Search found 64 matches

Sat Mar 17, 2018 8:57 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Water and Cell Diagrams
Replies: 3
Views: 383

Water and Cell Diagrams

Would you ever include H2O in a cell diagram if it were part of a half reaction? For example, how would you write this reduction half reaction in a cell diagram: NO3- + 2H + e - ==> NO2 + H2O?
Sat Mar 17, 2018 8:37 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Inert Electrodes
Replies: 1
Views: 165

Inert Electrodes

When do you need an inert electrode? If you have an inert electrode, do you put it on both sides of a cell diagram or just one?
Fri Mar 16, 2018 11:35 am
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: Homework Problem 9.25
Replies: 1
Views: 293

Homework Problem 9.25

"If SO2F2 adopts a positionally disordered arrangement in its crystal form, what might its residual molar entropy be?" I understand how to do the math for this, but whenever I input the numbers in my calculator: 1.38 x 10^-23 ln(6^6.02 x 10^23), I get an "overflow error". Is this...
Mon Mar 12, 2018 12:21 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Collision Theory
Replies: 3
Views: 256

Re: Collision Theory

The higher the speed, the more likely particles are to collide and break/form bonds. Activation energy is the energy required to break/form bonds, so the higher the activation energy, the more difficult it will be to break/form bonds because the reaction requires a large input of energy before the p...
Mon Mar 12, 2018 12:20 am
Forum: Zero Order Reactions
Topic: Half life
Replies: 7
Views: 784

Re: Half life

Yes, just plug your given values into the equations and solve!
Mon Mar 12, 2018 12:19 am
Forum: Kinetics vs. Thermodynamics Controlling a Reaction
Topic: Equilibrium constant
Replies: 3
Views: 414

Re: Equilibrium constant

K = k/k' where k' is the rate constant for a reaction's reverse form.
Wed Mar 07, 2018 2:48 am
Forum: First Order Reactions
Topic: Pseudo First Order Reactions
Replies: 2
Views: 208

Pseudo First Order Reactions

How do we know if something is a pseudo-first-order reaction? Or would that be specified?
Wed Feb 28, 2018 12:57 am
Forum: Method of Initial Rates (To Determine n and k)
Topic: Homework Problem 15.99
Replies: 1
Views: 180

Homework Problem 15.99

"15.99 Which of the following plots will be linear? (a) [A] against time for a reaction that is first order in A; (b) [A] against time for a reaction that is zero order in A; (c) ln[A] against time for a reaction that is first order in A; (d) 1/[A] against time for a reaction that is second ord...
Tue Feb 27, 2018 10:49 pm
Forum: Kinetics vs. Thermodynamics Controlling a Reaction
Topic: Homework for Test 3
Replies: 2
Views: 249

Homework for Test 3

What homework problems cover 15.1-15.6 for Test 3?
Tue Feb 27, 2018 9:03 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Cell diagram order
Replies: 4
Views: 243

Re: Cell diagram order

I think that usually cell diagrams are written in the order of oxidation | reduction, although the solutions manual sometimes does have it in a different order. I would stick with oxidation | reduction because that's what we've mostly seen in lecture.
Mon Feb 26, 2018 5:25 pm
Forum: First Order Reactions
Topic: Equation in the Book vs. in Lecture
Replies: 2
Views: 216

Equation in the Book vs. in Lecture

The equation that the book gives for the integrated rate law of first-order reactions is [A] t = [A] 0 e -kt , but the equation for the integrated rate law of first-order reactions given in lecture is ln[A] = -kt + ln[A] 0 . Does it matter which equation we use since both are essentially the same? I...
Tue Feb 20, 2018 11:30 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Homework Problem 14.55b [ENDORSED]
Replies: 2
Views: 735

Homework Problem 14.55b[ENDORSED]

"A 1.0 m NiSO4(aq) solution was electrolyzed by using inert electrodes. Write (a) the cathode reaction; (b) the anode reaction. (c) With no overpotential or passivity at the electrodes, what is the minimum potential that must be supplied to the cell for the onset of electrolysis?" The solu...
Tue Feb 20, 2018 9:47 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Cell Diagrams [ENDORSED]
Replies: 2
Views: 208

Cell Diagrams[ENDORSED]

Does it matter what order the items in the anode side and cathode side of a cell diagram are written? For example, if you have Cl2 (g) and HCl (aq) on the anode side, would it make a difference if you wrote Cl2 (g) | HCl (aq) or if you wrote HCl (aq) | Cl2 (g) for the anode?
Tue Feb 20, 2018 9:09 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Expression for Q (Homework 14.37)
Replies: 1
Views: 182

Expression for Q (Homework 14.37)

I'm a little confused about how to calculate Q. I thought that the expression for Q was [product A]^a/[reactant B]^b using either the concentrations of the reactant and product or the partial pressures of the reactant and product. However, when doing 14.37a and c, the solutions manual combines the c...
Wed Feb 14, 2018 12:15 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: delta H for Isothermal Reversible Expansion [ENDORSED]
Replies: 1
Views: 213

delta H for Isothermal Reversible Expansion[ENDORSED]

Is it possible to find delta H for an isothermal reversible expansion?
Tue Feb 13, 2018 8:33 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: delta U for Isothermal Expansion of Ideal Gas
Replies: 2
Views: 342

Re: delta U for Isothermal Expansion of Ideal Gas

The equation for an ideal gas is U=3/2*nRT. When you're calculating the change in internal energy, the number of moles and the gas constant are constant values and that means the change in internal energy is dependent upon temperature change. But in an isothermal expansion temperature doesn't chang...
Tue Feb 13, 2018 8:29 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: delta G for Phase Changes
Replies: 1
Views: 241

delta G for Phase Changes

Does delta G = 0 for phase changes at the proper melting/boiling points? For example, homework problem 9.91b says that the standard Gibbs free energy for the vaporization of water at 100°C is 0. Why is this? Does this apply for the fusion/vaporization of other substances at their melting/boiling poi...
Tue Feb 13, 2018 7:43 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Homework Problem 9.81
Replies: 1
Views: 218

Homework Problem 9.81

"Which is the thermodynamically more stable iron oxide in air, Fe3O4(s) or Fe2O3(s)? Justify your selection." The solutions manual says to look at the reaction converting Fe3O4 to Fe2O3, calculate the delta G for that, and then based on the delta G/spontaneity of the reaction, you'll know ...
Tue Feb 13, 2018 6:43 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: delta U for Isothermal Expansion of Ideal Gas
Replies: 2
Views: 342

delta U for Isothermal Expansion of Ideal Gas

Why does delta U = 0 for the isothermal expansion of an ideal gas?
Tue Feb 13, 2018 2:31 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: isothermal system [ENDORSED]
Replies: 3
Views: 341

Re: isothermal system[ENDORSED]

There is no temperature change because energy from heat transfer is equal to energy from work. If they weren't equal, then the energy change from the heat transfer would result in a temperature change. However, because they are equal, the energy supplied by work makes up for that would-be temperatur...
Tue Feb 13, 2018 2:29 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Delta G Units
Replies: 1
Views: 182

Re: Delta G Units

The rule that I usually follow is to use kJ for standard delta G of a whole reaction because the products aren't necessarily only present in 1 mole. Therefore, you can't say that standard delta G of a reaction is necessarily going to be kJ/mole; all that "standard delta G" implies is that ...
Thu Feb 08, 2018 2:42 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Self-Test 9.7A
Replies: 2
Views: 293

Re: Self-Test 9.7A

Trouton's law says that the standard entropy of vaporization are commonly 85 J/(K*mol). It talks about entropy, not enthalpy. The question, on the other hand, asks you to solve for the enthalpy of vaporization of bromine liquid. The question asks you to apply the assumption that the entropy of vapo...
Tue Feb 06, 2018 2:52 pm
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: Example 9.10
Replies: 1
Views: 156

Example 9.10

Example 9.10 says, "Calculate the change in entropy of the surroundings when mercury freezes at 49 C; use Hfus(Hg) 2.292 kJ mol 1 at 49 C." It then goes on to solve the problem and sets T as (-49 + 273) K = 224 K. Are you just supposed to assume that the initial temperature is 0 C aka 273 ...
Tue Feb 06, 2018 1:39 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Self-Test 9.7A
Replies: 2
Views: 293

Self-Test 9.7A

I'm a little confused about how to apply Trouton's Law. For example, self-test 9.7A says "Use Trouton’s rule to estimate the standard enthalpy of vaporization of liquid bromine, which boils at 59 C." I thought that Trouton's Law said that most liquids have a standard enthalpy of vaporizati...
Mon Feb 05, 2018 9:58 am
Forum: Calculating Work of Expansion
Topic: Solving Reversible vs irreversible [ENDORSED]
Replies: 2
Views: 210

Re: Solving Reversible vs irreversible[ENDORSED]

I think the problems would say if they involve reversible or irreversible expansions.
Mon Feb 05, 2018 9:57 am
Forum: Calculating Work of Expansion
Topic: Naming Question [ENDORSED]
Replies: 5
Views: 400

Re: Naming Question[ENDORSED]

Reversible expansion is expansion that occurs as a result of infinitesimally small steps that can push the expansion either forward or backwards with the smallest of changes. This results in a maximum of work being done. In contrast, irreversible expansion is expansion that occurs as result of finit...
Mon Feb 05, 2018 9:53 am
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Entropy question [ENDORSED]
Replies: 2
Views: 179

Re: Entropy question[ENDORSED]

If the universe is constantly expanding, then entropy is also increasing with it.
Mon Feb 05, 2018 9:51 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Delta G equal Zero [ENDORSED]
Replies: 4
Views: 306

Re: Delta G equal Zero[ENDORSED]

Not necessarily, but delta G = 0 does mean that the reaction is at equilibrium.
Mon Feb 05, 2018 9:50 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: test question 7 [ENDORSED]
Replies: 6
Views: 431

Re: test question 7[ENDORSED]

delta H (fusion) of ice would have been given
Mon Feb 05, 2018 9:49 am
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Types of Disorders [ENDORSED]
Replies: 5
Views: 382

Re: Types of Disorders[ENDORSED]

I think that positional disorder has to do with how much space particles have to move around in. The greater the space, the greater the positional entropy because the more particles can move around. Thermal disorder has to do with temperature, so the higher the temperature, the greater the particle ...
Tue Jan 30, 2018 8:35 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: isothermal
Replies: 5
Views: 313

Re: isothermal

If an equation is isothermal, that means there is no heat transfer and therefore no change in temperature.
Tue Jan 30, 2018 8:34 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: unit of entropy
Replies: 11
Views: 638

Re: unit of entropy

The units are J/K (Joules per Kelvin)!
Tue Jan 30, 2018 8:34 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: 9.27 D
Replies: 3
Views: 221

Re: 9.27 D

It might also be easier to think about this in terms of PV = PV. Pressure and volume are inversely related. Therefore, the bigger the pressure (in this case, 2 atm instead of 1 atm), the smaller the volume (the volume for 2 atm would be half that at 1 atm).
Tue Jan 30, 2018 8:32 pm
Forum: Phase Changes & Related Calculations
Topic: Hw #8.49 standard state
Replies: 4
Views: 341

Re: Hw #8.49 standard state

My TA once said to assume that you're under standard conditions (298K and 1 atm) if the textbook doesn't say what temperature/pressure you're at, so I think you'd just assume that you're at standard state.
Tue Jan 30, 2018 8:31 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: heat capacities
Replies: 5
Views: 397

Re: heat capacities

The heat capacities for ice and water are different. Therefore, you would use the heat capacity of ice for ice and the heat capacity of water for water.
Sun Jan 21, 2018 4:13 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: ∆nRT in Reactions with no ∆n
Replies: 2
Views: 279

Re: ∆nRT in Reactions with no ∆n

If there is no change in the number of moles (no delta n), then that means that there is no change in volume (no delta V). Because w = -P(delta)V, if there is no delta V then there is no work done!
Sun Jan 21, 2018 4:04 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Homework Question 8.47
Replies: 4
Views: 223

Re: Homework Question 8.47

But that ultimately give you a negative work, whereas work done ON a system should be positive??? I just realized my mistake. Work done ON a system is positive, but EXPANSION work means that work is done on the surroundings/BY the system because the system is expanding. Therefore, work would be neg...
Thu Jan 18, 2018 4:42 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Homework Question 8.47
Replies: 4
Views: 223

Homework Question 8.47

For 8.47, why does the solutions manual use the equation (delta)H = U + P(delta)V instead of just U = q+ w? I'm also confused as to why work would be positive instead of negative because I thought that expansion work done ON a system is negative.
Mon Jan 15, 2018 5:01 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: U = q + w assumptions
Replies: 5
Views: 409

U = q + w assumptions

Does the equation (change in)U = q + w assume that there is constant pressure? Or does the pressure not have to be constant?
Sat Jan 13, 2018 4:22 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Irreversible vs. Reversible Processes
Replies: 3
Views: 223

Irreversible vs. Reversible Processes

Is the only difference between reversible and irreversible processes the fact that reversible processes are isothermal?
Sat Jan 13, 2018 4:17 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Enthalpy of physical change?
Replies: 3
Views: 502

Re: Enthalpy of physical change?

I'm not entirely sure, but I think that the 44.0 kJ/mol is just a value that was experimentally determined. I think that we would be given that in a table of H values or something if we needed it on a test (don't quote me on this though). The values given -- Hvap 40.7 kJ/mol at 100 degrees Celsius a...
Sat Jan 13, 2018 4:13 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Standard Enthalpy of Formation from Most Stable Form
Replies: 3
Views: 594

Standard Enthalpy of Formation from Most Stable Form

I know that the standard enthalpy of formation of an element in its most stable form is zero, but does that mean that the element begins in an unstable form and goes to its stable form, ex: N2 (l) ==> N2 (g)? Or would the element begin in its most stable form and then stay in its most stable form, e...
Wed Dec 06, 2017 3:00 am
Forum: Acidity & Basicity Constants and The Conjugate Seesaw
Topic: Ka and Concentration
Replies: 3
Views: 434

Ka and Concentration

Is Ka the same thing as [H3O+]?
Mon Dec 04, 2017 11:10 am
Forum: Properties & Structures of Inorganic & Organic Acids
Topic: Using The Equilibrium Arrow
Replies: 5
Views: 1386

Re: Using The Equilibrium Arrow

You use the equilibrium arrow for weak acids and bases because they don't fully dissociate. Strong acids and bases completely dissociate, so you would use a regular arrow for equations with strong acids/bases.
Mon Dec 04, 2017 11:08 am
Forum: Significant Figures
Topic: Significant Figures [ENDORSED]
Replies: 3
Views: 390

Re: Significant Figures[ENDORSED]

I'm not sure if you would get full credit, but try to round as little as possible in the course of solving the problem until you get to the very end so that your answer is as correct as possible.
Thu Nov 30, 2017 12:22 am
Forum: Naming
Topic: aqua = (OH2) or (H20)
Replies: 2
Views: 299

Re: aqua = (OH2) or (H20)

OH2 is written with the O first because the O is the atom with which bonds are formed.
Thu Nov 30, 2017 12:22 am
Forum: Non-Equilibrium Conditions & The Reaction Quotient
Topic: Quotient
Replies: 3
Views: 396

Re: Quotient

Q is the reaction quotient, while K is the equilibrium quotient. They both follow the same expression -- the product of the concentrations or partial pressures of each product raised to the power of its stoichiometric coefficient over the product of the concentrations or partial pressures of each re...
Sat Nov 25, 2017 8:22 pm
Forum: Ideal Gases
Topic: Units of Partial Pressure
Replies: 7
Views: 638

Re: Units of Partial Pressure

Partial pressure units should usually be atm. However, sometimes other units may be used, so be careful to convert then!
Sat Nov 25, 2017 8:21 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Polarity of NH3?
Replies: 4
Views: 8892

Re: Polarity of NH3?

NH3 is polar because it has 3 dipoles that do not cancel out. Each N-H bond is polar because N is more electronegative than H. NH3 is overall asymmetrical in its VSEPR shape, so the dipoles don't cancel out and it is therefore polar.
Wed Nov 15, 2017 12:46 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Regions of electron density
Replies: 4
Views: 319

Re: Regions of electron density

Yes, it does!
Wed Nov 15, 2017 12:45 pm
Forum: Hybridization
Topic: Notation
Replies: 6
Views: 447

Re: Notation

Px, Py, and Pz are simply the three orbitals in the P subshell (the three horizontal lines we draw). You don't necessarily need to write them out; usually we would just write P2, P3, etc. P(however many electrons are in the P subshell).
Wed Nov 08, 2017 3:35 pm
Forum: Bohr Frequency Condition, H-Atom , Atomic Spectroscopy
Topic: Principal Energy Level Change Sig Figs
Replies: 2
Views: 252

Principal Energy Level Change Sig Figs

When you're calculating the frequency emitted from a jump in principal energy levels (i.e. n=1 to n=3), how would you determine the significant figures needed for the answer?
Wed Nov 08, 2017 8:51 am
Forum: Lewis Structures
Topic: Organic Molecules [ENDORSED]
Replies: 1
Views: 224

Organic Molecules[ENDORSED]

Would we be expected to know how to draw the Lewis structure for a lot of organic molecules on the midterm? Or would we mainly be drawing Lewis structures for inorganic compounds?
Fri Nov 03, 2017 11:16 am
Forum: Ionic & Covalent Bonds
Topic: Covalent/ Ionic Bond
Replies: 5
Views: 424

Re: Covalent/ Ionic Bond

A metal and a non-metal will typically form an ionic bond, whereas non-metals bonding together will typically form a covalent bond!
Tue Oct 31, 2017 2:04 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Homework Problem 2.55
Replies: 5
Views: 388

Homework Problem 2.55

When the textbook refers to "Group 5 Transition Metals", does that mean Group 5 where Group 1 is the alkali metals? Or does it mean Group 5 where Group 1 is the first column of transition metals (Sc, Y, etc.)?
Mon Oct 23, 2017 5:47 pm
Forum: Student Social/Study Group
Topic: Post All Chemistry Jokes Here
Replies: 8252
Views: 1444674

Re: Post All Chemistry Jokes Here

Did you know that you can cool yourself to -273.15˚C and still be 0k?
Mon Oct 23, 2017 5:41 pm
Forum: Properties of Electrons
Topic: Rydberg Equation [ENDORSED]
Replies: 6
Views: 592

Re: Rydberg Equation[ENDORSED]

If the electron is going from n=4 to n=2, then n=2 would be the final energy state and n=4 would be the initial energy state.
Tue Oct 17, 2017 9:33 pm
Forum: DeBroglie Equation
Topic: Particle-Specific/Light-Specific Equations
Replies: 1
Views: 215

Particle-Specific/Light-Specific Equations

In class, Professor Lavelle mentioned that the De Broglie equation can only be used for particles with rest mass, like electrons, and therefore can't be used for light. Are there any other equations that can only be used for particles with rest mass or that can only be used for light?
Mon Oct 16, 2017 9:32 pm
Forum: Properties of Light
Topic: Problem 1.23
Replies: 3
Views: 293

Re: Problem 1.23

Thompson W 3B wrote:I believe the conversion is given on a sheet of constants and equations, which is attached to the tests. Here is the sheet: https://lavelle.chem.ucla.edu/wp-conten ... ations.pdf

Thank you so much! I didn't see that earlier.
Mon Oct 16, 2017 8:09 pm
Forum: Properties of Light
Topic: Problem 1.23
Replies: 3
Views: 293

Problem 1.23

Problem 1.23 gives the energy of gamma ray photons in kiloelectronvolts. Do we have to know the conversion between KeV and joules or do you guys think that energy will be given to us in J?
Sun Oct 15, 2017 9:58 pm
Forum: Properties of Light
Topic: Speed of Light Constant
Replies: 18
Views: 1039

Re: Speed of Light Constant

2.998 x 10^8 m/s is more accurate, but Professor Lavelle uses 3.00 x 10^8 in his lecture slides and example problems, so I don't think using 3.00 x 10^8 would be an issue.
Tue Oct 10, 2017 11:53 pm
Forum: DeBroglie Equation
Topic: Post-Module Assessment Q. 34
Replies: 4
Views: 320

Re: Post-Module Assessment Q. 34

I think your answer is like correct, meaning approx. close to the answers given b/c when you round the answer you got and change it to scientific notation, you should get 1.00 * 10^5 m/s and since it's way less than speed of light, which if the fastest speed known so far, answer should be reasonabl...
Sat Oct 07, 2017 10:01 pm
Forum: Balancing Chemical Reactions
Topic: Determining order of elements to work with when balancing equations [ENDORSED]
Replies: 2
Views: 330

Re: Determining order of elements to work with when balancing equations[ENDORSED]

No, but sometimes starting with the least-occurring element when balancing a chemical reaction gives you an easier starting place for balancing a reaction.
Sat Oct 07, 2017 9:51 pm
Forum: SI Units, Unit Conversions
Topic: Significant Figures in relation to molar mass [ENDORSED]
Replies: 4
Views: 7598

Re: Significant Figures in relation to molar mass[ENDORSED]

Keep the molar mass as precise as possible while calculating. You should try to round as little as possible until you reach your final answer! Then you should be fine.

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