CHEMISTRY COMMUNITY

whitney_2C

Please post the entire problem. Question: The rate constant of the reaction between CO2 and H2O in aqueous solution to give the HCO3- ion is 1.5 x 10^10 L mol^-1 s^-1 at 25 degrees Celsius. Determine the rate constant at human body temperature (37 degrees Celsius), given that the activation energy ...

whitney_2C

Does anyone know why the solutions manual subtracted .59 from the Arrhenius equation expression?

whitney_2C

I also have test anxiety, it's not fun. Usually just forcing myself to do lots of practice problems and having a routine (eating breakfast, sitting in the front, having a certain pen/calculator, etc whatever works for you) helps give me some comfort so I can relax some at least. Good luck and know y...

whitney_2C

The Ecell being positive means that the reaction is favorable (it's spontaneous). Think of it more as energy the cell gives off (that could be used to do work) rather than the overall charge.

whitney_2C

I noticed the same thing. The equation is another form of the Arrhenius equation where the natural log is taken and the reverse reaction is taken into account I believe.

whitney_2C

The natural log of 1/something is the same as the natural log of that something times negative one. This is because of the log properties ln(1) = 0 and ln(A/B) = ln(A) - ln(B).

whitney_2C

I believe that Lavelle said that we can assume microscopic reversibility for the purposes of our class, however, I am not 100% positive.

whitney_2C

The rate constant is a coefficient of proportionality relating the rate of a chemical reaction at a given temperature to the concentration of reactants.

whitney_2C

k does change with temperature as it is temperature dependent like K and yes k is also different for different reactions.

whitney_2C

some jokes for you

whitney_2C

The order of a reactant tells us to what degree a reactant's concentration affects the rate of reaction. We determine it by using the formula below and a set of experimental data.

whitney_2C

Here's a handy table for the units of k that I like to refer to. This one is in dm^3 but this is the same as liters (L).

whitney_2C

Inert conductors are nonreactive solid metals that allow for electron transfer in the reaction between the oxidizing and reducing agents.

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Oxygen is usually assigned an oxidation state of ‒2 in its covalent compounds, such as CO, CO2, SO2, and SO3. Exceptions to this rule include peroxides (compounds containing the O2-2 group), where each oxygen is assigned an oxidation state of ‒1, as in hydrogen peroxide (H2O2), and OF2 in which oxyg...

whitney_2C

Here some steps for balancing redox reactions that I found helpful 1. Divide the equation into oxidation and reduction half reactions. 2. Balance all elements besides hydrogen and oxygen. 3. Balance O by adding H2O to the appropriate side of each equation. 4. Balance H by adding H+. 5. Balance the c...

whitney_2C

We aren't assuming that the Gibbs Free Energy is equal to 0, rather we are putting in 0 for its value to see when T\Delta S and \Delta H are equal to one another in order to determine at what temperatures the Gibbs Free Energy would be negative (spontaneous reaction) or positive (non spontaneous rea...

whitney_2C

Yes, standard conditions assume 298.15 K or 25 degrees Celsius, which one you use in your calculations just depends on the kind that you are doing.

whitney_2C

You know to use C_v or C_p depending on what the givens are in the question. If the system is at constant pressure then use C_p and if it is at constant volume then you use C_v.

whitney_2C

you can solve for $\Delta G$ using $\sum \Delta G(products) - \sum \Delta G(reactants)$ because Gibbs Free Energy is defined by the state functions enthalpy and entropy, and it too is a state function.

whitney_2C

Understanding it would definitely make problem solving easier so I would try to as much as you can. I took some notes in my discussion with the graph labeled, a picture of it is shown below. Hope it helps!

whitney_2C

K B is the Boltzmann constant and it provides a measure of the amount of energy (i.e., heat) corresponding to the random thermal motions of the particles making up a substance. The link below sends you to a video if you want to learn more about it. https://www.khanacademy.org/science/in-in-class11th...

whitney_2C

If you're dealing with bond enthalpies then you use the equation: $\Delta Hrxn = \sum H (bonds broken)-\sum H(bonds formed)$

whitney_2C

The equations he will give us on the midterm can be found on his website or on your first test on the front page. Here's a link to the constants and equation sheet! https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14B/Constants_Equations.pdf You can find this on the left hand side of hi...

whitney_2C

We can also assume it is at 1 atm when the question says that the reaction is occurring under standard conditions.

whitney_2C

Yeah I also found it slightly confusing but my TA mentioned that we were doing the middle part of 4 first then moving to the beginning. We have covered 4C, 4D, 4E, and on Friday I believe we finished 4A so we are moving into 4B. After that we're going on to end of Topic 4 in the book for the thermod...

whitney_2C

Also the \ln (\frac{V2}{V1}) part comes from the properties of logarithms because when you take a definite integral its F(b) - F(a) so for \int_{V1}^{V2} (\frac{1}{V})dV you get \ln (V2) - \ln (V1) which by log properties is the same as \ln(\frac{V2}{V1}) , th...

whitney_2C

In my notes I have - reaction at constant P: that involve changes in the number of moles of gas then P\Delta V is significant - reaction at constant P: that involve solids and liquids, then P\Delta V is insignificant so you're right, it's not significant when solids and liquids are involved because ...

whitney_2C

Specific heat capacity is the amount of heat required to raise 1 gram of a substance by one degree (i.e. 1 degree Kelvin). It is a state property because it is a property independent of how the substance was prepared (in this case heated).

whitney_2C

Here's a link to an example of a pH calculation where you do have to include Ka2, might help you out more than my explanation. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Acids_and_Bases/Monoprotic...

whitney_2C

If it is not justified then you would have to do another ICE table, calculate the equilibrium concentration of [H3O+] for the second deprotonation, and then to get the pH you add the [H3O+]s and take the -log of the over all concentration.

whitney_2C

You can tell that this reaction is endothermic because in order for a diatomic halogen to become two atoms the bond between the atoms needs to be broken and this requires energy, so you can predict that equilibrium will shift toward the products (this is because the extra energy that the heat gives ...

whitney_2C

We never really change X to zero because if X is 0 then that means that some of the concentrations for reactant/products at equilibrium are zero which doesn't make sense, but we do neglect the X if it is added or subtracted from a concentration in our K expression if the K value is less than 10^-3.

whitney_2C

Q is different from Kc because Kc indicates the ratio of products to reactants at equilibrium while Q indicates the ratio of products to reactants at any time during the reaction (such as when it has not yet reached equilibrium) we can then use Q to figure out which direction a reaction is sitting t...

whitney_2C

yes, just be careful because some problems will give concentrations/values for product and reactant and in those cases you have to solve for Q to find out which direction the reaction is favoring before finishing the ice table to solve the actual problem.

whitney_2C

I definitely think that memorizing those rules is the best way to go because it's hard to know which reaction is being favored without them if you are given concentrations/partial pressures for both products and reactants in a problem. The way that I remember them is actually the opposite of how Lav...

whitney_2C

I definitely understand, I also found his websites to be slightly confusing to navigate at first. Trust me though you'll get the hang of it after the two weeks or so. As for outside of lecture prep, I personally found doing all the homework problems he assigns, going to Lyndon's review sessions (he'...

whitney_2C

Two sig figs is actually right because the value 0.031 only has two sig figs since neither of the zeros are significant. If the number were .0310 then it would have three sig figs though because 0s that come after non-zeros in decimals are significant.

whitney_2C

In general how can you tell if determine if a reaction is exothermic or endothermic? You would have to calculate the enthalpy or \Delta H of the reaction to be able to know if a reaction is endothermic or exothermic. If the enthalpy is negative then it is exothermic and if it is positive then it is...

whitney_2C

My TA said we can use either and both naming conventions are on the ligand names in coordination compounds sheet on Lavelle's website! I guess you could consider "cyanido-kN" as "more" correct since it's the new form that IUPAC decided on, but for the final we can use whichever w...

whitney_2C

Kw is the equilibrium constant for water. The constant varies depending on temperature but I don't think we'll ever be asked to calculate it because it's a constant that we use to solve problems. It's given on our formula sheet!

whitney_2C

I think memorizing them and their charges is a good idea because the names/formulas and charges aren't given in the homework problems so I think questions on the final about coordination compounds are going to be formatted similarly

whitney_2C

A molecule with a double bond will not always have a higher bowling point then one with more single bonds. This is because the physical properties like boiling and melting points are determined by intermolecular forces not intramolecular forces. Intermolecular forces are bonding forces between molec...

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A metal could be outside the brackets if it is bound to a coordination compound but the metal with the ligands attached to it that makes up the coordination compound is always inside the brackets.

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Being polydentate is different than a coordination compound's coordination number. If something has a coordination number of two then this means the metal center has 2 ligands attached to it, which doesn't necessarily mean that the ligands themselves are bidentate, tridentate, etc. the same goes for...

whitney_2C

I was wondering the same thing but Patricia Poths talked about this in her review session on Monday. She said they are the same it is just that IUPAC changed the way that we name some of the ligands recently to ending in ido rather than o. She also said that we can use either on the final and not lo...

whitney_2C

K is an equilibrium constant which is a number that expresses the relationship between the amounts of products and reactants present at equilibrium in a reversible chemical reaction at a given temperature. K a values are equilibrium constants that correspond to acids in their reactions with water an...

whitney_2C

You can determine which is the acid and which is the base from the Bronsted-Lowry definition of a base an acid. According to Bronsted-Lowry, an acid is a species that donates a proton (H+) and since HI has a H in it, it is the only one of those species that can donate an H+ via breaking the H-I bond...

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Remember that a salt is a metal bonded to a non-metal and so typically you can figure out what salt is made by combining the metal in the base (like K in KOH) with the nonmetal in the acid (like Cl in HCl). In this example, the salt in the products would be KCl and the OH- and H+ left over combine t...

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A ligand is a species that has at least one set of unpaired electrons that it uses to form a coordinate covalent bond with a metal atom. Monodentate means that it has one set of unpaired electrons that it donates to create the bond whereas bidentate means that it has two sets of unpaired electrons t...

whitney_2C

There are lots of possible ligands out there so I doubt he expects us to memorize the names for all of them, but it would be helpful for you to know the common ones like amine, chloride (and other halides), aqua, hydroxide, and cyanide.

whitney_2C

Ammonia changes to amine when ammonia bonds. It's similar to how non metals have different names when bonded to cations (NaCl is sodium chloride rather than sodium chlorine). We're going to learn more about naming in Wednesday lecture so that should help! We'll also learn more about ligand naming in...

whitney_2C

The worked out answers are on the class website, it's on the right side underneath exam information!

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The bond angles in trigonal bipyramidal are 180 degrees, 90 degrees, and 120 degrees. The 180 degrees refers to the bond angle between the axial atoms, the 90 degrees refers to the bond angles between an axial atom and an equatorial atom, and the bond angle 120 degrees refers to that between the equ...

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https://phet.colorado.edu/en/simulation/molecule-shapes

whitney_2C

If you click on this link you can download a software that shows molecules in 3D and you can move them around so you have a better idea of the shapes and angles! If that doesn't help you with remembering molecule bond angles for different shapes then I would just memorize them.

whitney_2C

Yep you're right the bond angle would change because the new electrons being introduced to the molecular structure would interest and repel the electrons already there.

whitney_2C

I'm pretty sure don't have to calculate them!

whitney_2C

I believe the maximum number of electrons that would be present if n=2 would be 8 electrons because when n=2, l could be 0 or 1 (s or p), therefore there is an s orbital and a p orbital in the 2nd shell and s can hold 2 electrons and p can hold 6 so 2 + 6 = 8. This would mean that the maximum number...

whitney_2C

O^- would still be smaller because S^2- has a third shell which is significantly larger than O's 2nd shell.

whitney_2C

I believe that the exam will specify when to draw resonance structures. When it does ask you to draw resonance structures, it wants you to draw out the different possibilities for electron/atom arrangement where formal charges are closest to zero. This is kind of hard to put in words though so I att...

whitney_2C

^^ the first image is an example of a pi bond and the second shows some examples of possible sigma bonds

whitney_2C

I think that Lavelle is going to go over this soon in class so maybe he can make it more clear, but basically a sigma bond is a single bond and pi bonds are any additional bonds between the same two atoms following the single bond. For example a double bond between two carbons would have 1 sigma bon...

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Ionic bonds are bonds in which one atom (the nonmetal) takes an electron from another atom (the metal) so that both have an octet whereas covalent bonds are where nonmetals share electrons to get to their octets. https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&a...

whitney_2C

The elements in the s-block are significantly more reactive than those in the p-block and this is because of their very low ionization energies whereas the p-block elements do have high electron affinities but still are not comparable to s-block's ionization energies. To see how reactive group 1 met...

whitney_2C

Electrons get pulled in when there is a higher positive charge (there are more protons) which is why they get smaller. This explains why cations are smaller than anions. Cations have lost some amount of electrons but still have the same number of protons as the neutral atom (this is why it's still t...

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Atomic radius decreases across a period because as you go across a period the effective nuclear charge (# protons) increases bringing the electrons closer to the nucleus. While more electrons are also being added, the radius still decreases because additional electrons added across a period are in t...

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A nodal plane is a part of a wave function where the probability of finding an electron is zero. You can see that p and d orbitals have nodal planes by looking at the shapes of those wave functions and where they narrow. The number of nodal planes is equal to angular momentum quantum number (l).

whitney_2C

The presence of a dipole moment is dependent on electronegativity and the structure of a compound. Personally I like to determine if a dipole moment occurs by drawing the lewis structure of a compound then looking at electronegativity values of the elements in it and comparing them. Lots of times yo...

whitney_2C

All of the equations we have gone over in lecture will be given. Here's a link to the equations and constants we're given! https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14A/constants_equations.pdf Make sure you know c = \lambda \nu , E = h\nu , \lambda = \frac{h}{mv} , and En = \frac...

whitney_2C

Electrons aren't always ejected. They are only ejected if the light hitting the metal has a sufficient amount of energy (it's wavelength has to be small enough i.e. it has to have high frequency). A really important thing to remember is that the intensity (amplitude) of the light wave does not have ...

whitney_2C

You'll also find that after doing chemistry for a while you'll just naturally memorize the molar masses of elements and compounds. After doing IB chemistry I had the molar masses of carbon, nitrogen, water, hydrogen, sodium hydroxide, and many more memorized without having to even study them because...

whitney_2C

You can choose any 7 you like! It would make the most sense to do quantum mechanics questions though since that is what we are working on right now. Don't worry though you don't have to do questions on things we haven't learned yet.

whitney_2C

Yes, there are a variety of sig fig rules for calculations based off of division, addition, subtraction, and even for when you're taking logarithms (I think we'll do that in our acid base unit). If you're answering a question, your sig figs should be based off of your least precise measurement, that...

whitney_2C

The energy itself is not negative. The negative sign just tells you that that is lost rather than gained. It's kind of like velocity being directional. Negative velocity doesn't mean that the item moving is moving negatively it just indicates what direction it is moving in. Hopefully that helps :)

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