CHEMISTRY COMMUNITY

Frank He 4G

The universe in the way we use it sometimes in this class is to refer to the thermodynamic universe or the parts that are relevant to our consideration of the thermodynamics of something we're studying. It is not the same as the thermodynamics of the entire universe. Thus the S of the universe can a...

Frank He 4G

Molar heat capacity is just the heat capacity per mole, but the specific heat is a little less obvious in that it's per gram.

Frank He 4G

Yeah, I would expect that to be an error. Especially as there is a vertical line that is supposed to indicate they're different states.

Frank He 4G

I think it might be because the gibbs free energy of formation for Cl2 is 0, since it's already in its most stable form.

Frank He 4G

We should always work in kelvin just to be safe, so I'd convert to kelvin if needed and keep it that way. If the problem asks for a value using Celsius, then convert it back at the end.

Frank He 4G

so the reaction looks like its gonna be COBr2 <> CO(g) + Br2(g) you have the final concentration of CO solve using ICE table I figured it was an ice table question, but how would we solve if we weren't given K? Unless the og poster accidentally left K out? Well do we? I could be wrong about this, b...

Frank He 4G

Molecules need to collide in the right combination with the right energy in the right orientation. If the temperature is increased, then molecules move faster and are more likely to collide in the right way.

Frank He 4G

Either we have cubic equations where we can make the approximation, or we don't. We are definitely not expected to solve any cubic equations in this class.

Frank He 4G

Since Q is calculated similarly, when Q is higher than K, we know that there are more products than reactants at Q. If there are more products than at equilibrium, then the reaction has to be proceeding in reverse to go to equilibrium. And the reverse is also true.

Frank He 4G

Yeah, Chatelier's principle would explain this. Generally, the less particles floating around, the less energy has to be used to compress the system, so the side with the least moles would be favored.

Frank He 4G

For this problem, it seems like we don't have to know the reversibility. I solved it correctly myself under the assumption that the Ssur simply equals the standard entropy of vaporization of the substance (which is given) adjusted for the amount of the substance. So I suppose the underlying assumpti...

Frank He 4G

Looking at the example, it says free expansion. When something is said to freely expand, it means that the external pressure is 0. That means that w = -PV = 0.

Frank He 4G

Be sure to understand that the sign conventions for work, in our class, imply that w means work done by the system. And work is like a transfer of energy. So if work done by the system on the surroundings is energy transferred from system to the surroundings, then a negative work will mean energy tr...

Frank He 4G

I think trying to intuitively understand what work is is also important. From what I understand, it's like the energy used or transferred to do something. So if you do work, then you're expending energy in order to make something happen. Thus if a system does (positive sign) work on the surroundings...

Frank He 4G

Unless there has actually been a HW or test problem that made us do that before, I wouldn't expect it.

Frank He 4G

Yes, if you know the order of the slow step, you can determine the rate and assume that the overall rate of the reaction is equal to it.

Frank He 4G

The net energy change of a reaction includes the activation energy, not the other way around, for both endothermic and exothermic reactions. In general, if you are trying to determine the net energy change from a reaction profile, all you need to look at is the initial and final energies. Remember t...

Frank He 4G

Well it makes sense to me that since a solid heterogeneous catalyst cannot be blended with the reactant perfectly, then we probably want to at least maximize the surface area.

Frank He 4G

Will we always be given whether a solution is basic or acidic for balancing redox reactions? If we aren't, what are all the ways we'd be expected to know to determine if the solution is acidic or basic?

Frank He 4G

Since kinetics is about the rate of the reaction and thermodynamics tells you about how favorable it is, I would say that if a reaction is determined as favorable (spontaneous), then what's left is determining how fast it happens. Since a reaction like diamond turning into graphite can be favorable ...

Frank He 4G

Is there a certain amount within an integer that we should get in order to assume it counts for experimental error and round it? Like, say on the exam I get n=1.75. Is that close enough to round to 2? If not, what is the general cutoff for being close enough? As others have suggested, we're probabl...

Frank He 4G

A unimolecular elementary step corresponds to a first order rate law. A bimolecular elementary step corresponds to a second order rate law, and so on and so on. As such, they are directly correlated with each other and can be used interchangeably to describe a reaction. Is there a good reason why w...

Frank He 4G

By manipulating the Arrhenius equation, we can work out that temperature is inversely related to the activation energy. However note that the activation energy for a reaction doesn't change if you raise the temperature of the system, since the activation energy just tells you how much energy it take...

Frank He 4G

Also note if you were ever confused that A naught is how we refer to [A]0, and that the little 0 symbol, or naught, does often mean the initial or starting point when it is in the lower position (whereas it would usually mean under standard conditions if it were in the upper position).

Frank He 4G

Activation energy is essentially the energy input needed to break the bonds of a reactant so that new bonds can form to create the product, so if the activation energy, or energy required to make the reaction happen, is high, then naturally it would take longer for a reaction to happen, which means ...

Frank He 4G

Also as it was introduced in lecture, in thermodynamics we just looked at individual states of a reaction which never took into account the variable of time, while in kinetics, we look at the speed at which reactions happen and do involve time.

Frank He 4G

A form of the equation with whole numbers exists as well, but we usually have the form with fractions since it neatly matches up with the stoichiometry of the balanced reaction. That is, if we have a reaction where we know the amount of species X changes at 1/2 the speed of another species Y, or (1/...

Frank He 4G

Personally, I've always just memorized it as reduction being a reduction in the charge. So if the charge is reduced, then that has to mean an electron is gained. But does this always work? for example in molecules in which the net charge stays the same, but the number of electrons in each atom is d...

Frank He 4G

Another way to imagine this is that a reaction such as that actually has two steps. One where the bonds of the reactants are broken, and one where the bonds of the products are formed. To break bonds, you need to put in energy.

Frank He 4G

The unmodified Van't Hoff equation actually suggests that we're using it under conditions of constant temperature and when the reaction is at equilibrium. You're thinking about the equation we derived in class, where there was a change in temperature: lnK2/K1 = -delta H naught/R * (1/T2 - 1/T1) In t...

Frank He 4G

We also derived in class a form of the equation that lets us calculate the K of a reaction when we only know the delta H naught, which goes like this: lnK2/K1 = -delta H naught/R * (1/T2 - 1/T1)

Frank He 4G

Personally, I've always just memorized it as reduction being a reduction in the charge. So if the charge is reduced, then that has to mean an electron is gained.

Frank He 4G

At equilibrium, the delta G of the reaction is 0. Think about G as the potential energy of a substance. When a reaction is at equilibrium, then the potential energy of the reactants and the potential energy of the products has to be equal, so there is no change in G. This is then useful for deriving...

Frank He 4G

I don't know if we will encounter problems where we have to calculate the delta G naught or delta H naught in order to then use the Van't Hoff equation, but usually what we do with this equation is calculate K for a reaction at a different temperature when we know the delta H naught of a reaction, u...

Frank He 4G

If you're given a problem, it should say whether it's reversible or irreversible. Otherwise, another method of determining whether a process is reversible is whether the delta S of the universe is 0 or positive. Irreversible processes always have a positive delta S of the universe.

Frank He 4G

If they do not explicitly say, and they give you the delta S of the universe, it may be possible to infer, as the only way for something to be reversible is for the delta S of the universe to be 0. Any irreversible process results in a positive delta S of the universe.

Frank He 4G

Note that exothermic/endothermic refers to heat (exergonic/endergonic refers to free energy). Although exothermic reactions usually have a positive delta G, it's not always the case.
By definition a system is exothermic when the delta H is negative, and endothermic when the delta H is positive

Frank He 4G

That equation can be applied to any situation. It's just telling us that the entropy of the surroundings and the system can be added to get the total change in entropy of the universe. If we're considering an irreversible reaction that happens spontaneously, it will have a positive delta S of the un...

Frank He 4G

Yes, exothermic reactions usually have a negative delta G.

Frank He 4G

One would need a perfect pure crystal of a molecule or atom at 0 K to have 0 entropy. If it is not, then there are possible chances for molecules or atoms existing in different positions.

Frank He 4G

First, thank you for explaining the ATP part of the biological examples. :) Can someone explain the same for osmotic pressures? I'm not sure we went over this either, but osmotic pressure is basically the pressure needed so that water doesn't pass through a membrane. It's calculated using the conce...

Frank He 4G

The change in internal energy is represented by delta(U). Delta(U) is equal to the energy transferred by heating (q) + the energy transferred by compression (w). delta U = q + w This is good to note. Delta U is importantly not just internal energy but the change in internal energy. We are calculati...

Frank He 4G

A different way to think about this is to understand that heat and work are defined as the transfer of a property. You can't say that something is at a certain heat or has a certain work, only that heat was gained or lost, or that work was done by or on it. So if something can't be in a certain stat...

Frank He 4G

A closed system means that matter cannot go in or out, but heat can. If the system is contained within something that can expand or contract, then the volume can change. An example we often use is a piston. A different example could be a balloon, though we can not as easily control the pressure of a...

Frank He 4G

I haven't seen combustion reactions where H2O was given as a liquid though.

Frank He 4G

If the problem doesn't give you the bond enthalpies, then you must use one of the other methods depending on what information is given. This thread may help you understand which method is the most suitable depending on the information given in the problem: https://lavelle.chem.ucla.edu/forum/viewtop...

Frank He 4G

The enthalpy of the reaction for glucose in a combustion reaction versus one with cellular respiration is completely dependent on what the other reactants it is interacting with and the relative bond energies of those reactants and products. If the net reaction still simplifies down to the same thi...

Frank He 4G

A state function is basically just a descriptor or categorization of a value or property, just as we'd categorize velocity as a value having to do with motion. To understand a state property, we need to understand what a path function is. A path function describes a value or property that essentiall...

Frank He 4G

Why would the reverse reaction of vaporization, melting and sublimation be exothermic? Well first, the reason why vaporization, melting, and sublimation are endothermic is because bonds are being broken, which takes energy to happen. So in the reverse reaction, bonds are being formed, and it is exo...

Frank He 4G

Think about it this way (using the mountain example we went over in class): the distance we traveled to get to an altitude depends on whether we took a shorter or longer route. That means it's a path function, since we'd have to factor in how long their route was and can't simply take the final and ...

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