Search found 42 matches

Sat Mar 17, 2018 11:03 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: why is the entropy change at constant pressure greater than the entropy change at constant volume?
Replies: 3
Views: 535

why is the entropy change at constant pressure greater than the entropy change at constant volume?

Why is the entropy change at constant pressure greater than the entropy change at constant volume? I understand mathematically by looking at the formula, but I was curious why conceptually?
Sat Mar 17, 2018 5:29 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Test 2 Question 8b
Replies: 9
Views: 851

Re: Test 2 Question 8b

Haocheng Zhang 2A wrote:I asked my high school chemistry teacher this question, he said we don't have to convert them to the same unit becasue equilibrium constant is unitless, so I am wrong. Sorry about that.

Do we still use molarity and bars then or do we convert it to atm or some other pressure unit?
Sat Mar 17, 2018 1:46 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Test 2 Question 8b
Replies: 9
Views: 851

Re: Test 2 Question 8b

You need to convert bar to M. We need to keep unit the same. Think of the idea gas equation PV=nRT, then we have P/RT=n/V. The number of mole divided by volume is molarity, so P/RT=Molarity. That's how we convert bar to M. Hope it helps! You had to assume standard conditions, so temperature = 298 K...
Fri Mar 16, 2018 10:18 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: What do we use for the axis of reaction profiles?
Replies: 1
Views: 173

What do we use for the axis of reaction profiles?

In this class do we us potential energy, enthalpy or gibb's free energy for the y-axis?
Do we use reaction progress or reaction coordinate for the x-axis?
Fri Mar 16, 2018 10:15 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: What are the units of activation energy?
Replies: 2
Views: 1499

What are the units of activation energy?

Do we use J/mol or J for activation energy?
Thu Mar 15, 2018 2:30 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: How do you use a combined equilibrium equation?
Replies: 1
Views: 250

How do you use a combined equilibrium equation?

From the Wednesday discussion electro-chemistry test question 8, how do you figure out which units of pressure to use in the mixed equilibrium expression? We can't use PV=nRT to find molarity because we don't know temperature.
Thu Mar 15, 2018 2:08 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Test 2 Question 8b
Replies: 9
Views: 851

Re: Test 2 Question 8b

How do you know temperature? It isn't stated in the problem.
Thu Mar 15, 2018 4:58 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Standard Gibbs
Replies: 3
Views: 449

Re: Standard Gibbs

I'm pretty sure that standard refers to the standard state of the molecule (Ex: gas, liquid or solid). It is also at 273.15K and 1 atm pressure.
Thu Mar 08, 2018 3:10 pm
Forum: Second Order Reactions
Topic: Half Life for First Order vs Second Order
Replies: 5
Views: 627

Re: Half Life for First Order vs Second Order

The half life for first order is constant whereas the half life for second order reactions are double the previous half life. Hence, you can not apply the method to second order reactions.
Thu Mar 08, 2018 3:06 pm
Forum: General Rate Laws
Topic: integrated rate law to determine order
Replies: 2
Views: 214

Re: integrated rate law to determine order

By manipulating the equation to the y = mx + b format, you can compare your equation to the integrated rate laws for each order, and then determine the order of your expression.
Wed Mar 07, 2018 6:27 pm
Forum: First Order Reactions
Topic: First Order Integrated Rate Law
Replies: 3
Views: 260

Re: First Order Integrated Rate Law

The first equation you pointed out and the second equation are equal. It is just converting between ln and log.
Tue Mar 06, 2018 1:07 am
Forum: General Rate Laws
Topic: rate constants
Replies: 7
Views: 459

Re: rate constants

the rate constant is constant for a specific reaction at a specific temperature. If the reaction is different then the activation energy would be different, and if the temperature is different then that would influence K as well.
Tue Mar 06, 2018 1:03 am
Forum: General Rate Laws
Topic: Concentration and Rate Relationship
Replies: 4
Views: 301

Re: Concentration and Rate Relationship

The answer to your question depends on the specific rate law for the reaction you are discussing. Generally if you plug in the change and then compare the resulting rate laws, you should be able to figure out what the change in rate is.
Tue Mar 06, 2018 1:01 am
Forum: Experimental Details
Topic: units
Replies: 4
Views: 634

Re: units

It really doesn't matter whether you convert it to mol/L or not. Its pretty much up to personal preference and whatever helps you solve the problem.
Tue Mar 06, 2018 1:00 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: "Slow" Step Only?
Replies: 4
Views: 292

Re: "Slow" Step Only?

The rest of the reaction proceeds much faster than the slow step, hence the slow step is the only thing holding every thing back. For example, if you have a bunch of funnels that pour water into each other. The smallest funnel hole will determine how fast the water flows even if the funnels before a...
Tue Mar 06, 2018 12:58 am
Forum: Zero Order Reactions
Topic: slope
Replies: 3
Views: 399

Re: slope

the values themselves did undergo non-linear growth or decay, but we can modify the values to make the graph linear.
Sun Mar 04, 2018 9:36 pm
Forum: Method of Initial Rates (To Determine n and k)
Topic: Little Product
Replies: 4
Views: 346

Re: Little Product

If product is present then it can influence the reaction rate. Hence, we study the initial rates (when little to no product has formed) so that we can ignore the effects of products on the reaction rate.
Sun Mar 04, 2018 9:35 pm
Forum: First Order Reactions
Replies: 4
Views: 268

If you think about radioactive decay it almost always involves a large atom decaying into smaller ones or giving off gamma rays (energy), so it is probably safe to say that radioactive decay only has one reactant and thus is first order.
Sun Mar 04, 2018 6:39 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: Catalyst, Equation, and Rate Law
Replies: 4
Views: 2965

Re: Catalyst, Equation, and Rate Law

The poster above did a good job explaining the concept, so here is an example using an enzyme. Enzymes bring their substrates together and then alter their formation to make them more likely to react and thus start the reaction. However, it would be horribly energy inefficient if a cell had to creat...
Sun Mar 04, 2018 6:35 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: Catalysts
Replies: 3
Views: 278

Re: Catalysts

A catalyst just alters the environment or makes it more likely that a reaction will occur. Effectively, it reduces the activation energy "hill" so that the reactants can react with each other. This can be shown by enzymes. Enzymes bring their substrates together and then alter their format...
Sun Mar 04, 2018 6:32 pm
Forum: Method of Initial Rates (To Determine n and k)
Topic: When forming rate laws do we include solids and liquids?
Replies: 3
Views: 2868

When forming rate laws do we include solids and liquids?

When writing rate laws do we include solids and liquids in the rate law expression?
Wed Feb 21, 2018 11:59 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: When do we include H+ in the cell diagram? [ENDORSED]
Replies: 1
Views: 226

When do we include H+ in the cell diagram?[ENDORSED]

When do we include H+ in the cell diagram? Additionally, how do we know whether it is there to balance the charge or to partake in the redox reaction?
Wed Feb 14, 2018 8:52 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Why is the change in total entropy zero for a reversible free expansion?
Replies: 2
Views: 295

Re: Why is the change in total entropy zero for a reversible free expansion?

Ah sorry. I think I meant only reversible expansion.
Wed Feb 14, 2018 4:45 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Why is the change in total entropy zero for a reversible free expansion?
Replies: 2
Views: 295

Why is the change in total entropy zero for a reversible free expansion?

Why is the ΔStot = 0 for a reversible free expansion?
Wed Feb 14, 2018 10:07 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Why are standard free energies of formation zero for elements in their stablest form?
Replies: 2
Views: 223

Why are standard free energies of formation zero for elements in their stablest form?

I understand that ΔH⁰f is 0 for an element in its standard state.
But then by using the equation ΔG⁰f = ΔH⁰f - TΔS⁰f, shouldn't ΔG⁰f have some other value than 0? because elements in there standard states do have some ΔS⁰f value.
Ex: ΔG⁰f = 0 - (298K)ΔS⁰f ≠ 0
Tue Feb 13, 2018 4:26 pm
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: Is spontaneity determined by entropy or free energy?
Replies: 9
Views: 653

Is spontaneity determined by entropy or free energy?

I previously understood that a reaction was spontaneous when ΔG < 0?
But I was reading through the textbook and it stated that a reaction was spontaneous if ΔStot > 0. I though that a reaction did not have to be spontaneous if ΔS > 0?
Fri Feb 09, 2018 11:16 pm
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: How to calculate W [ENDORSED]
Replies: 6
Views: 466

Re: How to calculate W[ENDORSED]

W represents degeneracy and is the various micro states that the molecule can have at a specific energy.
Fri Feb 09, 2018 11:14 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Gibb's Free Energy
Replies: 2
Views: 184

Re: Gibb's Free Energy

Some real life examples of spontaneous processes are:
water freezing below 0°C
a boulder rolling down a hill
Fri Feb 09, 2018 11:06 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: work
Replies: 2
Views: 171

Re: work

When the volume of the system changes then the system has to do or have work done upon the system. For example if the system expands, it is doing work to more the surroundings out of the way. Hence, work is being done on the system. If a system is compressed, the surroundings are making the system s...
Wed Jan 24, 2018 10:21 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: What is the pressure value for standard states?
Replies: 2
Views: 267

What is the pressure value for standard states?

The textbook uses 1 bar for standard states but if I remember correctly we used 1 atm in class. Which one should we use on the exam?
Wed Jan 24, 2018 7:29 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Why is internal energy independent of volume for an ideal gas?
Replies: 1
Views: 561

Why is internal energy independent of volume for an ideal gas?

Why is internal energy independent of volume for an ideal gas?
Wed Jan 24, 2018 6:52 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: For a monatomic ideal gas, why are CP and CV are independent of temperature and pressure? [ENDORSED]
Replies: 1
Views: 516

For a monatomic ideal gas, why are CP and CV are independent of temperature and pressure?[ENDORSED]

In section 8.10, it states "Note that for a monatomic ideal gas, both CP and CV are independent of temperature and pressure"(281). Why is this so?
Wed Jan 24, 2018 6:32 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: What is the difference between internal energy and molar internal energy? [ENDORSED]
Replies: 2
Views: 1342

What is the difference between internal energy and molar internal energy?[ENDORSED]

What is the difference between internal energy and molar internal energy conceptually and mathematically?
Wed Jan 24, 2018 5:27 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Heat capacity question
Replies: 5
Views: 448

Heat capacity question

When do we use the following equations/what do they mean?
Ideal gas, Cp = (5/2) R
Ideal gas, Cv = (3/2) R
and what does R refer to?
Wed Jan 24, 2018 5:23 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Chapter 8.7 concepts and math question
Replies: 1
Views: 196

Chapter 8.7 concepts and math question

When do we use U = 3/2RT and when do we use U = 5/2RT and what does each equation represent? Additionally when do we take into account translational, rotational linear and rotational non-linear energies?
Fri Jan 19, 2018 9:06 pm
Forum: Calculating Work of Expansion
Topic: What exactly is reversible expansion?
Replies: 2
Views: 206

What exactly is reversible expansion?

Why is reversible expansion is called reversible expansion because it is usually in one direction? Also why does the pressure on the outside have to be the same as that on the inside?
Mon Jan 15, 2018 6:52 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Difference between heat(q), internal energy(U), and enthalpy(H)
Replies: 4
Views: 2469

Difference between heat(q), internal energy(U), and enthalpy(H)

I don't understand the conceptual difference between heat(Q), internal energy(U), and enthalpy(H). I understand that from a mathematical standpoint U contains Q but I am confused as to what exactly the difference is.
Mon Jan 15, 2018 5:23 pm
Forum: Calculating Work of Expansion
Topic: Will we be given conversion factors on the exam? [ENDORSED]
Replies: 3
Views: 273

Will we be given conversion factors on the exam?[ENDORSED]

Will we be given the values to convert between the different pressure units, converting from L*atm, and the various R values on the exams?
Thu Jan 11, 2018 6:09 pm
Forum: Phase Changes & Related Calculations
Topic: Intensive vs. Extensive
Replies: 5
Views: 640

Re: Intensive vs. Extensive

Intensive properties are properties that are not related to the size or mass of the system. (Think intrinsic). Extensive properties change with the mass or the size of the system. For example, for water will always boil at 100°C. Hence boiling point is an intensive property. However, the amount of e...
Wed Jan 10, 2018 7:22 pm
Forum: Phase Changes & Related Calculations
Topic: Why can state properties be added?
Replies: 11
Views: 705

Re: Why can state properties be added?

State properties are pathway independent. Meaning regardless of how the final product is achieved it will always have that particular value (in this case enthalpy). For example, if you wanted to make a NO 2 . The enthalpy of NO 2 will always be the same, regardless of how it was formed. Hence, we ca...
Wed Jan 10, 2018 7:13 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Why is fusion another name for melting?
Replies: 4
Views: 259

Why is fusion another name for melting?

I was curious why fusion and melting mean the same thing with regard to ΔHfus? Fusion and melting seem almost the opposite of each other seeing as how when something melts, some of the bonds in the solid break apart.
Wed Jan 10, 2018 7:00 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Is bond enthalpy the same as dissociation energy? [ENDORSED]
Replies: 2
Views: 121

Is bond enthalpy the same as dissociation energy?[ENDORSED]

In lecture today we learned that bond enthalpy is equal to the amount of energy required to break the bond. I just wanted to clarify if bond enthalpy the same as dissociation energy?