CHEMISTRY COMMUNITY

JonathanSung_2G

For me, the best way to visualize the breaking and formation of bonds is the draw the Lewis structures. It allows me to observe which bonds remain intact and which are broken; I can then move on to associating specific bond enthalpies to the bonds involved in the reaction. It does take a little bit ...

JonathanSung_2G

We differentiate between heat in the system and heat in the surroundings, and the net gain of heat in either tells us whether a reaction is exothermic or endothermic. Looking at the direction the heat moves, from the system to the surroundings or from the surroundings to the system, will help you de...

JonathanSung_2G

Yes, you can apply the Kw=[H3O+][OH-] formula to the aqueous solution, and the concentrations of each ion will change and compensate for each other depending on the acidity of the solution since Kw is a fixed value (in a neutral solution, the concentrations of both ions would be roughly 10^-7). Also...

JonathanSung_2G

Yes, as long as the surface that the steam comes into contact with is cooler than the steam's temperature, then the steam will condense. In general, remember that when bonds are formed, heat is released, and when bonds are broken, heat must be absorbed. In this case, bonds are formed, so heat is rel...

JonathanSung_2G

Whether heat is absorbed or released determines if the reaction is endothermic or exothermic. A reaction that absorbs heat will be endothermic with a positive delta H.

JonathanSung_2G

Whenever K is a really small value, less than 10^-4, we can assume that the change in concentrations of the compounds is very small. Also, I think that if there is ever a point where you are not sure whether or not you can assume x is extremely small, Dr. Lavelle said you can use the quadratic equat...

JonathanSung_2G

Yes, you can assume that x is small because of the low Kb value as well. The best way to approach problems like these in general is to write out an ICE table and fill in the proper values. In this problem since x is the concentration of OH-, you can use x to find the pOH and subtract the pOH from 14...

JonathanSung_2G

pH and pKa aren't interchangeable, but they correlate to the strength of an acid. For instance, if you are comparing the pKa of different acids, you can determine which acid is stronger depending on which pKa value is lower. pKa is constant for an acid, whereas the pH is dependent on the environment...

JonathanSung_2G

Yes, so when you do the ICE table, you would input the value of pressure given in bars and then solve the problem like you would if the value was given in atm.

JonathanSung_2G

Kw is also the product between Ka and Kb, and you would use Kw to find the values of Kb that correspond to Ka and vice versa. I believe that we use Kw in this manner on the sapling homework for Week 2, and we would use the calculated Ka or Kb values to find the concentrations at equilibrium. Kw is d...

JonathanSung_2G

I like to study periodically and plan out what I am going to do before the week begins. I find it helpful to take breaks in between study sessions just to reset the mental so I don't get burned out immediately. Having a plan in my mind helps me not stress out as much since I know what my goals are f...

JonathanSung_2G

I believe it is found through experimentation But can someone explain why [H3O+] and [OH-] concentrations need to be equal? For example we have 2H2O -> H3O+ + OH-. If we were to calculate the equilibrium constant, Kw = [H3O+][OH-] and we know that Kw=10^-14. The change from the initial concentratio...

JonathanSung_2G

I believe we use the conditions K<10^-3 and K>10^3 because they demonstrate the extremes where we can most accurately determine the shift in equilibrium as well as gauge the stability of the reactants and products at equilibrium. I think K<1 and K>1 are conditions that aren't specific enough to accu...

JonathanSung_2G

We use the gaseous phase of water when calculating the equilibrium constant because typically water in the liquid form is in excess and will not change throughout the reaction. When you see a substance in the gas phase in the chemical equation given, you use it in the equation to find the equilibriu...

JonathanSung_2G

I would also add that you would need to put the stoichiometric coefficients in front of the liquid anyway to balance out the chemical reaction, thus finding the correct stoichiometric moles of reactant and product. However, in terms of finding the equilibrium constant, you would not use the liquids ...

JonathanSung_2G

UA sessions are really helpful, and the UAs provide and go over worksheets that cover a specific topic. These sessions are useful especially before exams since the worksheets review all of the general material that will most likely be tested. Reading the textbook personally did not help me as much a...

JonathanSung_2G

Pure liquids and solids are not factored into the equilibrium constant expression. In your example, you will only need to use the equilibrium concentrations of the aqueous solutions and gases. Omit the solid reactant in the expression, so you would have the concentrations of both products over the c...

JonathanSung_2G

One method is to write out the Lewis Structure, and using the shape and positioning of the molecules, you can determine whether the molecule is polar or nonpolar. For CO2, the shape is linear with oxygen atoms on either side of the carbon atom, which means that CO2 is nonpolar. There is no net dipol...

JonathanSung_2G

In terms of looking at salts, you can tell which salts will behave as acids or bases. For example, NH4+ Cl- + H2O -> NH3 + H3O+ Cl-, chlorine does not affect the pH which means it has no effect as an acid or base. However, H3O+ concentration increases, which lowers the pH. You will need to see which...

JonathanSung_2G

Yes, ABCDE just represented the the ph at different volumes. The stoichiometric point is the only data point that we use to figure out the concentration of the sample.

JonathanSung_2G

Yes, in order for any protonation to occur, the pH needs to be higher than the pKa, assuming the compound in question is a weak acid. I think you can compare the protonation by weak acids based on the pKa, but because they are weak acids at heart, they won't act as strong acids because strong acids ...

JonathanSung_2G

Yes there is a pOH scale from 1-14. pH, pOH, and the concentrations of H+ and OH- can all be found given that you have at least one known value.

JonathanSung_2G

Like everyone here said, I think simply putting a space between the names inside and outside of the bracket is the proper way. For example, when given potassium hexacyanochromate(III), you know that potassium is outside of the bracket (to the left). And when you have pentaamminesulfatocobalt(III) ch...

JonathanSung_2G

Polyatomic ions can act as ligands, and ligands do not necessarily have to be polyatomic ions. Any molecule that is bonded to the center atom, usually a transition metal, acts as a ligand. The coordination sphere consists of the transition metal and all of the ligands that are bonded to it. It just ...

JonathanSung_2G

There are some basic oxidation number rules, including that hydrogen is +1 and chlorine is -1. For instance on the homework, we have to find out the oxidation number of cobalt in [Co(NH3)5Cl]CL2. We know hydrogen is +1 and chlorine is -1. Nitrogen's oxidation number cancels out with the 3 hydrogens ...

JonathanSung_2G

I would also add that the number of electron density regions and the coordination number do not always match. For instance, you can have a trigonal pyramidal structure with 4 electron density regions, yet the coordination number would be 3. You can have a square planar structure with 6 regions of el...

JonathanSung_2G

Ligands and transition atoms form the coordination complex. Biologically, coordination complexes like myoglobin and hemoglobin carry oxygen O2 in muscle tissue and the blood respectively. Enzymes and vitamins are also other examples of coordination complexes that have biological benefits.

JonathanSung_2G

Hybridization is based on the amount of regions of electron density. One lone pair acts as a region of electron density. A double bond and a triple bond both act as single regions of electron density as well. You can then determine what type of hybridization is around the atom if you know the total ...

JonathanSung_2G

I am going to go through Sapling, textbook problems, and UA worksheets. I personally wouldn't go through too much in one day, spread it out! I like to dedicate a few days to do the textbook problems, the sapling problems, and worksheets. My main focus is typically the textbook since it provides a wi...

JonathanSung_2G

Pi bonds do not allow the molecules to rotate. The p orbitals are connected end to end in two regions, and if one of the atoms were to rotate, then the resulting bond would snap. Sigma bonds, connected end-to-end, are the only bonds that allow the atoms to rotate.

JonathanSung_2G

You can also think of the shape of the molecule as well. BeCl2 has 2 Cl bonded to the central atom Be, and there are no lone pairs present around Be. Therefore, you know the shape is linear. The dipole moments between Cl and Be cancel out because the Cl molecules are as far away from each other as p...

JonathanSung_2G

The way I look at it is the more pi bonds you have, the shorter the bond length because more pi bonds signify a double or triple bond. However, by itself, a sigma bond is stronger than a pi bond due to a greater overlap between the orbitals. When the orbitals overlap end-to-end, there is a greater e...

JonathanSung_2G

If the lone pairs cancel out and the dipole moments cancel out, then you may have a nonpolar molecule with polar bonds. Don't assume that a molecule is polar simply because it has lone pairs, since the lone pairs signify a dipole moment but not necessarily a net dipole moment of the entire molecule....

JonathanSung_2G

You should figure out how many regions of electron density you have around the central atom. In the example XeF4, there are 6 regions of electron density around Xe. When 2 out of those 6 regions of electron density are lone pairs, you know that those 2 lone pairs are as far away from each other as p...

JonathanSung_2G

Covalent character is based on the electrons that are in the shared region between two ions. Because a small cation or a large anion have a high polarizability, this means that their electrons are more easily distorted. When a small highly charged cation is covalently bonded to a large anion, there ...

JonathanSung_2G

It also may be because oxygen is slightly more electronegative than chlorine, so the single electron would be paired with chlorine since oxygen is more likely to have its octet rule fulfilled. When this structure is drawn with the lone electron on chlorine, the formal charge is -1 on oxygen and +1 o...

JonathanSung_2G

Yes, there is no net dipole moment if the are dipole moments present cancel out. Think of it as you can have polar bonds within a molecule, but if these polar bonds cancel out, then the molecule is nonpolar as a whole, such as CO2.

JonathanSung_2G

I think one of the pointers I would give when drawing Lewis resonance structures is to understand the relationship between the octet rule and different atoms. In this example, I know that carbon has to satisfy the octet rule, so I will have to draw multiple resonance structures where the double bond...

JonathanSung_2G

In the lecture, from the left to right, the flasks contained C5H12, C15H32, and C18H38. The more atoms a molecule contains, the more electrons there are. The more atoms a molecule contains, the greater the size of the molecule. We know that polarizability is directly related to the number of electro...

JonathanSung_2G

Hydrogen has to be covalently bonded to an electronegative atom and close to another electronegative atom with free lone pairs to hydrogen bond, and these atoms are N, O, and F. You need atoms with high electronegativity to form a molecule that can attract hydrogens from another molecule, forming th...

JonathanSung_2G

You are correct in saying that AgF will be smaller in size, but I think because F is more electronegative, AgF will have more ionic character than AgI. Ag+ will pull more ions into the shared region from I- since I is not as electronegative as F. Thus AgI will have more covalent character than AgF, ...

JonathanSung_2G

Ba and Be are in the same column in the periodic table, but remember that electronegativity decreases going down the column. Therefore, because Ba is less electronegative than Be, then it will form a more ionic bond with Br.

JonathanSung_2G

I think that when drawing Lewis structures for molecules, we should factor in symmetry; a lot of times we want to draw atoms that are symmetrical on all sides. Also, if oxygen is the center atom, then that means that the other oxygens and nitrogens will have to bond to it, but we don't typically see...

JonathanSung_2G

Carbon is not as electronegative as nitrogen, which means that when hydrogen is bonded to carbon, there will not be a significant positive charge that would be used during bonding. Hydrogen bondings require a slight negative charge and a slight positive charge in order to form, and in this case for ...

JonathanSung_2G

Dr. Lavelle replaced two of the single bonds with double bonds because of the more favorable formal charges of sulfur and oxygen. When SO4 is drawn with single bonds, the formal charge of oxygen is -1 and the formal charge of sulfur is +2. When SO4 is drawn with double bonds, the formal charge of th...

JonathanSung_2G

I would also add that for some molecules, you can predict their structure based off of symmetry; the structure of something like PO4^3- would be easy to predict since you could draw 4 oxygen atoms surrounding the phosphorus atom. As everyone here has already stated, look to put the atom with the low...

JonathanSung_2G

Orbitals allow us to predict the location of electrons based on electron density. The orbital model helps us determine the probability of finding electrons and where we will have lower chances of finding electrons, such as nodal planes. It is a mathematical representation that helps us conceptualize...

JonathanSung_2G

Hi! Certain subshells do not exist because sometimes the energy level does not correspond to the angular momentum. 2d cannot exist because d orbitals only exist when the principle quantum number n is at least 3. g orbitals can only exist when the principle quantum number n is at least 5. Thus (a) an...

JonathanSung_2G

For me personally, I like to imagine the shapes of the orbitals (of course these are mathematical models and not concrete structures) and I also think of the electron configuration. Hund's Rule and the Pauli Exclusion Principle help me categorize the electrons into their respective subshells as well...

JonathanSung_2G

Yup, just always associate DeBroglie's with electrons since they have mass and light does not. Typically when you also see an electron's velocity in the problem you will most likely have to use DeBroglie's wave equation at some point as well.

JonathanSung_2G

n is the principle quantum number and represents the energy level an electron is located. l is the angular momentum of e and describes the shape of the orbital. n and l are not dependent on each other; you can have 4s, 4p, and 4d orbitals, where n=4 but l can be 1, 2, and 3 respectively.

JonathanSung_2G

The Balmer and Lyman series are also grouped together based at which level an excited electron will relax at. For the Balmer series, an electron would fall to n=2, and for the Lyman series, the electron would fall to n=1. Like the answer above said ^ anytime UV light or visible light are involved, y...

JonathanSung_2G

I believe he also described that Ψ is the orbital representation of the electrons, and when it is used in Schrodinger's Wave Equation along with the Hamiltonian double derivative (H), you can calculate the energy of the electron: HΨ=EΨ You can also use wave functions to predict the positioning of th...

JonathanSung_2G

Hi! So the probability of finding an electron is based on the electron density. For instance, at nodal planes between the orbitals of p, d, and f, the probability of finding an electron is zero because there is zero electron density. Likewise in actual orbitals, let's say p orbitals, there electron ...

JonathanSung_2G

a. Use the DeBroglie wavelength equation and plug in 9.11x10^-31kg for the mass of the electron. Make sure to convert the velocity of the electron to m/s by multiplying the velocity given by 1000. You should get 0.200nm as your wavelength. b. This part of the question is essentially telling you that...

JonathanSung_2G

Hi! Objects with wavelength properties are extremely small and are classified under quantum mechanics. Larger objects ranging from a baseball to a car are classified under classical mechanics. Classical objects do not exhibit wavelength properties. I believe that if the wavelength calculated is 10^-...

JonathanSung_2G

Hi! Both equations are very closely related. When we use the Rydberg equation, we are trying to find the frequency of light when an electron moves from one energy level to another (ex: n=5 to n=1). We can use the frequency to figure out the wavelength or energy of the electron using E=hv and wavelen...

JonathanSung_2G

Which of the following happens when the frequency of electromagnetic radiation decreases? Explain your reasoning. (a) The speed of the radiation decreases. (b) The wavelength of the radiation decreases. (c) The extent of the change in the electrical field at a given point decreases. (d) The energy o...

JonathanSung_2G

Hi! Basically when we calculate the empirical formula, we only want to use individual elements. In other words, instead of O2 you would simply use only O. Your calculations should look like this: 69.9 g Fe (1 mol Fe/55.85 g Fe)= 1.25 mol Fe 30.1 g O (1 mol O/16.00 g O) = 1.88 mol O Dividing by the l...

JonathanSung_2G

Balmer and Lyman series represent different sections on the atomic spectrum. Furthermore, they differ in the level that the excited electron relaxes at. If the electron comes to rest at n=1, then this is the Lyman series, and if it rests at n=2, then this is the Balmer series. This is overall how yo...

JonathanSung_2G

Usually you want to work with standard SI units, so unless the velocity is already in m/s, you don't need to make any additional conversions. In this problem, you also would want to work with kg for mass, which like everyone here as stated, is 9.11x10^-31 kg for an electron.

JonathanSung_2G

Too add on, an the behavior of an object as large as a car would be categorized under classical mechanics. A particle as small as an electron would be categorized under quantum mechanics. 6.94 x 10^-38m is such a small, insignificant wavelength for an object classified under classical mechanics, and...

JonathanSung_2G

The problem gives us a balanced equation in terms of symbols for the elements. On the reactants side, there are 2SiH4 molecules and 4H2O molecules. On the products side, there are 8H2 molecules and 2SiO2 molecules. To double check if the equation is balanced, we can see that there are 2 moles of sil...

JonathanSung_2G

Either format works, of course assuming the question doesn't specify which one you need. Another thing to remember is to make sure you convert to sig figs at the end when writing your final answer; converting to sig figs midway through solving the problem could potentially result in inaccurate numbe...

JonathanSung_2G

The volume in the denominator of the equation you listed is the volume of the entire solution. Let's say you need to find the molarity of 0.50 mol of X in 1L solution. You would use 1L as your volume when calculating the molarity. Think of it as you're already accounting for the volume of solute by ...

JonathanSung_2G

When diluting a solution, it is the concentration (molarity) of the solution that will change. Mathematically, if concentration c=n(moles)/v(volume), diluting the solution will increase the volume v but the moles n will stay the same. The moles of solute will only increase if more solute is added.

JonathanSung_2G

Even if the coefficients do not add up to the same value on the left and right hand side of the equation, the law of conservation of mass is still upheld. Even if the moles of molecules are not equal on both sides (4 vs. 3 in your example) we can still see that there are 2 moles of Na, 4 moles of H,...

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