CHEMISTRY COMMUNITY

DLee_1L

Yeah, so A should be a decimal with 1 indicating that every collision results in a correct orientation and 0 indicating every time there is a collision, it is not in the correct orientation

DLee_1L

Yeah, we haven't gone over how catalyst's act on the system mathematically, unless the problem tells us specifically that the catalyst lowers the Ea by x amount.

DLee_1L

The Boltzmann Distribution shows the distribution of the energy of particles based on their temperature. Similarly, the Arrhenius equation gives the rate of a reaction in terms of its activation energy and temperature. Both show the relationship between activation energy/kinetic energy and temperat...

DLee_1L

Yeah, if you think about it from a different perspective, a higher activation energy means that there is a higher barrier to overcome to complete that step of the reaction, therefore it happens less frequently and thus the whole reaction gets bottlenecked at that one step.

DLee_1L

k2K1 is equal to the overall k of the reaction because the overall k is unknown, but since the step 2 is suppose to be the limiting step (because the concentrations match up), by definition, k of overall is equal to the rate constant of the second reaction multiplied by the equilibrium constant of t...

DLee_1L

Not too sure about the first question, I think its because you are just trying to get the HOCl concentration for the actual rate law of the entire process so having an intermediate to get that value doesn't matter too much. For the second question, H20 is not included because its a liquid and theref...

DLee_1L

Yeah. Isn't it just the Arrenhius equation with the collision frequency being A?

DLee_1L

I believe you would have to calculate your own rate constant units so that the reaction rate is equal to mol/L*s. So if you had a reaction order of 1.7, Your units for the rate constant would be moles^2.7/s*L^2.7 to change out the moles^1.7/L^1.7 units of the molarities to obtain a mole/L*s units fo...

DLee_1L

A is usually a constant that refers to the frequency of correct orientation collisions there are. However, if you have the equilibrium constant, activation energy, and temperature, you will be able to calculate the A via the Arrhenius equation.

DLee_1L

You should use seconds since the reaction rate is usually in moles/L*s and the reaction constants are derived using those units in mind (e.g. 1st order rate constant units are in 1/s since 1/s multiplied by concentration to the first power (moles/L) equals moles/L*s)

DLee_1L

Confirmed, Professor Lavelle did not go over the derivations for Arrhenius, however, he did go over the derivations for 0,1,and 2nd order reactions and their half-life reactions. I don't think they'll be essential to know, but a good foundation to how things work if there is a concept heavy question...

DLee_1L

Yeah, you cannot find a half life of a substance if its concentration does not affect the rate law to begin with. That would be the equivalent of taking half of 0.

DLee_1L

Just to clarify, when calculating Q and K, we combine the concentration and partial pressures? or is there some conversion we have to do?

DLee_1L

I understand that the NO3-s are not involved in the reaction because the charges of N and O don't change so that leaves the two Ni2+2e- -> Ni equations. In the second compartment the OH- reacts with the Ni to produce Ni(OH)2. So I believe its because the Ni2+ + 2OH- -> Ni(OH)2 takes away Ni2+ in the...

DLee_1L

In addition, Graphite is sometimes used as an inert conductor, so that might pop up once in a while.

DLee_1L

Ohhh so in order to get n, you have to balance the half reactions and then subtract the electrons? You don't have to subtract the electrons, all you have to do is make sure the number of electrons given off and received by both half reactions are the same and then that is your n moles of electrons....

DLee_1L

The answer key does reverse the anode since the equation Ecell=Ecathode-Eanode accounts for flipping the sign of Eanode.

DLee_1L

For this question, why is Au in both half reactions and why are Au and Au3+ on different sides of the anode half reaction if in the original equation has them on the same side

DLee_1L

So does that mean that the negative ions from the salt bridge go to the anode and the positive ions from the salt bridge go to the cathode to keep the charges the same?

DLee_1L

Can the salt bridge eventually "run out" or stop dissolving due to the cathode and anode solution being too saturated with the the salt an electron buildup can occur and the cell stop working?

DLee_1L

Yeah, I think that's right, I remember for alkaline solutions, you do the same things as if it was acidic, but at the end, you add equal amounts of OH- to both sides to cancel out all the H+ by combining the H+ with OH- to create H20.

DLee_1L

For the first question, I believe you identify how many electrons are gained and lost by the least common multiple of electrons in the two half reactions (in this problem 6 electrons). Since the least common multiple of electrons is 6, you would have to multiply the oxidation half reaction by 3 to m...

DLee_1L

Yeah, you can't really tell if a reaction is exo or endothermic just by looking at the equilibrium constant. Enthalpy is the best way to see if a reaction is exothermic or endothermic because it is the measurement of how much heat is transferred. If enthalpy is negative, heat is leaving the system t...

DLee_1L

Yeah, so for reactions that take place in an acidic environment, you will probably end up with an equation that has H+ ions in it and for reactions that take place in a basic environment, you will probably end up with an equation that has OH- ions in it.

DLee_1L

Are you referring to the number of electrons gained/lost by the opposite half-reaction? In that case, we multiply each half reaction by the amount of electrons gained/lost by the opposite half reaction because then it becomes easy to add the two half-reactions because the amount of electrons gained/...

DLee_1L

For basic solutions, I remember something about adding OH- to balance basic reactions instead of hydrogen and needed H2O to if there is an uneven amount of Oxygen atoms.

DLee_1L

Based on what we did in class, I would say that a half reaction is split into an oxidation reaction and a reduction reaction. If you combine both reactions you should end up with the original reaction. From the example in class, is there a reason why the oxidation reaction only had the Fe giving of...

DLee_1L

To use the Van Hoff's equation, are there certain values that need to be assumed to be constant like pressure? Or does it not matter as long as we know the delta h, delta s, and temperature before and after?

DLee_1L

Yes, I believe that if the system is under both constant V and P (however, I think there will only be questions with one or the other) the change in internal energy will equal the change in enthalpy. And since P and V don't change, there is no expansion. However, there are cases when P and V are con...

DLee_1L

So does that mean that always look at the complexity (or number of atoms in a molecule) first and then go by the mass (heavier means more enthalpy) if they have the same amount of atoms and orientations?

DLee_1L

Reversible processes can be "turned" in any direction at the very slight perturbation of the system. The common one we went over in class was the slow isothermic expansion of gas in a piston chamber due to the addition of heat in gas which causes it to expand. Isothermic systems have temp...

DLee_1L

Plus, there are certain scenarios when the system is isothermal and thus delta U is equal to 0 such as when the gas expands in a container with a piston (with no pressure being exerted) and in a vacuum, therefore there is no work or heat transfer.

DLee_1L

Gibbs free energy is the energy in a chemical reaction that can be used to do work. Since enthalpy is the total heat content of a system, you would subtract entropy, which is the unavailable energy that is used for work, with temperature. This gives us the energy that can be used for work. So to cl...

DLee_1L

Does the substance in the bomb calorimeter get heat added to it? In my mind, I feel like that would mess up the measurement of the heat released by the actual reaction. I thought that the experiments in the bomb calorimeter had to spontaneously occur?

DLee_1L

The heat released by the chemical reaction will be equal to the heat absorbed by the calorimeter. So, (mass of calorimeter) x (C of calorimeter) x (change in temp) = (mass of substance) x (Cs) x (change in temp) For the calorimeter side of this equation, you would not include the mass of the calori...

DLee_1L

I think about it like a styrofoam cup being the calorimeter. If there is a chemical reaction within the styrofoam cup, then the styrofoam will absorb the heat from that reaction therefore, the amount of q absorbed by the calorimeter(styrofoam) is the q given off by the reaction.

DLee_1L

Technically, yes, because a negative heat capacity means that the system spontaneously loses energy

DLee_1L

You can use delta H and delta U interchangeably when the system has no work done on it and the only thing contributing to delta H is delta U

DLee_1L

Since Standard Reaction enthalpy is the formation of 1 mole of substances in the most stable form, is it not possible to calculate it based off of the table of bond enthalpies since it only gives the enthalpies of gases?

DLee_1L

Another way to look at it is that Endothermic reactions require heat so + energy/heat and Exothermic reactions give off heat so energy/heat.

DLee_1L

Yeah, High enthalpy means that a substance is highly reactive since it has more energy.

DLee_1L

YEs! Specific heat is J/(kg*K) and molar specific heat is J/(kg*m) so you would need to get the molar mass of the substance and do some dimensional analysis.

DLee_1L

I think Professor Lavelle mentioned phase changes in delta H calculations because the table of bond enthalpies refers to breaking bonds for gases, so you would need to calculate phase change in order to find delta H via bond enthalpies.

DLee_1L

Also, ICE tables are used to find the equilibrium concentrations for weak acids and bases that don't fully dissociate.

DLee_1L

When you increase the amount of N2 in a system, the amount of H2 doesn't change. Since there is a sudden injection of H2 in the system at equilibrium, the reaction will want to start favoring the product (NH3). Therefore, both N2 and H2 are getting consumed and H2 concentration decreases relative to...

DLee_1L

The example Dr. Lavelle gave in class was if you added Helium to a reaction occurring within a sealed container. While the pressure of the reaction is increasing, it has no effect on the equilibrium because adding gas doesn't have any effect on the actual concentration of either the products or rea...

DLee_1L

Can someone explain why adding an inert gas does not change the pressure of the system and the does not shift the chemical equation?

Thanks.

DLee_1L

For me, if the base contains an element in the first 2 columns of the periodic table and OH or an acid that contains an element in the halogen column, then its a strong acid/base and don't bother with the ice table. Anything else will not 100% dissociate and a ICE table is needed

DLee_1L

If it helps, I always look at heat as if it is another substance in the chemical equation when given a le Chatlier question. This makes it easier to see whether a heat increase or decrease will lead to a shift to products or reactants

DLee_1L

So does that mean all pure substances (liquids and solids) are considered solvents in a equilibrium reaction? or are solvents not even included in the equation?

DLee_1L

Can anyone explain to me why is temperature the only thing that can affect the equilibrium constant? I can kind of see why for Kp because of the PV/T balancing, but don't understand why temperature affects Kc?

DLee_1L

Yes, the concentration of the products will be larger because the high amount of reactants will turn into product to satisfy equilibrium. K stays the same though.

DLee_1L

Does Kc and Kp have to be in certain units or can we use any unit for concentration or pressure to get Kc or Kp?

DLee_1L

K is just the ratio of the amount of product versus amount of reactant. So if the product is 20 M and the reactant is 5M at equilibrium the units (M) would cancel out and Kc would equal 4

DLee_1L

No, its there is no actual bonding or transfer of electrons, its just that the polar sides are strongly attracted to each other

DLee_1L

Yeah, think of pOH is for OH just as pH is for H. Its just that pH is used more often.

DLee_1L

I guess we are to assume that cation metals always form a coordination complex with 4 or 6 h2o molecules (my TA said most always it is 6).

DLee_1L

Since Cl has a higher electronegativity, it will pull the electron pair it shares with the hydrogen closer to it, so the H will more easily dislocate and be donated to another molecule

DLee_1L

I guess a solution for acid rain would be "less pollution in terms of less co2 and so4 emissions"

DLee_1L

Because Zn has a charge of +2 and 2 nitrates have a combined charge of -2 so it evens out.

DLee_1L

So are NaO2 a d NaOH both count as oxides?

DLee_1L

a closer distance between the two atoms usually makes the sigma bond and bonds in general stronger. So the higher the attraction between 2 atoms, the stronger the sigma bond

DLee_1L

Sorry, for c) yeah, its because there is an OH and NH2 swapping places. Since there's no H atom transfer, not a bronsted-lowry acid&base. For d), it is a bronsted Lowry reaction because once you separate everything into the molecules, Nh4 gives one of its h atoms to nh2. Sorry about that, didn't...

DLee_1L

C) and d) aren't considered bronzed-lowry reactions because there is no simple transfer of a H atom in the chemical reaction.

DLee_1L

I believe so. the ligand naming convention is only if there is a complex with a transition metal in the center and that coordination compound is then bound to another molecule. Otherwise, the molecule will follow the naming conventions we already know.

DLee_1L

Usually, you can't tell from the shape of the ligand how many bonds it will have, its based on the number of lone pairs on the ligand. I think its better if the ligand complex has less 2x and 3x bonds so the ligand can twist into a shape that allows for the best bonds.

DLee_1L

Isn't it two electrons and not 2 electron pairs that make a coordinate covalent bond?

DLee_1L

alphabetic order, but disregarding the prefix. but for the formula, doesn't matter

DLee_1L

You should usually look at the anions first (especially if the cation is a transition metal) and then determine the oxidation state of cation since anion charges are usually the same (e.g. oxygen always has 2- state)

DLee_1L

Yes, more lone pairs would result in longer bond lengths. Dr. Lavelle also gave an order to how strong lone pairs and bonds affect each other: pair-pair>pair-bond>bond-bond.

DLee_1L

We can draw normally and then name the shape after. Lewis structures only focus on what bonds to what, not on electron repulsion

DLee_1L

So is the pi bond both the top and bottom electron areas? or are they both separate pi bonds?

DLee_1L

What exactly is the difference between hybridization and VSEPR theory then? Hybridization is used to calculate the number of electron pairs which directly relates to shapes of molecules, but VSEPR theory is the actual shapes?

DLee_1L

Also, sigma bonds are stronger and are directly between atoms, while pi bonds are weaker and are in a arc-like shape between atoms

DLee_1L

When we look at the hybridization of the CH3Br molecule, you will see that there is a tetrahedral shape. That means that the the pulls of opposing H atoms will cancel each other out, and the electronegativity of the Br will be mitigated by the opposing H atom.

DLee_1L

Also be mindful of the orientation of the molecule when analyzing for dipole-dipole interactions. For instance, a ccl4 molecule has the cl atoms canceling their charges out due to its tetrahedral shape so there is no dipole-dipole interactions.

DLee_1L

Don't memorize the table! Just know that F is the most electronegative element and electronegativity decreases as you go left and down the periodic table

DLee_1L

I think that the bond lengths are based on the probabilities of where the electron clouds are most likely to be. So when two atoms share electrons, the electron clouds are combined and the bond length is how long that probable electron cloud space is between the atoms

DLee_1L

I think the relationship is that Schrödinger's equation mathematically maps out the possible area that an electron can be in (an orbital) when it acts as a wave.

DLee_1L

I think of it as the sigma bond being the main bond and the pi bonds being the extra supporting bonds that give a bit but not as much strength to the bond

DLee_1L

I would assume just link them using the lines because if you grouped them together, that would be a functional group for o-chem, which we haven't covered yet.

DLee_1L

By definition, is titanium's valence electron count 2 because there are only 2 electrons in the highest energy level (4s) or is it four because the 4s and 3d shells are on a similar energy level?

DLee_1L

No, hydrogen bonding does not require a lone pair. It only requires a hydrogen positive side from one molecule and a very negative negative side (usually N,O, or F) from another molecule. An electron pair repulsion would not contribute too much to the polarity since it is temporary and pretty weak (...

DLee_1L

We might be asked to in order to determine the type of bond, but for now, we only need to know the trend.

DLee_1L

Technically, yes, isoelectric atoms have the same number of valence electrons, but a better definition would be having the same overall amount of electrons. (hydrogen and lithium are not isoelectric even though they have the same amount of valence electrons) In addition, even though there are an equ...

DLee_1L

I believe so, but it won't be too hard since all the resonance structures would be exactly the same just with a double or triple bond in a different place.

DLee_1L

Resonance structures are different ways to write the lewis structure of a molecule (usually when there are double or triple bonds that can be interchangeable). However, in reality, a molecule is not just one or the other resonance structure, but a combination of all of them due to electrons moving a...

DLee_1L

You can just count from left to right to find the number of valence electrons in the ground state for an atom (e.g.Titanium is the fourth element from the left, so it has 4 valence electrons)

DLee_1L

Just adding onto the previous explanation. Silver was used in the experiment because it had only one unpaired electron in the 5s orbital, which makes the silver atom's overall magnetic field easily influenced by it.

DLee_1L

Ground State means that the atom is at its lowest possible energy state, and that the electrons in the atom cannot lose anymore energy. In contrast an excited state is when electrons go to a higher energy level or shift orbitals.

DLee_1L

You should base your sig figs off of what the problem gives you I think. If 250 ml is your smallest given, your final answer should have 2 sig figs. I don't think constants like 273.15 K contribute to sig figs, but you might want to check that.

DLee_1L

I think there are two ryberg constants, one that is frequency (m/s) and one that is inverse wavelength. You can derive one from the other using the speed of light = frequency * wavelength equation.

DLee_1L

From my understanding, in the silver beam experiment, it was discovered that silver atoms went in two different directions because they had two different magnetic fields. These different magnetic fields are caused by the one unpaired electron spinning since a charged particle's movement causes a mag...

DLee_1L

The quantum mechanic description of light is that an object usually displays particle-like behavior, but wave-like behavior occurs when an object is small (>10^-15 m small). Through the photoelectric experiment, it was seen that light acted as more of a particle since increasing the amplitude/intens...

DLee_1L

Ironically, the Debroglie equation was derived from equations using light, however the wavelength of light cannot be found using debroglie's equation due to the equation requiring mass.

DLee_1L

Momentum is the inertia of a moving object. Just like a stationary heavier object has more inertia (the tendency of an object to stay or move in the same way as it has been), an moving object that is heavier or is moving faster has more momentum, thus p=mv.

DLee_1L

In a microwave oven, why are the microwaves dangerous if they have a lower frequency than visible light? Is it because they have a very high amplitude? If that's the case, then are microwaves being examined as waves, rather than photon particles?

DLee_1L

Also, the electron must be jumping from an energy level of n=>3 to n=2 to emit any visible light. The higher the initial energy level, the shorter the wavelength with the shortest being purple and longest being red.

DLee_1L

So technically, fractioned chemical equations (e.g H2 + 1/2O2 -> H2O) is balanced. But we multiply so that we have whole numbers because it's easier to look at and work with?

DLee_1L

Molarity is a measure of concentration and is usually used when the solvent is water and/or the amount of solute is relatively small compared to the amount of solvent used. Later on, we'll probably learn molality, which is a better concentration when calculating things like temperature changes or wh...

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