CHEMISTRY COMMUNITY

Angela Patel 2J

If you can find activation energy using the equation, I don't see why it wouldn't be a valid question on a test or final. Just plug other numbers into the Arrhenius equation and calculate Ea.

Angela Patel 2J

In regards to the Arrhenius equation that is: ln(k2/k1)=Ea/R(1/T1- 1/T2) One of the interpretation that the textbook stated was: "When T2 . T1, the right-hand side is positive, so ln(kr2/kr1) is positive, which means that kr2 > kr1. That is, the rate constant increases with temperature." ...

Angela Patel 2J

In regards to the Arrhenius equation that is: ln(k2/k1)=Ea/R(1/T1- 1/T2) One of the interpretation that the textbook stated was: "When T2 . T1, the right-hand side is positive, so ln(kr2/kr1) is positive, which means that kr2 > kr1. That is, the rate constant increases with temperature." ...

Angela Patel 2J

We use the term adsorption when there is a heterogenous catalyst. When there is a solid catalyst and a substrate (reactant) in solution, we say that the substrate/reactant is adsorbed because it would be attracted/attached to the catalyst in some way but would not combine or be absorbed into the cat...

Angela Patel 2J

Consider the equation Rate = k [A]^n [B]^m. For a table of experiments containing data on the initial [A], [B], and rate for each experiment, use the method of initial rates to determine n, m, and k. Determine m and n by setting up ratios of Rate = k [A]^n [B]^m for different experiments such that ...

Angela Patel 2J

It basically means that two of the same molecule are colliding and combining. This is a fast elemental step, so it will not determine the rate law of the overall reaction.

Angela Patel 2J

Yeah I think the oxidation number only changes by 1 for the indium so that is probably where the error is.

Angela Patel 2J

You are using the right stoichiometric coefficients. You should get that the balanced MnO4- reduction reaction has an E 0 of +1.51 V and the balanced Cl- oxidation reaction has an E 0 of +1.36 V. You then combine the reactions and do E 0 of the cathode minus the E 0 of the anode and get a positive E...

Angela Patel 2J

Would the slope of the graph be k?

Angela Patel 2J

Also remember that rate constants are temperature specific.

Angela Patel 2J

Can someone briefly summarize the method of initial rates for me? I was looking over my notes on this topic and I can't quite remember how to use it…

Angela Patel 2J

You can't just put gold ions in solution, you need it to be dissolved from another compound.

Angela Patel 2J

Since you were given a solid thing in the equation, I'm not sure the Pt is needed. You just need to figure out what's being oxidized vs reduced and put them on the correct side of the "salt bridge". I hope this helps :) I'm not entirely sure if I'm correct, but I remember a problem like t...

Angela Patel 2J

The pH meter is an example of a concentration cell. It measures the difference in potential of the inside of the pH meter and the solution it is placed in. Therefore, it depends on what solution it is placed in.

Angela Patel 2J

How does the salt from the salt bridge interact with the cathode and the anode solutions?

Angela Patel 2J

If you are talking about the half-rxns, then the reduction half-reaction is the one with electrons on the reactants side and the oxidation half-reaction is the one with electrons on the products side.

Angela Patel 2J

Does the porous disk/wall do the same thing as the salt bridge?

Angela Patel 2J

We know common oxidation numbers. For example oxygen always has an oxidation number of 2-. Hydrogen is typically +1. All group 1 metal cations have a charge of +1, etc. You can use these known oxidation numbers to find the oxidation numbers of other elements in compounds with these.

Angela Patel 2J

When the two compounds are in the same phase (for example both in aqueous solution) then you would use a comma. When they are in contact but are in different phases (like with a solid conductor and the metal in solution) you use a single vertical line.

Angela Patel 2J

But the battery still dies, so when does the salt bridge stop working?

Angela Patel 2J

The oxidation state changes, but there are also a certain number of molecules. So for carbon, even though the oxidation state goes from 2- to 1-, there are 2 carbon molecules so this would involve 2 electrons.

Angela Patel 2J

Because there are two moles of NO gas. Standard enthalpies of formation are given per mole, so you need to multiply each one by the number of moles when using the equation given on the formula sheet.

Angela Patel 2J

I think it's possible. Lavelle went over the work formula w = pressure * area * distance so it is fair game for the midterm.

Angela Patel 2J

It is possible to have a reaction where the temperature change is 0 but delta U is constant. Work and heat both affect temperature, so if their effects on temperature cancel out you could technically have a situation where there is no temp. change.

Angela Patel 2J

We can calculate entropy of vaporization at temperatures other than the boiling point. Just multiply the entropy of vaporization by the temperature (in Kelvin) and the # of moles of water.

Angela Patel 2J

For an adiabatic process, would the only change in temperature of the system be due to the amount of work done? Would the temperature change for an adiabatic process be less than the temperature change for a irreversible expansion? For example question 4.6 in the textbook, where it gives you a press...

Angela Patel 2J

Leonardo Le Merle 1D wrote:What are we meant to use if it is not reversible expansion?

You can use the equation S = nRln(v1/v2) if the pressure is not constant.

Angela Patel 2J

I'm not 100% sure, but it probably has to do with changes in entropy. Different molecular compositions for a solution would probably change entropy (and therefore gibbs free energy) as there would be a different number of states. I'm guessing that for the midterm we don't have to understand this exa...

Angela Patel 2J

For calculating delta S, you can substitute qrev for delta H. Remember that this is only true when there is constant pressure.

Angela Patel 2J

The kB constant can be expressed in terms of the gas constant (8.314 J/K*mol) divided by avogadro's number.

Angela Patel 2J

Because - delta H / T is the change in entropy of the surroundings (discussed earlier in lecture today) and this makes the equation easier to simplify into something that only has one variable. This way we can define spontaneity of a process in terms of only gibbs free energy.

Angela Patel 2J

Along with the tricks mentioned above, you can also determine whether work will be positive or negative based on expansion. If the system is expanding outward, work will be negative (w = -P(delta V)). On the other hand, if it is expanding inward, work will be positive. Hope this helped! Yes, work w...

Angela Patel 2J

Also, how would we know the sign of the internal energy?

Angela Patel 2J

How do we know that the pressure is constant for this system?

Angela Patel 2J

Basically, this is because steam has undergone a phase change from liquid to water vapor (vaporization). Vaporization requires a lot of energy. When steam touches your skin, it will release a lot of energy as it cools to form liquid water. Water, while at the same energy, does not have the same lat...

Angela Patel 2J

During reversible reactions, products and reactants are always being made (going back and forth), while in irreversible reactions products can't revert back to what it was as a reactant. I also think that during reversible reactions, the system stays at thermodynamic equilibrium, and during irrever...

Angela Patel 2J

why would q ever not be equal to delta H?

Angela Patel 2J

What about when we are calculating the change in internal energy in a system under constant pressure? Work is involved, do we always use the negative sign in the expression for work?

Angela Patel 2J

We will discuss it more when we start talking about thermodynamics. For now, all we need to know is that there is a difference in the amount of work done depending on whether the gaseous system is under pressure or not.

Angela Patel 2J

Is it ok to use either since we only care about the change in temperature? The change in temperature will be the same whether measured in kelvin or celsius.

Angela Patel 2J

Yep, because that represents having two moles of that reactant or product!

Angela Patel 2J

I would make sure to understand the concepts behind titrations, including why they are done. To remind you, titrations are used to determine the concentration of an acid or base in solution. An example would be an acid of unknown concentration that reacts with a base of known concentration. You woul...

Angela Patel 2J

If the textbook is talking about chemical reactions specifically, it means that reversible reactions will involve the most work. This is because the reaction will reach equilibrium (meaning the rates of the forward and reverse reactions are constant). Because the molecules are constantly reacting wi...

Angela Patel 2J

You would have to know the concentration of OH- if that was the only information you are given.

Angela Patel 2J

Basically, this is because steam has undergone a phase change from liquid to water vapor (vaporization). Vaporization requires a lot of energy. When steam touches your skin, it will release a lot of energy as it cools to form liquid water. Water, while at the same energy, does not have the same late...

Angela Patel 2J

What if you are given all gases, can you still use Kc? I know you can convert to Kp but why wouldn't we just use concentrations if they are given?

Angela Patel 2J

So Ka would be calculated for a compound by studying its equilibrium concentrations as if it were an acid, and Kb would be calculated for the conjugate base of that acid and its reaction is what I'm getting. Like the example with NH3 and NH4+ we did in lecture.

Angela Patel 2J

For example for an acid, we can find how much will dissociate in water by making the ICE table and finding the equilibrium concentration of the hydronium ion

Angela Patel 2J

Are Ka and Kb calculated differently?

Angela Patel 2J

Can you write out the problem?

Angela Patel 2J

What is chemical activity?

Angela Patel 2J

You know that the inital mass is equal to the final mass, and you can use this to solve for the initial concentration of NO

Angela Patel 2J

Yeah I can't seem to find it either. Maybe it's talking about table 5E.1. I might be wrong, but it seems like you should be able to do the problem without any extra information though?

Angela Patel 2J

I still don't really understand why solids aren't included, is it just because they don't have a concentration?

Angela Patel 2J

So we should convert all individual concentrations into pressures for gases using the ideal gas law, and not worry about converting K_c to K_p directly.

Angela Patel 2J

For reference: 8b) A newly designed laser pointer with a certain frequency is pointed at a sodium metal surface. An electron is ejected from the metal surface with wavelength 1.10 nm. What is the frequency of the light from the laser pointer? The work function of sodium is 150.6 kJ∙mol-1. I'll just...

Angela Patel 2J

Since we don't have to do calculations with K_a I would guess that we don't need to use ionization constants. They would only be given to us when we need to rank weak acids/bases in terms of strength.

Angela Patel 2J

are chelates only bidentate?

Angela Patel 2J

Is London dispersion the same thing as dipole-dipole or van der waals forces? Do we have to mention all of these

Angela Patel 2J

The same ligand binds to the transition metal more than once. Four nitrogen atoms in the ligand bind to iron, so it is tetradentate

Angela Patel 2J

The chemical equation is not that difficult to understand, just think about water and co2 in the atmosphere combining to form carbonic acid. I would know the formula and the concept behind it.

Angela Patel 2J

Two hydrogen bonds would not form between two atoms. A double bond consists of a sigma bond and a pi bond between two atoms that are sharing electrons. Hydrogen bonds are much weaker and can only form between hydrogen and oxygen/nitrogen/fluorine.

Angela Patel 2J

Why do strong acids have elements in group 1/2?

Angela Patel 2J

why is it called oxidation number if it is just the charge on the ion/transition metal?

Angela Patel 2J

You could probably figure it out if you drew out the lewis structure. But I doubt that we would be given any of these ambiguous cases since it was not mentioned in lecture or discussion (at least mine).

Angela Patel 2J

Do you use them just when the name starts with bi or di or is it just when it has bi, di, etc anywhere in the name?

Angela Patel 2J

I think it depends on the structure of the molecule. If the oxygen atom is the one binding to the metal on the left, then you would out OH2 in order to show the actual arrangement of the atoms in the coordination complex. Both are technically 'correct' but one can help you better visualize the struc...

Angela Patel 2J

In part a), how do we know that there are three potassium atoms?

Angela Patel 2J

They form complexes because they can accept multiple electron pairs from ligands. This is because of their partially-filled d subshells.

Angela Patel 2J

You can calculate formal charge but it's just easier to do it when you know the charges of the individual ligands. Plus, it's important to remember that transition metals don't often follow the octet rule, so it can be easier to find the charge of the transition metal using the "counting" ...

Angela Patel 2J

GFolk_1G wrote:I believe you would need more information to identify exactly which shape it is such as hybridization or specific numbers of lone pairs/bonding pairs

Yeah. You can't know which one it is based on the coordination number alone. You would have to be given other information.

Angela Patel 2J

Think about the examples that we looked at in lecture of monodentate vs bidentate vs tetradentate ligands. The number of bonds that a single ligand makes with the transition metal determines what we call it.

Angela Patel 2J

The polar bonds cancel each other out because the molecule has a linear shape. Always take shape into consideration when thinking about whether a molecule is polar or nonpolar!

Angela Patel 2J

So all resonance structures for a molecule have the same shape? Is this always true?

Angela Patel 2J

I think you should try to remember all of them since it will help you remember how the structure looks. It's probably not 100% necessary, but it would be helpful.

Angela Patel 2J

So every molecule will have london forces as a type of IMF?

Angela Patel 2J

Are we expected to memorize electronegativities in order to tell whether a molecule has a dipole? And what is the difference between dipole and london forces?

Angela Patel 2J

It's important to note that elements like carbon, nitrogen, and oxygen should not have expanded octets

Angela Patel 2J

505312292 wrote:
605110118 wrote:What is the difference between pi and sigma?

sigma is all single bonds and pi is the second bond in a double bond

I still don't really understand this concept, can someone explain it further?

Angela Patel 2J

How do we know what the central atom should be?

Angela Patel 2J

VPatankar_3L wrote:Also since Oxygen is more electronegative that Chlorine, it can carry the -1 charge.

Is oxygen more electronegative than chlorine? I thought it was the other way around?

Angela Patel 2J

The most important thing is to make sure that you are using the correct number of electrons. If there is multiple ways to do so, then you must look at formal charges and choose the structure that makes the most sense given what we know about electronegativity and bonding.

Angela Patel 2J

Small and highly charged cations are said to have very high polarizing power, because they will attract negative charge. For example, the Al3+ ion has very high polarizing power.

Angela Patel 2J

Are noble gases ever included in compounds? Do they ever share or donate electrons?

Angela Patel 2J

I think the diagram is just pointing out that you can make the Cl formal charge 0 when there are two double bonds with oxygen.

Angela Patel 2J

What about electronegativity? Does that count as a trend in the periodic table or is it just ionization energy / electron affinity?

Angela Patel 2J

Why would they be found in ionic compounds? I thought partial charge was when atoms share electrons unequally.

Angela Patel 2J

Why do radicals exist?

Angela Patel 2J

How do we know what the formal charge for the molecule should add up to? Is it always most stable if it adds up to 0?

Angela Patel 2J

I think electron affinities can be positive or negative depending on whether energy is released or used in order to form an anion.

Angela Patel 2J

Aluminum can also have 6 valence electrons. As well, the electrons for P, S, and Cl would be in the 3d state because this is a lower energy level than the 4s state.

Angela Patel 2J

Just remember that none of the lewis structures that we can draw for a molecule with resonance are actually correct, it is a blend of all of the possibilities as we discussed in class.

Angela Patel 2J

You can know by drawing it out. If the molecule has resonance, then it will have the same arrangement of atoms but different ways to share the electrons, i.e. different ways to place a double bond and still satisfy the octet rule/create the most stable molecule. However many ways you can draw it and...

Angela Patel 2J

Why can orbitals be referred to in terms of psi OR psi^2? Are they interchangeable?

Angela Patel 2J

There is no convention for which magnetic quantum number is associated with which plane for orbitals. Because of this, either the values will be randomly assigned to a specific orbital (given by the problem/textbook/etc) or we will just have to write all of the possible options and accept that it is...

Angela Patel 2J

I think in lecture Professor Lavelle mentioned that in the best case scenario Δp*Δx = h/4pi, but in most situations it's actually larger than h/4pi. In the example that he did with the baseball he wrote the final velocity as v = 3.2X10^10 m/s, so I would guess that we are looking at the best case sc...

Angela Patel 2J

Does calculating the De Broglie wavelength just mean calculating the wavelength of an electron?

Angela Patel 2J

Yes, you can look at the quantum numbers in order to identify whether or not a certain orbital can exist. We know that 2d cannot exist because the angular momentum quantum number can never equal 2 when n =2 (It must be 0 or 1, which correspond to the s or p orbitals). On the other hand, 6f can exist...

Angela Patel 2J

They technically provide the same data. Absorption line spectra display the wavelengths of light absorbed, and emission line spectra display the wavelengths of light emitted. You can measure either to get the same information, it just depends on what experimental tools were available.

Angela Patel 2J

Yes. We learned that it was unexpected, but changing to higher intensity light did not eject electrons, but changing the frequency of light emitted did. This demonstrates the particle-like qualities of light.

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