CHEMISTRY COMMUNITY

Rodrigo2J

O2(g), by itself, will have an oxidation number of 0.

Rodrigo2J

t can be in any unit of time depending on what is asked in the problem. Seconds is generally used because it is the SI unit we have associated with time. Just make sure that your units cancel and that the math makes sense.

Rodrigo2J

Yes, at equilibrium we know that delta G and E both equal 0.

Rodrigo2J

It is possible to use units of time that are not seconds. I think we use second in general as a benchmark.

Rodrigo2J

Kelvin

Rodrigo2J

E° is the cell potential under standard conditions (i.e. 298K, 1 atm, and 1M).

Rodrigo2J

You can identify the anode and cathode by writing out the half-reaction and looking at their reduction cell potentials. If the reduction cell potential is more negative, it is oxidizing, and if it is more positive it will be reducing.

Rodrigo2J

From my understanding, you add Pt(s) as the electrode if the half reaction (anode/cathode) does not have a solid metal or Hg(l).

Rodrigo2J

The units of Volts are actually equivalent to Joules/Coulomb. So, if you work out the math, the units should all cancel and you will get Joules.

Rodrigo2J

If Enaught is negative it means that it is not spontaneous (favorable).

Rodrigo2J

Not all redox reactions are acid-base reactions.

Rodrigo2J

the unites for delta G would be kilojoules.

Rodrigo2J

First, you have to find the moles of electrons being transferred by writing out the half reactions. Then you need to substitute all your known values into the equation delta G= -nFE.

Rodrigo2J

To find n, you would need to balance the redox reaction. Then you need to determine the number of moles of electrons being transferred in the reaction.

Rodrigo2J

I got the same answer you did, I think the solutions had an error.

Rodrigo2J

I'm sure that it probablt is a thing, but I don't think we have to worry about for the sake of this class.

Rodrigo2J

NH4Cl is a salt that dissociates into NH4+ and Cl-. The NH4+ has an extra proton that makes it acidic. This is why both HCl and NH4Cl contribute to the acidity.

Rodrigo2J

The purpose of the salt bridge to maintain a neutral charge on both sides of the battery.

Rodrigo2J

F is a given value known as Faraday's constant. It is equivalent to 96,485 C/mol. It's most likely on the equations sheet.

Rodrigo2J

You use celsius and kelvin depending on the units of the other constants being used.

Rodrigo2J

Based on what we did in class, I would say that a half reaction is split into an oxidation reaction and a reduction reaction. If you combine both reactions you should end up with the original reaction.

Rodrigo2J

If you do the algebra, you'll see that T cancels out.

Rodrigo2J

We add certain numbers of electrons depending on the number of electrons gained or lost. l. At least that's what I was taught. I'm sure Dr. Lavelle will cover it more in class.

Rodrigo2J

Because of the equilibrium constant, K, is dependent on temperature, we write T1/K1 and T2/K2 to show that K will be a different value at a different temperature.

Rodrigo2J

Entropy values will be given to us. If Dr. Lavelle wants us to know specific values he'll probably tell us.

Rodrigo2J

Think of it as what happens at each phase change. For example, you can hold an ice cube because it's solid and compact. When the ice melts it becomes water and it gets harder to contain. And when it becomes a gas (because of boiling) you can't really contain a gas because it diffuses throughout the ...

Rodrigo2J

Work is relative to the system. If the system is doing work then it is negative and if work is being done on the system then work positive.

Rodrigo2J

Gibbs free energy is a calculated value. They would either have to give us the information you described, or they would probably ask us to calculate each one in a multi step problem.

Rodrigo2J

An isolated system is simply a system where there is no heat flow into or out of the system and where the volume is cannot change.

Rodrigo2J

The problem tells you that you are raising the temperature of ice. Because Ice (H2O(s)) is the solid form of water, it will have a different heat capacity than H2O(l). You need to use the specific heat of ice which is c= 2.03J/g*c.

Rodrigo2J

In general, you need to create an ICE table with the information given. Then convert Pkb to Ka and set up an equation to solve for x. Since the 5% rule is not valid in this situation, you need to use the quadratic formula to solve. Once you find x, you can convert [H3O+] to pH. Then just subtract th...

Rodrigo2J

I guess you can ask Dr. Lavelle, but I doubt he will offer extra credit.

Rodrigo2J

You need to use PV=nRT to find the moles of Xe. Remember to convert your given information to the proper units. Then convert the moles to mass using the molar mass from the periodic table.

Rodrigo2J

As far as I know, anything we have learned in class, unless he specifically said otherwise, is fair game to show up on the midterm.

Rodrigo2J

This depends on what you define as the system and the surroundings. If work is done on the system it is positive. If work is done by the system it is negative.

Rodrigo2J

Partial pressure works similarly to the way that concentrations do in le chatelier's principle. For example, if you increase the partial pressure of a reactant, the equilibrium will shift towards the products. Likewise if you decrease the partial pressure of a reactant, the equilibrium will shift to...

Rodrigo2J

I think that the most significant difference would be in the number of bonds that connect the atoms. So a triple bond, which we know is shorter than a single bond, will require more energy to break it. Therefore, the bond enthalpy must be larger for the triplebonded substance.

Rodrigo2J

You always to convert between kelvin and celsius because they are not numerically the same.

Rodrigo2J

Normally, if this were a math class, I would say that not including the "+" sign implies that the solution is still positive. However, for the sake of enthalpy, it is vital that the difference between a "+" and "-" is shown. So, just remember to always put the plus sign...

Rodrigo2J

Yes, freezing is an exothermic reaction.

Rodrigo2J

To convert from pH to [H3O+] you just need to use the equation [H3O+]= 10^(-pH).

Rodrigo2J

if it asks for molar concentration then you just need to leave your answer in terms of molarity or (mol/L). The question should ask you for pH or pOH if they want that specific answer.

Rodrigo2J

Basically, the 5% rule can be used when the equilibrium constant is really small (less than 10^-3). It allows you to assume x on the denominator is negligible. So if we had Ka=(X^2)/(.5-x) and Ka was equal to 5*10^-8, then we could pretend that the x in the denominator isn’t there. So the equation w...

Rodrigo2J

I think you can only disregard a reactant from the equilibrium expression if it is a solid or liquid.

Rodrigo2J

Remember, in a chemical reaction the reactants will react with each other to create a product. In this case, because the partial pressure of CO2 is decreasing. then so does the partial pressure of H2 because it reacts with it.

Rodrigo2J

Normally, the question will specifically ask you to find either Kp or Kc. If the question is simply asking to write the expression for an equilibrium constant and doesn't give further direction, either Kp or Kc would be acceptable.

Rodrigo2J

Mostly, the purpose is to help us understand that in phases like solids and liquids we wouldn't use them to calculate K or Q.

Rodrigo2J

the coefficient becomes the exponent of whichever product or reactant it precedes. So as long as you keep the coefficient with the correct reactant or product it's assigned to you should be fine I think.

Rodrigo2J

By looking at the value of K, we predict whether the system is in equilibrium or not. In this case, K is a really big number so we know that the forward reaction is favored and therefore not at equilibrium.

Rodrigo2J

When a question asks us to identify the equilibrium expression for K, can we use either Kp (use partial pressures for the expression) or Kc (use concentrations for the expression)? or do we have to use a specific one in certain situations?

Rodrigo2J

Hi! I think the way we have to think about it is that if a substance is amphiprotic then it also has to be amphoteric.

Rodrigo2J

I'm pretty sure that the coordination number = the number of bonds so we wouldn't count lone pairs. Also I don't think that trigonal pyramidal would be a possible shape for a coordinate compound due to the fact that the central atom wants to accept electrons and having a lone pair wouldn't make sens...

Rodrigo2J

The molecule will be non polar as long as it doesn't have a net dipole moment. So as long as the three atoms around the central atom are the same then yes it should be non polar for a trigonal planar geometry.

Rodrigo2J

If you were to look at the formula for finding the energy of a photon we see that E=hc/lambda. We see from the equation that frequency and Energy per photon have an inverse relationship. If the frequency gets smaller, the energy per photon gets larger. If frequency gets bigger, energy per photon get...

Rodrigo2J

Hi!
If I'm not mistaken, I think the reason you're seeing cyanato in some cases is because they are talking about a cyanate ion [OCN-] and not a cyanide ion [CN-]. In the case of a cyanide ion you would write "cyano" whereas with a cyanate ion you would write "cyanato."

Rodrigo2J

I would suggest to simply try and memorize s many as you can simply because there aren't that many that the textbook shows.

Rodrigo2J

Essentially, the answer is yes. Due to H2Os amphoteric nature, it will act as the opposite of what it is reaction with. If it reacts with a base then it will be considered an acid and if it reacts with an acid it will become the base.

Rodrigo2J

I'm pretty sure Dr. Lavelle said to simply name the complex in alphabetical order.

Rodrigo2J

If I remember correctly, we write "ferrate" when the charge inside the coordination sphere is negative. We write Ferrous if the charge is neutral or positive. And the Roman numeral naturally comes right after the transition metal name in parenthesis.

Rodrigo2J

Even if we aren't given the charge of the transition metal, there is a way to determine it provided that we know the charges of the other ions/ligands. I doubt we would be given a problem where we can't at least determine the charge ourselves.

Rodrigo2J

If you were looking at a molecular formula, you can determine what is inside the coordination sphere by looking at what is on the inside of the brackets.

Rodrigo2J

I think anything that Dr. Lavelle covers in class is fair game to show up in the final, so it would be smart to remember the ones he spent the most time on (if anything).

Rodrigo2J

I think it is okay to write the formula either way, but when you're actually naming the molecule you would have to write it in alphabetical order.

Rodrigo2J

Sigma bonds overlap end to end on the same internuclear axis and pi bonds form when the lobe of one atoms orbital overlaps another.

Rodrigo2J

To determine hybridization simply count the number of regions of electron density around the central atom, or the atom in question.

Rodrigo2J

when talking about intermolecular forces we are talking about forces between molecules.

Rodrigo2J

Remember that in a sulfite ion there is a lone pair of electrons attached to the sulfur atom. Because lone pairs are more repulsive than atoms, they slightly change the bond angles between the atoms to slightly less than 109.5 degrees. We don't have know the exact bond angles, we just say that they ...

Rodrigo2J

Remember that hydrogen bonds do not occur within a single molecule rather they are present when multiple molecules with hydrogen atoms bonded to highly electronegative atoms (N, O, F) are next to each other.

Rodrigo2J

I don't think it will be necessary to know specific electronegativity numbers, but there are some cases in which you can use the the trends of electronegativity to determine if the molecule has a net dipole moment. If it does, it will be polar. If not then it will be non polar.

Rodrigo2J

I think it might depend on how the class does overall. He might look at the average, and if it's bad he might scale grades a little differently.

Rodrigo2J

the ion-ion attraction occurs between a cation and an anion. They are strong bonds.

Rodrigo2J

Effective nuclear charge is the net positive charge that valence electrons experience in a multi-electron atom. Generally as the number of electrons increases in an atom, the effective nuclear charge experienced by valence electrons decreases.

Rodrigo2J

what is more important in determining the relative boiling/melting point of a molecule: surface area (shape) of the molecule or the number of electrons?

Rodrigo2J

As far as I know, hydrogen bonds are always stronger than dipole-dipole and London dispersion forces.

Rodrigo2J

Also only highly electronegative atoms can form hydrogen bonds with hydrogen. These atoms are Nitrogen, Oxygen, and Flourine. I don't think CO can form a hydrogen bond with H2O because if you draw out the lewis structure for CO you will see that Carbon has a partial negative charge and oxygen has a ...

Rodrigo2J

For a hydrogen bond to occur, the hydrogen atom has to be bonded to two highly electronegative atoms. So, if we are looking at use one water molecule we cannot say that the bonds between the O and H are hydrogen bonds because there is only one highly electronegative atom. If we had 2 water molecules...

Rodrigo2J

Can someone explain why Boron and Aluminum (as examples) can be stable without a complete octet?

Rodrigo2J

Isoelectronic just refers to atoms/ions with the same number of electrons. Ex) Na+, F-, Ne — they all have the same # of electrons!

Rodrigo2J

Because oxygen is more electronegative than chlorine, it is “happier” with a -1 charge than chlorine would be.

Rodrigo2J

Everything we have learned so far except intermolecular forces I believe!

Rodrigo2J

Can someone explain what delocalized electrons are and maybe provide an example? Thank you!

Rodrigo2J

Yes! More electrons means more shells which means bigger atomic radius! But remember to take into consideration the growing number of protons as you go across the periodic table.

Rodrigo2J

B) s electrons tend to be better at shielding because they are closer to the nucleus. D) effective nuclear charge is the net positive charge experienced by VALENCE electrons. Think about it this way, if there are more shielding electrons between the nucleus and the outer shell, the net positive char...

Rodrigo2J

When we talk about delocalized electrons, we are talking about the fact that there is more volume which allows for electrons to occupy a space more evenly. We draw a circle inside the lewis structure to represent that the electrons are equally likely to be anywhere along the chemical bond.

Rodrigo2J

a) Mg3As2
b) In2S3
c) AlH3
d) H2Te
e) BiF3

Rodrigo2J

Cations are positively charged ions. Anions are negatively charged ions. The important thing to know about them is that cations tend to lose their electrons and anions gain those electrons.

Rodrigo2J

The Pauli Exclusion Principle basically states that no more than 2 electrons can occupy the same orbital at once and if an orbital does have 2 electrons, the spin will be paired.

Rodrigo2J

I think the wave function is meant to represent the orbitals, not a particular particle.

Rodrigo2J

Quick question: when we solve problems using the Heisenberg equation, we use the +/- value that comes after the given velocity or momentum value right? (For example, if we were given 5 m/s + or - 1m/s, we would use the +/- 1 m/s in the calculation correct?)

Rodrigo2J

Dr. Lavelle said that the mass of an electron was known, so I'm pretty sure it will be given. Photons do not have mass.

Rodrigo2J

For this problem, you have to remember that one sodium atom generates one photon, so the energy you calculate in part a is the same as the energy emitted by one sodium atom when it generates a photon. Knowing this just use the conversion (answer to part a in Joules)/1atom Na to find the answers to p...

Rodrigo2J

Hi! If you look in the textbook, you can find an equation that will help you find what you need! I think the equation was
$1/\lambda =R(1/n(initial)^2+1/n(final)^2)$
but please double check to make sure!

Rodrigo2J

For the sake of this class, will we only focus on the Hydrogen atom mode?

Rodrigo2J

Hi! I know you have to use the Rydberg equation to solve this problem. I don't recall if our professor showed us the equation in class.
$1/\lambda = R(1/n(initial)^{2}+1/n(final)^{2})$

Rodrigo2J

The speed of light is a constant so you don't have to worry about it changing.

Rodrigo2J

I had trouble with this question too! To put it rather simply, the lines are all grouped into a "series" based off of what lower energy level they are dropping down to and therefore how much energy they are emitting (photon). It's important to note that each frequency of light is associate...

Rodrigo2J

Accuracy is how close your answer is to the "correct answer," and precision is how consistent two or more of your answers are.

Rodrigo2J

Basically, in some of the questions that we are solving, we are just given the written out name of a molecule. Since this is the case, do we always assume that Nitrogen, Oxygen, hydrogen, etc. are always diatomic?

Rodrigo2J

I actually asked the TA this question and he said that the order of the elements doesn't necessarily matter as long as you have the correct subscripts and ratios for each atom. But just for a reference, if it is an ionic substance start with the metal and then the nonmetal(s), if it is an organic co...

Rodrigo2J

To summarize G7, the problem is pretty much asking you to find the real mass of KNO3 in a 510.g aqueous solution. They tell you that KNO3 is 5.45% of 510.g so that's a clue to find the mass of KNO3 in the solution. Furthermore, it's important to remember that the solution is aqueous which means that...

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