CHEMISTRY COMMUNITY

gabbymaraziti

For an elementary step, the coefficients double as the order of the reactants.
The order for a non-elementary reaction cannot be determined from the formula, only experimentally.

gabbymaraziti

Elementary steps make up each baby-step of the reaction, and sum to the total reaction. The total reaction is usually a non-elementary step.

gabbymaraziti

You also know that you can use the approximation if your equilibrium constant, K, is small enough.

gabbymaraziti

You use any products and reactants so long as they are not liquids or solids.

gabbymaraziti

What is the difference between activation energy and free energy of activation?

gabbymaraziti

Do you mean n in electrochemistry, for the number of moles of electrons being transferred? Or n in kinetics, for the order of the reaction?

gabbymaraziti

If the reaction is zero-order, K has units of M/s
If the reaction is first-order, K has units of 1/s
If the reaction is second-order, K has units of 1/M.s

gabbymaraziti

Increasing the concentration of a reactant does NOT increase the rate of the reaction. But as stated above, an increase in concentration would have a greater effect on a second rate reaction than a first rate reaction due to the exponent.

gabbymaraziti

We created a ratio of concentration 2/concentration 1, both of which were raised to the power of m. 2.0 was the value of that ratio (2.0^m). The ratio of the actual rates was also 2, so m must be 1.

gabbymaraziti

How do you know if a half reaction needs an inert electrode like Pt?

gabbymaraziti

I don't understand how the textbook found this answer. The questions asks us to write the chemical equation corresponding to a cell, and I found the correct oxidation and reduction equations, but not the correct full equation. reduction : Hg2(NO3)2 + 2e- --> 2Hg + 2NO3- oxidation : 2Hg + 2HCl --> Hg...

gabbymaraziti

Cell potential is greatest when the cell reaction first begins to occur. Following this, the concentrations change and the potential is no longer at its maximum value.

gabbymaraziti

I understand how to determine this based on Ecell values, but how would we determine this based off electrochemical properties??

gabbymaraziti

We should be given that information because that affects how we solve the problem.

gabbymaraziti

You add H2O to balance the oxygen, and H+ to balance the hydrogen in an acidic solution.

You add OH- to balance the oxygen, and H2O to balance the hydrogen in a basic solution.

gabbymaraziti

Does anyone know when you would flip the sign in a half relation? I know that if you flip the equation the sign flips, but then do you not do this when calculating E(cell)? like if they give you the E value in the problem and the equation has to be flipped do you flip the sign? The Ecell values giv...

gabbymaraziti

In an acidic solution, you add H+ to balance the hydrogen in the equation. In a basic solution, however, you add OH- to balance the hydrogens (and then add H2O to balance that change in oxygen).

gabbymaraziti

This is just a table, like what can be found in Appendix 2A for enthalpy, entropy, etc.

gabbymaraziti

There is no specific guideline for this, Lavelle just "curves" according to the final, total class averages. Not much hope though, considering the average on the midterm was fairly high.

gabbymaraziti

What is reaction A? The textbook keeps referring to a 'reaction A' in order to find the amount of moles in a reaction. Can someone please explain and/or provide me with the page in the textbook that introduces this?

gabbymaraziti

The textbook states "Not all electrode reactions include a conducting solid as a reactant or product. For example, to use the reduction 2H+ + 2e- --> H2 at an electrode, a chemically inert metallic conductor, such as an unreactive metal or graphite, must carry the electrons into or out of the e...

gabbymaraziti

In order to use Hess's Law, you should be given multiple reactions so that you can add/subtract them as needed. Maybe if you aren't given multiple equations, you could use the reaction enthalpies for the formation of all the products and reactants, and create the equations yourself (ie creating an e...

gabbymaraziti

A half reaction is when a redox reaction is broken up into the reduction or oxidation components.

gabbymaraziti

Jack Riley 4f wrote:delta S of the surroundings does equal 0 because it is free expansion, but delta S total must still increase so that only leaves delta S of the system to contribute to this. Thus, delta S total = delta S system =/= 0

Why does deltaS still have to increase?

gabbymaraziti

Can someone please explain how to derive/where these equations come from?

deltaU = 3/2nRdeltaT = 0
and
Utot = 3/2nRT
both for an ideal gas.

gabbymaraziti

Is a spontaneous process accompanied by an increase in entropy of the surroundings, or of the system?

gabbymaraziti

Why is deltaS of an irreversible process not equal to zero? For example, 4I.9 states that for the irreversible process, deltaS is the same as deltaS for the reversible process. This does not make sense to me, because the process in this example is also free expansion. Therefore, w= 0, deltaU= 0, and...

gabbymaraziti

Prof. Lavelle has said multiple times not to think of entropy as disorder. If we don't think of entropy as disorder, how should we think of it?

gabbymaraziti

I'm sure either are fine, but if you have a large number of joules (1000+), it might be more useful to provide the answer in kilojoules.

gabbymaraziti

Yes, liquids and solids are excluded from calculating K or Q.

gabbymaraziti

Cm is the molar specific heat of the compound.

gabbymaraziti

Chem_Mod wrote:What answer did you get? It might be an error because other students are getting different numbers as well. I got -.94kJ, not -.86kJ.

I got -.95kJ as well.

gabbymaraziti

Example 4A.4 in the textbook gives a change in temperature in celsius, then states that it is the same amount in kelvin. How is this possible?

gabbymaraziti

I've done this problem so many times and I can't get the right answer. Can someone please write out the solution? Water expands when it freezes. How much work does 100. g of water do when it freezes at 0 deg. C and pushes back the metal wall of a pipe that exerts an opposing pressure of 1070 atm? Th...

gabbymaraziti

q represents the transfer of heat to or from the system. Delta H represents the sum of the internal energy in the system, taking into account the system's pressure and volume when necessary.

gabbymaraziti

We should be given a table with a number of different values, including the heat capacity of various elements/compounds.

gabbymaraziti

The standard enthalpy of formation is the change in enthalpy that accompanies the formation of one mole of a compound from its elements. The standard enthalpy of reaction is the change in enthalpy that occurs in a system when one mole of matter is transformed by a chemical reaction.

gabbymaraziti

Delta U is the internal energy of the system/reaction, or the total store of energy in a system. One way delta U can be calculated is by subtracting the internal energy of the initial system from the internal energy of the final system.

gabbymaraziti

The enthalpy for an exothermic reaction is negative because the system/reaction loses energy, and the surroundings gain energy. Similarly, an endothermic reaction requires energy, so the enthalpy value is positive.

gabbymaraziti

Enthalpy is additive because it is a state function, meaning it can be added or subtracted.

gabbymaraziti

Enthalpy is a state function that keeps tract of the losses of energy during heat transfer. Heat itself, however, like work, is not a state function because it depends on the path used to go from the initial to the final state.

gabbymaraziti

You should calculate the enthalpy, then add the enthalpy of the phase change.

gabbymaraziti

The steam has much higher enthalpy than the liquid, because more heat is required to achieve a gaseous state. Therefore, when it comes into contact with your skin and quickly condenses, it releases a large amount of heat in comparison to that that would be released by the liquid state.

gabbymaraziti

What order did Prof. Lavelle recommend we read the textbook in, since he said he was going out of order for the Thermochemistry outline?

gabbymaraziti

5.39 part b asks us to find the new equilibrium concentrations of two gases when the volume of the flask they are contained in is reduced by half. The original concentration of one of the gases was 0.020 M. The solutions manual immediately finds that the new concentration of this gas is 0.040 M. How...

gabbymaraziti

Can we just use K and not specify Kc or Kp? Or would we be marked down?

gabbymaraziti

Manipulating the PV=nRT is helpful because P gives the partial pressure of gases, which we can use when the problem provides us only with molar concentrations.

gabbymaraziti

How can we convert between atm and bars? The examples in the book show that the conversion is not 1:1.

gabbymaraziti

Do you have to change the volume in order to change the pressure? Or would adding a non-inert gas affect pressure as well?

gabbymaraziti

I'm not sure how to start the calculations for Self Test 5I.3b. Can anyone explain how to begin? Hydrogen chloride gas is added to a reaction vessel containing solid iodine until its partial pressure reaches 0.012 bar. At the temperature of the experiment, K = 3.5 x 10^-32 for 2HCl(g) + I2(s) -> 2HI...

gabbymaraziti

I believe we would omit pure solids and liquids because Q is calculated in the same way as K.

gabbymaraziti

The reactants/products don't have to be in a specific order in the calculation of K itself, but the stoichiometric coefficients should be assigned to their respective reactant/product.

gabbymaraziti

I believe it is important so that we use the correct state when calculated K (using brackets for aqueous reactants/products, P for gaseous reactants/products, and knowing not to include pure solids and liquids in the calculation).

gabbymaraziti

Kc and Kp are the same, they are just used for reactions in different states.

gabbymaraziti

Q can be taken at any point in a reaction while K is taken when the reaction has reached equilibrium. If K = Q, then the reaction was in equilibrium at the time Q was taken. K and Q can be compared to each other in order to determine if the forward or reverse reaction is favored.

gabbymaraziti

When are we supposed to use ammine, and when should we use amine?

For example, ethylenediamine (en) uses amine and has NH2 groups, but the ligand HN3 is called ammine. What is the reasoning? What about NH4+?

gabbymaraziti

Are the ligands on the test (that we are expected to know) limited to the ligand table Prof. Lavelle created (named Ligand Names in Coordination Compounds)? Or are we expected to be able to recall more?

gabbymaraziti

I was hoping these wouldn't be on the final... but I'd like to know the answer as well.

gabbymaraziti

The more electron shielding an electron has, the lower the Zeff it experiences. Because the 1s orbital has more density near the nucleus than the 1p orbital, it is said to shield the 1p orbital from the full effective charge of the nucleus, Zeff. Key misunderstanding here is that the s and p orbital...

gabbymaraziti

HCLO2 would be the stronger acid, because the greater the # of oxygen atoms attached to the central atom, the stronger the acid.

gabbymaraziti

Do we need to know how to calculate the Ka, acid dissociation constant, and Kb, the base dissociation constant? Or are these the same as the K constant itself?

Also, is Kw within our scope?

gabbymaraziti

The Lewis structure of a Lewis acid typically has a positive charge, making them capable and likely to accept an electron pair. Lewis bases typically have a lone pair available to donate.

gabbymaraziti

Ammonia is considered a Bronsted base because it loses its lone pair and accepts a proton to become NH4+.

gabbymaraziti

pH cannot be negative, which is why the formula for calculating pH is -log(H3O+).

gabbymaraziti

Prof. Lavelle did spend a significant amount of time on cisplatin, so I think we should understand that and the distinction between cis and trans.

gabbymaraziti

How are we expected to know the charge of ions quickly (SO4 2-, CO3 2-)? Are we going to be given the ligand table shared by Prof. Lavelle on the final?

If not, what is the method for accurately determining the charge of an ion?

gabbymaraziti

The Lewis structure shows you how many single/double/triple bonds there are.
Single bond = 1 sigma bond
Double bond = 1 sigma bond + 1 pi bond
Triple bond = 1 sigma bond + 2 pi bonds

gabbymaraziti

It should be written sp^3d.

gabbymaraziti

I think they're two different acid-base theories; the difference is that a Lewis theory states that an acid is an electron acceptor and a base is an electron donor, and the Bronsted theory states that an acid is a proton donor and a base is a proton acceptor.

gabbymaraziti

I think the difference is that in a covalent bond, both atoms contribute electrons to the bond, but in a coordinate covalent bond, both electrons of the bond are donated from one of the atoms.

gabbymaraziti

The ligands should be ordered alphabetically

gabbymaraziti

Elements in the p-block of the 3rd period or lower can have expanded octets. I don't think we've learned the maximum amount of electrons they can hold.

gabbymaraziti

Regardless of how many electrons are held within the bond, they all act as a single unit because they all lie in the same region.

gabbymaraziti

The constants and equations sheet is on the class website, and that's the same one given for the midterm and final. You should check it out, and if there's anything that is not on there, you should memorize it.

gabbymaraziti

Lavelle uses the class average at the end of the quarter to assign final grades.. I'd like to know an approximate of what an A would be though, percentage wise.

gabbymaraziti

You should just calculate the formal charges of all the atoms, and use all of them to determine whether or not you need to change the structure (bonds/lone pairs).

gabbymaraziti

Yes, some polar molecules can be more polar than another. This is related to the molecule's differences in electronegativity as well as their symmetry.

gabbymaraziti

Boiling points relate to dipole-dipole interactions because as the strength of these interactions increases, more energy is required to separate the molecules, leading to a proportional increase in boiling point.

gabbymaraziti

It's easier understand this if you look at it visually, by drawing out the p-orbitals for each atom. N has one electron in each p-orbital, and O has one electron in 2 of the orbitals, and two electrons in one of the orbitals. N's electron configuration is more stable because its orbitals are half-fi...

gabbymaraziti

When a central atom has empty d-orbitals close in energy to valence orbitals, it can accommodate an expanded octet. Basically, atoms in the p-block of period 3+ can accommodate an expanded octet, but the size of the central atom also matters. It has to be physically capable of forming bonds to more ...

gabbymaraziti

I'd think the homework should be from Outline 3, considering the midterm only covered through 2D, and we haven't even really covered much of 2F or 2E yet.

gabbymaraziti

When drawing our Lewis structures, is it necessary to indicate if bonds are covalent or ionic?

gabbymaraziti

The electrons are not divided! Resonance just means that there are multiple possibilities for how the electrons are distributed around the structure.

gabbymaraziti

I think we're supposed to draw the one that is the most stable, i.e. lowest formal charges for each atom (I think that's what you're referring to by 'lowest energy').

gabbymaraziti

The midterm will include outlines 1 and 2, and most of the curriculum from outline 3. It stops where the Friday lecture ended.

gabbymaraziti

Why is the d-orbital filled before the s-orbital is filled for some atoms? For example, the electron configuration for Silver is [KR] 4d^10 5s^1. Why is the electron configuration not [KR] 4d^9 5s^2?

gabbymaraziti

You should use the simplified form unless specified otherwise (i.e. if the question wants to know which orbitals are occupied).

gabbymaraziti

Cations are smaller than their parents atoms because they have less electrons. Similarly, anions are larger than their parent atoms, because they have more electrons.

gabbymaraziti

You count the number of bonded electrons, not the number of pairs.

gabbymaraziti

If you purchased the textbook bundle from UCLA, there is a resource called Sapling Learning that is similar to Prof. Lavelle's focus topics, and I believe they have practice problems as well.

gabbymaraziti

Ionization energy decreases as the size of the orbital increases. This is because as the orbital size increases, electrons become farther from the nucleus, and therefore farther from its positive charge, making them easier to remove than core electrons.

gabbymaraziti

This has to do with Hund's rule. Basically, if more than 1 orbital in a sub shell is available, you should add electrons with parallel spins to different orbitals before filling up a singular orbital. This configuration has slightly lower energy than that of a paired arrangement because it maximizes...

gabbymaraziti

Also, I don't believe the conversions are provided on the tests (all the 10^x formulas).

gabbymaraziti

The unit for wavelength is meters... but, we aren't provided the conversions to nanometers, picometers, etc. on tests, so you should memorize the conversions for these.

gabbymaraziti

Sapling Learning is a similar kind of resource. You should have access to it if you purchased the textbook bundle through UCLA, and there are some learning objective courses on there.

gabbymaraziti

Can someone please explain the significant of the Balmer and Lyman series'? I understand they are both sequences of lines corresponding to atomic transitions, but what is their use in this level of chemistry? Are the series going to be incorporated into questions, or are they provided to increase th...

gabbymaraziti

I usually begin by circling the quantities and units given in the problem, and what type of unit I need to find. Then, I just find which equation allows me work between what is given and the unknown.

gabbymaraziti

De Broglie's equation is reserved for objects that mass, like electrons, cars, or baseballs. Since light is, by definition, mass-less, De Broglie's equation does not apply. The wavelength of light can be determined given the speed and frequency of the light wave, as wavelength = c/v.

gabbymaraziti

Wavelength depends on the speed that the object is moving at. The wavelength of any given electron is not constant.

Wavelength can be determined using the De Broglie equation: h (Planck's constant) / (mass)(velocity) of the given object.

gabbymaraziti

Anyone have any tips for distinguishing between formulas designated for electrons vs those related to light? I'm having trouble keeping all their meanings and uses straight.

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