CHEMISTRY COMMUNITY

Wilson 2E

Are there any equations associated with molecularity, or is it just something we need to know conceptually?

Wilson 2E

Since A is a constant, similar to k, it will either be given or potentially solved for, which can be done by rearranging the Arrhenius Equation

Wilson 2E

The rate determining step will be the slowest occurring step in the mechanism. For instance, if you're building a car and the engine takes the longest to install, it doesn't matter how fast you assemble the rest of the car, the final product won't be produced until the engine is in place.

Wilson 2E

Conceptually, is there a reason why only first order reactions do not depend on initial concentration for half life, while Zeroth and Second Order do?

Wilson 2E

Energy = -work/Charge;
Rearrange this to get work = -E*Charge
From the Farraday's constant, Charge = n*F
Therefore work = -n*F*E
At constant temp and pressure, work max = deltaG
Therefore deltaG = -n*F*E

Wilson 2E

Does microscopic reversibility just mean that the reverse reaction follows the same mechanism of the forward reaction, but backwards?

Wilson 2E

When calculating the rate of a multi-reactant reaction, why is it necessary to find the pseudo rate and increase the concentrations of all of the reactants but one?

Wilson 2E

Within the scope of this class at least, you can only calculate the order of a species when it is the only concentration changing.

Wilson 2E

You graph ln[A] vs time in order to get a linear graph, since it is ln[a] = -kt + c, not [a] = -kt + c. Using this you can find the value of k for the reaction, which would be the slope.

Wilson 2E

For the rate of the reaction, k[a][b], what happens if either a or b is a solid or liquid. Is it like the equilibrium constant where a product/reactant in a solid or gas phase is negligible?

Wilson 2E

Ahh I see, thanks for the help!

Wilson 2E

How can you calculate pH using the Nernst equation?

Wilson 2E

When balancing reactions in basic and acidic solution, will OH and H, respectively, be a product or reactant, or can it be either? If either, is there a pattern?

Wilson 2E

Is the method for writing out cell potentials for concentration cells the same as standard galvanic cells?

Wilson 2E

I think the primary difference between concentration cells and galvanic cells is that the aqueous solution and the electrodes are the same, just in difference concentrations. In this case, the cell potential isn't coming from the difference in standard potentials for the electrodes, but rather the d...

Wilson 2E

What does Faraday's constant represent?

Wilson 2E

If standard potential is a negative number, does this mean the reverse process is spontaneous, where electrons flow from right to left instead of left to right?

Wilson 2E

To find the half reaction, identify the change occurring in the oxidation numbers of the same elements. In Cr2O7 2-, Cr has an oxidation state of 6+, but on the right side Cr has a 3+ charge; Cr has gained electrons and been oxidized. For C2H5OH, C has a -2 oxidation state, while in C2H4O, C has a -...

Wilson 2E

Yes, since the cathode is on the right and anode on the left, subtracting the right value from the left value will result in the value of a positive cell potential.

Wilson 2E

In the example from Friday lecture where the solutions have aqueous and not solid conductors, Cu(s) | Cu2+(aq) || Fe3+(aq), Fe2+(aq) | Pt (s), why is platinum not included on the left side of the cell diagram as well?

Wilson 2E

Then aside from the element in its pure form, like in a compound NH3, FeO, etc. will the oxidation numbers for H and O consistently be +1 and -2

Wilson 2E

Will elements such as oxygen and hydrogen always have the same oxidation numbers (i.e. -2, +1)? Do only transition metals have varying oxidation state?

Wilson 2E

If a system is isochoric (constant volume), no work will be done since w = p*deltaV, and since there is no change in volume the w = 0.

Wilson 2E

In a reversible isothermal reaction, the total entropy of the system will equal 0, as the change in entropy inside the system will be counterbalanced by a change in entropy of the opposite sign by the entropy of the surroundings.

Wilson 2E

If we did have to solve for delta H naught, I assume you could do so either through the standard enthalpies of formation or even the deltaU = q + w formula.

Wilson 2E

Based on the equations nRln(V2/V1) and nCln(T2/T1), the only way to get a negative delta entropy would be to have the ln term be negative. This would occur by having a decrease in volume or temperature, which produces a value less that 1 for V2/V1 and T2/T1, giving a negative number when using ln.

Wilson 2E

As per the category name, there are obviously formulas for delta S due to volume and temperature changes. But is there any change in delta S if there is a change in pressure? Please explain why or why not

Wilson 2E

If a phase change does occur in the problem, the enthalpy of fusion/vaporization should be given (so long as it's not water, the values of which are on the formula sheet), which should indicate that you would need to include phase change calculations.

Wilson 2E

delta S with the degree symbol refers to the standard entropy value, the amount of entropy of one mole of a molecule. It is in units of J/(K*mol).

Wilson 2E

At a temperature where phase change occurs, heat energy is being used to separate molecules rather than change the temperature of the substance, resulting in a constant temperature.

Wilson 2E

The integral is necessary when P is not a constant. In the derivation of the integral, when P is constant it can be removed from the integral as a coefficient, becoming P * the integral of dv. The integral of dv is equivalent to v2 - v1 (delta V) which produces P*delta V.

Wilson 2E

Why do substances like O2 have a 0 standard enthalpy of formation?

Wilson 2E

Why does changing the amount of substance effect internal energy? Is it due to a change in enthalpy?

Wilson 2E

Does a reaction with a constant pressure count as an irreversible process?

Wilson 2E

What are the two degenerate states in the "dog-bone" example that Dr. Lavelle gave in lecture? What makes them different from each other?

Wilson 2E

The standard state is the phase a compound is in at standard conditions (25 C, 1 atm), and is typically indicated for an element on the periodic table.

Wilson 2E

Yes, it's just the change in heat between the final and initial states. The deltaH between states will be the same, only the sign will be different (e.g. solid -> liquid deltaH = 50, liquid -> solid deltaH = -50)

Wilson 2E

In lecture, Dr. Lavelle mentioned that enthalpy is a state function but that heat was not. What's the difference between the two?

Wilson 2E

How do you determine the value of the change in enthalpy between phase states?

Wilson 2E

However, using bond enthalpy to calculate the change in enthalpy is the most inaccurate of the three methods, as a lot of the calculations for bond enthalpies rely on averages rather than exact figures.

Wilson 2E

I know that Kc is a value without units, but can someone explain why that is? The units don't strictly cancel out all of the time.

Wilson 2E

Calculating for sig figs should only be done at the end of the problem to ensure the accuracy of your answer. While realistically rounding early might only throw the answer off by a couple hundredths, I'd say its better safe than sorry.

Wilson 2E

If I understood correctly, I believe that it is the Kb/Ka for the conjugate reaction, whichever produces either OH or H3O, respectively

Wilson 2E

Salts typically consist of an cation bonded to anion (e.g. NaCl, CaI)

Wilson 2E

For log sig figs, you take the number of sig figs from the number you log and have that many sig figs after the decimal point of your answer (e.g. log(3.800) = 0.5798).

Wilson 2E

The example Dr. Lavelle gave in class was if you added Helium to a reaction occurring within a sealed container. While the pressure of the reaction is increasing, it has no effect on the equilibrium because adding gas doesn't have any effect on the actual concentration of either the products or reac...

Wilson 2E

For a reaction to have K = 1, there must be equal concentration of reactant and product at equilibrium. This entails that neither side must be favored, something that rarely occurs as the purpose of a reaction is to achieve a more preferred, stable form.

Wilson 2E

Q taken at any point when the reaction is not at equilibrium indicates the direction that will be favored by the reaction. If Q is less than Kc, then there is an excess of reactants, and product will be created to balance that amount out. The same goes vice versa if Q is greater than Kc leading to m...

Wilson 2E

If Q is less than K, that means the Q ratio has a higher concentration of reactants due to the form [P]/[R], as a higher denominator produces a smaller number. To raise Q to the value of Kc, the amount of R must be reduced, which is done by creating more product, or favoring the right

Wilson 2E

Manipulating the ideal gas law (PV = nRT) into the form P = nRT/V, n/V is equal to concentration creating the formula P = concRT. Concentration is therefore equal to Pressure divided by R and T (Kelvin).

Wilson 2E

Is there a way to calculate the charge of a ligand, or is it something that just has to be memorized?

Wilson 2E

Yup, a bonding region can only have one sigma bond but multiple pi bonds.

Wilson 2E

Since oxygen is the most electronegative atom and has a negative charge, hydrogen attaches to oxygen in order to neutralize the charge and make the molecule more stable.

Wilson 2E

When an acid donates a hydrogen, it gains a negative charge from accepting an electron lone pair. The stability of the resulting anion determines if the acid is strong or weak. In electron withdrawing, electronegative atoms in the anion work to delocalize the negative charge and in doing so makes th...

Wilson 2E

Is the formation of hydrogen bonds considered an interaction between an acid and base?

Wilson 2E

Is it possible for ammonia to act as an acid by donating one of its hydrogen atoms, rather than acting as a base as is normally seen?

Wilson 2E

They are essentially just different ways of saying the same thing. An acid loses it's hydrogen (proton) in water to form hydronium (H3O), and in exchange gains the electron lone pair from the water molecule it interacts with.

Wilson 2E

Because organic acids typically don't have atoms with high electronegativity, the presence of which gives the attached hydrogen a greater partial positive charge and incentive for proton transfer, the hydrogen atom is less likely to form hydronium with water

Wilson 2E

Just going off of the language in the textbook, it seems that the two terms are interchangeable.

Wilson 2E

What gives strong acids/bases harmful qualities, ie. strong corrosive property?

Wilson 2E

A heme complex is the specific arrangement of a central Fe atom surrounded by 4 N atoms. If the central atom isn't Fe but is a different type of transition metal, then it isn't a heme complex but rather a different complex.

Wilson 2E

I think I understand how to use the bis-, tris-, tetrakis-, etc. prefixes but can someone provide an example where this is used?

Wilson 2E

Oxidation number for the transition metal is calculated by adding up the negative charges of the ligands attached to the central metal then comparing that number to the total charge of the atom. The oxidation number will be the difference between the total charge and the negative charges, as the pos...

Wilson 2E

When the book talks about the 2pz orbital as being the only one able to form a sigma bond, it only means with respect to the z axis. Since 2pz is the only orbital directly along the z axis, the 2px and 2py orbitals, if they are involved through double or triple bond, form pi bonds. However, the 2px ...

Wilson 2E

We most likely don't need to know it, but just in case the structure is a pentagonal formation on the equatorial plane along with one atom below and one above. Essentially trigonal bipyramidal, but instead of only 3 atoms in the middle it's 5 instead.

Wilson 2E

How does VSEPR apply when it comes to large molecules like carbon chains that don't really have a central atom?

Wilson 2E

Pentagonal Bipyramidal is AX7, so one more bonding site than octahedral (AX6).

Wilson 2E

Similarly, molecules that have more atoms will have decreased bond angles as there are more regions of electron density surrounding the central atom making it more cramped.

Wilson 2E

What causes different molecules with the same VSEPR shape to have slightly different bond angles from each other?

Wilson 2E

It's the energy generated from the interaction between two molecules. The more polar the molecules and the smaller the radius between them, the more energy present from the interaction. This type of energy is potential rather than kinetic as the molecules have this energy from their relative positio...

Wilson 2E

The stronger the interactions between molecules, the more energy/heat required to separate them from each other, which raises the boiling/melting points of the molecule.

Wilson 2E

Molecules with greater bond strength from their interactions will be harder to separate, thus requiring more energy/heat to break the bonds (raising the melting/boiling points) allowing a solid to become a liquid, or a liquid a gas.

Wilson 2E

Is the expansion of the octet rule limited only to an element's d-block, or can the f-block store electrons too? Any examples of this happening?

Wilson 2E

When using the formula for interaction energy how is the polarizability (alpha) calculated for each molecule?

Wilson 2E

I'm pretty sure hydrogen bond strength will vary depending on how polarized the atoms are that hydrogen interacts with. I'm not sure on the calculation for that specific number, but I assume -20kj/mol is just the strength of the attractive force on average.

Wilson 2E

Difference in electronegativity really only gives information about the character of the bond, whether it is more ionic or covalent, depending upon the constituent elements.

Wilson 2E

Fluorine only goes up to the second shell regarding orbitals, so it is unable to hold an expanded octet as higher numbered atoms can. Because of this, Fluorine can only hold a standard octet, and starting off with seven valence electrons, only has enough space in its valence to take one more electro...

Wilson 2E

Isoelectronic simply refers to two atoms that have the same amount of electrons. For instance, Neon has 10 electrons as does Fluorine-, which starts with 9 electrons but gains another to have 10 total, so these two atoms are isoelectronic.

Wilson 2E

Aside from that one atom donates both electrons to form a coordinate covalent bond, are there any other characteristics that differ between a coordinate and regular covalent bond?

Wilson 2E

Is there any particular reason why some atoms form coordinate covalent bonds? Is it just to from octets or as a result of electronegativity?

Wilson 2E

What effect is there on the nuclear charge when atoms gain expanded valence shells? Does moving electrons to the d-block increase electron shielding?

Wilson 2E

The emission and absorption spectra are essentially opposites of each other. The emission spectra will show the specific wavelengths given off, while the absorption spectra will show these specific wavelengths taken in by the atom. The wavelengths present in emission will be the ones absent in absor...

Wilson 2E

Expanded valence shells occur once you get to n=3 on the periodic table as at that point there start to be d-block orbitals. Elements after n=3 can use the d-block as a form of storage for the extra electrons allowing for the atom to be stable even outside the standard configuration of an octet.

Wilson 2E

Regarding electron affinity, because energy is released when an electron is added to an atom it makes the configuration of the atom more stable, which is why higher electron affinity (ie. more energy released) corresponds to the likelihood of the atom accepting an electron. Lower electron affinity m...

Wilson 2E

In addition, be careful not to mistakenly use this equation when regarding electrons, Einstein equation is only intended for photons.

Wilson 2E

Also, in orbitals where different configurations are present (ie. px, py, and pz for the p orbital), electrons will move to occupy each configuration and having parallel spin before pairing up within an orbital to maintain a lower energy state

Wilson 2E

That makes a lot more sense now. Thanks!!!

Wilson 2E

To add on, De Broglie's equation should only be used for particles/objects with mass, never with light. Regarding light and other forms of EM radiation, the main equations to be used will be either v*lambda = c and E = hv.

Wilson 2E

How is the Principal Quantum Number (n) calculated for a certain atom? Is it based off of the period of an element? If so does atom excitation affect n?

Wilson 2E

Can anyone explain why higher numbered shells have higher energy than lower number shells as well as the same for orbitals (ie. s has less energy than p which has less than d)?

Wilson 2E

De Broglie's equation (wavelength = h/p) is calculated using both Planck's constant (h) and momentum (p) to determine wavelength. Momentum equals mass times velocity (p=mv), therefore you can extend the equation to [wavelength = h/(mv)]. With this is mind, De Broglie's equation should be used if a q...

Wilson 2E

The energy of light is based off of the equation E=hv, where h = Planck's constant and v = frequency. Since v is the only variable that isn't constant, you're correct in stating that only an increase in frequency will change the energy of light, amplitude isn't even a factor in the energy equation.

Wilson 2E

Because of the particle quality of a photon, the interaction between photons and electrons occur one to one, that is the energy from only a single photon is transferred to one electron. If the energy of this single interaction does not meet the work function threshold, the electron will not be eject...

Wilson 2E

Each of the series are differentiated by the starting energy level of the electron belonging to the hydrogen atom. Lyman has a starting energy level of n=1, Balmer n=2, Paschen n=3, etc.

Wilson 2E

The quantization between energy states of electrons is due to the wave-like properties possessed by an electron. The electron is represented as a circular standing wave around the nucleus of an atom. The amount of energy absorbed by an electron will alter its wavelength and frequency, thereby changi...

Wilson 2E

Most elements on the periodic table are typically given to the hundredths place, so it's best to match that when using percentages so the number of sig figs match up.

Wilson 2E

Sulfide is on the right side of the periodic table, so it's the anion that will replace the OH. Use subtraction to figure out the mass of mystery element M. Then add that value to the mass of sulfur to find the total mass of the sulfide compound.

Wilson 2E

Also, when you're working with values with different numbers of sig figs, your answer should use the lowest amount of sig figs present (ie. 2.56 * 4.1 = your answer should be rounded off to two sig figs 10.)

Wilson 2E

When calculating the empirical formula of a compound and the masses of each element are given, is it necessary to convert the amount of each into a percentage of the actual mass then use the % out of 100g to find the mol ratio, or can you just go from the amounts given to the number of moles of each...

Wilson 2E

No. From the examples given in the textbook (pg F63) on combustion reactions, there isn't any symbol or change made to the basic format of the chemical equation regarding combustion specifically. So while, like you said a spark may be necessary to initiate the reaction it looks like that information...

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