Search found 50 matches

Tue Mar 13, 2018 10:25 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Rate Determining Step
Replies: 2
Views: 225

Re: Rate Determining Step

I think the rate determining step would be the slowest of the slow steps overall.
Tue Mar 13, 2018 10:23 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: Catalysts
Replies: 2
Views: 211

Re: Catalysts

Catalysts increase the k value in the rate law by lowering the activation energy of the reaction. They do not show up when writing the overall reaction and are not directly included in the rate law, only indirectly by changing the k value.
Tue Mar 13, 2018 10:18 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Replies: 4
Views: 360

Adsorption is when the catalyst is a solid and the reaction takes place on its surface. This image is an example to help visualize:
Sun Mar 11, 2018 9:52 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: 15.71
Replies: 2
Views: 173

Re: 15.71

H2O cannot be an intermediate because it is one of the reactants in the overall reaction.
Sun Mar 11, 2018 9:50 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: 15.65 (c)
Replies: 2
Views: 227

Re: 15.65 (c)

This can be seen in the Arrhenius equation: ln(k)=-Ea/RT+ln(A) where Ea is the activation energy. The slope of the line plotted on a ln(k) vs 1/T graph is -Ea/R. This means that for higher activation energies, the slope is more steeply negative so the ln(k) value, and also the k value, changes more ...
Sun Mar 11, 2018 9:18 pm
Forum: First Order Reactions
Topic: Pseudo First Order Reaction
Replies: 7
Views: 1084

Re: Pseudo First Order Reaction

We use pseudo first order reactions when we are studying a reaction with multiple reactants with concentrations that could affect the rate law. For example if the reaction in question is A+B-->P we would design an experiment to find out how the rate changes with the concentrations of each reactant. ...
Fri Mar 02, 2018 12:07 am
Forum: Kinetics vs. Thermodynamics Controlling a Reaction
Topic: Straight line
Replies: 7
Views: 821

Re: Straight line

Yes, the straight line is a best fit line to the data. When trying to determine the order of a reaction from the data, you need to plot the data and see what operation you need to do to make the best fit curve be a straight line instead of an exponential decay or other nonlinear graph. For example, ...
Thu Mar 01, 2018 10:31 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Salt Bridge [ENDORSED]
Replies: 1
Views: 171

Re: Salt Bridge[ENDORSED]

Some reactions, including concentration cells, use porous discs in the center instead of a salt bridge. This would be drawn as a single line, not a double line, in the center of the cell diagram.
Thu Mar 01, 2018 7:57 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Fractional Reaction Orders [ENDORSED]
Replies: 2
Views: 227

Fractional Reaction Orders[ENDORSED]

What does it mean for there to be a fractional (for example 1.5) reaction order and how does this happen? Will we have to know/work with reactions like this on exams? Also, is having a fractional reaction order common?
Sun Feb 25, 2018 7:23 pm
Forum: Second Order Reactions
Topic: Question about half-life regarding hw 15.35
Replies: 1
Views: 189

Re: Question about half-life regarding hw 15.35

I think you can't just do the half-life times four because the half-life is 50.5 s when [A]0=0.84 mol/L, but the half-life changes as the concentration changes, so later in the reaction it is a different value.
Sun Feb 25, 2018 7:09 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Electrochemical Series
Replies: 3
Views: 308

Re: Electrochemical Series

There is one in the appendix of the textbook on page A17 if you want an example.
Sun Feb 25, 2018 6:32 pm
Forum: Method of Initial Rates (To Determine n and k)
Topic: figuring out n and m
Replies: 3
Views: 342

Re: figuring out n and m

With the equations we have so far (as of end of week 7) I'm pretty sure that we need to be given the concentrations in order to calculate a numerical value for n and m.
Sun Feb 18, 2018 10:09 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Concentration Cells
Replies: 5
Views: 399

Re: Concentration Cells

Concentration cells can be used to make a simple battery (using only a difference in concentration not substance) but not a very strong one
Sun Feb 18, 2018 8:14 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Order in Cell Diagrams
Replies: 4
Views: 320

Re: Order in Cell Diagrams

Can you please clarify? In what context do you mean?
Sun Feb 18, 2018 7:58 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Nernst Equation
Replies: 4
Views: 441

Re: Nernst Equation

The Nernst equation allows us to calculate E based on the standard electric potential, temperature, and concentrations of a redox reaction.
Sun Feb 11, 2018 3:33 pm
Forum: Balancing Redox Reactions
Topic: E cell potiential
Replies: 2
Views: 171

Re: E cell potiential

Yes, n is referring to the number of moles of electrons transferred in the redox reaction, and this number can be found in the balanced equation.
Sun Feb 11, 2018 3:26 pm
Forum: Balancing Redox Reactions
Topic: Nature
Replies: 5
Views: 376

Re: Nature

In nature, and in the lab, all redox reactions involve both oxidation and reduction. This is due to the conservation of electrons. They have to go somewhere!
Sun Feb 11, 2018 3:22 pm
Forum: Balancing Redox Reactions
Topic: Reduction and Oxidation
Replies: 4
Views: 280

Re: Reduction and Oxidation

When the oxidation number increases, it is being oxidized, and if the oxidation number decreases, it is being reduced.
Sun Feb 04, 2018 10:29 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Open, closed, or isolated test 1 question
Replies: 10
Views: 928

Re: Open, closed, or isolated test 1 question

A vial filled with an organic solvent is a closed system (if the vial has a lid) because no matter can enter of leave the vial, but energy can enter it by heating or cooling it.
Sun Feb 04, 2018 6:29 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: delta G
Replies: 3
Views: 198

Re: delta G

I'm not really sure what you mean by minimum temperature (minimum temperature of what?), but delta G has to be zero at phase changes (ex. at boiling point and melting point) because this is the point at which the process goes from being spontaneous (negative delta G) to not spontaneous (positive).
Sun Feb 04, 2018 3:42 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Deriving Equations
Replies: 2
Views: 207

Re: Deriving Equations

I'm pretty sure Dr. Lavelle told us in lecture that we did not have to be able to mathematically derive the equations for the test but that being able to do so is helpful for understanding what the equations mean so that we can apply them correctly.
Sun Jan 28, 2018 10:03 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Isothermal Reversible
Replies: 3
Views: 483

Re: Isothermal Reversible

Yes, a change in temperature makes the reaction not reversible because all reversible reactions are isothermal. However, it is also possible to have an irreversible isothermal reaction.
Sun Jan 28, 2018 9:55 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Boltzmann equation: positional entropy or thermal entropy?
Replies: 4
Views: 257

Re: Boltzmann equation: positional entropy or thermal entropy?

According to the textbook the Boltzmann equation calculates "statistical entropy," which I think takes both positional and thermal entropy into account.
Sun Jan 28, 2018 9:51 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Replies: 1
Views: 154

I think that either this equation is assuming that the substance is a perfect crystal or that it is an approximation and that the positional entropy of the substance at 0K is negligible when compared with the deltaS term.
Sun Jan 21, 2018 4:58 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Units of Standard Enthalpies of Formation
Replies: 2
Views: 160

Re: Units of Standard Enthalpies of Formation

My TA writes it as kJ/mole of reaction, but said it doesn't really matter if you use kJ or kJ/mol. I think just make sure to keep track of your units in your work and use that.
Sun Jan 21, 2018 4:50 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Standard Enthalpies of Formation
Replies: 3
Views: 222

Re: Standard Enthalpies of Formation

Using standard enthalpies of formation and using bond enthalpies are actually two different methods of calculating the enthalpy of the reaction. If you are given standard enthalpies of formation (SEF), do the sum of the SEF of the products minus the sum of the SEF of the reactants to get the enthalp...
Sun Jan 21, 2018 4:32 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: state property
Replies: 3
Views: 245

Re: state property

Heat and work are not state functions because they depend on the path taken (like distance traveled), while enthalpy is a state function because it does not depend on what happens between the initial and final states (like elevation gain). For example, if there is a reaction in which there is no cha...
Sun Jan 14, 2018 10:32 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Extensive vs Intensive Properties
Replies: 3
Views: 197

Re: Extensive vs Intensive Properties

An extensive property depends on the amount of a substance there is, while an intensive property is the same no matter how much of a specific substance there is. Heat capacity depends on the amount of a substance because it takes more heat to raise the temperature of a larger sample so it is an exte...
Sun Jan 14, 2018 7:31 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Standard Reaction Enthalpy Definition
Replies: 7
Views: 371

Re: Standard Reaction Enthalpy Definition

Also, Dr. Lavelle said that standard reaction enthalpy is at 1 atm, but my TA said it was at 1 bar. Which is it?
Sun Jan 14, 2018 7:24 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.57
Replies: 1
Views: 128

Re: 8.57

You can only use the products-reactants method if you are given the heats of formation and this problem gives heats of combustion instead. The answer book manipulates (adds, reverses, multiplies by a constant) the given combustion equations until getting the equation: C2H2 + 2H2 --> C2H6. The same o...
Sat Dec 09, 2017 7:58 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Change in Equilibrium Constant
Replies: 2
Views: 310

Re: Change in Equilibrium Constant

Yes the equilibrium constant will change if the stoichiometric coefficients are halved. For example for the reaction 2A <--> 3B K1 = [B]^3 / [A]^2 which is not the same as the equilibrium constant of the reaction A <--> 3/2B K2 = [B]^3/2 / [A] because the coefficients become the exponents in the K e...
Sat Dec 09, 2017 7:19 pm
Forum: Significant Figures
Topic: Logarithms
Replies: 3
Views: 293

Logarithms

Can someone please explain the sig fig rules for logarithms? And why do they make sense? Why can't we just use the number of sig figs in the given numbers in the question?
Sun Dec 03, 2017 6:34 pm
Forum: Amphoteric Compounds
Topic: d-block elements
Replies: 1
Views: 240

Re: d-block elements

I found a past post that might help: "Re: particular d-block metals form amphoteric oxides? Postby Chem_Mod » Sun Aug 21, 2011 10:46 am We covered metal oxides and hydroxides (e.g., NaOH) which are basic compounds. We also covered non-metal oxides (e.g., H2SO4, H3PO4, etc.,) which are acidic co...
Sun Dec 03, 2017 6:16 pm
Forum: Properties & Structures of Inorganic & Organic Acids
Topic: difference in acids
Replies: 3
Views: 353

Re: difference in acids

Also, the Lewis definition is more general than the Bronsted definition (not all Lewis acids are Bronsted acids, but all Bronsted acids are Lewis acids)
Sun Nov 26, 2017 7:41 pm
Forum: Naming
Topic: 17.31 part c
Replies: 2
Views: 262

Re: 17.31 part c

[Co(NH3)4(H20)2] has an overall charge of +3 and each bromide ion has a charge of -1, so three bromide ions are needed to balance the charges in this ionic compound, hence the subscript 3.
Sun Nov 26, 2017 7:34 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: 4.11
Replies: 1
Views: 166

Re: 4.11

The answer book says that the "arrangement of electron pairs will be trigonal bipyramidal ... the actual structure is described as a seesaw" for this problem (page 98). When classifying the shape of a molecule we do not count lone pairs, so we get seesaw, but if we did count lone pairs as ...
Sun Nov 19, 2017 4:47 pm
Forum: Naming
Topic: Hg(CH3)
Replies: 2
Views: 300

Re: Hg(CH3)

I looked online and could not find dimethylmercury being referred to as dimethylmercury(0). Where did you see it called that? But you are right that it is Hg(II).
Tue Nov 14, 2017 9:14 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Chapter 4 #25
Replies: 1
Views: 260

Re: Chapter 4 #25

This question is actually really misleading because while it looks like you can place the Cl atoms opposite each other and therefore cancel the dipole moments if you just use the Lewis Structure, if you look at the molecule's shape you can see that this doesn't work. CH2Cl2 has four regions of elect...
Sun Nov 12, 2017 5:10 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Trigonal Bipyramidal
Replies: 2
Views: 178

Re: Trigonal Bipyramidal

The non-central atoms in a trigonal bipyramidal molecule don’t have to be the same. For example PF3Cl2, where P is the central atom and both F and Cl surround it.
Wed Nov 08, 2017 10:48 pm
Forum: Bond Lengths & Energies
Topic: Homework Problem 3.87
Replies: 5
Views: 536

Re: Homework Problem 3.87

F is the smallest atom among F, Cl, and Br, meaning that the internuclear distance between the fluorine and carbon would be the smallest even while ignoring any effects by the relative electronegativities. This means the F-C bond is the shortest of the three.
Sat Nov 04, 2017 10:19 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Trigonal planar vs. Trigonal bipyramidal
Replies: 4
Views: 798

Re: Trigonal planar vs. Trigonal bipyramidal

Wed Nov 01, 2017 8:59 pm
Forum: Ionic & Covalent Bonds
Topic: Ionic vs. Covalent Bonds
Replies: 3
Views: 470

Re: Ionic vs. Covalent Bonds

Also, some molecules with polar covalent bonds are overall nonpolar due to their symmetry. For example, CO2 is a nonpolar molecule even though the C to O bond is polar covalent (electronegativities: 3.5-2.5=1 which is between .5 and 1.5). This is because it is symmetrical, so the partial negative ch...
Sun Oct 29, 2017 9:36 pm
Forum: Lewis Structures
Topic: Placement [ENDORSED]
Replies: 6
Views: 468

Re: Placement[ENDORSED]

What if there are multiple of the atom with the lowest ionization energy (for example 4 carbon atoms)? Which one would be central and how would they be arranged? Can we predict the shape of the Lewis structures for complicated molecules?
Tue Oct 24, 2017 10:27 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Shells, Subshells, and Orbitals
Replies: 4
Views: 488

Re: Shells, Subshells, and Orbitals

To elaborate on orbitals, there are three orbitals in the p-subshell (px, py, and pz) that are determined by the ml quantum number. You can think of the orbitals as the different "orientations" that the subshell can have. The d-subshell has five orbitals. Each orbital can have two electron...
Sun Oct 22, 2017 1:29 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Cartesian axes [ENDORSED]
Replies: 2
Views: 420

Re: Cartesian axes[ENDORSED]

The cartesian axes are the x-,y-, and z-axes representing the three physical dimensions. In other words they are directions in space (forward-backward, left-right, and up-down). The orientations of the orbitals are defined along these axes in order to make the math work. There is no "universal&...
Wed Oct 18, 2017 9:45 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Ground state vs excited state
Replies: 3
Views: 386

Re: Ground state vs excited state

If an electron is in its ground state, for example 1s, and is excited, is there a difference in energy between being excited to the 2s vs 2p orbitals? We were told in class that the first quantum number is the one that determines energy but also that electrons fill the s orbitals before the p orbita...
Sat Oct 14, 2017 9:17 pm
Forum: Properties of Electrons
Topic: Wave Properties of Electrons [ENDORSED]
Replies: 4
Views: 429

Re: Wave Properties of Electrons[ENDORSED]

Does that mean that electrons have different wavelengths? How would you find the wavelength or frequency of an electron?
Tue Oct 10, 2017 9:02 pm
Forum: Properties of Light
Topic: Problem 1.4
Replies: 3
Views: 297

Re: Problem 1.4

I'm pretty sure that this is a trick question and none of the answers are correct (but I think the wording is open to the possibility of there being no correct answer) A) This isn't true because all EM radiation travels at a constant speed in a vacuum and they did not say that there was a medium B) ...
Sat Oct 07, 2017 5:53 pm
Forum: Significant Figures
Topic: Sig Figs in a problem with addition & multiplication [ENDORSED]
Replies: 5
Views: 1640

Re: Sig Figs in a problem with addition & multiplication[ENDORSED]

If the problem has both addition and multiplication, do we use the addition/subtraction rule or the multiplication/division rule for determining the number of sig figs in the final answer?
Thu Oct 05, 2017 5:50 pm
Forum: Accuracy, Precision, Mole, Other Definitions
Topic: Formula Units Usage
Replies: 3
Views: 433

Re: Formula Units Usage

A formula unit is a group of ions. You can think of it as a "molecule" of an ionic compound such as NaCl (table salt). When the textbook asks for the amount of a compound in formula units it is asking for a count of formula units or "How many formula units are there in this sample.&qu...