CHEMISTRY COMMUNITY

oliviahelou

It depends on the order. But for whichever order, you use the "t1/2" to calculate k for the reaction, then you use the k and manipulate equations to calculate t from [A]0, [A]t, and k.

oliviahelou

it is a reaction RATE. So it must be positive, as we cannot have a negative speed, it does not make sense.

oliviahelou

We know that products are formed so (positive) while reactants are consumed so (negative).

oliviahelou

average : over a time interval
instantaneous : slope of the tangent line to a specific point on the plot of the graph

oliviahelou

k1 and k2 are for forward reactions (1 and 2). k' is for the the reverse reaction.

oliviahelou

No, the change in entropy is not the same in reversible and irreversible pathways.

oliviahelou

Degeneracy is W:
It is the number of orientations that the atoms can take in a given molecule.

oliviahelou

It's because heat is a built-in function, does not always have to be a constant!

oliviahelou

It's delta U= w +q!
heat + work!

oliviahelou

When delta G is negative, the reaction is spontaneous.

oliviahelou

This only means that the reaction will be spontaneous in the reverse direction.

oliviahelou

Your second equation refers to when the temperature is standard. If you have any other temperatures than 25 degrees C, use the first equation.

oliviahelou

Yes it is always 0 (standard).

oliviahelou

Oxygen in molecule: -2
a neutral element that is not part of a compound has an oxidation number of 0
monoatomic ions have oxidation states equal to their ionic charge

Work around these major rules and you'll find the oxidation number.

oliviahelou

You check the reduction potentials, the species with the highest reduction potential will be the most prone to be reduced and will be a good oxidizing agent.

oliviahelou

You can:
- do work on the system
- heat or cool system
- add remove the substance

oliviahelou

Isolated systems do not exchange heat or matter with the surroundings whereas closed system do.

oliviahelou

That means that work is done by the system, as it is losing energy (there's a negative sign).

oliviahelou

A closed system is defined by a system where we can transfer energy in or out of it but not matter. (could gain or lose energy through heat or work)

oliviahelou

delta U = 0 only when w + q = 0, or in isolated systems as there is no change in internal energy (first law of thermodynamics)

oliviahelou

It all depends on the scenario we are in. When there is no transfer of heat, q=0 so U=w.
When there is no work (compression / expansion), w will be = to 0, which would mean that U=q.

oliviahelou

It depends on the amount of substance you are working with. As opposed to an intensive property that does not depend on the amount of substance.

oliviahelou

1 degree celsius is equal to 1 degree Kelvin, so when measuring delta T for instance, the difference in Kelvin will be the same as the difference is Celsius so you can use either, they would be equivalent.

oliviahelou

You are correct!

oliviahelou

Yes, and then you can use the qcal= -qreaction to find the heat of the reaction.

oliviahelou

Extensive properties: depend on the quantity of a substance
Intensive properties: do not depend on the quantity of a substance

However, both are physical properties of matter.

oliviahelou

Once steam has touched your skin, it immediately phase changes to liquid (a process that releases heat). But boiling water does not phase change once it touched your skin.

oliviahelou

That's what I got as well!

oliviahelou

A reaction that releases heat = exothermic
A reaction that requires heat = endothermic

oliviahelou

When you say that something is a state property, that means that it does not depend on the path taken to be prepared.

oliviahelou

Indeed they are calculated the same way.
pH: level of H3O+ concentration
pka: relative strength of the acid.

oliviahelou

In order to maintain equilibirum!

oliviahelou

No they do not.

oliviahelou

If Q<K, that means that the reaction will tend to form more products.

oliviahelou

All you need to know is that the higher the PH, the weaker the acid. So it is only logical that if we have a strong acid, the PH will be lower.

oliviahelou

We use the reaction quotient when we don't know for sure if the system is at equilibrium. if K=Q, then the system is at equilibrium.

oliviahelou

Yes, they are equivalent with the only difference that you must change the signs.

oliviahelou

I believe the answer to your question is the phases of condensation and freezing. Those are the main ones we covered in lecture.

oliviahelou

Sublimation is endothermic (requires heat)

oliviahelou

As we covered in the lecture in week 3,
Condensation is a form of exothermic reaction (releases heat).

oliviahelou

It is simply the standardized temperature (the one mostly considered for the norm od reactions).

oliviahelou

Ka: a strong acid has a high Ka value and a weak acid will have a lower Ka value.
pKa: it is kind of the opposite, the lower the pKa value, the stronger the acid.

Logically, it would be the same for the kb and pKb values for bases

oliviahelou

Because this is an exothermic reaction, the production of reactants will be favored.

oliviahelou

It is the conjugate of acids and bases that are charged: if gained a positive atom, it will have a positive charge; and the same for negative.

oliviahelou

You are right, the change would be 2x.

oliviahelou

It is not the concentrations that are equal, but the ration of the product to reactant that will be equal at equilibrium.

oliviahelou

K<Q = numerator is large.

= too much product --> reaction will favor the formation of reactants.

oliviahelou

We did not cover in class how K changes, but we know that if we change the conditions in which a reaction takes place (aka temperature), the equilibrium constant would change.

oliviahelou

left (reactants) <=> left (products)

oliviahelou

Lies to the right --> reaction favors products

whereas

lies to the right --> reaction favors reactants

oliviahelou

Hello,

You can only calculate the equilibrium constant (K) at the equilibrium point of the reaction. However, you can calculate the reaction quotient Q at any point in the reaction.

oliviahelou

It is bent because of the lone pair of electrons on the oxygen atom.

oliviahelou

a seesaw shape is a shape with 4 molecules and 1 lone pair.

oliviahelou

Hey! It's bent because the lone pair electrons push the atoms down.

oliviahelou

Draw the dipole moments between all the atoms of the molecules. If they cancel out, the molecule is nonpolar; if the dipole moments do not cancel out, the molecule is said to be polar.

oliviahelou

I think it's something you have to memorize, look up the VSEPR model charts and go from there!

oliviahelou

you need to know the rules to name a compound. Once you get how it's done, it shouldn't be hard for u to name a complex after seeing its molecular formula. We covered this topic in lecture.

oliviahelou

All you have to know is that the more bonds, the stronger.
triple > double > single (bonds)
closer atoms -- furthest

oliviahelou

a compound containing a ligand (typically organic) bonded to a central metal atom at two or more points.

oliviahelou

the coordination number is the number of atoms, ions, or molecules that a central atom or ion holds as its nearest neighbours in a complex or coordination compound or in a crystal.

oliviahelou

What’s the relationship between geometry and coordination number complexes ?
(Ex Which geometry or geometries are common for complexes with a coordination number of 6?)

oliviahelou

The first bond in a triple bond is a sigma bond, however, the 2 others are necessarily Pi bonds!

oliviahelou

It's all about drawing the dipole moments in a lewis structure. Meaning if the dipole moments of 2 of the same atoms cancel out, the molecule would not be polar! (vectors)

oliviahelou

We didn;t cover such cases in class, but logically, if a lone pair of electrons results in a certain repulsion, if we only had 1 electron, we would have less of a repulsion effect.

oliviahelou

Here's an example of an octahedral shape molecule: (Mo(CO)6)

oliviahelou

Lone pairs have a high repulsion effect. They will decrease the bond angles as they will repulse the bonds of the central atom with the circling atoms.

oliviahelou

Hey,

Does anyone know how I can review my questions for midterm 2? How can I see where I went wrong?

oliviahelou

AXE only takes into consideration the central atom. In fact, a molecule's shape relies on the central atom (how many atoms/ lone pairs or electrons are around it)

oliviahelou

electron arrangement: includes lone pairs and lone electrons

shape: only includes the atoms in the molecule

oliviahelou

By drawing the Lewis structure, you notice that some atoms have dipole moments that cancel each other out.

It sometimes does not only rely on the electronegativity difference between 2 atoms, but also on the dipole moments.

Hope this helps!

oliviahelou

Fluorine is electronegative. You must only fill its valence shell without giving it a formal charge.

oliviahelou

There are a few values of bond angles that I think you should know going in the exam, the main ones, and you should also note that lone pair electrons change the bond angles in a specific shape.

oliviahelou

The most stable form of a Lewis structure is when the overall formal charge is 0. Therefore, you must always opt for the option that gives a formal charge closest to 0.

oliviahelou

When using single bonds results in too many valence electrons, it's always good to switch to double bonds.

oliviahelou

I understood that there are a few atoms that are an exception to the octet guideline, but in what sense? what makes them so different?

oliviahelou

Honestly, practice makes perfect. Doing exercises that involve comparing electronegativities will help you determine which atoms are more electronegative than others. Additionally, you can always type on google "periodic table electronegativity" and have all the values displayed in front o...

oliviahelou

Hi!

What is an oxidation number? And how do we find it?

oliviahelou

To find the volume or concentration while doing a dilution use the M1V1=M2V2 EQUATION. However, when you're looking for a volume in general, just use the other formula, as u don't have an initial and final volume and molarity.

oliviahelou

You should round it to 0.237!

oliviahelou

Honestly, just use a periodic table you're comfortable and familiar with. As long as it has all the necessary information, it should be good.

oliviahelou

Honestly, there is no general guideline to balancing. You just have to keep trying until you get it done and balanced. Practice makes perfect.

oliviahelou

It is more of an "octet guideline" because there are many exceptions. But in covalent bonds, atoms share electrons until a noble gas configuration is reached. So; 1. count valence e- for each atom 2. choose atom with the lowest ionization energy as the central atom 3. arrange atoms symetri...

oliviahelou

A mole is a unit that describes a specific number. Molecules are uniquely an assembly of atoms.

oliviahelou

It's all about trial and error. Keep adding the necessary stoichiometric coefficients until you get a balances equation with the same number of elements on each side.

oliviahelou

You can easily look up the electronegativity of an atom, you are not meant to know them by heart. Just look up "electronegativity of O" and compare.

oliviahelou

In order to see which molecules can form H bonds, you need to make sure that the stronger atom (O for instance) has a negative charge while the H atom has a positive charge.

oliviahelou

You can find everything you need on Professor Lavelle's website. I reckon reading a document in which there's all the information about grades.

oliviahelou

Hey!

Does anyone know when do we get our results after the exams? Specially for midterms 1?

Thank you!

oliviahelou

Hey!

Every single atom has an infinite amount of electron shells, and each electron can leave the ground state to go to any of these infinite electron shells.

oliviahelou

Hey!

This could come up in the midterms as a conceptual question or a concrete application.

oliviahelou

Hey!

I am finding it extremely hard ti conceptualise and concretely understand what the s-, p-, d-, and f- orbitals are... If someone could explain in a very easy and understanding way, I would so much appreciate it.

(Keep in mind that I did not take High School Chemistry)

oliviahelou

Hey!

If someone could help me understand the difference between wave and particle, I've been struggling so much to differentiate them...

oliviahelou

I personally advise you to rewatch the lectures on ccle (x 1.5 speed) and do the achieve homework. You can also ask your TA for extra worksheets!

oliviahelou

Look at the bonds between the atoms, and compare the electronegativity between atoms

oliviahelou

I see that a lot of people gave you interesting replies, however, you can always rewatch the professor's class, he deals with concrete examples and shows you what he does step by step! There's also a module on his website that guides you through the process!

oliviahelou

Try to find the methodology in previous examples the professor gave in class and mirror it in your solving!

oliviahelou

water can either be an acid or a base!

oliviahelou

Honestly it depends. In general, some countries use F and some use C. However for chemistry, just know how to convert from one to the other!

oliviahelou

If I paid attention in class, Professor Lavelle told us he would always give the molecular formulas for the class 14B. However, it is normal to make mistakes, and therefore if the formula was not given, look it up!

oliviahelou

Hey,
Lambda represents the wavelength, it is measured in nm.

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