CHEMISTRY COMMUNITY

Venya Vaddi 1L

The anode is always oxidation and goes on the left. The cathode is always reduction and goes on the right.
In general the format is:

electrode on the anode side| oxidation half reaction || reduction half reaction| electrode on cathode side

Venya Vaddi 1L

Yes, it is on the Kinetics outline. In general, I think we have to be able to find the integrated rate law by setting the unique rate expression equal to the overall rate law for a reaction with one reactant.

Venya Vaddi 1L

H+ is usually always included in the cell diagram to indicate acidic conditions if H+ if part of the half reaction.

Venya Vaddi 1L

Why is the sulfate ion half reaction not considered in this problem when determine the oxidation and reduction half reactions?

Venya Vaddi 1L

Pseudo-first order reaction just means that the reaction is first order with respect to one of the reactants, but the overall reaction order can be different. The overall reaction can be second order and still be pseudo-first order with respect to each reactant.

Venya Vaddi 1L

We will get a table of standard reduction potentials on the test which will have the reduction E values and the half reactions.

Venya Vaddi 1L

It helps to look at the table of standard reduction potentials so that you know which half reactions to use. You use the half reactions that involve Au+/Au and Au3+/Au. After balancing the equation, some of the Au cancels out and you are left with the net overall reaction.

Venya Vaddi 1L

I think we are just assuming that they are independent of temperature so that we can use those values at two different temperatures in the calculations to solve for K values when temperature changes.

Venya Vaddi 1L

Adiabatic means there is no heat transfer (q=0) and isothermal means that there is no change in temperature (delta T=0).

Venya Vaddi 1L

This is set by definition in order to measure the standard cell potentials of the other half reactions since we can only measure E as a difference between the two electrodes used.

Venya Vaddi 1L

For test 2, do we need to be able to balance redox reactions using H⁺/OH^-/H₂O in acidic/basic solutions, or will we only need to balance the reactions for batteries?

Venya Vaddi 1L

Chemical to electrical energy is a galvanic cell, which has a negative delta G since it is spontaneous.
Electrical to chemical energy is a voltaic cell, which has a positive delta G since it is not spontaneous.

Venya Vaddi 1L

Electrons always flow from the anode (where oxidation takes place) to the cathode (where reduction takes place) in both galvanic and voltaic cells.

Venya Vaddi 1L

How does a porous disk complete the same function as a salt bridge?

Venya Vaddi 1L

A buffer can resist pH changes upon the addition of an acid or base. It is often made up of an acid and its conjugate base.

Venya Vaddi 1L

What we covered in class about entropy is in sections 4F-4I in the seventh edition.

Venya Vaddi 1L

Work is equal to zero since it is free expansion and delta U equals zero since it is isothermal. Thus q is also zero, since delta U=q+w. If q is zero and temperature is constant, then the entropy of the surroundings=-q/T, which equals zero.

Venya Vaddi 1L

Is delta U equal to zero for both reversible and irreversible reactions? If not, under what conditions is delta U equal to zero?

Venya Vaddi 1L

I usually check the units of the other terms in the equation to determine which value of R I need to use

Venya Vaddi 1L

The enthalpy of formation of O2 is zero since it is in its standard state.

Venya Vaddi 1L

Knowing the number of microstates gives you the W value to use in the equation S=k_B*ln(W).

Venya Vaddi 1L

I'm not sure what section corresponds to the lecture in the sixth edition, but it is section 4G in the seventh edition.

Venya Vaddi 1L

When using the equation w=-P*delta V, the product of the units of pressure (generally atm) and volume (generally Liters) is L.atm. However, since work is defined as the energy to act against some force, the convention is to express work in units of energy (Joules), which is why we must convert L.atm...

Venya Vaddi 1L

The naught signifies that the given enthalpy value is under standard conditions (1atm, 25 degrees C). When standard conditions are used in the experiment/calculation of enthalpy, then the naught symbol can be included.

Venya Vaddi 1L

For part a: delta H is 765 J and the final temp is 343 K.
for part b: delta H is 1275 J and the final temp is 373 K.

Venya Vaddi 1L

The textbook shows the derivation of Cp,m=Cv,m+R.

For a monatomic gas, Cv,m=delta U/delta T, which gives 3/2*R. (this derivation is also shown in the book on page 265 of the seventh edition in detail).

Using this result for Cv,m and the above formula, Cp,m is 5/2*R.

Venya Vaddi 1L

Will the bond enthalpy values always be positive for reactants and negative for products, or are there any exceptions to this?

Venya Vaddi 1L

Work is equal to the change in internal energy of the system, so ΔU= +28 J as calculated in part a of this question.

Venya Vaddi 1L

For an ideal gas, Cp is greater than Cv because the Cp system expands (since pressure is constant and volume is not) so the Cp system does work.

Venya Vaddi 1L

Heat (q) is a measure of thermal energy transferred. Temperature is a measure of the average kinetic energy of atoms or molecules.

Venya Vaddi 1L

When Professor Lavelle was explaining why steam causes severe burns, I think he said that when the steam comes into contact with steam, it takes a longer time for it to become a liquid and releases heat in the process. Can someone explain/clarify this concept?

Venya Vaddi 1L

If Ka<10^-3, the acid is considered weak. Similarly, if Kb<10^-3, the base is considered weak.

Venya Vaddi 1L

I think other molecules can undergo autoprotolysis as long as there is a proton transfer between two identical molecules where one acts as a Bronsted base and the other acts as a Bronsted acid.

Venya Vaddi 1L

For acids that are polyprotic, there is a different pKa value each time the acid loses an H+. pKa1 just denotes the pKa value for the first H+ ion the acid loses and is used to distinguish it from its other subsequent pKa values.

Venya Vaddi 1L

I think you would just calculate [H3O+] using Ka for the stronger acid because it changes the pH of the solution. "If a really really weak acid is added to water and the change in [H3O+] is very small (like 10-9), then we say that the pH is essentially unchanged. 10-7 + 10-9 is really close to ...

Venya Vaddi 1L

Is there an error in the 7th edition solution manual for 6B.9? For (i), when I calculate -log[H3O+], I got -0.176, not +0.176 like the solution manual says. Can someone explain how to get the correct answer?

Venya Vaddi 1L

Use Kp when even partial pressures and Kc when given concentrations. You can convert using PV=nRT and can also convert between Kp and Kc but that is more complicated, but is necessary when you are given Kp and concentration values (or the other way around).

Venya Vaddi 1L

Another possible scenario is that if the pressure was changed by decreasing/increasing volume and there are the same number of moles of gas on both the reactants and products sides of the equation, then the system would be unaffected and would not favor a particular side.

Venya Vaddi 1L

Does anyone know why all the pressure values are divided by 100 in the K expression in problem 5.35 (7th edition)?

Venya Vaddi 1L

You can also use the units:

P in atmospheres (atm)
V in liters (L)
n in moles
T in Kelvin (K)
R= 0.08206 L.atm.K^-1.mol^-1 (constant)

Venya Vaddi 1L

In the second container, since the concentration of O₂ is greater the partial pressure is greater. Pressure and concentration are directly proportional as shown by P=(conc.)RT, so if concentration is greater, then pressure is also greater (given that T is the same).

Venya Vaddi 1L

Q is written the same way as K is. Q=[products]/[reactants] and both solids and liquids are excluded from the expression.

Venya Vaddi 1L

How do we determine the hybridization on the lone pair? For example, if O is bonded to two atoms and has two lone pairs, wouldn't it just be sp3 or is there another specific notation for the lone pair I am missing? In the same way that you do when determining shape, count the regions of electron de...

Venya Vaddi 1L

I think we just need to know that an amphoteric compound can either donate an H+ or accept an H+. To draw these compounds you would just draw the Lewis structure and depending on whether it is acting as an acid/base in the reaction, you would draw the appropriate conjugate pair as having accepted an...

Venya Vaddi 1L

The base is the compound that can accept a proton, so its conjugate acid is the compound that is formed after accepting that proton. It is a conjugate acid because if it is put in water it will act as an acid and donate the proton.

Venya Vaddi 1L

A Bronsted acid is defined as a proton donor, while a Lewis acid is an electron pair acceptor.

Venya Vaddi 1L

Is there an error in the solutions manual for 6B.9 (7th edition)? The concentration of H30+ is given as 1.50 M in the problem, and using the information to calculate pH I got -0.176 but the solutions manual writes +0.176, which does not correlate to the hydronium ion concentration provided.

Venya Vaddi 1L

Intermolecular forces are described as having negative energy values since they are attractive forces.

Venya Vaddi 1L

In general, all molecules have dispersion forces.

If there is an H bonded to a N, O, or F atom there will be hydrogen bonding.

If the molecule has a net dipole (polar), there will be dipole-dipole interactions.

Venya Vaddi 1L

The equatorial bonds are in the same plane while the axial bonds are located above/below the plane. For example, a trigonal bipyramidal molecule has two axial bonds (the ones drawn on the top and bottom) and three equatorial bonds (the ones that are 120 degrees apart).

Venya Vaddi 1L

In the case that there are two central atoms, I believe you would write the VSEPR formula for each central atom separately since we identify molecular shape around each central atom separately as well.

Venya Vaddi 1L

I believe Dr. Lavelle said that he will generally ask for molecular shape, not the shape of the electron geometry.

Venya Vaddi 1L

I think Dr. Lavelle said we do not need to know how to draw the shapes of the hybridized orbitals, but we do need to be able to identify the ones used.

Venya Vaddi 1L

Starting from the third period, elements can expand their octet since there are more orbitals available. d orbitals are first available starting in the n=3 energy level. Once the s and p orbitals are filled in an energy level, the next available orbitals for the electrons are the d orbitals. The low...

Venya Vaddi 1L

Because of the pi bond between the two C atoms, C2H4 cannot rotate. The pi bond essentially locks the molecule in place or makes it stationary. The pi bond must be broken in order for the C atoms to be able to rotate, and the molecule will stop rotating once the pi bond is reformed. I don't think th...

Venya Vaddi 1L

I think the dash is usually drawn to show that the atom is going away from you and the wedge is usually drawn to show the atom is coming towards you.

Venya Vaddi 1L

A single bond is always one sigma bond. If you have a double bond, one of the bonds will be a sigma bond and the other one will be a pi bond. If you have a triple bond, one of the bonds will be a sigma bond and the other two bonds will be pi bonds. This is because only one sigma bind can form, so th...

Venya Vaddi 1L

These prefixes are used when naming isomers. Cis means same side and trans means on the other side. I don't think we need to know this nomenclature since we have not covered it yet.

Venya Vaddi 1L

The see saw shape occurs when there are 5 regions of electron density, specifically 4 bonding pairs and 1 lone pair. Lone pairs are more repulsive, so to reduce the effect of the repulsion the lone pair interacts with only 2 bonds at 90 degrees versus a different arrangement, which would be less sta...

Venya Vaddi 1L

These prefixes are used in naming isomers. Cis- means "this side" and trans- means "the other side of." I don't think we are required to know this nomenclature yet since we have not really gone over it.

Venya Vaddi 1L

The equation given for dipole moments was:
dipole moment= charge x distance between atoms (bond length)

Venya Vaddi 1L

Both London forces and dipole-induced dipole forces have an energy of -2 kJ/mol. When asked to compare the intermolecular forces between two compounds, which one is stronger: London forces or dipole-induced dipole forces?

Venya Vaddi 1L

In the solutions manual for the 7th edition, the answer to part a 2E.25 is that CH₂Cl₂ is a polar molecule. Can someone explain why CH₂Cl₂ is polar? Wouldn't the dipole moments cancel, making this molecule nonpolar?

Venya Vaddi 1L

We won't be asked to calculate electronegativity, but it is calculated using measured values of electron affinity and ionization energy.

Venya Vaddi 1L

If an atom or molecule is more polarizable, does this mean that its dispersion forces are stronger?

Venya Vaddi 1L

I attached pictures of my notes from lecture on Wednesday 11/7. We covered strengths and lengths of bonds and attractive forces. Hope this helps!

Venya Vaddi 1L

Chemical bonds form when the valence electrons of one atom interact with the valence electrons of another, resulting in the sharing of electrons.

Venya Vaddi 1L

The electron configuration for Sb is [Kr]4d 10 5s 2 5p 3 . The valence electrons are the electrons in the outermost shell. In the case, the outermost shell is n=5. Thus, the valence electrons are the five electrons in 5s and 5p. Since 4d is completely filled in Sb they are not valence electrons. 3d ...

Venya Vaddi 1L

NH2, CH2, and COOH are all groups, so after counting the number of electrons I placed those groups together. Then I drew the bonds between the atoms within the groups and then between the groups to get the final Lewis structure.

Venya Vaddi 1L

Can someone explain why triple bonds are shorter than double bonds?

Venya Vaddi 1L

The elements starting from the third period (P, S, Cl) can hybridize the d orbitals and thus have more than an octet.

Venya Vaddi 1L

I think the arrow from the diagram in Dr. Lavelle's powerpoint was just to visually show how the fluoride ion is added to BF₃ to form BF₄^- with a coordinate covalent bond.

Venya Vaddi 1L

Any element with 1 electron (such as Hydrogen or ions with one electron Li²⁺) would have degenerate orbitals (meaning that the orbitals in a given shell have the same energy).

Venya Vaddi 1L

When we discussed the building up principle, the notes say to "add Z electrons with no more than two electrons per orbital." Does Z refer to the atomic number? I just wanted to make sure that the notation is correct.

Venya Vaddi 1L

Can someone explain why electron affinity is the highest for elements in the top right of the periodic table?

Venya Vaddi 1L

Because of shielding, the outer electrons feel reduced electrostatic attraction, resulting in an effective nuclear charge. Shielding explains why it is easier (in other words, requires less energy) to remove the outer electrons because the outer electrons have less of an electrostatic attraction fro...

Venya Vaddi 1L

Intensity is the amount of energy delivered per unit time per unit area.

Venya Vaddi 1L

Which one of the following statements is correct in describing Heisenberg's uncertainty (indeterminacy) equation? A. The less precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa. B. The more precisely the position is determined, the more pre...

Venya Vaddi 1L

When Dr. Lavelle drew energy levels during lecture, higher energy levels were closer together. Is there a point when the distance between energy levels is so small that they are no longer distinct levels?

Venya Vaddi 1L

Are the terms wave function and orbital essentially interchangeable? In other words, do theses two terms refer to the same thing?

Venya Vaddi 1L

The orbitals are not named alphabetically. The s, p, d, and f stand for sharp, principal, diffuse and fundamental, respectively.

Venya Vaddi 1L

Hertz (Hz) and s^-1 are equivalent units. You can use either one for frequency.

Venya Vaddi 1L

In lecture, Dr. Lavelle also mentioned that one photon interacts with one electron.

Venya Vaddi 1L

The meter was defined in 1963 as 1,650,763.73 wavelengths of radiation emitted by krypton-86 (it has since been redefined). What is the wavelength of this krypton-86 radiation? To what region of the electromagnetic spectrum does this wavelength correspond (i.e. infrared, ultraviolet, x-ray, etc.)? W...

Venya Vaddi 1L

In the lecture, Dr. Lavelle mentioned that the description of light as photons explains the intensity profile of black body radiation. Can someone explain what black body radiation is?

Venya Vaddi 1L

Can someone explain the condition E(photon) >= E(energy to remove e-) that was covered in lecture on Monday?

Venya Vaddi 1L

I think you have to use the mass of an electron, which is 9.10938356 × 10^-31 kilograms.

Venya Vaddi 1L

3.00x10⁸ m/s is a rounded value. Generally in calculations 2.998x10⁸ m/s is used since it has a greater number of sig figs and is not rounded off, so it is slightly more accurate than using a rounded value.

Venya Vaddi 1L

Both terms are commonly used interchangeably and we will not get marked down for using one term over another.

Venya Vaddi 1L

In these problems, you always want to make sure you are multiplying by a whole number to ensure you have whole numbers of atoms.

Venya Vaddi 1L

You are calculating the reactant moles in order to compare the amount of reactants available to the ratios in the balanced equation to determine which reactant is limiting. You cannot compare the quantities in grams since your balanced equation gives you molar ratios.

Venya Vaddi 1L

I asked my TA this question in discussion today and he said for this first test we do not need to worry about writing the states of matter in a balanced equation; it is something we will go over later.

Venya Vaddi 1L

In my discussion section today, my TA mentioned that 1 ton equals 1000 kg.

Venya Vaddi 1L

Something that helps me keep track when I am balancing an equation is to list each atom in the reaction and write how many of it are on the reactant side to its left and how many of it are on the product side to its right. I also generally start with balancing the elements in one compound, and I usu...

Venya Vaddi 1L

Hey!

Unfortunately the score for the Pre-Assessment on the modules does not show up.

However, the questions in the Post-Assessment are the same as the ones in the Pre-Assessment and you will be able to see a score once completing the Post-Assessment.

Hope that helps!

Venya Vaddi 1L

I am not sure if you are referring to problem E3 in the 6th or 7th edition, but here is an explanation for E3 in the 7th edition. Based on the given molar masses, 1 mole of gallium atoms is 70 grams, and 1 mole of astatine atoms are 210 grams. In order for the two pans to be equal, the masses on eac...

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