Search found 30 matches

by Philip Lee 1L
Wed Mar 13, 2019 10:42 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: ΔG equation
Replies: 1
Views: 38

ΔG equation

How is the following equation derived?

ΔG = ΔGo + RTln(Q)
by Philip Lee 1L
Wed Mar 13, 2019 10:39 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: K = k/k'
Replies: 3
Views: 61

K = k/k'

K = k/k'

Is this statement only true for a second order reaction of the form A + B --> C + D?
by Philip Lee 1L
Wed Mar 13, 2019 10:36 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Equilibrium constants with partial pressures and concentrations?
Replies: 2
Views: 38

Equilibrium constants with partial pressures and concentrations?

When writing equilibrium constants for reactions, can we include both partial pressures and concentrations for the species?

For example:
2H+(aq) + 2Cl-(aq) --> Cl2(g) + H2(g)

What is the equilibrium constant for this reaction? Do we use the (aq) species? (g) species? Both?
by Philip Lee 1L
Sun Mar 10, 2019 9:08 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Verifying reaction mechanisms
Replies: 1
Views: 44

Verifying reaction mechanisms

When verifying reaction mechanisms, how do you decide whether to find a rate law for the net rate of formation of a species or the rate law for the net rate of decomposition of a species?
by Philip Lee 1L
Sun Mar 10, 2019 2:10 pm
Forum: General Rate Laws
Topic: Unique rates
Replies: 1
Views: 30

Unique rates

When we write a rate law, it the rate the unique rate for the reaction?
by Philip Lee 1L
Sun Mar 03, 2019 12:16 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Why do we use concentration cells?
Replies: 2
Views: 56

Why do we use concentration cells?

Cell potential of a concentration cell at equilibrium is 0. When it is not at equilibrium, the cell potential tends to be low. Why do we use concentration cells at all? Do concentration cells have any advantages when compared to normal galvanic cells?
by Philip Lee 1L
Wed Feb 27, 2019 11:40 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Cathode/Anode, +/-
Replies: 2
Views: 47

Cathode/Anode, +/-

The textbook says the following: "A commercial galvanic cell has its cathode marked with a + sign and its anode with a - sign. Think of the + sign as indicating the electrode at which the electrons enter and 'add to' the cell and the - sign as indicating the electrode at which the electrons lea...
by Philip Lee 1L
Tue Feb 26, 2019 5:07 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: ΔG is slope? [ENDORSED]
Replies: 1
Views: 94

ΔG is slope? [ENDORSED]

On page 403 of the 7th edition textbook, there is a graph of Gibbs free energy vs progress of reaction (pure reactants on left, pure products on right). The book says that "ΔG is the slope of the line at each composition, whereas ΔG o is the difference between the standard Gibbs free energies o...
by Philip Lee 1L
Sat Feb 23, 2019 5:39 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: enthalpy of formation vs standard reaction enthalpy
Replies: 1
Views: 26

Re: enthalpy of formation vs standard reaction enthalpy

This is because the reactions in 4J5 are formation reactions while the reactions in 4J7 are not formation reactions. A formation reaction is a reaction where a species is synthesized from its elements in their most stable forms. For example, the formation reaction of NH 3 (g) in 4J5 (1/2)N 2 (g) + (...
by Philip Lee 1L
Sat Feb 23, 2019 5:27 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Standard Gibbs Free Energy
Replies: 1
Views: 49

Standard Gibbs Free Energy

Why is ΔGo fixed for a given reaction and temperature? In reality, is it possible to perform a reaction with all species in their pure forms? Or is it a theoretical value?
by Philip Lee 1L
Sat Feb 23, 2019 5:24 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Midterm Question 1A Sig Figs?
Replies: 1
Views: 50

Midterm Question 1A Sig Figs?

For Q1A on the midterm, why does the value of Q p only have one significant figure in the answer key? I see that there are some values given in the question that have only one significant figure (T = 700 K and K p = 3 x 10 4 ). However, these values are not used in the actual calculation of Q p . On...
by Philip Lee 1L
Fri Feb 15, 2019 3:18 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Why do we care about ΔG
Replies: 3
Views: 86

Why do we care about ΔG

"The great importance of the introduction of Gibbs free energy is that, provided that the temperature and pressure are constant, the spontaneity of a process can be predicted solely in terms of the thermodynamic properties of the system " (7th Edition, pg 330) Doesn't the equation ΔS tot =...
by Philip Lee 1L
Fri Feb 15, 2019 3:11 pm
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: Self-test 4I.4A
Replies: 2
Views: 139

Re: Self-test 4I.4A

Why is it that when the two values are similar we can say that ΔStot = 0?
I thought that ΔStot = ΔSsys + ΔSsurr.
What are the system and surrounding in this problem?
by Philip Lee 1L
Mon Feb 11, 2019 12:22 am
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: Self-test 4I.4A
Replies: 2
Views: 139

Self-test 4I.4A

"Confirm that liquid water and water vapor are in equilibrium when the temperature is 100. °C and the pressure is 1 atm. Data are available in Tables 4C.1 and 4G.1." I'm not sure how to show that ΔS tot = 0 at 100. °C Relevant data from the two tables (maybe... again, I'm not really sure h...
by Philip Lee 1L
Sat Feb 09, 2019 12:31 am
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: ΔS = nRln(v2/v1)
Replies: 1
Views: 42

ΔS = nRln(v2/v1)

During lecture, an equation for ΔS was derived using a previously given equation that relates entropy with degeneracy. The steps are as follows: Given: S = k B lnW 1) ΔS = S final - S initial 2) ΔS = k B lnW 2 - k B lnW 1 3) ΔS = k B ln(W 2 /W 1 ) 4) ΔS = k B ln(V 2 /V 1 ) 5) ΔS = nRln(V 2 /V 1 ) I ...
by Philip Lee 1L
Sat Feb 09, 2019 12:20 am
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Nonspontaneous Changes
Replies: 1
Views: 44

Nonspontaneous Changes

The textbook says that "a nonspontaneous change can be brought about only by doing work" (7th edition 296).

Can nonspontaneous changes be brought about by a transfer of heat?
by Philip Lee 1L
Sat Feb 09, 2019 12:16 am
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Units for Entropy
Replies: 3
Views: 74

Re: Units for Entropy

From searching online for a bit, it seems like "disorder" is supposed to be unitless. This makes sense as it is simply a measure of the number of possible microstates in a system. Entropy and disorder are not the same thing. It is probably difficult to understand the physical meaning of en...
by Philip Lee 1L
Sat Feb 02, 2019 12:16 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Self-test 4A.1A
Replies: 1
Views: 28

Self-test 4A.1A

"Water expands when it freezes. How much work does 100. g of water do when it freezes at 0 o C and pushes back the metal wall of a pipe that exerts an opposing pressure of 1070 atm? The densities of water and ice at 0 o C are 1.00 g/cm 3 and 0.92 g/cm 3 , respectively" (7th edition pg. 247...
by Philip Lee 1L
Fri Feb 01, 2019 11:56 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: breaking double bonds?
Replies: 6
Views: 80

breaking double bonds?

Suppose a reactant molecule has a double bond between two carbon atoms, and the product molecule has only a single bond between these two carbon atoms. When doing calculations with bond enthalpies, why does the double bond break completely and then reform into a single bond (rather than just one bon...
by Philip Lee 1L
Fri Feb 01, 2019 11:25 pm
Forum: Phase Changes & Related Calculations
Topic: PΔV for solids and liquids
Replies: 5
Views: 85

PΔV for solids and liquids

In lecture today, Dr. Lavelle explained that PΔV is negligible for reactions that involve solids and liquids at constant pressure. However, the book states the following: "The expression w = -P ex ΔV applies to all systems. A gas is easiest to visualize, but the expression also applies to an ex...
by Philip Lee 1L
Thu Jan 24, 2019 5:36 pm
Forum: Calculating pH or pOH for Strong & Weak Acids & Bases
Topic: 6A.19 7th edition of the textbook
Replies: 1
Views: 137

Re: 6A.19 7th edition of the textbook

I think 10-15 is correct. Probably just a typo in the textbook.
by Philip Lee 1L
Thu Jan 24, 2019 5:31 pm
Forum: Calculating pH or pOH for Strong & Weak Acids & Bases
Topic: 6B.9
Replies: 1
Views: 116

Re: 6B.9

I think I got similar answers to what you got. I believe that the answer given in the back is a typo.

Textbook answer for (i):
[H3O+] = 1.50
[OH-] = 1.50 x 10-14

[H3O+] x [OH-] should be equal to 1.0 x 10-14
1.50 x 1.50 x 10-14 clearly does not equal 1.0 x 10-14
by Philip Lee 1L
Thu Jan 24, 2019 5:17 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Strong vs. Weak Acids
Replies: 3
Views: 66

Re: Strong vs. Weak Acids

Instead of looking at concentrations, I think you need to look at equilibrium constants to determine whether or not an acid or base is considered strong/weak. For example, any acid with a Ka > 103 is considered to be a strong acid. For bases, you need to consider Kb values.
by Philip Lee 1L
Thu Jan 17, 2019 8:20 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Weak acids
Replies: 2
Views: 57

Weak acids

If a really really weak acid is added to water and the change in [H 3 O + ] is very small (like 10 -9 ), then we say that the pH is essentially unchanged. 10 -7 + 10 -9 is really close to 10 -7 , so we say pH is 7. If a stronger acid is added to water and the change in [H 3 O + ] is larger, do we ne...
by Philip Lee 1L
Thu Jan 17, 2019 8:05 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Net Ionic Equations for Equilibrium Constants
Replies: 2
Views: 47

Re: Net Ionic Equations for Equilibrium Constants

I believe you should always use the net ionic equation for finding equilibrium constants since the species that appear on both sides of the equation do not change/participate in the reaction. If you do not use the net ionic equation, the species that appear on both sides should still cancel each oth...
by Philip Lee 1L
Thu Jan 17, 2019 8:01 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Sig Figs
Replies: 3
Views: 82

Re: Sig Figs

Yes, that is correct! You should also keep in mind the sig fig rules when solving problems with log, such as pH problems. The number you get after applying log to a certain value should have a number of decimal places equal to the number of sig figs in the original value. e.g. -log (1.0 x 10^-7) = 7...
by Philip Lee 1L
Thu Jan 10, 2019 12:31 pm
Forum: Non-Equilibrium Conditions & The Reaction Quotient
Topic: Question about self-test 5I.2A
Replies: 1
Views: 44

Question about self-test 5I.2A

A mixture of H 2 , N 2 , and NH 3 with partial pressures 22 kPa, 44 kPa, and 18 kPa, respectively, was prepared and heated to 500. K, at which temperature K = 3.6 x 10 -2 for the reaction 3H 2 + N 2 <--> 2NH 3 . Decide whether ammonia tends to form or decompose. (7th edition, page 418) The value of ...
by Philip Lee 1L
Wed Jan 09, 2019 8:58 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Section 5H.2 Composite Equations
Replies: 1
Views: 25

Re: Section 5H.2 Composite Equations

In this case, the second reaction has to be squared because it was multiplied by a factor of 2 before it was combined with the first reaction.
by Philip Lee 1L
Mon Jan 07, 2019 6:30 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: 6th edition hw problem 11.31
Replies: 2
Views: 57

Re: 6th edition hw problem 11.31

The value of Q is very similar to the value of K. The difference between the two is that Q is the value of (product activities)/(reactant activities) at ANY stage of a reaction, not just at equilibrium. If Q is less than K, that means (products)/(reactants) at that stage is less than at equilibrium....

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