## Search found 51 matches

Sun Mar 18, 2018 10:54 am
Forum: Kinetics vs. Thermodynamics Controlling a Reaction
Topic: Kinetically controlled reactions
Replies: 2
Views: 291

### Kinetically controlled reactions

If a reaction has a high activation energy, does that mean it will be under kinetic control?
Sat Mar 17, 2018 4:03 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Reversible reactions
Replies: 1
Views: 252

### Reversible reactions

Is a reversible reaction always isothermal?
Thu Mar 15, 2018 4:34 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: 14.101 Concentration Cells
Replies: 1
Views: 166

### 14.101 Concentration Cells

The problem states that the concentration of K+ ions inside a nerve cells is about 20-30 times greater than the concentration of K+ ions outside the cell and then asks us to find the potential difference between the inside and outside of the cell. I understand that we need to use the equation: E = E...
Wed Mar 14, 2018 10:48 am
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: max work & ∆G
Replies: 1
Views: 158

### max work & ∆G

Under what conditions is the maximum work of a battery always equal to ∆G?
Sun Mar 11, 2018 11:03 pm
Forum: *Enzyme Kinetics
Topic: Enzyme
Replies: 5
Views: 1500

### Re: Enzyme

In every case I can think of, enzymes act as catalysts by lowering the activation energy of the reaction and speeding up the reaction. Cells use a lot of energy, so it is in their best interest to evolve enzymes that will speed up their chemical processes as opposed to slowing them down. Enzymes and...
Sun Mar 11, 2018 11:00 pm
Forum: Method of Initial Rates (To Determine n and k)
Topic: rate law
Replies: 5
Views: 492

### Re: rate law

Yes! Rate laws can ONLY be determined by experiment because it is impossible to know the order of a reaction or reactant simply by looking at the chemical equation. We can come up with proposed rate laws based on how we think things will react, but we need experiments to prove that these proposed ra...
Sun Mar 11, 2018 10:57 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Cell Diagrams
Replies: 6
Views: 373

### Re: Cell Diagrams

The anode is always on the left side of the cell diagram, and the cathode is on the right! Usually the parts of the cell diagram are written left to right, reactants going to products, but in some cases the order changes based on states of matter. Solids (such as inert electrodes) always go on the v...
Mon Mar 05, 2018 10:16 am
Forum: Second Order Reactions
Topic: Half-life of second order reactions
Replies: 2
Views: 467

### Half-life of second order reactions

I know that for first order half life problems, when the question give us the half life and asks us to find the time it takes until 1/8 of the original substance is left, we can simply find how many half-lives it has gone through and then multiply the half-life by that number. For example if the hal...
Mon Mar 05, 2018 10:08 am
Forum: Second Order Reactions
Topic: Half-lives
Replies: 2
Views: 264

### Re: Half-lives

You still need to use the first order integrated rate law because in this case you know t (1/2) and you are given a time, therefore you can use the equation: A(t) = A(0)e^-kt to find the number of molecules at 8 seconds, given by A(t). Because you found t (1/2) to be 5 seconds, you can plug 5 into o...
Sun Mar 04, 2018 4:07 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: slow reaction mechanisms
Replies: 4
Views: 267

### Re: slow reaction mechanisms

Like the saying a team is only as good as it's weakest link, the reaction rate is only as fast as it's slowest step. Thus, for a reaction with multiple steps the slowest step will be the rate determine step like we discussed in class.
Sat Feb 24, 2018 1:53 pm
Forum: Method of Initial Rates (To Determine n and k)
Topic: Order
Replies: 5
Views: 312

### Re: Order

As mentioned above, the order is the power to which the concentration of the reactant or product is raised in the rate law. The overall order of the reaction also helps you know what the time versus concentration graph is going to look like for that reaction, which I think we are going to cover more...
Sat Feb 24, 2018 1:46 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Inert Electrodes [ENDORSED]
Replies: 3
Views: 344

### Re: Inert Electrodes[ENDORSED]

Inert electrodes are necessary when you are dealing with a reaction taking place between aqueous solutions, liquids, or gasses- essentially any reaction where you have no solid present. When there is no solid present in the reaction there is nothing available to transfer electrons, so you need to ad...
Sat Feb 24, 2018 1:38 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 14.23 (c)
Replies: 3
Views: 195

### Re: 14.23 (c)

In general, order is not especially important as long as you have the anode reaction on the left side of the cell diagram and cathode reaction on the right side of the diagram. The order of each side is firstly dependent on the state of the reactants and products, in general solids come first, then ...
Tue Feb 20, 2018 3:49 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Rule of Thumb for Cell Diagrams
Replies: 3
Views: 193

### Re: Rule of Thumb for Cell Diagrams

Sometimes the cell diagrams use comas instead of lines to separate two reactants, why is this? When do we use a solid line and when do we use a coma?
Tue Feb 13, 2018 4:49 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: ∆U equation
Replies: 2
Views: 178

### ∆U equation

I was on chemistry community and I came across a UA worksheet on which the equation ∆U = 3/2nR∆T was used to solve for internal energy. I don't remember learning this in lecture and this equation isn't on the equation sheet, so should we know this equation? If so, when can we use this equation as op...
Tue Feb 13, 2018 4:44 pm
Forum: Calculating Work of Expansion
Topic: When to use w= -P∆V
Replies: 1
Views: 180

### When to use w= -P∆V

Does your system have to be at constant pressure and isothermal in order to use the equation w = -P∆V? I've come across a few problems where I tried to use it but got it wrong, was wondering if the system changing temperature is the reason I'm missing these questions?
Sat Feb 10, 2018 12:25 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Van't Hoff Equation
Replies: 3
Views: 208

### Re: Van't Hoff Equation

From my understanding the equation is not on the equation sheet but you should either memorize it or know how to derive it from the fact that
∆H - T∆S = -RT ln (K). It's pretty helpful for specific problems given K and ∆H, but I doubt you'll have to show the derivation if you do use the equation.
Sat Feb 10, 2018 12:20 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Units Confusion [ENDORSED]
Replies: 3
Views: 229

### Re: Units Confusion[ENDORSED]

When you're confused on units, usually the best thing to do is to track your units throughout the problem. This means writing out all the units for every number you put into your equation and then determining which units cancel out in the final answer. It also might be helpful to review which units ...
Sat Feb 10, 2018 12:13 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: entropy of irreversible and reversible reactions.
Replies: 2
Views: 239

### Re: entropy of irreversible and reversible reactions.

For a reversible reaction, the key is that the ∆S(total) is equal to 0. Because ∆S + ∆S (surroundings) = ∆S (total), when ∆S (total) is 0, this means that ∆S and ∆S (surroundings) have opposite signs. For example, if ∆S = 0.15 J/K for a reversible reaction, then ∆S (surroundings) must be equal to -0...
Mon Feb 05, 2018 9:22 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Reversible vs Irreversible reactions
Replies: 2
Views: 139

### Reversible vs Irreversible reactions

Can someone please explain the difference between reversible and irreversible reactions in terms of equations? How do I identify each system and how do we know which equations to use for each? For example problem 47 from chapter 9 (9.47) asks us to determine the entropy for an expanding gas by a) is...
Mon Feb 05, 2018 9:16 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Q vs. K
Replies: 2
Views: 157

### Re: Q vs. K

Finding K and finding Q are done through the exact same process, [products]^coefficients/[reactants]^coefficients, so yes both are raised to their stoichiometric coefficients. K is simply what we call the ratio when the reaction is at equilibrium, and Q is what we call the ratio at all other times t...
Mon Feb 05, 2018 9:10 pm
Forum: Balancing Redox Reactions
Topic: acronym for reduction
Replies: 5
Views: 516

### Re: acronym for reduction

As someone already mentioned "LEO the lion says GER" is a good way to remember that Losing Electrons = Oxidation and Gaining Electrons = Reduction! Another helpful acronym for remembering which part of the redox reaction occurs at the cathode vs the anode is "Red Cat" = REDuction...
Thu Jan 25, 2018 11:55 am
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Replies: 6
Views: 252

### Re: Confusion about entropy formula

I am also confused about reversible reactions, but I believe it may mean a reaction that can occur forwards and backwards without an increase in entropy.
Thu Jan 25, 2018 11:47 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Heat and temp difference
Replies: 2
Views: 192

### Re: Heat and temp difference

I'm not entirely sure what your question means, but for isothermal, reversible expansion of a gas we know that the gas is expanding at a constant temperature, therefore the system is doing work because the gas is expanding (changing volume). We know that work can be calculated by finding P∆V, so if ...
Wed Jan 24, 2018 4:27 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: homework 8.67
Replies: 3
Views: 262

### Re: homework 8.67

By definition, an enthalpy of formation can be found by writing the formation reaction for the compound with all the reactants in their naturally occurring states and a total production of one mole of the product. Once you have an accurate formation reaction written you can then use bond enthalpies ...
Fri Jan 19, 2018 8:30 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Difference between heat(q), internal energy(U), and enthalpy(H)
Replies: 4
Views: 2406

### Re: Difference between heat(q), internal energy(U), and enthalpy(H)

To answer the most recently asked question on this thread, heat (q) is simply a measure of the energy released or absorbed by a system. Enthalpy (∆H) is very similar except it measures the heat released or absorbed at a constant pressure. Just q does not necessarily imply that the reaction is at con...
Fri Jan 19, 2018 8:23 pm
Forum: Calculating Work of Expansion
Topic: 8.25
Replies: 1
Views: 100

### 8.25

This question is essentially a 2 part question, the first which asks us to find the specific heat capacity of the calorimeter which I understand how to do. The C(calorimeter) = 0.478 kJ/˚C The second part says that 100.0 mL of 0.200 M HBr(aq) was mixed with 100.0 mL of 0.200 M KOH(aq) in the same ca...
Fri Jan 19, 2018 8:08 pm
Forum: Calculating Work of Expansion
Topic: 8.13- negative signs?
Replies: 2
Views: 123

### 8.13- negative signs?

Question 13 from chapter 8 problems reads: In a combustion cylinder, the total internal energy change produced from the burning of a fuel is -2373kJ. The cooling system that surrounds the cylinder absorbs 947 kJ as heat. How much work can be done by the fuel in the cylinder? I understand that ∆U = q...
Thu Jan 11, 2018 9:42 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Heat Capacity: molar versus specific
Replies: 3
Views: 2093

### Heat Capacity: molar versus specific

I understand that there are two types of heat capacities, molar heat capacity and specific heat capacity. What is the difference between the two and what type of problems require each one/when do you use one as opposed to the other?
Thu Jan 11, 2018 9:33 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Standard Enthalpies vs. Bond Enthalpies Equations
Replies: 1
Views: 99

### Re: Standard Enthalpies vs. Bond Enthalpies Equations

Standard enthalpies take the form H= H(products) - H(reactants) because in this case the ∆H refers to the enthalpy required to form each one of these molecules/compounds. In this situation every ∆H has it's appropriate + or - sign depending on if the formation is endo- or exothermic, which means all...
Thu Jan 11, 2018 9:17 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Celsius the same as Kelvin? [ENDORSED]
Replies: 9
Views: 2611

### Re: Celsius the same as Kelvin?[ENDORSED]

Like Deap mentioned, for this question all that matters is the delta T (change in temperature) not the actual units of temperature so you can use kelvin or celsius because the change in temperature will be the same regardless of which units you use. For example the change between 35ºC and 20ºC would...
Thu Dec 07, 2017 12:12 pm
Forum: Shape, Structure, Coordination Number, Ligands
Topic: How to tell if bidentate, tri- etc
Replies: 5
Views: 880

### Re: How to tell if bidentate, tri- etc

@Fenared Another way to find out if something is monodentate, bidentate, tridentate, etc is to look for specific atoms that you know usually have lone pairs attached to them when they form compounds, such as N. One N in a compound usually corresponds to one binding site, so if the molecule has three...
Thu Nov 30, 2017 9:34 pm
Forum: Identifying Acidic & Basic Salts
Topic: Identifying strong acids and bases
Replies: 2
Views: 362

### Re: Identifying strong acids and bases

A strong acid or base is one that disassociates completely in water, such as HCl or NaOH. These acids and bases will be nearly 100% H+ ions and Cl- ions or Na+ and OH- ions respectively. The best way to determine if an acid/base is strong or weak is to memorize them, but there are only 6 strong acid...
Thu Nov 30, 2017 9:19 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Le Chatelier's Principle
Replies: 2
Views: 261

### Re: Le Chatelier's Principle

As a general rule, when you decrease the volume and thus increase the pressure under which the reaction is taking place, the reaction will favor the side with less moles of gas. If the pressure is increased and the product side of the reaction has less moles of gas, the reaction will favor the produ...
Sun Nov 26, 2017 2:14 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: 4.23
Replies: 2
Views: 195

### Re: 4.23

I'm fairly certain that having a double bond between Sb and O would be acceptable as well since making a double bond between Sb and O gives both atoms a formal charge of zero.
Sun Nov 26, 2017 2:09 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: chemical equilibrium part 2 module #12
Replies: 1
Views: 125

### chemical equilibrium part 2 module #12

In the post assessment module for equilibrium part 2, question 12 reads: Consider the following reactions at 1200 K, for which you know Kc = 1.7 x 10^-3 Br2 (g) = 2Br (g) Your experimental setup is able to measure the equilibrium concentration of Br2 based on its color, but you are unable to measure...
Sat Nov 18, 2017 9:59 pm
Forum: *Molecular Orbital Theory (Bond Order, Diamagnetism, Paramagnetism)
Topic: Delocalized Pi Bond
Replies: 2
Views: 197

### Re: Delocalized Pi Bond

A delocalized pi bond occurs in a molecule that has one or more resonance structures with different placement of double bonds, and therefore different placement of the pi bonds. One of the most common examples we have seen in class is benzene, formed by a ring of carbon atoms with a hydrogen atom on...
Sat Nov 18, 2017 9:48 pm
Forum: Hybridization
Topic: 4.91 Benzyne
Replies: 2
Views: 255

### Re: 4.91 Benzyne

As mentioned in the comment above, the reason this molecule is highly reactive because the triple bond between the 2 carbons not attached to hydrogens are being forced into a shape that their sp hybridization would not normally produce. Because they are in a ring with all the other carbons, which ar...
Thu Nov 09, 2017 1:31 pm
Forum: Lewis Structures
Topic: N2O LEWISH STRUCTURE
Replies: 3
Views: 466

### Re: N2O LEWISH STRUCTURE

The key here is actually electronegativity, because even after considering formal charge you have two lewis structures each with a -1, +1 & 0 formal charge just on different elements. In this case, the element that most wants to have the -1 formal charge is the element with the highest electrone...
Thu Nov 09, 2017 1:13 pm
Forum: Lewis Structures
Replies: 3
Views: 294

One good way to determine which element will accept the radical is to consider formal charge. If the radical will bring the center atom closer- or all the way to- a formal charge of 0 then it is most likely that the radical will go on that atom. You can also consider electronegativity because the mo...
Thu Nov 02, 2017 10:14 pm
Forum: Lewis Structures
Topic: 3.33 Part C
Replies: 3
Views: 258

### Re: 3.33 Part C

As a follow up question, how do you know that O is going to form a double bond with N and not F? Maybe I'm missing something obvious here but how do we determine which atom will form a double bond with the central atom?
Thu Nov 02, 2017 10:10 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Writing Electron Configurations for ions
Replies: 1
Views: 179

### Writing Electron Configurations for ions

In the beginning problems from chapter 3, number 5 asks us to write electrons configurations for ions like Cu+ and Ga+. I assumed that the configuration for Cu+ would look something like [Ar] 3d^8 4s^2, but the solutions manual has the solution as [Ar] 3d^10. Why do the electrons fill the 3d subshel...
Thu Nov 02, 2017 10:08 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Writing Electron Configurations for ions
Replies: 2
Views: 267

### Writing Electron Configurations for ions

In the beginning problems from chapter 3, number 5 asks us to write electrons configurations for ions like Cu+ and Ga+. I assumed that the configuration for Cu+ would look something like [Ar] 3d^8 4s^2, but the solutions manual has the solution as [Ar] 3d^10. Why do the electrons fill the 3d subshel...
Tue Oct 24, 2017 9:51 pm
Forum: Quantum Numbers and The H-Atom
Topic: D-Orbital
Replies: 4
Views: 449

### Re: D-Orbital

Realistically, the purpose of the notation is only to give us a better idea of the location of the electron in each orbital, and is not particularly necessary for the electron configuration. I think that we don't usually bother with notation in the d-subshell because there are so many possible posit...
Tue Oct 24, 2017 9:44 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: ch 2, 47, part d
Replies: 2
Views: 257

### ch 2, 47, part d

Question 47 from chapter 2 asks us to predict which orbital an electron will be removed from to form a 1+ ion given certain elements. I understand parts a-c, but d asks about Au and the solutions manual states that the electron will be removed from the 6s orbital which confuses me since Au is not fi...
Fri Oct 20, 2017 12:17 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: 2.19: Part d, Subshells
Replies: 1
Views: 142

### 2.19: Part d, Subshells

For question 19 in chapter 2, part b and c ask you to find m vales for an electron in a given subshell. Part b asks about the 6d-subshell, which from what I understand has 5 lobes and part c asks about the 3p subshell which has 3 lobes if I am correct. However part d asks how many subshells are in a...
Fri Oct 20, 2017 11:46 am
Forum: Heisenberg Indeterminacy (Uncertainty) Equation
Topic: Chapter 1 #43 [ENDORSED]
Replies: 7
Views: 565

### Re: Chapter 1 #43[ENDORSED]

As a follow-up question about this question, in the solution manual they use an h value of (1.054 x 10^-34 Js) which is not the usual value we have been using in class? Can someone explain this difference?
Fri Oct 13, 2017 3:58 pm
Forum: Photoelectric Effect
Topic: photoelectric effect post-module assessment [ENDORSED]
Replies: 5
Views: 433

### Re: photoelectric effect post-module assessment[ENDORSED]

To clarify, the question is asking about the energy (in J) required to remove one sodium atom. However, they gave you the work function in KJ/mol and you want J/sodium atom which is why you must divide your answer by avogadro's number. Hope this helps!
Fri Oct 13, 2017 3:23 pm
Forum: Properties of Electrons
Topic: Converting Between keV and Joules
Replies: 3
Views: 1052

### Converting Between keV and Joules

When I was working on problem 23 from chapter 1, the question states that the gamma ray photons emitted have a energy of 140.511 and asked for the wavelength of the gamma rays. When I checked the solution manual it showed that before plugging the given value into any equation you must multiply the g...
Wed Oct 04, 2017 11:39 am
Forum: Significant Figures
Topic: Sig Figs in 0.0380 [ENDORSED]
Replies: 5
Views: 519

### Re: Sig Figs in 0.0380[ENDORSED]

So to be clear, 57,000 would have 2 sig figs whereas 57,001 would have 5. And 0.003 would have 1 sig fig whereas 2.003 would have 4 sig figs.
Wed Oct 04, 2017 11:32 am
Forum: Significant Figures
Topic: Rounding [ENDORSED]
Replies: 2
Views: 273

### Re: Rounding[ENDORSED]

I believe you could also write this number as 4.8 x 10^-3 and still have 2 sig figs, just as another option