Search found 32 matches

Sun Mar 18, 2018 3:16 pm
Forum: Phase Changes & Related Calculations
Topic: carbon
Replies: 4
Views: 374

Re: carbon

You must take deltaH sublimation into account of Carbon when finding delta H when using bond enthalpies. This is because the bond enthalpies in the table are given for bonds in gas phase, but the reaction has carbon in solid form.
Sun Mar 18, 2018 3:14 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Phase changes and entropy
Replies: 5
Views: 519

Re: Phase changes and entropy

You can replace q with delta H when the system is at constant pressure. We often use the formula deltaS=deltaH/T when we are calculating the entropy change at boiling point or other phase changes. Just make sure when you do this, you use the correct T (the boiling point temperature).
Sun Mar 18, 2018 3:11 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Replies: 5
Views: 533

Adiabatic does not mean that the system is necessarily irreversible or reversible. It simply means that q=0 because there is no change in temperature. This means that the change in internal energy is solely dependent upon work.
Sun Mar 18, 2018 3:09 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Pseudo Rate Laws
Replies: 5
Views: 714

Re: Pseudo Rate Laws

We can say that a reaction is a "pseudo 0/1/2 order" when we are examining one reactant and other others are in excess. In this case, the rate depends on that one reactant and is the pseudo order of whatever that reaction's exponent is. It is not the overall order because the other reactan...
Sat Mar 10, 2018 10:45 pm
Forum: Calculating Work of Expansion
Topic: internal energy [ENDORSED]
Replies: 1
Views: 144

Re: internal energy[ENDORSED]

Internal energy is 0 when the process is isothermal. This means when the temperature is constant. The internal energy is constant when the problem tells you it is. For example, in the midterm there was a question that told us that the system moves back to its original internal energy meaning that th...
Sat Mar 10, 2018 10:40 pm
Forum: Calculating Work of Expansion
Topic: reversible expansion [ENDORSED]
Replies: 1
Views: 148

Re: reversible expansion[ENDORSED]

If you look at these graphs, you can find your answer. We know that work is the area under the curve. The straight line across representing the constant external pressure in an irreversible process cuts off the area whereas the changing pressure allows for more work done in a reversible process.
Sat Mar 10, 2018 10:32 pm
Forum: First Order Reactions
Topic: Integrated Rate Laws
Replies: 4
Views: 396

Integrated Rate Laws

Whenever we use the integrated rate law formulas, are we assuming that it is for a reaction with only one reactant? Or are using the equation with respect to one reactant in the reaction?
Sat Mar 03, 2018 9:32 pm
Forum: General Rate Laws
Topic: Integrated Rate Law Units
Replies: 4
Views: 323

Re: Integrated Rate Law Units

It is easier to not use mol/L in some cases; however, make sure that your concentration units match up with your K units when using the rate equation because that is super important. When using the rate equation, it is probably easiest to go ahead and convert your concentration units to mol/L.
Sat Mar 03, 2018 9:22 pm
Forum: General Rate Laws
Topic: writing rate law
Replies: 3
Views: 269

writing rate law

Why are we able to write a rate law using the slow elementary step but not the general balanced reaction? Also, how do we use the elementary step to write the rate law?
Sat Mar 03, 2018 9:19 pm
Forum: General Rate Laws
Topic: Unique Reaction Rates [ENDORSED]
Replies: 33
Views: 78407

Re: Unique Reaction Rates[ENDORSED]

If all of the stoichiometric coefficients were different, the unique rate can still be the same using any of them to calculate it because we are multiplying the (change in concentration/change in time) by 1/coefficient. This therefore allows us to calculate the unique rate using any of the species i...
Sun Feb 25, 2018 6:03 pm
Forum: Kinetics vs. Thermodynamics Controlling a Reaction
Topic: activation energy
Replies: 4
Views: 265

activation energy

If a reaction has a higher activation energy, does that mean that the reaction is necessarily slower? Or does it solely mean that it needs more energy?
Sun Feb 25, 2018 5:59 pm
Forum: Method of Initial Rates (To Determine n and k)
Topic: figuring out n and m
Replies: 3
Views: 309

figuring out n and m

What are you supposed to do hypothetically if we are not given any constant initial concentrations so we cannot divide the reactions in order to find n or m?
Sun Feb 25, 2018 5:54 pm
Forum: General Rate Laws
Topic: Rate law equations
Replies: 3
Views: 286

Rate law equations

What is the difference between the equations Rate=k x [concentration]^a and (-1/a)*(d[A]/dt)?
Sat Feb 17, 2018 11:36 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: salt bridge
Replies: 3
Views: 280

salt bridge

What is the purpose of a salt bridge in voltaic cells? If the salt bridge were removed, what would happen to the cell?
Sat Feb 17, 2018 11:32 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: notation for galvanic cell
Replies: 2
Views: 166

notation for galvanic cell

What do the commas mean when we do the notation with the metals and the lines? In other words, in the short hand cell notation what does a comma signify versus a line?
Sat Feb 17, 2018 11:28 pm
Forum: Balancing Redox Reactions
Topic: balancing hydrogen
Replies: 1
Views: 146

balancing hydrogen

I am a little confused. I don't understand what the book was saying about balancing hydrogen for an acidic versus basic solution. For a basic solution, what side do we had OH- to and what side to we add H2O to?
Fri Feb 09, 2018 10:09 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: ΔG=0
Replies: 2
Views: 219

Re: ΔG=0

When delta G equals 0, the system is in equilibrium. Delta G also equals zero at phase changes.
Tue Feb 06, 2018 12:07 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Gibbs energy for gas and liquid phase
Replies: 1
Views: 166

Re: Gibbs energy for gas and liquid phase

This happens because the molar entropy of the gas phase is greater. As temperature increases, the TS portion of G=H-TS increases as well, decreasing G. If the S (entropy) is greater then an even larger amount is going to be subtracted resulting in a sharper decrease.
Tue Feb 06, 2018 12:01 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: 9.15
Replies: 4
Views: 328

Re: 9.15

The pressure at a phase change is constant so we can change the formula deltaS=q/T to deltaH/T. You then plug in the delta H fusion of water and divide it by the temperature given (298.15 K). You then need to multiply this number by the number of moles (1.00) to fix the units.
Sun Feb 04, 2018 12:13 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Maximum work
Replies: 2
Views: 223

Re: Maximum work

When temperature and pressure are not constant, I do not believe there is relationship between maximum work and the change in free energy. I know that when temperature and pressure are constant, the maximum work is equal to the change in free energy because all of the energy free and available is us...
Sun Feb 04, 2018 12:08 am
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Change in Entropy of Universe
Replies: 3
Views: 301

Re: Change in Entropy of Universe

Delta S universe is equal to 0 only when the system us at equilibrium. So yes, only when a reversible process is taking place. When delta S universe is positive, the process is spontaneous.
Sat Feb 03, 2018 1:21 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Thermal and positional disorder
Replies: 2
Views: 234

Re: Thermal and positional disorder

Rather than types, thermal and positional disorder can be thought of components of entropy. These are characteristics that describe entropy. If a system were to increase in entropy, its atoms would collide more vigorously and randomly, resulting in the energy of the system to disperse randomly. The ...
Sun Jan 28, 2018 4:10 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: degeneracy
Replies: 3
Views: 210

degeneracy

I am a little confused about how to find the number of degeneracies for a gas and what the degeneracy really is. How do we calculate this value and what does it have to do with the second law of thermodynamics?
Sun Jan 28, 2018 3:58 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: which w formula to use
Replies: 2
Views: 137

Re: which w formula to use

We use -PdelaV when the EXTERNAL pressure is constant. This means that the system is irreversible. If the system is at equilibrium (P internal = P external) then deltaV changes infinitesimally, meaning makes small fluctuations. This second situation is a reversible process because the system is at e...
Sun Jan 28, 2018 3:53 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Entropy based on initial temp
Replies: 3
Views: 172

Entropy based on initial temp

At the end of lecture on Friday, Dr. Lavelle mentioned that if two gases (one at a lower temperature than the other) had the same amount of energy added to them, the change in entropy would be much greater for the gas that started at a lower temperature. Why is this? Why does the magnitude of deltaS...
Sat Jan 20, 2018 5:43 pm
Forum: Calculating Work of Expansion
Topic: Reversible vs. Irreversible Process
Replies: 2
Views: 611

Reversible vs. Irreversible Process

I'm a little confused about what makes a process reversible versus irreversible. How do we know when to use -PdeltaV rather than the equation with ln and what makes a process considered as reversible? Dr. Lavelle mentioned something about the volume changing infinitesimally and I am not sure about h...
Sat Jan 20, 2018 5:33 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Heat capacity for monatomic ideal gas
Replies: 2
Views: 143

Re: Heat capacity for monatomic ideal gas

They should be independent of temperature and pressure because technically the heat capacity should be about the same for an element in each state of matter regardless of the circumstances. The heat capacity is per grams degrees Celsius so it should take the same amount energy, no matter the amount ...
Thu Jan 18, 2018 10:55 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Gas in a piston with regards to pressure
Replies: 3
Views: 194

Re: Gas in a piston with regards to pressure

My TA said that the pressure increases not decreases though. Also why do you think that T is constant? I thought that it might increase because heat was added.
Thu Jan 18, 2018 10:11 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Gas in a piston with regards to pressure
Replies: 3
Views: 194

Gas in a piston with regards to pressure

In discussion this week, my TA did this problem with the class: Gas in a piston does 171 KJ of work. 242 KJ of heat are added. Find deltaU. Is the final pressure of gas higher or lower than the initial? I understand how to find deltaU with q+w and we ended up getting 71 KJ. I am confused, however, a...
Sat Jan 13, 2018 9:27 am
Forum: Phase Changes & Related Calculations
Topic: Heat Curve of Water
Replies: 3
Views: 229

Heat Curve of Water

Will somebody explain why the vaporization section in the heat curve of water is much longer than the fusion part? In other words, why does it take longer for a liquid to become a gas as than a solid to become a liquid?
Sat Jan 13, 2018 9:21 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Periodic Trend for Heat Capacity
Replies: 2
Views: 454

Re: Periodic Trend for Heat Capacity

I do not believe that there is a periodic trend for the heat capacity because the elements in the periodic table are solids, liquids, and gases and different states of matter of an element have different heat capacities.
Fri Jan 12, 2018 4:02 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Enthalpy formula from day 1
Replies: 2
Views: 152

Enthalpy formula from day 1

Why does delta H = qp at constant pressure? I know we were told to memorize this, but I don't understand where it came from.