## Search found 48 matches

Mon Mar 05, 2018 10:46 am
Forum: General Rate Laws
Topic: Rate laws vs average rate
Replies: 2
Views: 94

### Re: Rate laws vs average rate

Thank you! That helped a lot :)
Sun Mar 04, 2018 10:49 am
Forum: General Rate Laws
Topic: Rate laws vs average rate
Replies: 2
Views: 94

### Rate laws vs average rate

Hi can someone explain to me the difference between the average rate of a reaction and the rate laws? when should I use each equation?
Wed Feb 28, 2018 6:11 pm
Forum: Student Social/Study Group
Topic: Post All Chemistry Jokes Here
Replies: 7337
Views: 823892

### Re: Post All Chemistry Jokes Here

If you're not part of the solution,

.
.
.
.

You're part of the precipitate.
Wed Feb 28, 2018 6:05 pm
Forum: Student Social/Study Group
Topic: Post All Chemistry Jokes Here
Replies: 7337
Views: 823892

### Re: Post All Chemistry Jokes Here

Wed Feb 28, 2018 5:49 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: changing sign of of standard cell potentials
Replies: 4
Views: 130

### Re: changing sign of of standard cell potentials

So, if we are using the reduction electrode potentials, we do not change the sign when we imput the voltages into the equation?
Tue Feb 13, 2018 3:58 pm
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: Molar entropies
Replies: 5
Views: 340

### Re: Molar entropies

okay, thanks, also, what is the difference between standard molar entropies and statistical entropies?
Mon Feb 12, 2018 3:01 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: W
Replies: 2
Views: 131

### Re: W

But, I know it might not be asked on the exam, Im just wondering, how do we determine the number of arrangements? for example CO can be arranged as CO and OC, but it can also be vertically arranged with O on top and C in the bottom and vice versa. What about the z plane? and all of the other planes?
Mon Feb 12, 2018 2:57 pm
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: Molar entropies
Replies: 5
Views: 340

### Re: Molar entropies

Okay, I get that conceptually, but how do we calculate them? Do we measure them experimentally or?
Mon Feb 12, 2018 11:12 am
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: Molar entropies
Replies: 5
Views: 340

### Molar entropies

I am confused, what are standard molar entropies and what do we use them for?
Mon Feb 12, 2018 7:42 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: W
Replies: 2
Views: 131

### W

I understand W = number of arrangements ^number of atoms, but how do we determine the number of arrangements?
Sun Feb 11, 2018 9:36 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Practice Midterm 3A
Replies: 3
Views: 153

### Re: Practice Midterm 3A

Also, take in mind that if T is increasing, there will be a change in P or V to make up for it, given that PV = nRT.
Sun Feb 11, 2018 9:35 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Practice Midterm 3A
Replies: 3
Views: 153

### Re: Practice Midterm 3A

Use Cv when the volume is constant and Cp when the pressure is constant.
Sun Feb 11, 2018 9:33 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: "Ideal"
Replies: 6
Views: 205

### Re: "Ideal"

I believe the term "ideal" is used to describe gases, not systems. Ideal gases are those in which the forces of attraction between the molecules are negligible, and the molecules occupy a very small fraction of the space (taken to be as zero). You must know a gas is ideal before getting in...
Sun Feb 11, 2018 12:45 pm
Forum: Calculating Work of Expansion
Topic: Isothermal Irreversible Expansion
Replies: 3
Views: 144

### Re: Isothermal Irreversible Expansion

Sorry! Yes, absolutely, I meant Irreversible expansion.
Wed Jan 31, 2018 8:49 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Celsius or Kelvin
Replies: 5
Views: 149

### Re: Celsius or Kelvin

When in doubt, it is always best to convert the temperature to Kelvin by adding 273.15.
Wed Jan 31, 2018 8:46 pm
Forum: Calculating Work of Expansion
Topic: Work Equations [ENDORSED]
Replies: 9
Views: 307

### Re: Work Equations[ENDORSED]

It depends in the situation. If the volume is constant, ΔV = 0 so w = 0. If the pressure is constant, w = -P(ext)ΔV. If you have an isothermal reversible expansion (both pressure and volume change by infinitely small amount), w = -nRTln(V2/V1). I hope that helps?
Wed Jan 31, 2018 8:42 pm
Forum: Calculating Work of Expansion
Topic: Isothermal Irreversible Expansion
Replies: 3
Views: 144

### Re: Isothermal Irreversible Expansion

Yes! absolutely. If the system expands fast, and the energy lost by work is replaced by heat, you can have an isothermal reversible expansion. Take in mind that, reversible expansion will do more work overall however (the area under the curve is larger).
Sun Jan 21, 2018 5:26 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: ∆H equation
Replies: 3
Views: 123

### Re: ∆H equation

There are two different equations. The first one, ΔH = mCΔT is found using the specific heat capacity of the substance, and m stands for the mass of the substance in grams. The second one, ΔH = nCΔT is found using the molar heat capacity of the substance, and n stands for the number of mol of the su...
Sun Jan 21, 2018 5:22 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: ΔH = ΔU + PΔV
Replies: 2
Views: 128

### Re: ΔH = ΔU + PΔV

Sat Jan 20, 2018 1:05 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: ΔH = ΔU + PΔV
Replies: 2
Views: 128

### ΔH = ΔU + PΔV

I dont understand why enthalpy is defined as ΔH = ΔU + PΔV. Can someone please explain where this equation came from to me?
Sat Jan 20, 2018 1:03 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Enthalpy
Replies: 4
Views: 90

### Re: Enthalpy

Bond enthalpies refer to the energy required to break a specific bond. The values given for diatomic molecules are absolute values, while the values given for any other bond enthalpy are averages. The enthalpy of formation of a given substance represents the heat released or absorbed when one mol of...
Sat Jan 20, 2018 12:52 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: ∆H equation
Replies: 3
Views: 123

### Re: ∆H equation

If you are referring to the equation q= nCmΔT, the molar heat capacity of the substance will tell you how many joules are necessary to increase the temperature of one mol of the substance by 1 degree Celsius or Kelvin. By substituting each value into this equation, you will get a ΔH in joules per mo...
Wed Jan 17, 2018 6:57 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Energy Change in an Isolated System
Replies: 4
Views: 149

### Re: Energy Change in an Isolated System

The internal energy of the system will not change, because no work or heat can be applied to the system, so there is no point in finding delta u. Delta U for this case will always equal 0.
Wed Jan 17, 2018 6:05 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Question 8.29
Replies: 2
Views: 121

### Question 8.29

In relation to this question, why do more complex molecules tend to have a higher specific/molar heat capacity? What are other factors that affect how high is the heat capacity of a substance?
Wed Jan 17, 2018 5:24 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Q 8.13
Replies: 9
Views: 303

### Re: Q 8.13

I don't understand why the final answer is negative. Can someone explain it to me? thanks!
Sun Jan 14, 2018 12:54 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Standard Enthalpy of Formation (pure substance)
Replies: 3
Views: 336

### Re: Standard Enthalpy of Formation (pure substance)

Remember that the standard enthalpy of formation of any substance is the change in heat that takes place when one mol of such substance is formed from its constituent elements in their standard state. O2 (g) is formed from O2(g), so, to form one mol of O2 (g), one mole of O-O bonds need to be broken...
Sun Jan 14, 2018 12:47 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Bond Enthalpies
Replies: 3
Views: 149

### Re: Bond Enthalpies

Absolutely! If you have a molecule in which a double bond can exist in two different positions, the enthalpy of both bonds is affected by resonance. The bond enthalpy of each bond should be about 1.5 times the enthalpy of the single bond.
Sun Jan 14, 2018 12:39 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Reaction Enthalpy vs. Enthalpy of Formation
Replies: 2
Views: 99

### Re: Reaction Enthalpy vs. Enthalpy of Formation

A reaction enthalpy refers to the total heat emitted or absorbed by any chemical reaction at a constant pressure. The enthalpy of formation of a substance X refers to the change in heat that takes place when one mol of X is formed by its constituent elements in their standard states. In other words,...
Sun Dec 10, 2017 2:10 am
Forum: Administrative Questions and Class Announcements
Topic: Final
Replies: 3
Views: 277

### Re: Final

how do we know which of the four locations we should go to?
Sun Dec 10, 2017 2:07 am
Forum: Hybridization
Topic: sp3 hybridization
Replies: 6
Views: 333

### Re: sp3 hybridization

Lone pairs count as an area of electron density in such way that, when hybrid orbitals are formed, each lone pain occupies one of these new orbitals. In H2O, the lone pairs each occupy one sp3 orbital. The two other sp3 orbitals form covalent bonds with the two hydrogens. This results in four sp3 or...
Sun Dec 10, 2017 2:00 am
Forum: Ideal Gases
Topic: Value of R
Replies: 3
Views: 524

### Re: Value of R

Be careful not to mix up this constant with Ryderberg's constant. The use of the same letter for their abbreviation often leads to confusion.
Sun Dec 10, 2017 1:57 am
Forum: Calculating the pH of Salt Solutions
Topic: When to use ICE table?
Replies: 3
Views: 1479

### Re: When to use ICE table?

ICE tables allow you to find the equilibrium concentrations of any equilibrium reaction, using only the initial concentration of the reactants. They also help you to find Ka and Kb for weak acids and bases, which will allow you to determine their relative strengths.
Sun Dec 10, 2017 1:50 am
Forum: Administrative Questions and Class Announcements
Topic: Saying Thank You to Dr. Lavelle
Replies: 200
Views: 45352

### Re: Saying Thank You to Dr. Lavelle

Dr Lavelle, Thank you for coming to class in a good mood everyday. Thank you for your patience and for the effort you put in giving us all possible tools for us to succeed in your class. Your explanations are simple and straightforward, you make difficult chemistry easy to understand. See you next q...
Sat Nov 18, 2017 10:57 pm
Forum: Shape, Structure, Coordination Number, Ligands
Topic: Defining a Ligand
Replies: 3
Views: 181

### Re: Defining a Ligand

A ligand is a molecule with a lone pair of electrons. It forms coordinate (dative) bonds with transition metal cations. If it only has one lone pair, it is called "monodentate", if it has more than one lone pair and can form more than one coordinate bond, it is called "polydentate&quo...
Sat Nov 18, 2017 10:54 pm
Forum: Shape, Structure, Coordination Number, Ligands
Topic: Understanding sigma and pi bonds?
Replies: 8
Views: 308

### Re: Understanding sigma and pi bonds?

Given that pi bonds form above and bellow the internuclear axis, they hold the atoms in place, making it impossible for them to rotate.

I also wanted to add that single bonds are sigma bonds, and triple and double bonds are pi bonds.
Sat Nov 18, 2017 10:45 pm
Forum: Shape, Structure, Coordination Number, Ligands
Topic: Chelating Complexes
Replies: 5
Views: 216

### Re: Chelating Complexes

Chelating complexes are, in oder words, polydentate ligands. The lone pairs that will form the coordinate bonds must be at the right distance from one another so that they can both bond to the TM cation. The Ideal distance is two atoms in between the lone pairs for linear complexes. However, in the ...
Sat Nov 18, 2017 2:53 pm
Forum: Shape, Structure, Coordination Number, Ligands
Topic: Polydentate ligands and shape
Replies: 1
Views: 131

### Polydentate ligands and shape

EDTA is a hexadentate ligand, so it forms six coordinate bonds with Fe(III). Would that make the structure of the complex octahedral even when the six bonds formed are with the same ligand? Would the coordination number still be 6 even if iron is binding to one ligand?
Fri Nov 17, 2017 9:07 am
Forum: Hybridization
Topic: Hybrid orbitals
Replies: 4
Views: 209

### Re: Hybrid orbitals

When I first read the textbook I was a bit confused as well with regards to what those equations meant and how they related to hybrid orbitals. After reading it a few times, I think the textbook uses those equations better explain how they derive a hybridized molecule. The signs of each of the orbi...
Thu Nov 16, 2017 12:56 pm
Forum: Hybridization
Topic: Hybridization and lone pairs
Replies: 2
Views: 168

### Hybridization and lone pairs

Water has four regions of electron density, so it should have 4 sp^3 orbitals. Can oxygen's lone pairs of electrons occupy two sp^3 orbitals, or do they make oxygen have a different type of hybridization? Is there a different rule for hybridization of central atoms with lone pairs?
Thu Nov 16, 2017 12:44 pm
Forum: Hybridization
Topic: Hybrid orbitals
Replies: 4
Views: 209

### Hybrid orbitals

In the textbook, hybrid orbitals are described as a linear mix of atomic orbitals, as given by the following formulas
h1 =s +px +py +pz, h2= s -px -py +pz, h3= s -px +py -pz, but I don't understand what the formulas tell us, can someone explain them to me? Thank you!
Thu Nov 16, 2017 12:37 pm
Forum: Dipole Moments
Topic: Dipole Moments
Replies: 2
Views: 143

### Dipole Moments

Is there any way to find out what the magnitude of a dipole moment is?
Sun Nov 05, 2017 11:10 am
Forum: Electronegativity
Topic: Comparing Electronegativity of Elements
Replies: 5
Views: 406

### Re: Comparing Electronegativity of Elements

The general trend for electronegativity is that it decreases down the group and increases across the period. The elements in the p block (except the noble gasses) tend to have high values of electronegativity because they want to gain electrons to become isoelectronic with a noble gas. On the other ...
Sun Oct 29, 2017 12:56 pm
Forum: Quantum Numbers and The H-Atom
Topic: Spin magnetic quantum number
Replies: 1
Views: 111

### Spin magnetic quantum number

Hi! I dont understand why is it more favorable for electrons in the same orbital to have oposite spins. Does it lower the orbital's potential energy?
Sat Oct 28, 2017 6:18 pm
Forum: *Particle in a Box
Topic: En= (n^2*h^2)/(8*m*L^2)
Replies: 1
Views: 222

### En= (n^2*h^2)/(8*m*L^2)

Hello! I was wondering what type of questions we might get in which we will have to use this equation? Also, should we know how to apply this formula for next week's test?
Sat Oct 28, 2017 5:25 pm
Forum: *Shrodinger Equation
Topic: Shrodinger Equation Applications [ENDORSED]
Replies: 7
Views: 356

### Re: Shrodinger Equation Applications[ENDORSED]

Hi! I have a question, does the wave function work as a visual representation of what atomic orbitals look like? In other words, do atomic orbitals look like their respective wave functions?
Mon Oct 23, 2017 11:04 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Chromium and Copper Electron Configuration Exceptions [ENDORSED]
Replies: 5
Views: 435

### Re: Chromium and Copper Electron Configuration Exceptions[ENDORSED]

Cr and Cu are exceptions because they are more 'stable' with a 4s^1 configuration, this makes me wonder, is there any way we can measure or investigate stability experimentally?
Mon Oct 23, 2017 10:51 pm
Forum: Bohr Frequency Condition, H-Atom , Atomic Spectroscopy
Topic: Negative Energy Near the Nucleus [ENDORSED]
Replies: 2
Views: 159

### Re: Negative Energy Near the Nucleus[ENDORSED]

Taking in mind that at n= infinity the potential energy of the electron is zero, the potential energy of the electron might be more negative as we approach the nucleus because the electron requires more energy to be released.
Thu Oct 12, 2017 8:44 pm
Forum: Einstein Equation
Topic: Experimental determination of wavelength/energy
Replies: 1
Views: 153

### Experimental determination of wavelength/energy

In order to find E we need v, and in order to find v we need E for E=hv. So, how are either E or v experimentally determined?