CHEMISTRY COMMUNITY

Kathryn Wilhem 1I

With a temperature change, the K changes, so then the pH would also change because the concentration of H+ changes.

Kathryn Wilhem 1I

The slow step is the one with the slowest rate, k. The reaction can only be as fast as the slowest step, so the reaction rate becomes that of the slowest step.

Kathryn Wilhem 1I

For the snowflakes, delta H is negative because the snowflakes lose heat, and delta S is negative because going from a liquid to a solid is less chaotic. Negative delta H minus a negative delta S makes a negative delta G. For the sublimation, delta H is positive because heat is going in, and delta S...

Kathryn Wilhem 1I

In concentration cells, it is the same metal being reduced and oxidized so when you subtract the E of the anode from the E of the cathode, you will get 0.

Kathryn Wilhem 1I

Using leChatelier's principle, when the temperature is changed, depending on whether the reaction is endothermic or exothermic, the reaction will favor either the reactants or the products. We use the equation -deltaH/R(1/T2 - 1/T1) = ln(K2/K1) to find what the new K is when the temperature is chang...

Kathryn Wilhem 1I

In the Nernst equation, there are the constants R and F which should be given. Temperature is usually 298K. You can find n using the standard reduction potentials and K can be found with the concentrations or partial pressures. Ecell can also be found using standard reduction potentials.

Kathryn Wilhem 1I

You calculate the delta S for each reaction of formation, because if delta S is more negative, then delta G will be more positive. If you look at the equation for Gibbs Free Energy: delta G = delta H - T delta S, the delta S will need to be more negative to make delta G positive. If delta G is posit...

Kathryn Wilhem 1I

I have found it helpful to create my own outline of what we have learned based on the lecture notes, and reinforcing it with the relevant chapters in the textbook and Dr Lavelle's outlines.

Kathryn Wilhem 1I

You should ask your TA what homework problems they want, and in general, you should usually follow what we are learning in class.

Kathryn Wilhem 1I

There is a chart in the 7th edition pg. 330 that I found helpful for this. It's table 4J.1. I can write it here also: Delta H Delta S Spontaneous? negative positive yes, delta G < 0 negative positive yes, if TdeltaS < deltaH and deltaG < 0 positive positive yes, if TdeltaS > deltaH and deltaG < 0 po...

Kathryn Wilhem 1I

K is the [P]/[R] so if K is very large, the [P]>[R]. Because the concentration of products is greater at equilibrium, the products must be more stable, because the reaction favors them.

Kathryn Wilhem 1I

The reaction is with a polyprotic acid, so it can donate more than one proton, so there will be more than one reaction. They are asking for the second reaction, so you must first write the reaction with the first proton donated, then write the reaction with the second proton donated.

Kathryn Wilhem 1I

The units of q, delta U, and delta H depend on the other units in the problem, but they will always be in either J or kJ.

Kathryn Wilhem 1I

Problem 4F.11 asks you to find the change in entropy when the volume changes from 3L to 0.5L and the temperature changes from 291.5K to 301.1K. When calculating delta S for the temperature change, the solutions used delta S = nRln(T2/T1). Why didn't they use delta S = nCln(T2/T1)?

Kathryn Wilhem 1I

If delta H is positive that means that the reaction is endothermic. In an endothermic reaction, if the temperature is increased the reaction will favor the forward reaction.

Kathryn Wilhem 1I

Work is being done by the system against the atmosphere, so work should be positive.

Kathryn Wilhem 1I

The heat capacity equation varies depending on whether there is constant pressure or constant volume. You use q=nCT, but for constant pressure, C=(5/2)R, and for constant volume, C=(3/2)R. W = - P x ∆V is used to find work, which is then used in the equation ∆U = q + w. For ∆U = q + w, if volume is ...

Kathryn Wilhem 1I

Going off of the equation sheet on the website, we should know:
Ideal gas, Cp = (5/2) R
Ideal gas, Cv = (3/2) R
Ideal gas, U = 3/2 nRT
∆U = q + w
q = n C ∆T
W = - P x ∆V
PV = nRT
w= -nRTln(V2/V1)
Those are all of the Thermochemistry equations that I could find on the sheet.

Kathryn Wilhem 1I

He was saying that a boulder could just roll down the hill on its own, so it wouldn't require work to roll down the hill. But, the boulder will not roll up the hill on its own, so it will require work to be pushed up the hill.

Kathryn Wilhem 1I

Look at the units of the two constants and see which one will give you the units of what you are solving for.

Kathryn Wilhem 1I

Cv is used when there is constant volume and Cp is used when there is constant pressure.

Kathryn Wilhem 1I

The book states that the molar heat capacity increases with molecular complexity. The molar heat capacity of nonlinear molecules is higher than linear molecules because nonlinear molecules can rotate about three axes, not just two axes. I don't think we have gone over this in class yet.

Kathryn Wilhem 1I

He talked about how the third method to solve for enthalpy is to use standard enthalpies of formation. You take the standard enthalpies of formation of all the reactants and subtract them from all the standard enthalpies of formation of all the products. This can be used to solve for the standard re...

Kathryn Wilhem 1I

The first method is to use multiple chemical reactions and add them together to find the enthalpy of a certain reaction. The second method is to use bond enthalpies adding the bonds broken with the bonds formed. This method is the most accurate for diatomic molecules, and the rest are averages, so t...

Kathryn Wilhem 1I

For parts (i) and (ii) of the problem, we are given the [H30+] = 1.50M and [OH-] = 1.50M respectively. When I calculated the pH and pOH, I found that the answer was -0.176. When we know that taking the log of the concentration will give us a positive answer, do we just not put a negative sign in fro...

Kathryn Wilhem 1I

How do we know if a reaction is endothermic or exothermic? Can we only know if we are given delta H?

Kathryn Wilhem 1I

You need to use the formula M1V1=M2V2 because the volume of the container was changed. You have the original volume and concentration, and the new volume, so you must solve for the new concentration.

Kathryn Wilhem 1I

Question 5J.13 asks if there will be more ammonia will be formed if an equilibrium mixture containing 2.23 mmol N2 and 6.69 mmol H2 in a 500. mL container is increased in temperature from 600. K to 700. K. K=1.7x10^-3 at 600. K and K=7.8x10^-5 at 700.K. How does this problem relate to LeChatelier's ...

Kathryn Wilhem 1I

In the solutions, they are using moles instead on molarity in the ICE table. They then convert to molarity after they have done the ICE table.

Kathryn Wilhem 1I

Question 5I.13 Part C asks whether the Cl2->2Cl or F2->2F reaction is more thermodynamically stable. How do we know which is more stable?

Kathryn Wilhem 1I

The double arrows represent equilibrium reactions.

Kathryn Wilhem 1I

In hydrogen bonding, the N, O, and F on one molecule form a bond with the H on another molecule, so there needs to be an H bonded to N, O, or F for the molecule to have hydrogen bonding IMF's. For example, in a molecule with an O-H bond, it is polar, so when another O-H bond nearby will have its H t...

Kathryn Wilhem 1I

A hydrogen bond is about 10% of the typical covalent bond strength, making it a much stronger attractive force than dipole-dipole and london dispersion.

Kathryn Wilhem 1I

You would draw the normal shape of the ligand that is a chelate, and then the lone pairs would bond to the central metal atom, forming a ring.

Kathryn Wilhem 1I

For 6B.9 part a row i and ii, we are given a chart giving just the [H3O+] for i and [OH-] for ii and we have to find the pH, pOH, [H3O+], and [OH-]. For i, we are given [H3O+] = 1.50M. If we use the formula pH = -log[H3O+], then we get a negative pH, and the same goes for using pOH = -log[OH-] in ii...

Kathryn Wilhem 1I

Can someone please explain c and d of 6A.17? It says to state whether As2O3 and Bi2O3 are acidic, basic, or amphoteric. Why are they amphoteric?

Kathryn Wilhem 1I

For part a, there are three trigonal planar shapes connected by each carbon atom. For part b, you must look at the hybridization of each atom to determine what kind of bond is used. For the C-H bonds, C is 2sp2 hybridized and H is just a 1s orbital, so it is a sigma bond. For the C=C bond, both C's ...

Kathryn Wilhem 1I

If you look at the valence electrons of B and F, Boron has 3 valence electrons and Fluorine has 3(7) valence electrons, totaling 24 total electrons to use in your lewis structure. If you use 6 electrons for B-F bonds, then you have 18 left over. F won't be stable unless it has a complete octet, so y...

Kathryn Wilhem 1I

You would just say slightly less than what the normal bond angle is.

Kathryn Wilhem 1I

I am confused on what is used to determine whether an atom is positively or negatively charged in a dipole moment. Is it related to formal charge?

Kathryn Wilhem 1I

Pi bonds do not allow atoms to rotate as well. If the atoms rotate, the pi bonds will easily break. However, sigma bonds do allow the atoms to rotate.

Kathryn Wilhem 1I

An atom is trigonal pyramidal if it has four areas of electron density but only has three bonding electron pairs, or AX3E. An atom is t-shape if it has five areas of electron density but only has four bonding electron pairs, or AX3E2.

Kathryn Wilhem 1I

If we disregard the formal charges when drawing our lewis structure will it be wrong? It could possibly be wrong if there is a better lewis structure that will give the most atoms a formal charge of zero, or if an atom that is not as electronegative as another has a negative formal charge. It would...

Kathryn Wilhem 1I

Bond length also affects the dissociation energy, or the energy required to break a bond. Single bonds require less energy to break than double or triple bonds.

Kathryn Wilhem 1I

We will probably be going more in depth and learning more this week.

Kathryn Wilhem 1I

The uncertainty would be twice the value of the difference in speed, so it would be 0.8 m/s.

Kathryn Wilhem 1I

Typically, when the electronegativity difference between the two atoms is greater than 2, an ionic bond forms. If the electronegativity difference is less than 1.5, it is a covalent bond. However, all ionic bonds have some covalent character.

Kathryn Wilhem 1I

The p orbitals have a shape with two lobes, and depending on whether it is px, py, or pz, you would draw the two lobes on the x axis for px, the y axis for py, and the z axis for pz.

Kathryn Wilhem 1I

Lowest energy does not have to do with ground state. Lowest energy is when all of the electrons are most stable, so when the octet of valence electrons is filled. This is only ground state for noble gases.

Kathryn Wilhem 1I

The further away an electron is away from the nucleus, the easier it is to remove. Once the electron that is furthest away from the nucleus is removed, the next farthest would be removed. However, this electron is closer to the atom than the first electron, making it harder to remove, and increasing...

Kathryn Wilhem 1I

S, p, d, and f orbitals all have an angular momentum number (l) associated with it.
For s, l=0
For p, l=1
For d, l=2
For f, l=3
And so on. The l is a subshell, so each energy level (n) has different subshells that are allowed, giving you the s, p, d, and f orbitals.

Kathryn Wilhem 1I

I don't think we need to know specific wavelengths, but we do have to know the order from longest to shortest wavelengths.

Kathryn Wilhem 1I

You must calculate the threshold energy of the photon using the equation: E=h(frequency). They give you the frequency and h is a constant, therefore you can calculate the threshold energy of the photon.

Kathryn Wilhem 1I

S-orbitals do not have nodal planes, so they have even electron density distribution, making the shell symmetric. Every other orbital has at least one nodal plane, meaning there is zero electron density distribution and making the shell not symmetric. This explains why the p-orbitals have a dumbbell...

Kathryn Wilhem 1I

The Heisenberg Equation is (delta p, the uncertainty of momentum) times (delta x, the uncertainty of position) is greater than or equal to 0.5 times h/2pi.

Kathryn Wilhem 1I

When I went to a workshop the UA described the energy in the light like a glass of water. In the glass, the water is the energy needed to remove an electron, the threshold energy. The extra volume of the glass above the water is the kinetic energy that is given to the electron.

Kathryn Wilhem 1I

The energy of the photon must be greater than or equal to the energy to remove an electron. Light sources with long wavelengths have a lower frequency. The lower frequency does not have enough energy to remove an electron, even if it has high intensity. Intensity is also the amplitude of the wave. T...

Kathryn Wilhem 1I

You divide the percentages by the molar mass of each element. For example, divide 63.15g C by 12.01 g/mol.

Kathryn Wilhem 1I

I did the chem module for empirical and molecular formulas and I was having trouble on problem 19. I calculated the moles of each element: for C I got 3.283 moles, for O I got 3.286 moles, and for H I got 7.802 moles. So, I got a ratio of 1:1:2.38. I figured I would multiply by 3 to get 3:3:7, but t...

Kathryn Wilhem 1I

I did the module for limiting reactants, and I kept getting confused on problems 30, 32, and 33. These were the problems with the formula 2A + 1B --> 3C. For 30, the question is If you have 1 mole A and 1 mole B, which is the limiting reactant? Wouldn't B be the limiting reactant because there are l...

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