Search found 55 matches

Thu Mar 14, 2019 10:09 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Standard cell potential and equilibrium
Replies: 3
Views: 106

Standard cell potential and equilibrium

Why does a positive standard cell potential correspond to K>1?
Thu Mar 14, 2019 9:44 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Bronsted Neutralization Reaction
Replies: 1
Views: 121

Bronsted Neutralization Reaction

The solutions manual says that the proper redox reaction for the formation of water from hydronium and hydroxide ions involves the two following reactions: O2(g)+4H+(aq)+4e->2H2O(l) (cathode) and O2(g)+2H2O(l)+4e->4OH- (anode). Using these two reactions gives you an overall standard cell potential o...
Thu Mar 14, 2019 9:41 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Bronsted Neutralization Reaction
Replies: 1
Views: 79

Bronsted Neutralization Reaction

The solutions manual says that the proper redox reaction for the formation of water from hydronium and hydroxide ions involves the two following reactions: O_{2}(g)+4H^{+}(aq)+4e\rightarrow 2H_{2}O(l) (cathode) and O_{2}(g)+2H_{2}O(l)+4e\rightarrow 4OH^{-} (an...
Thu Mar 14, 2019 9:05 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Conductivity
Replies: 1
Views: 80

Conductivity

How can you tell if a metal (or a solid in general) is conductive? Is there a way to check on the periodic table, or are there some general rules of thumb?
Thu Mar 14, 2019 8:19 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Hydronium
Replies: 2
Views: 112

Hydronium

When do you include hydronium ions in your galvanic cell diagram? And, because I've heard different things from different UAs, can we combine these ions with the reduced anions rather than writing them as two separate aqueous solutions? For example, if your cathode contained H+ and NO3-, both aqueou...
Wed Mar 13, 2019 9:39 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Multistep entropy problems
Replies: 2
Views: 318

Multistep entropy problems

****This is an important question because I've heard two different things from two different UAs and you probably have too!**** In an entropy problem in which both temperature and volume are changing, you isolate the two entropy changes. When you calculate the entropy change for temperature, do you ...
Wed Mar 13, 2019 9:33 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Stot
Replies: 1
Views: 256

Stot

What exactly is Stot? Is this a combination of the change in entropy of surroundings minus the change in entropy of the system? Furthermore, how can you use Stot to determine spontaneity?
Wed Mar 13, 2019 1:02 am
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: 7th edition 4F.11
Replies: 4
Views: 486

Re: 7th edition 4F.11

Still have this question. Is it possible to use R instead of C when calculating the entropy of reversible isobaric compression due to change in temperature, as the solution manual says?
Tue Mar 12, 2019 12:13 am
Forum: Method of Initial Rates (To Determine n and k)
Topic: k prime?
Replies: 1
Views: 127

k prime?

What is k prime and how does it differ from k? I've seen k' used in multiple contexts- for example, used as the rate constant for the rate law of a reverse reaction and used as the rate constant for the rate law of an isolated concentration in a termolecular reaction.
Tue Mar 12, 2019 12:07 am
Forum: General Rate Laws
Topic: slow step
Replies: 11
Views: 567

slow step

How do we determine which step in a multistep reaction is the slow step? Or will that always be given?
Mon Mar 11, 2019 12:54 am
Forum: Method of Initial Rates (To Determine n and k)
Topic: Pneumonic for remembering units
Replies: 1
Views: 81

Pneumonic for remembering units

Does anyone have a good way to remember what the units of k are for zero/1st/2nd order reactions?
Mon Mar 11, 2019 12:52 am
Forum: Method of Initial Rates (To Determine n and k)
Topic: Unique rate?
Replies: 7
Views: 218

Unique rate?

Is the "unique rate" of a reaction the same thing as k? If not, what is it and how do we find it if it isn't given?
Mon Mar 11, 2019 12:50 am
Forum: Second Order Reactions
Topic: Graphs of second order
Replies: 1
Views: 118

Graphs of second order

What does the graph of a second order reaction tell us, and how does it relate to the differential rate law and integrated rate law for a second order?
Sat Mar 09, 2019 8:52 pm
Forum: General Rate Laws
Topic: Reaction intermediates?
Replies: 1
Views: 82

Reaction intermediates?

I know Prof. Lavelle mentioned that intermediates are not products, but what exactly are they? How do we account for them when consolidating the steps of a multistep reaction?
Sat Mar 09, 2019 6:24 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Gases and Liquids in Cell Diagrams
Replies: 3
Views: 182

Re: Gases and Liquids in Cell Diagrams

H20 shouldn't be included in the cell diagram! Also, if you're ever in doubt about what to put in your cell diagram, just make sure you have included each component of the half reactions. So you should have the solid, any ions involved in reduction/oxidation, and the conductor (if it is separate fro...
Sat Mar 09, 2019 6:05 pm
Forum: Zero Order Reactions
Topic: General “order” question
Replies: 6
Views: 451

Re: General “order” question

The order refers to the exponent of concentrations of different species that must interact in order to form a product. For example, a zero order reaction, such as the evaporation of water or the decomposition of a single solid, does not depend on concentration and thus has an exponent of zero. If a ...
Sat Mar 09, 2019 5:41 pm
Forum: Zero Order Reactions
Topic: Zero order
Replies: 10
Views: 681

Re: Zero order

Another example could be the evaporation of water. The rate at which liquid water becomes water vapor does not depend on a "concentration," but rather on a multitude of other things (amount of water/temperature/pressure/etc)
Thu Feb 28, 2019 2:17 pm
Forum: Balancing Redox Reactions
Topic: 7th edition 6K.3
Replies: 2
Views: 88

7th edition 6K.3

This problem involves balancing the redox reaction Cl2(g)->HClO(aq)+Cl2(g). The balanced reduction half reaction reports aqueous Cl- ions as products, even though they are not included in the skeletal equation at all. Could someone please clarify why that is? Do the HClO molecules dissociate in wate...
Thu Feb 28, 2019 2:33 am
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: "Strongly oxidizing"
Replies: 5
Views: 226

"Strongly oxidizing"

If, for example, Cu2+(aq) has greater oxidizing ability than H+(aq), does that mean that it has an increased ability to act as an oxidizing agent and reduce the h2 species or that it has the greater ability to BE oxidized by another reducing agent? The inverted language is getting more and more diff...
Thu Feb 28, 2019 12:22 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 7th edition 6L.5
Replies: 1
Views: 69

7th edition 6L.5

In part D) of this problem, you are asked to write a cell diagram for the skeletal equation Au+(aq)->Au(s)+Au3+(aq). In the solutions manual, the Au+(aq) isn't included in the anode and cathode, only in the cathode. Why? I understand that it is both the oxidizing and the reducing agent, so why isn't...
Mon Feb 25, 2019 10:11 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Stability and free energy
Replies: 1
Views: 88

Stability and free energy

Why does a positive free energy correspond to instability? I would assume that a negative free energy and thus spontaneous decomposition/formation would correspond to instability.
Mon Feb 25, 2019 1:16 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Porous disks
Replies: 1
Views: 54

Porous disks

Under what conditions could a porous disk be used instead of a salt bridge? And why would this need to be porous only for anions?
Mon Feb 25, 2019 1:13 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Positive vs negative voltage
Replies: 1
Views: 104

Positive vs negative voltage

Why do we always want to know voltage as a positive value rather than a negative value?
Mon Feb 25, 2019 1:09 am
Forum: Balancing Redox Reactions
Topic: Rules for redox reactions
Replies: 3
Views: 119

Rules for redox reactions

In general, what are the rules for charges? I know that pure elements always have a charge of zero, and oxygen is always 2-, unless if it is paired with hydrogen? Or something along those lines?
Tue Feb 12, 2019 10:34 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Extensive v. Intensive Property of Heat Capacities
Replies: 3
Views: 169

Re: Extensive v. Intensive Property of Heat Capacities

Look at the units- specific heat capacity is typically in units of J/g*C, and molar heat capacity is typically in units of J/Kmol. This means that in order to find the actual amount of heat energy in Joules, you need to multiply SHC by temperature and mass, and you need to multiply MHC by temperatur...
Tue Feb 12, 2019 10:31 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Delta U
Replies: 2
Views: 136

Re: Delta U

Delta U is zero when q and w are both zero. So, if there is no work done by or on the system, and there is no transfer of heat energy into or out of the system, the change in internal energy will be zero.
Tue Feb 12, 2019 10:29 pm
Forum: Calculating Work of Expansion
Topic: Uses of different formulas
Replies: 3
Views: 170

Re: Uses of different formulas

W=-Pext(deltaV) is used when there is a constant external pressure outside of a system, and the system irreversibly expands into its surroundings, doing a certain amount of work. W=-nRTlnv2/v1 is used for a much more nuanced system, in which the external pressure is not constant but is instead subje...
Tue Feb 12, 2019 1:44 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Focus 4C.13 (7th ed)
Replies: 1
Views: 59

Re: Focus 4C.13 (7th ed)

You end up with essentially the same answer either way, but at this point it's a matter of pride and I need validation that my way makes sense
Tue Feb 12, 2019 1:39 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Focus 4C.13 (7th ed)
Replies: 1
Views: 59

Focus 4C.13 (7th ed)

When calculating the heat required to melt the ice and then heat the resulting liquid water to its final temperature, why do we multiply the mass of ice and the specific heat capacity of liquid water to calculate the energy required to heat the water? Since liquid water and ice have different densit...
Tue Feb 12, 2019 1:11 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Problem 4C.3 (7th ed.)
Replies: 1
Views: 52

Problem 4C.3 (7th ed.)

Even though the problem asks you to find the final temperature and the change in enthalpy at both constant pressure and constant volume, is there even such thing as an enthalpy change when pressure is not constant? In that case, would it just be a change in q? If so, why is the problem phrased like ...
Tue Feb 12, 2019 1:03 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Reasons for Heat Capacity
Replies: 2
Views: 135

Reasons for Heat Capacity

What are some molecular or even atomic factors that influence a compound's heat capacity? For example, why does copper have a higher heat capacity than water, and why does CO2 have a higher heat capacity than CO?
Tue Feb 12, 2019 12:05 am
Forum: Administrative Questions and Class Announcements
Topic: Week 6/7 homework
Replies: 2
Views: 161

Week 6/7 homework

Which topics can we turn in completed problems for for weeks 6 and 7, respectively? Will you accept any, as we will have spent most of both weeks studying previous material?
Sun Feb 10, 2019 9:06 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Polyprotic acid question 2.0
Replies: 1
Views: 80

Polyprotic acid question 2.0

When can you ignore the second deprotonation of a polyprotic acid? In the solutions manual it says you can ignore when Ka2 is significantly less than Ka1, but how much is "<<"?
Sun Feb 10, 2019 6:22 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Edition 7 #5.61
Replies: 2
Views: 152

Edition 7 #5.61

In this question, why does decreasing the amount of aqueous glucose cause a shift in the equilibrium concentration? Even though it is a product, it is technically not a gas, and I don't understand how you can include it in a K calculation.
Sat Feb 09, 2019 9:09 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: General entropy question
Replies: 9
Views: 227

General entropy question

What qualities/characteristics of a molecule or a substance increase entropy?
Sat Feb 09, 2019 5:59 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Cv/Cp versus R
Replies: 4
Views: 218

Cv/Cp versus R

When calculating ΔS, when should we use R and when should we use Cv and Cp, or (3/2)R and (5/2)R, respectively?
Sat Feb 09, 2019 5:58 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: 7th edition 4F.11
Replies: 4
Views: 486

7th edition 4F.11

I'm confused as to why the solutions manual doesn't indicate that you use (5/2)R instead of R when calculating the entropy change of non-isothermal irreversible expansion. Isn't the pressure kept constant? And the gas should be assumed ideal?
Sat Feb 09, 2019 5:51 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Ideal Behavior
Replies: 3
Views: 173

Re: Ideal Behavior

I was going to ask the same thing, should we always take "ideal conditions" to mean 1 mol of gas and pressure=1 atm?
Sat Feb 09, 2019 3:17 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: delta H
Replies: 3
Views: 155

Re: delta H

ΔH is the change in enthalpy, or the amount of heat released or absorbed under conditions of constant pressure, and ΔH* is the standard reaction enthalpy of a reaction in which all reactants and products are in their standard states (for a gas, 1 atm/for a solution, 1 M). ΔH is measured in kJ and ΔH...
Sat Feb 09, 2019 3:13 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Degeneracy
Replies: 4
Views: 208

Re: Degeneracy

We did a problem at a PL session where the number of states wasn't explicitly given, but we were given the chemical formula of the molecule. In this case, you would draw out every possible orientation of the molecule. Think Lewis structures/molecular geometry. For example, the molecule SO2F2 has six...
Sat Feb 09, 2019 2:54 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Delta H and q
Replies: 3
Views: 193

Re: Delta H and q

Also, enthalpy is specifically defined as the amount of heat absorbed or released under conditions of constant pressure. q represents heat absorbed/released, so qsubp is the same thing as enthalpy. qsubv is NOT the same thing as enthalpy.
Sat Feb 09, 2019 2:50 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Gas constant, R
Replies: 3
Views: 133

Re: Gas constant, R

Make sure to pay attention to the units as well as the context of the problem. For example, if a question asks you to find the work done during reversible expansion, you would use the equation W=-nRTln(v2/v1). Work is always presented in J or kJ, so take a look at the other side: n is in moles, temp...
Sat Feb 09, 2019 2:42 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Reversible Exapansion
Replies: 2
Views: 104

Re: Reversible Exapansion

Reversible expansion occurs when the external pressure of a system's surroundings is very slightly decreased in a series of many, many steps. As the pressure decreases in this manner, the gas inside the system is able to slowly expand due to a difference between internal and external pressures. It i...
Thu Jan 31, 2019 12:33 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Comparing SHC
Replies: 3
Views: 174

Comparing SHC

Would a molecule with more atoms have a higher specific heat capacity than a molecule with fewer atoms? Or would they have the same specific heat capacity but different molar heat capacities?
Thu Jan 31, 2019 12:31 am
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Perfect system
Replies: 3
Views: 231

Perfect system

How would we go about creating a "perfect system" or at least approximating one? What variables would we need to control? I'm a bit confused as to what this actually means. Are all isolated systems perfect? Sorry this is a lot of questions in one post.
Thu Jan 31, 2019 12:24 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Phase changes
Replies: 2
Views: 118

Phase changes

Could someone explain why we need to include the "enthalpy of phase changes" when calculating the amount of heat in a reaction? Could you possibly give an example of how we would apply this concept?
Fri Jan 25, 2019 1:32 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Le Chatelier's Principle question
Replies: 4
Views: 212

Le Chatelier's Principle question

What is the difference between changing pressure by adding an inert gas and changing pressure by decreasing volume when you are trying to use Le Chatelier's principle to calculate reaction dynamics? Why?
Fri Jan 25, 2019 12:52 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: First row question
Replies: 3
Views: 200

First row question

What is the molecular reasoning for why the presence of row 1 elements in acids do not affect the pH? I understand that they don't have to be accounted for when calculating, for example, the pH of a solution of nitrous acid and potassium nitrite, but why is that? Just curious.
Thu Jan 24, 2019 10:06 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Polyprotic acid question
Replies: 1
Views: 68

Polyprotic acid question

How do we know when an acid will continue to ionize after an initial ionization? Will every single acid with multiple hydrogen atoms ionize a second/third/etc. time, and if not, how can we tell the difference between those that will and those that will not?
Fri Jan 18, 2019 12:41 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Why do stronger bases have conjugate acids with larger pKa values?
Replies: 4
Views: 213

Re: Why do stronger bases have conjugate acids with larger pKa values?

Try thinking about this on a molecular level. When a strong acid is deprotonated, the resulting conjugate base is remarkably stable. This could be due to inductive effects (shifting of negative charges around atoms with large EN differences) or resonant structures (decreased average negative charge ...
Fri Jan 18, 2019 12:32 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Calculating PH of weak acids and bases
Replies: 5
Views: 154

Re: Calculating PH of weak acids and bases

The Ka is incredibly small (less than 10^-3), and so we don't need to use the quadratic equation because we assume the molar concentration of the original protonated molecule is going to be essentially unchanged at equilibrium. Though some of the molecule will be deprotonated, it is such a small num...
Thu Jan 17, 2019 11:44 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Focus 6A #19 part C
Replies: 1
Views: 44

Focus 6A #19 part C

I understand how to use the formula for Kw to find the molar concentration of hydroxide ions when given the molar concentration of hydronium ions, but I don't understand how, in the solutions manual, #19C is calculated. The 3.1 mol/L is multiplied by a factor of 10^-3, but I don't see why that would...
Thu Jan 10, 2019 1:45 am
Forum: Ideal Gases
Topic: Value of K
Replies: 6
Views: 208

Re: Value of K

Think about the math behind what K actually is: a ratio of products to reactants. Therefore, when K=1, that means that the products and the reactants have an equal concentration/partial pressure (for example, 5/5=1). When K>1, that means that the ratio is "top-heavy" and there are more pro...
Thu Jan 10, 2019 1:32 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Calculating K
Replies: 3
Views: 56

Calculating K

When writing an expression for the equilibrium constant, does it matter if you use partial pressure instead of concentration or vice versa? And do you use the molecule's coefficient in the chemical formula as an exponent for both partial pressure and concentration?
Thu Jan 10, 2019 1:00 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Multiplying Equilibrium Constants?
Replies: 1
Views: 83

Multiplying Equilibrium Constants?

Under what circumstances do you multiply two K values? I was doing problem #3 in Focus 5H and I'm not quite sure why you're supposed to multiply the K values instead of adding them.