CHEMISTRY COMMUNITY

MHarrold_1E

For a reaction with multiple reactants, will k' be the same for every reactant? How do we know what value of k' to use when finding k?

MHarrold_1E

When using cell notation, how do we know when to use a comma or a single line to separate species?

MHarrold_1E

What does it mean when E(cell) is negative? Does that mean the forward redox reaction is not favorable, but the reverse is favorable?

MHarrold_1E

When calculating standard reduction potential for Zn(s)--->Zn2+(aq), the table only has the standard reduction potential for Zn2+(aq)--->Zn(s) which is -0.76. Since we want the reverse reaction, do we just change the sign?

MHarrold_1E

Does anyone have any resources or tips they can share to help with assigning oxidation numbers? Thank you!

MHarrold_1E

Since cell potential must always be positive, and delta G can be solved for using -nFE, I assume that delta G is always negative. Does this mean that it is not possible to have a non spontaneous reaction? Also, are reverse reactions possible?

MHarrold_1E

Ecell is determined by cell potential of the cathode minus the cell potential of the anode. So, if there is more concentration of the cathode, we know that the cell potential will be greater than the standard cell potential. If there is more concentration of the anode, we know that the cell potentia...

MHarrold_1E

Is the voltage the same as the maximum work done and the maximum potential difference?

MHarrold_1E

I thought the cell potential has to be always positive, so how can we have a negative delta G for a spontaneous reaction?

MHarrold_1E

Is the standard cell potential the same as the standard reduction potential?

MHarrold_1E

Because we were given the pressure in torr, we need to multiply by 1atm/760torr. We can then use this value to calculate work which would be the change in volume multiplied by the pressure in atm. The units come out to be L.atm. The units for work must be in joules, so you must multiply by (8.314mJ....

MHarrold_1E

I like to think of it this way. When the heat capacity is lower, the slope will be steeper because it cannot "hold" a lot of heat before it begins a phase change. When the heat capacity is higher, it is able to "hold" more heat before undergoing a change phase.

MHarrold_1E

I believe that i represents the current going to zero, because the maximum potential difference is when there is almost no current flowing or very little.

MHarrold_1E

Is the potential of the anode and cathode referring to how much it pulls electrons towards it? Is the potential of the anode ever negative?

MHarrold_1E

In the very first redox reaction example, where does the 8H+ come from?

MHarrold_1E

Megan Sparrow 1A wrote:Maybe the molar values were already accounted for in the specific problems you are referencing. On Dr. Lavelle's equation sheet, moles should be included in the equation you mentioned. Do you mind sharing which questions specifically ask for this?

Sorry, of course! 4F.17

MHarrold_1E

I sometimes get confused if I should use the pressure/volume/temperature of the system or the surroundings. Is there a rule of thumb that I should know or could help? Thank you!

MHarrold_1E

In the textbook, some problems do not give the number of moles when asking to determine change in entropy. In the solution manual I saw that they calculated entropy by using ΔS = C*ln(T2/T1). We have used the equation ΔS = n*C*ln(T2/T1), so I was wondering if we just omit moles when it is not given?

MHarrold_1E

ΔS and ΔH must have the same sign. When there is an increase in entropy there is also an increase in enthalpy because heat had to be put into the system. When bonds are broken, solid to liquid, energy must be put into the system. When a system is at a higher energy state, the entropy is higher as we...

MHarrold_1E

Why do we use a different equation for calculating the change in entropy during a phase change? How do we know when to use ΔS = ΔH/T or ΔS = n*C*ln(T2/T1)?

MHarrold_1E

Could someone please explain how we know that the internal pressure of the system is at 3 atm because the y-axis only gives us the external pressure? Thank you!

MHarrold_1E

Oxygen difluoride is a colorless, very poisonous gas that reacts rapidly and exothermically with water vapor to produce O2 and HF: OF2(g) + H2O(g) ---> O2(g) + 2HF(g) change in enthalpy = -318kJ What is the change in internal energy for the reaction of 1.00mol OF2? I know to use the ideal gas equati...

MHarrold_1E

I was also confused about this question. I know that you solve for the final temperature using q=nC(Tf-Ti), but what do we plug in for C. I saw in the solutions that C was replaced with 5/2R. Could someone please explain why that is?

MHarrold_1E

I believe that the external pressure is the pressure that is outside of the system. I think of the system as a container, so everything outside of the container is the external pressure. I wouldn't think of it as the pressure pushing the piston because the direction of the piston depends on whether ...

MHarrold_1E

In the last example in lecture #11, Dr. Lavelle used the ideal gas law in order to solve for w. I am still confused as to why he subtracted that value instead of adding it because the equation is +w. It would be great if someone could explain this! Thanks!

MHarrold_1E

When solving for work in the equation to find the change in internal energy, what value do we use for -P? Do we use the internal pressure or external pressure?

MHarrold_1E

When calculating the internal energy change, we use q, the energy transferred by heating. When the volume is constant the change in internal energy is equal to the energy of heat transferred by heating? Does q and qv differ in values or is it just different notation to denote that the system has a c...

MHarrold_1E

When solving for the work of expansion, the equation uses the integral from V1 to V2. Are these values the initial volume of the container and then the final volume of the container after expansion?

MHarrold_1E

A reaction vessel of volume 0.500 L at 700. K contains 1.2mmol SO2(g), 0.50 mmol O2(g), and and 0.1 mmol SO3(g). At 700. K, Kc=1.7×10^6 for the equilibrium
2SO2(g)+O2(g)⇌2SO3(g). (a) Calculate the reaction quotient Qc.

In the solution manual, why were all of the mmol divided by 0.5?

MHarrold_1E

What conditions must be true for a a reversible expansion. How about irreversible expansion? Also, what happens when the pressure inside and outside the system are the same?

MHarrold_1E

I know that the change in temperature of a system is negative if the reaction is exothermic and positive if it is endothermic, but I am having a hard time understanding q(sys) + q(sur) = 0. Could someone please explain that equation to me? Thanks!

MHarrold_1E

I believe the main different is in the names itself, one measures molar heat capacity at a constant pressure while the other measures molar heat capacity at a constant volume. These values for a substance will be different, so it all depends on what you are trying to find.

MHarrold_1E

What helps me is to look at the system instead of its surroundings. For the ice example, when it melts, the system must gain heat. Because the system is gaining heat (requires heat), the reaction is endothermic. In phase changes, it may be easier to identify the reaction as exothermic or endothermic...

MHarrold_1E

It depends on whether you are using concentrations or partial pressures to solve for the equilibrium constant. Use Kp if you are using pressure and Kc if you are using concentration. You can use either to solve for the equilibrium constant of a reaction with only gases.

MHarrold_1E

Could someone please walk me through this problem? I am not sure which equations to use. Thank you! An ice cube with a mass of 51.2 g at 0.0 ∘C is added to a glass containing 406 g of water at 45.0 ∘C . Determine the final temperature of the system at equilibrium. The specific heat capacity of water...

MHarrold_1E

How does Ka or Kb relate to the strength of an acid or base?

MHarrold_1E

In the textbook, I read that weak acids have a small Ka value and a big pKa value. This makes sense to me because pKa is closely related to pH. However, for bases it says the same thing, that weak acids have a small Kb value and big pKb value. I thought that a bigger pKb value would result in a high...

MHarrold_1E

Are the numbers at the top of the phase diagram that go horizontally across the change in enthalpy when water goes from solid to liquid and liquid to vapor? If so, why are there three numbers if there are only 2 phase changes?

MHarrold_1E

I am a little confused about the water and steam example. Would steam burn more because the change in enthalpy would be a larger negative value, meaning it is releasing more energy and therefore heat?

MHarrold_1E

Amphoteric means that a molecule can act as an acid or base in a reaction. Amphiprotic means that a molecule can either donate or accept a proton H+. In my opinion I believe that if a molecule fits one of these definitions it must have to fit the other.

MHarrold_1E

Percent ionization is the percent of a compound that has dissociated. For strong acids and bases, the percent ionization is 100% because they completely dissociate. For weak acids and bases we can find the percent ionization by dividing the amount of acid or base ionized by the initial amount of aci...

MHarrold_1E

I believe that Dr. Lavelle was just adding to what he said before which was that solids and liquids are never included in the equilibrium expression. Water is constantly transferring protons between water molecules, but since H2O is a much more stable molecule than OH and H3O, the concentration of H...

MHarrold_1E

I was wondering when and how we are suppose to use Kw. Do we use it to find the concentration of OH and H30 when an acid and base reaction is at equilibria? Does the reaction have to be a neutralization?

MHarrold_1E

Is it possible to get a cubic equation when the K>10^-3, if so, what do we do from there? Also, if K<10^-3 and we do not get a cubic function, do we still omit -x/+x?

MHarrold_1E

If an equation has both an aqueous solution and gas, I believe that it would be easier to solve for the equilibrium constant in terms of concentration, so Kc. I would only use Kp if everything is given in pressure. Even in reactions that contain all gases, it might still be easier to calculate Kc us...

MHarrold_1E

Could someone please explain why the reactants would be favored by an increase in total pressure? I originally thought that there would be no change.
H2O(g) + C(s) ----> H2(g) + CO(g)
Thank you!

MHarrold_1E

Is saying that the reaction is "going forward" , "favors the forward reaction", and "shifts right" all the same thing? Also, when the reaction favors the products does that mean the forward reaction is favored?

MHarrold_1E

When finding the equilibrium constant of a gas, do we use the ideal gas law to find the partial pressure or concentration (n/v)? Essentially what values do we use to calculate equilibrium constant?

MHarrold_1E

For us to say that the equilibrium is shifted to the left/right, in the module the professor said that the K value needs to be greater/less than 10^3/10^-3. However, in the post-assessment, one of the question asked about the K value when the equilibrium is shifted to the right and answer is K>1. D...

MHarrold_1E

If the concentration of reactants is larger, the reaction favors the reactant side. If the concentration of the products is larger, the reaction favors the products side. By "favor", I believe it means that the reaction would prefer to go one way instead of the other. If the products have ...

MHarrold_1E

Hi! The molar concentration of pure substances (liquids and solids) do not change molar concentration. Because of this, we do not need to include pure substances when calculating the equilibrium constant. Simply put, just leave solids and liquids out when calculating equilibrium constant since solid...

MHarrold_1E

Dr. Lavelle mentions that K is related to the energy difference between the reactants and products, and I was wondering what he means by energy difference. How stable the structure of each molecule is?

MHarrold_1E

When neutralization occurs, when the concentration of H30+ is higher than OH- than the solution is acid. Will we need to know how to calculate the concentration? How else would we find out if the solution is acidic or basic?

MHarrold_1E

If it is a strong acid, can we assume that the concentration of the acid is equal to the concentration of H30+? Can we also assume the the concentration of a strong base is equal to the concentration of OH-?

MHarrold_1E

Are all salts either a weak acid or weak base? How do we know if they affect the pH of the solution or not?

MHarrold_1E

I am really struggling with putting acids in order from weakest to strongest. The same with bases. How can we determine the strength without the pKa or Ka?

MHarrold_1E

What exactly is a conjugate acid or base? Do all acids have a conjugate base?

MHarrold_1E

Coordination compounds are an organometallic complex. They form when a transition metal cation form complexes with ligands, any compound that interacts with a transition metal and donates an electron pair. The bonds that form with ligands and transition metals are coordinate covalent bonds (we touch...

MHarrold_1E

They are the same thing! Just for reference: dispersion, London, induced dipole-induced dipole, and van der waal forces are all the same thing. All interactions between molecules have this force.

MHarrold_1E

The coordination number is the number of ligands bonded to the transition metal. Remember that only the elements within the square brackets are part of the coordination compound, so anything outside will not contribute to the coordination number or shape. I made this mistake! When you find the coord...

MHarrold_1E

I believe that having more bonded atoms would make the london dispersion forces greater because the size of the molecule is larger. The larger the molecule, the stronger the dispersion forces. This is because the electrons are held less tightly to the nucleus since they are further away.

MHarrold_1E

In lecture #25, Dr. Lavelle mentions that Fe wants to form an octahedral. How do we know what shape each transition metal prefers?

MHarrold_1E

When Dr. Lavelle does the example of NiCl2 dissolving in water, how many polar covalent bonds are allowed to form with the O and Ni? Does it have to be an octahedral coordinate compound?

MHarrold_1E

I think resonance does not affect the strength of the pi bond, but I am not completely sure and don't take my word for it. I think that the size of the atoms could affect the strength of the pi bonds as larger atoms have a longer bond length due to its size. Yes, if the pi bonds are not as strong th...

MHarrold_1E

Because ionic bonds are stronger, they have a higher boiling point. The stronger the bond is, the harder it is to boil. Ionic bonds are stronger than covalent bonds because a lattice structure of positively charged ions and negatively charged ions form. This lattice structure makes the bonds between...

MHarrold_1E

What is the difference between electron shape and molecular shape?

MHarrold_1E

Because sigma bonds interact end to end, I believe that they would be longer than pi bonds. Also, we know that double bonds are shorter and stronger than single bonds, so I would assume that pi bonds are stronger than sigma. But all pi bonds must also have a sigma bond, so I am a little confused abo...

MHarrold_1E

I believe that sigma bonds correlate with the s-orbital while pi bonds correlate with the p-orbital. There can only be one pi bond when two atoms are bonded because the it can only interact end to end one time.

MHarrold_1E

Just to clarify, lone pairs are as far away as possible from each other? Also, how do you know where the lone pairs are when drawing shape?

MHarrold_1E

I believe that the d orbital has 2 nodes due to the possible regions of electron density.

MHarrold_1E

Only the b,c, and d molecules have dipole-dipole interactions because those molecules are polar. Because a and e have symmetry, there is not one side of the molecule that is more negative or positive.

MHarrold_1E

I use the electronegativity trend and shape to help me find out if the molecule is polar. So, if a molecule is polar, that means one side is hogging the electrons more than the other. So if more electronegative atoms are on one side of the molecule, then it is polar. If the electronegative molecules...

MHarrold_1E

If the energy level is n=2, it has the s sub-shell that contains one orbital and p sub-shell which contains 3 orbitals, so I believe that there would be 4 total orbitals in the at energy level n=2.

MHarrold_1E

From the examples I have done, the lone electron went on the central atom. I am not sure if this is always the case though.

MHarrold_1E

What exactly is an electric dipole moment and how do we calculate it? Also, how do we know the magnitude of the charge in dipoles?

MHarrold_1E

Just to clarify.... when an ion is highly polarizable or has high polarizing power, does that mean it has more covalent character. I am a little confused about polarizability because when something is polar doesn't that mean one side has a negative charge and the other has a positive? Sorry, I hope ...

MHarrold_1E

Essentially yes. A dipole moment is when there are polar bonds within a molecule. When a molecule has a dipole moment or is a dipole, there is a separation of positive and negative charges.

MHarrold_1E

I don't think that the shape matters if you have the correct central atom. I usually just put each atom evenly around the central atom and try to make it as symmetrical as possible. I think we will be getting more into shapes during our next unit!

MHarrold_1E

Elements in period 3 and below are allowed to break the octet rule. Sometimes this is necessary to fit in order to fit all the valence electrons. The most stable Lewis structure will be the one with the lowest formal charge.

MHarrold_1E

When an anion is larger, does that increase or decrease ionic character?

MHarrold_1E

If there are 3 or more elements in a molecule, I was wondering how you know which atoms are bonded together.
For example, in COCl2, O and Cl are bonded to C. How do we know that Cl and C aren't bonded to each other?
Sorry, I don't know if I am asking clearly...

MHarrold_1E

Jordi M 2F wrote:Would this be the case with the 5s and 4d orbitals as well?

Yes, it would be the same for 5s and 4d. The 5s orbital will fill before the 4d orbital even though in an electron configuration the 4d will be written before the 5s.

MHarrold_1E

I believe that they all increase even for a H atom because when the energy level increases, so does the possible values for the quantum numbers.

MHarrold_1E

We use formal charge to find the most stable structure of a molecule. And for the octet rule, every atom must have 8 electrons except H that only has two. Exceptions to the octet rule occur with elements from period 3 and below. Those elements are allowed to have expanded octets because the d-shell ...

MHarrold_1E

When you draw resonance structures, do you have to show every possible position for the pair of electrons or is showing one structure enough?

MHarrold_1E

I was wondering if someone could walk me through this problem...

What is the minimum uncertainty in a helium atom's velocity if the position is known within 1.3 A.

Thank you!

MHarrold_1E

When an atom has high electron affinity does that mean that it wants to gain an electron? Also, does high electron affinity mean that energy is released when an electron is gained?

MHarrold_1E

What causes the diffraction pattern is two or more waves interacting with each other. If we only had one slit, the wave would not be interacting with another wave, so it would not cause diffraction patters. When you have two or more slits, the wave passing through each of the slits can interact with...

MHarrold_1E

If the velocity of an object is 10 m/s (+/- 2), does that mean the change in velocity is 4?

MHarrold_1E

I am a little confused about the different planes of orbitals. When writing electron configuration is it necessary to write px, py, and pz? Also, what are the planes (subscripts) for d and f orbitals?

MHarrold_1E

You can use the de Broglie equation to find wave-like properties of any object with velocity. If you have the mass and velocity of an object, you can find wavelength. You can also manipulate the equation depending on the information you are given in the problem.

MHarrold_1E

It would be great is someone could clarify Hund's rule...does it say that the electrons fill the different orbitals within the same subshell with a parallel spin before electrons pair within an orbital?

MHarrold_1E

C is saying that as the frequency decreases, the wave itself lengthens. Since the wave is lengthened, the slope at a given point decreases which would make sense because the slope would not be as steep. Hope this helps!

MHarrold_1E

Spectral lines appear when the frequency of light entering the atom is equal to the energy different between two different energy levels of the atom. I am not sure if I would say that there is one wave for every spectral line, but I would say that there is one wavelength for every spectral line. I b...

MHarrold_1E

I was just wondering how we know that an electron acts like a wave. I know that when an electron passes through a crystal, it shows diffraction. What does that exactly mean and how does it prove that electrons act like a wave?

MHarrold_1E

I believe that Dr. Lavelle told us that wave like properties are no longer detectable at 10^-15 m or less. I think he was just trying to make the point that all moving objects have wave like properties, but since the wave length is so small, we cannot detect or notice it.

MHarrold_1E

Using the de Broglie relationship, we are able to determine the wavelength of something that is moving with its velocity and mass. When the mass of the object is large, the wavelength will be so small the it is undetectable. We still know that there are wave like properties because of the way we der...

MHarrold_1E

I had trouble understanding the model at first, but after studying it for a while I believe that I get it. When an atom absorbs light, an electron is excited to a different energy level. An electron can only be exited at a higher energy level for a very short amount of time, so the electron then rel...

MHarrold_1E

In the equation E(n)=-hR/n^2, what does E(n) represent? Also, does n represent the final or initial energy level of the electron?

MHarrold_1E

The electron in a hydrogen atom is excited to the n=7 shell and emits electromagnetic radiation when returning to lower energy levels. Determine the number of spectral lines that could appear when this electron returns to the lower energy levels, as well as the wavelength range in nanometers. Could ...

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