CHEMISTRY COMMUNITY

vanessas0123

For w, when is it positive? Is it when work is being done on the system or when the work is being done by the system?

vanessas0123

Balancing acidic reactions means adding H2O to balance oxygens, and H+ to balance hydrogen.
Balancing basic reactions means adding H2O to balance oxygens, and OH- to balance everything else.

vanessas0123

My TA put it in her mailbox and she posted the grades on my.ucla!

vanessas0123

Ecell is the cell potential for the half reaction.
E standard cell is the overall cell potential (Ecathode - Eanode)

vanessas0123

A dead battery means that the reaction is at equilibrium, so Ecell and delta G is 0.

vanessas0123

When would you use E=E°-(RT/nF)lnQ vs E=E°-(0.05916V/n)logQ ?

Also, I understand that you can substitute RT/F for 0.025963 for the ln equation when its under standard condition - 25 degree celsius. Is this correct?

vanessas0123

In the cell diagram Hg|Hg2^2+||NO3-,H+|NO|Pt, the half reactions are like this: 3e- + NO3- -> NO (red potential: 0.96V) 2Hg -> Hg2^2+ + 2e- However, only the reduction potential of 2e- + Hg2^2+--> 2Hg is given (0.79V). Why don't we flip the sign since its supposed to be an oxidation reaction (0-->1+...

vanessas0123

How do you find N in the Nernst equation?

vanessas0123

Whats the difference between Ecell and Ecell° ?
Under which conditions would you need to find Ecell and Ecell° 2 and which equations would you use?

vanessas0123

2NO3-(aq)+8H+ (aq)+6 Hg(l)-> 3Hg2^2+(aq)+2NO(g)+4H2O If the cell diagram is Hg(l)|Hg2^2+||NO3-(aq),H+(aq)|NO(g)|Pt(s), how do you find the cell potential? If I am correct, the cell potential of the cathode is 0.96V. How do you find the potential of the anode or Hg(l)/Hg2^2+(aq)? I cannot find it in ...

vanessas0123

When and where would you add Pt(s) into the cell diagram? Could you give an example?

vanessas0123

They are both aqueous, so it's separated by a comma!

vanessas0123

The salt bridge allows for transfer of electron and charge from anode to cathode, balancing the charges between the half cells.

vanessas0123

In acidic solution: You add H2O to balance the oxygen first, and H+ to balance the hydrogen.
In basic solution: You add OH- to balance the oxygen first, and H2O to balance the hydrogen.

vanessas0123

How do you find n in -nFE?

vanessas0123

When would you flip the sign in a half reaction? Do you flip the sign of the given reduction potential is the reaction is oxidation?

vanessas0123

Appendix 2B or just google it!

vanessas0123

I think it's 5 posts per week by Sunday night (every week of the quarter).

vanessas0123

What are half reactions?

vanessas0123

A salt bridge neutralizes the charges of the half cells by allowing for ion transfer.

vanessas0123

Can someone outline the steps of balancing a redox reaction?

vanessas0123

Most of the time H has a charge of +1. However, when its bonded to a cation or a metal, it will have a charge of -1.

vanessas0123

When would you use the equation deltaSrev/T? And does this only work for reversible reactions?

vanessas0123

What is the difference between delta S and delta S total?

vanessas0123

Yes! S=kBlnW, and W= degeneracy aka (# of orientations/positions)^(# of atoms/ molecules).

vanessas0123

What's the difference between the entropy of the surrounding, system, and total entropy? And how would you find these values in isothermal reversible and irreversible expansions?

vanessas0123

You should check the constants list and if it explicitly states that it's at STP and not 298K!

vanessas0123

What's the difference between delta U and delta H?

vanessas0123

An ideal gas in a cylinder was placed in a heater and gained 5.50 kJ of energy as heat. If the cylinder increased in volume from 345 mL to 1846 mL against an atmospheric pressure of 750. Torr during this process, what is the change in internal energy of the gas in the cylinder? Which equation would ...

vanessas0123

How do you know if the work is done on or by the system? Which one would be negative and which one would be positive?

vanessas0123

Because q and w are both in kilojoules, you don't need to do anything with the units!

vanessas0123

A constant-volume calorimeter was calibrated by carrying out a reaction known to release 3.50 kJ of heat in 0.200 L of solution in the calorimeter (q 5 23.50 kJ), resulting in a temperature rise of 7.32 8C. In a subsequent experiment, 100.0 mL of 0.200MHBr(aq) and 100.0 mL of 0.200M KOH(aq) were mix...

vanessas0123

pKa = -logKa
pKb = -logKb

pka + pkb = 14

vanessas0123

You use Kc when given concentrations, and Kp when given partial pressures.

vanessas0123

x is too small if it's < 10^-3.

vanessas0123

What does it mean for a gas to be ideal / act ideally?

vanessas0123

Look at the units you have and match it with the R constant's units!

vanessas0123

When you reduce a reactant, why does the equilibrium shift in the reverse direction?

vanessas0123

Can you calculate Kp if you are not given the individual partial pressures? ICE table is only for concentration right?

vanessas0123

At 500. K, the equilibrium constant for the reaction Cl2(g)+ Br2(g)--> 2 BrCl(g) is Kc 5 0.031. If the equilibrium composition is 0.495M Cl2 and 0.145M BrCl, what is the equilibrium molar concentration of Br2?

vanessas0123

You use the Principle any time a change is made to the reaction at equilibrium. The reaction will shift to minimize the effect of the change due to Le Chatelier's Principle.

vanessas0123

You calculate Kc if given concentration. Kp if partial pressures are given.

vanessas0123

Q can be calculated at any point during the reaction, including the equilibrium point. Thus, if Q = K, that means that the reaction is at equilibrium.

vanessas0123

Le Chatelier's principle states that a system in equilibrium will shift right / forward or left / backwards in response to a change in pressure, temperature, volume, concentrations etc. in order to minimize its effects on the reaction. The reaction will shift in order to keep a "balance".

vanessas0123

Can you use ICE tables to calculate partial pressures for gases as well?

vanessas0123

I don't think it matters that much as long as the topics are relevant! Practicing acid and bases would be helpful though.

vanessas0123

At 500. K, the equilibrium constant for the reaction Cl2(g) Br2(g) ∆ 2 BrCl(g) is Kc 0.031. If the equilibrium composition is 0.495 mol L 1 Cl2 and 0.145 mol L 1 BrCl, what is the equilibrium concentration of Br2?

vanessas0123

Yes. You calculate it the same way as K, except the reaction is simply not at equilibrium.

vanessas0123

Solids and liquids don't have concentrations nor partial pressures.

vanessas0123

What's the difference between Q and K?

vanessas0123

Q is used at any point of the reaction, while K is calculated when the rxn is at equilibrium.
If Q<K, then the forward reaction will proceed (to make more products).
If Q> K, then the reverse reaction will proceed.

vanessas0123

Large K value (K>10^3) means that there are more products @ eq and that the "equilibrium sits to the right."
Small K value (K<10^-3) means that there are more reactants @ eq and that the "equilibrium sits to the left."

vanessas0123

[concentration].
(pressure) for when there are subscripts / coefficients before the products or reactants.
K values don't have units because they cancel out, from what I know.

vanessas0123

Why is H2CO3 more acidic than H2GeO3 (besides the fact that carbonic acid is a known strong acid)?

vanessas0123

Why is HClO2 more acidic than HBrO2? I initially thought HBrO2 was more acidic as it has a larger atomic radius (negative charge more dispersed --> more stable ... etc). However, Cl is more electronegative than Br, which makes it more acidic. When would you look at atomic radius vs electronegativity?

vanessas0123

Section 6C.4 of the textbook says that the greater the number of oxygens attached to the central atom (or the greater the oxidation number), the stronger the acid. Why?

Thanks :)

vanessas0123

Why is it that when the conjugate base of an acid has more electronegative atoms, it is more acidic?

vanessas0123

Question 6A.13

Draw the Lewis structure or symbol for each of the following species and identify each one as a Lewis acid or Lewis base

Why are Ag+ and BF3 lewis acids? How do you determine this through drawing the lewis structure?

vanessas0123

Why is the hybridization of P in PCl6 sp3d2?

vanessas0123

How do you figure out which direction to draw your dipoles in a lewis structure?

vanessas0123

Resonance structures are different ways that the Lewis structure can be drawn. For example, one structure might have a double bond on an atom while another has a double bond on another atom. Both would work thus they are resonance structures.

vanessas0123

Intermolecular forces are the significant factors affect melting points! The stronger/ more intermolecular forces, the higher the melting/boiling points.

vanessas0123

Why are sigma bonds are generally stronger than pi bonds? What is their difference?

vanessas0123

In a molecule, do you draw the arrow/ dipole towards the δ- charge or the δ+ charge?

vanessas0123

A is the central atom, so it's always 1.
X is the number of atoms bonded to the central atom.
E represents the number of lone pairs

vanessas0123

Triple bond strength > double bond > single bond.

Think of having one rope being pulled vs two ropes - the one with the two ropes is stronger thus shorter!

vanessas0123

3F! Honestly anything is fine, as long as it is close to what we are learning in class.

vanessas0123

Why's the best / most stable resonance structure the one with the least formal charge?

vanessas0123

There are london forces among all molecules due to the random movement of electrons!

vanessas0123

Double/triple bonds have a stronger attraction, thus they are shorter than single bonds!

vanessas0123

Double/triple bonds have a stronger attraction, thus they are shorter than single bonds!

vanessas0123

Sum up to the overall charge!

vanessas0123

What are sigma and pi bonds and what's the difference?

vanessas0123

Why is the ionization energy of nitrogen higher than that of oxygen's? Doesn't IE increase from left to right on the periodic table?

vanessas0123

nitrogen is more stable, thus it takes more energy to remove an electron, compared to oxygen.

vanessas0123

Electronegativity is the tendency of an atom to attract a bonding pair of electrons, while electron affinity is the amount of energy released when an electron is added to a neutral atom.

vanessas0123

I'm pretty sure the equation will not be provided on the front page of our test, but it might be helpful to memorize it. You could definitely complete lewis structure questions without using it though.

vanessas0123

The octet rules states that atoms prefer to have a full thus stable electron shell, which includes 8 electrons.This rules applies when drawing lewis structures!

vanessas0123

Probably on chemical bonds since that's the topic for this week!

vanessas0123

What is the difference between resonance and lewis structures? When would you need to draw resonance structures?

vanessas0123

Atomic radii should be only measured /compared in the ground state

vanessas0123

It is used to solve for the energy of the photon. You also use it with the C=λν equation a lot.

vanessas0123

Why is the electron configuration of tungsten [Xe]4f14 5d4 6s2?

vanessas0123

It's a set mass! So are the masses of protons and neutrons.

vanessas0123

This is just to clarify:

s orbital --> l = 0, n=1
p orbital --> l = 1, n= 2

so when electron moves from n=1 to n=2 the energy of the electron increases because it's absorbing energy instead of emitting energy (like when it moves down levels) right?

vanessas0123

What does Heisenberg Uncertainty Equation measure and when would you use it?

vanessas0123

Can someone explain when you would use DeBroglie's Equation and what it means?

vanessas0123

Yes! The units for E is joules but it's also joules per photon.

vanessas0123

It won't be tested! He is just going over the concept in class.

vanessas0123

Hz is unit for frequency, and it equals to 1 cycle / second.

vanessas0123

They'll tell you the specific metal and give you the work function, or you will be able to calculate the work function for that specific metal.

vanessas0123

Can someone explain when you would use Rydberg's Equation / what types of questions you use it in?

vanessas0123

Kennedi3K wrote:Does anyone know when the scores for the test will be up ? Thanks!

You can see your test scores once your TA hands it back in discussion. I think you can also view it on CCLE!

vanessas0123

You get your test back the discussion the week after you take your test! At least that's what happened for my discussion.

vanessas0123

1eV (electronvolt) = 1.6022x10^-19J (joules), so 1keV (kilo-electronvolt) = 1.6022x10^-16J. The energy in the formula E=hv is in joules, so you need to convert first!

vanessas0123

Avogadro's number is 6.022 × 10^23 and it indicates the number of atoms or molecules in a mole of any substance!

vanessas0123

I usually use the full number the periodic table gives you in all my calculations, then use sig figs for the final answer.

vanessas0123

Avogadro's constant is 6.02x10^23, and you use it when you want to determine how many "formula units" (you might see this term in the textbook!) are in a mole. Most times it is used to calculate how many atoms / molecules are in a mole of substance.

vanessas0123

Here are some examples, hope this helps:

567 = 3 sig figs
100264 = 6 sig figs
0.0034 = 2 sig figs
3.21002 = 6 sig figs
190000 = 2 sig figs

vanessas0123

You should avoid rounding until the very end! Keep and use the exact values in your calculator while you're calculating, and round / use sig figs for the final answer.

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