## Search found 30 matches

Thu Mar 15, 2018 10:27 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Elementary reactions
Replies: 6
Views: 389

### 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: 188

### 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: 177

### 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: 1231

### 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: 592

### 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: 333

### Re: 15.27

Since it's a first order reaction, we use t1/2 =$\frac{ln2}{k}$ 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: 1294

### 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: 268

### 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: 257

### 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: 217

### 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: 240

### 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: 162

### 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: 213

### Re: When to use this equation

We use $w = -P\Delta V$ for irreversible conditions (constant P). We use $w = -nRTln\frac{V2}{V1}$ 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: 325

### 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 ($q = mc\Delta T + n\Delta Hvap$)
Mon Feb 12, 2018 1:19 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Isothermal expanision and deltaU [ENDORSED]
Replies: 2
Views: 188

### Re: Isothermal expanision and deltaU[ENDORSED]

Isothermal means that temperature is constant. For an ideal gas (ideal = isothermal conditions), $\Delta U = \frac{3}{2}nR\Delta T$. Because $\Delta T$ = 0, $\Delta U$ = 0,
Fri Feb 09, 2018 1:11 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: DG = -nFE
Replies: 3
Views: 307

### 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: 251

### 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: 88

### 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: 636

### Re: Work Equations[ENDORSED]

If the problem states it's irreversible, then we use $w= -P\Delta V$. If it's reversible, we use $w=-nRTln\frac{V2}{V1}$.
Thu Feb 01, 2018 10:15 am
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Celsius or Kelvin
Replies: 5
Views: 318

### 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: 5
Views: 633

### Re: Entropy change when Temperature changes[ENDORSED]

For 9.5, we use $\Delta S = \frac{q}{T}$ to calculate $\Delta S$ for the 800K reservoir losing 40kJ of energy (negative value) plus $\Delta S$ 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: 137

### 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: 729

### Re: Irreversible vs. Reversible[ENDORSED]

In a reversible pathway, pressure is changing and more work is done (w=-nRTln$\frac{V2}{V1}$). In an irreversible pathway, pressure is constant and less work is done (w=-p$\Delta V$).
Fri Jan 26, 2018 2:11 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Iron Rusting: Exo or Endo? [ENDORSED]
Replies: 4
Views: 5557

### 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
Replies: 5
Views: 333

### 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: 270

### 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: 372

### Re: Delta H and q Interchangeability

$\Delta H$ = 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: 282

### 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: 226

### 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 = $\Delta H$).
Thu Jan 11, 2018 10:27 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Heat Capacity vs. Specific Heat Capacity
Replies: 6
Views: 318

### 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 $q=mc\Delta T$