CHEMISTRY COMMUNITY

Angel Chen 2k

gibbs free energy of formation is always zero for pure elements in their standard state.

Angel Chen 2k

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...

Angel Chen 2k

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...

Angel Chen 2k

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.

Angel Chen 2k

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...

Angel Chen 2k

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.

Angel Chen 2k

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.

Angel Chen 2k

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.

Angel Chen 2k

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

Angel Chen 2k

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.

Angel Chen 2k

The oxidation number is the charge the atom would have in a molecule or an ionic compound if the electrons were completely transferred.

Angel Chen 2k

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...

Angel Chen 2k

The SI unit for change in Gibbs free energy would just be the SI unit for Gibbs free energy (kJ/mol).

Angel Chen 2k

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.

Angel Chen 2k

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...

Angel Chen 2k

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.

Angel Chen 2k

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.

Angel Chen 2k

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 ...

Angel Chen 2k

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 ...

Angel Chen 2k

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...

Angel Chen 2k

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.

Angel Chen 2k

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...

Angel Chen 2k

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...

Angel Chen 2k

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...

Angel Chen 2k

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.

Angel Chen 2k

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.

Angel Chen 2k

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...

Angel Chen 2k

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.

Angel Chen 2k

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...

Angel Chen 2k

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.

Angel Chen 2k

It might not be possible on Earth because other than CO2, there are other pollutants in the air that result in slightly acidic rain.

Angel Chen 2k

I do not think so. It should be a weak base according to the definitions of Lewis acid and base.

Angel Chen 2k

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!

Angel Chen 2k

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...

Angel Chen 2k

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!

Angel Chen 2k

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!

Angel Chen 2k

It does matter. It helps us know the direction of the flow of electrons.

Angel Chen 2k

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...

Angel Chen 2k

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.

Angel Chen 2k

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 ...

Angel Chen 2k

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.

Angel Chen 2k

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.

Angel Chen 2k

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.

Angel Chen 2k

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 ...

Angel Chen 2k

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.

Angel Chen 2k

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.

Angel Chen 2k

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...

Angel Chen 2k

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...

Angel Chen 2k

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...

Angel Chen 2k

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.

Angel Chen 2k

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.

Angel Chen 2k

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.

Angel Chen 2k

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.

Angel Chen 2k

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.

Angel Chen 2k

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...

Angel Chen 2k

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...

Angel Chen 2k

Homework is handed in weekly in the discussion sections. So if your discussion is on Tuesday, you hand in the homework on Tuesday.

Angel Chen 2k

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...

Angel Chen 2k

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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