## Search found 31 matches

Wed Mar 13, 2019 10:01 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Isothermal
Replies: 5
Views: 207

### Re: Isothermal

But if work is done, can't delta U change? Because delta U = q + w. And isothermal means there is no heat exchanged. Therefore q is 0 and delta U = w. If work is any value non zero, delta U will be too, so even if temperature is constant U will change with work. *Edit: Please ignore what I've said a...
Wed Mar 13, 2019 9:57 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Monoatomic Gas
Replies: 3
Views: 363

### Re: Monoatomic Gas

It depends? A monoatomic ideal gas can be at constant T or constant P.
Wed Mar 13, 2019 9:55 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Stot
Replies: 1
Views: 262

### Re: Stot

I believe deltaS total (aka S universe) is the deltaS surr plus deltaS sys. If deltaS total is positive, the process is spontaneous, if deltaS total is negative the reaction is non spontaneous, and if deltaS total is 0, the reaction is at equilibrium.
Thu Mar 07, 2019 1:19 am
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: 14.93a (6th ed)
Replies: 1
Views: 78

### Re: 14.93a (6th ed)

It's because the cell relies on the concentration gradient to power it. The reduction part would for example have an ion added to electrons to make a solid. Therefore it contributes the ion and its corresponding concentration to the reactant side, whereas in the oxidation part it is the opposite, an...
Thu Mar 07, 2019 1:04 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Ecell
Replies: 13
Views: 414

### Re: Ecell

As Dr. Lavelle stated in lecture, if you don't want to remember the equation of cathode - anode, just make sure you write the reduction half reaction and find the E°, then write the oxidation held reaction and find the E°, making sure that when you look at the standard reduction tables to find the f...
Thu Mar 07, 2019 12:56 am
Forum: Van't Hoff Equation
Topic: test 2
Replies: 7
Views: 562

### Re: test 2

My TA mentioned that if you use the equation you can determine the new [H+] concentration and then compare it to 10^7. I think if it's higher then it's more acidic, if it's lower its more basic.
Wed Feb 27, 2019 3:45 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: E and Spontaneity
Replies: 2
Views: 81

### Re: E and Spontaneity

The cell reaction is spontaneous when E is positive, because in the equation deltaG = -nFE, if E is positive, G is negative, which is a spontaneous process.
Wed Feb 27, 2019 3:40 pm
Forum: Balancing Redox Reactions
Topic: 14.5 6th edition
Replies: 2
Views: 120

### Re: 14.5 6th edition

I believe it might have something to do with the fact that this reaction is taking place in a basic solution. Therefore you would follow the balancing rules for basic solutions of adding H2O to balance out the O's and then H2O to the side that needs H's and then OH- to the other side you put the H's...
Wed Feb 27, 2019 3:36 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Cell diagram
Replies: 1
Views: 79

### Re: Cell diagram

Yes, by using a comma you are denoting that the two species are in the same phase, for example Fe3+ (aq), Fe2+ (aq). However, if they are in different phases, say you need to put in a conducting solid like Pt, you would denote it as Fe3+ (aq), Fe2+ (aq) I Pt(s). Different phases are separated by the...
Wed Feb 20, 2019 4:24 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Gibbs and concepts
Replies: 3
Views: 135

### Re: Gibbs and concepts

In addition, another concept that is good to keep in the back of your mind which Dr. Lavelle covered in lecture is that the numerical value of G has no bearing on how fast the reaction is happening. For example, a reaction with delta G = -344 kJ/mole is not necessarily happening at a faster rate tha...
Wed Feb 20, 2019 4:20 pm
Forum: Balancing Redox Reactions
Topic: Separating the equation
Replies: 5
Views: 154

### Re: Separating the equation

The separated reactions are called half-reactions, and they generally help you visualize the transfer of electrons with respect to oxidation and reduction. For balancing equations, people who are really familiar with these reactions could figure out how to balance the charges and mols of elements in...
Wed Feb 20, 2019 4:15 pm
Forum: Balancing Redox Reactions
Topic: how to balance
Replies: 5
Views: 167

### Re: how to balance

Yeah, so you would have to take both into consideration. In the end, not only does the number of moles of each element have to be the same, the charges on each side also have to be the same. For example, in the redox reaction (Zn) + (Ag^+) ---> (Zn^2+) + (Ag), it might seem that there is one mole of...
Thu Feb 14, 2019 12:03 am
Forum: Calculating Work of Expansion
Topic: Reversible expansion
Replies: 4
Views: 236

### Re: Reversible expansion

It's also good to note that reversible reactions always do more work or have more work done by irreversible reactions, since reversible reactions is the max work done.
Wed Feb 13, 2019 11:57 pm
Forum: Calculating Work of Expansion
Topic: Integrals
Replies: 4
Views: 166

### Re: Integrals

Yeah, I don't think deriving formulas are a requirement of this class, although it definitely helps you understand the concepts better and in the case you that you don't have equations in front of you, you can always get from place to place by deriving formulas.
Wed Feb 13, 2019 11:52 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Changes in Pressure
Replies: 5
Views: 182

### Re: Changes in Pressure

It has to do with the equation PV=nRT. P and V are inversely related, so in the formula, you would also have to make sure your terms are inverses. It should be that if you have P1 corresponding to V1 and a P2 corresponding to a V2, which you can find one by the other through PV=nRT, deltaS=nRln(P1/P...
Thu Feb 07, 2019 12:37 pm
Forum: Calculating Work of Expansion
Topic: 6th edition 8.27 part b
Replies: 3
Views: 139

### Re: 6th edition 8.27 part b

Hmmm, well for the number of mols you get from PV=nRT using the values discussed should be .307 mols. I am not sure how you got 3.068 x 10^-5 mols. After getting the mols though, since it is a reversible isothermal expansion, you use w=-nRTln(Vf/Vi). If n=.307 mols, R=8.314 J/(mol K), T=305 K and Vf...
Wed Feb 06, 2019 6:57 pm
Forum: Calculating Work of Expansion
Topic: 6th edition 8.27 part b
Replies: 3
Views: 139

### Re: 6th edition 8.27 part b

The volume should be 4.29L, not the final volume, since you want to find the number of moles at the start of the expansion.
Wed Feb 06, 2019 6:49 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: #25 Homework 6th Edition
Replies: 1
Views: 80

### Re: #25 Homework 6th Edition

It should be the power of however many molecules there are. So if there's one molecule, its 6^1 and therefore the degeneracy is 6. However if it's 1 mole of the SO2F2 then you would have 6^Avogadro's number which is 6^6.02x10^23. The question asks for residual molar entropy, so it is asking for 1 mo...
Wed Feb 06, 2019 6:42 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: Isothermal reaction
Replies: 3
Views: 257

### Re: Isothermal reaction

As the previous posts mention, isothermal means constant temperature. Similarly there is also the term isobaric, which means constant pressure.
Tue Jan 29, 2019 12:51 am
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Le Chatelier's Principle
Replies: 3
Views: 216

### Re: Le Chatelier's Principle

Yes, Le Chatelier's Principle's definition is as follows: to counteract a change in condition, a system in equilibrium will shift as to minimize the change's effect. If a system was not at equilibrium, you could think of it like in the middle of a change, and then system will shift to be in a state ...
Tue Jan 29, 2019 12:44 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: HW Week4
Replies: 4
Views: 206

### Re: HW Week4

Hi all, I was wondering if anyone knew the sections for the 6th edition that Dr. Lavelle has lectured. I ran into a similar problem when I was doing the homework before the lectures on this topic started and then realized the lectures didn't go in the same order as the textbook.
Tue Jan 29, 2019 12:38 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Cv versus Cp
Replies: 4
Views: 187

### Re: Cv versus Cp

Yes, Cp is always greater than Cv. This is because when temperature changes under constant pressure there is extra work being done to change the volume. If all else is constant, when temperature increases, volume must increase too. This is as opposed to constant volume in Cv.
Wed Jan 23, 2019 4:51 pm
Forum: Non-Equilibrium Conditions & The Reaction Quotient
Topic: Strength of acids based on Ka and pKa
Replies: 4
Views: 502

### Re: Strength of acids based on Ka and pKa

The rule is this: The larger the Ka, the stronger the acid. The larger the Kb, the stronger the base. The smaller the pKa, the stronger the acid. The smaller the pKb, the stronger the base.
Wed Jan 23, 2019 4:48 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Hess's Law
Replies: 5
Views: 110

### Hess's Law

Hey does anyone know what Hess's Law is? Did they go over that in Lavelle's 14A?
Wed Jan 23, 2019 4:14 pm
Forum: Phase Changes & Related Calculations
Topic: How to tell acids and bases
Replies: 8
Views: 248

### Re: How to tell acids and bases

It also depends on which definition of acids and bases you are using. In this class I believe we mainly just use the Brønsted-Lowry definition of acids and bases, which follows: A Brønsted-Lowry acid is anything that is a proton donor (when I say proton donor in this context it means an H+ ion, not ...
Wed Jan 16, 2019 10:56 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: pH
Replies: 5
Views: 160

### Re: pH

Also if pOH is given and it asks for you to give the pH, know that pKw= pH + pOH. This is derived from the equation where Kw= [H+][OH-].
Wed Jan 16, 2019 10:46 pm
Forum: Non-Equilibrium Conditions & The Reaction Quotient
Topic: Percentage Ionization
Replies: 3
Views: 164

### Re: Percentage Ionization

Elaborating on the previous replies, since percent ionization for acids are [concentration of H+ at equilibrium][/concentration of HA initial] multiplied by 100, it is the amount of the original acid that has dissociated in percent.
Wed Jan 16, 2019 10:41 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Endothermic Reaction
Replies: 4
Views: 148

### Re: Endothermic Reaction

That is kinda confusing, huh. Normally Kw will not change unless temperature changes, so in the case of 25 degrees C Kw is 1.00 x 10^-14 but when it is 50 degrees C Kw is 5.48 x 10^-14. I'm not sure about thinking about Kw in terms of adding heat an the reaction being endothermic or not, but for sur...
Wed Jan 09, 2019 10:44 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: The relationship between Kc and K
Replies: 2
Views: 110

### The relationship between Kc and K

Can somebody explain conceptually why when Kc is greater than K the gaseous mols of reactants are greater than the mols of products and the reaction proceeds forward?
Tue Jan 08, 2019 6:12 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Concentrations of products/reactants
Replies: 6
Views: 237

### Re: Concentrations of products/reactants

It also matters if it's Kc or Kp. While Kc does refer to the ratio of concentrations to their respective powers of products and reactants, Kp will refer to the ratio of partial pressures to their respective powers of products and reactants.
Tue Jan 08, 2019 6:04 pm
Forum: Ideal Gases
Topic: R in PV=nRT
Replies: 34
Views: 685

### Re: R in PV=nRT

Different problems will have different units, so as long as you match the unit to the corresponding R value you should be good. Here is a list with some R values:

0.0821 (atm L)/(mol K)
0.0831 (bar L)/(mol K)
8.314 (J)/(K mol)
62.36 (torr L)/(mol K) or (mmHg L)/(mol K)
1.987 (cal)/ (mol K)