CHEMISTRY COMMUNITY

Paula Dowdell 1F

This is from an old midterm. Can someone tell me what equation or relationship was used to find q in part b? Thanks.

Paula Dowdell 1F

How are ∆U and ∆H related?

Paula Dowdell 1F

How do you know how big the energy barrier is going to be, besides by knowing which step is slow (which makes the energy barrier largest for the slow step).

Paula Dowdell 1F

How do we draw a reaction profile? (For example the one in problem 15.89)

Paula Dowdell 1F

Does anyone know why they used kilojoules in this problem? When i use regular joules (R= 8.314 J/Kxmol) I get a completely different answer.

Paula Dowdell 1F

Standard formation enthalpy refers to the formation of a compound from elementary compounds, while bond energy is applicable when considering the formation of a molecule from isolated atoms.

Paula Dowdell 1F

The balloon expands because the kinetic energy increases due to the warmer temperature, making the molecules move faster and hit the inside of the balloon harder and more frequently.

Paula Dowdell 1F

Reactions with a neg. ∆G release energy (they can proceed without an energy input (are spontaneous)).Reactions with a positive ∆G need an input of energy in order to take place (are non-spontaneous).

Paula Dowdell 1F

Hi, In part (b) why is it that 3.5h is used for the value of "t" instead of 2s? I thought we would use 2s because the question asked "(b) If [N2O5]0 = 0.0567 mol L-1, what will be the concentration of N2O5 after 2.0 s? " The work in the solutions manual is as follows: [N2O5]=0.05...

Paula Dowdell 1F

Hi! Can someone explain to me how they determined that [B] is second order?
I determined [A] by solving as I have previously seen: 1.25^a/2.5 = 8.7/17/4 => 0.5^a=0.5 (first order)

Paula Dowdell 1F

Why is it that half lives of 0-order reactions and 2nd-order reactions are "not used"? (pg. 631 in the book)

Paula Dowdell 1F

For the problem shown in example 15.4 we are supposed to, "Confirm that the reaction is first order in C3H6 and calculate the rate constant". The example then tells us to "Plot the natural logarithm of the reactant concentration as a function of t. If a straight line is obtained, the ...

Paula Dowdell 1F

Can someone explain to me how to determine what kind of order a reaction is? Thanks.

Paula Dowdell 1F

Can someone explain to me why the H+ is added in this cell diagram?

The answer is: Pt(s)|Fe3+(aq), Fe2+(aq)||H+(aq), MnO4-(aq), Mn2+(aq)|Pt(s)

Paula Dowdell 1F

I am also wondering this.

Paula Dowdell 1F

How do we know that this salt bridge is necessary?

Paula Dowdell 1F

Write the half-reactions and devise a galvanic cell (write a cell diagram) to study each of the following reactions: (b) H+(aq) + OH-(aq) --> H2O(l), the Brønsted neutralization reaction Are we given the half reactions for this problem or is there some way to figure it out without? I don't know when...

Paula Dowdell 1F

Why is it that the cell diagram is set up like Ag(s)|AgBr(s)|Br-(aq)||Ag+(aq)|Ag(s) ? I thought that it would be set up like Br-(aq)|AgBr(s)|Ag(s)||Ag+(aq)|Ag(s) The cathode shows that Ag+ is reduced to form Ag, which is why they are in the order of Ag+,Ag but how does the order on the anode show th...

Paula Dowdell 1F

I need help with this too! I do not understand how to get the anode half reaction.

Paula Dowdell 1F

A Galvanic cell converts chemical energy into electrical energy. A voltaic cell is an electrochemical cell that uses a chemical reaction to produce electrical energy.

Paula Dowdell 1F

It's good to note that work is zero if applied force is zero (W=0 if F=0).

Paula Dowdell 1F

A state function describes the equilibrium state of a system. Internal energy, enthalpy, and entropy are state quantities because they describe quantitatively an equilibrium state of a thermodynamic system, irrespective of how the system arrived in that state.

Paula Dowdell 1F

Hi! This relates to question 9.73 in the book: "Under what conditions, if any, does the sign of each of the following quantities provide a criterion for assessing the spontaneity of a reaction? (a) G ; (b) H ; (c) S ; (d) delta S total" I thought that if delta S total was negative the reac...

Paula Dowdell 1F

Can someone explain the reasoning behind this to me? "Which substance in each of the following pairs has the higher molar entropy: (a) 1 mol CO2(g) at 25 C and 1 bar or 1 mol CO2(g) at 25 C and 3 bar" A: CO2(g) at 1 bar, because disorder increases with volume. I thought bar corresponded to...

Paula Dowdell 1F

But how do you know that "for a change in temperature, the energy supplied as heat is related to the increase in temperature by the heat capacity, C". Is that always true?

Paula Dowdell 1F

What does an increase in disorder correspond to? An increase or decrease in entropy?
And why?

Paula Dowdell 1F

"During the test of an internal combustion engine, 3.00 L of nitrogen gas at 18.5 C was compressed suddenly (and irreversibly) to 0.500 L by driving in a piston. In the process, the temperature of the gas increased to 28.1 C. Assume ideal behavior. What is the change in entropy of the gas?"...

Paula Dowdell 1F

Hi!

How come in question 9.9 you use just R instead of 3/2R when determining the change in entropy?
The question is: Calculate the entropy change associated with the isothermal expansion of 5.25 mol of ideal gas atoms from 24.252 L to 34.058 L.

Thanks!

Paula Dowdell 1F

it's good to note that the entropy of a system decreases only when it interacts with some other system whose entropy increases in the process.

Paula Dowdell 1F

It is always good to note that if you increase temperature, you increase entropy.

Paula Dowdell 1F

What Do You Get When You Cut An Avocado Into 6X10^23 Pieces?
Guacamole

Paula Dowdell 1F

just a fun fact to note: the entire universe is an isolated system!

Paula Dowdell 1F

The second law of thermodynamics states that an isolated system's entropy never decreases!

Paula Dowdell 1F

Another way of thinking about it is that an adiabatic process is a process where the net heat transfer to the working gas is zero vs an isolated system, which is a system that is completely shut off from the surroundings.

Paula Dowdell 1F

Hi!
I'm confused on the work calculation in 8.9!

w= -((750 Torr)/(760 Torr x atm^-1)) ((1846mL - 345mL)/(1000mL x L^-1)) = -1.48 L x atm

where did the 760 Torr & the 1000mL come from in the denominators of this work calculation?

Thanks!

Paula Dowdell 1F

Hi!

I am confused about how they found the heat required (delta T) in example 8.3 from the book on page 270. Can someone walk me through this part?

Thanks!

Paula Dowdell 1F

Can somebody explain Hess' Law and its use to me?

Paula Dowdell 1F

To measure heats of reactions, we enclose reactants in a calorimeter, then we initiate the reaction, and then measure the temperature difference (before and after the reaction). The temperature difference allows us to evaluate the heat released in the reaction.

Paula Dowdell 1F

It would be very helpful to google the "heating curve" graph!

Paula Dowdell 1F

why is it that in the reaction: "B(OH)2 + 2 H2O(l) ⇌ H3O+(aq) + B(OH)4-(aq)", B(OH)2 serves as a lewis acid. I thought that it was donating electron pairs? Can someone explain this to me in detail?

Paula Dowdell 1F

Why is it that in 12.33 the solutions manual says the mass of Na2O is 18mol/L?
I'm confused because the concentration of the original solution is also 18mol/L and i did not think mass and concentration were the same thing.
Thanks!

Paula Dowdell 1F

Can someone walk me throught the steps (including the net ionic equation) of figuring out whether NH4I(am) + KNH2(am) -> KI(am) + 2NH3(l)
can be classified as a rxn between bronsted acids and bases?

Paula Dowdell 1F

Hello on J5 c) I am very confused!

Why is the net ionic eq of LiOH + HI -> H2O + LiI equal to OH + H -> 2H2O? I thought it would just be OH + H -> H2O.
Thanks!

Paula Dowdell 1F

For the same reasons NO is x-0.240

Paula Dowdell 1F

Hi! I am having trouble figuring out how to approach this problem. Can someone walk me through their way of going about this problem? What is delta H? Q: Predict whether each of the following equilibria will shift towards products or reactants with a temperature increase: a) N2O4(g) ⇌ 2NO2(g), delta...

Paula Dowdell 1F

It is helpful to note that a pH balance of 7 is ideal for neutral water solutions! In this situation, acids and bases neutralize each other's effects and characteristics.

Paula Dowdell 1F

Hi! I was wondering when the syllabus would be posted for 14B. I am assuming that the 40minute tests we take in discussion will also be a part of 14B and knowing the dates of those tests would be helpful as I am signing up for my winter quarter work shifts soon! Thank you!

Paula Dowdell 1F

tetraamminediaquacobalt(III)bromide

How come the answer, "[Co(NH3)4(H20)2]Br3" Bromide has a subscript of 3?

Paula Dowdell 1F

What are the common Latin Stems of elements we are expected to know?

Paula Dowdell 1F

Taylor 1F wrote:We have to memorize them, but we only need to know like common ones(i.e numbers 1-8 prefixes)

Where can you find these numbers 1-8 prefixes?

Paula Dowdell 1F

Are we expected to memorize the latin stems of elements? I know you can find them in appendix 2D but do we need to know all of them? Or are there just a few common ones we should know?

Paula Dowdell 1F

Hi I'm a little confused on when to know if the question is asking for a shape that ignores or includes lone pairs in its answers. 4.11 asks to give the "VSEPR formula and predict its shape" and by judge of it's answers they are taking lone pairs into consideration since the answers are a)...

Paula Dowdell 1F

something that might be helpful: Lone pairs of electrons take up more space than bonded pairs!

Paula Dowdell 1F

I would note that you need to first determine if the bonds between the atoms are polar! Some shapes will cause dipoles to cancel out.

Paula Dowdell 1F

but are metalloids considered non-metals for periods 3 and on?

Paula Dowdell 1F

Hi,
I read in the textbook that period three and on "non-metal" atoms may expand their valence shells. Are metalloids in this case considered non-metals?

Paula Dowdell 1F

Hey! I know the answer to this question is B, but can someone explain to me why A, C, and D are false? 21. When a solution is diluted what is the main concept that one must be aware of? A. The moles of solute change. B. The moles of solute stay the same. C. The volume of solvent stays the same. D. T...

Paula Dowdell 1F

I'm confused by this question because when i divide 0.0594mol H/0.16996 I get 2.99 (approximately 3) and then when dividing 0.16996mol C/0.16996 I get 1. This leaves me with CH(sub)3 when I really should be getting C(sub)2 H(sub)6
What am I doing wrong?

Paula Dowdell 1F

hi!
What are the coefficients in chemical equations called?
A. Mass-ratio coefficients

B. Stoichiometric coefficients

C. Molar coefficients

D. Reaction coefficients

I thought they were called Molar coefficient but turns out I am wrong! What is the right answer?

Paula Dowdell 1F

Hi! The question is how many milliliters of the stock solution would you need to use to make 250 mL 0.650 M NaOH solution.
How come the answer is 81.2mL? Thanks!

Paula Dowdell 1F

What does size mean when referring to octet exceptions?

Paula Dowdell 1F

When do you know that your lewis structure has a good formal charge?

Paula Dowdell 1F

Hi! I am having some trouble understanding this concept. In the textbook Example 2.1 shows how to calculate the probability of finding an electron, but I am still confused. The question/answer for the problem is as shown below: Can someone walk me through this? Q: Suppose an electron is in a 1s orbi...

Paula Dowdell 1F

What is the difference between ionization and ionization energy? Is there a limit to ionization energy?

Paula Dowdell 1F

Can somebody explain the concept of particle in a box to me?

Paula Dowdell 1F

the extent of change in the electrical field is referring to the amplitude!

Paula Dowdell 1F

It is important to note that all of these instances (a)-(d) can be answered by using E=hv and v(lambda)=c (a) speed does not decrease because it is constant (b) the wavelength does not decrease because frequency is proportional to (lambda)-1 (c) Yes! as frequency decreases the waves get wider and th...

Paula Dowdell 1F

Could somebody explain to me the difference in Thomson and Rutherford's models of the atom?

Paula Dowdell 1F

Hi I am having trouble with part b of E.29. (a) 0.0417 mol CuCl2 x 4H2O (b) The questions asks how many moles of Cl- ions are present in the sample. The book says to double the amount in part (a) but i don't really understand why. If it is asking specifically how many moles of Cl- ions are present w...

Paula Dowdell 1F

I had the same problem! Just note that the question says "metal hydroxide". You can assume that M=metal and hydroxide= (OH)2.

Paula Dowdell 1F

Hi! In question Self-Test E.2A I am trouble figuring out which number of significant figures to use. Question: The mass of a copper coin is 3.20g. Suppose it were pure copper. (a) How many moles of Cu atoms would the coin contain, given a molar mass of Cu of 63.55 g/mol? (b) How many Cu atoms are pr...

Paula Dowdell 1F

Is there any reason why you have to change pm to m? Do you typically change to m in most chemistry problems?

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