Search found 50 matches

by Guangyu Li 2J
Sun Mar 18, 2018 2:27 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Reaction Graphs
Replies: 3
Views: 242

Re: Reaction Graphs

The greater the activation energy, the longer reactants cost to reach the energy barrier, and the slower the reaction rate. For the endothermic reactions, since products need to overcome a higher energy barrier, the forward reaction is slower than the reverse reaction; For the exothermic reactions, ...
by Guangyu Li 2J
Sun Mar 18, 2018 2:23 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Catalyst and Intermediate
Replies: 5
Views: 310

Re: Catalyst and Intermediate

Yes. It is really easy to confuse these two ideas. Reaction intermediate is a molecule that plays a role in the reactions but does not appear in the overall reaction equation. On the other hand, the catalyst is a molecule that speeds up the chemical reactions by providing an optional pathway, or mor...
by Guangyu Li 2J
Sun Mar 18, 2018 2:10 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Catalysts
Replies: 3
Views: 244

Re: Catalysts

First, you need to make sure that if a certain molecule speeds up the reaction. Second, you need to check that if the molecule is not consumed in the process of the reaction. If a given molecule satisfied both of the two standards, it is a catalyst.
by Guangyu Li 2J
Mon Mar 12, 2018 2:01 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Activation Energy
Replies: 4
Views: 187

Re: Activation Energy

If the kinetic energy of the molecules reaches the activation energy, they will be able to collide with each other effectively and allow the reactions to proceed. The reactions can be speeded up by lowering the energy barrier of the reactions. One of the most significant examples of this point is th...
by Guangyu Li 2J
Mon Mar 12, 2018 1:55 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Collision Theory
Replies: 3
Views: 182

Re: Collision Theory

The collision theory believes that chemical reactions are led by the molecular collisions. Only those with high enough energy and appropriate direction, or called activated molecules, can collide with each other and proceed chemical reactions. Based on the molecular knowledge in 14A, we know that on...
by Guangyu Li 2J
Mon Mar 12, 2018 1:35 am
Forum: General Rate Laws
Topic: Determining Orders
Replies: 2
Views: 139

Re: Determining Orders

Yes. Given that only one reactant's concentration changes, you can compare how many times the reactant's concentrations changes and how many times the reaction rate changes. If the times reaction rate change is the same as that of concentration changes, it is a firsr-order reaction. If the times rea...
by Guangyu Li 2J
Mon Mar 05, 2018 3:05 pm
Forum: Second Order Reactions
Topic: Limiting Step
Replies: 7
Views: 455

Re: Limiting Step

The slow step is associated with higher activation energy because it needs to overcome the higher energy barrier, while the fast step is associated with lower activation energy because it needs to overcome the lower energy barrier. As a result, the slow step always the limiting step because it alway...
by Guangyu Li 2J
Mon Mar 05, 2018 1:46 pm
Forum: First Order Reactions
Topic: Half-life of first order reactions [ENDORSED]
Replies: 4
Views: 229

Re: Half-life of first order reactions [ENDORSED]

Half-life represents the time that a material is decayed to half of its original content. This variable is actually independent of the concentrations. More specifically, the time a given material decays from its original content to half of it is the same as that used to decay from half of it to a qu...
by Guangyu Li 2J
Mon Mar 05, 2018 1:31 pm
Forum: Kinetics vs. Thermodynamics Controlling a Reaction
Topic: Half-life
Replies: 4
Views: 198

Re: Half-life

Half-life usually describes the decay of discrete entities, and it describes the time that a material left only half of its original content. As a result, T does not always equal to 1/2. For example, it can be 1/4 which is the 1/2 of the 1/2 of the original material after two half-life time intervals.
by Guangyu Li 2J
Fri Feb 23, 2018 12:37 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Finding n
Replies: 15
Views: 761

Re: Finding n

According to the textbook, n is the number of electrons transferred in moles. Commonly we can first list the half-rxns to identify how many moles of electrons are transferred.
by Guangyu Li 2J
Fri Feb 23, 2018 12:27 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Spontaneous Directions [ENDORSED]
Replies: 7
Views: 258

Re: Spontaneous Directions [ENDORSED]

The values of standard potential can help us determine whether the process is spontaneous. When Eo is >0 the process is spontaneous and tend to produce products while when Eo <0 the process is not spontaneous and tend to produce the reactants. The example of 14.5 can help you better understand this ...
by Guangyu Li 2J
Fri Feb 23, 2018 12:17 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Cell Diagrams [ENDORSED]
Replies: 7
Views: 281

Re: Cell Diagrams [ENDORSED]

Inert species allow the electrons transportation. In addition, in the cell diagrams, inert species should always be in the outermost locations. It is not usually involved in the calculations.

Hope this helps!
by Guangyu Li 2J
Tue Feb 20, 2018 2:32 am
Forum: Balancing Redox Reactions
Topic: Oxidation Numbers
Replies: 6
Views: 256

Re: Oxidation Numbers

The oxidation state of oxygen element such as O2, P4, S8 is zero (0).
The oxidation state of oxygen in its compounds is usually -2, except for compounds like H2O2, and Na2O2, in which the oxidation state for O is -1.
by Guangyu Li 2J
Tue Feb 20, 2018 2:28 am
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: redox
Replies: 7
Views: 284

Re: redox

If the oxidation number of an atom is greater in the product, then it lost electrons and the substance was oxidized and it is the reducing agent. If the oxidation number of an atom is less, then it gained electrons and was reduced and it is the oxidizing agent.
by Guangyu Li 2J
Tue Feb 20, 2018 2:24 am
Forum: Balancing Redox Reactions
Topic: Oxidation Agents
Replies: 4
Views: 141

Re: Oxidation Agents

Reductions Agents are oxidized in the half-reactions who lose electrons.
Oxidizing Agents are reduced in the half-reactions who gain electrons.
by Guangyu Li 2J
Sun Feb 11, 2018 5:34 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Electrical energy to chemical energy
Replies: 2
Views: 127

Re: Electrical energy to chemical energy

A significant example is an Electrolytic Cell. Compared with a galvanic cell, an electrolytic cell has a battery to offer electricity for the electrochemical cell system. Important examples of electrolysis are the decomposition of water into hydrogen and oxygen, and bauxite into aluminum and other c...
by Guangyu Li 2J
Sun Feb 11, 2018 5:28 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Galvanic Cell
Replies: 3
Views: 118

Re: Galvanic Cell

Galvanic Cell is an electrochemical device that generates electricity through the spontaneous redox reaction taking place in the cell. Cathode receives the electrons and has reduction reaction.
by Guangyu Li 2J
Sun Feb 11, 2018 5:10 pm
Forum: Balancing Redox Reactions
Topic: Salt Bridges
Replies: 14
Views: 378

Re: Salt Bridges

A salt bridge is a connection of between the oxidation and reduction half-cells of a galvanic cell. It maintains electrical neutrality within the internal circuit, preventing the cell from rapidly running its reaction to equilibrium. In other words, only if a salt bridge is present, electricity can ...
by Guangyu Li 2J
Fri Feb 02, 2018 5:20 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Free Energy and Pressure
Replies: 4
Views: 140

Re: Free Energy and Pressure

The Gibbs free energy depends on pressure as well as on temperature. The pressure dependence of the Gibbs free energy in a closed system is given by the combined first and second laws and the definition of Gibbs free energy as dG=-SdT+Vdp

Hope this helps!
by Guangyu Li 2J
Fri Feb 02, 2018 5:17 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: State functions [ENDORSED]
Replies: 7
Views: 239

Re: State functions [ENDORSED]

State functions is a property that depends only on the current state of the system and is independent of how that state was prepared. In addition, if the components of a function are all state functions, then the overall equations is a state function. For the U=q+w, since q and w are both not state ...
by Guangyu Li 2J
Fri Feb 02, 2018 5:06 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Spontaneous
Replies: 14
Views: 419

Re: Spontaneous

Just a suggestion to view question 9.51 on the textbook: Why exothermic reactions are spontaneous? Exothermic reactions are commonly spontaneous reactions because the result is an increase in the entropy of the surroundings. Using the mathematical relationship △G=△H-T△S, it is pretty clear that if △...
by Guangyu Li 2J
Fri Jan 26, 2018 11:09 am
Forum: Phase Changes & Related Calculations
Topic: Closed Systems [ENDORSED]
Replies: 8
Views: 481

Re: Closed Systems [ENDORSED]

A closed system can have energy exchange with the surroundings

According to the equation: △U=q+w, you can both haet the container (closed system) or do work on the system.

Hope this helps!
by Guangyu Li 2J
Fri Jan 26, 2018 11:02 am
Forum: Phase Changes & Related Calculations
Topic: Heat and Work relation
Replies: 6
Views: 255

Re: Heat and Work relation

The value of work is not necessarily negative. According to what the textbook states in CH8: First Law of thermodynamics, △U=q+w 1. the q and w is determined by the realistic circumstance 2. the positive and negative sign is assigned to represent that if the work is done to the system and the system...
by Guangyu Li 2J
Fri Jan 26, 2018 10:55 am
Forum: Phase Changes & Related Calculations
Topic: Names of phase changes
Replies: 4
Views: 201

Re: Names of phase changes

Fusion (from solid to liquid) vs. Freeze (from liquid to solid)
Vaporization (from liquid to gas) vs. Condensation (from gas to liquid)
Sublimation (from solid to gas) vs. Deposition (from gas to solid)
by Guangyu Li 2J
Sun Jan 21, 2018 12:23 am
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Calorimeter
Replies: 10
Views: 446

Re: Calorimeter

Most people believe that the calorimeter was first used by French chemist Antoine-Laurent de Lavoisier to determine the calorific value of a guinea pig. The heat released by the guinea pig melted the surrounding snow of the calorimeter that proves that the exchange between the system and the surroun...
by Guangyu Li 2J
Sun Jan 21, 2018 12:12 am
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Difference between Closed and Isolated
Replies: 10
Views: 707

Re: Difference between Closed and Isolated

In thermodynamics, the systems without both matter exchange and energy exchange are isolated systems In thermodynamics, the systems without matter exchange with the surroundings are closed systems, but there can be some energy exchange. In other words, if there is an energy exchange is the main diff...
by Guangyu Li 2J
Sun Jan 21, 2018 12:07 am
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Perfect Isolated System
Replies: 3
Views: 136

Re: Perfect Isolated System

Actually there is no single system is totally isolated, including the universe itself. However, when the system is isolated enough, we can describe it as "completely isolate", especially over short time frames or small distances.

Hope this helps!
by Guangyu Li 2J
Fri Jan 12, 2018 12:04 am
Forum: Phase Changes & Related Calculations
Topic: Temperature during phase changes [ENDORSED]
Replies: 4
Views: 275

Re: Temperature during phase changes [ENDORSED]

In a simple way, since the melting process requires a large amount of energy, the temperature change when melting is not significant. Just want to add something: in physics, we divide matters into crystals and non-crystal(amorphous matters) When crystals are melting, the temperature does not change....
by Guangyu Li 2J
Thu Jan 11, 2018 11:59 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Hess's Law vs. Using Bond Enthalpies
Replies: 3
Views: 130

Re: Hess's Law vs. Using Bond Enthalpies

For both Hess' Law and Bond Enthalpies, we are supposed to figure out the enthalpy of certain reaction through the changes of enthalpies (Ex. The NO2 example in the lecture). However, according to different circumstances, we have to choose using the bond enthalpies (when given the Lewis structure or...
by Guangyu Li 2J
Thu Jan 11, 2018 11:54 pm
Forum: Phase Changes & Related Calculations
Topic: Enthalpy is a state property
Replies: 13
Views: 483

Re: Enthalpy is a state property

A State Property is not dependent on path taken to obtain that state but is determined by its state.

E.x. Enthalpy is a state property
Altitude is a state property
WOrk is not a state property
by Guangyu Li 2J
Sat Dec 09, 2017 3:55 am
Forum: Conjugate Acids & Bases
Topic: Conjugate Base of CH3COOH
Replies: 3
Views: 294

Re: Conjugate Base of CH3COOH

It should be correct because the textbook does indicate the conjugate base of CH3COOH as CH3COO-. I suppose the purpose of writing in this style is to indicate that the CH3COOH is the proton donor who donates one proton, and it becomes CH3CO- after losing that proton and becomes the conjugate base o...
by Guangyu Li 2J
Sat Dec 09, 2017 3:51 am
Forum: Bronsted Acids & Bases
Topic: Strong vs. Weak
Replies: 5
Views: 234

Re: Strong vs. Weak

From the equation, you can see how many hydrogen atoms are protonated.

Typically, all the hydrogen atoms in strong acid are protonated; on the other hand, a small fraction of hydrogen atoms in weak acid are protonated.

Hope this helps.
by Guangyu Li 2J
Sun Dec 03, 2017 12:04 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Gases [ENDORSED]
Replies: 3
Views: 211

Re: Gases [ENDORSED]

First I agree with what the previous post stated that no gas is actually ideal gas under real situation even though noble gases are close to ideal gases. Some more comments on how to identify ideal gases: 1. The gas molecules don't attract each other. 2. The volume of gas molecules can be neglected....
by Guangyu Li 2J
Sat Dec 02, 2017 11:58 pm
Forum: Ideal Gases
Topic: Partial pressures
Replies: 4
Views: 397

Re: Partial pressures

Dalton Law of Partial Pressure is the theorem for the ideal gases specifically. First of all, the gases suitable for this law must be ideal gases. According to this law, if the gases in the container don't react with each other, every kinds of gases are distributed evenly in the container. The press...
by Guangyu Li 2J
Sat Nov 18, 2017 11:42 pm
Forum: Ionic & Covalent Bonds
Topic: Ionic & Nonpolar/polar
Replies: 3
Views: 482

Re: Ionic & Nonpolar/polar

Actually the difference of electronegativity determines whether the chemical bond is ionic or covalent. When the electronegativity difference is larger than certain level, the electrons would be attracted by the atom with a larger electronegativity and an ionic bond is formed. However, when the elec...
by Guangyu Li 2J
Sat Nov 18, 2017 11:37 pm
Forum: Bond Lengths & Energies
Topic: Bond Order
Replies: 2
Views: 236

Re: Bond Order

Based on my understanding, bond order is the measurement of the stability of chemical bonds. For a chemical between two atoms, the larger bond order is, the stabler the chemical bond is; the smaller bond order is, the weaker the chemical bond is. When the bond order is zero, the chemical bond cannot...
by Guangyu Li 2J
Sun Nov 12, 2017 4:03 pm
Forum: Dipole Moments
Topic: Dipole moments
Replies: 6
Views: 366

Re: Dipole moments

1. What is dipole? Between atoms connected by a covalent bond, due to the difference in the electronegativity, atoms share electrons unevenly. The atom with a larger electronegativity attracts electron more strongly and becomes partially negative, while the atom with a smaller electronegativity attr...
by Guangyu Li 2J
Sun Nov 12, 2017 3:51 pm
Forum: Electronegativity
Topic: Electronegativity and electron affinity
Replies: 3
Views: 208

Re: Electronegativity and electron affinity

The electron affinity refers to the energy change when an electron is attracted by a neutral atom or molecule in the gaseous state to form a anion. The electronegativity means atoms' ability of attracting electrons. Commonly, the electronegativity increases from the left of the period to the right, ...
by Guangyu Li 2J
Sun Nov 05, 2017 12:14 am
Forum: Lewis Structures
Topic: Bases and Acids
Replies: 3
Views: 177

Re: Bases and Acids

In general, Lewis acid is the "electron accepter" and Lewis base is the "electron donor". Commonly, most of cations and metal ions are Lewis acids (accepting electrons) most of nonmetal ions, neutral molecule with unpaired electrons (e.g. NH3), and some molecules with C=C bonds a...
by Guangyu Li 2J
Sun Nov 05, 2017 12:04 am
Forum: Bond Lengths & Energies
Topic: Bond Lengths [ENDORSED]
Replies: 8
Views: 531

Re: Bond Lengths [ENDORSED]

A single bond only has a pi bond. A double bond contains a pi bond and a sigma bond. Whereas a pi bond is stronger and difficult to break, a sigma bond requires more energy to sustain the interaction between atoms. As a result, in order to form a sigma bond, two atoms have to be closer to each other...
by Guangyu Li 2J
Sun Oct 29, 2017 3:19 pm
Forum: Electronegativity
Topic: Trends in periodic table
Replies: 6
Views: 679

Re: Trends in periodic table

Trends are not applied to noble gas. That is because noble gases has already reached their most stable structure.
by Guangyu Li 2J
Sun Oct 29, 2017 3:13 pm
Forum: Ionic & Covalent Bonds
Topic: Ionic vs. Covalent Bonds [ENDORSED]
Replies: 8
Views: 435

Re: Ionic vs. Covalent Bonds [ENDORSED]

Covalent Bonds are formed when two atoms shared their outermost electrons. When the electronegativity are different, two atoms will share electrons unevenly and the covalent bond will be a polar covalent bond. However, it is a limit of the electronegativity difference that when it is above 2.0, the ...
by Guangyu Li 2J
Sat Oct 21, 2017 2:37 pm
Forum: Properties of Light
Topic: test 2 topic
Replies: 9
Views: 596

Re: test 2 topic

Commonly, tests mainly cover what Dr. Lavelle taught in classes. So you don't need to worry about that too much. But actually the content of 1.1 section is not complicated, which is about the process that how scientists discover the atomic structure. It includes: Dalton's Structure (Atom), J.J.Thomp...
by Guangyu Li 2J
Sat Oct 21, 2017 2:30 pm
Forum: Photoelectric Effect
Topic: Unexpected Result [ENDORSED]
Replies: 4
Views: 354

Re: Unexpected Result [ENDORSED]

For the lights with long wavelength, they can hardly remove electrons from the metal surface even by increasing the intensity. This unexpected phenomenon hints that energies of the emitted electrons to be independent of the intensity of the incident radiation. Instead, the energies required to remov...
by Guangyu Li 2J
Sat Oct 21, 2017 2:15 pm
Forum: *Black Body Radiation
Topic: Black Body Radiation [ENDORSED]
Replies: 5
Views: 1062

Re: Black Body Radiation [ENDORSED]

According to Wikipedia, A black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. Actually, it is not necessarily to be white, but a physical body that doesn't favor certain wavelength rather than others.
by Guangyu Li 2J
Fri Oct 13, 2017 11:46 pm
Forum: Bohr Frequency Condition, H-Atom , Atomic Spectroscopy
Topic: Using the Rydberg Equation
Replies: 4
Views: 255

Re: Using the Rydberg Equation

The given solution turns E/h to v first that we can set up an equation. Since we know the one of the n, we can figure the other one through the formula more easily.
by Guangyu Li 2J
Fri Oct 13, 2017 11:36 pm
Forum: Properties of Light
Topic: Difference Between Photon and Electron
Replies: 5
Views: 364

Re: Difference Between Photon and Electron

1. Difference in the definition: Photons are the basic units in the electromagnetic radiation that move in the speed of light in vacuum; Electrons are parts of the atom and move around the nucleus in certain orbitals. 2. Difference in the charge: Photons are neutral while electrons are negatively ch...
by Guangyu Li 2J
Fri Oct 13, 2017 11:24 pm
Forum: General Science Questions
Topic: Copenhagen Interpretation
Replies: 1
Views: 428

Copenhagen Interpretation

In today's lecture, the professor mentioned Bohr Frequency Condition and Heisenberg (maybe in my discussion class, I'm not pretty sure). Well, these two people reminded me of a theory that I read on another book—Copenhagen Interpretation. The definition that I remembered was: when one sees another p...
by Guangyu Li 2J
Fri Oct 06, 2017 5:53 pm
Forum: Empirical & Molecular Formulas
Topic: F1
Replies: 3
Views: 338

Re: F1

1. Taking the Carbon as example, the relative atomic mass of Carbon is approximately 12.01. Since there are 10 Carbon atoms, you need to multiply 12.01 by 10 when you calculate the mass of carbon. Then you divide this result by the total mass which is 152.2264 and you will get the corresponding mass...
by Guangyu Li 2J
Fri Oct 06, 2017 5:44 pm
Forum: Balancing Chemical Reactions
Topic: Balancing Chemical equations too slow... any advice? [ENDORSED]
Replies: 18
Views: 1540

Re: Balancing Chemical equations too slow... any advice? [ENDORSED]

I have a fast way for balancing equations which is specific to the atoms of odd numbers. You can multiple them by 2 first: An example from the Internet before is: FeS2+O2=Fe2O3+SO2 a. First, you can multiple Fe2O3 by 2, then you get FeS2+O2=2Fe2O3+SO2 b. Then, you can balance the iron atoms and you ...

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