CHEMISTRY COMMUNITY

Erika Patel 3I

Why is the $\Delta U$ of an isothermal reaction equal to 0? I thought that isothermal would just mean that q=0, but this isn't the case in the entropy problems that we solved.

Erika Patel 3I

Gibbs free energy is a measure of the maximum work that a system can do. Enthalpy is a measure of the total heat of a system. Gibbs free energy is equal to enthalpy minus temperature x entropy.

Erika Patel 3I

Yes, because the relationship is Keq = k/k' you should be able to manipulate the equation given k_reverse and K_eq to find k_fwd.

Erika Patel 3I

I think this would usually be given to us in the problem, or it would be the step in the reaction mechanism that matches to the overall rate law from the experimental data.

Erika Patel 3I

This question asks why there is a discrepancy between the Gibbs free energy of vaporization of water calculated at 100C from Appendix A and the actual value. Why should the actual value be delta G = 0 at 100C? Why is the gibbs free energy zero for the normal boiling point?

Erika Patel 3I

The unique rate law takes into the stoichiometric concentrations of the reactants and products so is the same for all reactants and products in the reaction.

Erika Patel 3I

Part C asks: 2A --> B + C given that [A] 0 = 0.153M and that after 115 s the concentration of B rises to 0.034M. Determine the rate constant for the first order reaction expressed for the rate of loss of A. I am confused about where to begin solving this problem since we are given an initial concent...

Erika Patel 3I

How would you derive the units of a rate constant for a zero-order reaction?

Erika Patel 3I

When you are looking to find the order with respect to each species, you need to isolate where only that species has concentration changing. For B, you would use experiments 2 and 3 since only its concentrations are changing. When you divide the rate of experiment 3 by experiment 2, you get 5.84... ...

Erika Patel 3I

To balance this reaction, you would need to add 2 H2O to the side with the BrO- so that the number of oxygens are equal. Then you would add 4OH- to that side and 4 H2O to the opposite side to balance the hydrogens. Finally, you balance charges by adding electrons. 2 of the waters should cancel out.

Erika Patel 3I

How would be balance an equation for the formation of Fe2O3 * 3H2O? I know that the reactants would be Fe, O2, and H2O.
I thought that it would be 4Fe + 3 O2 + 6 H2O --> 2 Fe2O3 * 6H2O, however this is the correct answer. Does anyone know how to go about doing this?

Erika Patel 3I

When writing cell diagrams, I thought that reactants were always written first and then products, however this isn't the case in all of the textbook problems. Could someone explain why this is and how to tell when you need to write products before reactants?

Erika Patel 3I

Yes, this is because Ecell is an intensive property and it is measured per mole of electrons. So if you double the number of electrons in the equation, your Ecell would still be the same, because it is per mole of electrons (essentially you would be dividing the supposed larger E value by 2 so get t...

Erika Patel 3I

One thing to remember is that in H2O2, oxygen has a -1 oxidation number. When balancing these half reactions, you are in basic solution, so you will be adding OH- to get the final balanced equations, not H+. You should get Cl2O7 + 6H20 + 8e --> 2 ClO2- + 3 H20 + 6OH- and H2O2 + 2 OH- --> O2 + 2 H2O ...

Erika Patel 3I

Could someone re-explain why E cell (standard) = 0V for a concentration cell?

Erika Patel 3I

Could someone explain how to determine which metal is the cathode and which is the anode?

Erika Patel 3I

In order to count the number of orientations, we would look at the different ways the atoms can be arranged in a molecule. For a simple example, nitric oxide could be written NO or ON. For more complex molecules, it is best to draw the Lewis structure and think about the different ways the molecule ...

Erika Patel 3I

At equilibrium, the system has reached a rate where the forward and reverse reactions are occurring at equal rates. A system also tends towards minimizing free energy available to do work, and when it is at equilibrium, this energy is at a minimum (which is zero).

Erika Patel 3I

Why is the entropy of surroundings equal to zero for isothermal irreversible expansion but not for reversible expansion since neither has a temperature change?

Erika Patel 3I

What is a simple definition for internal energy and what it actually means, besides just the sum of heat and work?

Erika Patel 3I

When we say that a gas is ideal, we assume that there are no intermolecular attractions or repulsions between the gas particles, that they are all equally sized and of negligible mass, and that their collisions are elastic. Also, the gas will expand to fill its entire container and the average kinet...

Erika Patel 3I

Hi, I am a bit stuck on how to start solving textbook problem 4.15. I wrote out my balanced equation of 2HCl (aq) + Zn (s) \rightarrow H 2 (g) + Zn 2+ (aq) + 2 Cl - (aq) the I know that the problem will involve the q=mc \Delta T equation, however I am not sure where to go from here. ...

Erika Patel 3I

For a calorimeter problem at constant pressure, how do we know that q_total=0, i.e. that q_sys=-q_surr?

Erika Patel 3I

Could you please post the answer key?

Erika Patel 3I

Cp values will always be slightly higher than Cv values because under constant pressure the system is increasing temperature but also doing work of expansion (expanding its volume to keep the pressure the same) against an external pressure. Under a constant volume system, there is no work of expansi...

Erika Patel 3I

equation should be $\Delta G=\Delta G + RT lnQ$ where the second delta G is delta G °

Erika Patel 3I

This question asks for delta G of the reaction at 298K when the partial pressures are P N2 = 0.3 bar, P H2 = 0.3 bar and P NH3 = 0.750 bar given that delta G ° = -32.8 kJ/mol for the reaction N2 (g) + 3 H2 (g) \rightleftharpoons 2 NH3 (g). To solve this problem, I was using the equation \Delta G = \...

Erika Patel 3I

Using these two different values for R depends on the units given in your problem/what units you are working with. If you are given pressure and volume values in atmospheres and Liters then it makes sense to use R = 0.08206 atmL/molK, but if you already have units in Joules then it makes sense to us...

Erika Patel 3I

This problem asks us to determine which of the compounds are stable with respect to decomposition into their elements under standard conditions. I wrote my first equation as PCl 5 (g) \rightarrow P(s) + 5/2 Cl 2 (g). However, the textbook has the equation reversed. I am confused why that would be th...

Erika Patel 3I

A system doing work on the surroundings will have a negative value for w, because it is losing energy is push against the surroundings. Since w=-P\Delta V or w=-\Delta nRT , work will be negative when there is either an increase in the volume of the system or increase in the number of moles of gas. ...

Erika Patel 3I

For textbook question 4D.7, we are asked to find the change in internal energy and must use the equation \Delta U=\Delta H-\Delta nRT , which, as we discussed in the example problem during lecture, applies to systems with a constant external pressure. However, this problem did not specifically state...

Erika Patel 3I

For part a, you are given the heat required to vaporize 0.579 mol of methane. In order to find the enthalpy of vaporization of methane, which is a measurement of heat PER MOLE, you need to divide the 4.74 kJ by the 0.579 mol of methane. For part b, you will do the same thing, except first you must c...

Erika Patel 3I

For this question, I understand that we are supposed to calculate the different between having six resonance bonds between the carbons versus 3 C-C single bonds and 3 C=C double bonds. However, I don't understand why having a higher bond energy makes the resonance structure more stable and less reac...

Erika Patel 3I

When calculating Kp (or Q) we will always use pressure in bars. However, since bars and atm have a roughly 1 to 1 conversion, we can use either of these units and come to the same answer.

Erika Patel 3I

ClO4- is the conjugate base of a strong acid, and therefore will have a negligible effect on the pH. NH4+ is the conjugate acid of a weak base (making it a stronger conjugate acid) and will therefore cause the solution to be acidic.

Erika Patel 3I

In lecture today we went over how constant pressure systems have a higher molar capacity than constant volume systems for gases but I didn't quite catch why. Could someone explain why this is the case?

Erika Patel 3I

How can we determine when to use an ICE table versus the henderson-hasselbach equation, specifically for buffer solutions?

Erika Patel 3I

For an acidic solution, if the pH is larger than the pKa value, then the system will shift to create more H 3 O + to lower the pH and gain equilibrium. In that case, the system would be charged. For a basic solution, if the pH is larger than the pKa, the system will shift to reduce the OH - concentr...

Erika Patel 3I

This question refers to an adiabatic process where no energy is transferred as heat in a closed system. I am confused why it is always true that w=0 and always false that w < 0. Even though no heat is being transferred, closed systems allow for the exchange of energy with the surroundings. Why can't...

Erika Patel 3I

For this problem, it is helpful to recognize that \Delta V=-Axd . In this case, Area would be equal to \pi r 2 , where r is half the diameter. This is because we have a tube which would have a cross section as a circle. The value of d would be 20 cm. From there you should be able to calculate the vo...

Erika Patel 3I

During the chemical equilibrium unit, we used an approximation when Kc < 1x10^-4 yet when doing acid-base problems we learned today that we can approximate generally when Ka < 1x10^-3. Why is there this difference? And what does it have to do with the 5% rule?

Erika Patel 3I

Question 3 on Achieve asks to find the percent ionization, pH and pOH for lactic acid given a 0.1125 M solution and Ka = 8.40x10 -4 . I correctly calculated the percent ionization to be 8.28%, however I am confused as to why we can calculate the pH using the x-value of H 3 O + concentration. I thoug...

Erika Patel 3I

I am confused when to use this equation provided in the textbook: pH = 1/2 (pKa1 + pKa2) versus when to just assume that the second de-protonation is negligible to the pH value and only use Ka1. Does anyone know when specifically to use that equation?

Erika Patel 3I

Hi, so for this problem H2SO4 is going to first de-protonate in water into H3O+ and HSO4-. This would be a complete deprotonation, so the H3O+ concentration would equal the original concentration of H2SO4, which is 0.05 M. Next, you would use the equation for the second deprotonation (which is a par...

Erika Patel 3I

When you are adding two equations together, it is helpful to see if there are any intermediate substances that will be consumed during the overall reaction. For instance, if your two equations are [A] + [B] \leftrightharpoons [C] and [C] + [D] \rightleftharpoons [E] then your overall equation would ...

Erika Patel 3I

We can only approximate that the initial and final molar concentrations for the reactants will be the same when Kc is less than or equal to 10 -4 . When Kc is larger than this, the change in x will be significant enough that the molar concentration of the reactants at equilibrium will vary from the ...

Erika Patel 3I

Hi! In the audio-visual focus topics Dr. Lavelle said that in this class we will assume the activity coefficient is equal to one and consider activity to be equal to concentration. The activity for solid and liquid is 1 because the molar concentration of a pure substance (liquid or solid) does not c...

Erika Patel 3I

I understand that the equilibrium constant is temperature dependent, so a change in temperature will cause the equilibrium constant (K) to change. Does this still mean that when the temperature of a reaction occurs it will either shift to form more products or reactants or is only K changing? Simila...

Erika Patel 3I

I know the difference between Kc and Kp is that Kc deals with equilibrium concentrations and Kp deals with partial pressures, but the book also differentiates between just K and Kc. What would be the difference here? Also can gases only be represented using Kp or can they also be expressed using Kc?

Erika Patel 3I

This question gave the thymine structure and asked how many protons the base can accept. I am confused why the oxygens with 2 lone pairs cannot accept a proton. The answer is that only the two nitrogens can accept a proton.

Erika Patel 3I

What would be the order of increasing boiling point temperatures for N2, NO2 and NO? I understand that N2 would have the lowest boiling point because it only experiences LDFs, however I am not sure how to differentiate NO2 and NO since both experience dipole-dipole interactions.

Erika Patel 3I

What would be the order of increasing boiling point temperatures for N 2 , O 2 and NO? I understand that NO would have the highest boiling point because it experiences dipole-dipole forces. To determine the difference between N2 and O2 we would look at size since the larger molecule would be more po...

Erika Patel 3I

I know ethylenediamine has no charge and EDTA has a charge of 4-, but what is the charge on diethylenetriamine?

Erika Patel 3I

How would you determine the order of boiling temperatures for N2, O2 and NO since all of these compounds are linear and only experience LDFs?

Erika Patel 3I

Would the porphyrin ligand be considered chelating since there are 3 spaces in between the two lone pairs on the nitrogen? In class we discussed having a lone pair-spacer-spacer-lone pair to be a chelating ligand. Does that mean that there is too great an angle for this to occur in porphyrin?

Erika Patel 3I

You would expect the bond angle to increase because as you gain s character you are moving more towards an sp² hybridization rather than an sp³ hybridization and the sp² has a higher bond angle.

Erika Patel 3I

Given the thymine structure, we were asked to determine how many protons this base could accept. I understood that each nitrogen could accept one H+ ion, however I am not sure why the two oxygen atoms in the structure cannot accept protons since they have electron pairs available for bonding.

Erika Patel 3I

I have seen explained that HF is a stronger acid than H2O because F is more electronegative than O. However, I am confused about why we can use electronegativities for comparison because when looking at HF vs HCl, F is more electronegative than Cl but HCl is considered the stronger acid since it can...

Erika Patel 3I

Why would HCl be a stronger acid than HF? I thought that since F is more electronegative than Cl this would make the bond more polar and therefore make HF a stronger acid. Why is it that bond strength takes over in this case?

Erika Patel 3I

This question asked for the systematic name for [Co(NH₃)₅Cl]Cl₂.

Using the rules in the textbook, I got pentaammine chlorido cobalt (III) chloride however this is not correct. I was wondering if anyone could tell me what part of this naming needs to be corrected.

Erika Patel 3I

Why can CO₃^2- be both bidentate and monodentate?

Erika Patel 3I

Larger, more complex compounds may have multiple central atoms, in which case you would determine the VSEPR notation around each atom.

Erika Patel 3I

Lone pairs take up more volume than bonding atoms, so they will repel these other bonded atoms and thus change the bond angles to take up more space.

Erika Patel 3I

Why is NO₂^- ambidentate and not bidentate even though the nitrogen and oxygens have lone pairs?

Erika Patel 3I

First you need to draw the Lewis structure of the molecule to determine the number of bonds between the two atoms. Then you use the atomic radii of the two atoms for that specific number of bonds and add them together to get the bond length.

Erika Patel 3I

Water can accept and become H₃O⁺ or lose and become OH^-.

Erika Patel 3I

Why do the halogens only like to form single bonds and typically won't form double or triple bonds?

Erika Patel 3I

Can a nonpolar molecule have dipole-dipole forces if it has polar bonds?

Erika Patel 3I

One of the review sheets has ion-dipole interactions stronger than H-bonds and the other has the opposite. Which of these two interactions are stronger and why?

Erika Patel 3I

This is because hydrogen has a smaller atomic radius than oxygen. Hydrogen doesn't follow the periodic trend from left to right across the table if that is what confused you.

Erika Patel 3I

Figure 2D.11 gives you the covalent radii of each atom in these structures. For each of these, you have the central atom (C, Si, or Sn) bonded to 4 fluorine atoms. Therefore, you know that your bonds are going to be C--F, Si--F, and Sn--F. To find the bond length, you simply add the radii of the cen...

Erika Patel 3I

I believe iron would have two valence electrons. If we look at its electron configuration, it is [Ar] 3d 6 4s 2 . The two electrons in the 4s orbital would be the valence electrons since this is the highest energy shell and valence electrons come from the s and p orbitals. The presence of the 3d orb...

Erika Patel 3I

Can someone explain why oxygen is more electronegative than chlorine in the perchlorate ion? I am also still confused on how to determine which Lewis structure is most plausible based on the oxidation number. I understand that chlorine has an oxidation number of +7, but I'm not sure where to go from...

Erika Patel 3I

This question asked to explain why the vapor pressure of diethyl ether is greater than that of water in terms of the strength and type of intermolecular forces present. To my knowledge, water is able to hydrogen bond with other molecules while diethyl ether is only able to have London interactions a...

Erika Patel 3I

This question asked to draw the Lewis structure and typical contributions to resonance for chloric acid HClO 3 . I was curious why there would be multiple structures for this compound since there is one structure where all of the formal charges on the atoms are zero but then on the other ones some o...

Erika Patel 3I

This is because NO 3 - has resonance. Drawing a single lewis structure, the nitrogen will form a double bond with one of the oxygens because this will reduce the formal charge and make the structure more favorable. As we learned, these electrons are really delocalized around all the atoms, the syste...

Erika Patel 3I

This question asks to draw the Lewis structures that contribute to resonance hybrid of nitryl chloride, ClNO 2 . For this drawing, I had the nitrogen as the central atom since it has the lowest ionization energy, then I had the oxygens across from each other and the chlorine coming off the nitrogen....

Erika Patel 3I

Why does the Lewis structure for glycine, H₂C(NH₂)COOH have the double bond attached to the free oxygen and not the oxygen attached to the hydrogen atom?

Erika Patel 3I

Today in class we drew the Lewis structure for the sulfate ion and determined that sulfur can have more than an octet because the 3d states can be involved in bonding. I was a bit confused about this since sulfur only goes up to 3p 4 in terms of its electron configuration and does not have any 3d or...

Erika Patel 3I

Valence electrons are simply the ones in the outermost shell (principal quantum number). So in an electron configuration that has 3d⁶ 4s² for instance, the valence electrons would be in the 4s orbital.

Erika Patel 3I

Hi, I think the equation you are using is wavelength times frequency equals the speed of light, but this doesn't apply to any particle (or electron), just light. If you are trying to find the velocity of an electron, you should use the de Broglie equation which is wavelength = h/mv. Rearranging this...

Erika Patel 3I

Half filled subshells are more stable because of the symmetric distribution of electrons in each orbital. If you think of the electron configuration diagram, each orbital has one unpaired electron with the same (parallel) spin. This state reduces repulsions and therefore increases stability.

Erika Patel 3I

In this problem, it states that electron affinity is equal to the difference in energy of the incident photon (the incoming light) and the ejected electron. Here, the equation would be similar to the one we used in class with the work function: E photon -E e = E affinity . Therefore, E affinity =E p...

Erika Patel 3I

Hi, since this is a diatomic atom, you would double the atomic mass. Then you would need to convert to kg/mol (dividing grams by 1000 to get to kilograms). However, this is for 1 mol of chlorine molecules and we are being asked to find the wavelength for a single molecule. Since the conversion is 6....

Erika Patel 3I

I am unsure how to approach this problem and was hoping someone could help me out. I converted each product amount to moles of C and H respectively but am not sure if that is necessary or what to do from there. PCBs were once widely used industrial chemicals but were found to pose a risk to health a...

Erika Patel 3I

In class, Dr. Lavelle emphasized that the equations E=hv and E=pc can only be applied to light, yet they are used to derive the de Broglie equation \lambda =\frac{h}{p} , which cannot be applied to light, only objects with rest mass. Can someone explain this to me? How can equations that only apply ...

Erika Patel 3I

In the textbook, the section on orbital shapes in topic 1D covered the radial distribution function. I was wondering if someone could explain what this means and how to apply it because I didn't really understand the book's definition.

Erika Patel 3I

When you are using the Rydberg equation, recall that it is v = R[1/n 1 2 - 1/n 2 2 ]. This equation uses the frequency of the light, however what you have set up uses the wavelength. Therefore, you first need to convert 434 nm to its corresponding wavelength using the speed of light equation c= \lam...

Erika Patel 3I

For me, it helped to look at the one diagram from class which had the electromagnetic spectrum with each type of light and its corresponding wavelength. Since making popcorn had the longest wavelength, this would be found in the microwave region, which has the longest wavelengths on that graph. Sunt...

Erika Patel 3I

I believe you are actually looking at question 14 in the textbook. Regardless, for part a, when n=6, l could equal 0, 1, 2, 3, 4, or 5 so there are therefore 6 possible values for l. Remember that the value of l essentially gives you the shape of the orbital, where 0 is s, 1 is p, and so on. For par...

Erika Patel 3I

The boundary surface is a smooth surface enclosing the electron cloud and it is used to draw orbitals, indicating the probability of where the electron could be located. I believe the boundary surface is something more conceptual than used for calculations, but someone else may be able to answer the...

Erika Patel 3I

Yes, you are on the right track! From there you need to find the change in energy (E final -E initial -- i.e. E 1 -E 7 ) when going from energy level n=7 to n=1. Then you can combine the equations E=hv and c = \lambda v to form E=\frac{hc}{\lambda } . Rearrange for wavelength, plug in your energy va...

Erika Patel 3I

As you may know, placing metal objects into a microwave can generate sparks. Two of your friends are arguing over the cause of sparks. One says that microwaves "herd" electrons into "pointy" areas of the metal object from which electrons jump from one part of object to another. O...

Erika Patel 3I

Hi, could someone explain the difference between atomic and molecular spectroscopy? My understanding so far is that atomic spectroscopy deals with the EMR emitted/absorbed by atoms while molecular spectroscopy deals with molecules. Are there any other distinctions or differences in the graphs create...

Erika Patel 3I

Today in discussion we talked about how the equation E = \frac{-hR}{n^{2}} says that E ∝ - \frac{1}{n^{2}} . I have written in my notes that this relationship tells us that as you increase the energy level (say from n=1 to n=2) then the energy also increases. I am a bit confused about how this conce...

Erika Patel 3I

Today in class we went over that either absorption or emissions spectroscopy can be used to calculate the change in energy for an electron to go from one energy level to another, and thus also the frequency of light needed to energize an electron from one principle quantum level to another. However,...

Erika Patel 3I

Basically, as an electron gets farther and farther from the nucleus (going to higher energy levels), the nucleus has a weaker hold on that electron (its effective nuclear charge is reduced for that electron). Eventually, you get to a point when the electron is so far away that there is no attraction...

Erika Patel 3I

If you recall from the equation c= \lambda v, the speed of light = wavelength x frequency. What this tells us is that frequency and wavelength are inversely proportional (as one increases, the other decreases) but that the speed of the electromagnetic radiation (EMR) is constant at 3.0x10 8 m/s. A h...

Erika Patel 3I

In order to find the number of photons, you need to divide the total energy (in Joules) for the 32W lamp (think of this as every single photon) by the energy for an individual photon (which you said that you calculated in joules). The units of joules will cancel out and you should be left with the n...

Erika Patel 3I

The E K = 1/2 mv 2 equation gives the velocity of the resultant ejected electron and is simply incorporated into the equation E photon - E threshold (work function) = E k . What this equation basically says is that the ejected electron will have energy equal to the energy of the light of the incomin...

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