Search found 47 matches

Sun Mar 18, 2018 11:50 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: maximum potential
Replies: 1
Views: 230

Re: maximum potential

Emf represents maximum voltage because it represents a state where no current flows (use a voltmeter with an extremely high resistance) so the potential difference is at its maximum.
Sun Mar 18, 2018 11:49 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: work, cell potential, and free energy
Replies: 1
Views: 224

Re: work, cell potential, and free energy

Free energy in a closed system = max non-expansion work
Cell potential has to be maximized for the maximum non-expansion work to be done
Sun Mar 18, 2018 11:47 pm
Forum: Balancing Redox Reactions
Replies: 6
Views: 478

In basic solutions, you add OH-.
Sun Mar 18, 2018 11:47 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: Activation Energy Temperature Dependence
Replies: 5
Views: 632

Re: Activation Energy Temperature Dependence

I think it does have to do with the frequency factor more than the activativation energy -- thank you.
Thu Mar 15, 2018 7:56 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Final 2008, Q3
Replies: 1
Views: 198

Final 2008, Q3

We have a galvanic cell with the following half-rxn's: Cu2+ (aq) + 2e- ---> Cu (s) standard cell potential = 0.34V H2 (g) ---> 2H+ (aq) +2e- Part D: Na2S is added to the Cu2+ half-cell, CuS forms, the new [Cu2+] = 1*10^-16 M. What is the cell voltage? I understand that we use the Nernst equation for...
Mon Mar 12, 2018 1:05 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: Heterogeneous Catalyst and Rate Law
Replies: 1
Views: 158

Heterogeneous Catalyst and Rate Law

Are heterogenous catalysts included in the rate law? I would think yes, since the reaction rate should be proportional to their surface area.
Mon Mar 12, 2018 11:22 am
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: Activation Energy Temperature Dependence
Replies: 5
Views: 632

Activation Energy Temperature Dependence

Why is activation energy independent of temperature? I would think that it would decrease as temperature increases because of the increased rate of collisions?
Mon Mar 12, 2018 10:41 am
Forum: Reaction Mechanisms, Reaction Profiles
Replies: 2
Views: 198

I'm pretty sure we won't be asked to do a steady state calculation. But to answer your question, a problem involving steady state could be structured exactly the same way as a problem that involves the pre-equilibrium approach. The steady-state approach is based on the assumption that the concentrat...
Mon Mar 12, 2018 10:40 am
Forum: Balancing Redox Reactions
Topic: Half-reactions
Replies: 4
Views: 287

Re: Half-reactions

This is only true in a GALVANIC CELL - where the chemical reaction is spontaneous. In an electrolytic cell, this is not what we would do (we'd actually do the opposite I'm quite sure), because the reaction is non-spontaneous.
Sun Mar 11, 2018 8:42 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Steps
Replies: 3
Views: 217

Re: Steps

It is usually something you will be given. You can't determine it simply by looking at the steps of a reaction.
Mon Mar 05, 2018 11:10 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: slowest step determines rate of overall reaction
Replies: 3
Views: 260

Re: slowest step determines rate of overall reaction

Here's an analogy to add to the above answer: Say you are doing a group lab report. You can only turn in your lab report when all members are done with their parts. Thus, you can only turn in the lab report when the member who takes the longest to do his/her part finishes. So you can say that the ra...
Mon Mar 05, 2018 10:57 am
Forum: First Order Reactions
Topic: Rate of consumption vs concentration
Replies: 2
Views: 199

Re: Rate of consumption vs concentration

For a first-order reaction, the rate of consumption vs concentration graph would be a line with a positive slope.
Mon Mar 05, 2018 10:49 am
Forum: Zero Order Reactions
Topic: Units of k
Replies: 12
Views: 999

Re: Units of k

Zero-order: M/s
First order: s^-1
Second order: L/(mol*s)

These units are derived from the rate law -- we know the rate of reaction must be M/s, so the units of k change to accommodate that.
Sat Mar 03, 2018 11:37 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Q vs. K
Replies: 8
Views: 586

Re: Q vs. K

In much of what we do, Q is used when we want to calculate some sort of potential at non-standard conditions (Gibbs Free Energy, Nernst Equation). K is used in our calculations when we want to calculate the potential at standard conditions, usually.
Sat Mar 03, 2018 11:34 pm
Forum: Kinetics vs. Thermodynamics Controlling a Reaction
Topic: Kinetics vs Thermodynamics
Replies: 7
Views: 467

Re: Kinetics vs Thermodynamics

A system is thermodynamically stable depending on its thermodynamic potential energy (Gibb's, internal energy, enthalpy). Just because a reaction is thermodynamically stable, though, does not say anything about kinetic stability (that has to do with mainly activation energy and pre-equilibrium condi...
Sat Mar 03, 2018 11:22 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: reaction mechanisms reaction rate
Replies: 2
Views: 159

Re: reaction mechanisms reaction rate

The coefficients of a reaction don't tell us about the mechanism of a reaction, which is what a rate law describes -- EXCEPT if the reaction is elementary (one step). In your specific question, the rate law was determined experimentally.
Sat Mar 03, 2018 11:20 pm
Forum: General Rate Laws
Topic: writing rate law
Replies: 3
Views: 252

Re: writing rate law

You can't just use the balanced equation to write the rate law because the reaction has many steps that are not reflected in the reaction (except in an elementary reaction). The rate law is an expression of the MECHANISM of a reaction -- each individual step -- which the stoichiometric coefficients ...
Sat Feb 24, 2018 9:01 am
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: 14.37a [ENDORSED]
Replies: 3
Views: 272

Re: 14.37a[ENDORSED]

Thank you!

One follow-up question, though: in this concentration cell, can the partial pressure of H2 on the products side of the equation be different from the partial pressure of H2 on the reactants side? (Both sides of the equation don't have H2 with a partial pressure of 1 bar)
Fri Feb 23, 2018 9:34 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Solving for Q when given molarity and partial pressure
Replies: 3
Views: 499

Re: Solving for Q when given molarity and partial pressure

Both partial pressures and concentrations can only be entered into Q when the concentration is in M and partial pressure is in bars.

You can use expression equating Kc and Kp to understand why this is possible.
Thu Feb 22, 2018 10:11 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: double lines between anode and cathode
Replies: 3
Views: 260

Re: double lines between anode and cathode

In a galvanic cell, the purpose of a salt bridge is to maintain the charges in the anode and cathode so the reaction can continue while ensuring that half-cells do not mix (if they did mix, they would directly oxidize/reduce the other, electrons wouldn't pass through a wire, no work would be done). ...
Thu Feb 22, 2018 9:37 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: 14.37a [ENDORSED]
Replies: 3
Views: 272

14.37a[ENDORSED]

In this problem, we are able to determine that it is a concentration cell with the OVERALL reaction: 2H+ (aq, 1 M) + H2 (g, 1 atm) ----> 2H+ (aq, 0.075M) + H2 (g, 1 atm) The solution manual includes the H2 gas in the expression for Q. I was wondering why it does when the partial pressures of H2 are ...
Mon Feb 19, 2018 7:51 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Problem 14.11e -- Mercury Electrode
Replies: 1
Views: 120

Problem 14.11e -- Mercury Electrode

e. Write the half-reaction and the balanced equation in each of the following galvanic cells: Pt (s) | Sn^{4+}(aq), Sn^{2+}(aq)\left | \right |Cl^{-}(aq)|Hg_{2}Cl_{2}(s)|Hg(l) So, it seems as though mercury, in its liquid form, is used as an electrode ...
Fri Feb 16, 2018 4:25 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: s vs. delta s
Replies: 7
Views: 414

Re: s vs. delta s

S is the entropy of a substance while delta S is the change in entropy. We use S to calculate delta S of a reaction (entropy of products - entropy of reactants). Delta S is generally more useful than S as we can calculate changes in entropy for various processes.
Fri Feb 16, 2018 4:18 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Boiling point at room temperature
Replies: 5
Views: 278

Re: Boiling point at room temperature

We know the vaporization of water is non-spontaneous at room temperature, so we must first heat it up to 100, boil it, and then cool it back down. A lot of the water would condense -- you are right -- but like much of what we do, this is a simplification that allows us to calculate the change in a s...
Fri Feb 16, 2018 4:01 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Delta G standard
Replies: 4
Views: 290

Re: Delta G standard

The SI units for Gibb's Free Energy are Joules. However, as mentioned in class, we could also think of the units as Joules/mols of reaction.
Wed Feb 14, 2018 2:16 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Maximum Energy Able to Do Work [ENDORSED]
Replies: 3
Views: 283

Re: Maximum Energy Able to Do Work[ENDORSED]

Maximum useful (nonexpansion) work is associated with Gibb's Free Energy.
Fri Feb 09, 2018 9:19 am
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Temperature Dependence of Entropy
Replies: 3
Views: 480

Re: Temperature Dependence of Entropy

Yes. If you look at the P-V graph of a reversible isothermal expansion, P and V can change pretty heavily over a long period of time. That's true for temperature change in this case too.
Wed Feb 07, 2018 11:41 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Entropy Concept
Replies: 2
Views: 208

Re: Entropy Concept

An endothermic reaction can certainly increase the total entropy of a system. That's why certain endothermic processes are spontaneous: the dissolution of salt in water for example. Remember, this is possible because spontaneity is a function of total entropy, not just the entropy of a system (this ...
Wed Feb 07, 2018 11:31 am
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Temperature Dependence of Entropy
Replies: 3
Views: 480

Re: Temperature Dependence of Entropy

The temperature dependent entropy change equation isn't exactly derived from the reversible isothermal work equation. But, it is similar in a sense: to get that equation, we assume that the change in temperature is infinitely small, that the process is isochoric (volume doesn't change). Then, we sum...
Tue Feb 06, 2018 7:20 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Entropy of a Irreversible Process
Replies: 2
Views: 173

Entropy of a Irreversible Process

Say you have a reversible version of a process and an irreversible version of the same process. Since entropy is a state function, the system will have the same entropy change for both processes. Yet, the total entropy of the irreversible process is greater than that of the reversible process. How i...
Sun Feb 04, 2018 12:27 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: What does reversibility have to do with entropy?
Replies: 1
Views: 131

Re: What does reversibility have to do with entropy?

During a reversible process, the entropy of an isolated system is constant -- this is because the reaction is always at equilibrium.
Fri Feb 02, 2018 1:49 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Internal Energy and Spontaneity
Replies: 3
Views: 180

Re: Internal Energy and Spontaneity

The spontaneity of a reaction doesn't have a lot to do with internal energy, which is the thermodynamic potential. Only when S and V are constant will a spontaneous reaction reduce internal energy -- not at constant P or T. Rather, we need to look at Gibb's Free Energy which accounts for a non-const...
Fri Feb 02, 2018 1:33 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: 2. At the temperature of a phase transition, the transfer of heat is reversible."
Replies: 2
Views: 178

Re: 2. At the temperature of a phase transition, the transfer of heat is reversible."

Let's look at the vaporization of water to answer your question. At 100 degrees Celsius, water is at its boiling point. That doesn't mean it is steam, though -- we must first break all the intermolecular interactions for it to become steam. Rather, at 100 degrees Celsius, the water and steam are act...
Fri Feb 02, 2018 1:24 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Ideal Situation vs. Real Situation
Replies: 2
Views: 152

Re: Ideal Situation vs. Real Situation

A non-ideal gas intermolecular attractions, meaning that the particles will attract each other, reducing total microstates and entropy. In an ideal gas, there are no intermolecular attractions so the gas has the largest number of microstates. Entropy of an ideal gas > Entropy of a real gas. This dis...
Tue Jan 23, 2018 12:15 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Enthalpy Change in a Closed System
Replies: 1
Views: 900

Enthalpy Change in a Closed System

Say you have a reaction that produces a net 2 mol of gas in an closed system.

My question is, does the enthalpy of the system change in the above reaction, when it is in a closed system? If it does, can someone explain why that is true, in conceptual terms?
Tue Jan 23, 2018 12:00 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: HW #41 vs #21
Replies: 3
Views: 233

Re: HW #41 vs #21

The negative sign comes from one entity gaining the heat that is lost by the other. In my opinion, it makes the most sense if the negative sign goes on the entity that is gaining heat. But, it really does not matter where you put it, as long as it is in the equation. Switching the side of the negati...
Tue Jan 23, 2018 11:56 am
Forum: Calculating Work of Expansion
Topic: expansion vs non expansion
Replies: 1
Views: 123

Re: expansion vs non expansion

No, they are not the same things. A reversible process is a process that is always at equilibrium -- and for it to be equilibrium, changes in the process must be infinitely small and occur infinitely slowly. Furthermore, this cannot happen with a real gas, only an ideal one. An irreversible process ...
Tue Jan 23, 2018 11:37 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Work and Volume
Replies: 2
Views: 159

Re: Work and Volume

The above poster is correct, but there are also many types of non-expansion work that don't require a change in volume. For example, an electric current can do work. At its most basic, work is movement against an opposing force.
Sat Jan 20, 2018 1:19 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Constant Volume vs Constant Pressure
Replies: 6
Views: 305

Re: Constant Volume vs Constant Pressure

For systems that are at constant volume, no work is done, since the change in volume is 0 (w = - (external pressure) * change in volume). In these systems, pressure increases as a result of heat. We know that pressure increases because of Boyle's law. For systems at constant pressure, work is being ...
Wed Jan 17, 2018 1:55 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: work vs heat
Replies: 4
Views: 196

Re: work vs heat

Work and heat are not the same. Work is done when motion is created against an opposing force, while heat is the transfer of energy due to a temperature difference. Work requires an agent -- someone or something must do work. Heat does not require one. Furthermore, when work is done, energy is trans...
Wed Jan 17, 2018 1:48 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: internal energy
Replies: 5
Views: 277

Re: internal energy

Internal energy is a state property because it is a property intrinsic to a system. And as said above, the value of internal energy is not dependent on how that value was reached -- it is path independent.
Wed Jan 17, 2018 1:46 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Free Expansion
Replies: 2
Views: 116

Re: Free Expansion

Free expansion occurs when there is nothing for the gas to "push against." The primary instance in which this occurs is in a vacuum -- the external pressure is 0, and so a change in volume of the gas does not produce any work. It is worth noting that free expansion for ideal gases is an is...
Wed Jan 17, 2018 1:36 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.51 energy "released"
Replies: 4
Views: 298

Re: 8.51 energy "released"

The answer is positive because it refers to "energy released." The reaction releases +3292 kJ/mol, but the enthalpy of the reaction per mole of TNT is -13618 kJ/mol. In this case, the sign of enthalpy just has to do with the way the question is worded.
Fri Jan 12, 2018 9:30 pm
Forum: Phase Changes & Related Calculations
Topic: Heat and work not state functions
Replies: 7
Views: 528

Re: Heat and work not state functions

One more way to think about it: There are lots of ways to change the energy of system, and those ways of adding energy to a system can be thought of as heat or work. Now, there are lots of combinations of ways you can add energy to a system to achieve a certain state. And depending on how the energy...
Fri Jan 12, 2018 9:16 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Enthalpy formula from day 1
Replies: 2
Views: 150

Re: Enthalpy formula from day 1

I'm pretty sure enthalpy is really just a convention created by scientists to make calculations convenient (most reactions occur at constant pressure). It is really hard to explain the equations related to enthalpy from a conceptual standpoint. This one can be derived, but it requires the First Law ...
Fri Jan 12, 2018 9:10 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Δ H=q
Replies: 3
Views: 239

Re: Δ H=q

Another way to think about state properties is to think about them as properties that belong to a system. For example, internal energy "belongs" to a system and so it is a state property. In contrast, heat is not a state property because it is simply energy that is transferred in and out o...
Fri Jan 12, 2018 8:17 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Difference between delta H and delta U
Replies: 2
Views: 20166

Re: Difference between delta H and delta U

Delta H is the change in enthalpy and Delta U is the change in internal energy. Internal energy is the amount of energy a system has. However, we must realize that a system doesn't exist in a vacuum. Rather, there must be some sort of change in volume/pressure because of the presence of a system. If...