Search found 59 matches

by Angel Chen 2k
Fri Mar 15, 2019 5:11 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Gibbs Free Energy of Formation
Replies: 2
Views: 155

Re: Gibbs Free Energy of Formation

gibbs free energy of formation is always zero for pure elements in their standard state.
by Angel Chen 2k
Fri Mar 15, 2019 4:59 pm
Forum: First Order Reactions
Topic: pseudo first order
Replies: 2
Views: 55

Re: pseudo first order

Pseudo rate laws are generated because rate laws cannot be used for reactions with more than one reactant since changes in both reactants, most of the times, occur simultaneously. With Pseudo rate laws, the concentration of one reactant must be in large excess ( so the concentration is essentially c...
by Angel Chen 2k
Fri Mar 15, 2019 4:51 pm
Forum: Environment, Ozone, CFCs
Topic: Homogeneous Catalysts and Ozone
Replies: 2
Views: 321

Re: Homogeneous Catalysts and Ozone

Ozone is a relatively unstable molecule that constantly breaks down into O2 and O, with the presence of NO(as a catalyst)the process speeds up. Homogenous catalysts constantly interact with the reactant to form an intermediate substance, then interacts with another reactant to regenerate the origina...
by Angel Chen 2k
Mon Mar 11, 2019 12:16 am
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Melting of ice
Replies: 9
Views: 206

Re: Melting of ice

You can determine if a reaction is spontaneous by examining mainly two factors, the entropy and whether the reaction is exothermic or endothermic. Then by plugging in these values into the Gibss free energy equation, if the answer is negative, then the reaction is spontaneous.
by Angel Chen 2k
Mon Mar 11, 2019 12:04 am
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: Catalysts Effect on Overall Rxn vs. Step of Rxn
Replies: 2
Views: 65

Re: Catalysts Effect on Overall Rxn vs. Step of Rxn

A catalyst will appear in the steps of a reaction mechanism, but it will not appear in the overall chemical reaction since it is not a reactant or product. Catalysts would only lower the activation energy of a specific reaction since they are highly selective and would not speed up the overall react...
by Angel Chen 2k
Sun Mar 10, 2019 11:59 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: molecularity
Replies: 8
Views: 120

Re: molecularity

Molecularity is the number of molecules that take part in the rate-determining step. So depending on the prefix, it refers to how many species are in the reaction.
by Angel Chen 2k
Sun Mar 10, 2019 11:57 pm
Forum: Kinetics vs. Thermodynamics Controlling a Reaction
Topic: molecularity
Replies: 4
Views: 214

Re: molecularity

Molecularity is the number of molecules that are in the rate determining step. Which is different from the order of reactants which is the sum of the powers to which the reactant concentrations are raised in the rate law.
by Angel Chen 2k
Sun Mar 10, 2019 11:53 pm
Forum: General Rate Laws
Topic: Rate constant k
Replies: 5
Views: 96

Re: Rate constant k

The rate constant is equal to the rate of a reaction at the singles concentration of the reactants.K depends on the nature of the reactant, temperature, and the presence of catalyst.
by Angel Chen 2k
Mon Feb 25, 2019 1:30 am
Forum: Balancing Redox Reactions
Topic: Adding OH- and H20 into reactions
Replies: 3
Views: 61

Re: Adding OH- and H20 into reactions

In basic solution, you first balance O by using H2O, then balance H by adding H2O to the side of each half-reaction that needs H , and then add OH - to the other side of the equation
by Angel Chen 2k
Mon Feb 25, 2019 1:25 am
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Signs of delta G and E
Replies: 4
Views: 80

Re: Signs of delta G and E

For a spontaneous reaction, you usually have to look for values of delta H, delta S and T and results in a negative delta G value. With the Gibbs free energy equation, one of the factors for a reaction to be spontaneous is having a negative delta G.
by Angel Chen 2k
Mon Feb 25, 2019 1:21 am
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Oxidation number
Replies: 2
Views: 55

Re: Oxidation number

The oxidation number is the charge the atom would have in a molecule or an ionic compound if the electrons were completely transferred.
by Angel Chen 2k
Wed Feb 20, 2019 12:37 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Positive vs Negative E value
Replies: 5
Views: 483

Re: Positive vs Negative E value

You determine the Ecell value from the two half-reactions you write from either the cell diagram given or the balanced chemical equation. The formula for calculating the Ecell is E(cathode) - E(anode) ( these values can be obtained from the book standard E value chart ). For example, electrolytic ce...
by Angel Chen 2k
Wed Feb 20, 2019 12:32 am
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Gibbs and Activation Energy
Replies: 4
Views: 299

Re: Gibbs and Activation Energy

The SI unit for change in Gibbs free energy would just be the SI unit for Gibbs free energy (kJ/mol).
by Angel Chen 2k
Wed Feb 20, 2019 12:27 am
Forum: *Free Energy of Activation vs Activation Energy
Topic: Spontaneous reactions
Replies: 7
Views: 1064

Re: Spontaneous reactions

It is Gibbs free energy that determines spontaneity, not the enthalpy of the reaction. The reaction would only be spontaneous if Gibbs free energy is negative.
ΔG=ΔH−TΔS
This equation could be positive even if the enthalpy is negative ( exothermic) with a high enough temperature.
by Angel Chen 2k
Tue Feb 12, 2019 12:00 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: W in adiabatic
Replies: 1
Views: 63

Re: W in adiabatic

Because work can be done in the adiabatic process even though the heat transfer does not exist. According to the first of thermodynamics, thermal energy will also change when work changes and heat does not change. Also, work can make volume changes. It explains the delta V in the equation you mentio...
by Angel Chen 2k
Mon Feb 11, 2019 10:47 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: When can you assume 1 mol?
Replies: 1
Views: 56

Re: When can you assume 1 mol?

You assume 1 mol when you are short of information. In some problems, you can't solve it unless you have the number of moles, so you assume 1 mol.
by Angel Chen 2k
Mon Feb 11, 2019 10:24 pm
Forum: Calculating Work of Expansion
Topic: Question on the units of work
Replies: 3
Views: 57

Re: Question on the units of work

The units depend on how the question is asked. So just make sure your the unit of your answer match what is asked in the question.
by Angel Chen 2k
Mon Feb 04, 2019 8:38 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: PV=nRT
Replies: 5
Views: 134

Re: PV=nRT

First of all, this equation can only be used on ideal gas. 1) The volume of gas particles are really small, which can be neglected 2) Gas particles are always moving around randomly in the system 3) There are no interactions between the particles 4) The collisions between gas particles and the wall ...
by Angel Chen 2k
Mon Feb 04, 2019 8:26 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Internal heat
Replies: 2
Views: 47

Re: Internal heat

In the constant-pressure process, the gas can still do the work. Also, when it interacts with other systems with different temperatures, heat can be transferred between them (if neither of them is an isolated system). Therefore, since both Q and W can be changed in this process, the internal energy ...
by Angel Chen 2k
Mon Feb 04, 2019 8:19 am
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: why is the entropy change at constant pressure greater than the entropy change at constant volume?
Replies: 3
Views: 227

Re: why is the entropy change at constant pressure greater than the entropy change at constant volume?

In the isochoric change process, the gas does not do any work; on the other hand, in the isobaric change process, the gas does work to expand the gas. In addition, according to the 2nd law of thermodynamics, entropy in the isolated system always stays the same or increases. Therefore, since gas does...
by Angel Chen 2k
Mon Jan 28, 2019 1:12 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Exothermic vs. Endothermic
Replies: 10
Views: 187

Re: Exothermic vs. Endothermic

The two posts before are really standard definitions. In other words, you can simply regard the exothermic reaction as a "releasing heat" process. On the other hand, you can regard endothermic reaction as an "absorbing heat" process.
by Angel Chen 2k
Mon Jan 28, 2019 1:10 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Adiabatic and diathermic walls
Replies: 4
Views: 73

Re: Adiabatic and diathermic walls

For a system with an adiabatic wall, there is no heat or substance exchange. Only work can be done to change the internal energy. On the other hand, for a system with a diathermic wall, there is only heat exchange. I believe that we only need to understand the concepts of these two terms. More speci...
by Angel Chen 2k
Mon Jan 28, 2019 1:05 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: heat capacity
Replies: 7
Views: 110

Re: heat capacity

Heat capacity is used to measure the heat absorbed or released when a given kind of materials experience a change of temperatures. It could be expressed by the equation: Q=mc*delta T (T is in Kelvin temperature scale). The materials with higher heat capacities require more heat to have a certain tem...
by Angel Chen 2k
Sun Jan 20, 2019 11:17 pm
Forum: Non-Equilibrium Conditions & The Reaction Quotient
Topic: Removing products
Replies: 10
Views: 161

Re: Removing products

Removing the products would just affect Q since K only changes when temperature changes. According to one part of the Le Chatelier's principle , when a system at equilibrium is disturbed by a change in concentration of one of the components, the system will shift its equilibrium (Q would change ) un...
by Angel Chen 2k
Sun Jan 20, 2019 11:10 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Changes in Temperature
Replies: 8
Views: 127

Re: Changes in Temperature

In an endothermic reaction ( delta T> 0) heat is absorbed so equilibrium shifts to the right. In an exothermic reaction ( delta T<0 ) , heat is released , the reaction goes towards the left.
by Angel Chen 2k
Sun Jan 20, 2019 11:06 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: 5%
Replies: 5
Views: 98

Re: 5%

The 5% rule is essentially a technique that allows us to simply calculations ( do not have to use the quadratic formula) when we are able to recognize that the change in composition ( x) is less than 5 % of the initial value.
by Angel Chen 2k
Mon Jan 14, 2019 12:20 am
Forum: Ideal Gases
Topic: R [ENDORSED]
Replies: 5
Views: 124

Re: R [ENDORSED]

From my perspective, I do not think that you need to know much about the definition of R or memorize the value of R. They will be given in the tests. However, one thing you need to know is how to apply different values and units under different circumstances. It all depends on the settings of the qu...
by Angel Chen 2k
Mon Jan 14, 2019 12:09 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: K vs Q
Replies: 4
Views: 74

Re: K vs Q

To summarize the posts above, K is for the reaction which reaches it equilibrium, while Q is for the reaction which does not reach its equilibrium. Comparing the values of K and Q can indicate us the position of equilibrium and the direction that the reaction proceeds.
by Angel Chen 2k
Mon Jan 14, 2019 12:07 am
Forum: Ideal Gases
Topic: ICE Table
Replies: 4
Views: 93

Re: ICE Table

For the initial value, reactant has a given value, while products are zero. In the reaction, reactant will decreases for a given amount x, while products will increase by an amount proportional to x. Finally, when the reaction is complete, the reactant is "some value-x", while the products...
by Angel Chen 2k
Sun Dec 09, 2018 3:32 pm
Forum: Conjugate Acids & Bases
Topic: Conjugate Bases
Replies: 7
Views: 289

Re: Conjugate Bases

To add on, Conjugate acid/base pair are compounds that differ by one H+. After a reaction, all acid/base would have a conjugate base/acid.
by Angel Chen 2k
Sun Dec 09, 2018 3:30 pm
Forum: Air Pollution & Acid Rain
Topic: Basic Rain
Replies: 5
Views: 242

Re: Basic Rain

It might not be possible on Earth because other than CO2, there are other pollutants in the air that result in slightly acidic rain.
by Angel Chen 2k
Sun Dec 09, 2018 3:22 pm
Forum: Amphoteric Compounds
Topic: Amphoteric Compounds
Replies: 1
Views: 203

Re: Amphoteric Compounds

I do not think so. It should be a weak base according to the definitions of Lewis acid and base.
by Angel Chen 2k
Mon Dec 03, 2018 11:28 pm
Forum: Hybridization
Topic: Hybridization
Replies: 11
Views: 185

Re: Hybridization

Trigonal planar is just one possible molecular structure of sp3d. Actually, the specific structure varies depending on the number of bonding regions and nonbonding regions (or lone pairs). I recommend you to memorize the table of hybridization on textbooks.

Hope this helps!
by Angel Chen 2k
Mon Dec 03, 2018 11:23 pm
Forum: Conjugate Acids & Bases
Topic: conjugate acids
Replies: 3
Views: 98

Re: conjugate acids

I want to explain this point more generally. Since a Lowey-Bronsted acid is a proton donor, the acid gives out a proton (normally H+) to become a conjugated base. On the other hand, as a proton acceptor, the base received the proton to become a conjugated acid. In summary: 1. If you want to find the...
by Angel Chen 2k
Mon Dec 03, 2018 11:15 pm
Forum: Bronsted Acids & Bases
Topic: Strong Acids
Replies: 4
Views: 77

Re: Strong Acids

You can compare the values of Ka and pKa. For Ka, the larger the Ka, the greater the acidity; the smaller the Ka, the less the acidity. For pKa, the larger the pKa, the less the acidity; the smaller the pKa, the greater the acidity.

Hope this helps!
by Angel Chen 2k
Sun Nov 25, 2018 11:58 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: pi-bonds
Replies: 7
Views: 119

Re: pi-bonds

Basically, a pair of double bonds consists of one sigma bond and a pi bond. Triples bonds contain one sigma bond and two pi bonds.

Hope this helps!
by Angel Chen 2k
Sun Nov 25, 2018 11:56 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Molecular Shape
Replies: 4
Views: 80

Re: Molecular Shape

It does matter. It helps us know the direction of the flow of electrons.
by Angel Chen 2k
Sun Nov 25, 2018 11:50 pm
Forum: Hybridization
Topic: hybridization
Replies: 2
Views: 60

Re: hybridization

Basically, you can only look at the bonding regions and non-bonding regions. More specifically, one lone pair are considered as one non-bonding region, two lone pairs are considered as two non-bonding regions, etc. On the other hand, one bond, two bonds, or three bonds between the central atom and a...
by Angel Chen 2k
Sun Nov 18, 2018 10:47 pm
Forum: Hybridization
Topic: E- Promotion and Hybridization
Replies: 2
Views: 154

Re: E- Promotion and Hybridization

Electron promotion is related to hybridization. The hybridized orbital has both s characteristics and p characteristics. Its energy is between the s orbital and the p orbital. As a result, the electron promotion is essential for s orbital electrons to promote to hybridized orbital with higher energy.
by Angel Chen 2k
Sun Nov 18, 2018 10:41 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Polarity in Molecule
Replies: 3
Views: 86

Re: Polarity in Molecule

When a molecule has no dipole moment(the dipole moment is symmetric), it is nonpolar ( for example, CO2). To be considered as a polar molecule, the difference in electronegativity of the atoms must be large. Using the water molecule as an example, the vertical components of the dipole moment points ...
by Angel Chen 2k
Sun Nov 18, 2018 10:33 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: lone pair influence on shape
Replies: 12
Views: 222

Re: lone pair influence on shape

The presence of lone pair around the central atom affects the bond angle. A lone pair of electrons always tries to repel the bonded electrons, when electron pairs move away from each other, the shape of the molecule is affected.
by Angel Chen 2k
Sun Nov 11, 2018 11:31 pm
Forum: Resonance Structures
Topic: Resonance Structures
Replies: 3
Views: 101

Re: Resonance Structures

In the resonance structures, only electrons are pushed or shifted. I guess what you said "double or triple bonds are shifted" means the electrons that constitute the pi bond(s) are shifted. In this way, the resonance structures stabilize the molecules through electrons delocalization.
by Angel Chen 2k
Sun Nov 11, 2018 11:28 pm
Forum: Electronegativity
Topic: Electronegativity
Replies: 8
Views: 265

Re: Electronegativity

One exception that I want to mention is hydrogen. The electronegativity of H is 2.1, which is larger than B and smaller than C.

Hope this helps.
by Angel Chen 2k
Sun Nov 11, 2018 11:22 pm
Forum: Octet Exceptions
Topic: List of octect exceptions
Replies: 6
Views: 186

Re: List of octect exceptions

For the first row, the ground state form of He is a duet instead of the octet. It is the most stable form for the atoms of the first several elements. For the first two rows, we do not normally exceed the duet or the octet. For the atoms the elements below, sometimes they can exceed the octet. Hope ...
by Angel Chen 2k
Sun Nov 04, 2018 9:08 pm
Forum: Formal Charge and Oxidation Numbers
Topic: Formal Charge
Replies: 8
Views: 169

Re: Formal Charge

Based on the definition of the formal charge, we can get an easier way to calculate it.

(Number of available valence electrons)-(dashes and dots)

--dashes are half of the bonding electrons (or bonding pairs)

--dots are the non-bonding electrons.

Hope this makes the calculation easier.
by Angel Chen 2k
Sun Nov 04, 2018 9:04 pm
Forum: Sigma & Pi Bonds
Topic: delocalized pi bond
Replies: 5
Views: 1180

Re: delocalized pi bond

In a resonance hybrid structure of a given molecule, you can say the delocalization of electrons stabilize the structure of this molecule. The bonds electrons could move over are delocalized pi bonds.
by Angel Chen 2k
Sun Nov 04, 2018 9:00 pm
Forum: Lewis Structures
Topic: Lewis Acids vs Bases
Replies: 2
Views: 66

Re: Lewis Acids vs Bases

Lewis acid is a species that accepts an electron pair while Lewis base is a species that donates an electron pair. The Lewis acid and base theory is in contrast with the Bronsted-Lowry Acid and Base theory, in which Bronsted-Lowry Acid (BLA) is the proton donor while Bronsted-Lowry Base (BLB) is the...
by Angel Chen 2k
Sun Oct 28, 2018 11:57 pm
Forum: Trends in The Periodic Table
Topic: s-block and p-block reactivity?
Replies: 6
Views: 122

Re: s-block and p-block reactivity?

I don't think ionization energy and reactivity are directly inverse. Low ionization energy only indicates the high reactivity with atoms with low electronic affinity. More specifically, the reactions between them are formed through ionic bonding. However, it does not indicate the reactivity of react...
by Angel Chen 2k
Sun Oct 28, 2018 11:45 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Orbital momentum vs spin momentum
Replies: 1
Views: 54

Re: Orbital momentum vs spin momentum

Both of them are used to describe the motion and location of electrons in space around the nucleus. I think you are talking to the angular (orbital) momentum, which is used to describe the shape of electron cloud, which refers to s,p,d,f. On the other hand, the spin momentum represents the spin dire...
by Angel Chen 2k
Sun Oct 28, 2018 11:39 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: 4s state
Replies: 2
Views: 62

Re: 4s state

Because orbitals with a lower n+l value are filled first. For the 4s orbital, the n+l value is 4+0=4 , while for the 3d orbital , the n+l value is 3+2=5. This is why the 4s orbital is filled first.
by Angel Chen 2k
Sun Oct 21, 2018 11:19 pm
Forum: DeBroglie Equation
Topic: De Broglie
Replies: 12
Views: 514

Re: De Broglie

The De Broglie equation indicated that the relationship between wavelength and momentum is inversely proportional. The equation is wavelength is equal to planck's constant divided by momentum( mass*velocity). You use to De Broglie equation to calculate de Broglie wavelength.
by Angel Chen 2k
Sun Oct 21, 2018 11:16 pm
Forum: Photoelectric Effect
Topic: Work Function
Replies: 16
Views: 215

Re: Work Function

Work function is essentially the energy required to eject an electron. The equation of the work function is work function = h* Vo ( with Vo being the minimum frequency needed to eject an electron). Sometimes work function us given in eV so make sure to covert eV into Joules to avoid unit confusions.
by Angel Chen 2k
Fri Oct 19, 2018 12:05 am
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: angular nodes
Replies: 1
Views: 59

Re: angular nodes

In wave functions, at points of zero amplitude called nodes, which are surfaces with zero electron density, the wave function changes signs at the nodes.
by Angel Chen 2k
Sat Oct 13, 2018 7:05 pm
Forum: Photoelectric Effect
Topic: Properties of Light
Replies: 5
Views: 118

Re: Properties of Light

The intensity of a light source is measured as the square of the amplitude of the wave, which is the height above the centerline of light waves.
by Angel Chen 2k
Sat Oct 13, 2018 7:00 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Visualizing Orbitals in Atoms
Replies: 3
Views: 111

Re: Visualizing Orbitals in Atoms

Orbitals are actually the regions where we can possibly find electrons. In contrast to the "circle orbitals" we usually refer to, the orbitals in chemistry are more likely to be three-dimensional regions. More specially, s orbital is like a sphere, while p orbital is like a double-headed l...
by Angel Chen 2k
Sat Oct 13, 2018 6:48 pm
Forum: Properties of Light
Topic: De Brogile Equation
Replies: 6
Views: 58

Re: De Brogile Equation

To elaborate off of Samatha's reply, De Brogile wavelength equation is actually the development of previous atomic models. For example, Dalton's model, J.J. Thompson's plum pudding model, Lutherford's nucleus model, and etc. De Brogile proposed that any matter have the properties of both waves and p...
by Angel Chen 2k
Fri Oct 05, 2018 9:22 pm
Forum: Properties of Light
Topic: Homework Question
Replies: 2
Views: 76

Re: Homework Question

Homework is handed in weekly in the discussion sections. So if your discussion is on Tuesday, you hand in the homework on Tuesday.
by Angel Chen 2k
Fri Oct 05, 2018 9:18 pm
Forum: Limiting Reactant Calculations
Topic: Comparing Calculated Moles step M7(b)
Replies: 2
Views: 91

Re: Comparing Calculated Moles step M7(b)

Since the question is asking for the mass of boron, you calculate the mass of Boron produced from 125kg of B2O3 and 125kg of Mg. You use 2:1 ad the ratio for calculating the mass of boron produced from B2O3 and the ratio you use for the mass of boron produced from Mg is 2:3. These ratios come from t...
by Angel Chen 2k
Fri Oct 05, 2018 12:10 am
Forum: SI Units, Unit Conversions
Topic: 6th Edition E27, First Part
Replies: 5
Views: 90

Re: 6th Edition E27, First Part

you use the Avogadro's constant to convert from mass per mol to mass per molecule. It should be (1/6.022x10^23 molecules.mol^-1 ) so you would be able to get g.molecule^-1 for the mass of the H2O molecule.

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