Search found 30 matches
- Thu Mar 15, 2018 10:27 am
- Forum: Reaction Mechanisms, Reaction Profiles
- Topic: Elementary reactions
- Replies: 6
- Views: 848
Re: Elementary reactions
In an elementary reaction, you can use the coefficients as exponents when writing the rate law for the elementary step (cannot be done for overall or non-elementary rates) because it's directly related to molecularity.
- Thu Mar 15, 2018 10:21 am
- Forum: Reaction Mechanisms, Reaction Profiles
- Topic: Reaction Mechanisms
- Replies: 2
- Views: 383
Re: Reaction Mechanisms
The two ways to find a reaction mechanism are: 1) direct computation with differential equations (we aren't doing this method) 2) identifying the rate-limiting step and comparing the rate law of the rate-determining step to the overall rate. There are 2 methods the textbook discusses: steady-state a...
- Thu Mar 15, 2018 10:14 am
- Forum: Reaction Mechanisms, Reaction Profiles
- Topic: Slow Steps
- Replies: 2
- Views: 366
Re: Slow Steps
The example from lecture was trying to identify which proposed mechanism agrees with the given rate law. If the second step is slow and its rate law contains an intermediate, then we use the equilibrium constant K expression to substitute for the intermediate (we assume the reaction before the rate-...
- Thu Mar 08, 2018 10:23 am
- Forum: Reaction Mechanisms, Reaction Profiles
- Topic: What is Molecularity?
- Replies: 9
- Views: 2427
Re: What is Molecularity?
Molecularity is the number of species/molecules that react in an elementary step. A unimolecular reaction (k[A]) involves 1 molecule (that reacts with the solution, container walls, photons, etc). A bimolecular reaction (k[A]2) involves 2 species/molecules colliding.
- Thu Mar 08, 2018 10:16 am
- Forum: First Order Reactions
- Topic: Equation variations
- Replies: 9
- Views: 1174
Re: Equation variations
These equations are the same, just rearranged using log rules, so it does not matter which you use. They are just written differently to make it easier to solve for different terms.
- Thu Mar 08, 2018 10:03 am
- Forum: Method of Initial Rates (To Determine n and k)
- Topic: 15.27
- Replies: 2
- Views: 587
Re: 15.27
Since it's a first order reaction, we use t1/2 = and solve for k with the given half life. Once you have k, we can use the integrated rate law (ln[A] = -kt + ln[A]0). If the concentration of A decreases to 15% of the original, we can use [A]0 = 1 and [A] = 0.15 to solve for t.
- Thu Mar 01, 2018 10:19 am
- Forum: Zero Order Reactions
- Topic: slope
- Replies: 9
- Views: 4723
Re: slope
For a zero order reaction, the graph of [A] vs time is negative because the reactant concentration is decreasing as the reaction proceeds. For a first order reaction, the graph of ln[A] vs time is negative because as [A] decreases, ln[A] also decreases. For a second order reaction, a graph of 1/[A] ...
- Thu Mar 01, 2018 10:15 am
- Forum: Method of Initial Rates (To Determine n and k)
- Topic: What is the order of the reaction? [ENDORSED]
- Replies: 4
- Views: 515
Re: What is the order of the reaction? [ENDORSED]
The order of a reaction is the sum of the individual orders with respect to each reactant, and it tells you about the relationship between reactant concentrations and rate.
- Thu Mar 01, 2018 10:11 am
- Forum: Method of Initial Rates (To Determine n and k)
- Topic: Units for Concentration
- Replies: 4
- Views: 539
Re: Units for Concentration
Unless the problem specifies units, it doesn't matter whether you use mol/L or mmol/L because the answers will be equivalent.
- Thu Feb 22, 2018 10:13 am
- Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
- Topic: 14.55
- Replies: 2
- Views: 428
Re: 14.55
Dr. Lavelle discussed 2 methods to calculate Ecell. The first method is to use E o cell = E o cathode - E o anode, and you wouldn't switch the signs for the E o here. The second method is to add E o of the two half-reactions together, and you would switch the sign for the oxidation reaction E o beca...
- Thu Feb 22, 2018 10:06 am
- Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
- Topic: PH meter
- Replies: 3
- Views: 444
Re: PH meter
A pH meter is an example of an ion-selective electrode. The pH meter has a thin-walled glass membrane that measures the difference in [H+] from in the standard inside the pH meter and the solution is is placed in. This [H+] difference is used to find the pH of the solution.
- Thu Feb 22, 2018 10:00 am
- Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
- Topic: Concentration Cells
- Replies: 2
- Views: 342
Re: Concentration Cells
From the lecture example- if you have a concentration cell with the anode side= 0.1M solution and cathode side = 1.0M solution, the concentration of the 1.0M solution will decrease (reactant) and the concentration of the 0.1M solution will increase (product). With the equation E = E^{\circ} - \frac{...
- Mon Feb 12, 2018 1:35 pm
- Forum: Calculating Work of Expansion
- Topic: When to use this equation
- Replies: 3
- Views: 429
Re: When to use this equation
We use for irreversible conditions (constant P). We use for reversible conditions (P is not constant).
- Mon Feb 12, 2018 1:22 pm
- Forum: Concepts & Calculations Using First Law of Thermodynamics
- Topic: Enthalpy of fusion [ENDORSED]
- Replies: 4
- Views: 1044
Re: Enthalpy of fusion [ENDORSED]
The total energy put into the ice cream is equal to the energy needed to raise the temperature from -2.8 to 0degC plus the energy needed to melt it ()
- Mon Feb 12, 2018 1:19 pm
- Forum: Concepts & Calculations Using First Law of Thermodynamics
- Topic: Isothermal expanision and deltaU [ENDORSED]
- Replies: 2
- Views: 352
Re: Isothermal expanision and deltaU [ENDORSED]
Isothermal means that temperature is constant. For an ideal gas (ideal = isothermal conditions), . Because = 0, = 0,
- Fri Feb 09, 2018 1:11 pm
- Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
- Topic: DG = -nFE
- Replies: 3
- Views: 586
Re: DG = -nFE
n refers to the number of moles of electrons (which should be the same in the oxidation and reduction half reactions if they're balanced)
- Fri Feb 09, 2018 1:06 pm
- Forum: Balancing Redox Reactions
- Topic: Anode vs cathode
- Replies: 6
- Views: 641
Re: Anode vs cathode
An easy way to remember this is with "AN OX" (anode/oxidation), "RED CAT" (reduction/cathode)
- Thu Feb 08, 2018 10:46 pm
- Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
- Topic: Cell diagrams
- Replies: 1
- Views: 238
Re: Cell diagrams
Yes, when writing out cell diagrams, we use a comma to separate substances of the same phase in contact. We use a single vertical line to indicate different phases in contact (including porous disks), and double vertical lines to indicate a salt bridge.
- Thu Feb 01, 2018 10:18 am
- Forum: Calculating Work of Expansion
- Topic: Work Equations [ENDORSED]
- Replies: 9
- Views: 1358
Re: Work Equations [ENDORSED]
If the problem states it's irreversible, then we use . If it's reversible, we use .
- Thu Feb 01, 2018 10:15 am
- Forum: Concepts & Calculations Using Second Law of Thermodynamics
- Topic: Celsius or Kelvin
- Replies: 5
- Views: 727
Re: Celsius or Kelvin
For problems with ln \frac{T2}{T1} , temperature has to be in K because the conversion factor is additive (+273) and won't cancel out. In comparison, the units don't really matter for problems with ln \frac{V2}{V1} or ln \frac{P1}{P2} because the conversion factors are multiplied (like x1000mL) and ...
- Thu Feb 01, 2018 10:04 am
- Forum: Entropy Changes Due to Changes in Volume and Temperature
- Topic: Entropy change when Temperature changes [ENDORSED]
- Replies: 6
- Views: 1374
Re: Entropy change when Temperature changes [ENDORSED]
For 9.5, we use to calculate for the 800K reservoir losing 40kJ of energy (negative value) plus for the 200K reservoir gaining the 40kJ of energy (positive value).
- Fri Jan 26, 2018 2:42 pm
- Forum: Concepts & Calculations Using Second Law of Thermodynamics
- Topic: Relating Degeneracy and Volume
- Replies: 2
- Views: 318
Re: Relating Degeneracy and Volume
Each gas molecule only having 2 possible positions was just for the example in lecture. For a CO molecule, the atoms can be configured as [C-O] or [O-C]. I think the number of possible positions depends on the specific problem.
- Fri Jan 26, 2018 2:36 pm
- Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
- Topic: Irreversible vs. Reversible [ENDORSED]
- Replies: 10
- Views: 1478
Re: Irreversible vs. Reversible [ENDORSED]
In a reversible pathway, pressure is changing and more work is done (w=-nRTln). In an irreversible pathway, pressure is constant and less work is done (w=-p).
- Fri Jan 26, 2018 2:11 pm
- Forum: Thermodynamic Systems (Open, Closed, Isolated)
- Topic: Iron Rusting: Exo or Endo? [ENDORSED]
- Replies: 4
- Views: 12724
Re: Iron Rusting: Exo or Endo? [ENDORSED]
Iron rusting is a reaction with oxygen to create iron oxide. Forming bonds is exothermic because it releases energy (vs breaking bonds, which is endothermic because it requires energy).
- Thu Jan 18, 2018 10:27 am
- Forum: Phase Changes & Related Calculations
- Topic: Question about Problem 8.41
- Replies: 5
- Views: 599
Re: Question about Problem 8.41
The liquid water will lose heat and its temperature will decrease, and the ice will absorb that heat and melt/temperature will increase. This is why the q(water) is negative and q(ice) is positive.
- Thu Jan 18, 2018 10:20 am
- Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
- Topic: Bond Breaking or forming
- Replies: 5
- Views: 569
Re: Bond Breaking or forming
If you're using bond enthalpies, then after you draw the Lewis structures you just need to look at the individual bonds in the reactants/products to see which bonds are broken from the reactants and which bonds need to form to make the products
- Thu Jan 18, 2018 10:13 am
- Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
- Topic: Delta H and q Interchangeability
- Replies: 5
- Views: 591
Re: Delta H and q Interchangeability
= q (heat) at a constant pressure
- Thu Jan 11, 2018 1:08 pm
- Forum: Phase Changes & Related Calculations
- Topic: Why steam burns are worse explained
- Replies: 5
- Views: 676
Re: Why steam burns are worse explained
After 100 degrees, vaporization is complete so the temperature of steam can be much greater than 100 deg (think of the phase change diagram from lecture). Because the temperature is higher, there is a greater change in enthalpy.
- Thu Jan 11, 2018 10:34 am
- Forum: Phase Changes & Related Calculations
- Topic: General Question
- Replies: 3
- Views: 323
Re: General Question
Heat (q) is the transfer of energy as a result of temperature change. Heat at a constant pressure is equal to enthalpy (qp = ).
- Thu Jan 11, 2018 10:27 am
- Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
- Topic: Heat Capacity vs. Specific Heat Capacity
- Replies: 6
- Views: 1016
Re: Heat Capacity vs. Specific Heat Capacity
Heat capacity tells you how much heat is necessary for a temperature change. Molar heat capacity is in J/K-mol and specific heat capacity is in J/C-g. Depending on what units (mol or g) are given in a problem, use the molar heat or specific heat for the "c" value in