## Search found 61 matches

Fri Mar 13, 2020 9:15 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Delta S total
Replies: 3
Views: 32

### Re: Delta S total

To calculate total change in entropy (S) you add change in S of the surroundings and change in S of the system.
Fri Mar 13, 2020 6:54 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: Equilibrium Constant
Replies: 2
Views: 11

### Re: Equilibrium Constant

A catalyst does not affect equilibrium constant. Only temperature, in think, affects K.
Fri Mar 13, 2020 6:52 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Slow Step Mechanism
Replies: 7
Views: 38

### Re: Slow Step Mechanism

I'm not sure if early or later steps are more common but I've seen more problems with the later step being the slower reaction rate which would create a "bottle neck" or a back up in intermediate reactants
Fri Mar 13, 2020 5:28 pm
Forum: Second Order Reactions
Topic: Second Order Rxn
Replies: 2
Views: 20

### Re: Second Order Rxn

The second order reaction has a positive slope on a graph with 1/[A] on the y axis and 1/time on the x axis
Fri Mar 13, 2020 5:24 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Reduction Potential
Replies: 4
Views: 20

### Re: Reduction Potential

The anode has the reduction potential that is more negative I believe
Thu Mar 12, 2020 2:31 pm
Forum: First Order Reactions
Topic: Half Life Equations
Replies: 10
Views: 42

### Re: Half Life Equations

The 0th, 1st, and 2nd order equations are provided on the equation sheet but the equation sheet doesn't specify which one is which. So if you are doing your test with the equation sheet I recommend labeling each one so you don't mix them up.
Fri Mar 06, 2020 10:04 pm
Forum: Balancing Redox Reactions
Topic: spontaneous redox reactions
Replies: 4
Views: 42

### Re: spontaneous redox reactions

A way to think about this is that if the reaction in the battery/cell is non spontaneous and needs energy added to it, it defeats the purpose of a battery which is supposed to release energy spontaneously.
Fri Mar 06, 2020 9:54 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Arrhenius Equation
Replies: 6
Views: 56

### Re: Arrhenius Equation

I think that we use it to figure out the rates of reactions with different temperatures.
Fri Mar 06, 2020 9:48 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Determining the cathode/anode
Replies: 9
Views: 79

### Determining the cathode/anode

How would you determine the cathode or anode of a cell by only knowing the E value of the half reactions (assuming the you don't know which electrode is being oxidized or which is being reduced)?
Fri Mar 06, 2020 9:43 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Cell potentials and concentration
Replies: 3
Views: 38

### Re: Cell potentials and concentration

When you take the equation E = E(standard) - (RT/nF)lnQ and you change the concentration of products or reactants then you change the value of Q which is equal [P]/[R]. Therefore if you change a concentration, whether it be of products or reactants, your will change the cell potential (E).
Fri Mar 06, 2020 9:37 pm
Forum: Administrative Questions and Class Announcements
Topic: Notes on 3/04/20 (wednesday)
Replies: 2
Views: 47

### Re: Notes on 3/04/20 (wednesday)

That section in the notes reads "Step 1 must be the rate determining (slowest) step, since overall rate is proportional to the concentration of [NO2]^2.
Slowest step determines rate of reaction: rate = k1[NO2]^2"
Wed Mar 04, 2020 9:06 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Standard Cell Potential
Replies: 2
Views: 23

### Standard Cell Potential

I was under the impression that standard cell potentials were supposed to be positive but in 6N.1 the standard cell potentials of the cathode and anode cause it to be negative. Did I confuse two different types of problems? Is there a type of problem where the answer (in volts) needs to be positive?
Fri Feb 28, 2020 8:52 pm
Forum: General Rate Laws
Topic: Deriving the laws
Replies: 5
Views: 47

### Re: Deriving the laws

I don't think that we will be tested on deriving since in the last midterm we were given the already derived equations we would need.
Fri Feb 28, 2020 8:26 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Difference in concentration
Replies: 2
Views: 24

### Difference in concentration

I remember hearing in lecture, regarding galvanic cells, that a difference in concentration between two solutions causes an electric flow. I don't really understand what this means. Could someone elaborate?
Fri Feb 28, 2020 8:02 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Metals in galvanic cell
Replies: 1
Views: 16

### Metals in galvanic cell

I'm confused about how you would know which metal in the galvanic cell acts as the cathode that receives electrons and which acts as the anode that gives electrons. I'm assuming that the more electronegative element becomes the cathode, but in a cell with zinc and copper, copper is more electronegat...
Fri Feb 28, 2020 7:28 pm
Forum: Balancing Redox Reactions
Topic: Acidic and Basic solutions
Replies: 11
Views: 67

### Acidic and Basic solutions

Are redox reactions balanced differently when they're taking place in an acidic solution?
Fri Feb 28, 2020 1:49 pm
Forum: Kinetics vs. Thermodynamics Controlling a Reaction
Topic: diamond
Replies: 4
Views: 55

### Re: diamond

Diamond is kinetically stable in relation with graphite because there is an activation barrier for it to become graphite and energy would need to be added. It is thermodynamically unstable in relation to graphite because it is at a higher energy level then graphite.
Fri Feb 21, 2020 8:17 pm
Forum: General Science Questions
Topic: THE RETURN OF DJ LAVELLE [ENDORSED]
Replies: 6
Views: 111

### Re: THE RETURN OF DJ LAVELLE[ENDORSED]

I'm in the 3pm lecture, but if it's the same song he played for us it's called Hazey by Glass Animals
Fri Feb 21, 2020 8:14 pm
Forum: Van't Hoff Equation
Topic: what does it do?
Replies: 13
Views: 153

### Re: what does it do?

The Van't Hoff equation helps us find how K (the equilibrium constant) changes when temperature changes.

(P.S. I was confused about what equation you were referring to in your post. Even though it was in the Van't Hoff equation section, I recommend being specific in the posts just to avoid confusion)
Fri Feb 21, 2020 8:02 pm
Forum: Calculating Work of Expansion
Topic: when work is 0
Replies: 5
Views: 73

### Re: when work is 0

I don't think that work is necessarily 0 in a reversible, isothermal process. The values that are equal to 0 are delta U (change in internal energy) and delta H (change in ethalpy).
Fri Feb 21, 2020 7:50 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Salt Bridge
Replies: 4
Views: 41

### Salt Bridge

I understand that the salt bridge keeps both sides neutral, but why do both sides need to be neutral? What would happen if the salt bridge wasn't there?
Fri Feb 21, 2020 1:55 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Voltage and Current
Replies: 2
Views: 18

### Re: Voltage and Current

A voltage (a potential difference in charge) causes a current. A current is the transfer of electrons from the anode (oxidized) to the cathode (reduced).
Fri Feb 21, 2020 1:41 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: E
Replies: 5
Views: 23

### Re: E

If E is positive, then the reaction goes in the forward direction and is spontaneous.
Fri Feb 21, 2020 1:38 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: E v E(standard)
Replies: 4
Views: 29

### Re: E v E(standard)

E(standard) is under standard conditions (1 atm pressure and 0 degrees C or 273 K) while E is more general and under any conditions depending on the problem.
Fri Feb 14, 2020 10:50 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Entropy in reversible and irreversible
Replies: 3
Views: 35

### Re: Entropy in reversible and irreversible

For reversible you would use delta S = nRln(V2/V1). For isothermal irreversible free expansion you would use the same equation. The difference is that the change in entropy of the surroundings is - delta S for reversible and 0 for irreversible.
Fri Feb 14, 2020 8:06 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: State functions
Replies: 5
Views: 60

### Re: State functions

The state functions so far in class are the ones that appear with the Delta sign in front. Just remember that once in a while they don't have the delta sign in front. By the way these are G, H, U, and S so far. You could also use the word G.U.S.H. to remember.
Fri Feb 14, 2020 7:49 pm
Forum: Calculating Work of Expansion
Topic: work = 0
Replies: 14
Views: 139

### work = 0

Why does work equal 0 when, for example, a system pushes a piston out within a vacuum? If the piston is moving wasn't work done to move it?
Fri Feb 14, 2020 1:56 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Delta H
Replies: 10
Views: 105

### Re: Delta H

I would have to see an example of a problem but if it is a reversible, isothermal process and temperature is constant, then Delta H would be 0 in all cases because there would be no heat transfer in and out of the system.
Fri Feb 14, 2020 1:50 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Delta U = n*Cv,m*deltaT
Replies: 7
Views: 86

### Re: Delta U = n*Cv,m*deltaT

My TA sent us a list of equations and Cv wasn't changed to Cp when pressure is constant so Delta U = n*Cv,m*deltaT under constant pressure and constant volume. I was just wondering why since I thought you would need to use Cp under constant pressure.
Wed Feb 12, 2020 11:01 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Delta U = n*Cv,m*deltaT
Replies: 7
Views: 86

### Delta U = n*Cv,m*deltaT

Why does Delta U = n*Cv,m*deltaT apply for both constant pressure and constant volume?
Wed Feb 12, 2020 9:40 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Bond Enthalpy
Replies: 2
Views: 28

### Re: Bond Enthalpy

When calculating Enthalpy of a reaction, its the other way around. You add up the bond enthalpies of the products and then subtract the sum of bond enthalpies of reactants. When you are given enthalpies of formation, the products keep the same sign because they are being formed while you subtract th...
Fri Feb 07, 2020 5:33 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Entropy + Disorder
Replies: 3
Views: 25

### Re: Entropy + Disorder

Entropy really mean how many different ways stuff in a system can be reconfigured. So hypothetically if you have a box with a lot of space and objects inside of it, the system could have a lot of entropy because there are many ways you can rearrange the objects. On the other hand, something that's l...
Fri Feb 07, 2020 5:25 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Molecular Interlude
Replies: 1
Views: 18

### Re: Molecular Interlude

You accounted for the for Um(rotation, nonlinear) but also need to take Um(translation) into account. They are both the same but you need to add them together in order take all motion into account. Adding them would give you 3RT which would then give you the answer in the book.
Fri Feb 07, 2020 4:56 pm
Forum: Phase Changes & Related Calculations
Topic: relationship between heat capacity and heating curve
Replies: 6
Views: 33

### Re: relationship between heat capacity and heating curve

So if you take the heating curve, you have Q on the x axis and temperature of a substance on the y axis. If you add a little bit of heat energy (Q) and the temperature increases a lot, then the slope will be steep. This means that the substance has a low heat capacity because it did not take much he...
Fri Feb 07, 2020 5:21 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Combustion Reactions
Replies: 4
Views: 37

### Re: Combustion Reactions

I believe that when you are balancing a combustion equation you usually want to have a 1 for the coefficient of the hydrocarbon, or the thing you're burning, even if it means that the O2 has a coefficient that's not a whole number, such as 1/2.
Fri Feb 07, 2020 5:16 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: molar heat capacity
Replies: 2
Views: 22

### Re: molar heat capacity

When you get a specific value, that is supposed to be an exact heat capacity so your answer would be more accurate. When you're not given that value, you can estimate using 3R/2, 5R/2, etc. The way you know which one to use is my checking if the molecules are monoatomic (1 atom), diatomic (2 atoms),...
Fri Jan 31, 2020 11:43 pm
Forum: Phase Changes & Related Calculations
Topic: Reversible and Irreversible
Replies: 5
Views: 48

### Reversible and Irreversible

What is the difference between reversible and irreversible processes in terms of thermodynamics. Does it have to do with volume changing back to what it was before? I'm confused on the text book's explanation. And also why do reversible processes do more work than irreversible ones?
Fri Jan 31, 2020 8:29 pm
Forum: Phase Changes & Related Calculations
Topic: DELTA H Vs Q
Replies: 7
Views: 48

### Re: DELTA H Vs Q

I believe that is correct because DELTA H depends on the amount of the substance that reacted.
Fri Jan 31, 2020 8:09 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Max Heat Capacity
Replies: 4
Views: 19

### Re: Max Heat Capacity

I'm not sure what "max" heat capacity refers to but a system's heat capacity is the amount of pressure needed to increase the temperature of 1 gram or 1 mole of the substance by 1 degree Celsius. If dealing with mass, its how much energy is needed to increase 1 gram of a substance. If deal...
Fri Jan 31, 2020 1:36 am
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: state property
Replies: 8
Views: 45

### Re: state property

I don't understand state functions other than that its just final minus initial, but examples of pathway functions are heat and work.
Thu Jan 30, 2020 1:52 am
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: 4b.3b
Replies: 3
Views: 33

### Re: 4b.3b

You are right that you subtract the energy from heat (492 J) from the total energy (982 J) to get the measure of work. The answer should be +490 J. You probably made a small mistake when you calculated for this problem.

But you're right: delta U = w + q
Fri Jan 24, 2020 10:33 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Reaction Enthalpies calculation
Replies: 2
Views: 14

### Re: Reaction Enthalpies calculation

I know that for the 2nd method (Bond enthalpies) only the the bond enthalpies in diatomic molecules are accurate (i.e. O2, N2, F2). All the other ones are averages for various molecules making it the least accurate.
Fri Jan 24, 2020 10:29 pm
Forum: Phase Changes & Related Calculations
Topic: Enthalpy
Replies: 4
Views: 38

### Enthalpy

I'm a little confused about enthalpy. From what I understand, enthalpy is the capacity to transfer heat. Is this the same as energy being the the capacity to do work? The lines between these concepts are a little blurry for me.
Fri Jan 24, 2020 10:25 pm
Forum: Phase Changes & Related Calculations
Topic: Heat, energy, and work
Replies: 2
Views: 22

### Re: Heat, energy, and work

In thermodynamics, heat is the energy transferred as the result of energy difference. Heat transfers from high temperature regions to low temperature regions. Work is the process of achieving motion against an opposing force, such as gravity. Energy is the capacity of a system to do work. You can ad...
Fri Jan 24, 2020 10:07 pm
Forum: Phase Changes & Related Calculations
Topic: Bond enthalpy
Replies: 4
Views: 28

### Re: Bond enthalpy

I believe that there is a correlation. It would make sense since if there is a higher bond enthalpy, it would be expected for the atoms to be held closer together and therefore have a shorter bond length. I'm not 100% sure though.
Fri Jan 24, 2020 8:36 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Q and K
Replies: 4
Views: 37

### Re: Q and K

Q is the state of the relationship between the concentration of product and reactant at a given time. K is the equilibrium constant which is where the reaction is approaching. Usually K is given. You can also find it by dividing [P]/[R] when the reaction is at equilibrium (and the concentrations of ...
Fri Jan 24, 2020 1:17 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Finding OH concentration
Replies: 2
Views: 26

### Finding OH concentration

I'm confused on how to find [OH-] from only knowing [H30+]. In problem 6B.9, when [H3O+] is 1.5M, the answer for [OH-] is supposed to be 1.5 x 10^-14 but I keep getting 6.67 x 10^-15. What I'm doing is taking Kw = [H3O+][OH-] and then dividing [H3O+] on each side but it gets me the wrong answer.
Thu Jan 23, 2020 3:34 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: 6B.3
Replies: 2
Views: 17

### 6B.3

For this problem, I understand that the expected pH is 1.6 because you plug in the concentration of HCl(aq) into -log(x). However, I'm confused why the actual pH is 1.7. Is the solution 200mL or is it 250mL?
Sun Jan 19, 2020 6:45 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: 5% rule
Replies: 3
Views: 31

### 5% rule

I keep hearing the 5% rule, but I'm still confused about what it is and when to use it.
Sat Jan 18, 2020 12:33 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Difference between K and Q?
Replies: 11
Views: 56

### Re: Difference between K and Q?

Q and K have the exact same formula. Q can be any value depending on the state of the reaction. K is Q when the reaction reached equilibrium.
Fri Jan 17, 2020 9:16 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: pH values of weak and strong acids
Replies: 6
Views: 41

### Re: pH values of weak and strong acids

A higher pH means a lower concentration of Hydrogen ions (protons). To go from pH to [H3O+], you calculate 10^(-pH). Therefore the higher the pH value, the lower the concentration of [H3O+] and the lower the acidity.

I'm not sure if this answers what you were asking, but I hope it helps.
Fri Jan 17, 2020 8:58 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Dilution
Replies: 1
Views: 21

### Dilution

If you dilute a solution that is, for example, 1 mL to 10 mL, will [H3O+] and [OH-] both be divided by 10? My question is based off 6B.11.
Fri Jan 17, 2020 11:32 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Negative pH
Replies: 2
Views: 21

### Negative pH

Can you have a negative pH? If so, what does that mean conceptually? Does a concentration of 1 mean that 100% of the solution is H30+? And if that is true then how does that make sense?
Fri Jan 17, 2020 11:07 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Value of Kw
Replies: 6
Views: 38

### Value of Kw

For problems that don't say what Kw is, do we just assume that it is 1.0*10^-14 and that the pH scale is 1 to 14? Or is there some way to calculate Kw when just given, for example, [H30+]?
Fri Jan 17, 2020 8:28 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Acid to conjugate base
Replies: 1
Views: 19

### Acid to conjugate base

When going from an acid to a conjugate base or a base to a conjugate acid, is the difference in H ion always 1 or can multiple H ions be added or removed when becoming the conjugate base/acid?
Sun Jan 12, 2020 6:50 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Reaction Quotient Q
Replies: 6
Views: 29

### Re: Reaction Quotient Q

I believe the question you are working on will usually tell you whether you are finding Q or K. From the problems I've done, you are often given a K value at the beginning and are told to find Q in order to compare it to K.
Thu Jan 09, 2020 10:37 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Difference between -> and <->
Replies: 4
Views: 40

### Difference between -> and <->

When calculating Q , does it matter whether it's written as R<->P or R->P? Is it different because R->P doesn't have a reverse reaction rate(or does it)?
Thu Jan 09, 2020 8:15 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: 5G.7
Replies: 5
Views: 47

### Re: 5G.7

This is Daniela Shatzki 2E:

There is a button that says "New Topic" that looks similar to the button that says "Post Reply." It should be near the top and bottom of the page and shows up on pages like "Equilibrium Constants & Calculating Concentrations."
Thu Jan 09, 2020 7:56 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Composite Equations
Replies: 2
Views: 32

### Re: Composite Equations

I'm not really sure why either, but I think it makes some sense when you think about adding two equations that both have K=1. If you multiply like you're supposed to, then K is still 1 and you still have an equal number of reactants and products. If you add the K's, then you would get 2 which means ...
Thu Jan 09, 2020 5:36 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: K values and stability of product
Replies: 2
Views: 18

### Re: K values and stability of product

Also, if K is less than 10^-3, then the equilibrium lies towards the left and favors the reactants more. For this class specifically, I believe that a K value between 10^-3 and 10^3 isn't considered to significantly favor either reactants or products.
Wed Jan 08, 2020 9:11 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Multiples of Chemical Equation
Replies: 3
Views: 28

### Multiples of Chemical Equation

I understand that if the equilibrium constant, K, is 10 and you multiply 2 through the equation, then K becomes 10^2. For example if you have Br2 + 3F2 <--> 2BrF3 and multiply it so its 2Br + 6F2 <--> 4BrF3, then you would make the initial K to the power of 2. However, does this work with every numb...