CHEMISTRY COMMUNITY

Hasmik Dis 2F

I think it's application-based. You can always ask Lavelle you're interested in being one :)

Hasmik Dis 2F

I was wondering the same thing. I loved 14A and 14B because of chemistry community and all the interactive study session!

Hasmik Dis 2F

I was expecting worse. However, it wasn't that bad. There were a few harder questions that required deep thinking.

Hasmik Dis 2F

Thanks everyone:))

Hasmik Dis 2F

For the lecture example about partial pressures:
How come we don't also include the Cl2, when calculating the gibbs free energy of the reaction?

Hasmik Dis 2F

How do you know that the changes in carbon, that's where I'm stuck. Is it something you just know/memorize? The changes in carbon are determined by the oxidation numbers. So for example, we have to see how C's oxidation number changes in C2H5OH --> C2H4O. This is determined by the other elements in...

Hasmik Dis 2F

Can someone show me what they did for Self Test 6K.1A?
I keep getting the wrong answer.

Self-test 6K.1A Copper reacts with dilute nitric acid to form copper(II) nitrate and the gas nitric oxide, NO. Write the balanced net ionic equation for the reaction.

Hasmik Dis 2F

In his lecture, Lavelle was saying how the initial rates are the maximum rates. Can anyone explain what he means by this and why is it relevant?

Hasmik Dis 2F

To add on , this essentially applies to La Chatelier's principle. Whenever you add something, the system will favor the direction that will undo the effect. For example, in his lecture, Lavelle mentioned how in a reaction of 2Al (s) + 3Mn 2+ (aq) = 2Al 3+ (aq) +3Mn(s), increasing [Mn 2+ ] where > 1....

Hasmik Dis 2F

I reccomend watching Tyler Dewitt's videos. He is super helpful for chemistry. Here's a link. to one of his videos for balancing redox reactions if you want to check him out. You can also see his other videos too

https://www.youtube.com/watch?v=v5sDNmYCaqo&t=12s

Hasmik Dis 2F

This is because if we were to add more water, it wouldn't make a big difference since there's already a huge amount. We can thus exploit this property to our advantage.

Hasmik Dis 2F

I really like the acryonym LEO the lion as a way to remember loss of e- equals oxidation. This is great for when to recognize which half reactions are oxidizing or reducing.
Also, it is really helpful to memorize the oxidation rules and numbers

Hasmik Dis 2F

Yes. The nerst equation "quantatively determines the direction of electron transfer under non-standard conditions." Non-standard being non equilibrium. So, if we were to use the nerst equation, which connects the concept of Gibbs Free energy with the reaction quotient q, we see that delta ...

Hasmik Dis 2F

I'm planning on doing premed since working in the medical field has always been a goal of mine. I've been watching a lot of podcasts and watch youtube videos about the premed life and learned to prepare myself for the difficult years to come

Hasmik Dis 2F

I think this should be memorized, it will save you a lot of trouble. Usually H-X acids are strong, such as HCl, while X-OH bases are strong, such as NaOH.

Hasmik Dis 2F

I think if H+ were to be a product then that would equate an acidic solution, which translates to a lower pH. This is because an acidic solution essentially means having more H+ in solution, so if it were to be a product, it would make sense for it to have a lower pH.

Hasmik Dis 2F

I believe that species that lie higher than H2, based on the standard reaction potentials are stronger reducing agents, whereas the lower ones is stronger oxidizing agents.

Hasmik Dis 2F

Hi. Just like the above answer says, all you really do is treat it like a normal equilibrium problem. Think of the pressure as a "partial pressure" or a concentration. Given this, all you do is plug it into the quotient reaction.

Hasmik Dis 2F

It's not that it is always equal, but rather when it is equal, that means no energy is being transferred since the maximum potential difference is reached.

Hasmik Dis 2F

The electrochemical series is when the electrodes with their ions are arranged based on their standard reaction potentials. The smaller the atomic number, the stronger oxidizing power.

Hasmik Dis 2F

Do we have to memorize the oxidation numbers of elements? For example, in sapling week 7/8 question 8, I was wondering would we have to know the oxidation number for Cl2 or K?

Hasmik Dis 2F

Why is it that a higher atomic number means higher molary entropy?

Hasmik Dis 2F

Yes. Delta U has to be 0 because an isothermal means constant temperature.

Hasmik Dis 2F

(a) Calculate the heat that must be supplied to a copper kettle of mass 400.0 g containing 300.0 g of water to raise its temperature from 20.0 °C to the boiling point of water, 100.0 degrees C?

How come we add the heat capacities of both the kettle and water to get our final answer?

Hasmik Dis 2F

Potassium perchlorate, KClO4, is used as an oxidizer in fireworks. Calculate the heat required to raise the temperature of 10.0 g of KClO4 from 25°C to an ignition temperature of 900°C. The specific heat capacity of KClO 4 is 0 .8111 J⋅K−1⋅g − 1. So in this problem, I know that we have to do the hea...

Hasmik Dis 2F

So the book said that when there are no other changes, then delta U = w (work). However, the next section also proceeds to say that when energy is transferred through heat (and no other source) then delta U = q. What I don't quite understand: Is q considered our "w" in this case? Or is it ...

Hasmik Dis 2F

I think it does because internal energy can only be changed with work. This essentially with effect ∆U.

Hasmik Dis 2F

Yes! A zero q means no transfer of heat, while positive values for heat mean absorption (just like endothermic reactions)

Hasmik Dis 2F

So the first equation is used for calculating work when pressure is not constant[b], while the other equation is used to calculate work when pressure is [b]constant

Hasmik Dis 2F

In lecture we said how it is almost impossible to have an isolated system. Why is that so? Also, if isolated systems are so hard to obtain, how do we study them and its behaviors? Why are isolated systems important?

Hasmik Dis 2F

Have you checked the solutions manual for errors?

Hasmik Dis 2F

Isobaric is basically a process in which pressure remains constant. How may this happen? Usually when we allow volume to expand/contract in a way to balance pressure changes. This can be used for calculating w, as said in the previous answer.

Hasmik Dis 2F

An isolated system would no longer be labeled as an isolated system if we witness energy changes. It would then be either a closed/open system based on insulation (and other factors). In order for an isolated system to be considered isolated, no energy changes must occur.

Hasmik Dis 2F

The main difference between a closed system and an isolated system is that a closed system can exchange energy with its surroundings, while an isolated system cannot exchange anything with its surroundings. An example of a closed system would be a sealed beaker with no insulation. Since there's no i...

Hasmik Dis 2F

Combustion can actually apply to anything if only it adheres to its characteristics. In order for it to be a combustion reaction, it must have an oxidant (usually oxygen), fuel , and heat. So, while it is mostly going to be the burning of hydrocarbons, it can also be other compounds as well.

Hasmik Dis 2F

Yes^ What Ashley said it absolutely correct. The main thing to remember is that bond enthalpies are the least accurate method because bond enthalpies are averages of many other bonds. This also relates to the concept of resonance that we learned in chem 14A, if you'd like to refer back to that as we...

Hasmik Dis 2F

Basically enthalpy is used to measure the change in heat under constant pressure. What makes enthalpy itself so important is the fact that is it a state property, meaning that it is not dependent on the path taken to obtain that state. This allows us to solely measure the change in heat, despite how...

Hasmik Dis 2F

Steam has more energy than water in its liquid state. This is because it has the additional heat obtained from the vaporization of water. In the heating curve, this is demonstrated by a steep slope going up to water in the vapor phase. When steam gets on the skin, it begins to condense and gives off...

Hasmik Dis 2F

I believe bond enthalpies are less accurate because it is an average value. I think it has something to do with resonance also: the bonds are just averages of possible electron locations.

Formation enthalpies are more technical and accurate.

Hasmik Dis 2F

I came across a problem: figuring out bond enthalpies from standard enthalpies of formation. How would you tackle this problem?

Hasmik Dis 2F

Can anyone please give me a rundown of water's heating curve that Lavelle spoke about in his lecture? I keep trying to grasp the concept but I don't understand it.

Hasmik Dis 2F

I like to think of endothermic as having heat as a reactant. If heat is a reactant, just treat it like a normal la chatelier's concept. If we raise the temperature, we're adding heat. What does that mean? Adding a reactant. Adding a reactant means to favor products.

Hasmik Dis 2F

A careless laboratory technician wants to prepare 200.0 mL of a 0.025 M HCl(aq) solution but uses a volumetric flask of volume 250.0 mL by mistake. (a) What would the pH of the desired solution have been? (b) What will be the actual pH of the solution as prepared? I checked a past chemistry communit...

Hasmik Dis 2F

The value of K w for water at body temperature (37 °C) is 2 .1 × 10^-14 . (a) What is the molar concentration of H3O + ions at 37 °C? (b) What is the molar concentration of OH − in neutral water at 37 °C? So for this problem, I know that Kw = [OH-][H30+]. At first, I thought of doing x^2 = Kw (which...

Hasmik Dis 2F

Does anyone know why when creating ICE charts, and we have a decrease of "-2x" we still have to include the "2" as an exponent when writing out the constant equation? For example, if my product is -2x, and whenever it's time to calculate either K or for X, we would then do produc...

Hasmik Dis 2F

The exponents mathematically change the answer completely. Also, it is important to realize that those exponents also represent the ratios of which each substance exists in the reaction. Account for the different ratios will also change your answer than merely dividing out the substances.

Hasmik Dis 2F

Condensation is the process of turning water vapor into liquid. This of course requires a loss of heat. Therefore, whenever you lose heat, a reaction is exothermic. Think of the prefix "exo" as meaning "exit."

Hasmik Dis 2F

I believe you also have to consider the fact that phase changes represent no temperature change.

Hasmik Dis 2F

Enthalpy basically measures the heat flow at a constant pressure. Since heat is not a state property at constant pressure, we can't really use it to reveal heat flow in a system. Therefore, enthalpy is an energy quantity that measures the amount of heat absorbed/released (which is what I meant when ...

Hasmik Dis 2F

The above answers are all correct. I got confused over this problem but the one thing I forgot is that when it asks for the total pressure, you have to add up all the partial pressures. So, don't forget that last step and then from there, you're all set! It's so cool seeing students have the same ex...

Hasmik Dis 2F

I always use cross multiplying and for me it's kinda easier. It's funny because I never use the quadratic equation to solve these problems. I just cross multiply, then plug it into the "plysmlt" function on my calculator!

Hasmik Dis 2F

Temperature is the only thing that can change K. When we speak of endothermic, we put heat as a reactant. Therefore, if we increase the temperature, we're adding more reactants, so the reaction will favor products. I think this Is what increases K. If it's exothermic, heat is a product, so increasin...

Hasmik Dis 2F

If we added more reactants, then the biggest change would be the direction of the reaction. In this case, since we added more products, Le Chatelier's principle says that our reaction will favor reactants. On the other hand, more reactants obviously mean reaction will favor products. So, instead of ...

Hasmik Dis 2F

I think with Le Chatelier's principle, if you spend some time really studying and understanding the concept, you don't even have to memorize it. You already intuitively will know what effects the changes have by drawing off the knowledge that embedded in your mind. For this topic, I tried memorizing...

Hasmik Dis 2F

For this problem, you should really utilize the plot. The plot shows you the ratios at which the substances are created and decomposed.

Hasmik Dis 2F

Hi! I would love if anyone could help me with this problem. 5I.13. (a) In an experiment, 2.0 mmol Cl 2 (g ) Cl 2 (g ) was sealed into a reaction vessel of volume 2.0 L and heated to 1000. K to study its dissociation into Cl atoms. Use the information in Table 5G.2 to calculate the equilibrium compos...

Hasmik Dis 2F

Did you figure out how to do part (a) of that problem? Because I couldn't figure it out

Hasmik Dis 2F

I believe that the pressure will increase because adding an inert gas means that the inert gas' partial pressure will also contribute to the overall pressure. The volume however stays constant because it does not react with anything.

Hasmik Dis 2F

Did anyone else get K = 3.91 as the answer while the book says Kc= 3.88? I know my answer is close, but I'm not sure why it's not the precise answer. Any thoughts?

Hasmik Dis 2F

So for this textbook problem (Focus 5I question 5I.17), when we get to the point of solving for x after completing the ICE table, I found out that we assume x in ".114-x" is so small and so we should just disregard it (hence the reason I crossed it out in red). But, I don't understand how ...

Hasmik Dis 2F

Table 5G.2 has two constants, K and Kc. What's the difference between the two and how would I know which one I should use? For example, for problem 5I.13 in the textbook, I don't know which constant I should use.

Hasmik Dis 2F

So in his third lecture, Dr. Lavelle mentioned how adding an inert gas won't change the volume because it does not react with anything. That I understand. What I'm kinda confused about is how it is possible for a change in pressure to have zero effect on volume? I thought there was an inverse relati...

Hasmik Dis 2F

So he did say that he's going to go over this when we get to thermodynamics, but to sort of give a brief answer: exothermic basically means that the reaction is going to release heat ("ex" heat "exits"), while endothermic means that the reaction will absorb heat. So that means: E...

Hasmik Dis 2F

So I was wondering: are there specific rules as to when we can use PV=nrt? I know that if you have pressure and temperature you can, but is there a certain condition that has to be fulfilled for us to use it or can we just use it whenever we have pressure and temperature? Also, what's the connection...

Hasmik Dis 2F

Which axis is the nodal plane for d-orbitals located?

Hasmik Dis 2F

Which of the following species are radicals? (a) NO 2; (b) CH3 ; (c) OH; (d) CH2O. Basically, I did the lewis structures and noticed that (c) and (d) are radicals since CH3 and OH have 7 valence electrons. However, when I checked the textbook answers, it said only a and b are radicals. I thought it ...

Hasmik Dis 2F

Basically changing the pH will alter the shape of receptors/activation sites. When enzymes are no longer able to fit into their activation sites (substrats) then they can no longer do what their function is. This is the denaturing of an enzyme- blocking its ability to function by changing shape.

Hasmik Dis 2F

So what's the difference between hydroxide bases and oxide bases? I'm not even sure if my question makes sense haha I'm just really confused about the concept. Can anyone explain oxides/hydroxides and their importance/differences?

Hasmik Dis 2F

So I read somewhere that the Cl in the salt NH4Cl (just an example) does not affect the solution's pH. Something about Cl being a spectator ion? I don't really get the concept behind this, does anyone have any idea? Thanks!

Hasmik Dis 2F

If pH is less than pKa, then the environment is going to be more acidic. Thus, it won't make sense if the acid ionizes (by ionize I mean protonated) because all it's doing is making an acidic environment more acidic. So, it's going to be neutral. Hope this helped.

Hasmik Dis 2F

The lone pair is what makes NH3 a weak base. Oxygen, being more electronegative than the nitrogen, will be more attracted to the lone pair on the nitrogen than nitrogen itself. This causes ammonia to let go of these electrons more easily- thus making it a weaker base. I guess I am using the Lewis ba...

Hasmik Dis 2F

An example would be on sapling. For example, if we were to find the oxidation of cobalt in [Co(NH3)4 Cl2] Cl we would want to make sure our charges add up. The overall compound is neutral. The Cl outside has a -1 charge. Cl2 would be -2 and NH3 is neutral. So far, we have a charge of -3, but since o...

Hasmik Dis 2F

Basically a ligand is a compound that interacts with a transition metal. So technically yes. But also It depends whether the atoms are in the coordinate sphere and if it is bonded to the central metal. For example, in [Co(NH3)] Cl3, the Cl3 would not be a ligand since it is outside the brackets, and...

Hasmik Dis 2F

I believe the greek prefixes are the most important one to know when naming a ligand: 1: Mono (but we don't really use this since it's like using 1x in math) 2 : di 3 : tri 4: tetra 5: penta 6: hexa Also, if the ligand already has a Greek prefix in the name (ex: ethyleneDIamine) then we would use 2:...

Hasmik Dis 2F

Hi! I know that some people might have different numbers/questions depending on their account on sapling. Here is how I got the answer to name my first compound: Name the following compound: [Co(NH3)3 Cl3] First, since all of the compounds are in the bracket, I am not so worried. Since NH3 is my li...

Hasmik Dis 2F

This is because it depends on whether you use the Bronsted Acid definition- which states that any species that can donate a proton is an acid. Whereas a lweis acid is a species that can accept a nonbonding electron pair. Given this, in some situations, the difference in acid definitions do not corre...

Hasmik Dis 2F

Dr Lavelle was showing us the pH chart in the beginning of today's lecture. I wanted to know if we'd have to memorize that chart? Would we have to know the pH levels of certain products?

Hasmik Dis 2F

I remember from Dr. Lavelle's lectures that pi bonds do not let bound atoms to rotate. This is also why proteins can change shape/form- because of their single bonds.

Hasmik Dis 2F

So actually the shape should be trigonal pyramidal since the central atom is surrounded by three atoms. It would be trigonal bipyramidal if it also had a lone pair (four electron densities- three of it being atoms). Given this, trigonal planar bond angles are typically 120 degrees because that's the...

Hasmik Dis 2F

So basically, it is true that shapes only consider atoms, but this is for naming shapes. If we have a lone pair, it does influence the shape, but in terms of naming, we use atom positions. Let's take :NH3 for example. Since there's three atoms around the central atom, it would normally be trigonal p...

Hasmik Dis 2F

So basically this question asks which of the following molecules are linear and XeF2 was one of the molecules that were linear. This molecule has three lone pairs around Xe while the F atoms have a full octet. When I checked the solution, it said how this molecule is linear because the three pairs a...

Hasmik Dis 2F

So in one of Dr. Lavelle's lectures (lecture 20), he mentioned how "lone pairs do influence molecular shape but only atom positions are used to name the shape." What did he mean by this?

Hasmik Dis 2F

I know this may seem like an abstract tip : but think of it as “things.” How many things are around phosphorus ( the lone pair counting as “one thing”)? There’s 4. So then we can look at the hybridization answers. We ask ourselves, which answer is equivalent to 4? That would be s1p3- if you were to ...

Hasmik Dis 2F

Although there is some repulsion, the whole point of the VESPR model is to make sure that we get the least repulsion possible. The two fluorine atoms coming closer together is the option to make this possible.

Hasmik Dis 2F

If electrons in a pi bond are confined to one region, it not as delocalized. On the other hand, whenever two orbitals overlap, the electrons can be free to spread over all atoms: this is delocalization!

Hasmik Dis 2F

Another tip with viscosity is to compare water and syrup ( I feel that real examples help more). Water of course flows more easily, but syrup has a thicker consistency so it doesn’t flow as smoothly. Syrup is thus more viscous.

Hasmik Dis 2F

Basically a good rule of thumb is to remember that axial atoms that are connected to the rings at a 90 degree angle, while equatorial involves angles smaller than that.

Hasmik Dis 2F

So on #12 for the Sapling homework, it asked about hydrogen bonding. I know the two requirements needed for a hydrogen bond to occur: one site must have a highly electronegative atom with a lone pair (that being N, O , or F) and the other being a hydrogen atom bonded to another highly electronegativ...

Hasmik Dis 2F

Hi Janna! So I think the issue is that B(OH)3 is under lewis base. I believe it is a lewis acid because as you can tell from the lewis structure, B can still accept electrons because it doesn't have a full octet (which is one of the Boron exceptions). Feel free to correct me if I'm wrong! I just tho...

Hasmik Dis 2F

I had a question for this problem too: did we have to know how to write out the compounds? Will the midterm test us on this? Also, will we have access to the electronegativity chart on the midterm?

Hasmik Dis 2F

The outline sheet on Dr Lavelle's website for Outline 3 said that we should be able to draw lewis structures of inorganic, organic, and biological compounds. I was wondering: what's the difference between these three? Also, It said we should be able to "Draw line structures of organic compounds...

Hasmik Dis 2F

Dr Lavelle had a great explanation for this in one of his lectures. It is important to also note that some compounds may have higher boiling points than others due to the extent of polarability. For example, CH4 has a higher boiling point than CCL4. This is because since CCl4 has more electrons, it ...

Hasmik Dis 2F

Yea that value is just a given in the problem. I believe we're going to learn about lattice energy/change in energy during thermochemistry. But, as of right now, it's pretty much a given value. How they obtained it is a topic we're going to learn about later in chemistry.

Hasmik Dis 2F

An oxidation number is basically the electron amount that an atom gains/loses whenever forming a bond with other atoms. There's usually set guidelines for assigning oxidation numbers. For example, group one is always +1, group 2 is +2, oxygen is always -2 (except for in hydrogen peroxide). There's m...

Hasmik Dis 2F

*cries in autofill*
Haha I actually kind of have my UCLA ID memorized, but not entirely. Only the first 6 numbers XD
Will definitely work on that!

Hasmik Dis 2F

^ Flourine's small size does have associations with its electronegativity. Since the atom is extremely close to fulfilling its octet, the nucleus is going to tightly hold onto the electrons. This is why it also has a relatively smaller radius than the other atoms in the same group (the nuclear charg...

Hasmik Dis 2F

A good way to think about coordinate covalent bonding would be the BF3 example in Dr. Lavelle's lecture. In BF3, the B has a lower octet (6 valence electrons). Thus, it had room to bond with a F- (which could provide BF3 with the last two electrons to fill the octet). The BF3 then covalently bonds w...

Hasmik Dis 2F

In the lecture, he only introduced us to the formula and what each aspect of it meant. So, I believe as long as you understand the formula conceptually you will be fine. Otherwise, if he really expected us to know how to solve a problem using that formula, then he probably would have done some pract...

Hasmik Dis 2F

It depends on what classes you're taking. I know Chem 14BL fills up fast, so if you wanted to take that with Chem 14B then I'd sign up for both first pass b/c 14B is a co-requisite for 14BL. I need to take LS30B though and that fills up faster so I'm using my first pass on it, but I'm not sure if I...

Hasmik Dis 2F

Also, I wanted to add to Eva's answer the fact that Na+ has a stronger positive nuclear charge than Na. Therefore, that stronger pull the nucleus has on the electrons can account for the smaller radius.

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