Search found 30 matches

Tue Mar 13, 2018 7:39 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Bomb Calorimeter
Replies: 2
Views: 374

Re: Bomb Calorimeter

Constant volume does mean that delta V is equal to 0. Since a bomb calorimeter is an isolated system, Delta U is equal to 0 because no heat or work is being transferred/done.
Tue Mar 13, 2018 7:38 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Standard Hydrogen Electrode
Replies: 1
Views: 135

Re: Standard Hydrogen Electrode

The standard hydrogen electrode is the cathode or the anode depending on what else is in the system (like copper or zinc). How you can tell whether the standard hydrogen electrode is the cathode or anode is based on the cell potential. The cell potential should be positive, so according to the equat...
Tue Mar 13, 2018 7:34 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Test 1 #3
Replies: 4
Views: 426

Re: Test 1 #3

Since it is at the same temperature, you know that Delta U is equal to zero. However, you also know that work is happening because the balloon is expanding, therefore indicating a change in volume. According to Delta U = q + w, for Delta U to be zero, there must also be heat being transferred in ord...
Wed Mar 07, 2018 1:08 pm
Forum: First Order Reactions
Topic: Question 15.23
Replies: 3
Views: 267

Re: Question 15.23

You find the concentration of A used up, which is stoichiometry since you're given the concentration of B that rises. Then you subtract the concentration of A used up from the initial concentration of A for the final concentration. After that, since it's a first order reaction you use the equation l...
Wed Mar 07, 2018 11:06 am
Forum: General Rate Laws
Topic: Rate Laws
Replies: 3
Views: 249

Re: Rate Laws

I think for this class, we consider the initial concentration of the reactant when finding the rate law because it is easier to study when there is little product formed.
Wed Mar 07, 2018 9:48 am
Forum: Method of Initial Rates (To Determine n and k)
Topic: 15.29
Replies: 2
Views: 186

Re: 15.29

You are using the equation ln[A] = -kt + ln[A]0 to find k. However, first you must use stoichiometry to find the concentration of A used up to form the given mols of B. Then you subtract the concentration of A used up from the initial concentration of A to find the final concentration of A. After yo...
Sat Mar 03, 2018 8:23 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Step Speed
Replies: 3
Views: 204

Step Speed

Will we ever have to figure out whether a step in a reaction is fast, very fast, or slow? Or is that always given to us? Also, is it possible to have two slow steps in a reaction?
Fri Mar 02, 2018 10:00 am
Forum: First Order Reactions
Topic: Pseudo-First-Order Reactions [ENDORSED]
Replies: 3
Views: 255

Pseudo-First-Order Reactions[ENDORSED]

Will we be expected to do calculation problems regarding pseudo-first-order reactions? If so, what are some examples of what we might need to know?
Thu Mar 01, 2018 9:38 pm
Forum: Method of Initial Rates (To Determine n and k)
Topic: 15.23 & 15.29 [ENDORSED]
Replies: 2
Views: 239

Re: 15.23 & 15.29[ENDORSED]

You need the final mols of A, but you are given the concentration B has risen to. You first calculate the amount of mols of A that need to be used up for B to have that concentration. Then you subtract that value from the initial mols of A to find the final mols of A.
Tue Feb 20, 2018 10:19 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: 14.31
Replies: 6
Views: 394

Re: 14.31

You use E(cathode)-E(anode) to find the standard E. If E is positive, it favors the products so k>1.
Tue Feb 20, 2018 10:14 pm
Forum: Balancing Redox Reactions
Topic: 14.3
Replies: 2
Views: 181

Re: 14.3

You add 5e- because you need to balance out the charge. Since the left side is Mn7+ and the right side is Mn2+, you need 5 electrons to make the charges on both sides equal. You know it is Mn7+ because oxygen has a 2- charge, so O4 should have an 8- charge. However, MnO4 only has a 1- charge overall...
Tue Feb 20, 2018 10:11 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Platinum
Replies: 1
Views: 86

Re: Platinum

You add platinum when there is no solid metal to conduct the electron flow in the galvanic cell.
Mon Feb 12, 2018 3:56 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Practice Midterm Karen Leung #8 [ENDORSED]
Replies: 4
Views: 428

Re: Practice Midterm Karen Leung #8[ENDORSED]

I think you should use delta S = nCvln(T2/T1), with n as the total mols of gas and then add your answer from part a) to get the total entropy. However, I don't know if we can assume that the gases are ideal, since it doesn't say so in the problem. If they aren't, then we can't calculate Cv.
Mon Feb 12, 2018 3:52 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: practice test 6A&6B
Replies: 1
Views: 132

Re: practice test 6A&6B

For 6a), the reaction is exothermic, so delta H is negative. We are also given that the temperature is very low. Based on the equation delta G = delta H - T(delta S), if delta S is positive, then the reaction is spontaneous since the answer will be negative. Even if delta S is negative, since the te...
Mon Feb 12, 2018 3:34 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: entropy
Replies: 4
Views: 352

Re: entropy

In the book, it usually has delta S as the entropy of the system, while it specifies the entropy of the surroundings and total entropy with delta Ssurr and delta Stot respectively. Delta Stot would therefore be delta S + delta Ssurr.
Wed Feb 07, 2018 10:40 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: 11.21
Replies: 2
Views: 211

Re: 11.21

For this question, you don't need Q because the problem is assuming the reactants and products are at equilibrium. To find the standard Gibbs free energy, you just have to use the equation delta G = -RTlnK. Hope this helps!
Tue Feb 06, 2018 7:33 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Salt Bridge
Replies: 7
Views: 415

Salt Bridge

Is there always a salt bridge? If not, when would a salt bridge be used?
Tue Feb 06, 2018 7:31 pm
Forum: Balancing Redox Reactions
Topic: Basic vs. Acidic
Replies: 6
Views: 322

Re: Basic vs. Acidic

In terms of balancing, when the reaction takes place in a basic solution you use OH- to balance the chemical reaction. When the reaction takes place in an acidic solution, you use H+ to balance.
Mon Jan 29, 2018 7:34 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Pressure and Volume on Spontaneity
Replies: 2
Views: 287

Re: Pressure and Volume on Spontaneity

Yes, pressure and volume do affect the spontaneity of a reaction because they contribute to entropy. For example, when a system is expanding, there is more area and translational energy. This results in a higher entropy.
Mon Jan 29, 2018 7:22 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: What is the relationship between reversibility and spontaneity of a reaction?
Replies: 3
Views: 289

Re: What is the relationship between reversibility and spontaneity of a reaction?

There is no relationship between reversibility and spontaneity of a reaction. It depends on entropy and Gibbs free energy.
Mon Jan 29, 2018 7:20 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Degeneracy and Entropy
Replies: 3
Views: 309

Re: Degeneracy and Entropy

Degeneracy refers to the number of different states energy can be contained in. The more states (higher degeneracy), the higher the entropy, because there is more randomness.
Mon Jan 22, 2018 4:44 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: 109
Replies: 2
Views: 306

Re: 109

For this problem they are finding the volume of the coal in cm^3 and using the given density to find the grams of carbon burned. Since the delta H given is in kJ/mol, you need to convert the grams of carbon burned to the mols of carbon burned, which is why they have the (12.01 g/mol) in the equation...
Mon Jan 22, 2018 4:39 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Exothermic Reactions
Replies: 2
Views: 132

Re: Exothermic Reactions

An exothermic reaction means that heat is being released from the system into the surroundings, which in this case would be the reactor. Since heat is being released, the reactor temperature will increase. Therefore, to maintain a constant temperature in the reactor, it will need to be cooled. Hope ...
Mon Jan 22, 2018 2:26 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Calculating Entropy for Irreversible Energy Transfers [ENDORSED]
Replies: 1
Views: 151

Re: Calculating Entropy for Irreversible Energy Transfers[ENDORSED]

For homework problem 9.1, you can use the equation Delta S = q/T to calculate the change in entropy. Just make sure to keep your units constant.
Wed Jan 17, 2018 4:43 pm
Forum: Calculating Work of Expansion
Topic: Free Expansion
Replies: 2
Views: 182

Free Expansion

The book mentions the concept of free expansion, and I understand that w=0. What would be an example of free expansion? Would we ever encounter a problem with free expansion in this class?
Mon Jan 15, 2018 4:32 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.67b
Replies: 1
Views: 126

Re: 8.67b

The number +717 kJ/mol is given in the problem itself, and it is the standard enthalpy of sublimation of carbon. Since it is given to us directly in the problem, I don't think it is in the tables or the appendix.
Mon Jan 15, 2018 4:26 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Enthalpy Trends
Replies: 2
Views: 204

Re: Enthalpy Trends

Typically a higher bond enthalpy means that it takes more energy to break/form the bond. Stronger bonds therefore have higher enthalpies.
Wed Jan 10, 2018 3:33 pm
Forum: Phase Changes & Related Calculations
Topic: Enthalpy of Freezing
Replies: 4
Views: 520

Enthalpy of Freezing

Is the symbol for enthalpy of freezing -∆Hfus or is there another symbol we should use?
Tue Jan 09, 2018 11:36 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Standard Reaction Enthalpies [ENDORSED]
Replies: 1
Views: 124

Standard Reaction Enthalpies[ENDORSED]

Are standard reaction enthalpies always molar enthalpies? Also, I am a bit confused on the term "standard state" and what that implies.
Mon Jan 08, 2018 3:30 pm
Forum: Phase Changes & Related Calculations
Topic: Enthalpy and Volume of a System
Replies: 2
Views: 147

Enthalpy and Volume of a System

I understand that a change in enthalpy of a system takes into consideration heat released or absorbed at constant pressure. However, is there a relation between enthalpy and volume of the system?