CHEMISTRY COMMUNITY

haileyramsey-1c

On Dr. Lavelle's review list it says Lyndon's Monday workshop is 6-8 but someone had posted it was 7-9. I was wondering which time it is at for today

haileyramsey-1c

You set 1/2[A]0 to the half life time because you are wanting to find when the concentration of A is half of its initial value so no matter what the concentrations are it will always be 1/2[A]0. Then you would solve for t. For first order t1/2 = 0.693/k as well.

haileyramsey-1c

In concentration cell problems they typically give you all the information and you plug it into the equation in the previous post. Or, you may have to calculate concentration using the other information, but it is important to remember that in concentration cells Eºcell is always zero.

haileyramsey-1c

Can a zero order reaction have a catalyst or be influenced by one? If so would k just depend on the amount of catalyst?

haileyramsey-1c

How do you determine which step is slow and which is (or are) fast?

haileyramsey-1c

When is a reaction zero order or how do we determine that it is?

haileyramsey-1c

To determine the number of electrons being transferred you look at the oxidation numbers of the substances. Then after writing and balancing the half reactions (since the number of electrons must be balanced) that is the number transferred.

haileyramsey-1c

In this situation you look to see which species has the higher (more positive) standard reduction potential. Sometimes that is H2O and sometimes that is the metal ion but by comparing you will know.

haileyramsey-1c

If you are asking about flipping the sign of the reducing agent then that happens because the reducing agent is the substance being oxidized. In the book they use Ecell=Ecathode+Eanode, so you flip the sign of the thing being oxidized because it is the reverse reaction, but in class Lavelle went ove...

haileyramsey-1c

Since oxygen always has an oxidation number of 2- it determines the oxidation number of the other atoms present. In this case in order for overall charge to be 1-, Mn must be 7+.

haileyramsey-1c

If the problem indicated the reaction occurred under acidic or basic conditions, does the H+ or OH- have to be a reactant or does it not matter?

haileyramsey-1c

Why is it in this question for the cell diagram KOH is written when potassium is not mentioned in the question? Does it matter what ion you use or why is it K?

haileyramsey-1c

In writing the cell diagram you exclude coefficients but include subscripts which can indicate diatomic molecules. Since coefficients are just telling you the number of that molecule it is not important, but the subscripts show you that actual makeup. Therefore the cell diagram is not 'balanced'/=.

haileyramsey-1c

In this problem n is 2 because you must look at the coefficients of the atoms that are changing in oxidation number. Since it is 2Ce 4+ --> 2Ce 3+, 2 moles of electrons are being transferred.

haileyramsey-1c

I was wondering on this question why is one of the half reaction between O3 and O2 not between the O3 and the O3 in the BrO3- since O2 and O3 and both zero but the O3 in BrO3- has a 2- charge.

haileyramsey-1c

Since n is the number of electrons transferred you can look at the charges of the atoms in the reactions and see what is changing. More directly, by writing out the half reaction and balancing it to see how many are transferred since the number of electrons has has to cancel that is the number trans...

haileyramsey-1c

I think the best way to do this is to write out the original reaction and see which parts are changing in oxidation number. In this reaction the potassium and chloride ions are not partaking in the reaction as their oxidation states don't change.

haileyramsey-1c

In class we learned about Φ, the electric potential difference in galvanic cell. But are you given the values of Φ or is there an equation to calculate it?

haileyramsey-1c

During class, Dr. Lavelle did a few examples calculating cell potential (E). But i was wondering why you don't flip the sign of for the oxidation half reaction? I know he touched on this during lecture but I am still confused.

haileyramsey-1c

The oxidation number is 0 for ozone since it is a form of elemental oxygen the same applies for O3 as for O2.

haileyramsey-1c

I was wondering why the anode is negative when oxidation is occurring there meaning the reaction is losing electrons which would make it more positive?

haileyramsey-1c

Typically the problem will indicate it. For example if the problem gives you delta H at constant pressure then delta H = q or if the problem says explicitly that 50J heat transferred. Or, the problem may have two steps and in one you find w and in the order you use w to find q.

haileyramsey-1c

Since, H2O is not a solvent as a gas it is included in the K expression. But in this chem class we tend to exclude H2O as most often it is the solvent involved in the reaction which has no effect on the reaction.

haileyramsey-1c

When calculating the heat required for ice to vaporize you break it into steps. Step one would be heating to 0 degrees celsius (if the initial temperature is less than zero). Then step two would be for phase change solid to liquid. Then step three would be heating it as a liquid to 100 degrees celsi...

haileyramsey-1c

In a review session the TA said that it doesn't really matter. But based on what the question is asking answer in per mole or just kJ.

haileyramsey-1c

Examples of extensive properties are gibbs free energy, internal energy, heat capacity, enthalpy, and entropy. Examples of intensive properties are specific heat capacity and molar heat capacity.

haileyramsey-1c

The flat line represents the phase change and the vertical/slanted line represents the heating (enthalpy increase).

haileyramsey-1c

so the surrounding is the universe? The universe is the surrounding plus the system. The system is what we are observing or interested in where as the surrounding is the components around it. In a calorimeter the water the substance is dissolved in is they surroundings and the substance is the syst...

haileyramsey-1c

The R value allows for Cp to be related to Cv since they are not the same values. But, for ideal gases if you know one value you can determine the other using the R value ratios.

haileyramsey-1c

A car engine is an open system. Gases can leave the system and heat can be transferred. But also, matter and energy can enter the system. So, since matter and energy can be transferred it is an open system.

haileyramsey-1c

It’s also important to know if the reaction in the question is reversible or irreversible. For reversible reactions you use w=-nRTln(V2/V1) and for irreversible reactions you use w=-p(ex)deltaV

haileyramsey-1c

Extensive properties are dependent on the amount of substance present. Intensive properties do not depend on the amount present. Enthalpy depends on the mass of the substance and is therefore an extensive property. But, molar enthalpy is an intensive property.

haileyramsey-1c

Delta H is the change in enthalpy of a system during a reaction. But q, is the energy transfer.

haileyramsey-1c

The standard enthalpy of formation is zero because it is naturally occurring in its diatomic state. Since this state is preferred as it is more stable it requires no energy to form. Standard enthalpy of formation is the change in enthalpy when one mole of a substance is formed at 25 degrees celsius ...

haileyramsey-1c

Alison Trinh 1E wrote:What does DeltaH represent?

Delta H is the change in enthalpy of the system during a reaction. The delta H can indicate whether the reaction is endothermic or exothermic based on whether delta h is positive or negative. A negative delta H indicates it is exothermic and thermodynamically favorable.

haileyramsey-1c

Delta U is the change in energy of the internal system. Also, Lavelle said he would discuss this Wednesday.

haileyramsey-1c

For this question, I was wondering why they divided everything by 100 rather than using the number given on y axis of graph? Also, is this something you should always do if given a graph or did I miss something?

haileyramsey-1c

A reaction is exothermic if the ΔH is negative (as it is thermodynamically favorable) which means it is releasing heat in the process. A reaction is endothermic if the ΔH is positive which means heat/energy is required to have the reaction proceed.

haileyramsey-1c

Strong acids and base will completely dissociate in water/solution into their ions so it is not reversible. But, weak acids and bases do not dissociate completely and have the ability to be reversed.

haileyramsey-1c

In some circumstances, after using values from the ICE table you eliminate the x in the denominator of the K equation but in other situations you don't. When exactly should or shouldn't you? Is there a cut off value?

haileyramsey-1c

For this question, I had the same issue but when I was looking at the equation I noticed there are no H atoms in the products so I think it is supposed to be 2HD ⇌ H2 + D2. The, there would be the same number of moles of gas on each side of the equation resulting in the answer no change.

haileyramsey-1c

Is there ever a situation where a solid, liquid, or aqueous solution would be involved in the equilibrium constant expression? or are they always excluded.

haileyramsey-1c

The Law of Mass Action gives us the equation K= [C]^c[D^]d/[A]^a[B]^b. The law of mass action allows us to use the ratios of reactants and products of pressures and concentrations to determine the constant.

haileyramsey-1c

When making an ICE chart for a reaction that proceeds in reverse I know the x would be negative but how would you know the reaction is reverse and nor forward? Would it tell you?

haileyramsey-1c

Adding acids and bases do not affect the K value because the concentration must still multiply to 1.0x10^-14. Kw represents the equilibrium constant of water and although concentration of the acid and base in solution may change water is in large excess that it remains unchanged and unaffected by a ...

haileyramsey-1c

I was wondering what situations you would use Kc vs Kp in and why some reactions with all gases use Kc not Kp?

haileyramsey-1c

The equilibrium changes with temperature and can be reached at different temperature but the equilibrium constant will be effected. As temperature increases the value of K decreases.

haileyramsey-1c

A catalyst speeds up a reaction allowing it to reach the activation energy to be achieved faster. But, the catalyst is not consumed in the reaction.

haileyramsey-1c

Solids and liquids are not included in the reaction quotient because at equilibrium they do not affect the reactant amount. Their activity in the reaction is 1.

haileyramsey-1c

When Q>K a reverse reaction will be favored but when Q<K a forward reaction will be favored.

haileyramsey-1c

K represents the equilibrium constant when the reaction is in equilibrium. Q represents the reaction quotient and is the quotient of activities of products and reactants. Typically you compare the determined q value to a given k value to determine which direction the reaction proceeds.

haileyramsey-1c

A high Ka and a low pKa indicate a strong acid. Since, pKa is the negative log of Ka it means the acid is more fully dissociated in water.

haileyramsey-1c

Yes you should know how to calculate pOH and pH. But, both are -log[OH-] or -log[H+] and make sure that if asked for pH and you find pOH that you subtract the pOH from 14 since pOH + pH =14.

haileyramsey-1c

For pH sig figs still consider the number given in the problem (ex: 2.00g) then find the pH and use three sig figs excluding those left on the decimal (ex: 1.675 not 1.70). The sig fig rules for pH are slightly different but I believe Lavelle posted a guide to them.

haileyramsey-1c

Basic oxides are oxides that form a base in water and acidic oxides are oxides that form an acid when in water. Basic oxides are generally group 1 and 2 elements (example: CaO + H2O --> Ca(OH)2). Acidic oxides are typically groups 14-17 (example: SO2 + H2O --> H2SO3)

haileyramsey-1c

Ligands can be neutral or negatively charged with a lone pair available. They donate the lone pair to the metal cation that is involved.

haileyramsey-1c

Hydrogen bonding occurs when there is a hydrogen bonded to a N,O, or F atom and is attracted to another N,O, or F atom or vise versa. So, the number of hydrogen bonding sites consists of the number or lone pairs on the N,O or F molecules (because they must essentially share electrons with the H atom...

haileyramsey-1c

Unlike other species, transition metal ions have an empty valence shell orbitals allowing them to accept a pair of electrons. They can also form different complexes as transition metals often have multiple oxidation states.

haileyramsey-1c

The lewis structure of acrylonitrile is attached. To calculate the bond angles you must consider each set of atoms' VSEPR (for example the C,H,H, and C is trigonal planar and therefore the angles are 120 for that set). You repeat this process for each set of at least 3 atoms because for a bond angle...

haileyramsey-1c

Vapor pressure is the pressure caused by liquids evaporating. Typically vapor pressure decreases as IMF increases because they can more easily become a gas.

haileyramsey-1c

A nodal plane are regions where there is zero likelihood of finding an electron. In a sigma bond it is a symmetrical sphere and the electron density is cylindrically symmetrical around the axis (imagine a beach ball). In a pi bond there is electron density on either side of the internuclear axis but...

haileyramsey-1c

A compound can be amphoteric if it can accept or donate a proton and it depends on what the compound is reacting with. For example a water molecule reacting with HCl acts as a bronstead base and accepts a proton. But, when water is reacting with NH3, it acts as a bronstead acid and donates a proton.

haileyramsey-1c

In I3- there are three lone pairs around the central atom. The shape is linear because all three of the lone pairs are in the same plane (equatorial) which causes their dipole moments to cancel because they are of equal magnitude and in equally opposing directions.

haileyramsey-1c

In I3- there are three lone pairs around the central atom. The shape is linear because all three of the lone pairs are in the same plane (equatorial) which causes their dipole moments to cancel because they are of equal magnitude and in equally opposing directions.

haileyramsey-1c

A pi bond is not two bonds but it does overlap in two places because there is a nodal plane. Since pi bonds overlap side to side there will be overlap above and below the internuclear axis but this is still considered one bond.

haileyramsey-1c

NaCl has a very large difference in electronegativity compared to HCl resulting is stronger dipole dipole forces. Also, NaCl is larger and has more electrons than hydrogen therefore has stronger London dispersion forces. Since, both of these intermolecular forces are stronger than those in HCl, the ...

haileyramsey-1c

In sp2, the s and p orbitals combine to form 3 orbitals of equal energy all within sp2. So, it's not really that there is an s and two p still separate but now three orbitals of energy in between s and p but closer to p (because 2 electrons are coming from p).

haileyramsey-1c

The p orbital(s) and s orbital combine to form hybridized orbitals of equal energy. In some instances, unhybridized p orbitals from pi bonds.

haileyramsey-1c

I think of you may be thinking of why oxygen's ionization energy is less than nitrogen. This is because nitrogen has a half filled p orbital which is more stable and ideal than oxygen's 4 electrons in the p orbital.

haileyramsey-1c

There isn't a cut off in polar and non-polar molecules as a polar molecule. But, for covalent bonds the difference in electronegativity is less than 1.5 and for ionic bonds the difference in electronegative is greater than 2.

haileyramsey-1c

Dipole moments happen when there is a difference in charge due to an unequal sharing of electrons between atoms within a molecule. This causes one part of the molecule to become slightly negative and the other slightly positive. They from due to attractions between other molecules.

haileyramsey-1c

Any element that is in the third period or later or after can have an expanded octet. Typically the central atom has the expanded octet. The elements that like specific bonds are H, O, N, and C. Hydrogen likes 1 bond, oxygen likes 2 bonds, nitrogen likes 3 bonds, and carbon likes 4 bonds.

haileyramsey-1c

Pi bonds are bonds formed by side by side overlap of two p orbitals. Sigma bonds are two electrons in a cylindrically symmetrical cloud between two atoms. Pi bonds occur in p orbitals and sigma bonds occur in s orbitals.

haileyramsey-1c

The lewis structure for PO4 3- would be with P in the center and 4 oxygen surrounding, 3 of which connected with a single bond and one with a double bond. You wouldn't put 4 double bonds as you want to distribute the charge. So, having a charge of 3- is really high and not favorable. Also, oxygen is...

haileyramsey-1c

Although they are very similar they are slightly different. Electron affinity is something measurable whereas electronegativity is more of a concept that you apply to atoms/molecules. Electron affinity has the same trend (although electronegativity is not truly a trend) where it is increasing as it ...

haileyramsey-1c

Typically yes the formal charge would become negative. But in CO, the carbon has a negative 1 charge and oxygen has a positive 1 charge. You would think that this should be swapped but you first must satisfy the octet before trying to move or change formal charges. In some cases the more electronega...

haileyramsey-1c

In an ionic bond electrons are being donated and in covalent bond electrons are being shared. Ionic compounds are very soluble in water as they are polar like the water. Covalent compounds are non-polar and not soluble in water.

haileyramsey-1c

Ionic compounds tend to be highly soluble as they are polar. But, covalent compounds tend to not be soluble in water as they share electrons more equally and typically are non-polar. Water, which is polar, can dissolve polar compounds (like dissolves like) so ionic bonds are more soluble in water.

haileyramsey-1c

It is equally as favorable as both scenarios result in an octet. If it gains four electrons it would be more difficult for the nucleus to hold on to the electrons strongly as it now has four additional electrons. But nonetheless, carbon wants to have an octet any way how.

haileyramsey-1c

Also when a structure has resonance it means that it is in between all forms drawn. This is why resonance structures bonds aren't the length of a single/double/triple bond because the delocalized electrons are moving between atoms giving each bond a different length than may be expected.

haileyramsey-1c

What it means when the lyman series is n=1 is that the electron is returning to that state. Yes, n=3 is correct as when you use Rydberg's equation you get 3.

haileyramsey-1c

For this problem, you know the nfinal is 1 because UV light is the Lyman series. But you need to first use v=c/λ to find the frequency. Then you use Rydberg equation to find n2 which is 3.

haileyramsey-1c

The Rydberg equation is used to determine the wavelength of light when an electron is changing energy levels. It gives the wavelengths of the spectral line.

haileyramsey-1c

The energy levels get close and closer together due to a higher effective nuclear charge. As energy levels go up, the pull on the electrons decreases causing a greater distance between nucleus and outermost orbital. There is such a large gap because there is a greater energy change.

haileyramsey-1c

In the p-orbital the subscripts of x, y, and z are the different orbitals within the p sub shell and each one has a different orientation. The x, y, and z tell us where the electrons are located and essentially each one is a different axis.

haileyramsey-1c

When going from Na to Mg, for example, there is an increase of one electron. So, for the electron configuration in Na it is 1s^2 2s^2 2p^6 3s^1 and Mg is 1s^2 2s^2 2p^6 3s^2. The electron is added to the outer most orbital (in this case the 3s orbital).

haileyramsey-1c

Elements in the s-block have low ionization energy because if one or two electrons (depending on if they are in group one or two) are removed, then the atom becomes stable as it has the full octet. Elements in p-block on the other hand have higher ionization energies especially those in group 17, as...

haileyramsey-1c

Ionic bonds typically have the highest boiling points due to their charges. Then, as mentioned, hydrogen bonds create very high boiling points. This is because it requires the greatest amount of energy to break the intermolecular bonds. Dipole-dipole bonds have a lower boiling point compared to hydr...

haileyramsey-1c

For this problem, first convert to number of joules in 2 seconds so each step after would be in terms of 2 seconds. Then use the equation E=(hc/λ) to find joules/photon. Then in order to find number of photons use the number of joules in 2 sec and the answer from previous step and the units cancel s...

haileyramsey-1c

The nodal plane is the region around the nucleus where you won't find any electrons. The location of the nodal plane(s) are found using Schrodinger's equation to find shape of orbital.

haileyramsey-1c

The photon must match the energy gap to cause a transition. If the energy is above or below it doesn't match the gap so there will be no transition.

haileyramsey-1c

For this question it is asking about similarity between lines in series. The lines within a series share the same quantum number for the lower energy level (in which the electron returns to after being excited). So if electrons were to return to the n=1 then they are apart of the Lyman series (visib...

haileyramsey-1c

Heisenberg's equation stated qualitatively means there is a limit to the accuracy to which the momentum and position of a particle can be known simultaneously. Also, we do not need to know Schrodinger's equation for this course but it tells us where orbitals come from and uses the idea that an elect...

haileyramsey-1c

For this question, I said that the lines within a series share the same quantum number for the lower energy level (in which the electron returns to after being excited). This number is associated with the Lyman series for n=1 or Balmer for n=2.

haileyramsey-1c

The emission and absorption spectra are the same. They must line up and match because the energy emitted or absorbed would be the same. Since the question asks for the initial and final energy levels of the electron so the transition would be from n=1 to n=3

haileyramsey-1c

The lines within a series share the same quantum number for the lower energy level (where the electrons return to after being excited). Since they share this it creates the groupings which then can be identified based on the quantum number where UV is Lyman series, visible is Balmer series and so on.

haileyramsey-1c

When you increase the intensity of light you increase the number of photons emitted. As intensity increase the amplitude increases. Yes, intensity is like brightness in that intensity is the amount of energy delivered in a certain time to a certain area.

haileyramsey-1c

I'm assuming this is regarding the photoelectric effect so I hope this helps. If light had only wave-like properties, then increasing the intensity of light should release more electrons. But if light behaved like a particle then as intensity increases the number of electrons increases not their spe...

haileyramsey-1c

What does Dr. Lavelle mean when he says "the energy of an electron is quantized"?

haileyramsey-1c

If both products produce the same amount of product you would still have to look at the molar ratios. For example if in a balanced equation one substance was 3 moles and the other was 1, you would have to multiply the first substance by 3 to determine the needed number of moles. So, just because the...

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