CHEMISTRY COMMUNITY

Fdonovan 3D

Can someone explain to me the ATP hydrolysis question from the final? I’m not sure how the value of Q was supposed to be determined I just felt like I wasn’t picking up on some given information.

Fdonovan 3D

For exothermic reactions is the value of q negative or positive?

Fdonovan 3D

How do you calculate pH given only the Kw?

Fdonovan 3D

How do we determine rate laws of the reverse reaction from the rate of the forward reaction?

Fdonovan 3D

How do you calculate the half life of a zero order rxn?

Fdonovan 3D

For the reaction 3/2 O2 -> O3, why do we use the stoichiometric coefficient of 3/2? Would it still be correct if we multiply both sides by 2, or is that not the simplest form of the equation?

Fdonovan 3D

Can someone please explain to me why we assume Cu/Cu2+ is the anode? In the question it writes the cell as M(s)|M+(aq)||Cu2+(aq)|Cu(s) so I assumed that the Cu/Cu2+ would be the cathode but why is that not the case?

Fdonovan 3D

Why is n=1 for this problem? When I write out the half equations I have 2 electrons for each, but in the solution manual it says that n should be equal to 1.

Fdonovan 3D

Under what circumstances do we need to add in inert electrodes to our cell diagram? (Like Pt(s) for example)

Fdonovan 3D

For question 6M.1, if Cu(s)|Cu2+ is the anode, why do we keep the standard potential as +0.34 V instead of reversing it to Cu(s) -> Cu2+ +2e- which would give us a standard potential of -0.34 V.

Fdonovan 3D

Initially at t=0 (before the reaction even starts) there aren't any products at all, therefore initial reaction rates only depend on reactant concentration.

Fdonovan 3D

Do we need to know how to find the slope of the tangent line to get an instantaneous rate?

Fdonovan 3D

Because reactants get depleted over time, the rate of the forward reaction will decrease. Initially, there are usually a lot of collisions between molecules which slows as more reactants go to products.

Fdonovan 3D

I'm just a little uncertain what it means for a reaction to be "first order". What does this tell us about the reaction?

Fdonovan 3D

Are rate constants always positive? And if so why is this the case?

Fdonovan 3D

Can someone please explain the purpose of the salt bridge in the galvanic cell?

Fdonovan 3D

I think that the standard notation is anode on the left and cathode in the right. I believe this is always the case for cell diagrams.

Fdonovan 3D

O3 is the oxidizing agent because it gets reduced to O2.

Fdonovan 3D

Can someone please explain how Le Chatelier’s Principle applies to cell potential? I was a little confused when it was explained in class.

Fdonovan 3D

Why is it that felt G is equal to the max work?

Fdonovan 3D

Under what conditions is delta S equal to zero?

Fdonovan 3D

Suppose that 50.0g of water at 20 degrees C is mixed with 65.0g of water at 50 degrees C at constant atmospheric pressure in a thermally insulated vessel. Calculate delta S and delta S total.

How would we find the delta S for the surroundings in this problem?

Fdonovan 3D

What is the difference between delta S and delta S total? How do we calculate each of them?

Fdonovan 3D

Why is it that sometimes units for delta H are only kJ and sometimes they are kJ/mol? Is there a specific circumstance when each applies, or is it always kJ/mol and sometimes the mol just isn’t written?

Fdonovan 3D

Under what conditions is q equal to delta H just to clarify?

Fdonovan 3D

Calculate the final temperature and the change in enthalpy when 765 J of energy is transferred as heat to 0.820 mol Kr(g) at 298 K and 1.00 atm (a) at constant pressure (b) at constant volume. Treat the gas as ideal. Looking at the answer key, it says to use n(5/2 R) delta T and n(3/2 R) delta T res...

Fdonovan 3D

A constant volume calorimeter was calibrated by carrying out a reaction known to release 3.50kJ heat in a .200 L of solution in the calorimeter resulting in a temperature rise of 7.32 degrees C. In another experiment 100.0 mL of 0.200 M HBr and 100.0 mL of 0.200 M KOH mixed in the calorimeter and th...

Fdonovan 3D

State functions such as internal energy have the same value for a particular state no matter how that state came about. Work and energy are not state functions because these properties do depend on how a state is brought about since accomplishing the same task in two different ways will result in di...

Fdonovan 3D

What exactly does it mean for the expansion of a gas to be “isothermal”? How does this impact which equations we should use?

Fdonovan 3D

Does anyone have any tips on how to remember when to use each equation? I’m having a tough time trying to keep them all straight and know when to use each of them.

Fdonovan 3D

Total enthalpy is equal to the sum of the enthalpies of the steps of the reaction. I think that Dr. Lavelle said we don’t need to know too much about this though.

Fdonovan 3D

ayushibanerjee06 wrote:Why are bonds that are formed negative in bond enthalpy calculations?

Forming bonds releases energy which means it is an exothermic reaction (negative delta H).

Fdonovan 3D

Some of the equations will be on the formula sheet and you can sometimes use those to derive other equations you need to if you can’t remember. Plus practicing problems will help you learn the equations you use most often!

Fdonovan 3D

Bomb calorimeters are isolated systems (constant volume) and constant pressure calorimeters are open systems (constant pressure.

Fdonovan 3D

When something goes from a solid to a liquid instead of liquid to solid in fusion, why is it still going go be a positive enthalpy value? Enthalpy is positive for solid to liquid and negative for liquid to solid. Going from a solid to a liquid is an exothermic reaction, meaning heat is absorbed by ...

Fdonovan 3D

Work is not a state function because it is dependent on the distance traveled by an object, thus its value would change based on the total distance traveled to get from the initial to the final state.

Fdonovan 3D

Are we going to have to know whether an acid or base is strong or weak just by its name or chemical formula? If so, how do we do know?

Fdonovan 3D

How do we know when we have to use the Ice box versus when we can just use the given concentrations and K value to find the concentrations of missing molecules.

Fdonovan 3D

In 6B.9, there is a table that gives you either the [H30+] concentration, the [OH-] concentration, the pH, or the pOH. In the first part of the table, it is given that [H3O+] = 1.50 mol/L and you have to calculate the other things in the table. To get the pH, I did -log[H3O+] = -log(1.5) and I got a...

Fdonovan 3D

Did Dr. Lavelle say why exothermic reactions favor the reverse reaction when heated and endothermic reactions favor the forward reaction? I'm just curious.

Fdonovan 3D

Temperature matters because it changes the value of K for the reaction. As temperature increases, K decreases and as temperature decreases, K increases. Exothermic forward reactions favor the reverse reaction when heated, while endothermic forward reactions favor the forward reaction when heated.

Fdonovan 3D

Yes, pOH + pH = 14 so you can just do a little algebra to rearrange that equation to solve for what you're looking for.

Fdonovan 3D

I don't think solids or liquids are taken into account when counting moles because they aren't affected by pressure change in the way that gases are. Gases behave differently under different pressures since it affects how much contact is made between molecules, while solids and liquids are already i...

Fdonovan 3D

Why do we use the ice box if the x is negligible when adding/subtracting? I'm confused why we use it if we end up taking the Xs out anyway.

Fdonovan 3D

I think Kp because they're gases you need to use the partial pressures.

Fdonovan 3D

Think of concentration (C = n/V). The number of moles (n) does not change, but the volume decreases (since the pressure increases), therefore the concentration (C) increases. If the pressure doubles, for example, then the volume would be cut in half causing the concentrations of both reactants and p...

Fdonovan 3D

I don't think we have gotten this far in class yet, but generally if ∆G < 0 then the reaction is exothermic, and if ∆G > 0 then the reaction is endothermic. The reaction is Δ G = Δ H − T Δ S. (I only know this from bio so I'm not sure if this will be the same or not for chem but I'm sure we'll learn...

Fdonovan 3D

Use Le Chatelier's principle when there are changes to the concentrations of products or reactants, changes to pressure, or changes to temperature. These are the only ones we have covered in class so far, so I'm not sure if the list goes beyond this.

Fdonovan 3D

0.10 mol O3 is placed in a 1.0L vessel and reaction 2 O3 --> 3 O2 is allowed to reach equilibrium. Then, 0.5 mol O3 is placed in a second 1.0L vessel at the same temperature and allowed to reach equilibrium. (c) Will the ratio of P O2/ P O3 be the same? The answer is that they will not be the same, ...

Fdonovan 3D

I believe we are supposed to just assume that the gases in the chemical equations are ideal gases. Assuming this allows us to use the PV = nRT equation.

Fdonovan 3D

How can you tell if a compound is amphoteric?

Fdonovan 3D

i'm just checking to make sure i did it right because i think there are some problems with the solutions manual, but for (iv) does: [H30+] = 5.62x10^-15 [OH-]=1.78 pH=14.25 pOH=.75 ? also, for (iii), does [OH-] = 5.62 x 10^-14 or -15? for (iv): pOH = 0.75 (given) [OH-]= 10^-(0.75) = 0.0178 M pH = 1...

Fdonovan 3D

Can someone explain why boric acid is a Lewis acid and an electron pair acceptor? Also does this mean that the conjugate base of boric acid is not stable since boric acid is a weak acid? A Lewis acid (by definition) is an electron pair acceptor. Boron is able to accept an electron pair because it o...

Fdonovan 3D

When do you use bond strength over electronegativity (and vice versa) when finding out which acid is stronger? According to the book, use bond strength when comparing acids w/ central atoms that are in the same group (Ex. HCl vs Hf --> the H-Cl bond is longer and easier to break since Cl is larger ...

Fdonovan 3D

For determining the strength of a base, use the conjugate acid. You can use the basic principles for deciding which acid is stronger (including electronegativity, bond strength, # of Oxygens on central atom, or electronegativity of groups attached to central atom depending on the specific acid you a...

Fdonovan 3D

Which is the strongest acid, H2CO3 or H2GeO3? (Justify your answer) In the answer key, it says H2CO3 is the stronger acid because C is more electronegative. However, I thought that when comparing acids in which the only difference is the central atom, you compare the bond strengths for elements in t...

Fdonovan 3D

BrO- would be the stronger base because it is the conjugate base of a weak acid, HBrO. A weak parent acid always indicates a strong conjugate base that must be a relatively good proton acceptor, which can relatively easily form HBrO molecules when added to water in the rxn: BrO- + H2O --> HBrO + OH...

Fdonovan 3D

Are we supposed to be able to determine the conjugate acid for any base we are given (or vice versa)?

Fdonovan 3D

Are there specific characteristics of molecules that allow you to identify them as amphoteric? If so, what are they?

Fdonovan 3D

I missed class Wednesday and am not sure what a conjugate seesaw means. Can someone briefly explain this?

Fdonovan 3D

Since weaker bonds can release H+ more readily, compounds with weaker (longer) bonds are stronger acids.

Fdonovan 3D

Acid rain happens when specific compounds, such as nitrogen oxide or sulfur dioxide, are released into the air. These pollutants react with water and oxygen in the air, forming acidic compounds that result in acid rain.

Fdonovan 3D

I'm having some trouble remembering all the rules for naming-- does anyone have any tips/tricks to make this easier at all? Anything helps!

Fdonovan 3D

I think it is a ring structured amino acid that is important in hemoglobin and maintaining pH in the blood.

Fdonovan 3D

This might be a stupid question, but is there a difference between Lewis acids and regular acids?

Fdonovan 3D

Why is it that transition metals have so many oxidation states?

Fdonovan 3D

Dipoles cancel if the molecule is completely symmetrical (like in the trans-dicloroethane example from class).

Fdonovan 3D

While we're on the topic-- what are sigma and pi bonds and why are they special?

Fdonovan 3D

The VSEPR formula for square planar is AX5E, and I think this is probably just rare because not a lot of molecules have five bonding pairs on the central atom plus an additional lone pair.

Fdonovan 3D

I think the point is to show you the lone/bonding pairs so that you can find the VSEPR formula (Ex. CCl4 is AX4 which would tell you that its shape is tetrahedral). I also remember Prof. Lavelle saying that if you put atoms symmetrically around the central atom, that's a pretty safe bet for getting ...

Fdonovan 3D

Are there any rules that are different for ions or do you just treat them the same as any other molecule for the VSEPR model?

Fdonovan 3D

Polar bonds are usually between atoms with pretty different electronegativities (like H -- O), while nonpolar bonds are between atoms with similar electronegativities (like O--O or like C -- H).

Fdonovan 3D

Draw the Lewis structure, VSEPR formula, molecular shape, and bond angles for I₃-.
Would the structure of this be bent? Or where exactly do the 3 lone pairs go?

Fdonovan 3D

I'm not sure if memorization is the best method for this but that's the method I'm doing.. Linear is easy-- it's 180 degrees because they're literally just in a line. Beyond that though, it's easier to see if you have a 3D model in front of you (which I'm sure you don't) but I'll just write out the ...

Fdonovan 3D

Is it always necessary to draw the Lewis structure when using the VSEPR model? Or are there times when you don't really need it?

Fdonovan 3D

Isopropyl alcohol, used in rubbing alcohol, is a compound containing only C, H, and O. What is the empirical formula of isopropyl alcohol if you find that 0.255 g of the compound gives 0.561 g of CO2 and 0.306 g of H2O when burned completely in excess oxygen? Can someone please help get started on t...

Fdonovan 3D

^^ I agree. Just count the number of electrons that you should have in theory, and then if you end up having extras after all the atoms have their octet then the atom capable of having an expanded octet would get these extra electrons.

Fdonovan 3D

Generally, if two atoms have similar electronegativities (such as C and H that both have half full valence shells) then they will form a nonpolar covalent bond since neither has more pull on the electrons. In contrast, atoms with fairly different electronegativities (such as H and O) form polar bond...

Fdonovan 3D

Carbon always forms 4 bonds (shares electrons)! I'm not sure if there are any exceptions to this but I do remember Dr. Lavelle saying that.

Fdonovan 3D

I think that if they ask us for it we can just draw multiple variations of the molecule. So if it was O-N=O and they said draw the resonance structure you would just do O=N-O (this is missing the lone pairs of course!)

Fdonovan 3D

Do we need to know how to calculate dipole moments? If so how do we do that?

Fdonovan 3D

Are double bonds typically longer than single bonds, or vice versa? Are there any general rules about this or does it vary?

Fdonovan 3D

Is there a way to determine bond length besides experimental results?

Fdonovan 3D

How can you determine if an atom is likely to have more than an octet? Is there a trick or do we just memorize that phosphorus, sulfur, and chlorine have this ability?

Fdonovan 3D

What does it mean that electrons are delocalized? And why don't Lewis structure capture this effect?

Fdonovan 3D

Can ions have a formal charge of zero? Or will they always have a nonzero charge because the ions themselves are charged?

Fdonovan 3D

Basically, on a very small scale, the path that is taken by an electron is not exactly certain therefore the velocity and momentum aren't 100% accurate either. The Heisenberg Indeterminacy Equation is used to account for this limit on how accurate the velocity and momentum can be.

Fdonovan 3D

Would anyone mind explaining how the short hand for electron configurations works? For example Chromium, Cr: [Ar]3d⁵ 4s¹
How do you choose the element to base the shorthand off of?

Fdonovan 3D

In period 5, the 4d state has lower energy than the 5s state after 2 electrons have occupied the 5s state. Is there a reason this is the case or is it just one of those things that just is?

Fdonovan 3D

I never did electron configurations in high school and I am super lost. In my notes I have that for carbon when there are 6 electrons, the configuration is 1s² 2s² 2px 2py. Could someone please explain to me where these numbers came from?

Fdonovan 3D

How do you know if an electron will spin up or spin down? It depends on the values for n,l,m, right? The direction of the electron's spin can be determined by the spin magnetic number, ms (the s is a subscript but I couldn't figure out how to do that on here). The value of ms can be either +1/2 whi...

Fdonovan 3D

Why do electrons in the outermost shell have the greatest amount of energy?

Fdonovan 3D

In my notes I wrote "electrons in atoms & molecules absorb or emit quanta of energy, which gives atomic (& molecular) spectra. I'm not exactly sure what atomic spectra even means, would anyone mind explaining it in layman's terms?

Fdonovan 3D

Light exhibits both qualities of waves and particles, though not at the same time. The diffraction of light upon hitting obstacles point towards wave behavior, as light is able to bend around objects. On the other hand, the photoelectric effect demonstrates the particle like behavior of light as lig...

Fdonovan 3D

Diffraction is what sets light (and other things that travel in waves, ie. sound) apart from particles. Light is able to bend around obstacles and in doing so certain diffraction patterns emerge. The way the pattern is arranged depends on how the light spreads around an object.

Fdonovan 3D

In constructive interference, the amplitude of the resulting wave is equal to the sum of the maxima of the two individual waves. In destructive interference, the two waves "cancel each other out" so resulting wave has an amplitude equal to the difference between the individual waves.

Fdonovan 3D

I'm having some trouble with M.15-- could anyone help me out? It says Al reacts with Cl₂ to form AlCl₃. I was able to write out the equation and balance to get 2Al + 3Cl₂ --> 2AlCl₃, but I'm having trouble figuring out how to find what mass of AlCl₃ can be produced given that the reaction began with...

Fdonovan 3D

I see your point-- they don't seem all that useful to me either. But I think they are used to gauge the ratio of elements in a given compound. Really it's just a simple form of notation that doesn't have a lot of practical applications, but it is helpful for getting to the molecular formula.

Fdonovan 3D

Avogadro's number represents the number of particles in a mole. If you know how many particles are in a given substance, this number can help you figure out how many moles there are. You can measure anything in moles really-- you could measure your food in moles if you really felt like it.

Fdonovan 3D

Based on the homework questions, the main units we were using were grams, liters, and moles. Be sure that you pay attention to the given information, as sometimes they give data in milligrams or milliliters so you'll have to convert to the standard units in order to use the information in equations....

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