Search found 31 matches

by Julia Cheng 2J
Sat Mar 17, 2018 1:17 pm
Forum: Third Law of Thermodynamics (For a Unique Ground State (W=1): S -> 0 as T -> 0) and Calculations Using Boltzmann Equation for Entropy
Topic: S and delta S
Replies: 4
Views: 283

Re: S and delta S

You would usually use delta S to find the change in entropy of a reaction (so some change to the system has to happen in order for there to be a delta S). We've only been using S to find the entropy of a molecule as a result of its structure (so there doesn't have to be any change in its state).
by Julia Cheng 2J
Sat Mar 17, 2018 1:14 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: qreaction and qcalorimeter
Replies: 3
Views: 359

Re: qreaction and qcalorimeter

Kind of... the calorimeter is a very specific type of surroundings though.
by Julia Cheng 2J
Sat Mar 17, 2018 1:13 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Open, closed, or isolated test 1 question
Replies: 10
Views: 588

Re: Open, closed, or isolated test 1 question

Rachel N 1I wrote:Why is the contents of an ideal cooler at 3 deg Celsius considered isolated and not closed?

In an ideal cooler, no energy is exchanged with the surroundings (because ideally its contents will stay cold without absorbing warmth from the outside), so it's isolated.
by Julia Cheng 2J
Fri Mar 09, 2018 10:29 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Finding the intermediate(s)
Replies: 5
Views: 173

Re: Finding the intermediate(s)

An intermediate is produced in an earlier step and then used in a later step. It isn't an intermediate if it is used in an early step and produced in a later step.
by Julia Cheng 2J
Fri Mar 09, 2018 10:26 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: What is Molecularity?
Replies: 9
Views: 754

Re: What is Molecularity?

Also, the greater the molecularity, the less likely the reaction is to happen since the probability of that many molecules colliding is less.
by Julia Cheng 2J
Fri Mar 09, 2018 10:23 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Pre-Equilibrium Approach Methodology
Replies: 2
Views: 121

Re: Pre-Equilibrium Approach Methodology

In the slow step, we find the unique reaction rate according to the formation of products, which is equivalent to the overall rate law. So, we can set them equal.
by Julia Cheng 2J
Fri Mar 02, 2018 10:57 am
Forum: First Order Reactions
Topic: Pseudo First Order Reaction
Replies: 5
Views: 484

Re: Pseudo First Order Reaction

A pseudo first order reaction is actually a higher order reaction in which the concentrations of all the reactants except one is super high. This way, only the change in concentration of one reactant will be significant and you can solve for its concentration as if it was a real first order reaction.
by Julia Cheng 2J
Fri Mar 02, 2018 10:51 am
Forum: Method of Initial Rates (To Determine n and k)
Topic: Order of the Reaction [ENDORSED]
Replies: 3
Views: 141

Re: Order of the Reaction [ENDORSED]

You can use the reaction to figure out order only if it is an elementary reaction.
by Julia Cheng 2J
Fri Mar 02, 2018 10:48 am
Forum: Kinetics vs. Thermodynamics Controlling a Reaction
Topic: Average rate [ENDORSED]
Replies: 6
Views: 468

Re: Average rate [ENDORSED]

As long as the forward reaction is progressing faster than the reverse reaction, the rate of the change in concentration of reactants will be negative because they are being consumed.
by Julia Cheng 2J
Fri Feb 23, 2018 11:06 am
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Table 14.1
Replies: 1
Views: 120

Re: Table 14.1

The more positive the reduction potential, the greater its ability to be reduced. Since the table is arranged in order of large to small reduction potential, it works out.
by Julia Cheng 2J
Fri Feb 23, 2018 11:05 am
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: 14.33b homework
Replies: 3
Views: 179

Re: 14.33b homework

For this problem, we know deltaG for Tl3+/Tl. However, for b we want to know the deltaG for Tl3+/Tl+ in order to find whether or not Tl is disproportionate. We use the reaction potential for Tl+/Tl to calculate deltaG for the half-reaction and then combine the deltaG's for Tl3+/Tl and Tl+/Tl in a He...
by Julia Cheng 2J
Sun Feb 18, 2018 8:48 pm
Forum: Balancing Redox Reactions
Topic: Balancing
Replies: 3
Views: 148

Re: Balancing

To determine oxidation numbers, remember that the oxidation number of oxygen is always +2 unless you're looking at a peroxide (I think). Then you just determine the oxidation numbers of the rest of the atoms in the molecules based on the charge of the molecule. The oxidation numbers of all the atoms...
by Julia Cheng 2J
Sun Feb 18, 2018 8:42 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: "Molar" form/ Pure number
Replies: 2
Views: 131

Re: "Molar" form/ Pure number

I'm pretty sure the r stands for "reaction." So rather than this being a standard deltaG, this is the deltaG of the reaction and the number of moles of electrons in the reaction.
by Julia Cheng 2J
Sun Feb 18, 2018 8:40 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: 14.9
Replies: 2
Views: 150

Re: 14.9

It helps to write out all your units when you're doing a problem so you can see what cancels out for your final answer.
by Julia Cheng 2J
Fri Feb 09, 2018 10:55 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Conducting Metal
Replies: 3
Views: 112

Re: Conducting Metal

Yes, a conducting metal/solid is necessary for electrons to be transferred between half-reactions.
by Julia Cheng 2J
Fri Feb 09, 2018 10:54 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Galvanic Cell
Replies: 3
Views: 118

Re: Galvanic Cell

Yes; it uses spontaneous redox reactions to generate electrical energy.
by Julia Cheng 2J
Fri Feb 09, 2018 10:51 am
Forum: Balancing Redox Reactions
Topic: Standard Reduction Potential
Replies: 3
Views: 137

Re: Standard Reduction Potential

Just make sure that the number of electrons cancel out and the charges are balanced and you should be good to calculate the standard reaction potential by adding up E for each half reaction
by Julia Cheng 2J
Fri Feb 02, 2018 10:45 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: entropy to surroundings
Replies: 3
Views: 133

Re: entropy to surroundings

Keep in mind that the second law is referring to the total entropy, aka the entropy of the universe. So, the entropy of the system or of the surroundings can increase without violating the second law as long as the total entropy is not negative.
by Julia Cheng 2J
Fri Feb 02, 2018 10:41 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Endergonic vs Endothermic
Replies: 2
Views: 120

Re: Endergonic vs Endothermic

For the equation deltaG=deltaH-TdeltaS, for a negative deltaH (exothermic), as long as the TdeltaS term is negative and larger in magnitude than deltaH, deltaG will be positive and the process will be endergonic.
by Julia Cheng 2J
Fri Feb 02, 2018 10:38 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: 9.75
Replies: 1
Views: 87

Re: 9.75

I believe that a trans isomer has a smaller W than its corresponding cis crystal. A trans isomer and its mirror across a certain axis are indistinguishable, but you can tell the difference between a cis crystal and its mirror, so the cis crystal has a greater W.
by Julia Cheng 2J
Fri Feb 02, 2018 10:32 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: 9.73
Replies: 1
Views: 72

Re: 9.73

S, G, and H change depending on the conditions. So, S, G, and H indicate spontaneity for whatever conditions they are given for in the problem.
by Julia Cheng 2J
Fri Jan 26, 2018 10:32 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Heat and temp difference
Replies: 2
Views: 138

Re: Heat and temp difference

Isothermal just means that there's no change in temperature of the system, but there is still an exchange of heat between the system and the surroundings.
by Julia Cheng 2J
Fri Jan 26, 2018 10:28 am
Forum: Phase Changes & Related Calculations
Topic: Names of phase changes
Replies: 4
Views: 200

Re: Names of phase changes

Don't forget that when you are solving for enthalpy change for exothermic phase changes, you just use negative delta H for their endothermic counterparts.
by Julia Cheng 2J
Fri Jan 26, 2018 10:13 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Internal Energy in a Vacuum [ENDORSED]
Replies: 5
Views: 240

Re: Internal Energy in a Vacuum [ENDORSED]

Also, q is zero in free expansion because no heat exchange occurs between the system and surroundings.
by Julia Cheng 2J
Sat Jan 20, 2018 11:39 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: PV=nRT
Replies: 4
Views: 191

Re: PV=nRT

If the system expands or is compressed, there can only be a change in the number of moles if the system is open.
by Julia Cheng 2J
Sat Jan 20, 2018 11:30 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Assigning system/surroundings
Replies: 5
Views: 232

Re: Assigning system/surroundings

The system will always be wherever the reaction is occurring, and the surroundings are everything else.
by Julia Cheng 2J
Sat Jan 20, 2018 11:29 pm
Forum: Calculating Work of Expansion
Topic: Reversible vs Irreversible
Replies: 4
Views: 149

Re: Reversible vs Irreversible

Reversible work is done very slowly and requires the maximum amount of energy.
by Julia Cheng 2J
Fri Jan 12, 2018 11:30 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Liquids and Solids
Replies: 3
Views: 228

Re: Liquids and Solids

Yes, even for the same substance, its standard enthalpy of formation is different depending on what state it is in. On tests and in the textbook, the standard enthalpies of formation are given and specific to whatever physical state the molecule is in, so we have to pay attention to that.
by Julia Cheng 2J
Fri Jan 12, 2018 11:23 am
Forum: Phase Changes & Related Calculations
Topic: Expansion and change in pressure
Replies: 4
Views: 190

Re: Expansion and change in pressure

Since the internal energy of the gas is less, the molecules have less energy and experience fewer collisions with each other and the walls of the container. So, the temperature and pressure are less. I'm not sure, but with your answer, I believe that the flaw is that the pressure could be greater ev...
by Julia Cheng 2J
Fri Jan 12, 2018 11:14 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Most Stable Forms
Replies: 4
Views: 212

Re: Most Stable Forms

The standard enthalpy of formation is 0 when the element is in its purest form. So even though water is in liquid form commonly, its standard enthalpy of formation is not 0 because it's not in its purest form.

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