CHEMISTRY COMMUNITY

Guangyu Li 2J

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, ...

Guangyu Li 2J

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...

Guangyu Li 2J

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.

Guangyu Li 2J

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...

Guangyu Li 2J

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...

Guangyu Li 2J

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...

Guangyu Li 2J

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...

Guangyu Li 2J

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...

Guangyu Li 2J

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.

Guangyu Li 2J

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.

Guangyu Li 2J

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 ...

Guangyu Li 2J

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!

Guangyu Li 2J

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.

Guangyu Li 2J

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.

Guangyu Li 2J

Reductions Agents are oxidized in the half-reactions who lose electrons.
Oxidizing Agents are reduced in the half-reactions who gain electrons.

Guangyu Li 2J

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...

Guangyu Li 2J

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.

Guangyu Li 2J

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 ...

Guangyu Li 2J

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!

Guangyu Li 2J

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 ...

Guangyu Li 2J

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 △...

Guangyu Li 2J

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!

Guangyu Li 2J

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...

Guangyu Li 2J

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)

Guangyu Li 2J

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...

Guangyu Li 2J

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...

Guangyu Li 2J

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!

Guangyu Li 2J

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....

Guangyu Li 2J

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...

Guangyu Li 2J

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

Guangyu Li 2J

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...

Guangyu Li 2J

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.

Guangyu Li 2J

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....

Guangyu Li 2J

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...

Guangyu Li 2J

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...

Guangyu Li 2J

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...

Guangyu Li 2J

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...

Guangyu Li 2J

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, ...

Guangyu Li 2J

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...

Guangyu Li 2J

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...

Guangyu Li 2J

Trends are not applied to noble gas. That is because noble gases has already reached their most stable structure.

Guangyu Li 2J

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 ...

Guangyu Li 2J

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...

Guangyu Li 2J

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...

Guangyu Li 2J

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.

Guangyu Li 2J

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.

Guangyu Li 2J

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...

Guangyu Li 2J

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...

Guangyu Li 2J

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...

Guangyu Li 2J

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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