Search found 30 matches

by Jenny Cheng 2K
Sat Mar 17, 2018 4:27 pm
Forum: *Enzyme Kinetics
Topic: Enzyme
Replies: 5
Views: 476

Re: Enzyme

A homogeneous catalyst is a catalyst in the same phase as the reactant. A heterogeneous catalyst is a catalyst that is in a different phase than the reactant.
by Jenny Cheng 2K
Sat Mar 17, 2018 1:51 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Rule when writing cell diagrams?
Replies: 5
Views: 203

Re: Rule when writing cell diagrams?

In a cell diagram, the left side should be the anode, and the right side should be the cathode.
by Jenny Cheng 2K
Sat Mar 17, 2018 1:49 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Determining Slow Step
Replies: 2
Views: 811

Re: Determining Slow Step

The two important components to look for when determining reaction mechanisms are 1) that the sum of elementary steps = overall reaction and 2) that the mechanisms must agree with the experimentally determined rate law. The elementary step that has a rate law that matches the overall reaction's rate...
by Jenny Cheng 2K
Fri Mar 09, 2018 10:29 pm
Forum: Method of Initial Rates (To Determine n and k)
Topic: Negative Order
Replies: 7
Views: 1022

Re: Negative Order

A negative order means the concentration appears in the denominator of the rate law. Increasing the concentration of the species (usually a product) slows down the reaction.
by Jenny Cheng 2K
Fri Mar 09, 2018 10:27 pm
Forum: Method of Initial Rates (To Determine n and k)
Topic: unit of concentration [ENDORSED]
Replies: 4
Views: 306

Re: unit of concentration [ENDORSED]

I think the important thing to remember is to make sure that all your units match.
by Jenny Cheng 2K
Fri Mar 09, 2018 10:22 pm
Forum: General Rate Laws
Topic: 15.23 b
Replies: 3
Views: 153

Re: 15.23 b

If you make both sides of the equation ln([A]t/[A]0) = -kt negative, you will have: ln([A]0/[A]t) = kt. Making the natural log negative flips the content because:
ln([A]t/[A]0) = ln[A]t - ln[A]0
by Jenny Cheng 2K
Sat Mar 03, 2018 9:55 pm
Forum: Method of Initial Rates (To Determine n and k)
Topic: 15.15 [ENDORSED]
Replies: 5
Views: 226

Re: 15.15 [ENDORSED]

In general, rate = k * [A]^a * [B]^b ...
Thus, overall order = a + b + ...
by Jenny Cheng 2K
Sat Mar 03, 2018 9:49 pm
Forum: General Rate Laws
Topic: half-life
Replies: 8
Views: 279

Re: half-life

The greater the rate constant, the shorter is the half life. You can use half life to calculate the amount of reactant remaining in a reaction.
At any stage of a reaction, the concentration of A remaining after n half-lives is: (1/2)^n * [A]
by Jenny Cheng 2K
Sat Mar 03, 2018 9:43 pm
Forum: General Rate Laws
Topic: 15.3 partb ?
Replies: 3
Views: 141

Re: 15.3 partb ?

When you are not looking at the unique rate of the reaction, you must specify the species to which the rate refers. The species consumed or produced have rates that are related to the stoichiometry of the reaction.
by Jenny Cheng 2K
Fri Feb 23, 2018 2:35 pm
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: 9.5
Replies: 4
Views: 250

Re: 9.5

ΔS(total) = -q/T(1) + q/T(2)
The first reservoir loses heat, so the q should be negative. The second reservoir gains the heat that the first reservoir loses, so the q should be positive.
by Jenny Cheng 2K
Fri Feb 23, 2018 2:32 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: 11.111
Replies: 3
Views: 167

Re: 11.111

Since ΔG(1) is given, you can solve for k(1). The relationship between k(1) and k(2) is given, with k(2)=k(1)/10. You can solve for ΔG(2) using k(2). For this problem, you should use ΔG=-RT ln(k).
by Jenny Cheng 2K
Fri Feb 23, 2018 2:20 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: 14.27
Replies: 6
Views: 266

Re: 14.27

Gibbs Free Energy is a state function, so you can add intermediate components to reach your desired final value. Cell potential is NOT a state function, so you can't use the same addition method. However, you can relate Gibbs Free Energy and cell potential through ΔG=-nFE.
by Jenny Cheng 2K
Thu Feb 15, 2018 11:40 am
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: 9.15 Question
Replies: 2
Views: 152

Re: 9.15 Question

ΔH(reverse process)=-ΔH(forward process), so
ΔH(freezing)=-ΔH(fusion)
by Jenny Cheng 2K
Thu Feb 15, 2018 11:29 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Hess's Law
Replies: 5
Views: 249

Re: Hess's Law

Enthalpy is a state function, meaning you can calculate the desired value from the final and initial conditions, and you can also add components to reach the desired value. Hess's Law uses the fact that enthalpy is a state function to calculate ΔH.
by Jenny Cheng 2K
Thu Feb 15, 2018 11:27 am
Forum: Phase Changes & Related Calculations
Topic: Specific heat of water or ice?
Replies: 6
Views: 238

Re: Specific heat of water or ice?

To determine which specific heat capacity to use, you need to look at the phase of the substance. More specifically, if you are working on a problem in which you are calculating the heat required to change ice at -10 degrees C to liquid water at 0 degrees C, you would first calculate the heat associ...
by Jenny Cheng 2K
Fri Feb 09, 2018 2:20 pm
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: 9.37
Replies: 3
Views: 172

Re: 9.37

For this problem, you are finding ΔS(rxn) by using ΔS(m), the standard molar entropy (which is the entropy of 1 mol of substance). Use ΔS(rxn)=ΣnΔS(m)(products)-ΣnΔS(m)(reactants) I think ΔS(rxn) refers to the entropy of the entire reaction, and ΔS(f) refers to the entropy of formation of a substance.
by Jenny Cheng 2K
Fri Feb 09, 2018 2:10 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Finding entropy of vaporization from temperatures NOT = to boiling point [ENDORSED]
Replies: 3
Views: 156

Re: Finding entropy of vaporization from temperatures NOT = to boiling point [ENDORSED]

To find entropy of vaporization at temperatures NOT = to boiling point: example of finding entropy of vaporization of water at 85 degrees C (9.19) 1) increase T of water (from 85 degrees C to 100 degrees C) 2) find entropy of vaporization at boiling point (100 degrees C for water) 3) cool/decrease T...
by Jenny Cheng 2K
Mon Feb 05, 2018 12:43 am
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: determining number of microstates
Replies: 3
Views: 155

determining number of microstates

In general, how would you calculate the number of microstates of a more complicated molecule? Does anyone have an example?
by Jenny Cheng 2K
Fri Feb 02, 2018 8:58 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Problem 9.19 about Standard Entropy
Replies: 6
Views: 181

Re: Problem 9.19 about Standard Entropy

You should break this problem into 4 steps: 1) heat liquid water (ΔS=C * ln(T2/T1)) 2) vaporize water at boiling point (ΔS(vap) is given in the problem) 3) cool water vapor back to 85 degrees C (ΔS=C * ln(T2/T1)) 4) sum all the entropies from the above steps to find ΔS(total) You need to calculate e...
by Jenny Cheng 2K
Fri Feb 02, 2018 8:50 pm
Forum: Calculating Work of Expansion
Topic: Free expansion and irreversible expansion
Replies: 2
Views: 127

Re: Free expansion and irreversible expansion

No work is done in free expansion; an example of free expansion is a gas pushing against a piston and into a vacuum. Free expansion is an irreversible process, but it is not the same as irreversible expansion. For an irreversible expansion, P(external) does not equal P(internal). Reversible expansio...
by Jenny Cheng 2K
Fri Feb 02, 2018 8:44 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Free expansion
Replies: 2
Views: 146

Re: Free expansion

No work is done in free expansion because there is no external pressure; for example, free expansion occurs when a gas pushes against a piston and expands into a vacuum (in which no work is done). For a gas that is expanding by pushing against a piston and an external pressure (no free expansion), t...
by Jenny Cheng 2K
Sat Jan 27, 2018 7:34 pm
Forum: Phase Changes & Related Calculations
Topic: 8.40
Replies: 4
Views: 170

Re: 8.40

I also got about the same answer (using E(total) = E(raising temperature) + E(phase change)).
by Jenny Cheng 2K
Sat Jan 27, 2018 7:25 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: 8.65
Replies: 3
Views: 177

Re: 8.65

For this problem, first use Hess's Law: multiply the first thermochemical equation given by a factor of 2: 4NO(g) + 2O2(g) --> 4NO2(g) ΔH=(-114.1 kJ) * 2 Combine the two thermochemical equations given, and you'll see that the 4NO2 will cancel out and will not be in the final thermochemical equation:...
by Jenny Cheng 2K
Sat Jan 27, 2018 7:14 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: q isn't a state function
Replies: 3
Views: 208

Re: q isn't a state function

q isn't a state function because it isn't solely dependent on the initial and final states; the value of q depends on the pathway taken to reach the final q. Here is an example of why heat is not a state property: Consider raising the temperature of 50.0g of water from 25.0°C to 50.0°C. One method t...
by Jenny Cheng 2K
Fri Jan 19, 2018 5:04 pm
Forum: Calculating Work of Expansion
Topic: Integral from Today's Lecture
Replies: 3
Views: 143

Re: Integral from Today's Lecture

The integral represents the area under the curve, and I think the "infinite number of steps" is referring to infinitesimally small step sizes when calculating the area. This integral shows the relationship of work to pressure and volume through calculus, and I believe for chem 14B right no...
by Jenny Cheng 2K
Fri Jan 19, 2018 4:55 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: P delta V is significant?
Replies: 2
Views: 196

Re: P delta V is significant?

When there's a reaction at constant pressure that involves solids and liquids, a change in volume will be insignificant because V of the reactants will equal (or almost equal) V of the products. ΔV=0, so PΔV=0. Also, there is no expansion work. Thus, with constant pressure, for a reaction that invol...
by Jenny Cheng 2K
Fri Jan 19, 2018 4:45 pm
Forum: Phase Changes & Related Calculations
Topic: 8.39
Replies: 2
Views: 123

Re: 8.39

For this problem, you need to consider the energy needed for the phase change (melting/fusion) as well as the energy needed for the temperature change. Looking at a heating curve for H2O, solid H2O will melt at 0.0°C. The sum of the energy needed for the phase change and the energy needed for the te...
by Jenny Cheng 2K
Sat Jan 13, 2018 3:55 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Heat Not a State Property
Replies: 6
Views: 180

Re: Heat Not a State Property

Here is an example of why heat is not a state property: Consider raising the temperature of 50.0g of water from 25.0°C to 50.0°C. One method to raise the temperature is to supply energy as heat through an electric water heater. The heat can be calculated from the specific heat capacity of water: q =...
by Jenny Cheng 2K
Sat Jan 13, 2018 3:39 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: State function vs non state function
Replies: 2
Views: 788

Re: State function vs non state function

Internal energy is a state function. For example, in the first scenario, you raise the temperature of 50g of water from 25°C to 70°C. In the second scenario, you take the same amount of water at 25°C, raise the temperature to boiling point, vaporize the water, condense it, and eventually cool it to ...
by Jenny Cheng 2K
Sat Jan 13, 2018 2:53 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Hess' Law
Replies: 3
Views: 137

Re: Hess' Law

Hess's Law states that enthalpy changes are additive; the change of enthalpy at each step of a multi-step reaction can be added to determine the total change in enthalpy. Here is an example for how to use Hess's Law: 1. The first reaction shows one of the reactants from the overall reaction as a rea...

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