CHEMISTRY COMMUNITY

Amy Pham 1D

Emily Vainberg 1D wrote:For part A of this question, did you get that the rate of O2 was 1.32 moles.L-1.s-1 ?

Yes, this is correct! The rate of consumption of oxygen (O2) is 1.3 mols.L-1.s-1, with correct sigfigs.

Amy Pham 1D

A second order reaction is a chemical reaction whose reaction rate is dictated by a second order integrated rate law. A second order integrated rate law is an expression which gives you the rate at which the second order reaction will proceed, and it is dependent on either the concentration of a sin...

Amy Pham 1D

In a first order reaction, the reaction rate depends on the concentration of a single reactant. In a second order reaction, the reaction rate depends on either the concentration of two different reactants raised to the first power, or the concentration of a single reactant raised to the second power...

Amy Pham 1D

Just know that it is another method used to evaluate reaction mechanisms that results in the same conclusions. Even though we are not expected to know anything technical about it, just be aware that the steady-state approach is more complicated but also more flexible than the pre-equilibrium approac...

Amy Pham 1D

O2 is not a catalyst, simply a reactant in the overall reaction. The process itself refers to the conversion of oxygen to ozone, so oxygen is a simply a reactant in this process, otherwise there would be no reactants. So yes, nitric oxide (NO) is the only catalyst in this reaction.

Amy Pham 1D

I don't have the actual slides, but I do write down everything that is on them. Here is a link to my notes!

https://docs.google.com/document/d/10Bz ... sp=sharing

Amy Pham 1D

The handout does provide all of the baseline equations for deltaG, but it is hard to say if the list is exhaustive as there will always be scenarios where you will need to manipulate and use multiple equations to come to a sufficient answer method. So all the Gibb's Free Energy equations that you wi...

Amy Pham 1D

In order to understand the integrated rate law it is important to know where it came from, but I doubt Dr. Lavelle would ask us to specifically derive one equation from the other. If anything, developing an understanding of the relationship fosters a clearer understanding of reaction kinetics, though!

Amy Pham 1D

Electrolysis is one of the real life applications of electron transfer reactions: it uses the electron current in electrolytic cells to drive non-spontaneous redox reactions. In this way, electrical energy is actually being converted to chemical energy, with the example Dr. Lavelle presented in clas...

Amy Pham 1D

As long as both the top and bottom components of the Q ratio have the same units, there is no need to convert as they will cancel out and fit into the logarithm as such.

Amy Pham 1D

Lavelle said that the reaction from diamond to graphite was spontaneous but had a very high activation energy and would occur very slowly. Does this mean that over time, say a few trillions years, the diamond is slowly collecting enough energy to overcome the energy barrier? Or would the reaction e...

Amy Pham 1D

This would depend on the method that you use to find the reaction rate, which you would select based on the information provided to you in the question. If you are given multiple reactant/product concentrations and initial rates, then you would likely be using the method of initial rates and find th...

Amy Pham 1D

It seems that keeping the oxidized molecules on the left and the reduced molecules on the right takes precedence in writing out half reactions for redox reactions.

Amy Pham 1D

Reaction rates are positive quantities by convention, to consistently denote the rate at which a reaction is proceeding. It is true that the concentration of reactants is decreasing while the concentration of products is increasing in a forward reaction, but this all plays into the forward reaction ...

Amy Pham 1D

Yes, when there is no specification, you use platinum as your inert conductor acting as a electrode to transfer electrons in the cell.

Amy Pham 1D

We use half reactions to break down an overall reaction into two distinct portions of electron exchange: one for the oxidation reaction where electrons are being lost, and one for the reduction reaction where those electrons are being gained. In just the overall reaction we cannot see what is happen...

Amy Pham 1D

As stated in the answer above, note that oxygen does not always have its expected oxidation state of -2! Although that is what it is the majority of the time, there are exception cases that occur significantly frequently.

Amy Pham 1D

Definitionally, adiabatic systems are systems where there is no heat transfer taking place. So yes, the q for adiabatic systems will always equal 0.

Amy Pham 1D

There are many different methods to determine whether a reaction will proceed to favor reactants or products, or if it is at equilibrium: deltaG and comparing K to Q are two of the many. It all depends on what type of problem it is and what information is given, but yes these are definitely two meth...

Amy Pham 1D

Definitionally, oxidation is the loss of electrons while reduction is the gaining of electrons. A cathode by definition is the site from which electrons flow, while the anode receives this electron flow. Thus, oxidation is paired with the anode while reduction is paired with the cathode, simply by d...

Amy Pham 1D

For exothermic spontaneous reactions, we know that the exothermic property indicates that the change in enthalpy must be negative, and the fact that it is spontaneous implies that the change in entropy must be positive according to the second law of thermodynamics. Enthalpy and entropy are not direc...

Amy Pham 1D

Yes, for an isolated system the change in internal energy is zero since no matter or energy is exchanged with the surroundings. Because of this, q and w must equal 0 as well.

Amy Pham 1D

Perhaps this question can be revisited after we have covered this portion of the Gibb's Free Energy topic, but Dr. Lavelle actually hasn't covered those versions of the Gibb's Free Energy equation yet. The only one he has gone over so far has been the deltaG= deltaH - T*deltaS.

Amy Pham 1D

Negative values of deltaG indicate that the reaction has the free energy to carry out the process, and thus the reaction proceeds spontaneously. Positive values of deltaG indicate a nonspontaneous reaction, as it does not have the free energy necessary to allow the reaction to proceed.

Amy Pham 1D

It actually was stated by Dr. Lavelle that the combustion of glucose in a bomb calorimeter was an example of an isolated system. However, this implies that the change in internal energy is equal to zero, not just the internal energy. It is important to note what we are considering the system. Dr. La...

Amy Pham 1D

Meredithe DeGuzman4G wrote:At constant pressure, enthalpy is the amount of heat released or absorbed by the reaction.

Further, this is because at constant pressure the volume of reactants equals the volume of products, meaning that there was no expansion work in the reaction and the work term (w) in the equation equals zero.

Amy Pham 1D

Thank you for the reply, this clarifies a lot for the shift changes. In terms of Q, this makes sense, how about for all the different Le Chatellier’s Principles. Le Chatelier's Principle effectively states that systems adjust so as to minimize the effect of any changes. Adhering to this simple rule...

Amy Pham 1D

It turns out that there are many different explanations to this phenomenon, not discounting the simple fact that hot water evaporates more quickly than cold water, leaving less water to freeze--and thus requiring less heat loss--and allowing for a quicker overall freezing time. Another explanation t...

Amy Pham 1D

Both closed and open systems exchange energy with their surroundings, so the heat transfer for both cases could be calculated using the same equations of deltaU = q + w, with the appropriate equations for calculating q and w applying when there is constant pressure.

Amy Pham 1D

Your rationale for sig figs is correct! But I believe that when Dr. Lavelle did this in class example, he wanted to focus more on the method for solving calorimetry heat transfer types of problems than being too picky about sig fig rules. On a test I think the sig figs of the given values would like...

Amy Pham 1D

I highly doubt that we'll be required to memorize enthalpies of formation, so I assume they'll be given on a test. This would be a good question for Dr. Lavelle himself. Remember which ones can be cancelled because they're in their base state, though. You might have to memorize those, but I believe...

Amy Pham 1D

There is quite literally no difference between using Celsius or Kelvin in problems (such as these) where only change in temperature--not an absolute temperature--is involved. Whether you calculate the deltaT using Celsius or Kelvin will give you the exact same value since the temperature systems are...

Amy Pham 1D

Every liquid displays some degree of vapor pressure, in that there are always a certain amount of molecules being ejected from the surface of the liquid into the surroundings as a gas. The distinction comes when we realize that at higher temperatures, these ejections of molecules from liquid to gas ...

Amy Pham 1D

The value of the enthalpy of the phase change would likely be given, and all that we are expected to understand conceptually is that the enthalpy of phase change must be added to the rest of the bond enthalpies to accurately account for all enthalpy changes in the overall reaction.

Amy Pham 1D

Depends on the information given and the setup of the question. For example: if multiple rxn equations are given with their respective enthalpy changes, then you might suspect Hess's Law. If bond energies are given, then you might use bond energies. what exactly are bond energies? I think what was ...

Amy Pham 1D

Just go by the units that are given in the problem. Don't feel that problems dealing with gases have to use Kp simply because they are gases; Kc works identically.

Amy Pham 1D

It is normal for a pH or pOH value to be negative. If the concentrations are greater than 1.0, which in this case they are, then the pH/pOH values will be negative. This checks out mathematically.

Amy Pham 1D

And they do a good job of maintaining a solution at a constant pH because any acid or base that is added to the solution will be neutralized by the weak acid and conjugate base or weak base and conjugate acid in the solution, resulting in no change in levels of OH- or H3O+.

Amy Pham 1D

The only change in physical parameters that will result in a change in equilibrium constant is a change in temperature.

Amy Pham 1D

You are correct. Although carbonic acid is a polyprotic acid, it is not strong, so even the first protonation will not result in the same H3O+ concentration as the initial carbonic acid concentration. On the other hand, H2SO4 is the stronger acid and will dissociate.

Amy Pham 1D

In terms of calculation methods, there is no difference between Kc and Kp other than the fact that one of them uses concentrations and the other uses partial pressures, respectively. The way to formulate the expression remains the same. K is sometimes used generally to refer to either one of the two...

Amy Pham 1D

For our purposes, we use the Ideal Gas Equation to convert between partial pressures and concentrations so that we can calculate equilibrium constants with consistent units. You would use it by rearranging the equation, resulting in P=(n/V)RT where we know that (n/V)=concentration. So, you would now...

Amy Pham 1D

From an experimental standpoint, knowing the K of a reaction would allow us to determine when an reaction has reached equilibrium. With the knowledge of K, you could measure the concentrations/partial pressures of the reactants and products in the reaction and use them to calculate a value of Q. You...

Amy Pham 1D

P=partial pressure of the gas
V=volume of container
n=mols of substance
R=ideal gas constant
T=temperature

You would use this equation in our context to convert between partial pressures and concentrations (n/V) of reactants and products to be used in calculating equilibrium constants.

Amy Pham 1D

I believe that examining reactions by way of Le Chatelier's Principle functions under the assumption that reactants are in excess. It is a merely conceptual way of understanding how manipulating amounts of products/reactants affects reaction rates, without the added parameter of limiting reactants.

Amy Pham 1D

Little clarification, even when all the products and reactants are given in the gas phase you don't always have to use partial pressures to calculate the K value. If you are given concentrations, then using those and calculating K_c is fine as well.

Amy Pham 1D

Any time when you are given an equilibrium concentration (as opposed to just initial concentrations), you can use that value to determine the changes in concentration for each of the reactants and products. It is only when you don't know the equilibrium concentrations and aren't able to deduce the c...

Amy Pham 1D

I believe they would be, since only reactants and products that are in the solid and liquid phases don't contribute to equilibrium rate constants.

Amy Pham 1D

The chemical equation given is the overall reaction, but we can break it down into two steps that make up the overall multistep reaction, both step reactions being given in the table. First is 2 BrCl (g) ->/<- Br_2 (g) + Cl_2 (g), and the second reaction is H_2 (g) + Cl_2 (g) ->/<- 2 HCl (g). The K ...

Amy Pham 1D

Comparing K to 10^3 and 10^-3 will tell you which direction the reaction proceeds to. Values in between these numbers do not indicate any strong favoring toward products not reactants, but can still tell you which reaction (forward or reverse) is happening -slightly- more. If K<0, the reactants are ...

Amy Pham 1D

If the anion in an acid is stable, then it is able to exist stably on its own without the presence of the H+ ion (proton). So it more readily loses its proton in solution and dissociates more completely, thus making it a stronger acid.

Amy Pham 1D

Dr. Lavelle posted a reply that we may be asked about the conjugate seesaw on the final! We may be asked to identify the strong acids/bases and their weaker conjugate bases/acids with relation to each other, or to identify both the strong acid/base and the weaker base/acid with relation to the seesaw.

Amy Pham 1D

Based on the Brønsted–Lowry acid–base theory, a conjugate acid is formed by a base receiving a proton. A conjugate base is the opposite - it is the base formed when the acid donates a proton. So can a molecule become both a conjugate acid and base? If it gains a proton does that make it a conjugate...

Amy Pham 1D

Bronsted acids and bases are more precise definitions of what is considered an acid or a base as compared with the Lewis definitions. Bronsted definition deals with the donation/accepting of protons (H+ ions), while the Lewis definition deals with donation/accepting of electron pairs.

Amy Pham 1D

A salt containing an acid will lower the solution pH because they will give off H+ in water and increase the concentration of H_3 O+, thus lowering the overall solution pH.

Amy Pham 1D

Based on the Bronsted definition of acids and bases, acids are proton donors and bases are proton acceptors. (The Lewis definition deals with whether species donate or accept electron pairs.)

Amy Pham 1D

Those may have been the ones that he explicitly mentioned in class, but it is important to know conceptually what characterizes them as strong acids and be able to determine what other compounds are also strong bases based on this understanding.

Amy Pham 1D

It has two Cl's that bond with guanines in the DNA of fast-growing cells (both harmful and healthy), preventing them from replicating. This stops the cell division of both cancerous and healthy cells.

Amy Pham 1D

Kelsey Ash 1D wrote:Hybridization occurs when an atom needs to form more bonds than it would normally form in its ground state

And how would we know when an atom needs more bonds than normally found in its ground state? Is it based on bond angles?

Amy Pham 1D

In sigma bonds, the electron density is cylindrically symmetrical around the internuclear axis (the line connecting the nuclei of the atoms in question), resulting in a continuous, even distribution and no nodal planes. In pi bonds, there is a nodal plane at the internuclear axis, meaning that there...

Amy Pham 1D

The two classifications are synonymous, the terms mean exactly the same thing. When a central atom has two bonding regions and one lone pair its shape is bent/angular.

Amy Pham 1D

I think that both the structure w/ two double bonds and the structure w/ one double bond and one single bond are acceptable, so they are just giving answers for both. But wouldn't the structure with one double bond and one single bond not be ideal because it doesn't minimize formal charge? Technica...

Amy Pham 1D

In pi bonds, the two regions of electron density overlap side by side rather than end to end, creating a nodal plane including the internuclear region. This is likely what was meant when it was stated that pi bond electrons are more exposed, as the regions of electron density lie outside of the inte...

Amy Pham 1D

Any time in which there is an ionic bond between the atoms in a molecule, ion-ion interactions would be present as an attractive interionic force.

Amy Pham 1D

Hydrogen bonding is actually a very specific type of dipole-dipole interaction, between and H-N, H-O, or H-F bond and the lone pair of another electronegative element. So in that sense, yes: if a molecule has hydrogen bonding, it technically has dipole-dipole interactions as well.

Amy Pham 1D

Although I doubt we will be expected to memorize the exact values of specific dipole interactions, it is good to know that generally dipole-dipole interactions are around -2 kJ/mol.

Amy Pham 1D

Dipole interactions are present between dipole moments. A molecule that has dipole interactions must have dipole moments, so yes a molecule can have both. Finally, ion-ion interactions (not ion-ion dipoles, I think is what you are referring to) are the intermolecular interactions that are present be...

Amy Pham 1D

Yes, in the context of intermolecular forces it indeed represents the potential energy of the intermolecular interaction force. In other words, the interaction potential energy.

Amy Pham 1D

Not quite sure what is meant by this either, but if you are referring to electrons being locked in locations around an atom and being restricted from moving around, double bonds are much stronger than single bonds and the electrons involved in the bond are held more tightly.

Amy Pham 1D

Forces that affect boiling point are based on the strength of the intermolecular forces, so the ones that would most affect BP are the ones that are the strongest. So ionic interactions would have the highest effect, followed by hydrogen bonding, then ion-dipole interactions, and the last few all be...

Amy Pham 1D

The inequality for Heisenberg's indeterminacy places Planck's constant (an extremely small number) in the numerator to be divided further by 4pi, resulting in an even further miniscule value. You would have to multiply a conventional mass of everyday objects by an indistinguishable small number repr...

Amy Pham 1D

Will we have to know the actual values of electronegativity to determine the minute differences? I assume probably not.. My TA informed us that if electronegativity differences were asked to be determined/we needed to determine if bonds were covalent or ionic in order to draw a Lewis structure, the...

Amy Pham 1D

The charge of the anion doesn't play as much of a role as the size does since the shielding effect of multiple electrons is of less magnitude than that attractive force of the nucleus increasing with more protons. Since the size is more important, atomic radius is the aspect in focus when drawing co...

Amy Pham 1D

Resonance structures show the different versions of the Lewis structure that "blend" together to form the actual form of the molecule. They are necessary to show that the delocalized electrons are not just stationary in one spot, they are located in locations determined by the blending of ...

Amy Pham 1D

The electronegativity difference between the two is greater than that of the BO bond. You would either be able to tell this directly from numerical subtraction if the electronegativities were given to you, or by looking at the periodic table and seeing that C and O are farther from each other than B...

Amy Pham 1D

If the ion has an odd total number of valence electrons, then at least one of them must be unpaired and the species is thus a radical.

Amy Pham 1D

Indeed as stated above, ions can have charge but technically the name "formal charge" applies to the individual atoms in that ion. However, apart from terminology, a good guideline to apply to the idea you are referring to is that in a molecule or ion, the sum of the formal charges of the ...

Amy Pham 1D

Try to think of it not as an equation but just counting up the electrons! It is much less daunting that way, and it becomes far more intuitive. Just look at the atom in the Lewis structure and count how many dots you see as lone pairs, which wholly belong to that atom. Also count one for each bond y...

Amy Pham 1D

When writing the electron configurations for ions, look initially at the electron configuration for the neutral parent atom. In the case of Ru3+, this would be Ru's electron configuration being [Kr] 4d7 5s1. After finding this, you remove 3 electrons from the logical highest energy shells and subshe...

Amy Pham 1D

The two elements that he told us were exceptions were chromium (Cr) and copper (Cu), so I'm assuming silver would be another exception based on the fact that it is in the same group as copper?

Amy Pham 1D

Why is the electron configuration for Ag: [Kr]4d10 5s1 instead of [Kr]4d9 5s2? By the rules that we were taught, shouldn't it be the latter? Following this, I understand that starting with the correct electron configuration makes the configuration for Ag+: [Kr]4d10, with no unpaired electrons. I'm j...

Amy Pham 1D

Since the possibilities for the quantum number l are 0, 1, and 2, we have to account for that in our consideration of the possible values for ml. Accounting for all the possibilities, there are a total of 9 possible orbitals: ml being 0 for l=0, ml being -1, 0, or 1 for l=1, and ml being -2, -2, 0, ...

Amy Pham 1D

Since this is a chemistry class with a focus on the life sciences, most of the elements we will be dealing with fall into the s, p, and d blocks, which is why Dr. Lavelle said that those are the ones we should be focusing on when studying. They will be the ones that appear most frequently in example...

Amy Pham 1D

When writing out the electron configurations for atoms or ions, it is convention to be able to abbreviate the formula to the last noble gas before the element whose configuration is to be found. For example, for Mg it would be Ne and for Ti it would be Ar. From the atomic number of the noble gas you...

Amy Pham 1D

The only difference is in the cations and anions that form the ionic bonds. Cations are smaller than their parent atoms because they have a more positive charge and the nucleus holds more tightly to its electrons. And anions are bigger than their parent atoms because they have more electrons to expe...

Amy Pham 1D

The s-, p-, d-, and f- orbitals designate the shapes of the mathematical functions predicting the locations of electrons in atoms. The s- orbital has a spherical shape with no nodal planes and symmetric distribution, the p-orbital has two lobes on either side of the nucleus, d- orbital has three 4-l...

Amy Pham 1D

Additionally, the question would have to be dealing with electromagnetic radiation, not electron wave properties. A question relating to the latter would likely use the De Broglie equation or Schrodinger's wave function equation.

Amy Pham 1D

In class, Dr. Lavelle generalized the E=mc^2 equation to E=mv^2 (no longer dealing with photons) so that we could derive the equation for De Broglie's wave equation, wavelength=h/p with variables that cancel out. As he stated, there exists a better derivation of the equation that does not deal with ...

Amy Pham 1D

And in regards to your second question, multiplying the uncertainty in the position with the uncertainty in the momentum will result in a number that is always greater than or equal to h/4pi, the baseline uncertainty. This is the most optimized precision that the values of position and momentum can ...

Amy Pham 1D

Use the above commenter's guidelines, and just remember that the third equation is the De Broglie Equation for calculating wavelengths of particles, proving that all matter has wavelike properties. When you see particles and you are asked to find their wavelength, you know to use lambda=h/(mass x ve...

Amy Pham 1D

Activation energy is the energy difference between reactants and products in a chemical reaction (energy needed to make the reaction occur), while threshold energy is the energy required for reactants to undergo successful collisions (existing kinetic energy + activation energy). Thus, threshold ene...

Amy Pham 1D

By the technical chemistry definition, a chemical amount generally refers to the amount expressed in moles. From the textbook: "The amounts of atoms, ions, or molecules in a sample are expressed in moles, and Avogadro's constant, N(sub A), is used to convert between numbers of these particles a...

Amy Pham 1D

Further, when an atom absorbs energy its electrons jump up to higher energy levels in by quantized amounts, going up in specific levels through the electron orbitals. So yes, electrons of excited atoms gain energy.

Amy Pham 1D

Yes! That is the correct first step to finding the limiting reactant, then you would use the 0.0466 mols P4O6 and compare it with 0.0406 mols O2 you found in the previous step to find which one is the limiting reactant in the second reaction, P4O6 (s) +2 O2 (g) --> P4O10. As it turns out, O2 is the ...

Amy Pham 1D

mass of limiting reactant/molar mass of limiting reactant = mols of limiting reactant given mols of limiting reactant/given mols of excess reactant = calculated mols of limiting reactant/x, which is the amount of excess reactant used (in mols) (mols of excess reactant used) x (molar mass of excess r...

Amy Pham 1D

The upper limit of the correct range was, as the slides depicted, 720 nm. This is the correct number, 700 is just a simple round number to have a mental ballpark estimate.

Amy Pham 1D

When there is a catalyst taking part in a chemical reaction, do we include it when we write the balanced chemical equation for the reaction? For example, Question H7d from the Fundamentals homework: Write a balanced chemical equation for each of the following reactions. d) The reaction of ammonia ga...

Amy Pham 1D

Thank you for sharing! As I'm currently navigating my way around my first few days of classes here at UCLA as a first year, this was refreshing to read and reminded me of the bigger picture. Thanks and best wishes!

Amy Pham 1D

Especially after having done part a, part b can be done very quickly as you have already found the mols of K2S, the only compound in which sulfide ions are present. The mols of K2S are the mols of sulfide ion, and you would simply divide this number by 0.500L.

Amy Pham 1D

Yes, SnO2 would be named tin (IV) oxide, since oxygen's oxidation state is typically 2- and there are two of them, resulting in a 4- to be balanced out by tin's 4+. But Sn oxidation options are actually either 2+ or 4+, not 2+ and 3+. And just as a side note, the sign should be indicated after the n...

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