CHEMISTRY COMMUNITY

KarineKim2L

Thank you for a great 2 quarters Dr. Lavelle! I really enjoyed your humor and passion for chemistry! Your office hours were always extremely helpful, and I feel that I have learned so much in these past several months. I hope you get more sleep after we take the final tomorrow!

KarineKim2L

What is the activated complex model and what does it tell us?

KarineKim2L

When the temperature is increased, the molecules move faster, and therefore collide more often, causing the rate of the reaction to increase.

KarineKim2L

Why doesn't increasing the mass of the cathode change the Ecell? What does it mean to increase the mass of the cathode?

KarineKim2L

Where does the equation H=U+PV come from? Was there a derivation?

KarineKim2L

Kinetics is used to understand the rate of a reaction, while thermodynamics predicts whether the reaction will occur or not occur, and how favorable the reaction is. Thermodynamics, however, gives no indication of the speed of a reaction.

KarineKim2L

The one with the more positive or higher potential will be reduced, and the one with the more negative/smaller potential will be oxidized.

KarineKim2L

One way is to graph the reaction and see which equation results in a linear graph. A linear graph that plots [R] to time is zero order, ln[R] to time is first order, and 1/[R] to time is second order.

KarineKim2L

Why does the Work max equal delta G and are there any exceptions to this relationship?

KarineKim2L

For cell diagrams such as the one in problem 6L.3d, how do we know what our half reactions are?

KarineKim2L

The graph should be linear if the reaction is first order.

KarineKim2L

How does gibbs free energy relate to electrochemistry? What happens to the cell potential and the movement of electrons depending on the concentration of reactants and products?

KarineKim2L

The electrons move from the anode to the cathode, so the anode contains the oxidation half reaction and the cathode contains the reduction half reaction.

KarineKim2L

Test 2 covers the part of thermodynamics that wasn't on the midterm and all of the Electrochemistry outline.

KarineKim2L

Thermodynamic analysis is more useful for telling whether a reaction is favorable or not, by if you have a negative delta G. However, thermodynamics does not tell you the speed of a reaction, and this is where kinetics can be useful.

KarineKim2L

Yes. Whatever has an oxidation number that becomes lower in the products is reduced, and whatever has an oxidation number that increases in the products is oxidized.

KarineKim2L

They would have to give the state to you in the equation or tell you what state it is in the context of the problem.

KarineKim2L

We would need to know the common oxidation states of certain elements, for example O is usually -2 and H is usually +1. Using these and the total charge on the molecule, we can get the oxidation numbers of the transition metals. For example, in MnO4-, the oxidation number of Mn is +7, because the ox...

KarineKim2L

In addition to all of the above, the relationship between G not and G can be seen in the equation G not= G+RTlnQ.

KarineKim2L

We use platinum when we do not have a conducting solid in the reactants or products, such as when both the reactants and products are in solution (aq) instead of solid.

KarineKim2L

One half reaction is the oxidation reaction, and one half reaction is the reduction reaction. Whatever is losing electrons would be in the oxidation reaction, and whatever is gaining electrons would be in the reduction reaction.

KarineKim2L

No, because in O2, the oxidation number of O is 0. Similarly, in H2 the oxidation number of H is 0.

KarineKim2L

How can we assume that the standard delta H and standard delta S are constant when deriving the Van't Hoff equation? Do these values not differ with temperature?

KarineKim2L

For the example we did in class with MnO4 and Fe, how do we know that the hydrogens and the oxygens that formed water maintained the same oxidation numbers?

KarineKim2L

In what situations is delta H=0? Also, in what situations is delta U=0? Is delta U always 0 in isothermal reactions?

KarineKim2L

Will the questions always specify reversible or irreversible, and if it does not how do we know which equation to use?

KarineKim2L

Even at 0K, most molecules still have a degeneracy and therefore have residual entropy. If there is more than one way a molecule could be oriented or positioned, there will still be entropy at 0K.

KarineKim2L

Basically yes; Dr. Lavelle said we need to know everything on the first page of the outline for Thermodynamics.

KarineKim2L

I believe the two forms of that equation are q=mCAT and q=nCAT? M is for mass and n is for moles, so when given moles, you would use n along with Cn and when given mass, you would use mass along with Cm.

KarineKim2L

How do we know when to do the steps 1,2,3 for heating, phase change, and cooling such as question 12B on pizza rolls?

KarineKim2L

P stands for pressure, V for volume, n for the moles of gas, R is a constant, T is the temperature.

KarineKim2L

Q and K are both calculated by doing products over reactants. Therefore, If Q is greater than K, this means there are more products than there should be at equilibrium, so the reactant shifts left, to form more reactants. If Q is less than K, this means that there are more reactants than there would...

KarineKim2L

Internal energy is the energy contained within a system. Typically we are asked to find the change in U, internal energy, which we can do by using the equation Change in U= Delta H - Pressure Delta Volume.

KarineKim2L

When something is a state function, the value of it does not depend on the pathway of the reaction. For example, since enthalpy is a state function, we do not have to worry about the intermediates of the reaction to find the change in enthalpy, but can simply find the change in enthalpy by viewing t...

KarineKim2L

By stating that the pathway of a reaction does not matter for the total change in enthalpy, the Hess's law allows us to add the separate changes in enthalpies of separate reactions to get the total change in enthalpy of the sum reaction.

KarineKim2L

If X is less than 5% of the initial concentration, then the equilibrium concentration will not differ very much from the final even if you subtracted X. (Initial - X) would be close enough to just using the Initial concentration for the equilibrium concentration and therefore we can basically ignore...

KarineKim2L

I think you can pick them up at Young Hall from 9AM to 5PM every day next week. They have all of the tests there.

KarineKim2L

We can memorize the common strong acids and bases from the table in the textbook. Also, if the Ka is small, the acid is weak.

KarineKim2L

If you decrease the volume, pressure will increase. You can think of this logically; if you have a certain amount of material that takes up a certain amount of space, and you try to squish it into a smaller space, the pressure will increase.

KarineKim2L

If H2O is a gas, we include it in the ice table. If it is a liquid, we do not include it in the ice table.

KarineKim2L

The 5% rule applies to when you use the ICE table. After you have your K in the form of Xs, if K is less than 10^-3, then you can assume that X is so small that it will not make a difference. So for example, if you have K=X^2/(0.15 - X), then you can change it to X^2/0.15, as the X in the denominato...

KarineKim2L

Yes, some of the examples we did in class were on weak bases, and whenever x is less than 5% of the initial concentration, the approximation is valid.

KarineKim2L

Percent protonation is the concentration of the conjugate base or acid over the initial concentration of the acid or base.

KarineKim2L

If given the initial concentration and the pH at equilibrium, how do we find the Ka of the acid?

KarineKim2L

The short way to think about it is that if pressure is increased, the reaction shifts to the side that has less moles. However, Dr. Lavelle reviewed how change in pressure alters the Q in class, which provides the correct justification for the direction the reaction shifts.

KarineKim2L

Why is it that increasing the partial pressure by adding an inert gas does not affect the moles of reactant, volume, and product?

KarineKim2L

When Q=K, the reaction is at equilibrium, so the forward reaction is occurring at the same rate as the backward reaction.

KarineKim2L

According to the equation PV=nRT, pressure is directly proportional to concentration. I was wondering the same thing for when to use partial pressure and when to use concentration, but I believe usually it will be specified by the information given.

KarineKim2L

When asked to "Write the reaction quotient Q" and given several chemical equations, does it matter whether we use partial pressure or concentration as it is not specified? For example, on HW problem 5G.11, it does not specify, but some of the answers on the answer manual are in pressure an...

KarineKim2L

Why are solids and liquids not included in the calculation of Q and K?

KarineKim2L

As Dr. Lavelle mentioned, in order for a molecule to have a stable bidentate, it typically has to have a lone pair then two spacer atoms then a lone pair. H2O does not and therefore it is not large enough to wrap around and bond in two places.

KarineKim2L

I believe we only have to do calculations on pH of strong acids and bases and not on equilibrium of weak acids bases. However we do need to know how to calculate Ka and Kb.

KarineKim2L

NO3- participates in the reaction, but it does not change the acidity of the solution, and therefore does not affect the pH, so we can disregard it when considering pH levels. This is because NO3- is the conjugate base of HNO3, which is a strong acid and the conjugate bases of strong acids do not af...

KarineKim2L

How would we know how many valence electrons a transition metal has when we're drawing our lewis structures? For example, how many valence electrons would Hg have?

KarineKim2L

How can you tell that As2O3 is an amphoteric compound? Which atoms make it an amphoteric compound?

KarineKim2L

A ligand can bind at each lone pair. However, molecular shape must also be taken into account to see if the lone pairs are in a location that can be binded to another molecule. For example, Cl- has 4 lone pairs, but it cannot be a polydentate. It can only be a monodentate because its bond angles are...

KarineKim2L

If the resulting anion is stable, the molecule will more likely lose its proton. Molecules that have unstable anions will not want to release their proton; it will soon react again to regain the proton because it does not want to be an anion.

KarineKim2L

A bronsted acid is a proton donor, while a bronsted base is a proton acceptor.

KarineKim2L

An acid is strong because its proton can easily be donated. For example, HI is a strong acid because the bond between H and I is weak, and therefore the hydrogen proton can be easily removed, forming H3O+ when in water.

KarineKim2L

The weaker the bond between two atoms, the stronger the acid is. Therefore, HF is a weak acid, while HCl, Hbr, and HI are strong acids. HI is the strongest because the bond between H and I is the weakest.

KarineKim2L

The ligands have a set charge, as do the ions. So for example, for [Co(NH3)5Cl]Cl (There are 5 NH3s), then you know that the oxidation number of Cobalt is 2+. NH3 is always neutral, and the total charge for the coordination compound should be +1 because the charge of Cl is -1. +2 of Cobalt minus 1 f...

KarineKim2L

I believe that works. The number of electron dense regions should match up with how many total orbitals you have. For example, tetrahedral electron geometries will always be sp^3 hybridization.

KarineKim2L

Lewis acids are defined as the electron pair acceptor. Bronsted acids are defined as the proton donor. The Bronsted definition is only used for acids in which the hydrogen ion can transfer between molecules. HCl is a bronsted acid when reacting with water. Since HCl also accepts the electron pair, i...

KarineKim2L

Cisplatin works as a chemotherapy drug while trans-diamine-dichloro-platinum (II) does not because cisplatin is polar and therefore more reactive, and binds to the exposed Nitrogens on Guanine that result from the duplication of DNA. Cisplatin needs to bind to Guanine in 2 places for the bond to be ...

KarineKim2L

The number of orbitals in the hybridization should be the same as the number of electron densities around the central atom. Therefore, molecules with tetrahedral electron geometry will always be sp^3, trigonal planar will be sp^2, linear will be sp, trigonal bipyramidal will be sp^3d, and octahedral...

KarineKim2L

As the person above said, the more electronegative atom will have a partial negative, while the less electronegative atom will have a partial positive. Also, the dipole moment will be greater when the electronegativity difference is greater between the two atoms.

KarineKim2L

One trend that will likely be covered because we learned it after the midterm is that as the molecule gets larger, the london forces tend to get stronger because the molecule becomes more polarizable. This will relate to boiling points. For example, NaCl will have a higher boiling point than HCL bec...

KarineKim2L

Although trigonal planar and trigonal pyramid both result when 3 atoms are attached to the central atom, trigonal pyramid requires a lone pair on the central atom, while trigonal planar does not. In other words, trigonal planar has 3 regions of electron density, while trigonal pyramid has 4.

KarineKim2L

Hydrogen bonds are extremely powerful. This is because an extremely electronegative atom, such as O, pulls the electron on the Hydrogen atom towards them, leaving the H side partially positive. This partial positive H can react with the partial negative lone pairs of the O of another molecule.

KarineKim2L

I believe we just need to know which forces are stronger than others, and when certain intermolecular forces are present.

KarineKim2L

Lewis structures with resonance structures will have bond lengths that average the lengths of all of the bonds in the structure, as a resonance structure is the blended average of all different structures. However, if there is only one structure, then single bonds (denoted with a single line) are lo...

KarineKim2L

Yes, you can still draw dipole moments between the bonds of atoms if the bond is polar. In some cases, the polarity of the bonds in the molecule balance or cancel, making the molecule nonpolar.

KarineKim2L

This means that the intermolecular bonds are stronger, and thus the liquid is "thicker". It will not flow as well.

KarineKim2L

I believe that formal charges can tell you which resonance is most stable, and whether a resonance structure is energetically favorable or likely to occur, but it does not directly tell you whether there are resonance structures (which there usually are).

KarineKim2L

A dipole moment is the difference in charge between the two atoms involved in the bond. The greater the difference in electronegativity, the greater the dipole moment.

KarineKim2L

I believe the least electronegative element goes in the middle.

KarineKim2L

I believe they reference the same idea, but electron affinity is a measured value of the amount of energy released when an electron is added to an atom, while electronegativity is a chemical property calculated using electron affinity.

KarineKim2L

Generally, the smaller the atomic radius, the higher the electronegativity. So as you go right on the periodic table, atomic radius decreases and electronegativity increases. As you go down, atomic radius increases and electronegativity decreases. This is because the nucleus has a stronger hold on t...

KarineKim2L

When removing electrons from an atom to form an ion, you remove from the 4s orbital first. Thus, the 2+ ion electron configuration for Co^2+ and Fe^2+ would be a) and b) respectively, as the electrons in the 4s orbital are removed.

KarineKim2L

Electron affinity is how much that element wants to gain an electron. The trend for it on the periodic table is that as you move to the right, electron affinity increases, as the positive charge of the nucleus increases. However, the noble gases have very low electron affinity because they are alrea...

KarineKim2L

You would write the 3d10 first, because we always write electron configuration in ascending n order. Also, after there are electrons in the 3d orbital, its energy becomes less than the 4s orbital.

KarineKim2L

Molecules that have resonance structures are the blended average of the different resonance structures. So, when a molecule has resonance structures, I believe its electrons are not contained between two atoms, but move in a larger area, and are thus delocalized.

KarineKim2L

Atoms that have d orbitals can have expanded octets.

KarineKim2L

The S value is the number of shared electrons the atom in question has. For P in PCl5, there are 10 shared electrons, 2 in each bond symbolized by the single lines. Each Cl has 2 shared electrons, because there is only one single bond connected to each Cl atom.

KarineKim2L

l=0 (s) has 1 orbital, l=1 (p) has 3 orbitals, l=2 (d) has 5 orbitals, and l=3 (f) has 7 orbitals regardless of the value of n.

KarineKim2L

You can use the equation wavelength=h/p. p=momentum, which is mass x velocity. Plug in planck's constant, mass of the respective particles and velocity, and you can calculate wavelength.

KarineKim2L

I also learned it this way, and my TA mentioned that he also learned to write 3d before 4s. I believe it is just a difference in method in how to write the configuration. Writing the configuration in ascending n order makes it clearer when writing electron configurations for ions, as you just have t...

KarineKim2L

The element in the brackets [ ] is the noble gas from the group before the element you are creating the electron configuration for. This is shorthand because writing this noble gas(far right of periodic table) signifies the electron configuration up to that element. Then, you just have to write out ...

KarineKim2L

To do electron configuration, you look at the periodic table. The first row corresponds with the n=1 level, and the second row corresponds with the n=2 level. Basically, in each row, the 2 elements on the far left are the s1 and s2, while the 6 elements on the far right are the p1,p2,p3,p4,p5,p6. So...

KarineKim2L

For all electron configurations, I believe Dr. Lavelle wants us to write 3d before 4s regardless of which one is the higher energy level. I believe we simply write it in order of ascending n.

KarineKim2L

No I don't think so, they both apply to Hydrogen and refer to different regions, n=1 and n=2, just like you said.

KarineKim2L

In lecture, Dr. Lavelle said that a certain equation only works for H since that was the element used for the experiment. Which equation was that, and when he said that certain energy level jumps were certain groups of spectral lines, does that only apply when the element is H?

KarineKim2L

You could leave it in moles or convert to grams, either one, because you're trying to find the limiting reactant, so how much product you have will not be in your final answer. However, you need to be able to compare the different amounts of product that would theoretically be created using each amo...

KarineKim2L

Can someone explain the difference between constructive and destructive inference? I understood that constructive inference is waves in phase and destructive inference is waves out of phase, but I don't really understand what that means.

KarineKim2L

For multi-stage problems, are sig figs only graded for the final solution? Is it best to keep lots of figures for all of the steps and only round the final answer?

KarineKim2L

As the person above said, O is not a product of the reaction. Having 2 moles of CuO is very different from having an extra O atom. Also, typically in reactions, single O atoms are not produced.

KarineKim2L

When multiplying or dividing, you pay attention to the number with the least amount of sig figs, but when you do addition or subtraction, you pay attention to the number with the least values past the decimal point.

KarineKim2L

No, you do not need to convert the masses of each into percentages when masses are given, unless it asks for mass percent.

KarineKim2L

Because of the conservation of mass, you know that the mass on the reactants side has to equal the mass on the products side. So you add the molar masses of HA and XOH and subtract the molar mass of H2O to get the molar mass of XA.

KarineKim2L

First, you would see how many moles of each element is in the compound. In this case, 7 moles C, 15 moles H, 1 moles N, and 3 moles O. Multiply each by their respective molar masses. This gives you how many grams of that element is in this compound. Finally, divide each mass by the total mass of the...

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