CHEMISTRY COMMUNITY

Rebekah Alfred 1J

In determining the oxidizing agent and reducing agent, I would suggest figuring out the various oxidation states first.

Rebekah Alfred 1J

When E cell is equal to zero, the cell cannot do work.

Rebekah Alfred 1J

As this visual shows, the units for the rate constant change, but the units for the rate remain the same.

Rebekah Alfred 1J

Since neither Br(aq) and Br(g) are conducting solids, Pt(s) has to be included in the cell diagram.

Rebekah Alfred 1J

This video might help you understand how to distinguish between intermediates and catalysts:
https://www.brightstorm.com/science/chemistry/chemical-reaction-rates/tips-on-differentiating-between-a-catalyst-and-an-intermediate/

Rebekah Alfred 1J

The intermediate is not included in the rate law expression. You can recognize a species is an intermediate if it is in the steps, but not included in the overall reaction.

Rebekah Alfred 1J

When calculating Q for the Nernst equation, you can use both partial pressure and concentration. Before, all the values plugged into Q had to be all concentration values or partial pressure values.

Rebekah Alfred 1J

With average rate, there is a larger error because you are treating the curve as a line when taking two points into consideration. However, with instantaneous rate, you are using only one point to get the slope, which results in a more accurate slope.

Rebekah Alfred 1J

My TA mentioned that E cell can be negative sometimes and I believe there is a question with a negative E cell in section 6N.

Rebekah Alfred 1J

CMaduno_1L wrote:if you are referring to the reaction rate constant (lowercase k), then I believe that varying the temperature and solvent would affect its value.

An increase in temperature causes an increase in the rate constant (k) and a decrease in temperature causes a decrease in the rate constant (k).

Rebekah Alfred 1J

Galvanic and voltaic cells are the same thing, but with different names. However, in comparison to galvanic cells, electrolytic cells convert electrical energy into chemical energy while galvanic cells convert chemical energy into electrical energy. Here's a chart that shows the key difference betw...

Rebekah Alfred 1J

In a cell diagram, the anode (oxidation reaction) is on the left and the cathode (reduction reaction) is on the right.

Rebekah Alfred 1J

One of the UAs said that the values that will be given to us will be reduction potentials, not oxidation potentials.

Rebekah Alfred 1J

Bryce Barbee wrote:During discussion, my TA would tell us which one was the anode and which was the cathode. I think it is based off of left and right usually though.

My TA also would tell us which one was the anode and which one was the cathode.

Rebekah Alfred 1J

On the class website, it says that "Test 2 covers 2nd page of Outline 4: Thermodynamics and Outline 5: Electrochemistry and its Applications."

Rebekah Alfred 1J

In this example, 1 mole of electrons is transferred:

Rebekah Alfred 1J

C2H5OH and C2H4O have a net charge of zero, but (Cr2O7)-2 has a net charge of -2 and (Cr)3+ has a net charge of +3.

Rebekah Alfred 1J

This image might help you see the correlations better:

Rebekah Alfred 1J

To calculate K, you need to manipulate the first equation in this image to the last equation:

Rebekah Alfred 1J

Increasing the pressure of a system with an inert gas does not affect the equilibrium constant. Some examples of inert gases are Helium (He), Argon (Ar), Neon (Ne), Krypton (Kr), and Xenon (Xe).

Rebekah Alfred 1J

To add to the above statement, the energy lost by the system doing work is replaced by heat flow, q, into the system, which is why q=-w.

Rebekah Alfred 1J

In lecture, Dr. Lavelle mentioned this equation while explaining how a system loses energy as a result of doing work of expansion.

Rebekah Alfred 1J

Changes in internal energy are a function of initial and final states. When delta U is zero, a gas still can expand because it is a spontaneous process with an entropy increase.

Rebekah Alfred 1J

When the volume of the reactants is equal to the volume of the products, delta U is equal to delta H.

Rebekah Alfred 1J

Leslie Almaraz 4G wrote:What about an adiabatic system? is there a different equation for that?

In an adiabatic system, there is no heat transfer (q=0).

Rebekah Alfred 1J

A negative delta U occurs when the final internal energy is lower than the initial internal energy.

Rebekah Alfred 1J

We know if it is a spontaneous process if the delta S of the universe is positive, which means that the delta G of the system must be negative.

Rebekah Alfred 1J

Here's a visual that might help you:

Rebekah Alfred 1J

The substance with a greater temperature (hotter) will be the one transferring heat to the substance with a lower temperature (colder), which is why the negative sign will be given to the water (which is losing its heat).

Rebekah Alfred 1J

Here's a visual that shows an example of when work is negative:

Rebekah Alfred 1J

In a closed system, the system can exchange energy with the surroundings. In an isolated system, nothing exchanges with the surroundings, which means that we assume the insulation is effective and there is no energy being exchanged.

Rebekah Alfred 1J

A spontaneous reaction is a reaction that occurs in a given set of conditions without intervention. In lecture, Dr. Lavelle said that a spontaneous reaction means it is a favorable reaction.

Rebekah Alfred 1J

If you look at a heating curve, the slanted lines represent the temperature increasing, and the flat lines represent the phase change where there is no change in temperature. For example, on the first straight line, the leftmost point represents solid water at 0 degrees celsius, and on the most rig...

Rebekah Alfred 1J

Delta H is the change in enthalpy, delta U is the change in internal energy of the system, and delta S is related to entropy. The delta U (or delta H) and delta S of a system provides information on how the system will change.

Rebekah Alfred 1J

This is what a bomb calorimeter looks like: https://chem.libretexts.org/@api/deki/files/58277/c3f2def4f1ad04996c40999978baa644.jpg?revision=1&size=bestfit&width=435 This is what a calorimeter looks like: https://www.chem.fsu.edu/chemlab/chm1045/cpcalorimeter.png

Rebekah Alfred 1J

If K is small (K< 10^-3), we know that there are more reactants at equilibrium ("equilibrium sits to the left"). If K=1, both reactions are equally favorable, which is rare. If K is large (K> 10^3), there are more products at equilibrium ("equilibrium sits to the right"). For int...

Rebekah Alfred 1J

This visual shows how the proton transfer occurs between two water molecules:

Rebekah Alfred 1J

These are all the state functions that Dr. Lavelle mentioned during lecture: energy, pressure, volume, temperature, density, and heat capacity.

Rebekah Alfred 1J

Endothermic reactions use heat as a reactant because they require energy. Exothermic reactions use heat as a product because they release energy. This might be a helpful visual to see the role heat/energy plays in endothermic and exothermic reactions: https://i.ytimg.com/vi/APlWB5e2AP8/maxresdefaul...

Rebekah Alfred 1J

In the textbook, it says Hess's law states that "the overall reaction enthalpy is the sum of the reaction enthalpies of the steps into which the reaction can be divided."

Rebekah Alfred 1J

In the step up sessions I have attended, the UAs usually write out the questions for students to solve on the board instead of handing out worksheets or posting the questions online. I think the only time worksheets are handed out are in the workshops on Mondays, but they are not posted online.

Rebekah Alfred 1J

Use these Learning Outcomes to prepare for Test 1: Outline 1: Chemical Equilibrium https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14B/Chem14B_Outline1_Chemical_Equilibrium.pdf Outline 2: Acids and Bases https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14B/Chem14B_Outline...

Rebekah Alfred 1J

It is necessary to find Q before K because we need to discern whether the forward or reverse reaction is favored. For this question, Q=0.75 and, since Q < K, we know the forward reaction is favored.

Rebekah Alfred 1J

When Q < K at some time during the reaction, then [R] > [P] and the forward reaction is favored (which is the same thing as saying it "proceeds towards the products" and "proceeds to the right").

Rebekah Alfred 1J

Le Chatelier's principle states that chemical reactions adjust to minimize the effect of changes.

Rebekah Alfred 1J

This might be a helpful visual that shows how the total pressure of the gas mixture is the sum of the partial pressures of each individual gas:

Rebekah Alfred 1J

The activities of pure solids and liquids are equal to 1, which is why they don't affect the equilibrium constant and why you don't include them in your ICE table.

Rebekah Alfred 1J

When trying to understand why liquids and solids are not included in the equilibrium constant, remember that the activity, which Dr. Lavelle discussed when going over equilibrium constants, of any solid or liquid in a reaction is equal to 1.

Rebekah Alfred 1J

This website has a section called "The Effect of a Catalyst on Equilibrium" that might help you understand more thoroughly what happens if a catalyst is added: https://courses.lumenlearning.com/boundless-chemistry/chapter/factors-that-affect-chemical-equilibrium/

Rebekah Alfred 1J

Typically, you see what value in the question has the least amount of significant figures to determine how many significant figures your answer will need to be rounded to, which means you consider both the K constant and the initial concentrations.

Rebekah Alfred 1J

My TA and the UAs that held review sessions recommended knowing the most common strong acids and strong bases.

Rebekah Alfred 1J

You identify acidic and basic salts by identifying what kind of ions they have. If a salt contains a cation of a strong base and an anion of a weak acid (like NaOCl), the anion (ClO-) will acts as a base and increase the pH (making the salt basic).

Rebekah Alfred 1J

A Lewis acid is an electron pair acceptor and a Lewis base is an electron pair donor. From part a of the question, we find that SO3 would react with H2O2 to form H2SO5. Since Sulfur has a vacant orbital, we know that it would accept the electron pair, which makes it the Lewis acid.

Rebekah Alfred 1J

For these questions, it is important to know the most common strong acids and bases. When you see the salt (NH4)3PO4, you should recognize it comes from a weak base and strong acid, so when the salt is placed in water it would be acidic (pH<7).

Rebekah Alfred 1J

In determining the strength of an acid, there are two possible things you look at: 1. Bond Length (the longer the bond, the weaker it is) 2. Stability of the anion (which is the conjugate base)- it can be stabilized by a high electronegativity of one of the atoms and by resonance. You determine whic...

Rebekah Alfred 1J

Here's an image that depicts the example given about boron trifluoride:

Rebekah Alfred 1J

When determining which acid is stronger and the pKa is given, the acid with the lower pKa will be stronger.

Rebekah Alfred 1J

Here is an example of a polydentate ligand:

Ethylenediaminetetraaceticacid acid (EDTA)

Rebekah Alfred 1J

Here is an example that shows how a cis molecule and a trans molecule look:

Rebekah Alfred 1J

In lecture, we went over the example of 3 hydrocarbons at room temperature: Pentane (C5H12) was a mobile fluid at room temperature. Pentadecane (C15H32) was a viscous fluid at room temperature. Octadecane (C18H35) was a waxy solid at room temperature. In this example, it is clear how the number of e...

Rebekah Alfred 1J

The single unpaired electron on a radical is counted as a single region of high electron density, but it has a weaker repulsion than lone pairs. Therefore, the single unpaired electron on a radical does affect the shape of a molecule, but it does not affect the shape of a molecule as much as a lone ...

Rebekah Alfred 1J

A sigma bond by itself is stronger than a pi bond by itself because the atomic orbitals that form sigma bonds overlap more than the orbitals that form pi bonds. Double bonds are stronger than a single bond because they consist of one sigma bond and one pi bond, whereas a single bond consists of only...

Rebekah Alfred 1J

When the VSEPR formula is AX2E3 or AX2E4, remember that their shapes will be linear (not bent or angular) and they both will have a bond angle of 180 degrees.

Rebekah Alfred 1J

When the intermolecular forces are stronger, it requires more energy to break the bonds, thus making the melting/boiling point higher. For example, CHI3 will have a higher melting point than CHF3 because Iodine has a higher polarizability than Fluorine, which makes the intermolecular forces stronger...

Rebekah Alfred 1J

I think we should know the ones in the table that this link takes you to (the link was included in an email from Dr. Lavelle): https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14A/NamingCoordinationCompounds.pdf

Rebekah Alfred 1J

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

Rebekah Alfred 1J

Debora Fernandez Clemente_ 4H wrote:What are other topics other than the VSEPR models we should know for test two? or is only that?

These are a few of the topics my TA mentioned we should know for Test 2: hydrogen bonding, molecular shape, bond angles, dipole moments, polarity/non-polarity.

Rebekah Alfred 1J

Katherine Brenner 3H wrote:Can someone explain how we get that formula?

For example, NH3 will have a VSEPR formula of AX3E because it has three bonded atoms and one lone pair. CH3Cl will have a VSEPR formula of AX4 because it has four bonded atoms and no lone pairs.

Rebekah Alfred 1J

The lone pairs on the central atom influence molecular shape and, thus, affect the bond angles. Specifically, the addition of lone pairs force the bonding electrons closer together, making the bond angles smaller.

Rebekah Alfred 1J

I believe we should know the bond angles of the molecular shapes we go over during lecture (i.e. Linear-180 degrees & Tetrahedral-109.5 degrees). We predict bond angles when there are lone pairs on the central atom, which influence the molecular shape and affect the bond angle. For example, we c...

Rebekah Alfred 1J

Destiny_Ryales_3J wrote:Is this supposed to be on next week’s test?

My TA said that we should know about polar and non-polar molecules for Test 2.

Rebekah Alfred 1J

Examples of Dipole-Dipole Forces:

Rebekah Alfred 1J

The electronegativity of N, O, and F is important because it affects the strength of the hydrogen bond.

Rebekah Alfred 1J

When drawing Lewis structures, keep in mind that you should avoid placing a negative formal charge on the central atom.

Rebekah Alfred 1J

You generally want to avoid putting a negative formal charge on a central atom.

Rebekah Alfred 1J

This chart that compares sigma bonds and pi bonds could be helpful to you!

Rebekah Alfred 1J

Cl can have an expanded octet because it is located in Period 3. Starting from Period 3 (where the principal quantum number is n=3), the d-orbital becomes available and allows the additional electrons to go there, creating an expanded octet.

Rebekah Alfred 1J

A radical is a compound with an unpaired electron, which makes it highly reactive. Radicals are significant because they are exceptions to the octet guideline.

Rebekah Alfred 1J

Elements starting from period 3 can have an expanded octet because they have a d-orbital that the additional electrons can go into. In other words, the d orbitals become available beginning with the principle quantum number n=3.

Rebekah Alfred 1J

If an atom does not satisfy the octet rule, it means there are less than 8 electrons (i.e. the first 4 elements of the periodic table). An expanded octet refers to when there are more than 8 electrons; these are also known as exceptions to the octet rule. Examples of elements that typically have le...

Rebekah Alfred 1J

In the review session today, it was mentioned that the elements (with a d-orbital) in Period 3 and later can have an expanded octet.

Rebekah Alfred 1J

I think the main thing to take away from those values that were given to us is how the single bond length in C—C is longer that the double bond length between 2 Carbons.

Rebekah Alfred 1J

On the other hand, anions are bigger than their parent atoms because (as indicated by their negative charge) they gained electrons.

Rebekah Alfred 1J

He has 2 valence electrons, which means it has one electron pair. That is why the Lewis structure for it will look like this- :He

Rebekah Alfred 1J

For example, the electron configuration for Fe is [Ar]4s2 3d6 and we know that it has 2 valence electrons because of 4s2. We do not count the 3d6.

Rebekah Alfred 1J

Sartaj Bal 3H wrote:To express the wave function in three dimensions, the three quantum numbers (n, m, and ml) need to be used. This is known as the atomic orbital, or the region of space where the electron is located.

These atomic orbitals are mathematical functions that describe the behavior of the electrons.

Rebekah Alfred 1J

Light sources with long wavelengths (low frequency light) cannot eject electrons even with a high intensity light, which means it does not act like a classical wave. When the frequency increases (or you shorten the wavelength), the energy per photon increases as well, which causes the ejection of el...

Rebekah Alfred 1J

Thank you so much for taking the time to share your experience with us and answer our questions! Your advice is extremely helpful and motivating. Is there anything you didn't expect to experience as a medical student that you wish you knew before starting medical school? It wasn't really an experie...

Rebekah Alfred 1J

You would only be able to do this if the problem provides the molar mass of the molecular formula. This mass is usually provided in problems if they want you to find the molecular formula. You would then divide the given molar mass by the mass of the empirical formula mass. The answer you get is wh...

Rebekah Alfred 1J

It doesn't matter when solving problems for molarity/dilution, but it is is important to include when writing out the chemical equations.

Rebekah Alfred 1J

Does anyone know if Sapling is mandatory?

I don't think it is.

Rebekah Alfred 1J

If you want to practice determining how many significant figures a number has, this could be helpful:
https://www.khanacademy.org/math/arithmetic-home/arith-review-decimals/arithmetic-significant-figures-tutorial/e/significant_figures_1

Rebekah Alfred 1J

Hi! Here are some of the basic rules for significant figures: 1. All non-zero numbers are significant. Ex. 457 has 3 significant figures. 2. Zeros in between two non-zero numbers are significant. Ex. 9023 has 4 significant figures. 3. Trailing zeros (the zeros at the end) are only significant if the...

Rebekah Alfred 1J

In our Chemical Principles 7th edition textbook, the practice problems for each of the fundamental sections are located at the end of every section.

Rebekah Alfred 1J

When I do these type of problems, I often have to convert the units from grams to moles or milliliters to liters. So make sure you pay attention to the units before you start solving the problems using the equations M1V1=M2V2 and Molarity=moles of solute/liters of solution.

Rebekah Alfred 1J

When you are solving these type of problems, make sure you pay attention to the units! For this problem, some of the unit conversions you would need to do are converting the grams into moles and milliliters into liters.

Rebekah Alfred 1J

When balancing an equation, I first write out how many of each atom are on the side of the reactants and how many of each atom are on the side of the products. If you want to practice balancing chemical equations, this practice on Khan Academy might help you: https://www.khanacademy.org/science/chem...

Rebekah Alfred 1J

Here's an example of how to calculate molar mass: Let's say you need to find the molar mass of NaCl. This can be done by adding the atomic mass of sodium (22.99 g/mol) and the atomic mass of chlorine (35.45 g/mol). (22.99 g/mol) + (35.45g/mol)= 58.44 g/mol Therefore, the molar mass of NaCl will be 5...

Rebekah Alfred 1J

How do I tell what 0's are significant and which aren't? If you want to practice determining how many significant figures a number has, this could be helpful: https://www.khanacademy.org/math/arithmetic-home/arith-review-decimals/arithmetic-significant-figures-tutorial/e/significant_figures_1

Rebekah Alfred 1J

To see if the empirical formula is the same as the molecular formula or if it is just the ratio, you have to find the molar mass of the empirical formula. If this molar mass is the same as the molar mass of molecular compound, you know that the empirical formula and molecular formula are the same. F...

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