Search found 50 matches
- Sun Mar 15, 2020 7:34 am
- Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
- Topic: Delta S
- Replies: 8
- Views: 1332
Re: Delta S
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...
- Sat Mar 14, 2020 10:53 pm
- Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
- Topic: specific vs molar
- Replies: 4
- Views: 766
Re: specific vs molar
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.
- Sat Mar 14, 2020 10:48 pm
- Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
- Topic: Review slides
- Replies: 1
- Views: 147
Re: Review slides
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.
- Sat Mar 14, 2020 10:43 pm
- Forum: Gibbs Free Energy Concepts and Calculations
- Topic: Ppt Question
- Replies: 2
- Views: 274
Re: Ppt Question
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.
- Sat Mar 14, 2020 10:14 pm
- Forum: Ideal Gases
- Topic: T variable in pv=nrt [ENDORSED]
- Replies: 38
- Views: 1492
Re: T variable in pv=nrt [ENDORSED]
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.
- Sat Mar 14, 2020 10:10 pm
- Forum: Equilibrium Constants & Calculating Concentrations
- Topic: Calculating concentrations at equilibrium
- Replies: 4
- Views: 395
Re: Calculating concentrations at equilibrium
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...
- Sat Mar 14, 2020 9:38 pm
- Forum: Arrhenius Equation, Activation Energies, Catalysts
- Topic: Objective
- Replies: 2
- Views: 216
Re: Objective
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.
- Sat Mar 14, 2020 9:32 pm
- Forum: Non-Equilibrium Conditions & The Reaction Quotient
- Topic: cubic equations, assumption
- Replies: 3
- Views: 278
Re: cubic equations, assumption
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.
- Sat Mar 14, 2020 8:50 pm
- Forum: Non-Equilibrium Conditions & The Reaction Quotient
- Topic: Qc vs Kc
- Replies: 15
- Views: 5115
Re: Qc vs Kc
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.
- Sat Mar 14, 2020 8:02 pm
- Forum: Ideal Gases
- Topic: Partial Pressure
- Replies: 13
- Views: 670
Re: Partial Pressure
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.
- Sat Mar 14, 2020 7:19 pm
- Forum: Concepts & Calculations Using Second Law of Thermodynamics
- Topic: Determining the Change in Entropy of the Surroundings without Being Given the Reversibility
- Replies: 2
- Views: 220
Re: Determining the Change in Entropy of the Surroundings without Being Given the Reversibility
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...
- Sat Mar 14, 2020 6:45 pm
- Forum: Third Law of Thermodynamics (For a Unique Ground State (W=1): S -> 0 as T -> 0) and Calculations Using Boltzmann Equation for Entropy
- Topic: irreversible expansion of ideal gas
- Replies: 3
- Views: 366
Re: irreversible expansion of ideal gas
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.
- Sat Mar 14, 2020 6:23 pm
- Forum: Calculating Work of Expansion
- Topic: Increase/Decrease
- Replies: 6
- Views: 445
Re: Increase/Decrease
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...
- Sat Mar 14, 2020 4:59 pm
- Forum: Phase Changes & Related Calculations
- Topic: Negative Work
- Replies: 18
- Views: 1344
Re: Negative Work
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...
- Sat Mar 14, 2020 4:20 pm
- Forum: General Rate Laws
- Topic: integrals and derivatives
- Replies: 7
- Views: 554
Re: integrals and derivatives
Unless there has actually been a HW or test problem that made us do that before, I wouldn't expect it.
- Sat Mar 14, 2020 3:13 pm
- Forum: Reaction Mechanisms, Reaction Profiles
- Topic: Slow and Fast Step
- Replies: 7
- Views: 505
Re: Slow and Fast Step
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.
- Sat Mar 14, 2020 2:36 pm
- Forum: Arrhenius Equation, Activation Energies, Catalysts
- Topic: Activation Energy and Energy released/absorbed
- Replies: 3
- Views: 270
Re: Activation Energy and Energy released/absorbed
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...
- Sat Mar 14, 2020 2:10 pm
- Forum: Arrhenius Equation, Activation Energies, Catalysts
- Topic: heterogeneous catalyst
- Replies: 4
- Views: 309
Re: heterogeneous catalyst
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.
- Sat Mar 14, 2020 1:58 pm
- Forum: Balancing Redox Reactions
- Topic: Acid or base?
- Replies: 5
- Views: 475
Acid or base?
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?
- Sun Mar 08, 2020 11:35 pm
- Forum: Kinetics vs. Thermodynamics Controlling a Reaction
- Topic: kinetics vs thermo
- Replies: 3
- Views: 388
Re: kinetics vs thermo
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 ...
- Sun Mar 08, 2020 10:48 pm
- Forum: Second Order Reactions
- Topic: Non-integer orders
- Replies: 7
- Views: 1089
Re: Non-integer orders
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...
- Sun Mar 08, 2020 10:37 pm
- Forum: Reaction Mechanisms, Reaction Profiles
- Topic: Molecularity and rate laws
- Replies: 5
- Views: 393
Re: Molecularity and rate laws
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...
- Sun Mar 08, 2020 10:08 pm
- Forum: Arrhenius Equation, Activation Energies, Catalysts
- Topic: Temperature
- Replies: 4
- Views: 269
Re: Temperature
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...
- Sun Mar 08, 2020 9:55 pm
- Forum: Arrhenius Equation, Activation Energies, Catalysts
- Topic: A
- Replies: 8
- Views: 430
Re: A
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).
- Sun Mar 01, 2020 9:40 pm
- Forum: General Rate Laws
- Topic: Differential Rate Law
- Replies: 2
- Views: 174
Re: Differential Rate Law
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 ...
- Sun Mar 01, 2020 9:29 pm
- Forum: Kinetics vs. Thermodynamics Controlling a Reaction
- Topic: Kinetics vs. thermodynamics
- Replies: 4
- Views: 273
Re: Kinetics vs. thermodynamics
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.
- Sun Mar 01, 2020 9:25 pm
- Forum: General Rate Laws
- Topic: unique rate
- Replies: 3
- Views: 240
Re: unique rate
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/...
- Sun Mar 01, 2020 9:09 pm
- Forum: Balancing Redox Reactions
- Topic: oxidation vs reduction
- Replies: 19
- Views: 903
Re: oxidation vs reduction
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...
- Sun Mar 01, 2020 8:49 pm
- Forum: Kinetics vs. Thermodynamics Controlling a Reaction
- Topic: Spontaneity
- Replies: 4
- Views: 327
Re: Spontaneity
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.
- Sun Feb 16, 2020 12:40 pm
- Forum: Van't Hoff Equation
- Topic: Delta H and S naught
- Replies: 2
- Views: 136
Re: Delta H and S naught
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...
- Sun Feb 16, 2020 12:31 pm
- Forum: Van't Hoff Equation
- Topic: concept help
- Replies: 4
- Views: 247
Re: concept help
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)
- Sun Feb 16, 2020 12:25 pm
- Forum: Balancing Redox Reactions
- Topic: oxidation vs reduction
- Replies: 19
- Views: 903
Re: oxidation vs reduction
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.
- Sun Feb 16, 2020 12:21 pm
- Forum: Van't Hoff Equation
- Topic: equilibrium and gibbs energy
- Replies: 2
- Views: 121
Re: equilibrium and gibbs energy
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...
- Sun Feb 16, 2020 12:01 pm
- Forum: Van't Hoff Equation
- Topic: given values
- Replies: 3
- Views: 221
Re: given values
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...
- Sun Feb 09, 2020 8:51 pm
- Forum: Entropy Changes Due to Changes in Volume and Temperature
- Topic: reversible vs. irreversible
- Replies: 3
- Views: 134
Re: reversible vs. irreversible
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.
- Sun Feb 09, 2020 8:33 pm
- Forum: Calculating Work of Expansion
- Topic: Reversible vs Irreversible
- Replies: 3
- Views: 167
Re: Reversible vs Irreversible
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.
- Sun Feb 09, 2020 8:30 pm
- Forum: Concepts & Calculations Using Second Law of Thermodynamics
- Topic: Finding whether exothermic or Endothermic
- Replies: 4
- Views: 147
Re: Finding whether exothermic or Endothermic
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
By definition a system is exothermic when the delta H is negative, and endothermic when the delta H is positive
- Sun Feb 09, 2020 8:18 pm
- Forum: Concepts & Calculations Using Second Law of Thermodynamics
- Topic: Delta S Universe
- Replies: 3
- Views: 124
Re: Delta S Universe
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...
- Sun Feb 09, 2020 8:09 pm
- Forum: Gibbs Free Energy Concepts and Calculations
- Topic: System releasing energy
- Replies: 2
- Views: 84
Re: System releasing energy
Yes, exothermic reactions usually have a negative delta G.
- Sun Feb 09, 2020 8:00 pm
- Forum: Entropy Changes Due to Changes in Volume and Temperature
- Topic: Entropy at 0 Kelvin
- Replies: 3
- Views: 139
Re: Entropy at 0 Kelvin
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.
- Sun Feb 02, 2020 3:15 pm
- Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
- Topic: biological examples
- Replies: 7
- Views: 266
Re: biological examples
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...
- Sun Feb 02, 2020 2:49 pm
- Forum: Phase Changes & Related Calculations
- Topic: internal energy
- Replies: 11
- Views: 552
Re: internal energy
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...
- Sun Feb 02, 2020 2:22 pm
- Forum: Concepts & Calculations Using First Law of Thermodynamics
- Topic: Internal energy = State function
- Replies: 4
- Views: 230
Re: Internal energy = State function
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...
- Sun Feb 02, 2020 1:40 pm
- Forum: Thermodynamic Systems (Open, Closed, Isolated)
- Topic: Closed system
- Replies: 5
- Views: 201
Re: Closed system
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...
- Sun Feb 02, 2020 1:01 pm
- Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
- Topic: Combustion Reactions
- Replies: 5
- Views: 191
Re: Combustion Reactions
I haven't seen combustion reactions where H2O was given as a liquid though.
- Sun Jan 26, 2020 5:28 pm
- Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
- Topic: no bond enthalpies
- Replies: 1
- Views: 111
Re: no bond enthalpies
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...
- Sun Jan 26, 2020 5:09 pm
- Forum: Phase Changes & Related Calculations
- Topic: Combustion vs. Cellular Respiration
- Replies: 5
- Views: 213
Re: Combustion vs. Cellular Respiration
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...
- Sun Jan 26, 2020 4:59 pm
- Forum: Phase Changes & Related Calculations
- Topic: What is a state function?
- Replies: 3
- Views: 152
Re: What is a state function?
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...
- Sun Jan 26, 2020 4:39 pm
- Forum: Phase Changes & Related Calculations
- Topic: Phase changes: endothermic vs exothermic
- Replies: 12
- Views: 1095
Re: Phase changes: endothermic vs exothermic
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...
- Sun Jan 26, 2020 4:30 pm
- Forum: Phase Changes & Related Calculations
- Topic: State Properties
- Replies: 4
- Views: 129
Re: State Properties
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 ...