CHEMISTRY COMMUNITY

Elaine Pham 2E

You subtract instead of add because A is the reactant and it's getting used up. The B concentration that they give us is the concentration of B formed after the time we need to solve for. The concentration of B formed correlates to the concentration of A used up to produce B. Thus, you subtract the ...

Elaine Pham 2E

When you think of the cell potentials that are listed in the back of the book, think of them as specifically reduction potentials / as how likely a species is to be reduced. If you are ordering the species in terms of increasing oxidizing power, you would order them from lower reduction potential to...

Elaine Pham 2E

The molarity of B is given in the problem as 0.19.

Elaine Pham 2E

I think microscopic reversibility means that for the forward and reverse directions of a reaction, there are the same intermediates involved, so the forward and reverse reactions go on the same, single pathway.

Elaine Pham 2E

I believe there is a negative sign because they are asking for the rate of the overall reaction of NO2, which needs to be positive so they add the negative sign to make the value become positive. If they were asking for the rate of consumption of NO2, I think there would not be a negative sign in fr...

Elaine Pham 2E

For this question, since K >10^3, at equilibrium and K = concentration of products / concentration of reactants, then there are more products than reactants and the reaction lies to the right. Therefore, there is more SO3 (the product) at equilibrium, which then refers to its stability. Since there ...

Elaine Pham 2E

K changes with temperature, so for each reaction, when the reaction occurs at different temperatures, K is different.

Elaine Pham 2E

Reducing agents refer to substances that are being oxidized, which then in turn causes another substance to become reduced. Stronger reducing agents are lower in cell potential, so for this problem, you would want to rank the metals in order of highest cell potential to lowest cell potential as list...

Elaine Pham 2E

Lavelle will post on his website when the final review sessions start, but usually it is during week 10 that they do.

Elaine Pham 2E

Since the electrons are moving from the anode to the cathode, there is a charge difference between the two and the resulting charge buildup would cause the electrode reaction to stop. To prevent the reaction from stopping, the salt bridge allows ions to cross over and prevent the cathode from being ...

Elaine Pham 2E

We separate them with a comma because they are both in the same phase. The solid line is for when they are in two different phases, such as liquid and solid. I think it's just the rule.

Elaine Pham 2E

For polyprotic acids, the K for the reactions are different for each time, so you would only write the conjugate acid in the specific reaction you are looking at to find the equilibrium constant.

Elaine Pham 2E

For reversible expansion problems, since pressure isn't constant anymore, does that mean we cannot say that q=delta H? If we were asked to find q, would we have to use q=mCdeltaT?

Elaine Pham 2E

Yes I think so because reversible means the process is at equilibrium so there is no change in entropy.

Elaine Pham 2E

k_b is Boltzmann's constant and it's 1.38 x 10^-23 J/K. It's given on the constants and equations sheet online so I don't think we have to memorize it.

Elaine Pham 2E

The main reason in figuring out that the reaction is a combustion reaction is that for delta H that are listed, there is a subscript "c" that represents combustion. You then have to set up the combustion equations and use Hess' method to get the equation that is originally given and the ov...

Elaine Pham 2E

A positive delta U means the internal energy of the system you are looking at has increased while a negative delta U means internal energy of the system has decreased.

Elaine Pham 2E

For this problem, I think you just have to know (by drawing its lewis structure) that benzene has resonance structures. That means all of its bonds are the same length, which is between the length of a single and double bond between carbons. So since benzene has 6 carbon-carbon bonds of the same len...

Elaine Pham 2E

The unit for pressure is kg/(ms^2). The unit for volume is m^3. Since work = external pressure multiplied by change in volume, the units for work are (kg/(ms^2))m^3. When you simplify that, you get (kgm^2)/s^2 = Joules.

Elaine Pham 2E

The standard enthalpy of formation specifically refers to the amount of enthalpy for the formation of 1 mole of a substance, so that's why the units for it is kJ/mol. The standard reaction enthalpy has no mention of mols (hence no mols in the units, just kJ as the standard unit) and refers to the wh...

Elaine Pham 2E

These cations from group 1 and 2 are very weak Lewis acids, and they're so weak to the point that they don't have an effect on the water molecules around them. Therefore, you don't account for them when calculating pH.

Elaine Pham 2E

If you are given delta H of the equation, you can identify if the reaction is endothermic or exothermic. If delta H (which represents the change in enthalpy) is positive, then it is an endothermic reaction. If delta H is negative, then it is an exothermic reaction since energy is being released, not...

Elaine Pham 2E

Yes, in pure water the concentrations of hydronium and hydroxide are equal. When an acid or base is added though, these concentrations can change.

Elaine Pham 2E

For doing sig figs with pH, you start counting the sigfigs after the decimal point. For example, a pH of 1.11 has 2 sig figs or a pH of 10.000 has 3 sig figs.

Elaine Pham 2E

For this question, the HI and I2 are not really left over but have equilibrium concentrations that you need to solve for in order to get K. To get their equilibrium concentrations, you set up an ICE table. The 60% of H2 used up refers to the change in H2, "x", which you can get by multiply...

Elaine Pham 2E

I think Dr. Lavelle will address it in class but in the module he did say to use the estimation method when dealing with the concentrations with a power of 3.

Elaine Pham 2E

For the problem you mentioned in the textbook, the equation they gave us is actually a combination of 2 separate equations that you can find on the 11.2 table. The rule is that when there are 2 or more equations combined together, the overall equilibrium constant (K) is the product of the separate e...

Elaine Pham 2E

Cisplatin is used in chemotherapy for the treatment of cancer cells, which are characterized by rapid cell growth, because it can bind to DNA and prevent cell replication and thus stop cancer cells from growing. More specifically, cisplatin binds to the DNA base guanine. The nitrogens in guanine ess...

Elaine Pham 2E

It is bis when naming the complex because there are 2 molecules of oxalate in the complex. Since oxalate is polydentate, different prefixes must be used so you use bis- for when there's 2 of them.

Elaine Pham 2E

For #2, I think you got all the calculations right, but you might have gotten points off because of units. You needed to put meters after your calculation for the minimum delta x. But I'm not sure if that's just 1 point off or 2!

Elaine Pham 2E

Sigma bonds are able to rotate but pi bonds are not.

Elaine Pham 2E

Oxygen's ionization energy is lower than Nitrogen's despite the fact that ionization energy typically increases as you go across a period because of their electron configurations. Oxygen's electron configuration is 1s 2 2s 2 2p 4 . Nitrogen's electron configuration is 1s 2 2s 2 2p 3 . Because of Nit...

Elaine Pham 2E

It depends on the VSEPR shape. For a molecule with 5 bonded atoms and no lone pair around a central atom aka AX5 (trigonal bipyramidal), for the shape to become seesaw (AX4E), you remove the equatorial electrons first. As you keep removing the equatorial electrons from the trigonal bipyramidal shape...

Elaine Pham 2E

Usually, I think that the purpose of the expanded valence shell is to get the formal charge of the central atom to be neutral. But I'm not sure if it's necessarily that all atoms with an expanded octet when bonded to other atoms will have a formal charge of 0. When you're looking at XeF4 particularl...

Elaine Pham 2E

But wouldn’t the polarity between Sulfur and Fluorine overall “cancel out” to result in an overall nonpolar molecule? The dipole moments are all equal and pointing in opposite directions so won’t they “cancel out”? Does it have anything to do with the fact that Sulfur has a lone pair in the molecule?

Elaine Pham 2E

If 2 carbons that are in a pi bond do rotate, the bond will break. So for instance, in C2H4 (ethene), in the double bond between the 2 carbons, one of the bonds is a pi bond which restricts the molecule from rotating and makes it so that the whole molecule is on 1 plane. If one of the carbons were t...

Elaine Pham 2E

For question 4.25 in the 6th edition of the textbook, why is SF4 polar? I thought since the dipole moments pointing towards F cancel out, the molecule would be nonpolar.

Elaine Pham 2E

In addition, lone pairs, because of electron repulsion, "push" down onto the other bond angles to make them smaller.

Elaine Pham 2E

Why are the energies of the different interactions Dr. Lavelle talked about in class negative?
For example, the energy of an ion-ion interaction is -250 kJ/mol. I am a little confused why it's a negative number and not a positive one.

Elaine Pham 2E

The shape of a molecule contributes to strength of interaction because when the shape of the molecules makes it so that the distance between the 2 molecules is shorter and that there is more surface area interacting with each other, the strength of the interaction between the two will be stronger. F...

Elaine Pham 2E

I'm not sure how the values for electronegativity are calculated, but they are based on the element's electron affinity and ionization energy. I believe we don't need to know how to calculate it, we just need to know the definition and how it relates to covalent bonds and other topics.

Elaine Pham 2E

The rod shaped molecules have stronger bonds than the spherical ones because in the rod shaped molecules, there is more contact between the molecules and more surface area interacting with each other. As a result, the bond is shorter between the two molecules and stronger. As a result of the stronge...

Elaine Pham 2E

They can be seen as opposites. Polarizability refers to how an atom's electrons can get distorted or in other words feel the pull towards the positive nucleus of another atom. For example, the iodide ion has higher polarizability than a chloride ion because the electrons in the larger iodide ion are...

Elaine Pham 2E

Anions are larger than their parent atoms because an anion has more electrons than its parent electron. The increase in electrons leads to more electron repulsion, so electrons are more spread out from each other which causes the ionic radius to increase. Cations are smaller than their parent atoms ...

Elaine Pham 2E

For dilutions or calculating the molarity, the unit is always mol/liters. So if you were given the grams of the solute and you need to calculate the molarity, you would need to use the molar mass to change that to mols. Generally, I don't think we would be asked what the concentration of the solvent...

Elaine Pham 2E

When a gas-phase atom gains an electron, it gets closer to completing an octet or having a noble gas electron configuration. Having a noble gas electron configuration provides more stability for the atom, and thus with more stability the electron is now at a lower energy state. The change of the ele...

Elaine Pham 2E

With SO4^2-, you need to first look at the periodic table to determine how many valence electrons sulfur and oxygen have. Both of them are in Group 16, so they both have 6 valence electrons. Since the ionic compound has 4 oxygens, you do 4 x 6 = 24 to get the total number of electrons from oxygen in...

Elaine Pham 2E

The 2nd ionization energy is always higher than the first ionization energy because when removing the second electron, you are removing it from the cation of the element. It takes more energy to remove an electron from a cation than a neutral atom because the cation is holding onto the electrons mor...

Elaine Pham 2E

The effective nuclear charge refers to the net charge or attraction that an electron in an atom feels towards the positively charged nucleus. In multielectron atoms, the nuclear charge coming from the nucleus is called "effective" because other electrons that are closer to the nucleus shie...

Elaine Pham 2E

The Heisenberg Indeterminacy Equation basically tells us that we can't know 100% what both the momentum and position of a particle, such as an electron, is at one point in time, aka there is a limit on the accuracy to which the momentum and position of a particle can be known simultaneously. The act...

Elaine Pham 2E

The formula for kinetic energy is KE=(mv^2)/2. With this post-assessment problem, you can plug in the uncertainty of the velocity and the mass of an electron, which is 9.1 x 10^-31 kg (I just searched it up on Google), into the KE equation and get the uncertainty in kinetic energy. Keep in mind that...

Elaine Pham 2E

In lecture, Dr. Lavelle briefly mentioned a double derivative within the Shrodinger equation, but I was confused on that part. What/how does a double derivative relate to the equation?

Elaine Pham 2E

A wave does not necessarily "go" from constructive to destructive interferences. These types of interferences describe what happen when waves meet up with each other, or interact with each other.

Elaine Pham 2E

The photoelectric effect shows how in order for an electron to be removed from a metal, the wave of light needs to have a higher frequency/shorter wavelength. The level of intensity does not matter in this case. Thus, in the photoelectric effect, light is not acting as a classical wave and instead, ...

Elaine Pham 2E

The resulting marks from the experiment show the existence of a diffraction pattern from light waves. When the waves constructively interfere with each other, waves with bigger amplitudes are produced. The result of this is that the light that hits the wall is brighter. Ultimately, the existence of ...

Elaine Pham 2E

In L7 part B, when it's asking what mass of oxygen is needed to oxidize the tristearin, it's asking how much oxygen will be used up along with the 454 grams of tristearin in the combustion/oxidation reaction. It's the same method you used in part A, so you'll need to convert the 454 grams of tristea...

Elaine Pham 2E

Mass spectrum refers to the technique of mass spectrometry, which is a method that scientists use to measure the molar mass of elements or a sample.

In F17, the problem mentions "mass spectrum" to explain how the molar mass of the molecule was discovered.

Elaine Pham 2E

You need to divide the given molar mass by the empirical formula mass because the empirical formula is still just a ratio of the number of atoms in each element of the molecule or compound. It’s like the reduced version of the actual molecular formula, so when you divide the molar mass by the empiri...

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