CHEMISTRY COMMUNITY

MeeraBhagat

In the slides of the problems from previous exam questions that Lavelle sent out today, for slide 5, does anyone know why the K expression has a 2x in the numerator (products)?. The balanced chemical equation has all stoic coefficients of 1 so I don't know where that comes from.

MeeraBhagat

When given the overall rate law of a multi step reaction, sometimes one singular step of the reaction does not have all of the elements that make up the rate law. For example, if you are given the rate law: k[A]^2[C] and the reaction steps: 1. A+A—>D and 2. D+C—>B. You can see that the rate law for ...

MeeraBhagat

The reason it was in fraction form was because the first step of the reaction wanted to be written to get exactly one mole of product just to make it simpler. Therefore, to make this happen, there needed to be 1/2(O2) as a reactant. If you had started with 1 mole O2 then you would end up with 2 mole...

MeeraBhagat

Yes, they are different. The forward reaction is k and the reverse is k’. The equilibrium constant, K, is calculated by k/k’, which also corresponds to [reactants]/[products].

MeeraBhagat

It helps us know how many collisions occur with sufficient energy to cause a reaction to occur. The higher the temperature, the more collisions with sufficient energy.

MeeraBhagat

An intermediate is formed and then used up. A catalyst is what is needed to get a reaction going (so kind of like a reactant), and then it is eventually reproduced/shows up in another step of the reaction. It is never “consumed” (not the greatest word to use but...) without reappearing.

MeeraBhagat

yes, they are given but they are not labeled, so you will have to remember which equation corresponds to which order and also what they are used for.

MeeraBhagat

I believe it just shows the difference between the energy barrier for the forward reaction versus the reverse reaction. If the slow step is the determining factor for the rate of the reaction, then I guess you could say that it represents the slow step?

MeeraBhagat

I believe Lavelle already went over this in last week's lecture when explaining the variable A which takes into account the number of collisions with correct orientation.

MeeraBhagat

Depending on the different rate laws for the different ordered reactions, plotting the corresponding term on the y-axis against time will yield a straight line instead of a curve. For example, if given a graph with the y-axis as 1/ln[A] plotted against time, and the graph is a straight line, then yo...

MeeraBhagat

The limiting step is the slowest step, and it is also the step that defines the overall rate of the reaction.

MeeraBhagat

I’m not sure what you mean by halving or doubling the reactions, but the reason you manipulate and balance half reactions is so that you have equivalent number of electrons being taken from the oxidized substance as you are adding to the reduced substance. This number would be the same in both half ...

MeeraBhagat

Yes. Since all the half reaction standard potentials are given as reduction half reactions, you have to flip the equation of the substance at the anode (getting oxidized) which flips the sign of the E value. An easy way to know which one to flip is by choosing the one that would result in a more pos...

MeeraBhagat

You look at the change in charge of the substance. If something is being reduced from a 4+ charge to a 2+ charge, you know that the initial substance must have received 2 electrons (4+)+(2-)=(2+).

MeeraBhagat

Electrons are always flowing from the anode to the cathode. You can tell which substances are at the anode by determining which substance is losing electrons. The opposite goes for the cathode. In a cell diagram (written), the substances on the left of the double lines are the ones at the anode.

MeeraBhagat

In problem 6N.1 part b, why does the reduction of In 2+ —> In 3+ require 2 electrons instead of one?

MeeraBhagat

You would have to add a solid electrode (inert conductor) and the most common form of this is platinum or graphite.

MeeraBhagat

This table is used when trying to figure out which part of a reaction is doing the oxidizing or reducing. Whichever half reaction can reduce H+ is the half reaction that you should choose as the oxidation half of the reaction.

MeeraBhagat

The left side of this diagram already has Copper, so you do not need another electrode on that side. Platinum is added to the right side in order to serve as the electrode that can receive the electrons coming from the copper getting oxidized.

MeeraBhagat

The half reactions are given as reductions, and since a reaction requires oxidation and reduction, whichever half has the oxidation occurring would have a flipped equation and sign for the cell potential. If you were to switch the sides of the diagram, then both half reactions would change signs and...

MeeraBhagat

Electrons always go from the anode to the cathode because oxidation, or removal of electron, is always and only happening at the anode.

MeeraBhagat

Expansion work only corresponds to a change in volume. If there is no change in volume, there is no expansion work (or any work done at all if you are talking about an irreversible reaction).

MeeraBhagat

You need to separate the processes of the energy required to melt the ice, and the energy required to raise the temperature of the ice that has melted (or you could think of it as the energy coming out of the surrounding in order to change the temperature of the water that came from the melted ice a...

MeeraBhagat

Free energy is the energy that is available to do work, and Delta G is useful for telling us if the current state of a process is spontaneous (-delta G and system will do work) or not spontaneous (wont do work).

MeeraBhagat

By comparing the moles of gas on the reactant and products sides of the balanced equation, you can tell if the system has done expansion work. If moles of R > moles of P, the system has compressed. If moles R < moles of P, the system has expanded. This is all assuming that this is an open system and...

MeeraBhagat

Monoatomic gases have the potential to occupy more positions than a diatomic gas, and this would cause the degeneracy term for a monoatomic gas to be higher than a diatomic gas. Given the relationship between degeneracy and entropy, this would lead to a higher entropy for a monoatomic gas.

MeeraBhagat

Change in internal energy = 0 when there is no change in temperature (this is called isothermal). Delta U is = q when no work is being done on or by the system (constant volume). When a system is irreversible, external pressure is not equal to internal pressure. All real processes are irreversible (...

MeeraBhagat

It’s J/K which is similar to R which is J/K.mol, except the Boltzmann constant is per particle not mole

MeeraBhagat

There is also a delta for the entropy term because entropy is a state function.

MeeraBhagat

Gas>aqueous>liquids>solids because gases can occupy the most states while solids have one very defined volume or space that they occupy

MeeraBhagat

If the expansion of the gas is isothermal, then this means the temperature remains constant and delta U = 0 Could you explain why delta U would equal 0? I'm a little confused about the connection between temperature and delta U. Delta U = 0 because the temperature of the system is not changing, mea...

MeeraBhagat

Microstates describe all the different ways atoms can be arranged without a difference in energy.

MeeraBhagat

Work=0 when volume is not changing or when pressure is zero (vacuum).

MeeraBhagat

In an open beaker, the system is at constant pressure because there is no volume to contain any increase or decrease in pressure, and the pressure of the system will be the same as the atmosphere.

MeeraBhagat

Those are all correct. You will need to add S, which is entropy, and W, which is degeneracy. The equation sheet will be useful because if you understand the equations, then you will likely be able to deduce which term corresponds to which meaning.

MeeraBhagat

When bonds are formed, energy is released (an exothermic process), so the corresponding enthalpy term is negative. The opposite goes for breaking bonds.

MeeraBhagat

As long as you can figure out which bonds are broken and formed, then you do not need to draw out the Lewis structures and you can just refer to the table of bond enthalpy values.

MeeraBhagat

You can find the bond enthalpies of the reactants and products involved in breakage/forming of bonds in the reaction and then add those enthalpies to find the total enthalpy of the reaction. You can also calculate (total enthalpy of products) - (total enthalpy of reactants) which will yield the tota...

MeeraBhagat

Since the enthalpy of the vapor phase is higher than the enthalpy for the liquid phase of water, steam contains more heat which is then released (exothermic reaction) when it condenses, causing a more severe burn than liquid water alone.

MeeraBhagat

I would say its definitely dependent on the information given. Method 1: if they give you the reactions and their enthalpies, definitely use this. Method 2: If you don’t have the enthalpies but have the reactants, products, and the values of the enthalpies of bonds, use this method. Method 3: If giv...

MeeraBhagat

Since different phases have different enthalpies, you have to take that into account when doing a calculation because the table of bond enthalpies that you use for calculations has everything in terms of the gas phase.

MeeraBhagat

Solids and liquids are not included in the equilibrium constant equation because they do not have concentrations. They have densities. Therefore, they are negated when calculating the K value.

MeeraBhagat

Protonation occurs when a hydrogen ion is added to a molecule, and deprotonation happens when a molecule loses a proton. Bases are often protonated and acids are usually deprotonated. You calculate % protonation by dividing the product species that has been protonated/deprotonated by its conjugate f...

MeeraBhagat

Breaking bonds usually requires energy (endothermic), and forming bonds usually releases energy (exothermic). Also, if a reaction is spontaneous it is usually exothermic.

MeeraBhagat

ICE tables are used when you need to find out the equilibrium concentrations of products or reactants when only given the initial concentration of one. You use the balanced chemical equation when calculating the change (represented by x times the stoichiometric coefficient) and then solve for x to f...

MeeraBhagat

If the K value is less than 10^-3, then the concentrations of products created are small enough in comparison to the original concentration of the reactant(s) that you can consider it insignificant enough to leave the “-x” term out of the reactants when calculating K.

MeeraBhagat

You raise the value the concentration of a certain reactant/product to their respective coefficients in the balanced chemical equation.

MeeraBhagat

Q is used to evaluate the concentration of reactants and products at any given point in a reaction. If Q = K, then the reaction is in equilibrium at that time. If Q>K, then that means there are more reactants present at that time than there would be if the reaction was in equilibrium. If Q<K, that m...

MeeraBhagat

You cannot include solids and liquids in an equilibrium equation because they do not have pressures or concentrations (they have densities).

MeeraBhagat

Solids and liquids don’t have a concentration (they have density). Therefore it would not make sense to include them in an equilibrium expression.

MeeraBhagat

You should use numbers with the same units when you are doing calculations.

MeeraBhagat

Hybridization is an experimentally determined concept to explain why carbon can make four bonds. Since we know carbon can make four bonds, hybridization was developed as a model to explain how this happens. We invoke hybridization when we see that carbon forms 4 bonds.

MeeraBhagat

If a tetrahedral shape had all the same bonds in the same plane then the dipole moments could cancel. However, because it is a three dimensional shape, as Lavelle said, the only way for the dipole moments to cancel is if the central atom is bound to four atoms of the same element.

MeeraBhagat

The DeBroglie equation’s purpose it to calculate the wavelength of objects with a mass, as opposed to c=(wavelength)(frequency) which is unique for light. If given a velocity of an object, or if told to calculate the wavelength of an electron with x velocity, this is when to use the DeBroglie equati...

MeeraBhagat

When they’re asking for the longest wavelength of light, using the equation c=(wavelength)(frequency), you can see that this means looking for light with the smallest frequency. Frequency correlates to energy of the photon by the equation E=h(frequency) where the smallest possible frequency would me...

MeeraBhagat

Hydrogen bonds and dipole dipole interactions are listed as separate intermolecular forces. Although the reason for the interaction is because the H bonded to an electronegative atom develops a more positive dipole and can therefore interact with lonepairs and other atoms, dipole dipole interactions...

MeeraBhagat

Kayla Maldonado 1A wrote:#16 why is the trigonal planar shape considered nonpolar, how did we determine this ?

The problem mentioned that the "X" in the AX3 were all the same atoms. Their dipole moments would therefore cancel and the molecule would be nonpolar.

MeeraBhagat

Nikki Razal 4E wrote:for question 40c, how do you know whether o2 or o3 has the stronger bond and why?

o3 has resonance, which is stabilizing because the electrons are delocalized and makes the bonds stronger.

MeeraBhagat

Lauren Haight 1E wrote:for mini marshmallow 2b, why is the coordination number for Dihydroxoyoxolatocobalt (III) 4? Is it because oxalato is bidentate and can attach to cobalt at two places?

Yes, it is because oxalato is bidentate.

MeeraBhagat

Tiffany Chen 1K wrote:For #31 should the name be ...nickel(III) ion? The complex has a 2+ charge.

Yes I think it should be "ion" since its a positively charged molecule

MeeraBhagat

How do you calculate the charge of the Iron in the heme complex? I know it should be 2+, but don't the nitrogens that bind to the iron have an overall charge of 2+ making the iron 2-? I know that does not make sense since iron is a cation, but based on the math isn't that what the formal charge calc...

MeeraBhagat

Yes, biological examples will definitely be on the final. Any biological example he mentioned is fair game. The ones to know in depth based on the emphasis they were given in lecture are: cisplatin structure and how it works, ethylenediamine and EDTA relationship, Porphyrin and myoglobin relationshi...

MeeraBhagat

The heme complex is created by the binding of the porphyrin tetra dentate ligand to an iron ion. When a protein is bound, this creates myoglobin which is responsible for transport of oxygen throughout the bloodstream. This happens when an O2 molecule binds to the iron. Carbon Monoxide poisoning happ...

MeeraBhagat

EDTA has six binding sites which bind metals and remove them from a solution. The biological example would be to use EDTA to remove metals from the bloodstream.

MeeraBhagat

DHavo_3H wrote:Hi,

Where does cisplatin bond on the DNA to stop translation, and thus cause the cell to die? Thanks in advance!

The chlorines in the cisplatin bind to the guanines in the DNA strand to stop replication.

MeeraBhagat

The chlorines in the cisplatin molecule can both simultaneously bind to guanines on a DNA strand, which stops DNA replication, and therefore, cancer multiplication. The trans model cannot do this because the chlorines are on different sides, meaning they cannot both bind to the DNA strand at the sam...

MeeraBhagat

Induced Dipole - Induced Dipole interactions occur between any two molecules. Neither of the molecules has to be polar for this to occur. This means the same thing as Van Der Waals forces and London Dispersion Forces.

MeeraBhagat

hybridization just describes the mechanism of how those bonds actually form and in what form they exist. It does not change the shape.

MeeraBhagat

coordinate covalent bonds form when a ligand binds to a molecule and provides 100% of the electrons needed to create that bond.

MeeraBhagat

The cis form of the molecule has the chlorines on the same side of the cisplatin molecule, so it can effectively bind to the guanine with both chlorines stopping the cell division process and therefore eliminating cancer cells.

MeeraBhagat

Yes, it has all three. The 180 degree angle is the angle between the bond sticking straight up and straight down.

MeeraBhagat

Regions of electron density are the number of locations where electrons are more present. The number of regions is equivalent to how many atoms are bonded to the central atom plus any regions of lone pair electrons.

MeeraBhagat

The lone pair creates a stronger repulsion in the trigonal pyramidal shape, which is why the angle is less than 109.5. In a trigonal planar shape, there is no lone pair, so there is equal repulsion between the bonding regions creating equal 120 degree bonding angles.

MeeraBhagat

I believe they are just 120 and 90.

MeeraBhagat

I would know basic polyatomic ions. Other than that, we have not really gone over naming so I would assume he will provide the formula.

MeeraBhagat

Polarizability is the ability to be distorted by a cation, and polarizing is the act of distorting an anion electron cloud.

MeeraBhagat

Most conversion will be provided on the equations sheet.

MeeraBhagat

electronegativity describes the pulling power of the atom. Generally, across a period, electronegativity will increase. Down a group, it will decrease. This is due to effective nuclear charge and size of the atom. The smaller the atom, the closer to the nucleus the electrons can get, meaning that th...

MeeraBhagat

Why is it important to draw the resonance structure? it is important to draw all of the resonance structures because it allows us to better understand the concept of delocalization of electrons. It shows that there are certain types of bonds in the molecule, but that the bond is not located in one ...

MeeraBhagat

Covalent bonding occurs between two atoms that share electrons in order to complete each other's octet. With coordinate covalent bonding, a lewis base will coordinate covalently bond to a lewis acid and provide all of the electrons needed to complete the lewis acid's octet. In this case, the lewis a...

MeeraBhagat

Elements in the third energy level and higher can have more than an octet. This is because they have empty d orbitals available to hold the extra electrons.

MeeraBhagat

Lewis acids accept electron pairs and lewis bases donate that electron pair. The example that Dr. Lavelle used was with BF3 as a lewis acid and F- as a lewis base. Since Fluorine does not make double bonds, and since three fluorines bonded to a boron atom only gives boron 6 valence electrons, the on...

MeeraBhagat

As long as each atom in a molecule has an octet, including lone pairs, that is how you figure it out. When trying to determine whether to draw a lone pair or to add another bond, calculate formal charge to see what is optimal (always aim for formal charge of 0 on each atom).

MeeraBhagat

What did Dr. Lavelle mean when he said non-metals do not form cation because their ionization energies are too high? He meant that since their ionization energies are too high, meaning that a very large amount of energy is required to remove one of their valence electrons, they will not lose electr...

MeeraBhagat

Delocalization of electrons occurs in resonance structures. This is why, experimentally, all bond lengths are the same even if one of the molecule's bonds is a single and the other a double. The electrons do not stay localized to only one of the bonds, and instead they move around the molecule, caus...

MeeraBhagat

When there is resonance, the electrons are delocalized and not bound to a specific bond in the molecule. The bond lengths are blended.

MeeraBhagat

Double bonds are stronger, and therefore shorter, than single bonds. However, in resonance structures, each bond is somewhere between the length of a double and single bond.

MeeraBhagat

You cannot determine the spin of an electron, you just know that one spins up and the other spins down. The spins means more or less the direction of which the electron in moving around the nucleus. Electrons spin in opposite direction in the same subshell in order to maximize stability.

MeeraBhagat

Can someone explain the reason for why copper and chromium are exceptions to the normal filling rule?

MeeraBhagat

They both relate to the properties of each energy subshell. Each subshell can only hold 2 electrons, according to the Pauli Exclusion Principle. The two electrons in that subshell must have opposite spins as to maximize stability, according to Hund's Rule.

MeeraBhagat

Aside from not being able to have the same four quantum numbers, electrons with opposite spin in the same subshell will maximize stability.

MeeraBhagat

Can anyone explain how to list the orbitals for an element. Dr. Lavelle went over it in lecture but he did not explain the process fully. They follow the order of the periodic table. Hydrogen and Helium are in the first row, and have electrons in the s-block. The next two elements in the second row...

MeeraBhagat

Ami_Pant_3H wrote:For clarification, is this equation only used in terms of hydrogen atoms? If so, is there a more general equation that applies to other atoms?

only applies to hydrogen atoms

MeeraBhagat

Cassandra_1K wrote:For limiting reactants on the test do we have to explain how we know one is a limiting reactant or can we just specify which one?

just specify which one

MeeraBhagat

I don't think that every single simple step needs to be shown, but someone who does not know much about chemistry or who would be confused easily should be able to follow along and understand how you got from point A to B.

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