CHEMISTRY COMMUNITY

RylanPurificacion 3J

ΔU=0 when q+w=0, or q=-w. In other words, when all the heat gained by the system is expended as work. While this is true in many situations, it isn't guaranteed: For example, in this problem, the work expended should be lower than the heat gained if I did my math right, meaning that q=/=-w, and thus...

RylanPurificacion 3J

I've heard about an open, closed, and isolated system but what exactly is a perfect system? What characterizes such a system and are there any examples besides a bomb calorimeter? I would like to know as well, though I'm pretty sure that it's just another name for an isolated system. I checked thro...

RylanPurificacion 3J

Someone else shared a mnemonic for strong bases as well: "For strong bases, it's Lily and Nate RoBbed a Bank and Killed a Cessium of Strong Cats" (Li, Na, Rb, Ba, K, Cs, Sr, Ca)". Thanks to another chemistry community post here https://lavelle.chem.ucla.edu/forum/viewtopic.php?p=30443...

RylanPurificacion 3J

Does anyone know how to calm down your anxieties before you take this final? Since having test anxiety can be very distracting while you're trying to study or even while you're taking the test itself. Open to any tips:) Does anyone just zone out during the exam or get stuck in a brain fog and just ...

RylanPurificacion 3J

If you wanted a equation for it, the units would be in 1/(M^z-1*s), where z is the order for the reaction.

RylanPurificacion 3J

In a similar vein, for the second, since you multiply [A] by 3 and [B] by 1/2, and [B] is squared so it's by 1/4, you end up multiplying it with 3/4.

RylanPurificacion 3J

What you'll want to do to start this out is try and figure out the order of each reactant, so you'd find reactions where the concentrations of everything else are the same except the one you're looking for, and see how the change in concentration affects the rate. For example, if you wanted to find ...

RylanPurificacion 3J

Since you know it's a zero order reaction, you can use the equation [A]₀-[A]_i=-kt, where k is the given rate constant and t is the time you're looking for.

RylanPurificacion 3J

Your method should be right, as far as I'm aware. I did the same thing. I guess just double check to make sure everything is correct.

RylanPurificacion 3J

How do we know C is the zero order reactant? We know because in the context of the problem, there are two examples where the concentrations of [A] and [B] stay the same and [C] changes, yet their rates do not change. Since the rates do not change, that means that the concentration of [C] cannot aff...

RylanPurificacion 3J

I think it depends on whether the reaction is basic or acidic. For example, in MnO 2 -> MnO 4 - , the product side has more oxygens, so you'd need to add water on the reactants side to balance it out, and add hydrogen ions on the product side to balance that out. However, that's for an acidic soluti...

RylanPurificacion 3J

You haven't balanced the electrical charges. The oxidation side has 8 electrons and the reduction side has 6. Multiply the items in the oxidation half reaction by 3 and the items in the reduction half reaction by 4 to balance out the electrical charge.

RylanPurificacion 3J

Whenever one side has more Oxygens/Hydrogens than the other, you'd need to add water and either H+ or OH- to the reaction. Like when going from MnO2 to MnO4-, the extra 2 oxygens have to come from somewhere. So you'd add water to one side to provide the oxygen, and add H+ on the other side to balanc...

RylanPurificacion 3J

For a slightly different perspective, since E=E°-(RT/nF)*ln(Q), we can rewrite it as E=E°-((ln(Q)*RT)/nF). Both n and ln(Q) double, so you end up with E=E°-((2ln(Q)RT)/2nF), and the 2 on each side cancels out to our original equation.

RylanPurificacion 3J

To add onto the first responder's process, that's all you have to do for acidic conditions. However, for a reaction in a basic solution, you also have to add OH- ions on both sides to cancel out the H+ ions, which would also cancel out some of the H2O molecules. So in general, the reaction in basic ...

RylanPurificacion 3J

I don't know if there's one in the same order as LEO, but if you're fine with a completely different acronym there's OIL RIG, where it's Oxidation Is Losing electrons and Reduction Is Gaining electrons.

RylanPurificacion 3J

The first thing to note is that each of the reactions is a half reaction, so you would want to add them for a full reaction. From there, since you want each side to have an equal charge, you would balance the electrons (and make sure the rest of the equation is still balanced afterward). Hope that h...

RylanPurificacion 3J

Are you capitalizing the letters? Sapling can get pretty specific with that, and I noticed that I had to capitalize each element or else nothing would show up.

RylanPurificacion 3J

For this problem, I'm pretty sure you're able to just balance out the equation like we had done before so that there are equal amounts of all atoms on both sides. You could also check the charges afterward to make sure, but the former method worked for me. If that's not working, can you get more spe...

RylanPurificacion 3J

I don't think you need to memorize all of them, but there are some common elements to keep in mind. Oxygen is usually -2, Hydrogen is usually +1, Alkali earth metals are usually +2. I think also alkali metals are +1 and halogens are -1. Once you do have them, then it's usually just calculating it ou...

RylanPurificacion 3J

Is there a difference when a question says Cp,m/Cv,m or Cp/Cv? I saw the book used both when explaining this concept A Cp,m or Cv,m is just the pressure/volume molar heat capacity, or the heat capacity for a single mol. You'd have to convert one value to the other by multiplying by the number of mo...

RylanPurificacion 3J

Delta H and Delta S are usually related when that if one is increasing, the other is probably increasing as well. Delta H is just the measure of heat in the system, so if it's positive, then heat is going into the system. Delta S is the measure of entropy/the number of positions, and that usually in...

RylanPurificacion 3J

deltaU would be 0, but q and w wouldn't have to be zero. More generally, it ends up that q=-w, or that any heat gained by the system is expended as work, resulting in a net energy change of 0.

Isothermal just means that the temperature remains constant.

RylanPurificacion 3J

You can also intuitively think about it by knowing that by being positive, it would generally involve gaining heat and go from solid->liquid->gas, as well as being vice versa for negative. However, that should only be to make sure. Whatever you calculate should give the right sign anyways, due to th...

RylanPurificacion 3J

I don't know why I was doubting myself thinking there had to be something in the middle columns too. Thank you! could you explain why nothing goes in the middle columns? is it impossible for both dH to be negative and dS to be positive (or vice versa)? thank you! Nothing goes in the middle columns ...

RylanPurificacion 3J

In question 20 specifically, q=deltaU because it is at a constant volume, and so no work is being done. If w=0 and deltaU=q+w then deltaU=q.

RylanPurificacion 3J

Hi,

You would find the calorimeter constant by the equation C=q/deltaT. And to find q, you would just convert the mass of the compound to mols and then multiply with the heat of combustion. Hope that helped!

RylanPurificacion 3J

I know that reversible equations are also known as isothermic, and that irreversible ones are used when the system is being heated over a constant pressure.

RylanPurificacion 3J

You might need to multiply the -pressure times volume by 101 J to convert to joules. I was having trouble with this as well and this solved it, but why does this work? I don't really get where the conversion comes in. EDIT: Well seems the poster above me preemptively sniped my question with an answ...

RylanPurificacion 3J

I actually figured out this problem by remembering that grams can be roughly calculated as 1 g/ml, however I am confused as to why I don't need to include delta H fus in this equation, unlike previous sapling problems. Could someone explain to me when delta H fus is necessary? Thanks! Delta Hfus is...

RylanPurificacion 3J

For this specific example, we were given MCl3(s) ---> MCl3(aq) ΔH4 = -388.0, however when you break down the reaction we were looking at MCl3 changed from (aq) to (s), therefore we need to use the reverse of the reaction enthalpy we were given for that change. Hence you use -2 instead of just 2. Do...

RylanPurificacion 3J

A closed system is just a system where matter can't escape (like a sealed container), but an isolated system is a system where energy cannot escape (like a bomb calorimeter... Theroretically, anyways). In the closed system, let's say you're holding a sealed container of water, none of the water can ...

RylanPurificacion 3J

Constant pressure doesn't mean no pressure! It just means that there is no change in final and initial pressure. It could be both 0 or a nonnegative number. Why a nonnegative number specifically? Technically, pressure can never be less than 0, and technically it can. It depends on what your referen...

RylanPurificacion 3J

I believe that you would figure out this problem by utilizing the fact that enthalpy is the (change in energy of bonds broken) - (change in energy for bonds formed). In this reaction, the x2 is being broken and there are no bonds being formed: So it would be (bond energy of breaking X2) - 0 which wo...

RylanPurificacion 3J

Jamie2002 wrote:Constant pressure doesn't mean no pressure! It just means that there is no change in final and initial pressure. It could be both 0 or a nonnegative number.

Why a nonnegative number specifically?

RylanPurificacion 3J

To add on, you would specifically convert whenever your needed value is measured in moles. Since delta h is commonly measured in kJ/mol, it's likely that you would need to convert the mass to moles. However, in the future, if theoretical problems involve us converting to some other value measured in...

RylanPurificacion 3J

Kandyce Lance 3E wrote:For this problem, are [OH-] and [BH+] interchangeable ?

Yes, since it's a monoprotic reaction, both values taken in this problem are the same. Although it would probably be good form to use [OH-] and [BH+] properly, just to lessen the chances of messing up in the future when they're not.

RylanPurificacion 3J

It's also worth noting that ICE boxes can be used to solve equilibrium problems involving pressure as well, not just concentration. One of the problems in the previous Sapling HWs had that.

RylanPurificacion 3J

You know, that makes me curious. If it was pOH and pKb instead, would it still be the same thing, just reversed? As in bases have charged species dominant for pOH>pKb and neutral species dominant for pOH<pKb, and vice versa for acids. Intuition tells me yes, but I'm still not sure.

RylanPurificacion 3J

I had a lot of issues with this problem too. I think the biggest thing I needed to remember is that you don't just need the concentrations for [B], you need the concentrations for [B]initial. [B]initial is just [B]+[OH-], and then you'd use that value to find the percent ionization.

RylanPurificacion 3J

If Ka/Kb significantly favors one over the over (As in the value is k<10-4 or k>10^4), and the percent ionization is less than 5%, then it is safe to assume that x is neglectable. Mathematically, it means that it's small enough that you would essentially have the same concentration, and those ratios...

RylanPurificacion 3J

pH is just a convenient way to notate it. Saying that the pH is 2.15 is much more convenient than saying the concentration is 0.0070129. Because it's a negative log as well, we can quantitatively tell how strong an acid/base is. The inverse of -log(concentration) is 10^(-pH). If it is a strong acid ...

RylanPurificacion 3J

Mathematically, Kw=Ka*Kb=10^-14. So as one goes down, the other increases.

Conceptually, I'd imagine it has to do with how acids and bases are related. A strong acid dissociates into a weak base, for example. It's just due to how they're related. I hoped that helped.

RylanPurificacion 3J

You would use some rules of combining chemical equations to combine a few of the 4 listed into the top equation. The specific rules are -Switching the products and reactants [R] <-> [P] into [P] <-> [R] would make the new Kc value equal 1/Kc -Multiplying a chemical equation n([R] <-> [P]) = n[R] <->...

RylanPurificacion 3J

You know depending on which side the chemical equation is shifting. If you add more reactants initially, then the change for molecules on the left side would be -x (with stochiometric coefficients). The change on the products side would be +x (again with stochiometric coefficents). Same for if they ...

RylanPurificacion 3J

You know depending on the reaction equation. If the initial values are on the left side of the equation/you add reactants initially, then the change would be -x for all the molecules on that side (with stoichiometric coefficients), with the right side/products being +x (again with stochiometric coef...

RylanPurificacion 3J

SamayaJoshi1A wrote:Can someone explain what the term (aq) means? I know that it is aqueous, but how does that differ from a liquid? Is it because its a solution?

I believe the term aqueous is specifically referring to something dissolved in another liquid.

RylanPurificacion 3J

I’m confused on how this situation presented in 5J of the book can be true if adding gases doesn’t change pressure which then doesn’t effect concentration: “Suppose that the ammonia synthesis reaction, reaction A, has reached equilibrium. Now suppose that more hydrogen gas is pumped in. According t...

RylanPurificacion 3J

I haven't used it, but from what I could tell from searching it up, it is essentially the same thing. Just differences in naming.

RylanPurificacion 3J

In a general sense, whenever you have an equation that is going to equilibrium, it helps to make an ICE table. It was mostly used for concentrations, but it was also used for pressure in the homework, so it's not strictly limited to finding concentrations.

Edit: Got more specific.

RylanPurificacion 3J

I'm pretty sure the temperature is only there because any reaction only changes it's Kc value at a different temperature. Since the temperature is listed as 500 degrees Celsius both times, it's another way of saying you can use the listed Kc value for the problem. It doesn't factor into the calculat...

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