CHEMISTRY COMMUNITY

Hailey Kim 4G

I thought different species in the same state are separated by , not | ? Since Ag and AgBr are both solids, shouldn't it be |Ag(s), AgBr(s)| ?

Hailey Kim 4G

Kassidy Ford 1I wrote:^^^ does this mean that it is literally Gibbs free energy??

Yes, free energy of activation refers to Gibbs free energy.

Hailey Kim 4G

Reaction intermediates are formed in one step and then consumed in a later step of the reaction mechanism. Therefore, intermediates are not listed in rate laws.

Hailey Kim 4G

E cell is the cell potential at non-standard state conditions. You would use the Nernst Equation to find E cell.
E standard cell is the standard cell potential, which means concentrations are 1 M and pressures are 1 bar.

Hailey Kim 4G

No, that is not always the case. A good example is homework problem 7A.15. The given reaction is 2A(g) + 2B(g) + C(g) ---> 2G(g) + 4F(g). Even though A has a coefficient of 2, the data shows that reactant A is first order.

Hailey Kim 4G

You can compare experiments 1 and 3 since the concentration of B is changing and the concentration of C is constant.

Hailey Kim 4G

The standard reduction potential for Sn2+ + 2e- ---> Sn is -0.14V
The standard reduction potential for 2H+ + 2e- ---> H2 is 0.0V

Therefore, tin would be the anode because you would flip the sign to get a positive standard cell potential (+0.14V).

Hailey Kim 4G

You include Pt(s) when there is no solid metal conductor. For example, for problem 6M.11 (part a), the cell diagram would be:
| Pt(s) | Ti3+(aq), Ti2+(aq) || Co2+(aq) | Co(s)

Hailey Kim 4G

The number of electrons transferred is determined by your half-reactions. n is equal to the same number of electrons needed (in both the oxidation and reduction reactions) to balance out the charge between the reactants and products.

Hailey Kim 4G

Pt(s) is necessary because the oxidized and reduced species are in the same solution.

Hailey Kim 4G

Chem_Mod wrote:Can you please post the question.

6M.7 Arrange the following metals in order of increasing strength as reducing agents for species in aqueous solution:
(a) Cu, Zn, Cr, Fe

Hailey Kim 4G

6L.5 (d)
Au+(aq) ---> Au(s) + Au3+(aq)

Can someone please explain how to write the oxidation half-reaction? The solutions manual says the oxidation reaction should be:
Au3+(aq) + 3e- ---> Au(s)

But why isn't it Au3+(aq) + 2e- ---> Au+(aq) ?

Hailey Kim 4G

Pt(s)|O2(g)|H+(aq)||Cr2O7^-2(aq),H+(aq),Cr^3+(aq)|Pt(s)

How would you write the half reaction for the anode?

Hailey Kim 4G

6N.13 Calculate the reaction quotient, Q, for the following cell reactions, given the measured values of the cell potential. a) Pt(s)|Sn^4+(aq),Sn^2+(aq)||Pb^4+(aq), Pb^2+(aq)|C(gr) When I tried to calculate Q, I got 2.68 x 10^6, but the solutions manual says the answer is 10^6. Can someone please e...

Hailey Kim 4G

If you use n=2, make sure you square the concentrations of your products and reactants for your value of Q (due to the coefficient of 2 in the H+). E should equal 0.06 V.

Hailey Kim 4G

For part b, why does the solutions manual use n=2 instead of n=1 since there is only 1 mol of e- transfer? For K, I got 1x10^2 instead of 1x10^4.

Hailey Kim 4G

For homework problem 6M.7 part a, how do you know which standard reduction potential you should use for Cr? When Cr loses 2 e-, the standard reduction potential is -0.91, but when it loses 3 e-, it is -0.74. The one I use will determine if Cr is a stronger reducing agent than Zn.

Hailey Kim 4G

Gibbs free energy is in J/mol and temperature is in Kelvins. Therefore, you would use 8.314 J/mol.K in order to cancel out all the units and find a value for K (which does not have any units).

Hailey Kim 4G

H2 has a neutral charge because it is the most stable form of hydrogen. Other then that, the oxidation number of hydrogen is +1.

Hailey Kim 4G

A reaction is spontaneous when Delta(G) is greater than 0, or a negative value. When Delta(G) is a positive value, the reaction is not spontaneous, and the reverse reaction is favored. Finally, when Delta(G) is equal to 0, the reaction is at equilibrium.

Hailey Kim 4G

Since the heat required depends on the amount of the substance, heat capacity is an extensive property. Specific heat capacity and molar heat capacity are intensive properties because they are the heat required to raise the temperature of 1 g or 1 mol by 1 degree Celsius.

Hailey Kim 4G

Under acidic conditions, we are allowed to add H+ or H20 and under basic conditions, we are allowed to add OH- or H20. We do this to balance the number of atoms in the chemical equation. For example, in an acidic solution, if there is an oxygen atom in the given equation, you would have to add the a...

Hailey Kim 4G

Since internal energy (U) and entropy (S) are state functions, you can look at their initial and final values. Since they are the same (the system goes back to its original volume and pressure), the change of internal energy/entropy is 0.

Hailey Kim 4G

When the volume is constant, w=0. If the problem gives you the amount of energy that increased/decreased (q) after doing a certain amount of work (w) is when you can incorporate both q and w for the change in internal energy.

Hailey Kim 4G

Sartaj Bal 1J wrote:Why is the value of Cp,m 75.3 J K^-1 mol^-1 in this problem?

In this problem, H2O is a liquid, not a gas. Cp(liquid)=75.3 J/K*mol, which is given on our equations sheet.

Hailey Kim 4G

Can someone help explain why and how the equation for 3b includes (1/2)(m)(delta H of fusion)? I get what you would isolate to get the answer but I'm not sure how they got that part of the equation. For problem 3B, you have to use the equation q=mC(Delta T) but also the phase change equation q=m(de...

Hailey Kim 4G

Can someone show me step by step on how to solve for number 10. I did not understand why q(ice)=q(water). CHEM MOD: q(ice) = -q(water) DO NOT forget the negative sign. The heat released from one material in a system is absorbed by the other material in the system. For #10, why do we have to convert...

Hailey Kim 4G

Can heat capacities be negative for simple molecules?

Hailey Kim 4G

Since the equations for Cv and Cp are given on the equation sheet for monatomic gases, I would try to memorize the equation for linear and non-linear. For linear: Cv=(5/2)R, Cp=(7/2)R For nonlinear: Cv=3R, Cp=4R However, if you memorize the equation Cp=Cv+R, then you can use this equation to figure ...

Hailey Kim 4G

A bomb calorimeter measures specific heat at a constant volume.

Hailey Kim 4G

Delta H is equal to q when the heat released or absorbed is at a constant pressure.

Hailey Kim 4G

I tried to go to this review session also and when I got there the door was locked and the lights were turned off. I ran into the UA there and he also seemed confused as to why the room was not open.

Hailey Kim 4G

When calculating the first entropy change, heat is transferred FROM the reservoir at 800 K (meaning that heat is lost), which is why q is negative.

Hailey Kim 4G

In a closed system, energy can be exchanged with its surroundings. For example, if you put a lid on a pot, matter cannot be exchanged, but energy can still be transferred. In an isolated system, nothing is exchanged with its surroundings. An example of this is a thermos; however, it is not 100% perf...

Hailey Kim 4G

No system is ever 100% isolated.

Hailey Kim 4G

The change in internal energy is equal to the work (which was calculated in part A) in absence of other changes. My solution manual also said the answer was 28 J.

Hailey Kim 4G

Enthalpy, which is represented by delta(h), is heat at a constant pressure. Heat is represented by "q", and because the pressure is constant, it will have the subscript "p" for pressure, rather than "v" for volume.

Hailey Kim 4G

The change in internal energy is represented by delta(U). Delta(U) is equal to the energy transferred by heating (q) + the energy transferred by compression (w).

delta U = q + w

Hailey Kim 4G

The lower the pKa, the stronger the acid. The lower the pKb, the stronger the base. An easy way to check to see if you have the correct values is to add pKa and pKb and make sure the sum is equal to 14.

Hailey Kim 4G

I found the UA learning sessions to be the most helpful to prepare for the tests. It is also beneficial to do as many of the homework problems as you can.

Hailey Kim 4G

Megan Kirschner wrote:I also don't understand why that one is 2 sig figs. Is there a place I could get good review on sig figs?

I went to another review session and the TA said that there are only two sig figs because when dealing with pH, you look at the number of sig figs AFTER the decimal.

Hailey Kim 4G

Matt Sanruk 2H wrote:Do we get points off for not doing ICE boxes?

I would make an ICE box just to be safe. This way, you can show all your work and less likely to make errors.

Hailey Kim 4G

Can you please explain why for #2, the answers only have two sig figs?

Hailey Kim 4G

As a general rule of thumb, can we assume the approximation rule whenever K < 10^-5 or K < 10^-3? Which value of K is it? (assuming that you always check if the approximation is valid after solving for x) How do you check if the approximation is valid? Do you look at what you solved for x and see i...

Hailey Kim 4G

Yes, the pH can be less than 0 and greater than 14; however, is it more likely for the pH to fall in the 0-14 range.

Hailey Kim 4G

The pH increases as the solution becomes more basic (which means a higher concentration of OH- ions and a lower concentration of H+ ions) and decreases as the solution becomes more acidic (which means a lower concentration of OH- ions and a higher concentration of H+ ions).

Hailey Kim 4G

An inert gas does not chemically react, which means it does not change under given conditions. Noble gases are inert gases (examples include helium, argon, and neon).

Hailey Kim 4G

If a pH is less than 0 or greater than 14, that means the concentrations of H30+ or OH- are larger than one molar. However, the concentration of hydrogen ions are usually between 1 M or 10^-14 M.

Hailey Kim 4G

If a reaction is endothermic, that means it absorbs heat and heat is used as a reactant. In endothermic reactions, an increase in temperature (heat) leads to an increase in reactants. According to Le Chatelier's Principle, the reaction would then shift to the products to reach equilibrium.

Hailey Kim 4G

When/where can we pick up our Chem 14a final from last quarter?

Hailey Kim 4G

K is the equilibrium constant, while Q is the reaction quotient. They are calculated the same way but you can compare Q and K to determine if the forward reaction or the reverse reaction is favored. If Q<K at some time during the reaction, then reactants are favored. If Q>K at some time during the r...

Hailey Kim 4G

The value of K would be 1. However, it is very rare.

Hailey Kim 4G

Madeline Phan 1J wrote:when writing the expression for k, how do you know when to use brackets or to write p in front?

You use brackets to represent concentration and you use P to represent partial pressure. For a gas, we use its partial pressure and the equilibrium constant is denoted by Kp.

Hailey Kim 4G

Le Chatelier's Principle states that chemical reactions adjust so as to minimize the effect of any changes (such as the change in concentration, pressure, or temperature). When a reaction undergoes these changes, the equilibrium will shift and favor either the formation of products or reactants (dep...

Hailey Kim 4G

The ideal gas law allows you to be able to convert between partial pressure and concentration.

PV=nRT
P=(n/v)RT
P=(conc)RT

Hailey Kim 4G

There would be a dipole moment so it would be polar.

Hailey Kim 4G

You would look at the bond strength. For part a, HCl has a weaker bond strength than HF, which means it is a stronger acid. However, for part b, you would base the strength of the acid on the amount of oxygen atoms. HClO2 has more O atoms than HClO, which is why it is the stronger acid.

Hailey Kim 4G

I think it would be safe to write out the whole name.

Hailey Kim 4G

What are some important coordination compounds that we should know their biological significance for?

Hailey Kim 4G

Lewis acids would have the same qualities as Bronsted acids. However, a Bronsted acid donates H+, so there would have to be a proton transfer if a chemical reaction wants to be classified as a reaction between a Bronsted acid and base.

Hailey Kim 4G

What are some common drinks, food, or household items that we should be able to identify as acidic or basic for the final?

Hailey Kim 4G

Dipole moments cancel out in non-polar molecules. This means that the molecule is symmetrical. Polar molecules have dipole moments because there is a separation of charge. Tetrahedral molecules are only non-polar when all four atoms bonded to the central atom are the same element.

Hailey Kim 4G

The stronger the bond, the weaker the acid. For example, HF is a very weak acid because it has a very strong bond (it is also a large molecule). The higher the electronegativity, the greater the attraction. Increasing electronegativity decrease basicity (which means an increase in acidity) because t...

Hailey Kim 4G

A Bronsted acid is a proton donor and a Bronsted base is a proton acceptor. A Lewis acid is an electron pair acceptor and a Lewis base is an electron pair donor.

Hailey Kim 4G

A Bronsted acid is a species that is capable of donating a proton. HBr classifies as a Bronsted acid because it donates a H+ to water (the Bronsted base). HBr donates its proton to become Br-.

Hailey Kim 4G

Acids increase the concentration of H+ ions when dissolved in water, are proton donors, have a pH less than 7, and are sour in taste. Bases increase the concentration of OH- ions when dissolved in water, are proton acceptors, have a pH greater than 7, and are bitter in taste.

Hailey Kim 4G

The question is asking what type of force can occur for a certain molecule if it is paired up with that same molecule. So for SO2, since it is polar, it has dipole-dipole forces and London Dispersion forces. Dipole-induced dipole or ion-dipole forces would only occur if SO2 was paired with a differe...

Hailey Kim 4G

London Dispersion forces are also known as dipole-induced dipole interactions.

Hailey Kim 4G

The most electronegative elements are located in the upper right-hand corner of the periodic table. So as you move across a row from left to right, electronegativity increases. As you move down a group, electronegativity decreases. When drawing the lewis structure of a molecule, if it is symmetrical...

Hailey Kim 4G

Hydrogen bonding will ONLY occur is hydrogen is bonded to a nitrogen, oxygen, or fluorine atom.

Hailey Kim 4G

The lone pair in the seesaw shaped molecule affects all angles in the molecule. So the bond angles will be <90 degrees and <120 degrees.

Hailey Kim 4G

An easy way to determine if a molecule is polar or non-polar is to draw the Lewis Structure and see if it is symmetric or non-symmetric. If the Lewis Structure is symmetric, the molecule is non-polar, which also means it only has non-polar bonds. If the Lewis Structure is non-symmetric, the molecule...

Hailey Kim 4G

Q2B. A beam of light at constant intensity and increasing frequency is shown on a metal surface. A detector reads the number of electrons ejected from the sheet of metal. The graph below has a frequency of the incoming light on the X axis and the number of the ejected electrons on the Y axis. Draw t...

Hailey Kim 4G

For the midterm Q2A, I understand we need to convert 492 kJ.mol^-1 to J, but why do we first have to divide it by Avogadro’s number? When do we use Avogadro’s number?

Hailey Kim 4G

It helps if you draw out the Lewis Structure for both molecules. In diethyl ether, there is no hydrogen bonding, while in butanol, there is hydrogen bonding. Hydrogen bonding is the strongest intermolecular force, which means it has a higher boiling point.

Hailey Kim 4G

Both CHI3 and CHF3 are polar and have London Dispersion forces. However, CHI3 has a stronger London Dispersion force because CHI3 is a much bigger molecule than CHF3. Iodine has a greater molar mass than fluorine, and the bigger the atom, the stronger the intermolecular force. This would then requir...

Hailey Kim 4G

I don't believe so. I would only worry about the homework due in your discussion class.

Hailey Kim 4G

Madeline Phan 4H wrote:So for our midterm, we will just get a raw score for now?

Yes. Today in lecture, Professor Lavelle said that midterm scores will be released next Wednesday.

Hailey Kim 4G

When is test 2?

Hailey Kim 4G

How do you know that n1=1?

Hailey Kim 4G

for dino nuggets #4 why is the answer 2 sig figs and not 3? is it because 250 mL has only 2 sig figs? i'm a bit confused if that's the reason or if it's something else - if someone could clarify that would be great! thanks so much Yes, for #4, the answer only has 2 sig figs because 250 mL has only ...

Hailey Kim 4G

If you have a compound like ClO, which has 13 valence electrons, why would chlorine have the unpaired electron and not oxygen? What determines which atom gets the unpaired electron?

Hailey Kim 4G

When drawing the possible Lewis Structures for ClNO2, why can only the oxygens be double bonded, and not the chlorine?

Hailey Kim 4G

For part a, [Ar]3d^6 has a total of 24 electrons, which is chromium. However, since the unknown metal has an additional 3 electrons, you would find the element that has 27 electrons, which is cobalt. The answer to part a would be Co^3+. You can apply the same process to parts b, c, and d.

Hailey Kim 4G

How do you know if an element can exceed the octet rule?

Hailey Kim 4G

Yes. Odd-electron molecules, electron-deficient molecules, and hypervalent molecules are all exceptions to the octet rule. An odd number of valence electrons can lead to unpaired electrons, as well as the central atom having more or less electrons needed.

Hailey Kim 4G

How do you draw the lewis structure for bromine trifluoride? I know it has 28 valence electrons, but when I draw the lewis structure I can't figure out how to draw the bonds in a way to complete the octet.

Hailey Kim 4G

When drawing lewis structures, how do you know which atom is the central atom? I understand that for CCl4 the central atom would be carbon, but what about for compounds like NHF2?

Hailey Kim 4G

Which M^3+ ions (where M is metal) are predicted to have the following ground-state electron configurations:
a) [Ar]3d^6
b) [Ar]3d^5
c) [Kr]4d^5
d) [Kr]4d^3

I'm super confused on how to go about this problem.

Hailey Kim 4G

In row 5 on the periodic table, why does the electron configuration for Niobium (Nb) have "5s1" and not"5s2"?

Hailey Kim 4G

Does anyone know if Professor Lavelle is planning on uploading more video modules on to his website? Or is the "Heisenberg Uncertainty Principle" going to be the last video?

Hailey Kim 4G

How do you find the number of valence electrons a transition element has by using the periodic table?

Hailey Kim 4G

An easy way to find the number of valence electrons an element has is to use the periodic table. The group number (column) represents the number of valence electrons, excluding the transition elements in groups 3-12. Group 1 = 1 e- Group 2 = 2 e- Group 13 = 3 e- Group 14 = 4 e- Group 15 = 5 e- and s...

Hailey Kim 4G

When drawing Lewis Structures, how do you know when you're supposed to use a single or double bond?

Hailey Kim 4G

Yes. During my lecture class, Professor Lavelle worked the problems out on the board.

Hailey Kim 4G

When drawing spin, is there a reason why only half of the arrow point is used? I've seen it done both ways. In high school, I was taught to only draw half of the arrow point but Professor Lavelle draws the whole arrow point. I don't think it really matters as long as the arrow is pointing in the ri...

Hailey Kim 4G

Does anyone have any helpful study tips to prepare for the upcoming midterm? What topics should I review?

Hailey Kim 4G

Hund's rule states that due to e- repulsion, e- in the same subshell occupy different orbitals with parallel spin. This allows for the lowest energy state. All orbital shells must be occupied by one e- before an orbital can be paired up with two.

Hailey Kim 4G

If n=4, there would be four subshells: s, p, d, f.

Hailey Kim 4G

Homework for week 4 should be from the problems listed under the Quantum World since we are still covering topics from that unit.

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