CHEMISTRY COMMUNITY

RuchaDeshpande1L

Can someone explain how we are able to figure out the number of electrons transferred in a redox reaction based just off the change in oxidation number? For example, if we are given the balanced equation C6H12O6 + 6O2 —> 6H2O + 6CO2, and that carbon’s oxidation number in glucose is 0, each H is +1, ...

RuchaDeshpande1L

What is the difference between using a porous disk or a salt bridge in a cell to prevent charge buildup? Do the two essentially accomplish the same thing, or are there any major differences we need to be aware of?

RuchaDeshpande1L

What exactly is the difference between absorption and adsorption? How does adsorption work in relation to catalysts?

RuchaDeshpande1L

If we are given a rate law that involves a fraction of concentrations, such as $rate=k\frac{[A]^2}{[B]}$ , do we still add the exponent values to find the overall order of the reaction? Like in this case, would the overall order be 2 + (-1) = 1? Thanks for clarifying!

RuchaDeshpande1L

How exactly do we know that k, the rate constant, and K, the equilibrium constant, change with temperature? Is there a formula or relationship that relates these terms?

RuchaDeshpande1L

For the method of initial rates, do the stoichiometric coefficients of the balanced reaction equation matter in determining k or n?

RuchaDeshpande1L

Equations for the rates of reaction are built off of the form Rate = \Delta Concentration / \Delta Time . That is why, for example, the differential rate law of a first-order reaction is -d[A]/dt . However, the order of reaction is not need for this problem, as you are given the change in concentrat...

RuchaDeshpande1L

In the kinetics information sheet on Dr. Lavelle's website, there is a formula given for the integrated rate law of a first-order reaction involving 2A --> B. The equation given is ln[A] = -2k't + ln[A] 0 . How was this equation obtained and why is it different than the simple A --> B reaction? When...

RuchaDeshpande1L

I only know of one integrated rate law for first-order reactions. The integrated rate law, which is in linear form, is ln[A] t = -kt + ln[A] 0 . Plotting the natural log of concentration against time will produce a straight-line plot. I have only ever used and seen this one used, and as far as I kno...

RuchaDeshpande1L

Can someone go over how we determine the units of k for different orders of reactions? What impacts do the units make on our calculations?

RuchaDeshpande1L

In addition, Dr. Lavelle mentioned that enzymes can become saturated. This means that all active sites on all the available catalysts are filled with a substrate. If this is the case, then increasing the concentration of the substrate will have no effect on the rate of catalyzation, because all acti...

RuchaDeshpande1L

The solutions manual mentions that inert electrodes are necessary for gas/ion electrode reactions. What are all possible situations where we would need an inert electrode? Would we be marked down if we included an inert electrode in the cell diagram where it wasn’t necessary?

RuchaDeshpande1L

Figuring out why H+ was on the same side as O2 confused me at first too! However, when we attempt to balance the half-reaction for the anode, we can see that 5 H+ ions are needed on the left side to balance the number of hydrogen atoms on both sides. We now have 5 H+ on the left and 2H2O and H+ on t...

RuchaDeshpande1L

This confused me at first too! However, when we attempt to balance the half-reaction for the anode, we can see that 5 H+ ions are needed on the left side to balance the number of hydrogen atoms on both sides. We now have 5 H+ on the left and 2H2O and H+ on the right. Because we have H+ on both sides...

RuchaDeshpande1L

First off, just to clarify, the +5 isn't really a charge, it is an oxidation number. For the molecule BrO 3 - , we know that O typically has an oxidation number of -2. Because there are three oxygen atoms in the molecule, we can find that they collectively have an oxidation number of -6. We know tha...

RuchaDeshpande1L

What exactly is the electromotive force (emf)? What do we need to know about it in the context of this class, and why does it represent maximum potential difference?

RuchaDeshpande1L

While doing a few practice problems, I came across a question that asked about "what would be seen at each electrode after some time." The solutions said that the cathode would become "bigger with time" while the anode would become "smaller with time". What exactly does...

RuchaDeshpande1L

Can someone explain how we can derive the w_max = -nFE equation? I understand that delta G equals maximum work, but I do not understand where the original equation comes from.

RuchaDeshpande1L

Work done by expansion can be calculated by using w = -P * delta V. If delta V is zero, then there is no expansion occurring, and thus the overall work done is also zero. Note that under constant volume conditions, the change in internal energy of the system is given by q v and that the change in in...

RuchaDeshpande1L

Can degeneracy ever be zero? Why or why not?

RuchaDeshpande1L

The number of ways of achieving a given energy state is called degeneracy, W. The relationship between W (degeneracy) and S (entropy) is the Boltzmann Equation: S = kb * ln(W)

RuchaDeshpande1L

Why do we put the intiial pressure term above the final pressure term in the equation that solves for change in entropy? In other entropy equations, final volume and final temperature are in the numerator, while initial volume and initial temperature are in the denominator.

RuchaDeshpande1L

Labeling the gases in a system as "ideal" means that the molecules occupy no space and that intermolecular attractions are negligible. We use the concepts of ideal gas laws to approximate the best-case scenario in a system. An ideal system does not have to be under conditions fo constant v...

RuchaDeshpande1L

Why can there no longer be any change in a system when G (Gibbs free energy) is at a minimum?

RuchaDeshpande1L

Degeneracy refers to microstates of the same energy. Looking at Dr. Lavelle's example with the two-sided flask from 01/22 lecture, we can see that the four microstates arise from the two particles going to two different sides of the flask while still at equivalent energy states. We assign degeneracy...

RuchaDeshpande1L

Delta G, or the Gibbs Free Energy, tells us if the reaction occurs spontaneously or not. Because it is a calculated value from enthalpy and entropy, it actually doesn't always have to be negative. We know that spontaneous reactions occur when delta G is negative through the derivation Dr. Lavelle di...

RuchaDeshpande1L

When expanding the volume of a container full of gas in a vacuum, no work is done and so delta U is equal to zero. However, can someone explain why the gas expands spontaneously? Is it because of entropy, and why?

RuchaDeshpande1L

Can someone explain why no work can be done through expansion in a vacuum? And why does this result in the values of both q and w to be zero?

RuchaDeshpande1L

Dr. Lavelle's office hours can be found on the syllabus, Wednesday and Friday 2pm-3pm in Young Hall 3048A.

RuchaDeshpande1L

Using bond enthalpies, we can calculate the enthalpy of the reaction. Breaking bonds requires energy (positive change) while creating bonds releases energy (negative change). Therefore, we can find the net energy change by adding all the bond enthalpies of the bonds broken, and subtracting from that...

RuchaDeshpande1L

Heat capacity is the amount of heat required to change the temperature of an object by a certain amount. On the other hand, specific heat capacity is the amount of heat needed to raise the temperature of an object per unit mass of that object. We can see that heat capacity is an extensive property, ...

RuchaDeshpande1L

Using the standard enthalpy of fusion when solving for the energy needed for state changes can be found by multiplying the number of moles by the standard enthalpy of fusion. Standard enthalpy of fusion is the amount of heat needed to melt 1 mole of substance, so multiplying that value by the number...

RuchaDeshpande1L

The right-most number in the previous user's table displays the maximum number of electrons that specific subshell can accommodate. In your question, the quantum numbers n=2 and l=1 tell us that we are looking at a 2p subshell. However, we do NOT know the orbital number (ml) or the spin number (ms),...

RuchaDeshpande1L

Water has an exceptionally high heat capacity. This is caused by the weak hydrogen bonds among water molecules in the liquid form, constantly forming and breaking as the molecules slide past each other. In the gas form, water molecules are too far apart to interact like this. In the solid form, the ...

RuchaDeshpande1L

Heat capacity is the amount of heat required to change the temperature of an object by a certain amount. On the other hand, specific heat capacity is the amount of heat needed to raise the temperature of an object per unit mass of that object. Another way to think about this difference is to see tha...

RuchaDeshpande1L

I understand that if an exothermic reaction is occurring in a flask, energy is given off and thus the flask feels warm. However, can someone explain why an endothermic reaction occurring in a flask feels cool? I thought that endothermic reactions need to take in energy to occur.

RuchaDeshpande1L

EDTA has a charge of 4- but is still hexadentate because of the four charged oxygens and the lone pairs present on the two nitrogen atoms. The four single-bonded oxygen atoms with a negative charge will attach first to a central metal atom. It is also possible for the nitrogen lone pairs to bond and...

RuchaDeshpande1L

When naming a coordination compound, you name the ligands of the overall molecule in alphabetical order, regardless of the prefix that tells you how many of the ligands are present. For example, if you have Cl- and (H2O)2 as ligands, you would still put diaqua first before chloro, because A comes be...

RuchaDeshpande1L

Remember that an acid reactant has a corresponding conjugate base product and that a base reactant has a corresponding conjugate acid product. The conjugates basically tell you what the role of the product is in the reverse reaction. For example, an acid that donates a proton will produce a conjugat...

RuchaDeshpande1L

For the reaction represented by the equation AgCl(s) <—> Ag+(aq) + Cl-(aq), is the equilibrium constant Kc >1, <1, or approx. =1? Explain your answer. My thinking was that because Kc = [Ag+][Cl-] and there is no denominator of reactant concentration because AgCl is solid, Kc would probably be greate...

RuchaDeshpande1L

Will we need to know how to convert values of Kc to values of Kp, and vice versa? If so, how do we do this? Do you need to use the Ideal Gas Law (PV=nRT)? There were a few problems like this on a worksheet my TA gave me, so I just wanted to clarify.

RuchaDeshpande1L

When naming coordination compounds, we need to look at the charge of the overall transition metal complex, or the stuff that's in the square brackets. If that complex has a negative charge, we need to add -ate to the end of the metal name. In part (b), the complex has 3+ charge, making it positive, ...

RuchaDeshpande1L

Q is our reaction quotient, while K is our equilibrium constant. K is a fixed ratio value for a reaction at equilibrium at a certain temperature, and can be found by calculating K=[P]/[R] using the equilibrium concentrations. On the other hand, Q is calculated the same way but for the concentrations...

RuchaDeshpande1L

We can use the Lewis definition of acids and bases to explain the formation of coordination compounds. Ligands are Lewis bases, which means that they are electron pair donors. Metal cations are the centers of coordination spheres and are Lewis acids, or electron pair acceptors. The binding of Lewis ...

RuchaDeshpande1L

Bisoxalato is referring to the (C2O4)2 in the coordination sphere. [C2O4]2- is the anion oxalate; because it is an anion, we need to add -o to the end of the anion's name when naming coordination compounds. So in this case, oxalate becomes oxalato. Also, the bis- prefix designates that oxalate is a ...

RuchaDeshpande1L

On Dr. Lavelle's Chem 14A website, there is a link labeled "Naming Coordination Compounds". This document contains common neutral and anionic ligands used in coordination compounds. The charges are listed as well. I think it would be best to familiarize yourself with these ions as they do ...

RuchaDeshpande1L

The coordination sphere encompasses the central metal cation and all the ligands attached around it. From this, we can also find the coordination number, which is essentially the number of ligands attached to, or number of bonds around, the central metal. We can usually find this information by look...

RuchaDeshpande1L

When we are drawing an Aufbau diagram for a hybridized orbital, should we fill those orbitals completely, with two spin-paired electrons in every orbital before putting electrons in the 2p orbital? Or should we place one electron in each hybrid orbital and p orbital (parallel spins throughout) befor...

RuchaDeshpande1L

The Lyman series of an atomic emission spectrum always results from an electron demotion from any principal energy level n to n=1. The Lyman series wavelengths also fall under the section of the electromagnetic spectrum that is UV light. Therefore, it is safe to assume that whenever UV light is give...

RuchaDeshpande1L

Coordinate covalent bonds are when one atom provides two of its electrons to share between the atoms involved in the covalent bond. The example given in lecture was the tetrafluoroborate anion, BF4-, in which boron was able to complete its octet because the fluorine ion donated two of its own electr...

RuchaDeshpande1L

When asked which orbital is lowest in energy, should we look at the principal quantum number (n) or angular momentum quantum number (l, determines whether s,p,d,etc subshell)? As in, would 3d be lower in energy, or would 4s be lower?

RuchaDeshpande1L

Can someone please explain in-depth how effective nuclear charge (zeff) and atomic radius reate to each other? Why is it that when zeff increases, atomic radius decreases, and vice-versa? Thank you!

RuchaDeshpande1L

Each orbital in every subshell can hold at the most two electrons. The magnetic quantum number (ml) specifies a specific orbital within a subshell, and can have a value of -l <= ml <= +l. For example, within the p orbital (l=1) are the subshells ml= -1, 0, +1. In the given textbook problems, each va...

RuchaDeshpande1L

From what I understand, I think the atomc radius increases because the electron is getting arther away from the nucleus and thus does not feel the pull of the positively charged nucleus as strongly. The electron starts in the n=1 shell, as part of the 1s subshell. Once excited to the n=2 shell, or 2...

RuchaDeshpande1L

In the textbook, atomic spectroscopy was said to often result in spectral lines, some of which are grouped into specific series. For example, the Balmer series wavelengths represent visible light (electrons jump down to n=2) and the Lyman series wavelengths represent UV light (n=1). If in a problem ...

RuchaDeshpande1L

In addition, electrons and therefore matter have both particle and wave properties. This duality is shown through the double-slit experiment, where the constructive and destructive interactions of waves resulted in diffraction patterns on different parts of the surface. However, these wavelike prope...

RuchaDeshpande1L

So in this question, 1 meter is defined as 1,650,763.73 wavelengths of krypton radiation. So to find the wavelength, divide 1 meter by this number (1650763.73) to find the length of the wave, which turns out to be 606 nanometers, or 6.06 * 10^-7 meters. From there, you can just use E = (hc)/lambda a...

RuchaDeshpande1L

We are using E = 0 as a reference point in this case to represent the farthest an electron can be from its ground state. The negative sign basically means that an atomically bound electron has less energy than a free electron. Just remember that electrons in an atom do not ACTUALLY have negative ene...

RuchaDeshpande1L

Also, in questions involving mass percentages, it is important to thoroughly read the question to see which elements are involved and which respective percentages have been given in the question. For example, in a question that begins with "A compound of carbon, hydrogen, nitrogen, and oxygen h...

RuchaDeshpande1L

From what I understand of formula units, I think of them as “ionic molecules”. They are the lowest whole number ratio of ions represented in an ionic lattice. For example, one Na+ ion and one Cl- ion (NaCl) is the formula unit for the ionic compound sodium chloride. For more information you can chec...

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