CHEMISTRY COMMUNITY

Alex Chen 2L

You plot the integrated rate law, so the x axis label is time and the y label is 1/[A].

Alex Chen 2L

It will usually tell you in the problem, and you can use the pre-equilibrium method to determine the rate law. You can tell if it is an elementary step if another step occurs that uses a reactant/product from the step.

Alex Chen 2L

A reaction can only proceed as quickly as its slowest intermediate/elementary step. Therefore, fast reactions move towards equilibrium quickly and do not affect the progress of the slow step.

Alex Chen 2L

Assuming you are asking about the question on the final, the forward reaction rate and equilibrium constant was given. You can then use the equation k_forward/k_reverse = K to solve for the reverse rate constant.

Alex Chen 2L

According to the Arrhenius equation, you want a larger activation energy for a greater increase in k (rate constant) for a given increase in temperature.

Alex Chen 2L

We shouldn't need to know how to do such approaches. I believe he was just giving us background or addressing something we might have read in the textbook.

Alex Chen 2L

Check your units. The activation energy should be converted from kJ to J, so it cancels out with the units of R. Temperature should be Kelvin, not Celsius.

Alex Chen 2L

In the parentheses it should be 1/T1 - 1/T2, not the other way around.

Alternatively, put a negative sign in front to have -Ea/R and keep the order 1/T2 - 1/T1.

Alex Chen 2L

It is a measure of the rate of which molecules collide.

Alex Chen 2L

I believe you can. The assumption that the step before the rate-limiting one would be in equilibrium is only made to calculate the overall rate of the multi-step reaction.

Alex Chen 2L

In this question you have to consider water as either a reactant or product of the half-reactions. I find that it's easiest to figure out which component of each cell is the oxidizing/reducing agent first, in terms of which component is most likely to be reduced/oxidized, and then identifying how th...

Alex Chen 2L

I'm going to agree with you and say that the book made a mistake. Seems like they accidentally flipped the sign for E cell during calculations, from what I've seen in the solution manual.

Alex Chen 2L

In discussion, our TA said that the unbalanced redox reaction IO 3 - (aq) + Fe 2+ (aq) → I 2 (aq) + Fe 3+ (aq) can only be balanced within an acidic solution. I know that either an acidic or basic solution is required to balance the reaction since oxygen atoms are in the reactants but not the produc...

Alex Chen 2L

Refer to the example in the last paragraph in section 6L.3, page 552 of the textbook. The cell reaction in this case is a precipitation, caused by AgCl precipitate being formed from Al being oxidized into Al²⁺ ion, which reacts with the Cl^- ion in the anode of the cell.

Alex Chen 2L

In electrolysis, you are trying to produce solid, neutral Ni from its ion form as Ni 2+ in NiSO 4 (aq). To do this, you will need to supply electrons from water to reduce the Nickel ions (water is oxidized and electrons are in the products of the half-reaction), which becomes the anode reaction. The...

Alex Chen 2L

Entropy is q/t and q = -w. In isothermal irreversible expansions, w = nrtln(v2/v1). Put it together and you get (-nrtln(v2/v1)/t) In an irreversible expansion, work isn't calculated by nRTln(v2/v1) because it isn't the maximum work possible. However, entropy change can be directly calculated by nRl...

Alex Chen 2L

You might be thinking of the equation involving standard delta G's relation to the equilibrium constant K. Van't Hoff's Equation mainly relates a temperature change to its effect on K, if you already know the standard enthalpy change.

Alex Chen 2L

At equilibrium, delta G is equal to 0, and the reaction quotient Q is equal to the equilibrium constant K. You can substitute these values into the equation delta G = standard delta G + RTlnQ to relate K to standard delta G.

Alex Chen 2L

Delta H nought and delta S nought are always constant, because they are the changes in enthalpy/entropy in constant standard conditions (25 deg C, 1 atm). Therefore the Van't Hoff equation should always be accurate as long as those two values are known.

Alex Chen 2L

Yes, the reaction is proceeding in the reverse direction. The Gibbs free energy change for the forward reaction is positive, so the reverse reaction will proceed spontaneously to form more I2 in order to reach equilibrium. In the reverse direction, I2 is the product and I is the reactant.

Alex Chen 2L

I could be wrong, but I don't think having constant pressure really applies in this case since a phase change involves a phase that is not a gas. Also, no expansion work is being done.

Alex Chen 2L

Rita Chen 1B wrote:Does increasing the temperature always change the entropy of the surroundings?

If you are referring to increasing the amount of heat gained by the surroundings, yes. However, at higher temperatures the same amount of heat gained by the surroundings will result in a smaller change in entropy.

Alex Chen 2L

There are three parts of to this problem. First you would have to find the change in enthalpy from the He volume change, the change in enthalpy from the Krypton volume change, and then the change in entropy from the temperature change. You would add all these entropies together to get the total. To...

Alex Chen 2L

Cp refers to the molar heat capacity of gases at constant pressure (isobaric), which is equal to 5/2 times ideal gas constant R. Cv is at constant volume instead (isochoric), and it is a smaller value at 3/2 times R.

Alex Chen 2L

It depends on the kind of entropy change occurring. Since we are calculating delta S with the assumption that the gases are ideal, we can use PV = nRT to substitute for any information we don't know. For example, in the equation with the temperature change, you can substitute T2/T1 with P2/P1 or V2/...

Alex Chen 2L

Read the assumptions section in Example 4A.3 in the textbook. There isn't really an equation for an imperfect system - we simply assume no heat is lost to the container of the system, in other words 100% efficient heat transfer between what we define as the system and the surroundings.

Alex Chen 2L

In the example we did in class, we had an exothermic reaction with ΔH = -50kJ occuring in an open beaker producing net 8 moles of gas at 25C, and we had to calculate the change in internal energy. Since ΔU = ΔH - PΔV, we had to use the ideal gas law to calculate PΔV, setting it equal to ΔnRT since t...

Alex Chen 2L

In systems of constant pressure, qp is equal to the change in enthalpy, delta H. However, I'm a bit confused on how it differs from qv, or the energy transferred by heating in a system of constant volume. Does it still equal delta H or does it have to be calculated through q = mCdelta(T)?

Alex Chen 2L

When w = 0, it means the system has constant volume and no energy is being used for expansion/compression work. Therefore, the change in internal energy U only depends on the energy transferred through heat, q.

Alex Chen 2L

Can someone explain how the equation for work for a reversible expansion is derived? or be more specific when you say the integral is taken? Thanks! Lavelle has a PDF on his website that explains it here: https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14B/Integration_Rules.pdf Basica...

Alex Chen 2L

A state function is only determined by the current equilibrium of reactants and products, not the path it took to get there. For example, change in Enthalpy is a state function because you only take into account the final state minus the initial state. We can therefore add enthalpy changes when comb...

Alex Chen 2L

Steam releases a lot of energy when it hits your skin because it condenses from a gas phase to a liquid phase. The extra heat energy from this phase change is the reason why it hurts your skin a lot more than liquid water at the same temperature - water can rapidly cool to match the temperature of y...

Alex Chen 2L

The multi-step reaction should occur in a specific order (the products of the first reaction are involved as reactants of the second reaction, etc.), but you can simply combine all the reactants and products of each reaction into one overall reaction. Any components that are identical in both reacta...

Alex Chen 2L

Yes you must take a solid to liquid phase change into account if it appears in a chemical reaction. It is called the Enthalpy of fusion or melting.

Alex Chen 2L

The thermal energy is being used to overcome intermolecular forces or bonds in the sample in order for the phase to change. Since the energy is being used for that specifically, no energy is added to the temperature or actual kinetic energy from movement of molecules so the temperature remains the s...

Alex Chen 2L

According to the textbook, most polyprotic acids are weak acids even from the first deprotonation, so the second and following deprotonations have little effect on the pH of the solution. In numbers, if Ka2 is less than 1/1000th of Ka1, then the acid can be essentially be treated as a monoprotic aci...

Alex Chen 2L

In the textbook, they define K with no subscript the same as Kp.

Alex Chen 2L

Say the diluted solution has a concentration of X. The original solution was 5.00mL but diluted with water to 500.0mL, so the dilution ratio is 5.00/500.0 = 1/100. This means the diluted concentration is 1/100th of the original concentration, so you would multiply X by 100 to find the original conce...

Alex Chen 2L

The table is using K to represent Kp. For the reactions that the Kc and K values are the same, the moles of gas on both sides of the reaction is probably equal. This would mean that Kc=Kp, so therefore in the table, K is equal to Kc for those reactions. This is correct. Temperature changes can chan...

Alex Chen 2L

The pH of a salt solution depends on whether the dissolved salt has an ion that is a conjugate base of a weak acid, or a conjugate acid of a weak base. You would solve for the pH in the same manner as for a weak acid or base, just that the Kb or Ka value of the ion is equal to Kw/Ka or Kw/Kb of the ...

Alex Chen 2L

It depends on the number of moles of reactants and products. I think Lavelle mentioned that we aren't expected to solve cubic functions, but cubic functions will show up if you have 3 or more moles of reactant or product on one side of the reaction that are included in the equilibrium constant, sinc...

Alex Chen 2L

The equilibrium constant is equal between the two experiments. However, the ratio of O2/O3 is not the equilibrium constant - it is missing the stoichiometric coefficients. The correct constant would be (O2)^3/(O3)^2. Without the exponents, the ratio will not be balanced and therefore be different at...

Alex Chen 2L

This situation is called a heterogeneous equilibrium, and both gases and aqueous solutions have to be part of the equilibrium constant. Therefore, since aqueous solutions can only be measured in concentration, you would use the ideal gas law PV = nRT to convert the partial pressure of any gases into...

Alex Chen 2L

The question tests knowledge of the response of chemical equilibria to change. For part (a), the amount of O2 is different because more O2 is formed in the second container from the larger amount of O3. Therefore, the partial pressure of O2 in part (b) is also different because a higher amount of ga...

Alex Chen 2L

In terms of Kp and Qp, the unit you use for pressure doesn't matter as long as you stay consistent because the equilibrum constant and reaction quotient are both unitless.

Alex Chen 2L

Rishika Yadav 3D wrote:I think that because there are no lone pairs on the Fe, it will be tetrahedral

It's different for coordination compounds. Since the porphyrin ligand binds to Fe four times at the corners of a square, the shape is actually square planar.

Alex Chen 2L

Which of the following can be classified as reactions between Brønsted acids and bases? For those that can be so classified, identify the acid and the base. (c) CH3COOH(aq) + NH3(aq) --> CH3CONH2(aq) + H2O(l) The solutions manual says there is no proton transfer occurring in the reaction. I assume t...

Alex Chen 2L

I think the solutions are wrong. The pH on the table should be -0.176, but the table says +0.176. Also, a [H30+] concentration being greater than 1M means that the pH has to be negative, so I think the manual is wrong.

Alex Chen 2L

ZevMarx-Kahn3C wrote:Are there any examples we should know of that fall outside the 0-14 range?

I don't think there are any specific examples we should know, since I looked some up and they all look complicated. Just know that they are called superacids if the pH is negative (below 0).

Alex Chen 2L

Trichloroacetic acid has the same structure as acetic acid except that the 3 hydrogen atoms bound to the central carbon atom are replaced with chlorine atoms. When either molecule dissociates in water, an anion is formed with one oxygen atom holding a negative formal charge. The difference is that t...

Alex Chen 2L

Compounds that have weaker bonds holding the H atom and a more stable anion form after dissociating in water and giving up the H+ cation are more suitable, or at least stronger bronsted acids because they have a higher percent dissociation in water (more ions produced).

Alex Chen 2L

The coordination sphere is also represented by all the ligands inside the bracket of the molecular formula. For example, in [Co(Nh3)5CL]Cl.2H20, Cl and H20 are not in the coordination sphere.

Alex Chen 2L

The unused p orbital(s) can be used to form a pi bond in the molecule. In the example Lavelle gave in class (the molecule C2H4), the the two central carbon atoms each had sp2 hybridization and one separate p orbital to form the pi bond between each other.

Alex Chen 2L

I think the examples that Lavelle gave in lecture like trigonal bypyramidal for five dsp3 hybridized orbitals and octahedral for six d2sp3 hybridized orbitals aren't strictly tied to that hybridization. I'm pretty sure that See Saw also uses dsp3 hybridization, since the number of hybrid orbitals in...

Alex Chen 2L

Covalent bonds occur when atoms share electrons. In a sigma bond, a combination of s and/or p orbitals from two different atoms each have one unpaired electron, and the orbitals overlap head-to-head to pair the electrons and form the bond. Pi bonds are the same, but it has the p orbitals and the orb...

Alex Chen 2L

The number of hybrid orbitals is simply the number of regions of electron density around the central bonding atom. An sp4 hybrid orbital is not possible since there are only three p orbitals. If there are more than 4 regions of electron density, d orbitals would be used such as sp3d2 for 6 regions o...

Alex Chen 2L

AX2E3 forms a shape with lone pairs/bonding pairs similar to Trigonal Bipyramidal. However, all 3 lone pairs are in the equatorial angles of the shape, and the 2 bonding pairs with the 2 Iodine atoms are on opposite sides of the central Iodine atom on the axial angle. This results in the molecule sh...

Alex Chen 2L

Bonding pairs of atoms and lone pairs repel each other so that they are roughly equally spaced from each other. Based on the number of bonding pairs/lone pairs, the atom forms a unique 3D shape. There are some additional factors, including that lone pairs have higher repulsion than bonding pairs, th...

Alex Chen 2L

The arrangement of the atoms for shapes with only bonding pairs isn't directly based on the maximum bond angles between each atom, but on having equal spacing between each atom. If you visualize the 3d shape made by connecting the outside atoms, the shape it forms has edges that are of equal length ...

Alex Chen 2L

I don't think we have to memorize the bond angles for every shape, because for many shapes VSEPR can only predict the angle qualitatively rather than quantitatively. I do think it's good to memorize the angles with only bonding pairs (such as AX4 and AX5) though, since those will always have the sam...

Alex Chen 2L

VSEPR shapes that have lone pairs of electrons exert more repulsion on the atoms around the central atom, so they will have smaller bond angles. For example, AX2E2 has a smaller bond angle than AX4, since it has 2 lone pairs and 2 bonding pairs and therefore more repulsion when compared to the latte...

Alex Chen 2L

Polarizability increases as the volume occupied by electrons increases. Therefore, in terms of periodic trends, it tends to increase when going down and/or left in the periodic table.

Alex Chen 2L

Another way to think about it is that energy input is required to break interactions/bonds. Therefore, a positive energy input is required to overcome/cancel out the negative potential energy that an interaction has.

Alex Chen 2L

I know that induced dipole-induced dipole forces arise because of fluctuations in the electron cloud of atoms or molecules. But what exactly is represented by the attractive force (-alpha x alpha / r 6 )? If the attractive force is around -2kJ/mol, then is that amount of energy released when the int...

Alex Chen 2L

I understand how size and molar mass contributes to why the strength of interactions in H20 is greater than in H2S, but how does this explain why H20 has a higher melting point and boiling point than H2S? The oxygen atom in H20 is more electronegative than the sulfur atom in H2S, since its atomic r...

Alex Chen 2L

Alpha stands for the polarizability of each atom/molecule. Atoms with more electrons and larger size are more polarizable, and therefore will have a higher force for induced dipole-induced dipole interactions. r stands for the distance between atoms or molecules; note that the attractive force decre...

Alex Chen 2L

Thank you! Will this electron moving happen with all ions, or just chromium? I think the half-full and full -d orbital exceptions are for elements, including Chromium, Copper, and other elements in the same group as them. You can check these using the periodic table found in first pages of the text...

Alex Chen 2L

If a molecule has resonance structures, then we know that the compound is more stable than expected, since the real structure is an average of the resonance structures, resulting in shorter bond length and therefore stronger bonds.

Alex Chen 2L

Yes. For example, Chromium has the electron configuration [Ar]3d 5 4s 1 instead of the expected configuration [Ar]3d 4 4s 2 . This fact came mainly from experimental observation so it's not entirely known why it is more stable, but since the 3d orbital is at a lower energy level than the 4s orbital,...

Alex Chen 2L

Depends on what kind of exception. Elements in period 3 and above such as phosphorus, sulfur, and chlorine can hold more than 8 valence electrons due to having a d-orbital. This leads to molecules such as PCl 5 where phosphorus holds 10 valence electrons. On the other hand, elements such as boron an...

Alex Chen 2L

Yes, knowing the half full d5 and full d10 subshell exceptions for electron configuration is probably a good idea.

Alex Chen 2L

4s is filled before 3d when writing electron configurations for elements on the periodic table. However, 3d is at a lower energy level than 4s, so for example when forming cations you would remove electrons from 4s before the ones in 3d.

Alex Chen 2L

Is there a particular order the dots are written for each atom? Like, if I had an atom with 6 valence electrons, is there a specific way I must draw the electrons around the nucleus? Place the electrons on each side to individually first so that they are unpaired (for example Carbon has one dot for...

Alex Chen 2L

Period 2 non-metals (C, N, O, F) always will end up with an octet of valence electrons, since the -s and -p orbitals combined hold up to 8. Elements in period 3 and higher, such as Phosphorus, Sulfur and Chlorine, can have exceptions because they have access to the the -d orbitals that can hold addi...

Alex Chen 2L

Cations are ions with a positive charge, meaning that an element needs to have its electrons ionized (removed) to form one. Non-metals having a high ionization energy make them less favorable to have electrons removed and therefore less likely to form cations.

Alex Chen 2L

Yes, your answers are equivalent. Typically it's good practice to write your answers in scientific notation for larger exponents so that the number of sig figs is consistent, but in this case your answer actually has the same number of sig figs so it should be fine.

Alex Chen 2L

Electrons, as individual particles, spin and create a small magnetic field by doing so. An electron spinning clockwise is called "spin-up", while an electron spinning counterclockwise is called "spin-down". The two opposite spins also create oppositely charged magnetic fields.

Alex Chen 2L

I know that every orbital (represented by the first three quantum numbers) can support up to 2 electrons. However, s-, p-, d-, and f- orbitals have different shapes, so why is the number electrons for each orbital consistently two?

Alex Chen 2L

I believe you have to use the mass of an electron, not the mass of an hydrogen atom, because you're calculating the uncertainty in the speed of the electron itself. Also, the value for positional uncertainty that you found should be multiplied by two since you have to add the possibilities that the ...

Alex Chen 2L

I believe that diagram is simply a visual representation of electron configuration and of which orbitals are filled up given the number of electrons in an atom. Yes, he was filling up the orbitals as he went across the periodic table utilizing Hund's rule and the Aufbau principle. This is correct. ...

Alex Chen 2L

In the same way as with other waves or particles with wave-like properties, where constructive interference increases amplitudes of waves that are in phase, and destructive interference decreases amplitudes of waves that are out of phase. The fact that electrons have observable constructive and dest...

Alex Chen 2L

That's correct. The work function describes the threshold of energy required of photons hitting the metal to eject an electron, since each photon interacts with a single electron.

Alex Chen 2L

The line wavelength is your starting point. Using that, you should be able to find the frequency of that line of light, so you can use the equation E=hv to find the energy that the electron emitted as a photon. Furthermore, remember that because UV light has the highest energy of light, the electron...

Alex Chen 2L

I'm going to use light as the example we went over in lecture. Light has wavelike properties in that it oscillates and has a wavelength, frequency, and amplitude. However, it also has particle -like properties since the photoelectric effect demonstrates that light carries energy in discrete amounts ...

Alex Chen 2L

In terms of AMUs (atomic mass units), neutrons and protons both are 1 AMU in mass, and electrons are much smaller at ~0.00054 AMU. However, since you often need the mass in SI units to use in calculations, either the question or the equations sheet should provide the mass if it is necessary.

Alex Chen 2L

PranaviKolla3G wrote:How do we know how many sig figs our final answers should have?

The final answer should have the same number of sig figs as the number with the least number of sig figs used in calculations. In other words, the final number cannot be more precise than the least precise number used to obtain it.

Search found 86 matches