CHEMISTRY COMMUNITY

Ariel Cheng 2I

A pseudo first order reaction is like a second order reaction that acts like a first order reaction under certain conditions.

Ariel Cheng 2I

In an adiabatic reaction q =0 while in an isothermal reaction, delta T = 0.

Ariel Cheng 2I

It depends on whether C is given in per moles or per grams (molar heat capacity or specific heat capacity).

Ariel Cheng 2I

I believe that delta G is the measure of the maximum amount of energy available for work.

Ariel Cheng 2I

I think q = delta H under constant pressure. I also think that delta H implies constant pressure already so they can be used interchangeably but I am not sure.

Ariel Cheng 2I

When a system expands, it is doing work on the surroundings which means it would be losing energy. Therefore, work is negative. If the surroundings were to do work on the system, then work would be positive because the system is gaining energy.

Ariel Cheng 2I

To expand on the above, a homogeneous catalyst is a catalyst of the same phase as the reactants while a heterogeneous catalyst is one that is of a different phase than the reactants.

Ariel Cheng 2I

Remember you can also relate order numbers to individual reactants themselves or also the overall reaction order.

Ariel Cheng 2I

If you look at the Arrhenius equation, you will see that increasing temperature increases the number to which e is taken to the power of (T is in the denominator and the number is negative so the number becomes a smaller negative). This increases the value of K. If activation energy decreases, then ...

Ariel Cheng 2I

He said he would post the slides online but if you still want notes I can send them!

Ariel Cheng 2I

To continue the answers above, remember to use a single line when referencing a porous disc versus the double line used for a salt bridge.

Ariel Cheng 2I

When you are looking at a chemical reaction, the rate at which each reactant and product is changing might be different from one another. A unique rate would be the rate of change for any species in a chemical reaction divided by its stoichiometric coefficient. Unique rates should be the same for ea...

Ariel Cheng 2I

Sometimes in the homework, the solutions would include KOH instead of OH. Is there a reason for this and should we include salts in our cell diagrams too?

Ariel Cheng 2I

In the cell, electrons move from the anode to the cathode which would make the cathode extremely negatively charged. If left like this, electrons would not travel toward the cathode any longer due to the buildup of a negative charge there. The salt bridge helps maintain a neutral state for both solu...

Ariel Cheng 2I

I think the only way to be sure of spontaneity is when ΔG is negative (when ΔH −TΔS is negative) though I could be wrong.

Ariel Cheng 2I

A process is spontaneous when ΔG is negative (when ΔH < TΔS).

Ariel Cheng 2I

When we calculate the two independently, we are essentially splitting it up into steps which is allowed since entropy is a state function. First we change the volume and then we change the temperature to reach the final form.

Ariel Cheng 2I

I would assume that Gibbs free energy is an extensive property since the feasibility of a reaction depends on factors such as the amount of matter/availability of reactants, etc. Please correct me if I'm wrong!

Ariel Cheng 2I

If there is no change in volume, no work will be done. If you are referring to an isothermal process with constant pressure, then I'm pretty sure work = -(external P)(Delta V).

Ariel Cheng 2I

How do I know which R to use and when? Thanks.

Ariel Cheng 2I

Constant volume means that no work is done and so delta U is equal to q. When temperature is constant, delta U is equal to 0.

Ariel Cheng 2I

When do we use the equation U = 3/2 nRT? I saw this on the constants and equations sheet but I have no idea when to use it.

Ariel Cheng 2I

Either should be fine as long as you are consistent in your calculations and make sure that everything matches up properly.

Ariel Cheng 2I

I am not entirely sure about extrinsic versus intrinsic though I think they are similar. Intrinsic properties are ones that do not depend on outside factors / amount of substance such as boiling point/density whereas extrinsic properties depend on outside factors / amount of substance such as weight...

Ariel Cheng 2I

To build on, heat capacity is therefore an extensive property because heat capacity is the heat required to raise the temperature of an object by 1 degree celsius. The heat required to raise the temperature of an object depends on how much of the object there is or the amount of substance.

Ariel Cheng 2I

I think that the question is asking what the reaction favors when it is at equilibrium (as opposed to when we use Q to see which direction the reaction will go when it is not at equilibrium). I do not remember the exact numbers but I believe in the first few lectures Dr. Lavelle talked about the rea...

Ariel Cheng 2I

I think that you can ignore the x when you are adding it or subtracting it to some constant (e.g. 1.0-x) because the difference that x will make on the number is insignificant when x is that small. It is important to note that x does not equal zero though.

Ariel Cheng 2I

Because you know that the solution is neutral water (pH ~7), the hydronium and hydroxide concentrations must equal each other and balance each other out.

Ariel Cheng 2I

Concentration is dependent on the number of moles and volume (n/v). Though adding inert gas does change the pressure, it changes neither of the number of moles nor the volume which means concentration stays the same! If you changed pressure by changing the volume, then concentration would change and...

Ariel Cheng 2I

As said above, Q has the same formula as Kc but can be calculated at anytime, not just when a reaction is at equilibrium. To explain the comparison a little more, when Q < Kc, a forward reaction will be favored. This is because Q is calculated as [products] / [reactants]. If Q is smaller than Kc, we...

Ariel Cheng 2I

As the others mentioned before, the general rule is that you can disregard the X when the K value is smaller than 10^-3. Afterward, you can also confirm that the X you get is less than 5% of the initial concentration in order to double check that your approximation is valid.

Ariel Cheng 2I

They have the same formula but don't equal each other because K is calculated using molarities at equilibrium but Q is calculated using molarities at any point in time during the reaction. We can then use Q and compare it to K to see at where the reaction is at. If Q<K, then a forward reaction is fa...

Ariel Cheng 2I

Kp is the equilibrium constant calculated using partial pressure (of gases). Kc is the equilibrium constant calculated using concentration of products and reactions at equilibrium. You can convert between partial pressure and concentration through PV = nRT. where P = (n/v)RT and n/v is concentration.

Ariel Cheng 2I

We use brackets in our calculation for Kc because we are using the molarity/concentration which is denoted with brackets. I don't think its necessary to use brackets when solving for Kp because we are using partial pressure though I'm not sure.

Ariel Cheng 2I

AX5E is square pyramidal!

Ariel Cheng 2I

I find that drawing out the orbitals and filling in the electrons help. That way you can easily visualize whether there are unpaired electrons (paramagnetic) or no unpaired electrons (diamagnetic).

Ariel Cheng 2I

You use bis-, tris-, and tetrakis- when the ligand name already has di-, tri-, tetra-, etc or if they are polydentate. For oxalate, you would use bis-, tris-, etc as it is bidentate.

Ariel Cheng 2I

The greater the difference in electronegativity between two atoms, the greater the ionic character of their bond. I think the number mentioned in class was that if the value of electronegativity is greater than 2, than the bond is considered generally ionic.

Ariel Cheng 2I

If you are talking about the coordination number, the coordination number is the number of ligands bound to the central atom!

Ariel Cheng 2I

Do you mean what it means for it to be cis? Cisplatin is a chemotherapy drug that forms a coordination compound with DNA, particularly guanine, to stop cell division.

Ariel Cheng 2I

It would be best to, from the Lewis structure, figure out the shape of the molecule through VSEPR and use the electronegativity of each atom to determine the total dipole movement.

Ariel Cheng 2I

When bonds are formed, energy is released (negative) as atoms move toward a more stable state. The opposite, the breaking of bonds, would therefore require an input of energy (positive) in order to pull atoms away from that stable state.

Ariel Cheng 2I

The two are essentially the same but they relate the ions through different means. The Lewis definition relates two ions using the transfer of electrons. A Lewis acid is an electron acceptor while a Lewis base is the electron donor. The Bronsted definition relates the two through protons in which a ...

Ariel Cheng 2I

In a double bond, one will always be a sigma bond and one will always be a pi bond. In a triple bond, one will always be a sigma bond and two will be pi bonds. This is because, in order to form sigma bonds, the orbitals need to align end to end. Afterward, the lobes can only align in a parallel form...

Ariel Cheng 2I

Molecules with higher polarity have greater dispersion forces and therefore stronger dipole-dipole interactions. This greater intermolecular attraction force leads to a higher boiling point. This explains why water (with all its hydrogen bonds) has a high boiling point.

Ariel Cheng 2I

Looking at VSEPR notations might help. For AX4, a molecule without lone pairs, both its molecular and electron geometry are tetrahedral. For AX3E, its molecular geometry would be trigonal pyramidal but its electron geometry is tetrahedral.

Ariel Cheng 2I

Breaking bonds generally requires an addition of energy. The opposite, forming bonds, is a release of energy which explains the negative number.

Ariel Cheng 2I

An example would be AX3E. Its molecular geometry would be trigonal pyramidal but its electron geometry is tetrahedral. For those without lone pairs on the central atom such as AX4, both its molecular and electron geometry is tetrahedral.

Ariel Cheng 2I

A single bond will always be a sigma bond, a double bond will be a sigma bond and a pi bond, and a triple bond will be a sigma bond and two pi bonds. The first bond is able to overlap head-on which allows the bound atoms to rotate. However, the second and third bonds are forced to bond side by side ...

Ariel Cheng 2I

The 4s orbital comes before the 3d orbital because the 4s shell actually has a lower energy level than the 3d orbital. However, if the 4s orbital is filled (2 electrons), then its energy level rises to higher than the 3d orbital which is why we write it after the 3d (exceptions: Chromium and Copper).

Ariel Cheng 2I

I think that oxygen is more electronegative. Oxygen is smaller than chlorine, with chlorine having a whole other shell of electrons which means that valence electrons in oxygen feel a stronger force.

Ariel Cheng 2I

To add on to the previous posts, it is also most desirable to have the lowest possible charge on the central atom for stability though this is not always possible.

Ariel Cheng 2I

A lewis base simply donates an electron within a coordinate covalent bond and a lewis acid accepts an electron in a coordinate covalent bond!

Ariel Cheng 2I

This is because oxygen's 2p orbitals has 4 electrons (1 paired, 2 unpaired). Losing one electron from the pair would create a half filled stable orbital which is desired, hence the low ionization energy!

Ariel Cheng 2I

Unless the question asks you specifically to draw all the resonance structures, I think that one is okay.

Ariel Cheng 2I

In addition to the resources stated above, I also really enjoy going through worksheets from workshops, step-up sessions, etc. You can just look them up on chem community!

Ariel Cheng 2I

After the first electron is removed, the ratio of protons to electrons increases and therefore the pull on the remaining electrons increases. This raises the ionization energy.

Ariel Cheng 2I

I think something that would help would be to write out the units for all parts of the equation. In h/mv, h is given in kg.m^2.s^-1. In order to cancel out the kg, the mass "m" needs to be in kilograms as well. The velocity should cancel out m.s^-1 and leave meters for λ.

Ariel Cheng 2I

You would just solve this as you normally do. λ = h/p = h/mv h = 6.626 x 10^-34 m = 275 kg v = 34.72 m/s λ = (6.626 x 10^-34)/(275*34.72) = 6.94 x 10^-38 m A car does not have wavelike properties as only very small objects have measurable wavelike properties. As objects become larger, wavelike prope...

Ariel Cheng 2I

I would focus on the topics explored in the last four video modules and all the information given up until Friday's lecture.

Ariel Cheng 2I

Both work since wavelength is the inverse of frequency. It just depends on what you are trying to solve for in the problem! I think that something of this topic will most likely appear on the exam.

Ariel Cheng 2I

Maybe you could try balancing in steps one element at a time just in case a mistake is made so you can receive partial credit.

Ariel Cheng 2I

For addition and subtraction, you use the least number of decimal points from the given values and for multiplication, you use the least number of sig figs from the given values.

Ariel Cheng 2I

If you really dislike writing in pen, I would just ask your TA whether he/she cares! My TA said she did not mind whether we use pen or not.

Ariel Cheng 2I

I think here we need to use the M1V1 = M2V2 formula. I would first find the molarity of the original solution by converting 5.00 grams of KMnO4 into moles for the moles of solute and then divide it by the volume of the solution. (0.0316 mol/0.1500 L = 0.211 mol/L) Now you have the initial molarity (...

Ariel Cheng 2I

After identifying the limiting reactant, determine how much of the other reactant is used up by converting the grams of the limiting reactant to moles and, using the stoichiometric coefficients for the ratio, determining how many moles of the other reactant is used up. Convert that number to grams a...

Ariel Cheng 2I

After finding the empirical formula (C5H12O5), calculate its molar mass which should be about 152.15 g/mol. The question should give the molar mass of the compound. Divide that number by the 152.15 g/mol you just calculated to find the ratio between the molecular and empirical formula. Use that rati...

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