CHEMISTRY COMMUNITY

MinuChoi

I think it has to do with the fact that a first-order reaction's rate scales directly with the concentration (rate = k[A]). If you increase the concentration of the reactant, the rate will also increase by the same scalar. The half life isn't affected by this change because while there would be more...

MinuChoi

e^ln(x) = x

MinuChoi

The internal energy of an ideal gas is proportional to its mass and temperature (deltaU = 1.5*n*R*deltaT). Both mass and temperature are constant in an isothermal expansion, so the internal energy is constant (deltaU = 0). Internal energy, heat, and work are related by deltaU = q + w. deltaU = 0, so...

MinuChoi

Given the above statements are true: deltaG = deltaH - T*deltaS. If deltaS > 0, then T*deltaS is positive. Increasing T means that T*deltaS becomes larger, but it is subtracted. So deltaG would be decreasing as T increases. If your deltaG is positive before you start increasing temperature, then the...

MinuChoi

A's numerical value is unique for a reaction just like the rate constant k. It also has the same units as the rate constant. A unique first-order reaction has a unique rate constant k and frequency factor A, both with units s -1 (because it is a first-order reaction). k and A are related by the Arrh...

MinuChoi

Note that the A constant is a unique value for each chemical reaction, like the k constant.

MinuChoi

Also need to know: for a zero-order reaction, t_1/2=[A₀]/2k

MinuChoi

The half-life of a second-order reaction is t_1/2=1/k[A₀]

MinuChoi

Adding on, keep in mind that these units are the same for all rates regardless of order; because of this, the k constant of the reaction will have different units based on the reaction's order.

MinuChoi

A is a unique constant for a chemical reaction, similar to the k constant of a reaction.

MinuChoi

I noticed that some cell diagrams in the homework don't include salt bridges.. when is this possible? If you look at figures 6L2 and 6L3 in the textbook, there are cells in which the two half-reaction solutions have some contact with each other; so some cells have other means of ion transfer than a...

MinuChoi

For a first-order reaction, the rate is directly proportional to the concentration (e.g. if the concentration of a reactant was doubled, the rate of the reaction would be doubled.) For a second-order reaction, the rate is proportional to the second power (square) of the concentration (e.g. if the co...

MinuChoi

Units are change in concentration per second (mol x L^-1 x s^-1). Keep in mind that because of this, the reaction constant k may have different units for different rate laws.

MinuChoi

Integrated rate laws show reaction rate vs. time. Differential rate laws show rate as a function of the reactant concentration [R].

MinuChoi

Note that there are two sulfur atoms involved. The oxidation number of each sulfur in the reactants is +2. In the products. each is +6. Each sulfur has an oxidation number increase of +4, so there is a net transfer of eight electrons as shown in the balanced oxidation reaction.

MinuChoi

I believe the main difference is that at after you combine the half-reactions, you neutralize the H+ by adding hydroxide to each side of the equation.

MinuChoi

I think if you balance the half reaction O3 -> O2, you'll get O3 + 2H⁺ + 2e- = O2 + H2O. Notice that this actually is a reduction reaction since the ozone gains electrons (reactant side). So This would work as a half-reaction even though the oxidation numbers don't change.

MinuChoi

Adding on, keep in mind you add H2O to balance O regardless of acidic/basic reaction. It's the hydrogen/proton balancing that's different for acidic/basic.

MinuChoi

The ultimate purpose of the salt bridge is to keep the battery running (so it doesn't run itself to equilibrium quickly). It does so by maintaining charge balance as mentioned above.

MinuChoi

Keep in mind that any molecule made of only one element will have oxidation number of 0 if it's neutral charge.

MinuChoi

In an equation, you can also look at oxidation numbers. Oxidized elements will have an increased oxidation number in the products. Reduced elements will have a decreased oxidation number.

MinuChoi

Are you ignoring the H+ and H2O in the reactant and product? Do they not matter for this problem? They don't really matter for part a. To find the oxidation number of a single element in a compound, you don't really need to look at the other reactants. But you will definitely need to consider them ...

MinuChoi

Gibb's Free Energy is energy available to do work in reaction. It's different from other energy functions (like enthalpy) because it takes energy loss due to entropy into account, since real work is not 100% reversible.
It's mainly used to tell If a reaction is spontaneous.

MinuChoi

You can also look at the element's oxidation number before and after the equation. Elements whose oxidation number increases are oxidized, and vice versa for reduced. For 6K1: Given H is typically +1 oxidation and oxygen is typically -2 oxidation number, we can tell that our two elements undergoing ...

MinuChoi

I believe that is correct for almost all compounds. There are some rare exceptions like peroxides and metal hydrides.

MinuChoi

Not too sure about relation between intensive/extensive and state/nonstate, but state/non-state functions so far have usually in relation to equilibrium and energy processes (enthalpy, specific heat, and Gibbs free energy are all examples of state functions). Intensive/extensive properties are deter...

MinuChoi

I think you could find this by determining of the reaction is exothermic or endothermic reactions (endothermic reactions require a heat input). Sometimes you can tell this just by looking at the reaction. Ex. If the rxn is one reactant being broken into several products, it's probably exothermic bec...

MinuChoi

Adding on, this is a separate term from heat capacity, which is not measured per gram (The heat capacity of 5 grams of water is different than that of 10 grams of water).

MinuChoi

I thought that if it relies on the change in heat/transfer of heat then it is not a state function? Heat capacity's reliance on heat transfer is consistent (how the heat is added does not change the temperature outcome), so it is a state function. Keep in mind that enthalpy is also a state function...

MinuChoi

Adding on, because calorimeter's measure temperature changes, they will typically be seen in problems regarding specific heat capacities.

MinuChoi

Enthalpy will be positive because the molecules must absorb energy to loosen forces on each other and assume more free form.

MinuChoi

An irreversible expansion happens under a constant pressure, while the pressure for a reversible expansion will vary. So the pressure values will show differently on the graphs.

MinuChoi

Does anyone have any good videos explaining combustion reactions?

There's a brief explanation in the textbook section F63.

MinuChoi

what is the difference between extensive and intensive properties? just that they are affected by volumes?

Extensive properties change based on the amount present. Intensive properties do not.

MinuChoi

This equation is used to calculate the entropy of an object when we know how many ways the molecules in the system can be arranged without changes in energy. If we know that a molecule can be oriented in X number of ways , we can find the entropy of virtually any amount of that compound using the re...

MinuChoi

If you look at an ICE chart of the reaction, this would be the concentration of the conjugate product at equilibrium over the initial concentration of the acid/base.

MinuChoi

How do we know the heat capacity?

There's a formula for calculating it given heat and temperature (textbook p. 251). For specific heat capacities, those are unique for every substance.

MinuChoi

A state property is determined only by the "start" and "finish" values of a process. Basic example: If I walk 5m forward, then walk backward 2m, I am 3m forward from my original staring point. This displacement would be the same if I simply walked 3m forward; or if I walked 500m ...

MinuChoi

Pay attention to what the work is in reference to. Say you have a system and surroundings. If positive work is done BY the system, energy in the system is expended. This is equivalent to NEGATIVE work done ON the system. If negative work is done BY the system, energy is put into the system. This is ...

MinuChoi

Substances with higher heat capacity require more energy to raise its temperature and will have to expend more energy to cool down. If you're referring to heat transfer between two substances, the substance with a higher heat capacity will experience a smaller change in temperature because more ener...

MinuChoi

when trying to find the pH or poH of something, do We take the - log of the opposite concentration of the one we are trying to find?

No; pH = -log[H+] and pOH=-log[OH-].
But you can use pH to find pOH and vice versa because pH + pOH = 14.

MinuChoi

pH is a scale. It's logarithmic, so a linear increase in pH is not a linear increase in [H+].

MinuChoi

The 5% rule should be considered when you remove x- additions/subtracts to make the ICE equation easier to solve (when K <10^-3)

MinuChoi

The (b) equation is the original equation of the problem scaled by 1/2. If you multiple an entire equation by a scalar (in this case, 1/2), the K constant changes by being raised to that power (in this case, K=41 is raised to the power 1/2 to get K for the part b equation).

MinuChoi

Keep in mind that this problem is asking for mass, not concentration. So you don't need the MV formula because you already have the necessary concentration, but concentration x volume = mass. Concentration is independent of volume, so the 18 mol/L for the original 5mL solution applies also to the or...

MinuChoi

A common example of catalyst is enzymes, proteins that facilitate reactions in biological systems. For example, amylase is an enzyme in saliva that breaks starches from food into digestible sugars at a faster rate. Without enzymes like amylase, it would be much slower gaining energy from consumed fo...

MinuChoi

Whether or not product equilibrium concentration will increase depends on the reaction's K constant. For example, if you start with a higher concentration of reactants and products and the reaction's K value is very high, yes, product will be formed and the product concentration will be increased at...

MinuChoi

When a reaction goes right, it will begin producing more net product. One case that this will happen in this example is when reactant is added (N2 is increased)

MinuChoi

Some of the intermediate calculations are rounded in the answer book. I don't think the difference is that significant, though.

MinuChoi

The 2X change is correct. Remember, the Xs represent how much product is produced when reactant is consumed, and this is based on the stoichiometric coefficients. The 2x^2 is probably referring to the K constant. From his example, we used the ICE chart's equilibrium values to determine constant K = ...

MinuChoi

Also know where the bonds form.
A single bond has one sigma bond.
Double bonds have one sigma and one pi bond.
Triple bonds have one sigma and two pi bonds.

MinuChoi

pK = -logK. K is an equilibrium constant (Ka for acids, Kb for bases)
pKw = 14.
pKa + pKb = pKw
Stronger acids have a higher Ka value and a lower pKa value. Same goes for bases and Kb.

MinuChoi

You can just add zeros if you need more sig figs (say, 0.34 to 0.340). Also, as long as it isn't too small, you don't always have to bother with scientific notation (0.045 and 4.5x10^-2 would both likely be acceptable)

MinuChoi

That first 2 refers to the shell n=2: The orbitals in the valence shell are being hybridized (because only the valence electrons are involved in bonding) and the valence shell for carbon is n=2.

I don't think it's 100% necessary (in the textbook it's not used all the time).

MinuChoi

Both terms refer to substances that behave as acids and bases, but amphiprotic substances have hydrogen atoms (they can donate protons). So amphiprotic substances also are amphoteric, but not always vice versa. ex. Aluminum oxide is amphoteric but not amphiprotic, because it can react to both acid a...

MinuChoi

Coordination number is how many ligands are directly bonded to the central atom. This is basically everything in the coordination sphere, so coordination number = number of ligands in the bracket. For your example: in the bracket are 4 ligands (Br4) bound to the transition metal. The coordination nu...

MinuChoi

B12 specifically acts as a coenzyme in the production of said components.

MinuChoi

The sum of oxidation numbers of each component is the complex's overall charge. Anions have oxidation numbers equal to their charges. This includes anion ligands. Neutral ligands have oxidation number 0. Cations with fixed charges (like K[sup]+/sup]) have oxidation numbers equal to their charges. Fr...

MinuChoi

The cis-chlorine arrangement on cisplatin (the two chlorines are on the same side of the central atom) is what allows it to bind as a ligand to DNA molecules. This interaction interrupts DNA replication, so it can be used to treat cancerous cells. Transplatin does not have this cis-chlorine arrangem...

MinuChoi

I believe water is only monodentate because the available lone pairs are on one atom; even though there are two open lone pairs, the geometry would not favor two attractions to the same atom.

MinuChoi

Oxidation numbers are a measure of the relative number of electrons each atom in a molecule has. I believe it's measured because of how some atoms share electrons in bonds. When all the atomic numbers of the atoms in a molecule are added up, they equal the molecules overall charge. But keep in mind ...

MinuChoi

Adding on:
cis- means something like "on one side", and trans means "across".
cis-molecule are typically polar and trans-molecules are typically nonpolar.

MinuChoi

I don't think there is one. You could guess a possible shapes based on the coordination number, but its not reliable like VSEPR is (VSEPR doesn't apply to these complexes)

MinuChoi

Note that polydentate can refer to tridentate, tetradentate, etc (not mono- or bidentates). It depends on how many atoms in the ligand have lone pairs available.

MinuChoi

Adding on, note that because the sphere is the central atom and bonded ligands, not all components in a coordinate complex may be in the coordination sphere. That's why in the notation, the components in the coordinate sphere are bracketed.

MinuChoi

Polarizability in anions is how easily the electron cloud of an atom is distorted. It's a greater value for larger anions. For cations, polarizability is low but polarizing power is high (how easily it can attract the electrons from another atom. This value is typically greater for smaller, more pos...

MinuChoi

Repulsion between lone pairs > repulsion between a lone pair and a bonding pair> repulsion between bonding pairs

MinuChoi

In some shapes such as bent, it is physically impossible to have lone pairs on opposite sides of the atom. Keep in mind the tetrahedral geometry of bent molecules. For the t-shape, you'll want to look at the lone pair's angles with the other 3 bonds. In the t-shape position, the lone pairs have high...

MinuChoi

Adding on, minimal electrostatic repulsion is achieved when the electron-dense regions (bonds and lone pairs) on an atom are farthest from each other. There is an approximate maximum angle between bonds that can be achieved based on how many bonds are on that atom, and that is the basis of the bond ...

MinuChoi

We won't be asked to actually draw the dashes/wedges diagrams.

MinuChoi

I think there is a possibility of other electronegative atoms forming hydrogen bonds, but N/O/F are the specific elements for which the hydrogen bonds are favorable and the strongest, so N/O/F hydrogen bonds are the only ones we should be concerned with.

MinuChoi

How do we know when a molecule has resonance? After optimizing formal charges on the Lewis structure, you will typically find resonance if there is an arrangement of single/double bonds (maybe triple, though we haven't seen that in class yet) that can be moved around without affecting the overall d...

MinuChoi

Would you divide or multiply?

Multiply by 1000 when converting from kilojoules to souls.
Divide by 1000 for vice versa.

MinuChoi

We're not being tested on nomenclature, so while it's probably good to know the basic ones (like ionic ones such as magnesium sulfide), there shouldn't be any questions that we can't do if we don't know nomenclature.

MinuChoi

c is given as 3.00 x 10⁸ meters per second (m/s)

MinuChoi

What's the formal charge of NaOH?

NaOH is an ionic compound, so you would draw Na+ and OH- structures separately.
Looking at the structures and using the formal charge formula, you would get:
Na would have FC=1, H would have FC=0, and O would have FC=-1.

MinuChoi

For example, hw problem 2A.1a asks to give the number of valence electrons (including d electrons) for the element Sb. Since it asked to include d-block, the answer would be 15 valence electrons not 5 electrons correct? The valence electrons for Sb would be 5. Keep in mind that the d-block (4d) tha...

MinuChoi

For example is we had Br03- and we were adding up the electrons why would we add one instead of subtract one? A charge of -1 means that the compound has 1 extra electron (more electrons than protons means a negative charge). The Lewis structure would show an amount of electrons equal to the total o...

MinuChoi

Bond length probably won't work for that. But that form FC=V-(L+B) would work for B = number of bonds that atom has (+1 for singles, +2 for doubles), which is essentially the same thing as S/2. It's actually easier for me personally to do FC = V - (L+B) because I can simply count the number of bonds...

MinuChoi

You can use valence electron values to identify E. Count how many electrons are in the Lewis structure. It looks like the structure has 32 electrons based on the four bonds described. You also know the structure has an atom of element E, one oxygen, and three chlorines. You can subtract those atoms'...

MinuChoi

Does anyone know if we should know the elements' chemical symbols too? You'll be given a periodic table on the test, so memorizing all of the elements' symbols is likely not necessary. It's good to know most of the symbols for the more common atoms for efficiency, though. Maybe consciously make not...

MinuChoi

You should definitely know the order of types of radiation and an approximate range of frequencies/wavelengths for most of the radiations. I'm now sure if the specific frequencies need to be memorized, but you should probably at least be able to determine that, for example, 400-700nm light is in the...

MinuChoi

s, p, d, and f are subshells. One p-subshell has three orbitals.

3p is a subshell, but you can also specify a 3p-orbital.

MinuChoi

can someone explain to me what trends are? The periodic trends are traits of atoms that change based on the element and can be visualized on the periodic table. For example: atomic radius (basically how big the atoms are). If you put every element's atomic radius on a periodic table, that value wil...

MinuChoi

Can someone explain what a shell is vs a subshell? The shell is the n quantum number (broadest division of electrons in an atom). Shells correspond to the rows in the periodic table. The subshell is the s-p-d-f classification of orbital groups (quantum number l.) Shells are divided into subshells (...

MinuChoi

The quantum number m l tells us which orbital an electron is in. For example, if an electron is in an orbital in the 2p subshell: we know that the 2p-subshell (l=1) has 3 orbitals. These orbitals within 2p are denoted as m l =-1,0,1, which is based on the quantum number l. An electron in a 2p subshe...

MinuChoi

I recommend looking in the textbook pages 37-38 for a visual. You'll notice the nodal planes divide the lobes of the orbitals and that the nodal planes all cross the nucleus.

MinuChoi

The manual just gives the orbitals. You may have been looking at a different solution.

MinuChoi

In the 3p subshell, where l=1, does this mean ml=0? Can ml=0?

Yes. The possible magnetic quantum numbers m_l for electrons with l=1 (p-orbitals) are -1, 0, 1.

MinuChoi

You can use the equation E=hv, which refers to the energy of a single photon of light. In this problem, you're asked how many photons have a total given amount of energy. You're given the wavelength of the photons, so you can convert to frequency, use E=hv to find the joules per photon (E), and then...

MinuChoi

The limiting reactant of a chemical reaction is the reactant that is completely consumed to make the product (some amount of the other reactants will be leftover, or in excess). Say you have two given amounts of reactant that undergo a chemical reaction to make products. Using the balanced chemical ...

MinuChoi

If you're referring to the subtraction to find the leftover reagent, here's my work: Plug 12mol ClO2 into the equation to convert to mol BrF3 to determine how much BrF3 is consumed. According to the equation, 4mol BrF3 is consumed with 12mol ClO2. Subtract the 4mol BrF3 from the given 5mol BrF3 and ...

MinuChoi

I believe for the Balmer series, n=2 is the lower quantum level for electrons. It can be the final or initial state (n₁ or n₂), depending on whether light is being absorbed or emitted; but the wavelengths of spectral lines involved are the same for absorption and emission.

MinuChoi

Says in the textbook that, for the equation, the lowest value of n is the same for all lines in one series. For the Balmer series, the number is 2; for the Lyman series, 1. The lower value of n is the lowest energy level of electrons involved in the spectral lines. And because there are significantl...

MinuChoi

You can also try thinking of the decimals as fractions to help you decide what to multiply.
The decimals in the ratio for this problem are basically equivalent to thirds, so you would multiply by 3 to eliminate the fraction.

MinuChoi

By the law of conservation of mass, total mass of reactants used equals total mass of product formed. The total reactants (A and B) used have a combined mass of 12g, but there are 14g of product formed. Mass is conserved in chemical reactions, so the product should be mass 12g. The given result is n...

MinuChoi

You can convert micromoles to moles. The 1.44 micromoles of glucose is equal to 1.44x10^-6 moles of glucose. Now the units are more compatible with the molarity. Divide 1.44x10^-6 moles by the given 1.25x10^-3 mol/L measurement and the mole units will cancel out. [1.44x10^-6 moles]/[1.25x10^-3 mol/L...

MinuChoi

You can save balancing S for last, since it appears several times on one half of the equation.
And since S is its own particle on the reactants side, balancing S will be really easy once the Fe and P are balanced out.

MinuChoi

Does anyone know if the homework is graded on correctness? If the work is partially wrong or if the answer is incorrect, do we receive a lower grade for the homework that week? There's nothing on the syllabus about how strictly the homework's graded, but given we have access to the solutions manual...

MinuChoi

Hydroxide has -1 charge because of oxidation numbers. O's oxidation number is -2 and H's oxidation number is -1. You can also think of when water ionizes; it splits into a hydrogen ion (acidic) and hydroxide ion (basic). Hydrogen ions have charge +1 and water is neutral charge, so hydroxide has char...

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