CHEMISTRY COMMUNITY

Michelle Steinberg2J

Units are equivalent to 1/M^(overall order-1)*unit of time

Michelle Steinberg2J

"A catalyst is not consumed in the course of a reaction." I am confused as to why this is false. I understood that a catalyst is first found on the reactant side of the equation and then is later found as a product. Is it not consumed even though it goes from the reactants to the products...

Michelle Steinberg2J

"A catalyst is not consumed in the course of a reaction."
I am confused as to why this is false. I understood that a catalyst is first found on the reactant side of the equation and then is later found as a product. Is it not consumed even though it goes from the reactants to the products?

Michelle Steinberg2J

An intermediate is produced and then used. A catalyst, however, is used and then later produced.
Look at which sides of the equation they appear on :)

Michelle Steinberg2J

This sounds correct to me! We did a review problem with Professor Lavelle similar to this for the midterm.

Michelle Steinberg2J

I think the best way is to remember that something with the highest reducing power is the most negative. Conversely, something with the highest oxidizing power is most positive. You can look at the voltages provided and rank based on those values.

Hope that helps!

Michelle Steinberg2J

An intermediate is formed and then used.
A catalyst is used and then formed.

Michelle Steinberg2J

I'm not sure what equation to use for this problem -- there's so much information.
It would be really helpful if someone could walk through the steps. Thank you!

Michelle Steinberg2J

I am confused why the value of work is negative in this answer. The equation that I set up was 982J=492J + w because it says the internal energy increased (positive value) and the system absorbed heat (positive value). I would think that work is done on the system because I got a positive value. If ...

Michelle Steinberg2J

Hi I don't understand how to do this problem. The answer provided is 3.1%, but I don't see how to get there. If someone could walk through the steps that would be really helpful. Thanks.

Michelle Steinberg2J

First, I found the two half reactions on the list provided at the back of the test. I then flipped the reaction with the lower voltage (making it an oxidation half reaction because there must be one oxidation and one reduction) and that told me that the O3/O2, OH- is the oxidation half reaction. An...

Michelle Steinberg2J

I have a general question as to when to know to use the differential or integrated equations for first, second, or zero order.

Thanks.

Michelle Steinberg2J

Last year in chemistry I learned that the units of k = 1/M^(overall order-1)*unit of time.
:)

Michelle Steinberg2J

On my test we had to rank Zn, Mn, and Cr in order of increasing reducing power - I put Cr, Zn, Mn because the values would be -.74, -.76, and -1.18. But, I only got 1 point... Does anyone know what I might have lost a point on?

Michelle Steinberg2J

First, I found the two half reactions on the list provided at the back of the test. I then flipped the reaction with the lower voltage (making it an oxidation half reaction because there must be one oxidation and one reduction) and that told me that the O3/O2, OH- is the oxidation half reaction. An ...

Michelle Steinberg2J

Will we ever need to graph on a test? I went to Professor Lavelle's office hours and he quickly mentioned while explaining graphs that we won't be explicitly asked to graph. But, I think we should definitely have an understanding of what each of the graphs mean and be able to explain why they are c...

Michelle Steinberg2J

For this question, I tried solving for the order of [C] and got stuck. The solutions manual says that "C is independent of the rate." What does this mean?

Michelle Steinberg2J

Today in class Professor Lavelle mentioned that there are 3 ways to determine K. Based on what the question provides, you will determine which method is best to use:

1. Method of initial rates
2. If the graph shows a straight line, the slope of the plot is equal to -K
3. Half life

Michelle Steinberg2J

You can find the individual orders of a reaction using the "method of initial rates." You would find the overall order by adding together all of the individual orders that you have found in the previous step.

Michelle Steinberg2J

There have been a few homework problems that deal with determining the best oxidizing/reducing agent. How do you know?

Michelle Steinberg2J

You can look to see if there are H+ or OH- in solution. If there are H+, it would be acidic. If there are OH-, it would be basic.

I think Lavelle would prompt the reaction similar to how the textbook does, by saying if it is acidic or basic. But, if not, you can look for H+ or OH-.

Michelle Steinberg2J

"Copper reacts with dilute nitric acid to form copper(II) nitrate and the gas nitric oxide, NO. Write the net ionic equation for the reaction." I was wondering how they got to the final answer for this question. I don't see how they knew to make 3 Cu; I was able to balance it without doing...

Michelle Steinberg2J

When a question asks for the net ionic equation, what is the answer it is looking for? I guess my question is what is a net ionic equation and how does it differ from a balanced equation?

Thanks!

Michelle Steinberg2J

Yes! There should be electrons in both equations. Oxidation occurs when the charge increases, thus we have lost electrons (losing electrons=positive). Reduction occurs when the charge decreases, thus we have gained electrons (gaining electrons=negative). In an oxidation half reaction we see the elec...

Michelle Steinberg2J

mhuang 1E wrote:Are the posts counted weekly or is it cumulative throughout the quarter and then counted towards the end?

It says on the syllabus 3 points per week, so I think they are all counted at the end but we should have 3 posts per week. Hopefully that answers your question!

Michelle Steinberg2J

I understand why we use Cv, but I thought that Cv was (3/2)R? Isn't that what the constants & formulas sheet tells us?

Michelle Steinberg2J

I came across this practice problem and I'm not sure how to do it. Some help would be great! How many grams of water can be heated from 25.0 celsius to 100 celsius by the heat released from converting 49.7 g of PbO to Pb? The converting reaction is: PbO(s) + C(s) yields Pb(s) + CO(g) delta H = -106....

Michelle Steinberg2J

Every time I try plugging this into the calculator I get an error that says "overflow"... does anyone know what I'm doing wrong or is anyone else having the same error?

Michelle Steinberg2J

Does it also tell us that the reaction is able to happen without an addition of energy? I would also agree with the statement above. A spontaneous or favorable reaction will occur over time or naturally without any prompt from its surroundings. Dr. Lavelle used the example of a boulder going downhi...

Michelle Steinberg2J

This question asks to find the equilibrium constant for two different equations. It uses the values provided in Appendix 2A to find the delta g for the reaction. I'm confused how the solution manual goes from lnK= +184.42 to the final answer of k=1x10^80. Clarification on this final step would be mu...

Michelle Steinberg2J

If delta G is positive, it will favor the reverse reaction, or the reactants.

If delta G is negative, it will favor the forward reaction, or the products.

Michelle Steinberg2J

Would someone be able to briefly explain the difference between isobaric, isochoric, and isothermal? I have these definitions written down and I've read through the textbook, but I feel as though I've memorized the differences and don't fully grasp the concept. Thanks!

Michelle Steinberg2J

I also get confused between differentiating work and heat, so I understand! This is how I think about it: Work and heat are both forms of energy. The difference is how that energy is being transferred - it can be transferred with heat or with work. Heat is a form of energy transferred due to a diffe...

Michelle Steinberg2J

Ramya Lakkaraju 1B wrote:I just wrote the equation, delta U = q + w.

I also just wrote this equation. I honestly didn't even think to do any explaining. Hopefully that was sufficient!

Michelle Steinberg2J

I have a question about how to determine whether entropy is increasing or decreasing based on a chemical reaction? The solutions manual says an increase in moles on the product side shows an increase in entropy. Can someone explain this?

Michelle Steinberg2J

Can someone explain why a car engine is an open system?

Michelle Steinberg2J

Why do we use the specific heat capacity for water in the liquid state (4.18) to find the heat for the ice cube and the water?

Michelle Steinberg2J

Adding to this, there was a HW problem that showed different heating curve charts and had you choose which was the correct one based on information provided. If a substance has a lower heat capacity, it should have a steeper slope on a heating curve.

Michelle Steinberg2J

I am really confused by the negative sign that the solution manual puts on either the system or the surroundings. In the HW problem with the copper put into water, heat flow would go from the copper to the water... so why is the negative sign on the water? Isn't the copper the one that's losing heat...

Michelle Steinberg2J

What sort of concepts should we be comfortable with for internal energy and work? I'm confused what we will be tested on conceptually - I feel like internal energy and work deals mostly with calculations.

Thanks!!

Michelle Steinberg2J

Warda Sahib 2E wrote:Does Dr. Lavelle only test on things he's talked about?

In 14A it was pretty much if he didn't cover the topic in class then we do not need to know it for the test. The HW problems are pretty good indicators of what he will test on.

Michelle Steinberg2J

Also numbers 5,7,9,13?

Michelle Steinberg2J

Since Dr. Lavelle said no calculations of work or internal energy, will we not be tested on something like HW #3?

Michelle Steinberg2J

Can someone please explain why mercury in a thermometer is a closed system?

Michelle Steinberg2J

14 HW problems due this week in your discussion . Other weeks it will be 7 HW problems in your discussion.

Michelle Steinberg2J

These are just the overall equations that get you to the necessary product. That is, you must alter the equations provided in 65 to get to that overall equation. To do that, you can multiply the second equation by 1/2. This will allow 2 NO2 to cancel, and we want this to cancel because it is not inc...

Michelle Steinberg2J

I am having difficulty with determining the correct signs on energy changes. For example, HW problem 5, why is compression a positive value? Also, HW problem 7 says the system "absorbed 492 J of heat." Why is this a negative value?

Thanks!

Michelle Steinberg2J

If you focus in on the ice cube, you can see that it MUST be absorbing heat in order to melt. Thus, the system of the ice cube must be absorbing heat from its surroundings (the hand), and will have a positive delta H (change in enthalpy). It is helpful to know that an endothermic reaction (one in wh...

Michelle Steinberg2J

I remember that exothermic is the RELEASE of heat because of the ex- , which is similar to EXIT. Endo- itself means internal or within, so you can think of the heat being put "within" the system. Maybe this helps? The signs related to exo and endothermic reactions make sense once you under...

Michelle Steinberg2J

Section 8.3 of the textbook discusses expansion work. I'm have difficulty understanding expansion work vs. non expansion work. If someone could explain it further I would appreciate it!

Michelle Steinberg2J

So acids with longer bonds are stronger than ones with shorter bonds? For example, HCl has a longer bond than HF, but is stronger?

Michelle Steinberg2J

I have the same question! I'm assuming we will be provided with the necessary pKa and pKb values?

Michelle Steinberg2J

Today, we saw the example of $HCl\rightarrow (H+) + (Cl-)$

I am a little confused about what Professor Lavelle said about Cl- being more stable without the Hydrogen. How does this equation relate to what we learned today about conjugate bases and acids?

Michelle Steinberg2J

Similar to how we do not include solvents in the equilibrium constant "K" , we do not consider the solvents in Kw either. We consider the concentration unchanged, so we omit H20.

Michelle Steinberg2J

I learned a really awesome way to memorize Le Chatelier's Principle in high school. It puts it in layman's terms. "Add Away. Take Towards." If you ADD, the equilibrium will shift AWAY from where you added - AKA the opposite side of the reaction. If you TAKE, the equilibrium will shift TOWA...

Michelle Steinberg2J

For writing the reaction quotient, Q, or equilibrium expression, K, when do we write it as partial pressure with the capital P then the molecule as a subscript in contrast to as a concentration with the molecule surrounded in brackets? For example in 11.3, it asks to write the equilibrium expressio...

Michelle Steinberg2J

For part c and d of this question, the coordination number of the central metal depends upon the chelating compound. Are we required to know the chelating compounds and how many bonds they form, or will this information be given to us on the exam? I don't think we'll be given the information :( but...

Michelle Steinberg2J

In this question, does it matter the order of the elements within the brackets? My order is different than that in the solutions manual, does that make it wrong? I have just been putting them in order based on the name provided, taking it step by step, but after I have determined the elements and th...

Michelle Steinberg2J

How do you know to add -ate to the end of cobalt? What is the difference between "aquapentacyanidocobalt" and "aquapentacyanidocobaltate"?

Also, this question uses "cyanido" for (CN) whereas part (a) uses "cyano" for (CN). What is the difference?

Thanks.

Michelle Steinberg2J

Clarification: So when you have a double bond, the first bond would be consider sigma bond and the second one would be pi bond? For tripple bond, the first one would be sigma and the other two would be pi? Yes! This is correct. In lecture, Lavelle stated that a single bond (such as that between F-F...

Michelle Steinberg2J

Last year I learned something helpful that has stuck with me!

"SNAP"
Symmetrical
Nonpolar
Asymmetrical
Polar

This says that Symmetrical molecules (determined when you draw the Lewis Structure) are NON polar. Asymmetrical molecules are Polar. Generally this rule holds true.

Michelle Steinberg2J

On the most recent test there was a question that asked for all the possible values that a spin quantum number can take for a specific electron. How do you determine this? Is it always +/- 1/2?

Michelle Steinberg2J

Does anyone have any tips on memorizing all of the names for molecular shapes? I think my chemistry teacher last year gave us something to make memorizing a bit easier but I can't remember...

Thanks!

Michelle Steinberg2J

I think the most safe way to determine formal charge is using the formula Lavelle showed us in lecture: Formal Charge=Valence-(Lone+shared/2) But, I did learn a different way in my high school chem class that I've been using. Basically, you just look at the periodic table and look at how many valenc...

Michelle Steinberg2J

Does that apply for all row 3 atoms? That they can all accept more than 8 electrons? If so, can they all accept a total of 10, just like Sulfur? Thanks Any element that has an energy shell of n=3 or higher and is also a nonmetal can have an expanded octet. This is because they have access to the D ...

Michelle Steinberg2J

My question is related to unpaired electrons - spin up vs. spin down. How do you determine which way it will spin? Also, how do you know if your two electrons are paired OR parallel? In lecture we went over how paired electrons have opposite spin, which makes sense, but how do you know when two elec...

Michelle Steinberg2J

Hi! To find quantum numbers I find it helpful to take it step-by-step. 1. The principal quantum number is any positive integer. It is the energy shell. Ex. 1s, n=1. 3s, n=3. 2. ℓ is reliant on the principal quantum number AKA n. ℓ can be found with the equation n-1. It tells you the shape. It is hel...

Michelle Steinberg2J

In lecture we wrote down this equation ( HΨ=EΨ) as well as :
H=double derivative
E= energy of e-
Ψ= wave function representing e-

I'm unsure what this formula means or when to apply it. An explanation would be great!

Thanks!

Michelle Steinberg2J

As you move away from the nucleus, are you increasing or decreasing energy in the orbital? In other words, does an atomic orbital further from the nucleus have more or less energy than an orbital in close proximity to the nucleus?

Thanks!

Michelle Steinberg2J

Hi -

Would someone be able to explain why a light's wavelength determines whether or not a metal can eject an electron? I have memorized that long wavelengths cannot and short wavelengths can, but I do not fully understand why.

Thanks!

Michelle Steinberg2J

I was reading through the textbook and came across the Balmer series and the Lyman series. I am having difficulty designating the difference between the two and when to apply them. The textbook states: " The Balmer series consists of the lines with n1= 2 (and n2= 3, 4, . . .). The Lyman series ...

Michelle Steinberg2J

Hey! We did this question in office hours today and it took up the whole white board. Hopefully this explanation helps! Step 1. Convert grams of CO2, H2O, and N2 to moles. (You do this by dividing the number of grams by the molar mass) You should get .0155 mol CO2, .00965 mol H2O, and .003926 mol N2...

Michelle Steinberg2J

Here are some helpful tips/rules for Sig Figs! : 1. All numbers 1-9 inclusive count as sig figs 2. Middle zeros are always significant Ex. 409 3. Zeros before numbers 1-9 are not significant Ex. 0.00390 The first 3 zeros are not significant. 4. Zeros after all numbers 1-9 are significant IF AND ONLY...

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