CHEMISTRY COMMUNITY

Libby Dillon - 1A

Specifically for gases, the values of Cv and Cp will be a little different. This has to do with the vessel in which the system is able to react. If there is a constant volume, the vessel is sealed and you will be solving for Cv. In this case, the pressure will change as heat is added. If there is co...

Libby Dillon - 1A

When a reaction is exothermic and gives off heat, the surroundings absorb this heat. Similarly, in an endothermic reaction, the system absorbs heat from the surroundings. So, in an exothermic reaction, the heat "missing" from the system is given to the surroundings, and the total heat chan...

Libby Dillon - 1A

For a constant pressure calorimeter, the system is not sealed and air is able to escape the reaction vessel. This allows the system to remain at a constant pressure because the volume is able to change. Enthalpy values are given through constant pressure calorimeters. Constant volume calorimeters ar...

Libby Dillon - 1A

It likely will depend on what the problem is asking and what is given to you in the problem to figure out which of the three methods to use. Method 2 is the least accurate, but in some cases it will be the only option to solve the problem if only bond enthalpies are given/available.

Libby Dillon - 1A

Hess's Law has to do with enthalpies being a state function - meaning they can be treated as a simple value that can be added. If a reaction is reversed (products become reactants and vice versa), then the sign of the reaction enthalpy switches (negative/positive). If a reaction is multiplied by a s...

Libby Dillon - 1A

Yes, I believe the value of delta H (positive or negative) is the determining factor of whether something is endothermic or exothermic. In an endothermic reaction, heat is required, so delta H will always be positive. In an exothermic reaction, heat is released, so delta H will always be negative.

Libby Dillon - 1A

Yes! It was briefly mentioned in lecture that deltaQ or qp is equal to the change in heat, deltaH, or enthalpy - the amount of heat absorbed or released in a reaction.

Libby Dillon - 1A

Calculate the pH of 0.15M H2SO4 (aq) at 25 degrees celsius.

For this problem, I could only find the pKb of SO4 2- in table 6C.3. How would I solve the problem given this information, do I have to convert to Ka first? Thanks!

Libby Dillon - 1A

Usually it will be a negative x for the reactants and a positive x for the products. This is because the reactants are being used up and the products are being created. Make sure to take stoichiometric coefficients into consideration as well (if 1 mol of reactant goes to 3 mol of product, then it wo...

Libby Dillon - 1A

An exothermic reaction means heat is being released. During a step - up session the UA described heat as one of the products of the reaction which I found helpful. So, as more heat is released and the temperature decreases, there are essentially more "products", so the K value will increase.

Libby Dillon - 1A

I am also a little confused about this. I understand how we derived in lecture that both pH + pOH = 14 and pKa + pKb =14. Since pH and pKa both have to do with acidity, and pOH and pKb both have to do with basicity, what would cause these values to be different?

Libby Dillon - 1A

H20 is unique in that it sometimes plays a role in equilibrium concentrations and other times it does not. When H20 is in a gaseous state, include it in calculations. When H20 is in liquid form, omit it from any concentration equations. This is because liquids do not technically have a concentration...

Libby Dillon - 1A

Yes, because there is no written solution we can use to solve cubic equations, we omit x for parts of these questions when Kc is valued less than 10^-3. This allows us to remove enough variables and then easily solve the equation using algebra. I do not think Kc has to be significantly smaller than ...

Libby Dillon - 1A

Hello! I am also having difficulty with this question. I understand how to find x, but where do I factor in the new concentration of NO given in the problem (0.800 for my problem)? The feedback just says to determine the effect of adding NO and go from there, but I am unsure how to do this mathemati...

Libby Dillon - 1A

Yes! Concentration is simply the number of moles divided by the volume. So in this case you would take the number of moles given for each molecule and divide by the volume of the reaction vessel in liters, 3L. From here you plug each value into the formula to get your reaction quotient, Qc.

Libby Dillon - 1A

K does not change, so in this situation you would have to calculate Q again to see how adding more products or reactants affected the reaction quotient. If Q comes out to be lower than K, then the forward reaction is favored toward the products. If Q is greater than K, the reverse reaction is favore...

Libby Dillon - 1A

If a reaction is at equilibrium, the concentrations of the reactants and the products are unchanging. There could be a far greater amount of reactants than products, or vice versa. The important part is that the ratio is now the same (equilibrium constant). The reactants and the products will likely...

Libby Dillon - 1A

I believe K is the general term representing the equilibrium constant. Kp and Kc are used more specifically when dealing with either concentration equilibrium constant (Kc) or partial pressure equilibrium constant (Kp).

Libby Dillon - 1A

HF was discussed in lecture as a weak acid because it does not easily dissociate, like a strong acid (HCl) would. This is because F is very electronegative and has a shorter, stronger bond.

Libby Dillon - 1A

The Ka value for the first reaction does not involve any negatively charged ions in the example we did during lecture. This means the proton is not as tightly held as it will be in the second reaction, which involves a negatively charged ion. The proton will be more tightly held in the second reacti...

Libby Dillon - 1A

Yes! I was confused about this as well because in lectures it seemed to be used interchangeably. I was in a UA review session and he noted that an H+ is the same thing as a proton.

Libby Dillon - 1A

Water comes up in several problems because it is able to act as both an acid and a base. This depends if water, H2O, gains a H+ to form H3O+ (acid), or loses a H+ to form -OH (base).

Libby Dillon - 1A

Yes, an acid (strong or weak) will lower the pH of a solution while a base (strong or weak) will raise the pH of a solution. Remember a pH of 7 is neutral, 0 is the most acidic, and 14 is the most basic.

Libby Dillon - 1A

The first way explained in lecture to determine the strength or weakness of an acid is by looking at bond length. Larger atoms are going to have larger bonds, which are weaker and more easily broken. Longer bonds are more easily dissociated in water, which is a characteristic of a strong acid.

Libby Dillon - 1A

While oxygen is the most electronegative of the atoms shown, it is not what is being compared in this example. Each compound includes oxygen, so instead you compare the anions by the presence of Cl, Br, and I. Since Cl has the highest electronegativity of the three, it stabilizes the negatively char...

Libby Dillon - 1A

Strong acids are able to dissociate more, or completely in water. They will have longer bonds than weak acids, because longer bonds are more easily broken and can be dissociated. The example given in lecture in determining acid strength based on bond length was (weaker) HF<HCl<HBr<HI (stronger).

Libby Dillon - 1A

Lewis Acids and Bases can be identified as either electron donors or electron acceptors. So in a chemical reaction, the species accepting or gaining the electron pair is a Lewis Acid, like BF3 and H+. The species donating or losing the electron pair is a Lewis Base, like NH3 and OH-.

Libby Dillon - 1A

Another way to help differentiate between strong and weak acids is to remember that organic / carbon containing acids fall into the weak acid category. So acids such as Carbonic Acid, H2CO3 and Benzoic Acid, C6H5COOH are considered weak acids.

Libby Dillon - 1A

I know Cisplatin is used to treat an array of different types if cancer, so is this why it is considered a "general" chemotherapy drug? Or does that have to do with its structure / other components?

Libby Dillon - 1A

I am pretty sure in this situation it would count as two bonds. Even though it is a double bond and occurs at the same site, coordination number is a count of the number of actual bonds, so it would be counted as two.

Libby Dillon - 1A

Yes, I believe all polydentates are considered chelates. By definition, chelates bond to more than one site, so any -dentate above monodentate would be chelating (tridentate, hexadentate, etc.)

Libby Dillon - 1A

Pi bonds show up in the p orbital rather than the s orbital because they occur when two p orbitals overlap side by side. This results in bound atoms being unable to rotate and electron density both above and below the internuclear axis.

Libby Dillon - 1A

Sigma bonds interact end to end and allow bound atoms to rotate. They have cylindrical symmetry around the internuclear axis. Pi bonds overlap side by side and do not allow bound atoms to rotate without breaking bonds. Pi bonds result in electron density above and below the internuclear axis.

Libby Dillon - 1A

Bond angles can be generalized and predicted based on shape, but will slightly differ based on certain atoms. Also, lone pairs contain additional force which pushes bonding electron pairs closer together, making bond angles smaller. I do not think we have to be able to determine bond angles, but we ...

Libby Dillon - 1A

Tetrahedral and square planar both have four regions of electron density occupied by bonding pair electrons. However, square planar molecules also have 2 additional regions of electron density occupied by lone pair electrons, for a total of 6 regions of electron density. When only 4 of 6 regions are...

Libby Dillon - 1A

For H20, there are four regions of electron density, but two are occupied by lone pairs of electrons. This means the other two regions are occupied by bonded electron pairs, which is what we name shaped based off of. The name for this shape is bent or angular when two of four regions of electron den...

Libby Dillon - 1A

I believe this just has to do with repulsion forces and how much electron pairs repel each other compared to others. Lone pair - Lone pair demonstrates the highest level of repulsion, followed by lone pair - bonded pair, and lastly bonded pair - bonded pair. This is helpful in finding different shap...

Libby Dillon - 1A

Yes, lone pairs should be drawn when doing examples of VSEPR. Lone pairs are still regions of electron density and therefore have repulsion to bonding electron pairs. The molecular shape is influenced by lone pairs, but only atom positions are considered when actually naming the shape.

Libby Dillon - 1A

I believe you can still assign shape to molecules even if the bond angles are different. If the bonding pairs on the central atom are all with the same molecule, then we are able to figure out the bond angle. If not, then the bond angles will be slightly different because of differences in electrone...

Libby Dillon - 1A

Van Der Waal interactions are another term used for induced dipole - induced dipole interactions. They are temporary attractions caused by fluctuating electron densities, which result in fluctuating dipoles. They are always present and always attractive.

Libby Dillon - 1A

Hydrogen bonding occurs at very specific locations and many criteria must be present in order for a hydrogen bond to occur. Hydrogen bonds exist with molecules that contain Nitrogen, Oxygen, and Fluorine atoms. Additionally, there must be a Hydrogen atom covalently bonded to an electronegative atom,...

Libby Dillon - 1A

The most notable factor when determining bond lengths is how many are present for one bond (single bond, double bond, triple bond). The more bonds present, the shorter the bonds are. Therefore, single bonds are the longest, and triple bonds are the shortest. Also, the shorter the bond, the more tigh...

Libby Dillon - 1A

Yes, London dispersion will always be present to some extent in molecules. It is also known as induced dipole - induced dipole dispersion. This occurs when there is a temporary change in the distribution of electrons to form one more positively charged region, and one more negatively charged region,...

Libby Dillon - 1A

Electronegativity increases as you get closer to the top right corner of the periodic table, nearing Fluorine. Electronegativity is a measure of an atom's ability to pull in an electron. Electronegativity is directly related to both electron affinity and ionization energy. Is one increases, the othe...

Libby Dillon - 1A

The term "highly polarizable" refers to highly distorted electrons. With electrons more drawn to the other bonded atom, an ionic bond with more covalent character is formed. "High polarizing power" refers to the ions that cause large distortions. They are very similar concepts, b...

Libby Dillon - 1A

I believe you can know if an atom is able to have more than 8 electrons depending on its principle quantum number. Atoms with n=3 or higher have d-orbitals in valence shells that can accommodate additional electrons, as written in Dr. Lavelle's notes. This would include P, S, and Cl. Based on how ma...

Libby Dillon - 1A

Calculated formal charges are more stable the closer they are to zero, with formal charges of zero being the most stable. Bonds may even be rearranged in compounds to obtain a formal charge of zero. An example of this would be a lone pair changing to a single bond, both to complete an octet and have...

Libby Dillon - 1A

Electrons in resonance structures are delocalized. This means the Lewis structure for the particular atom can have multiple bonds in different equivalent locations, according to the notes from today. Delocalization refers to the electrons not being in one definite location, or localized. The electro...

Libby Dillon - 1A

Yes, I believe atoms with lone pairs displayed in their electron configurations are said to be more reactive than those with a full octet. Atoms with unpaired electrons are not as stable as those with all of their electrons paired. If an atom has lone pairs, the unpaired electrons will either form b...

Libby Dillon - 1A

Shorthand electron configurations are very helpful in saving time by not having to write several of the beginning states. You start with the preceding noble gas in brackets, then finish the configuration starting with the orbital directly following the noble gas.

Libby Dillon - 1A

I believe it is because starting from lowest energy and working to highest follows the "building up" or Aufbau principle. We start with the lowest energies in the 1s state and build up to higher energy states. This allows us to know which orbitals/states have more energy than others, and a...

Libby Dillon - 1A

When are we supposed to use E=hv, and when are we supposed to use E=1/2mv^2? Is E=1/2mv^2 only used for electrons, or can either be used? I am having some trouble for certain problems because I think I am using the wrong equation.Thanks!

Libby Dillon - 1A

4s has more energy than 3d and will be occupied first. However, for K and Ca, 4s comes before 3d. After the 4s state is occupied, electrons enter the 3d state, which is lower in energy.

Libby Dillon - 1A

I was able to figure out the first part of the question, the energy of the photon, but I can't seem to figure out which values to plug in to answer the second part, the energy of the electron. The E.coli bacterium is about 2.6 μm long. Suppose you want to study it using photons of that wavelength or...

Libby Dillon - 1A

I believe we only need to understand where Schrodinger's equation comes from and the underlying concepts it represents, not how to actually find values from it. We should know how it connects to Heisenberg's equation and the role it plays in orbitals. I am unsure how we will be tested on the concept...

Libby Dillon - 1A

You can find Δp and Δx by rearranging Heisenberg's Indeterminacy Equation to solve for each respective value. The units for momentum, p, should be kg*m/s. The units for position, x, should be m.

Libby Dillon - 1A

If the De Broglie equation results in a wavelength of 10^-15 or greater, it should have detectable wavelike properties. Anything with a wavelength smaller than this is said to not have detectable wavelike properties. This is because as mass increases, the wavelength decreases. Larger objects that us...

Libby Dillon - 1A

When you find the change in energy when an electron moves to a lower energy level, it is negative because the resulting energy will be lower than the initial energy. However when you are finding the energy emitted, it will be positive. This is the energy released as light when the electron moves to ...

Libby Dillon - 1A

So UV radiation would have shorter wavelengths than visible light, correct?

Libby Dillon - 1A

The Lyman series corresponds to UV radiation, which involves the n = 1 state for an electron. The Balmer series corresponds with the visible region and involves the n = 2 state for an electron. Infrared radiation involves the n = 3 state, but I am not sure if it has a series name.

Libby Dillon - 1A

Each photon must have sufficient energy (high enough frequency) to remove one electron. Increasing the intensity of light (the number of photons) will only increase the number of electrons emitted if each photon has a high enough frequency. One photon corresponds to one electron, therefore if there ...

Libby Dillon - 1A

I believe transferrable units means that when you are measuring quantized units like photons, there is no "in between" level or value. It must be n = 1,2,3, etc. and cannot be values such as n = 1.3, 3.4, etc.

Libby Dillon - 1A

While classical mechanics explains the behavior of large, visible objects, quantum mechanics explains the behavior of very small objects. Instead of being measured continuously, it will be quantized and measured in individual units (quanta / photons).

Libby Dillon - 1A

When determining a limiting reactant, it is possible (although rare in reality) that there is the perfect amount of each reactant to be fully used up without any excess. You would first convert the grams given of each reactant to moles based on their molar masses. From here, compare moles given to m...

Libby Dillon - 1A

Make sure when completing calculations you are using base SI units. These are units such as meters, seconds, and kilograms. This will ensure you are using the correct values in order to successfully cancel out other units. If you do calculations with centimeters, for example, you could end up with a...

Libby Dillon - 1A

I am looking to do some of the textbook fundamentals questions for extra practice, but I am having a hard time finding where they are in the e-text. Are they somewhere in the fundamentals section, or is there a certain page I should go to?

Libby Dillon - 1A

Empirical formulas are in the lowest terms. It is the amount of atoms from one element relative to the others. Molecular formulas, on the other hand, are the actual amount of atoms, or moles, of each element. When finding the molecular formula, calculate the molar mass of the compound and determine ...

Libby Dillon - 1A

When the amount of moles match up on each side, the equation is balanced. Unless each stoichiometric coefficient can be divided by the same number, it is in lowest terms and you are finished balancing.

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