CHEMISTRY COMMUNITY

Joanna Huang

When steam touches the skin and cools down, it must go through 2 processes: it must go through a phase change from gas to liquid, which requires a lot of energy (aka a lot of heat), and it must also cool down further to the temperature of the skin (releasing even more heat). Meanwhile, the boiling w...

Joanna Huang

We need at least 50 chem community posts by today midnight (100 posts if you had him last quarter).

Joanna Huang

140 points, with 24 questions and a combo of 4pt and 10 pt questions :)

Joanna Huang

Entropy approaches zero as temperature approaches absolute zero.

Joanna Huang

Bond enthalpies are the energy required to break a bond, therefore you are always inputting energy, and that means the enthalpy will be positive.

Joanna Huang

Does anyone have any tips for studying for finals? I'm so stressed and there seems to be so much to do, I don't know where to start :(

Joanna Huang

You find the number of moles after balancing the two redox reactions (oxidation and reaction).

Joanna Huang

K is the concentrations of the products over reactants AT EQUILIBRIUM, but Q is the concentrations of the products over reactants NOT AT EQUILIBRIUM.

Joanna Huang

Any tips on how to balance redox reactions? or anything tricky I need to watch out for?

Joanna Huang

A pH meter measures the acidity of things, and the scale goes up by multiples of 10. It measures the hydrogen ion concentration, and ranges from 1-14, with 7 being neutral, and anything above 7 basic, and any below 7 acidic.

Joanna Huang

The salt bridge helps maintain neutrality in each of the half reaction solutions, so the reaction will continue to work. Otherwise, the reactions will stop.

Joanna Huang

How can we tell whether an element has the best oxidizing or reducing power based on the reduction cell potential?

Joanna Huang

The half reaction that undergoes oxidation is always at the anode, and the half reaction that undergoes reduction is always at the cathode.

Joanna Huang

What does it mean when the cell is at max potential?

Joanna Huang

When balancing redox, how does one find out how many electrons to put on each side of the equation?

Joanna Huang

It depends on the units, but usually yes, we use 8.314!

Joanna Huang

If Q>K, the reaction shifts towards the reactants. If Q<K, the reaction shifts toward the products.

Joanna Huang

The noble gases walk into a bar. No one reacts. XD

Joanna Huang

The oxidizing agent is being reduced (aka gaining electrons) and is DOING the oxidizing, thus taking the electrons from another element (which is being oxidized). The reducing agent is the opposite.

Joanna Huang

Yep! Usually the question will say "standard room temperature" if they do not specify a temperature, meaning that it is at 25˚C or 298K.

Joanna Huang

A system is thermodynamically unstable when Gibbs free energy is positive, and thus not spontaneous.

Joanna Huang

What's the difference between open, closed, and isolated systems? And would it matter what kind of system it is when solving problems?

Joanna Huang

How can we tell whether a reaction is reversible or irreversible? And what are the different equations that we need to know in terms of reversible/irreversible?

Joanna Huang

In short, you look at the units that the question provided you, and choose the R based off of the units that you are given.

Joanna Huang

If work is done ON the system, the system is gaining energy, thus it will be positive. If the system does work, they lose energy. to express this in mathematics, we change the sign of the work to be either positive or negative.

Joanna Huang

There are so many equations and concepts that are floating around in my head, does anyone have a list of which equations we need for the midterm? Thank you!

Joanna Huang

How does knowing whether a system is endo/exothermic help determine the direction of work? Or do they have no correlation?

Joanna Huang

What do we use entropy to calculate? And how is entropy different from enthalpy (in terms of calculation)?

Joanna Huang

A 0.721 mol sample of SO2(g) initially at 298 K and 1.00 atm is held at constant volume while enough heat is applied to raise the temperature of the gas by 16.9 K. Assuming ideal gas behavior, calculate the amount of heat (q) in joules required to affect this temperature change and the total change ...

Joanna Huang

A 0.669 mol sample of CO2(g), initially at 298 K and 1.00 atm, is held at constant pressure while enough heat is applied to raise the temperature of the gas by 14.1 K. Calculate the amount of heat q required to bring about this temperature change, and find the corresponding total change in the inter...

Joanna Huang

I'm still kind of confused, how can we tell whether work is positive or negative?

Joanna Huang

If a problem gives us bond enthalpies, how can you use that to find the reaction enthalpy? And is the reaction enthalpy different from the standard reaction enthalpy?

Joanna Huang

I know that there are three methods for finding enthalpies, but I'm confused on what they are used on. Are they used to find bond enthalpies, or reaction enthalpies? Or neither?

Joanna Huang

Can the entropy of the system decrease, of does it always increase? (I'm kind of confused about the second law of thermodynamics and what it applies to).

Joanna Huang

Yeo, we assume the reaction is at standard temperature and pressure.

Joanna Huang

Steam is water vapor, and since gas-->liquid is when the gas molecules are forming bonds, thus releasing energy (exothermic reaction), the energy is given off as heat. This results in severe burns. Note that since it is changing states, even more energy is released.

Joanna Huang

Water condensing means it's going from gas to a liquid state, meaning the gas molecules are forming bonds and becoming a liquid. When bond formation happens, it is releasing energy, therefore it is exothermic.

Joanna Huang

I still don't really understand how a weak acid is different from a strong acid in terms of calculations? If the question gives a molarity for an acid, it doesn't matter whether it is weak or strong, you still use that molarity to solve right?

Joanna Huang

How does one convert from Ka to pH?

Joanna Huang

What is the relationship between Ka and pH?

Joanna Huang

Can someone explain what a monoprotic acid is and how that changes the reaction?

Joanna Huang

Can some explain why NH4ClO4, when placed in water, becomes acidic?

Joanna Huang

If the question gives you an initial concentration, then the change would have to be negative. If the question does not give an initial concentration, then it is zero, and thus it can only increase the concentration, and the change will then be positive.

Joanna Huang

If the enthalpy is positive, meaning the reaction is endothermic, how would that affect the equilibrium constant? And how about for an exothermic reaction?

Joanna Huang

If the question gives you the initial concentration of the products and reactants, then they add more moles to the reactants and let it go to equilibrium, which way does it shift and why?

Joanna Huang

How do we know when to use an ICE chart, and when to just do the normal equilibrium equation K=[products]/[reactants]?

Joanna Huang

What's the difference between Kc, Kp, Keq, and Q? Thank you!

Joanna Huang

Partial pressure would just be part of the total pressure. There can be multiple partial pressures of different gases in a mixture, which can add up to a total pressure.

Joanna Huang

Can someone explain to me why solids and liquids don't affect K? Thank you!

Joanna Huang

K>1 = products favored, since K=[products]/[reactants]. By the same logic, K<1 = reactants favored

Joanna Huang

Sometimes, the question will give a chemical equation with the molarity and the temperature of the reaction, and tell us to solve for K. Does temperature matter in this case?

Joanna Huang

You look at the electronegativity and the bond formation. If the molecule has uneven electronegativity, as in the different bonded atoms have different electronegativity and thus more electrons are pulled to one side, then the molecule is polar. If the bonds are symmetrical however, the charges canc...

Joanna Huang

Both are allowed, it doesn't matter which order you put it in. However, it's better to put the atom that is being bonded to first, which is oxygen, so OH2 is more preferred.

Joanna Huang

Hi Dr. Lavelle!! Thank you for the fun chem music and chem jokes at the end of each lecture :)) I really appreciated the fact that you tried to record the lectures in the lecture hall to let us experience what it would be like at school. Thank you so much for being our teacher!!

Joanna Huang

How do you calculate bond length? (assuming you have drawn out the Lewis structure)

Joanna Huang

Pi bonds are only applicable to covalent bonds, since ions don't share electrons and form bonds.

Joanna Huang

1) The oxidation number of a free element is 0
2) The number for the cation/anion is usually just their charge
3) the number for H is usually +1
4) the oxidation number for O is -2 unless it's in peroxides

That's what I have so far :D

Joanna Huang

Did you know that you can cool yourself to -273.15˚C and still be 0 k?

Joanna Huang

Are there any other bidentate ligands we need to know besides en?

Joanna Huang

I'm pretty sure the answer is triamminetrichlorocobalt(III)!! Someone double check me though :D

Joanna Huang

Does anyone have any tips for memorizing prefixes? Like do we need to know all of the prefixes?

Joanna Huang

Yep, I believe so! The bonds and the structure do not really correlate. You can have polar bonds, but the polarity can cancel out due to the non-polar structure.

Joanna Huang

Can someone explain what a dipole moment is, and what it depends on? I know a dipole dipole attraction depends on the polarity of the atom (the electronegativity). What's the difference between this and a dipole moment? And will we ever need to calculate this?

Joanna Huang

Is there a way to tell which is an ionic and which is a covalent, or even an acid, based off the lewis structure alone? Thank you!

Joanna Huang

Can we always assume that elements in the n=3 level or down can carry more than eight electrons? And if so, is there a set number to how many they can hold?

Joanna Huang

What is the definition of hybidization, and does formal charge play a role in determining this? Thank you!

Joanna Huang

Hi, I was wondering, to what extent do we have to memorize the different names for the VESPR models for the final? I know the basics, but there are so many combinations with lone pairs. Do we have to know all of them?

Joanna Huang

Yes, if the molecule is symmetrical, then the molecule is nonpolar. To determine this, just draw out the vectors on the lewis structure.

Joanna Huang

This always messes me up-- why is the C-H bond considered non-polar? They're really far apart on the periodic table, and if you follow the electronegativity trend on the periodic table, they should be different electronegativities? Unless this is an exception. If so, what are the other exceptions?

Joanna Huang

Polarizability is basically how much an atom's electron cloud can be manipulated and distorted. Thus, the polarizability trend usually follows the atomic radius trend-- the larger the radius, the more easy it is to manipulate the electrons (since the electrons are farther from the nucleus) and the m...

Joanna Huang

Dipole dipole interaction are between two molecule with oppositely charged ends. You can tell when they are oppositely charged when the atoms in the molecule have different electronegativities. This means that one of the atom in the molecule has a greater tendency to attract electrons, thus even tho...

Joanna Huang

The lower the ionization energy of an atom, the less of a hold that particular atom has on the electrons surrounding them, thus it will be easier for that atom to share its electrons and form a bond with other atoms. That's why the center atom always has the lower ionization energy.

Joanna Huang

I remember a question on Sapling where C and H bonded together was said to be a neutral atom. How are their electronegative values the same, if they're so far apart on the periodic table?

Joanna Huang

The midterm will definitely make it straightforward! Anything we need to know will be on the periodic table or the equation sheet they gave to us!

Joanna Huang

Can someone explain how to determine whether a molecule is a Lewis acid or base? In the sapling they talked about molecule structures and stuff in the answer, and I don't think we've gone over that yet... I know that a Lewis acid is an acceptor, but how do you know if it's an acceptor or now?

Joanna Huang

Dipole-Dipole: when two polar molecules are attracted through electrostatic attraction
Ion-Dipole: when an ion and a polar molecule attract each other
ion-ion: when a cation and anion bind together

Joanna Huang

I believe both Hydrogen bonding and Dipole-Dipole forces are at work! Since one molecule is strongly polar (F), the polar ends of the molecule will attract the hydrogen. Because hydrogen only needs one pair of electrons, it can also form a hydrogen bond with F, which is strongly electronegative.

Joanna Huang

Hi! Elements in the periodic table are general group together, where they share similar properties such as electron number. So yes, electron number does matter.

Joanna Huang

Yes, however the spin quantum number has to be +1/2 for one and -1/2 for the other electron.

Joanna Huang

A subshell would be the "s,p,d,f", and the orbital is the magnetic quantum number.

Joanna Huang

Can someone explain exactly what ionization energy and electron affinity is? I'm still a bit confused on what is it and the trends on the periodic table.

Joanna Huang

Yep, c in chemistry will always be known as the constant for the speed of light.

Joanna Huang

How can we tell whether an electron is excited or not through the electron configuration? And how can an electron be excited naturally?

Joanna Huang

We definitely need to know the basic information, such as where the spdf blocks are located on the periodic table, and how many orbitals are in each. s has 1 orbital, p has 3, d has 5, and f has 7. Each orbital can contain up to 2 electrons. "2s" stands for the 2nd line of the periodic tab...

Joanna Huang

Will we have to know in depth how to tell which atom has a smaller radius? I find it confusing how to determine which has a smaller radius between atoms (like oxygen) who experiences electron repulsion, atoms which has lost electrons (like Ne2-) or atoms which has gained electrons (like C2+), or lik...

Joanna Huang

I believe electron affinity is the energy that is released when and electron is added to an atom. Why is energy released, and not absorbed?

Joanna Huang

Yep, amplitude is only used in the wave model. Increasing the amplitude does not increase the amount of energy, thus, regardless of the amplitude (aka number of electrons), each electron can only react with another electron, so increasing amplitude does not make the electron eject off easier.

Joanna Huang

This differs depending on the metal. Each metal requires different work, or "threshold energy" levels in order for an electron to be ejected.

Joanna Huang

Hi! So in number 6 on sapling, they asked me to find the max number of electrons that was ejected from the metal surface. I used stoichiometry and found the max number of photons. When I put that answer in, it was right. Does this mean that the max number of electrons is the same as the max number o...

Joanna Huang

Yep! As seen in the equation E=hv, h is a constant (Planck's constant). Thus, when frequency increases, so does the energy, and vice versa.

Joanna Huang

Hi! So in order to find the maximum amount of electrons that could be ejected by the metal, the kinetic energy has to equal zero (if KE=0, that means that there is no excess energy ejecting the electrons thus the max number of electrons are ejected). For the KE to equal zero, the energy of the photo...

Joanna Huang

In the case of light, it is extremely fast moving compared to objects with mass, and the same laws for objects with mass cannot be applied to light. It is not the mass of the light that physically knocks electrons off, rather it is the energy of the particle that reacts with the electron. More speci...

Joanna Huang

According to my notes, the electron can still be ejected from the surface of the metal if the photon energy is equal to the energy needed to eject the electron from the metal.

Joanna Huang

Will the question ever give you the "intensity of light", and if so, where would this number fit into the equations? I know that the intensity is proportional to the number of photons, but will we ever have to solve any questions where the intensity would be provided to us and we would hav...

Joanna Huang

Is it safe to assume that if the problem gives us the "amplitude", they are talking about the intensity of light?

Joanna Huang

Protons and electrons differ massively in size, whereas protons and neutrons have almost the same mass. Electrons weigh an extremely tiny amount compared to protons.

Joanna Huang

In Professor Lavelle's lecture, he mentioned that our class would be mainly dealing with the wavelengths of anything from microwave to x-ray. The order of the list from largest to smallest wavelength would be microwave (1 mm), infrared (1 micrometers), visible light (aka the ROYGBIV colors, from 700...

Joanna Huang

Yep you ignore it! Once you figure out the limiting reactant, you ALWAYS use the limiting reactant to determine the moles in every other chemical in the equation. Thus, you ignore the other reactant, as that reactant will eventually be used up with extra reactant left over.

Joanna Huang

For week 1 Q10 on the sapling homework, I encountered a problem that depicted an organic chemical structure model, and it asked me to find the theoretical yield. I had to search up the formula for that structure. Will we be required to know how to find the molecular formulas for these structures, or...

Joanna Huang

The theoretical yield should always be 100%. As for the actual yield, if the number is not given, then the problem will always provide a method to solve for the actual yield. For example, the problem should usually tell you what the molar mass of the final product is, and using that number and the t...

Search found 102 matches