CHEMISTRY COMMUNITY

Matthew Tsai 2H

The negative just means that the reactant concentration is decreasing over time, which makes sense given that they are consumed as the reaction progresses.

Matthew Tsai 2H

Generally, k increases with temperature.

Matthew Tsai 2H

Yes, the overall rate would depend on the slowest step.

Matthew Tsai 2H

I don't believe any derivations or integrals will be tested; the formulas are all provided on the equation sheet.

Matthew Tsai 2H

You would use the same integrated rate law depending on the order of reaction and set concentration to 1/4, 1/8, etc. of the initial concentration and solve for t.

Matthew Tsai 2H

Can someone explain what emf is exactly and if we need to know anything about it other than that it represents max work?

Matthew Tsai 2H

Assuming temperature and pressure are constant, work can only be done electrochemically. In this case, max work can be calculated as the amount of charge (signified by nF, moles of electrons*charge in coulombs per one mole of electrons) multiplied by the potential difference they are traveling acros...

Matthew Tsai 2H

Whichever species has the higher positive standard reduction potential will act as the oxidizing agent and be reduced.

Matthew Tsai 2H

If the reaction is exothermic, increasing the temperature will decrease K, and if the reaction is endothermic, increasing the temperature will increase K.

Matthew Tsai 2H

I think both of these equations can basically be used interchangeably.

Matthew Tsai 2H

Eo is under standard conditions, while E changes as the reaction progresses.

Matthew Tsai 2H

Because intensive properties do not change when amount of substance is changed, this is most relevant when it comes to balancing redox reactions. Multiplying an entire half reaction by any constant does not change Eo.

Matthew Tsai 2H

In a galvanic cell, electrons flow spontaneously, but in an electrolytic cell, electrical energy drives electron flow in the opposite direction (nonspontaneous).

Matthew Tsai 2H

The value for the maximum electrical potential difference between the cathode and anode is the voltage of the cell.

Matthew Tsai 2H

Cations from the salt bridge travel to the solution surrounding the cathode, not the cathode itself, in order to balance out the negative charge buildup in solution as positive cations are precipitated out of the solution. Otherwise, buildup of charge would prevent electrons from flowing through the...

Matthew Tsai 2H

All galvanic cells are electrochemical cells; a galvanic cell specifically refers to an electrochemical cell in which a voltage difference across a cathode/anode drives a spontaneous redox reaction.

Matthew Tsai 2H

Delta G is zero at equilibrium because the concentrations of products and reactants are not changing and neither the forward nor reverse reaction is favored; given that the system isn't doing work, there is nothing that would cause a change in free energy of the system.

Matthew Tsai 2H

Delta G naught is a constant for a given chemical equation (measured under standard conditions) while delta G changes as the reaction progresses and conditions change.

Matthew Tsai 2H

As the reaction progresses, I is being converted to I2, which is shown as [I] decreases as [I2] increases proportionally. Eventually, once the system reaches equilibrium, the concentrations of the reactants and products remain constant.

Matthew Tsai 2H

∆Gº=-RTlnK is derived from the equation ∆G = ∆Gº + RT lnQ; if the reaction is at equilibrium and no work is being done, ∆G is equal to zero and Q=K which is where the equation comes from.

Matthew Tsai 2H

∆Gº refers to change in Gibbs free energy under standard conditions (usually this value will be given)
∆G refers to change in Gibbs free energy for the system (this is usually what we will be calculating for)

Matthew Tsai 2H

Any time there is no change in temperature, delta U is zero.

Matthew Tsai 2H

When a system does work, it loses energy in the process, so the value for w is negative.

Matthew Tsai 2H

When the change in internal energy is zero, it means that all the heat added to the system must be used by the system to do work.

Matthew Tsai 2H

Extensive properties depend on the amount of substance (heat capacity) while intensive properties are applicable to the substance in general (specific heat)

Matthew Tsai 2H

Generally we refer to everything from the perspective of the system, which means that positive work means that work is being done on the system. You can also think of doing work ON some system as adding energy to it, so it makes sense that work is positive.

Matthew Tsai 2H

It is important to note that the beaker does not insulate because this ensures that transfer of heat can still occur (otherwise it would be an isolated system).

Matthew Tsai 2H

The main difference is that heat exchange can still occur in a closed system but not in an isolated system.

Matthew Tsai 2H

When work is done BY a system, it is using energy in order to do the work, so w is negative. When work is done ON a system, the system absorbs energy so w is positive.

Matthew Tsai 2H

In this case, the only time you would need to integrate would be if asked to derive the equation, in which case you would need to understand each step of the integral on a conceptual level. For actual calculations you can just use the formula.

Matthew Tsai 2H

As an example, change in temperature from 0C to 100C = 100C. If you convert to K, you get 273K to 373K = 100K.

Matthew Tsai 2H

In the equation, q represents heat of the substance reacted inside the calorimeter and C(cal) is the heat capacity of the calorimeter. The idea is that when heat is lost by the chemical reaction, it is gained by the calorimeter and vice versa so the negative sign is necessary.

Matthew Tsai 2H

A closed system allows for exchange of heat with its surroundings (has the ability to release and/or absorb heat) while an isolated system does not.

Matthew Tsai 2H

The main difference is insulation. Even if a system is sealed and no exchange of matter is occurring, heat exchange can still occur (e.g. a heat pack gives off heat)

Matthew Tsai 2H

An example would be any sealed, insulated container. Technically insulation is never perfect, and a very minimal amount of heat exchange can still occur between system and surroundings, but not enough to have any significant impact on calculations.

Matthew Tsai 2H

What are some actual practical ways external pressure can constantly change during reversible expansion in order to remain equal to the pressure of the gas in the system?

Matthew Tsai 2H

Low pKa and low pH are generally associated with each other, while high pKa and high pH are also associated with each other. The same is true for pKb and pOH.

Matthew Tsai 2H

H2O could be included if it is in the gas phase and you are calculating Kp. Otherwise it should be excluded if it is liquid (which most of the time it will be).

Matthew Tsai 2H

A question that asks if a reaction is endothermic or exothermic should provide delta H, in which if delta H is positive, the reaction is endothermic and if it is negative, the reaction is exothermic.

Matthew Tsai 2H

Adding an inert gas does not affect the equilibrium constant because it does not affect the partial pressure of any reactants/products.

Matthew Tsai 2H

Also keep in mind x cannot be approximated to zero if it is by itself. So values like x, x^2, etc. would be kept but values like 0.1-x, 0.1+x, etc. can be approximated to 0.1

Matthew Tsai 2H

If a reaction is endothermic (requires heat), then addition of heat drives the reaction forward because the heat is used similarly to another reactant. The same concept applies if heat is removed, or also for exothermic reactions.

Matthew Tsai 2H

As mentioned in this thread, because the units in the numerator and the denominator are the same, you can think of K as "unitless" because it just indicates the direction and degree of reaction.

Matthew Tsai 2H

Based on lectures/homework problems, the main use of the ideal gas law has been to convert between pressure and concentration of a gas.

Matthew Tsai 2H

Regardless of conditions, solids and liquids will be pure substances with a concentration of 1, so for calculating the equilibrium constant of a reaction, they would have no effect.

Matthew Tsai 2H

When given a dissolution reaction, it would make sense that if the forward reaction is favored (which means the Ksp value is high), the reactant is highly soluble.

Matthew Tsai 2H

In lecture, Lavelle said that if the value is between the two limits neither the reactant nor the product is favored and it is in a sort of intermediate state. Does that mean that we most likely won't be asked a question on whether the reactants or products are favored if the K value falls between ...

Matthew Tsai 2H

There are several units for pressure, but as long as you stay consistent with the same unit, the Kp value should be the same.

Matthew Tsai 2H

For gases you should use Kp, because gases would have a "concentration" in the same way as aqueous solutions, as there is not really a solvent. Instead, the equivalent of concentration for a gas would be its partial pressure.

Matthew Tsai 2H

Yes, in this case the partial pressures would be different because the moles of gas are different but the volume of the containers that they are in is the same.

Matthew Tsai 2H

Nope, partial pressure can vary based on several factors (e.g. temperature, moles of gas, volume of container).

Matthew Tsai 2H

It may be more intuitive to memorize the charges of common ligands (from the chart) and use that combined with net charge of the compound/complex in order to calculate the charge of the transition metal cation.

Matthew Tsai 2H

Due to the smaller size of fluorine, the H-F bond is very strong, preventing much dissociation in water and thus preventing HF from donating a proton.

Matthew Tsai 2H

Bronsted acids are proton donors and Bronsted bases are proton acceptors. When an acid donates its proton it becomes the conjugate base and when the base accepts the proton it becomes the conjugate acid.

Matthew Tsai 2H

Not sure if you will be penalized, but it would probably be best to get used to including double arrows for equilibrium reactions.

Matthew Tsai 2H

The Lewis definition is just broader, so all Bronsted acids and bases will work as Lewis acids and bases as well. The only difference would be when dealing with compounds that don't have H+, in which case it would only work as a Lewis acid/base.

Matthew Tsai 2H

Coordination number just refers to the total number of bound ligands in the coordination sphere.

Matthew Tsai 2H

A ligand must have a lone pair; otherwise it would be unable to bind to the central cation and form a coordination complex.

Matthew Tsai 2H

A ligand is polydentate if it has multiple sites at which it can bind to a cation. The shape would look similar to an incomplete ring(s) with lone pairs at the ends.

Matthew Tsai 2H

Any compound is considered amphoteric if it acts as both an acid and a base, in other words it can either donate a proton or accept one (e.g. H2O)

Matthew Tsai 2H

Lewis bases should have a lone pair to donate while lewis acids are usually positively charged and therefore able to accept an electron pair.

Matthew Tsai 2H

If there are either one or two electron pairs on the central atom along with the two bonded atoms, it would result in bent shape. Otherwise, the molecule would be linear in shape.

Matthew Tsai 2H

When the central atom has one or more lone pairs, the higher strength of electron pair-electron pair and electron pair-bonded pair repulsion compared to bonded pair-bonded pair repulsion results in the bonds being pushed to form smaller angles.

Matthew Tsai 2H

I think it would be okay to just put AXE and the 1s would be redundant.

Matthew Tsai 2H

Yes, generally with the conditions necessary for hydrogen bonding to occur, which is hydrogen bonded to a very electronegative atom (F, O, or N), the resulting molecule would be polar.

Matthew Tsai 2H

You would get a T-shape with the formula AX3E2, so three bonded atoms (two along the y-axis) and one approximately perpendicular to them with two lone pairs.

Matthew Tsai 2H

An induced dipole refers to when a polar molecule induces a dipole moment in a nonpolar molecule when they are near each other. For example, if the nonpolar molecule is near a positive dipole of a polar molecule, its electrons would be attracted towards it, forming a negative dipole. While the nonpo...

Matthew Tsai 2H

If you visualize the electron cloud surrounding a molecule, it would make sense that the more electronegative atom(s) would pull the shared electrons closer to itself, forming a negative dipole moment on that region of the molecule and a positive dipole moment on the opposite end.

Matthew Tsai 2H

Forces that affect boiling point are based on the strength of the intermolecular forces, so the ones that would most affect BP are the ones that are the strongest. So ionic interactions would have the highest effect, followed by hydrogen bonding, then ion-dipole interactions, and the last few all b...

Matthew Tsai 2H

I believe h-bonding is the strongest type of intermolecular force that can exist in covalent molecules as it is the strongest dipole-dipole interaction. But in general, ion-ion attraction is the strongest intermolecular force due to the large charge difference that exists between atoms.

Matthew Tsai 2H

VSEPR stands for Valence Shell Electron Pair Repulsion, and VSEPR theory refers to the concept that molecular shape can be determined (at least for smaller molecules) based on the number of bonded atoms and lone pairs due to their tendency to maximize distance from each other. The AXE formula just i...

Matthew Tsai 2H

For intermolecular forces:
LDF/Van der Waals < dipole-dipole < h-bonds (the strongest type of dipole-dipole attraction)

Matthew Tsai 2H

Generally, if there is a particular part of the molecule that is more electronegative and the shape of the molecule does not allow for the charges to cancel (e.g. if the structure is not symmetrical), then the molecule is polar.

Matthew Tsai 2H

Simply put, a coordinate covalent bond occurs when one atom provides both electrons used in the bond, as opposed to one electron from each atom in a normal covalent bond.

Matthew Tsai 2H

The common exceptions of atoms that do not require at least a full octet are the first four elements, along with boron and aluminum. Otherwise, there aren't really any naturally occurring examples of an element that doesn't fill its octet so any resonance structures that break the rule wouldn't be v...

Matthew Tsai 2H

Yes; because these elements are located in the second period, they do not have access to the empty 3d orbital and cannot hold an expanded octet.

Matthew Tsai 2H

Yes, you would need to convert to kilograms because Planck's constant, h, uses kilograms.

Matthew Tsai 2H

The quantum number that you would want to use to determine the number of orbitals in a given subshell would be l, which identifies the actual subshell: s (l=0), p (l=1), d (l=2), or f (l=3). They have 1, 3, 5, and 7 orbitals respectively.

Matthew Tsai 2H

Threshold energy (represented by the work function) is relevant to the photoelectric effect because it refers to the energy required to remove a single electron from a certain metal surface. On the other hand, ionization energy is defined as the energy required to remove an electron from an atom in ...

Matthew Tsai 2H

In a coordinate covalent bond, one atom provides both of the bonding valence electrons.

Matthew Tsai 2H

Elements beyond the third row of the periodic table are able to hold extra valence electrons beyond the octet because they have an unfilled d-orbital.

Matthew Tsai 2H

Resonance just refers to the concept that a single molecule is not restricted to a rigid structure. Rather, it can change into unique resonance structures.

Matthew Tsai 2H

Yep, as previous answers stated, lone pairs just refer to a pair of electrons in an orbital that are not shared with another atom.

Matthew Tsai 2H

Yes, covalent bonds can only be formed between two nonmetals.

Matthew Tsai 2H

For the s and p block elements, the number of valence electrons is just the group number, but it can vary for the d block based on electron configuration. Charged ions may also carry extra or fewer valence electrons.

Matthew Tsai 2H

Because there are more electrons being shared between the atoms, there is a greater attractive force that pulls the atoms closer together, shortening bond length.

Matthew Tsai 2H

A nodal plane is just a planar area in which electrons cannot be located. Due to the spherical shape of the s-orbital, it does not have any.

Matthew Tsai 2H

I don't think we will be expected to calculate a specific value for effective nuclear charge, but to conceptually understand it, effective nuclear charge can be used to determine which energy state an electron is in due to the shielding effect.

Matthew Tsai 2H

Are there any specific equations or constants that aren't provided on the formula sheet but will be included on tests?

Matthew Tsai 2H

Intensity is determined by amplitude of a wave while frequency is determined by the length (specifically how short) a wave is. In the famous photoelectric effect experiment, it was determined that the energy of light is proportional to frequency, rather than the expected which was amplitude/intensit...

Matthew Tsai 2H

Like previous answers mentioned, momentum is mass * velocity. So, if an object is moving at a slow velocity but has a large mass, it can still have a substantial momentum.

Matthew Tsai 2H

Yes, as previous answers mentioned, they refer to the same thing. I believe "c" is used in general reference to some concentration while "M" is a more specific reference to molarity, which is a specific measure defined as mol/L.

Matthew Tsai 2H

The same rules for significant figures should still apply, and generally that would be anywhere from 4-5 sig figs based on what's given in the problem.

Matthew Tsai 2H

It's generally good practice to convert all the coefficients to integers, as that gives the most accurate representation of the chemical reaction.

Matthew Tsai 2H

For the first test, it won't be required to have the names and formulas of polyatomic ions memorized, but I believe they will not always be provided in future tests.

Matthew Tsai 2H

One angstrom is 10^-10 meters (m), or 100 picometers (pm).

Matthew Tsai 2H

Molarity is relevant when dealing with solutions. By doing molarity calculations, you can determine the moles of solute and how you may need to dilute the solution.

Matthew Tsai 2H

Both are generally used to refer to solution concentration. Molarity, as we know, is calculated as moles of solute/L of solvent. Meanwhile, molality is moles solute/kg solvent. I think most of the time though, molarity is the preferred measurement to use.

Matthew Tsai 2H

Molar mass refers to the mass in grams of one mole of a certain particle. For atoms, the molar mass is indicated on the periodic table (same as atomic mass). For molecules, just add up the molar masses of all its constituents.

Matthew Tsai 2H

As the previous answers mentioned, it's just a specific name for the number 6.022 x 10^23. Generally you would just think of it as a mole.

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