CHEMISTRY COMMUNITY

Faaizah Arshad 1H

The purpose of a pseudo first order reaction is to help you analyze the rate law of a reaction that depends on the concentration of more than one reactant species. The way it works is that you increase the concentration of one of the reactant species to be so high that any changes in its concentrati...

Faaizah Arshad 1H

Catalysts are not consumed during the course of a reaction. They are used to lower the activation energy so that more molecules, at the same temperature, have the minimum required energy to reach the transition state. Catalysts speed up the rate of the forward and reverse reaction, so they do not af...

Faaizah Arshad 1H

When you are calculating the heat it takes to change a substance from one phase to another phase, you need to separate it into different steps because of differences in specific heat capacity and due to the presence of phase changes. For example, if you are heating solid ice from -5 C to + 150 C, yo...

Faaizah Arshad 1H

Delta G is the maximum amount of work that a cell can perform. At standard conditions, we can use the equation delta G(naught) = -n F E(naught) to see that when E naught is a positive value, then delta G naught is a negative value and the redox reaction is spontaneous so a galvanic cell is used. On ...

Faaizah Arshad 1H

Pt stands for platinum. It should be included in the cell diagram when either or both of sides of the galvanic cell do not have a solid conducting metal as an electrode. Since platinum is inert, it would not be involved in the oxidation and reduction reactions. Some exceptions to this are that Hg (l...

Faaizah Arshad 1H

The standard EMF is the maximum potential difference between two electrodes and is denoted as E naught.

Faaizah Arshad 1H

For this problem, I would suggest drawing a reaction profile or visualizing one. First, the problem states that delta H = + 24kJ. This means the reaction is endothermic and would be represented as the distance from the energy of the reactants to the energy of the products on a reaction profile. Next...

Faaizah Arshad 1H

Thank you Dr. Lavelle!! I will never forget the amazing lessons I learned in your class.

Faaizah Arshad 1H

Sapling is a good way to get extra practice and can be helpful if you are struggling on particular kinds of problems since the solutions explains how to do problems step-by-step.

Faaizah Arshad 1H

Yes that is correct. If you increase the rate of the forward reaction, then the equilibrium constant K will increase. If you increase the rate of the reverse reaction, the equilibrium constant K will decrease.

Faaizah Arshad 1H

Oxidation is the loss of electrons. It occurs at the anode of a galvanic cell.
Reduction is the gain of electrons. It occurs at the cathode of a galvanic cell.

Faaizah Arshad 1H

e is wrong because k vs temperature is not linear. ln K vs 1/T is linear with a -Ea/R slope. g is linear because in the half life equation for a zero order reaction, [A] is in the numerator and is directly proportional to half life. h is not linear because in the half life equation for second order,...

Faaizah Arshad 1H

It is asking for the half life in hours.
Use t = 0.693/k to find the half life in seconds and then divide by 3600 seconds to convert to hours.

Faaizah Arshad 1H

Adding heat to a system increases the number of molecules with the minimum energy to overcome the energy barrier. So, raising temperature will increase the rate constant because more molecules are colliding with enough energy to make the reaction occur. Reactions with higher activation energy depend...

Faaizah Arshad 1H

Delta S surroundings is zero in an isothermal irreversible process (free expansion) because w = 0 since the system is not pushing against an external pressure. Also, since it's isothermal, delta U is 0. Since delta U = q + w, q must also be 0.

Faaizah Arshad 1H

Thank you so much for a wonderful quarter, Dr. Lavelle! I truly enjoyed being a student in your class. Your passion and dedication for teaching is evident through the numerous resources you have made available to us (UA sessions, TA office hours, Sapling, your discussion sections, etc). Thank you fo...

Faaizah Arshad 1H

Does temperature affect the frequency factor A? If so, how?

Faaizah Arshad 1H

The Nernst equation is key in telling us how concentration affects cell potential. When you increase the concentration of reactants, the Q term decreases. Taking the ln of value below 1 is negative. This means that you are adding E naught to a positive value, so the E cell potential will be more pos...

Faaizah Arshad 1H

Yes, it's a good idea to look at textbook questions that are listed in the syllabus outlines. These questions may appear on the final exam, as they have appeared on the midterms.

Faaizah Arshad 1H

The order does not matter when you are using commas. Just be sure to only use a comma if the phases are the same.
You use a Platinum electrode if you do not have a solid electrode for either a cathode or anode. The only exception to this is that Hg (l) can serve as an electrode.

Faaizah Arshad 1H

You can calculate k if you are given other values for the equation. For example, if you are given a data table with concentrations of reactants across different experiments and the initial rates for those experiments, then you can determine the order for each reactant, write a rate law equation, and...

Faaizah Arshad 1H

Yes, concentration is usually denoted in brackets. The units are typically moles/Liters or Molarity. In kinetics, the units are sometimes mmol/L also.

Faaizah Arshad 1H

I think you meant to write 2HD (g)--> H2 (g)+ D2 (g) When you increase the pressure of the system, you expect the reaction to favor the side with the less moles of gas to relieve the pressure (Le Chatelier's principle). However, in this case, there are 2 moles of gas in the reactants and 2 moles of ...

Faaizah Arshad 1H

The same molecule can be an oxidizing and reducing agent if you have two of those molecules as reactants. So, even though the molecule is identical, one is the oxidizing agent and the other is the reducing agent.

Faaizah Arshad 1H

Yes, using the delta G (naught) = -nFE(naught) equation, if standard E naught is positive, then delta G naught is negative, so the reaction is spontaneous.

Faaizah Arshad 1H

Add OH- and H20 in basic solution.
Add H+ and H20 in acidic solution.
If you're not told which solution you are in, assume acidic.

Faaizah Arshad 1H

The value of n in this equation is the number of moles of electrons that are exchanged in the oxidation and reduction reactions, so be sure to balance the redox reactions before solving for this.

Faaizah Arshad 1H

The E naught value is 0 if the solution of cathode and anode is at equilibrium.

Faaizah Arshad 1H

I believe it depends on which cell you are talking about. For galvanic cells, the anode is negatively charged, but for an electrolytic cell, the anode is positively charged.

Faaizah Arshad 1H

When a system does work (like pushing a piston to expand the volume), it is losing energy, so the value of work is negative.

When a system has work done on it (like compression), it is gaining energy, so the value of work is positive.

Faaizah Arshad 1H

You are correct, delta n is the number of moles of gas in the products minus the number of moles of gas in the reactants. You do not consider solids, liquids, or aqueous.
Also, delta n can be positive or negative.

Faaizah Arshad 1H

A molecule is thermodynamically unstable if its delta G is positive.
A molecule is thermodynamically stable if its delta G is negative.

Faaizah Arshad 1H

When you multiple L and atm, you can use the conversion factor that 101.325 J = 1 L-atm to convert to Joules.

Faaizah Arshad 1H

Dr. Lavelle did post a lecture yesterday. We had a Wednesday and Friday lecture this week.

Faaizah Arshad 1H

Can Delta S of the universe be negative? I thought that one of the laws of thermodynamics said that entropy of the universe is always increasing and that delta S total is always greater than or equal to 0. However, at one of the UA sessions, we were told that delta S of the universe is negative when...

Faaizah Arshad 1H

Ryan Hoang 1D wrote:I believe so, since the Se is the central atom, and all the other Os must revolve around it, theres no room for positional entropy. However, if I’m wrong, it’d be nice for someone to correct me!

What about the entropy? Would the entropy of SeO3 be higher because it has more atoms?

Faaizah Arshad 1H

What about the entropy? Would the entropy of SeO3 be higher because it has more atoms?

Faaizah Arshad 1H

Would SeO2 and SeO3 both have a W (degeneracy) value of 1 since the atoms around the Selenium are the same, so there would only be one microstate? If not, what would the correct degeneracy be?

Faaizah Arshad 1H

Once you have the number of moles, use the delta S = nC ln T2/T1 equation to find the change in entropy.

Faaizah Arshad 1H

Here is how I solved the problem: To find delta S of the system, use the standard entropy values that are given in the table. We know from the constants and equations sheet, delta S = q / T. At constant pressure, q = delta H of the system. The value of delta H will be equal and opposite for the surr...

Faaizah Arshad 1H

This one was tricky. I had to read the question carefully to predict. For delta G: We know if delta G is negative, it indicates that a reaction is spontaneous. In the given description, we are told that water vapor "readily" adsorbs onto the surface of the silica gel. This implies that the...

Faaizah Arshad 1H

You are very close. For this question, you are correct to use q = mCdeltaT to find the final temperature of the solution. Then use delta S = n C ln T2/T1 for each of the water solutions. However, make sure to use 75.3 J/K mol as the molar heat capacity of water liquid. You cannot use 4R because that...

Faaizah Arshad 1H

Here is how I solved question 7 for the the delta H of vaporization: 1. Use the given information in the problem to find kJ. Start by converting the given value of W into J/s (1 W = 1 J/s). Then multiply that by that by 60 sec/1min. Then multiply by the given minute value, so that you have the Joule...

Faaizah Arshad 1H

For an endothermic reaction, adding heat will shift the reaction to the right, causing more products to be formed, so K will increase. For an exothermic reaction, adding heat will shift the reaction to the left, so more reactants will be formed, and K will decrease.

Faaizah Arshad 1H

A negative sign in front of work means that the system is losing energy in the form of work because it is expanding.

Faaizah Arshad 1H

I solved the problem the same way that you did and got the correct answer, so I think the method works. However it seems that the book uses Cv and Cp because that is what is discussed in 4C with atoms, linear molecules, and nonlinear molecules.

Faaizah Arshad 1H

I'm not sure about part b, but for part a, we can say that delta U is equal to zero because we are told that the expansion is isothermal, which means that temperature remains constant. For an ideal gas in an isothermal process, internal energy is equal to 0.

Faaizah Arshad 1H

Entropy measures the randomness of molecules in a system, and has units J/K.
Enthalpy measures the energy and total heat in a system, and has units kJ/mol usually.

Faaizah Arshad 1H

Entropy is a measure of disorder in a system, while enthalpy is the measure of total heat in a system. Entropy is denoted by S, but enthalpy is denoted by H. Entropy is normally in J/K, while enthalpy is in kJ/mol.

Faaizah Arshad 1H

In part a, you did q(copper) + q(water) to find the total heat. For part b, you should use the value that you got for q when you do mCdeltaT of the water. Divide that by the total amount of heat that you calculated in part a. Multiply by 100%.

Faaizah Arshad 1H

This is a really interesting question! I searched the internet and learned that methanol and camphor can also undergo sublimation.

Faaizah Arshad 1H

By definition, delta U is the change in internal energy, while delta H is the enthalpy change. If there is no work being done, then delta U would be equal to delta H.

Faaizah Arshad 1H

q is the heat that is transferred to or from a system, and it is not a state function. However, H is the total heat of the system at constant pressure and it is a state function.

Faaizah Arshad 1H

You can assume that x is insignificant if the value that you receive for percent ionization is less than 5%. This would mean that your original substance barely dissociated, so the number that you would be subtracting from the initial is negligible.

Faaizah Arshad 1H

That's a good question. From what I've heard, the Math 31,32,33 classes typically go over calculus concepts. However, I am unsure whether there are other calculus classes at UCLA that would better complement the math we use in chemistry.

Faaizah Arshad 1H

The difference between the two equations is that in the first one, you use mass in grams, but in the second one you use number of moles. The equation you choose will depend on what the problem is asking.

Faaizah Arshad 1H

I believe we started talking about q and heat in Wednesday's lecture this week, but we will most likely do more calculations in Week 5 lectures.

Faaizah Arshad 1H

For strong polyprotic acids, you should consider both the first and second dissociation. But the first dissociation is assumed to go to completion (100%), so you only need the second Ka. For weak polyprotic acids, you only need to consider the second dissociation Ka2.

Faaizah Arshad 1H

Is this correct? : If a reaction contains a solid or a liquid substance, then increasing or decreasing the concentration of the solid or liquid will not affect the reaction. Le Chatelier's principle does not apply to solids and liquids? So, the equilibrium concentrations will not change, and the val...

Faaizah Arshad 1H

I think that whether you get burned by steam or not can depend on how fast the steam is coming out. For example, steam from pressure cookers ejects really fast, and the speed at which those gas water molecules hit your skin could affect whether you are burned. On the other hand, face steamers often ...

Faaizah Arshad 1H

H enthalpy is just the internal energy of a specific reactant or product. When you add delta in front of a symbol, it means "change." Delta H means the change in internal energy of a system (final - initial), or the enthalpy of products minus the enthalpy of reactants. Delta H is equal to ...

Faaizah Arshad 1H

Breaking a bond requires energy. Forming a bond releases energy. This is due to the fact that bonds form to allow atoms to be in the lowest energy (stable) position. Breaking a bond would cause two previously bound atoms to now be separate, and this may be an unstable position for the atoms. So, bre...

Faaizah Arshad 1H

If there is only one arrow that points in one direction, it means that the reaction will strongly proceed in that direction, but it does not mean that the reverse won't happen. For example, in a different lecture Professor Lavelle drew a very long arrow going from the reactants to products, and an e...

Faaizah Arshad 1H

For enthalpy of combustion, it helps me to remember that a combustion reaction has O2 as a reactant. The products are CO2, H2O, and sometimes N2. If your reaction has those reactants and products, then you are likely looking at a combustion reaction. When you are finding the total enthalpy of a comb...

Faaizah Arshad 1H

The standard state of an element is its phase at 25 degrees celsius and 1 atm or 1M.

Faaizah Arshad 1H

Steam causes severe burns because of the large negative enthalpy of condensation.

Faaizah Arshad 1H

One way to compare Q and K is that if Q>K then you have more products in the reaction that the amount of products there should be at equilibrium. Therefore, to reach equilibrium, the reaction will shift left. If Q <K then there are more reactants present than the amount of reactants that there shoul...

Faaizah Arshad 1H

When I use the ice table, I normally start with concentrations because I can directly calculate the equilibrium concentrations and plug them into the K expression to solve a problem. However, if you use moles, then you have to make sure to convert to concentrations later on, which you could possibly...

Faaizah Arshad 1H

In addition to what others have said above, you can also compare pH and pKa or pOH and pKb to determine if the dominant species in a reaction will be charged or neutral. We learned this towards the end of Chem 14A, and there are sapling questions on it as well, so it might be helpful to think of wha...

Faaizah Arshad 1H

You can find your sapling grade on the sapling website, by clicking gradebook. You can then verify if this grade is correctly entered into myUCLA by logging into your myUCLA and checking "exam and homework grades" for chem 14b.

Faaizah Arshad 1H

When you reverse a reaction, you should take the inverse of K, which means computing 1/K.

Faaizah Arshad 1H

You should include water in your equilibrium concentration expression if water is a product or if it is a reactant in the reaction and there is a solvent that is not water. In this case, water is not in excess and its concentration will not cancel on both sides. Therefore, it does affect the K value...

Faaizah Arshad 1H

Tables 6C.1 and 6C.2 include the Ka and Kb values for several chemical formulas. These can be found on page 461 and 462 of the textbook.

Faaizah Arshad 1H

Sometimes, I'll get a notification on the top right if someone has responded to my post. Otherwise, I'll just repeatedly check my account to see the responses.

Faaizah Arshad 1H

The answer key has a mistake; it should include a negative sign. It makes sense for the answer for this to be negative because when the [H30+] > 1 M, then the pH will be below 0. You are correct!

Faaizah Arshad 1H

Super acids are when the concentration of hydronium are above 1M and the concentration of hydroxide is smaller than 10^-14 M. Super bases are when the concentration of hydroxide is above 1M, so the concentration of hydronium is below 10^-14 M.

Faaizah Arshad 1H

An exothermic reaction is one in which heat is produced. If you think of "heat" as a product, then adding heat to an exothermic reaction would cause there to be more products than reactants so the reaction would shift to the left to produce more reactants.

Faaizah Arshad 1H

Volume doesn't change when adding an inert gas because you are not changing the amount of physical space there is of the container. You are simply adding gas that is unreactive to the container. There are more gas molecules that will collide (so the pressure has increased) but the space of the conta...

Faaizah Arshad 1H

If you add more reactant to a reaction that is already equilibrium, and you therefore perturb the system away from equilibrium, then according to LeChatelier's principle, the reaction will respond by producing products to minimizing the change and help return to the equilibrium state.

Faaizah Arshad 1H

For this question, the y-axis is shown in kPa. On Dr. Lavelle's constants and equations sheet, there is a conversion that says 101.3 kPa = 1 atm. Very similarly, 100 kPa = 1 bar. The textbook uses the second conversion to change kPa into bar.

Faaizah Arshad 1H

It's best to not have fractions as coefficients in your balanced chemical reaction. To turn the fractions into whole numbers, you can multiple the entire reaction by a common denominator.

Faaizah Arshad 1H

Changing pressure or volume does not change K. They can change the concentration of the substances in the reaction, but by the leChatelier's principle, the reaction will shift to minimize that change and return to the equilibrium concentrations of reactants and products. Changing temperature does ch...

Faaizah Arshad 1H

What are the bond angles for T shaped?

Faaizah Arshad 1H

Hi, are compounds containing silicon amphoteric (like SiO2)?
It seems like there wasn't a clear list of amphoteric compounds given, so I'm not sure which metalloids to identify as amphoteric.

Thanks

Faaizah Arshad 1H

Yes the ligand must follow the lone pair spacer spacer lone pair model in order to be a chelating ligand because that is the only formation that allows the ligand to "wrap around" a transitional metal cation

Faaizah Arshad 1H

You are correct that the second Chlorine is outside the brackets so it is not a ligand and is not part of the coordination sphere. However, you must take it into account when you are determining the oxidation number of the transition metal because the oxidation number of the transition metal will de...

Faaizah Arshad 1H

At least in the case of p orbitals, the px, py, and pz axes are 90 degrees from each other so that is why the orbitals will be at 90 degrees.

Faaizah Arshad 1H

That's a great mneumonic,. Heads up that HClO3 is also a strong acid; you left that out in the list.

Faaizah Arshad 1H

I'm not sure if you are asking in regards to a particular coordination compound (if you are, it would be helpful if you could write the entire formula), but in ethylenediamine, there are 2 NH2 and they are separated by two spacers so the entire molecule is bidentate because each N has a lone pair th...

Faaizah Arshad 1H

Why is formic acid stronger than acetic acid?
The textbook answer key says this: The Ch3 group in acetic acid has electron donating properties which means that it is less electron withdrawing than the H attached to the carbon in formic acid.

I'm struggling to understand this explanation. Thanks!

Faaizah Arshad 1H

Here's a list of the strong acids: HCl, HNO3, HI, HBr, H2SO4, HClO3, HClO4
Strong bases usually come from Group 1 and Group 2 hydroxides, like LiOH or Ba(OH)2

Faaizah Arshad 1H

Here is the link to the Final Review sessions: https://lavelle.chem.ucla.edu/wp-conten ... ns_14A.pdf

You can also locate it on the right of Professor Lavelle's Chem 14A website.

Faaizah Arshad 1H

A monodentate ligand is a lewis base that has one lone pair that it can donate, such as NH3. A bidentate or polydentate must follow the "lone pair, spacer, spacer, lone pair" rule. An example of a bidentate ligand is NH2CH2CH2NH2 because the two nitrogens have lone pairs that they can dona...

Faaizah Arshad 1H

Since shape can affect polarity and polarity can determine which intermolecular interactions a molecule can engage in, we can use shape and polar/nonpolar properties to predict IMFs. For example, in the case of water, since the molecular geometry is bent, the dipoles do not cancel, so water can exhi...

Faaizah Arshad 1H

The textbook puts 2 as the subscript for Zn(OH)2 and Zn(NO2)2. You can determine this by asking yourself what the ground state electron configuration of Zn. Since Zn has an e- configuration of [Ar]3d10 4s2, it will lose the two 4s electrons to reach a stable state. This means that Zn will become Zn2...

Faaizah Arshad 1H

In the textbook, anionic ligands were named as fluorido, chlorido, or bromido with the -ido at the end. However, in lecture and in the textbook answer key, the ide is completely dropped and replaced with o like chloro. When do you use -ido and when do you use -o?

Thanks!

Faaizah Arshad 1H

Yes, the ligands are in alphabetical order here. In the chemical formula, aqua came before oxalato.

Faaizah Arshad 1H

Yes, double bonds count as only one region of electron density.

Faaizah Arshad 1H

A delocalized pi bond is present in a molecule that has double bonds and also has resonance. For example, CO3 2- would have delocalized pi bonds because the multiple bond character is spread over three oxygen atoms.

Faaizah Arshad 1H

Dr. Lavelle briefly mentioned that hybrid orbitals are made from linear combinations, but he did not go into the details during lecture. The assigned homework problems do not discuss this either, so I would assume that we do not need to know it.

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