin, EDTA, myoglobin, ethylen...

Wendy 1E

SO3 is a nonmetal oxide and nonmetal oxides are acidic

Wendy 1E

In a neutralization reaction, acids and bases always produce salt and water.

Wendy 1E

Formic acid is stronger than acetic acid because acetic acid has a CH3 that is e- donating. This means that CH3 contributes electron density to the O-H bond. This makes it harder to remove the H. Thus it is weaker acid compared to formic acid.

Wendy 1E

Unlike trans-diamine-dichloro-platinum, Cisplatin has two Cls on the same side of the molecule that allows the Cls to bind to the same DNA strand to form 2 bonds. This blocks the enzymes used in DNA replication from replicating the DNA strand. Thus, it blocks DNA replication.

Wendy 1E

Light has wave-like properties all the time, along with particle-like properties.

Wendy 1E

Hybridization doesn't always occur when forming bonds. It only occurs when an atom needs to form more bonds than it would be able to in a ground state.

Wendy 1E

You add the suffix -ate to the metal name in a coordination compound when the net coordination compound has a net negative charge.

Wendy 1E

We can determine the number of hydrogen bonding sites between two molecules if there is an F, O, or N with an available lone pair. A hydrogen must be bonded to a F, O, or N.

Wendy 1E

Sigma bonds are also made from hybridized orbitals, whereas pi bonds are made only from leftover p orbitals.

Wendy 1E

Lavelle did not go into too much depth about this but I believe that you have to draw the Lewis structure. If the molecule has a tetrahedral shape, it is a tetrahedral complex. If the molecule has a square planar shape, it is a square planar complex.

Wendy 1E

D-orbitals do get involved in hybridization. This is why there are sp^3d orbitals and sp^3d^2 orbitals.

Wendy 1E

When the lone pair is in the equatorial plane, it is only perpendicular to two atoms. If the lone pair was in the axial plane, it would be perpendicular to three atoms. Therefore, having a lone pair in the equatorial plane would minimize the repulsions created by the lone pair.

Wendy 1E

You are correct in that the dipole moments only cancel for a tetrahedral if all the atoms bonded to the central atom are the same. This would make the molecule nonpolar. If one atom is different, the dipole moments do not cancel and the molecule is polar. However, it is possible for different atoms ...

Wendy 1E

This is correct. But, you also have to remember that F, O, and N also must have an available lone pair in order to be able to form a hydrogen bond.

Wendy 1E

I do not believe that we need to know these exact values. Just know the strength of each intermolecular force in comparison with other intermolecular forces. You should know that ion-ion forces are the strongest followed by hydrogen bonds, ion-dipole forces, dipole-dipole forces, and london dispersi...

Wendy 1E

If the central atom has lone pairs, it will cause more electron-electron repulsion, which will result in a molecule having a bond angle that is smaller than usual (i.e. less than 120 degrees). You can think of this as if the electrons are "pushing away" the other bonds, which results in a ...

Wendy 1E

The symbols represent partial charge. The negative partial charge goes on the most electronegative atom in a molecule, while the positive charge goes on the other atom. The dipole moments, which represent the arrows used, point from a positive partial charge to a negative partial charge (towards the...

Wendy 1E

Hydrogen bonds do have dipole moments because they typically form when a hydrogen atom bonds to a highly electronegative atom (F,O,N). When bonded to these specific highly electronegative atoms, the hydrogen bonds will have a dipole moment.

Wendy 1E

We do have to memorize the different shapes. However, Lavelle said that we are not required to memorize all of the bond angles as we will never be specifically asked to state them. The main thing that he wants us to remember are the shapes.

Wendy 1E

If given a molecule, how can you determine whether it has dipole-dipole interactions, dipole-induced dipole interactions, or induced dipole-induced dipole interactions?

Wendy 1E

In order to determine if a molecule is polar, you have to look at the shape and the differences in electronegativity. There is no cutoff in electronegativity. If the dipole moments all cancel, the molecule is nonpolar. If the dipole moments do not cancel, the molecule is polar.

Wendy 1E

What is the difference between ion-ion, ion-dipole, dipole-dipole, and dipole-induced dipole interactions?

Wendy 1E

How does induced dipole-induced dipole interactions relate to polarizability and electron distortion?

Wendy 1E

How does hydrogen bonding affect the melting points of compounds?

Wendy 1E

For each pair, determine which compound has bonds with greater ionic character: (a) HCl or HI; (b) CH4 or CF4; (c) CO2 or CS2?

For part a, I was wondering why HI has a lower ionic character than HCl. I am just confused as to why HCl has a larger difference in electronegativity than HI.

Wendy 1E

How do you determine the number of valence electrons in the d subshell? On A.1 part c, they asked for the number of valence electrons in Manganese and the answer was 7. How did they get to this answer?

Wendy 1E

Boron is also an exception to the octet rule as it can be content with 6 valence electrons, instead of an octet.

Wendy 1E

E=hv is used for the energy of a photon, or in this case, electromagnetic radiation.

Wendy 1E

Can someone please explain to me what dipole moments are and what they have to do with electronegativity?

Wendy 1E

Here's the link to the constants and equations sheet that Lavelle has on his website: https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14A/constants_equations.pdf If you cannot access the sheet through this link, you can find it on Lavelle's Chem14A classroom page. It is under the title...

Wendy 1E

I find it useful to remember that most periodic trends increase going upper right on the periodic table. This includes, ionization energy, electron affinity, and electronegativity. The only periodic trend that increases to the lower left is atomic radii and ionic radii.

Wendy 1E

A molecule with resonance refers to the fact that it has multiple bonds that can be placed in different locations within the molecule, without moving the individual atoms in the molecule. When a molecule has resonance, the electrons aren't localized at one particular atom, but rather they are deloca...

Wendy 1E

This is correct. The addition of an electron adds more electrons to the outermost shell. This increases the radius because there are now more electrons farther away from the nucleus.

Wendy 1E

I believe that you have ms and ml confused. Ms, which stands for the spin magnetic quantum number, is always either +1/2 or -1/2. +1/2 indicates that the electron's spin is up. -1/2 indicates that the electron's spin is down. Ml stands for the magnetic quantum number and refers to the orientation of...

Wendy 1E

I believe that for most electron configurations, we will have to write the entire configuration. However, if an element has an excessively long electron configuration, such as Strontium, we most likely will be able to use the shorthand notation.

Wendy 1E

Unfortunately, we do not. Luckily, professor Lavelle gives us enough time in class to take notes, and he doesn't rush us.

Wendy 1E

Why does electron affinity increase as we go towards the noble gasses? Electron affinity increases as we go towards the noble gases because the elements near the noble gases have the tendency to gain an electron due to their higher nuclear charge. In order for these elements to fill their shells, i...

Wendy 1E

As to your question on how many electrons will fit into a specific orbital, each orbital consists of two electrons. There is one s-orbital, thus there are 2 electrons in the s-sublevel. There are 3 p-orbitals, thus there are 6 electrons in the p-sublevel (3 p-orbitals* 2 e- per orbital). There are a...

Wendy 1E

As to the first part of your question, 3d orbitals have slightly higher energy than 4s orbitals, thus, the 4s orbitals get filled first. This is only true if there are no electrons in the 3d orbitals. If the 3d orbitals are filled, then the 4s orbitals will be higher in energy.

Wendy 1E

p = mv (v is the greek letter nu for frequency here, NOT velocity) lamda = h/p (lamda = wavelength) (h is Planck's constant) using these two equations, you can calculate the frequency, wavelength, and even the energy you can also use the uncertainty in momentum (Delta p) to find the uncertainty in ...

Wendy 1E

You are correct, the uncertainty would be 10 m/s. There is an error in the solution manual. If you go to the chem14A class website and scroll down to "Solution Manual Errors 7th Edition," you will see the corrected version.

Wendy 1E

h bar is a simplified form of Planck's constant, in which h bar is equal to h/2π. If you use the equation given in the textbook and substitute h bar as h/2π, you will see the equation that professor Lavelle gave us. This is because (h bar/2π)/2 is equal to h/4π. In regards to your question of which ...

Wendy 1E

Through experimentation, scientists determined that the minimum uncertainty of knowing both a particles momentum and position simultaneously is this value: h/4pi.

Wendy 1E

Use the equation that Lavelle gave us in the lecture E=(-hR/n^2) as the Rydberg equation. Lavelle didn't want us to use a different version of this equation that was given in the textbook, which is v=R{(1/n^2)-(1/n^2)}. He said that if we use this equation, we will just be plugging in numbers and we...

Wendy 1E

I believe that Dr. Lavelle mentioned this once in his lectures. A bond length between two molecules is about 10^-10m, which is equal to 1 Angstrom.

Wendy 1E

Within the De Broglie equation, the mass should be in kg instead of grams in order to cancel out with the units of Planck's constant. The units for Planck's constant are J.s. The SI units of J is kg*m^2*s^-2. Thus, the units for Planck's constant are also (kg*m^2*s^-2)*s. In order to cancel out kg i...

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