## Search found 43 matches

Sat Mar 17, 2018 9:41 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Isolated systems
Replies: 3
Views: 387

### Isolated systems

Are all isolated systems rigid like a bomb calorimeter (i.e. volume will never change?

Also, how does qsys = -qsurr if no heat is being exchanged?
Sat Mar 17, 2018 9:34 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Bomb Calorimeter
Replies: 6
Views: 523

### Re: Bomb Calorimeter

since bomb calorimeters have rigid walls, it is impossible for volume to change, so w = -PdV and dV = 0 so w = 0.
Sat Mar 17, 2018 5:36 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Anode and cathode in cell diagram
Replies: 3
Views: 262

### Re: Anode and cathode in cell diagram

The copper wire is the solid in the other one!
Sat Mar 17, 2018 1:20 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Anode and cathode in cell diagram
Replies: 3
Views: 262

### Anode and cathode in cell diagram

The following items are obtained from the stockroom for the construction of a galvanic cell: two 250ml beakers and a salt bridge, a voltmeter with attached wires and clips, 200ml of 0.008M CrCl3 (aq), 200ml of 0.12M CuSO4(aq), a piece of copper wire, and a chrome-plated piece of metal. How would you...
Sat Mar 17, 2018 12:32 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Delta S
Replies: 2
Views: 570

### Delta S

What exactly is the second law of thermodynamics in relation to delta S? I thought it was that delta s universe is always zero, but then a question on a practice final said that delta S total was positive so therefore a reaction was spontaneous. What is delta S total, if not delta S universe? Also, ...
Thu Mar 15, 2018 9:00 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: When to use standard potentials and when to balance
Replies: 2
Views: 187

### When to use standard potentials and when to balance

What is the difference was between skeletal reactions that you have to balance by adding H+ and OH- vs skeletal reactions that you can jut copy down from the standard potentials page? In theory, a half reaction is a half reaction, right?
Wed Mar 14, 2018 8:00 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: 15.109
Replies: 1
Views: 193

### 15.109

The rate constant of the reaction O(g) + N2(g) --> NO(g) + N(g), which takes place in the stratosphere, is 9.7 x 10^10 L mol-1 s-1 at 800 deg C. The activation energy of the reaction is 315 kJ mol-1. Determine the rate constant at 700 deg C. What is the first step here? Do you need to solve for lnA ...
Wed Mar 14, 2018 5:52 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Coefficients in pre-equilibrium
Replies: 1
Views: 155

### Coefficients in pre-equilibrium

When do you multiply by a specific rate law when finding pre-equilibrium? For example, in the example we did in class: 2NO + O2 --> 2NO and observed rate = k[NO]^2[O2] step 1 (fast): NO + NO --> N2O2 , rate = k1[NO]^2 step 2 (slow) N2O2 + O2 --> 2NO2 , rate = k2[N2O2][O2] we then went on to remove t...
Wed Mar 14, 2018 4:19 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: 15.79
Replies: 1
Views: 158

### 15.79

Hi, can someone explain how to begin solving 15.79? In the reaction of HBr with the reactive intermediate CH3CH==CHCH2 , at low temperatures the predominant product is CH3CHBrCH==CH2, but at high temperatures, the predominant product is CH3CH==CHCH2Br. (where == is a double bond) (a) Which product i...
Thu Mar 08, 2018 8:11 pm
Forum: General Rate Laws
Topic: 15.17
Replies: 2
Views: 203

### 15.17

The problem asks you you to look at a table and determine the order for each reactant. When the concentration of A is 10, rate = 2.0 when the concentration of A is 20, rate = 4.0 (all else equal) Since 2 to 4 could be doubling or squaring, can you tell from this much information what the order is of...
Thu Mar 08, 2018 7:29 pm
Forum: General Rate Laws
Topic: Problem 15.3
Replies: 4
Views: 300

### Problem 15.3

In 20. s, the concentration of nitrogen dioxide, NO2, decreases from 450 mmol/L to 320 mmol/L in the reaction 2 NO2(g) --> 2 NO(g) + O2(g). (a) Determine the rate of reaction of NO2.

How do you know how to calculate rate of reaction if you don't know the order?
Thu Mar 08, 2018 7:05 pm
Forum: General Rate Laws
Topic: Stoichiometric coefficients and order
Replies: 5
Views: 545

### Stoichiometric coefficients and order

I know that stoichiometric coefficients are only supposed to influence rate in the unique rate laws for each substance. But don't moles also affect rate? i.e. if you have NO + NO --> N2O2 Wouldn't the rate be k[NO][NO] aka k[NO]^2, aka second order? In general, how can you tell which rate law to use...
Thu Mar 01, 2018 11:48 pm
Forum: Kinetics vs. Thermodynamics Controlling a Reaction
Topic: Straight line
Replies: 7
Views: 684

### Straight line

Why does a straight line mean that the reaction is the correct order to match the graph? Is it because a straight line represents the best fit line?
Thu Mar 01, 2018 11:42 pm
Forum: General Rate Laws
Topic: Determining Order [ENDORSED]
Replies: 8
Views: 400

### Re: Determining Order[ENDORSED]

How do you find orders of the reactants?
Thu Mar 01, 2018 11:41 pm
Forum: General Rate Laws
Topic: Instantaneous Rate vs. Unique Rate [ENDORSED]
Replies: 2
Views: 159

### Re: Instantaneous Rate vs. Unique Rate[ENDORSED]

A unique rate is a rate of appearance/disappearance of any of the species in a reaction divided by its stoichiometric coefficient, so it should be the same for each species. I think Lavelle mentioned that he would explain this in class tomorrow!
Thu Feb 22, 2018 10:18 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Pressure in calculating Q
Replies: 4
Views: 242

### Pressure in calculating Q

How does pressure affect the calculation of Q? Is the equation different than just [P]^n/[R]^n ?
Thu Feb 22, 2018 10:17 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: anode vs cathode in non standard cell diagram
Replies: 5
Views: 334

### anode vs cathode in non standard cell diagram

In a cell diagram, is the anode always on the left and the cathode on the right? Is this only the case for galvanic cells? For the nernst equation problems, it has us choose which one is the anode based on the absolute value of Eo for the half reaction...does this mean that sometimes the cathode is ...
Wed Feb 21, 2018 10:58 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 14.19
Replies: 2
Views: 177

### 14.19

A student was given a standard Cu(s) 0 Cu (aq) half-cell and another half-cell containing an unknown metal M immersed in 1.00 m M(NO3)2(aq). When the copper was connected as the anode at 25 C, the cell potential was found to be -0.689 V. What is the reduction potential for the unknown M2 /M couple? ...
Wed Feb 21, 2018 9:06 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 14.15a [ENDORSED]
Replies: 3
Views: 272

### 14.15a[ENDORSED]

AgBr(s) <--> Ag+(aq) + Br-(aq)

anode: AgBr(s) + e- --> Ag(s) + Br-(aq)
cathode: Ag+(aq) + e- --> Ag(s)

Where did these half reactions come from? Shouldn't the anode be losing electrons? Also, where is the AgBr in the cathode equation?
Sun Feb 18, 2018 4:14 pm
Forum: Balancing Redox Reactions
Topic: Bruincast
Replies: 4
Views: 322

### Bruincast

Is there a way to view lectures if we aren't enrolled in lecture 1? That's the only one on bruincast.
Wed Feb 14, 2018 2:58 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Variables in reversible isothermal vs isolated system [ENDORSED]
Replies: 2
Views: 384

### Variables in reversible isothermal vs isolated system[ENDORSED]

What exactly are all of the values that equal zero in an isolated system (or bomb calorimeter) vs in a reversible isothermal system?

I know that:

isolated
w = 0

reversible
deltaH = 0
deltaU = 0
w = 0?

what happens to q, and delta S, in these? /anything else important that I am forgetting?
Wed Feb 14, 2018 12:37 am
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: #4 on practice midterm
Replies: 1
Views: 185

### #4 on practice midterm

You have a system consisting of 0.60 moles of an ideal gas contained in a 50.0L container at 1.0 atm. You just love chemistry to a fault, so you perform a series of steps to the system. First, you perform an isobaric compression of the container to 20.0L. Then, you pressurize the system to 8.0 atm u...
Wed Feb 14, 2018 12:27 am
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: #4 on practice midterm
Replies: 3
Views: 258

### Re: #4 on practice midterm

Thank you! How do you reduce the pressure from 8 atm back to 1 atm in the isothermal expansion stage?
Tue Feb 13, 2018 11:47 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: #4 on practice midterm
Replies: 3
Views: 258

### #4 on practice midterm

You have a system consisting of 0.60 moles of an ideal gas contained in a 50.0L container at 1.0 atm. You just love chemistry to a fault, so you perform a series of steps to the system. First, you perform an isobaric compression of the container to 20.0L. Then, you pressurize the system to 8.0 atm u...
Sun Feb 11, 2018 12:45 am
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: Relative entropy levels
Replies: 2
Views: 346

### Relative entropy levels

When looking at reactions and deciding whether entropy will increase or decrease from reactants to products, what are all of the contributing factors? I know that more moles, more gas relative to other states, dissolution, and complex molecules with a lot of positions will generally have more entrop...
Sun Feb 11, 2018 12:25 am
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: 2nd law -- conceptual
Replies: 3
Views: 227

### 2nd law -- conceptual

I know that the 2nd law of thermodynamics states that in an isolated system, entropy is always increasing.

How does this change in a non-isolated system? In normal problems, is it legal for deltaS surroundings or deltaS total to have negative values?
Sat Feb 10, 2018 5:28 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: 11.111
Replies: 2
Views: 211

### 11.111

A certain enzyme-catalyzed reaction in a biochemical cycle has an equilibrium constant that is 10 times the equilibrium constant of the next step in the cycle. If the standard Gibbs free energy of the rst reaction is 200. kJ mol 1, what is the standard Gibbs free energy of the second reaction? So k2...
Sat Feb 10, 2018 2:17 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Arithmetic in 11.19c
Replies: 2
Views: 186

### Arithmetic in 11.19c

Can someone please show the math they did in 11.19c?

Calculate k for CaCO3(s) --> CaO(s) + CO2(g) at 25 degrees C

k = 1x10^-23 but my calculator keeps rounding to zero and I want to see if I did it right.

Thanks.
Fri Feb 09, 2018 3:21 pm
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: 9.55?
Replies: 1
Views: 137

### Re: 9.55?

You use the gaseous phase because it is listed as gas in the problem. Aqueous means that it's dissolved in water, so the problem would specifically mention that its an NH3 solution if it wanted you to find the enthalpy of the aqueous form.
Fri Feb 09, 2018 3:18 pm
Forum: Balancing Redox Reactions
Topic: Salt Bridges and Porous Disks
Replies: 2
Views: 148

### Re: Salt Bridges and Porous Disks

I think when we discuss salt bridges we use them in general terms to mean that the system continues transferring voltage through the circuit, instead of just stopping when the reactant has been fully oxidized. So I think we can assume that no matter what it's made of, it will serve to transfer enoug...
Wed Feb 07, 2018 10:59 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Standard molar entropy (9.53)
Replies: 1
Views: 161

### Standard molar entropy (9.53)

How do you find the standard molar entropy of the reaction of NH3(l) --> NH3(g)? The tables in the book show the standard molar entropy for the gas, but nothing for the liquid. Is it assumed we can use the aqueous solution?
Mon Jan 29, 2018 3:46 pm
Forum: Calculating Work of Expansion
Topic: Can someone explain the relationship of temperature to U?
Replies: 1
Views: 107

### Can someone explain the relationship of temperature to U?

We learned that in constant temperature conditions, internal energy is constant. Why is this? Can heat and work change if T is constant?
Mon Jan 29, 2018 3:43 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: What is the relationship between reversibility and spontaneity of a reaction?
Replies: 3
Views: 261

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

If a reaction is irreversible, is it spontaneous? Is there any relation between these or are they calculated separately?
Thu Jan 25, 2018 1:17 pm
Forum: Calculating Work of Expansion
Topic: Integrals [ENDORSED]
Replies: 3
Views: 254

### Integrals[ENDORSED]

Can someone explain why we keep finding derivatives and integrals when deriving work equations?

I understand that reversible isothermal expansion involves changing energy at infinitesimal increments but that is about it.
Thu Jan 25, 2018 1:06 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Cv vs. Cp [ENDORSED]
Replies: 13
Views: 3888

### Re: Cv vs. Cp[ENDORSED]

They are listed on the formula sheet!
Ideal gas, Cp = (5/2)R
Ideal gas, Cv = (3/2)R
Both are constant because heat capacity for gases isn't dependent on temp or volume.
Thu Jan 25, 2018 12:57 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Bond enthalpies vs enthalpy of rxn - conceptual [ENDORSED]
Replies: 3
Views: 378

### Bond enthalpies vs enthalpy of rxn - conceptual[ENDORSED]

I know that breaking bonds is endothermic and making bonds is exothermic, so total avg enthalpy is bonds broken - bonds formed, aka (bonds in reactants ) - (bonds in products ). However, in general, total change in enthalpy is (enthalpy of products ) - (enthalpy of reactants ). This seems opposite o...
Thu Jan 18, 2018 1:13 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Lewis structures for bond enthalpies
Replies: 3
Views: 144

### Lewis structures for bond enthalpies

For questions on bond enthalpies, should we expect to have to know which bonds are broken and formed for each molecule/draw Lewis structures? I took 14A a while ago and I want to know if I should spend time reviewing that content for 14B tests.
Wed Jan 17, 2018 5:59 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Equations for gases using R
Replies: 4
Views: 203

### Equations for gases using R

I know Prof. Lavelle mentioned in class we didn't need to know the isothermal expansion equation on p. 266, but there are several boxes later on that talk about gases that we haven't learned about in class (all the stuff with PV = nRT, 3/2 RT, etc.). Can we expect to learn this on Friday or is this ...
Wed Jan 17, 2018 5:41 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Standard Temperature
Replies: 2
Views: 127

### Re: Standard Temperature

When a problem specifies an "initial temp" they are talking about the system. 25 degrees is is the standard temperature for the surroundings, not the system.
Mon Jan 15, 2018 11:39 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Pressure in enthalpy vs heat capacity
Replies: 2
Views: 153

### Pressure in enthalpy vs heat capacity

There are different equations for heat capacity in the cases of standard pressure vs standard volume. Does the same apply to enthalpy? Or do we always assume standard pressure for equations outside of a calorimeter?
Fri Jan 12, 2018 8:10 pm
Forum: Phase Changes & Related Calculations
Topic: Isothermal Expansion
Replies: 2
Views: 225

### Re: Isothermal Expansion

Follow up question - what is isothermic expansion?
Fri Jan 12, 2018 8:06 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Liquids and Solids
Replies: 3
Views: 248

### Re: Liquids and Solids

The back of the book does have standard enthalpy of formation for several compounds in multiple states. It is definitely listed for water in solid, liquid and gas forms (they just did the phase conversion for us).
Fri Jan 12, 2018 8:04 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: 20145

### Difference between delta H and delta U

What is the difference between delta H and delta U? and q for that matter