CHEMISTRY COMMUNITY

Anika Chakrabarti 1A

You can also use the Arrhenius equation to compare a reaction at two different temperatures and determine how a temperature change affects the activation energy or rate constant.

Anika Chakrabarti 1A

I think if you know the overall rate law that was determined experimentally, then the rate law of the slow step will be the same as the reaction rate law. You also might need to use the pre-equilibrium method if the rate law of the slow step has an intermediate.

Anika Chakrabarti 1A

If it is a concentration cell, diluting the side with a lower concentration would increase the concentration difference between the anode and the cathode, causing the cell potential to increase.

Anika Chakrabarti 1A

No, if you increase the mass of the solid it doesn't affect Ecell (I'm pretty sure this was one of the questions on test two).

Anika Chakrabarti 1A

A catalyst has to be there for a reaction to begin, whereas an intermediate is formed during one of the elementary steps and then used up in a later step.

Anika Chakrabarti 1A

Does anyone know if any of the other final review sessions have posted a worksheet beside Lyndon's?

Anika Chakrabarti 1A

First-order tells us that an increase in the concentration of the reactant is directly proportional to the rate, so doubling the reactant will double the rate. Second-order means that doubling the reactant will increase the rate by a factor of four, and zero-order means that the rate does not depend...

Anika Chakrabarti 1A

I'm pretty sure we won't have to figure out or guess the intermediates for a reaction.

Anika Chakrabarti 1A

I thought that zero-order does have a half-life. In my notes, I have that it's [A]initial / 2k.

Anika Chakrabarti 1A

I think this could mean when you have multiple reactants and the concentration of each reactant might affect the rate differently.

Anika Chakrabarti 1A

An integrated rate law represents the rate of a reaction with respect to time, so you can use it to find how quickly a reaction has progressed over a certain amount of time. A rate law only involves the initial rates.

Anika Chakrabarti 1A

Is there a way to determine from two half-reactions which one will be the anode and which will be the cathode? Is it always that the reaction with a more positive E will be the cathode?

Anika Chakrabarti 1A

When Ecell is positive, then everything on the right side of this equation is positive. Then, to solve for K you take e to the power of nFE/RT, which will give a large positive number.

Anika Chakrabarti 1A

Free energy is equal to the amount of energy that is available to do work. I think this means that the value of delta G is the maximum amount of work that a system can do.

Anika Chakrabarti 1A

Standard cell potential is the difference in reduction potentials between the cathode and anode when the reaction is occurring at standard conditions (1 M and 1 bar). Cell potential is when the reaction is occurring at conditions other than standard, and it changes when concentration changes, wherea...

Anika Chakrabarti 1A

You add Pt if there are no solids involved in the reduction or oxidation reactions.

Anika Chakrabarti 1A

I think it's better to be specific and include the charges. You could also write out the half-reactions to show that Fe3+ is gaining one electron and is being reduced.

Anika Chakrabarti 1A

I think that will be on the test. It is included in the part of the thermodynamics outline that we covered after the midterm.

Anika Chakrabarti 1A

The diagram tells us that the elements on the left side of || are involved in oxidation and the elements on the right are involved in reduction. With this information, you can figure out each half-reaction and determine which are reactants and products based on whether electrons are being lost or ga...

Anika Chakrabarti 1A

If G is positive for a galvanic cell, the redox reaction is unfavorable, and therefore electrons would not flow from anode to cathode. In order for a battery to work, G must be negative so that you get a positive voltage and the electron flow from anode to cathode is favorable.

Anika Chakrabarti 1A

You can use solid platinum as the electrode if there is no solid conductor in the reaction already. Some reactions occur with both reactants and products being aqueous, so you need to add a solid that can easily conduct electricity and will not interfere with the reaction, such as platinum or carbon...

Anika Chakrabarti 1A

It can be used to compare the equilibrium constants when the reaction takes place at two different temperatures.

Anika Chakrabarti 1A

Yes because a spontaneous reaction means that the reaction is more likely to proceed and more products are likely to form. If a reaction is not spontaneous, the reactants are unlikely to form products, so the reverse reaction is favored.

Anika Chakrabarti 1A

Delta U = 0 for reversible processes, since any work done is replaced by an equal amount of heat (q = -w)

Anika Chakrabarti 1A

I thought that the only way to have an isothermal irreversible expansion is if there is free expansion, meaning the external pressure is 0. Most irreversible expansions involve a temperature change.

Anika Chakrabarti 1A

It is used to calculate entropy if you know the number of ways particles can be arranged while still having the same energy (residual entropy).

Anika Chakrabarti 1A

If the system can only be arranged in one way, meaning it only has one possible microstate, then W = 1 (it doesn't matter how many particles are in the system because 1 raised to any power is always 1). Since the natural log of 1 is 0, then the entropy of the system is S = kBln(1) = 0.

Anika Chakrabarti 1A

Adding on to your comment, I think you would need a third component where you calculate q when cooling the substance back to the original temperature. Then, find the change in entropy for cooling and add all three entropy changes to find the total.

Anika Chakrabarti 1A

I find it helpful to remember the heating curve graph that Lavelle showed in class. If the reaction is going through a phase change, its temperature stays constant, so it wouldn't make sense to use the q = mC(delta T) since delta T would be 0. Instead, you use the hopefully given delta H value and m...

Anika Chakrabarti 1A

If you're finding q using the equation q = nC(delta T), you would use the value of Cp if the reaction is at constant pressure and the value Cv when at constant volume.

Anika Chakrabarti 1A

The monatomic values are on the constants and equations sheet. But for diatomic, it's not that hard to remember since you just add 1 to each of the ratios. I haven't seen any practice problems where we need to use polyatomic values, though.

Anika Chakrabarti 1A

What is the difference between standard enthalpy of formation and standard reaction enthalpy?

Anika Chakrabarti 1A

I think it is also important to know that the graph for an irreversible expansion will be a horizontal line (since pressure is constant), but the graph of a reversible expansion is a curved line (since pressure is changing). This means that the work done by a reversible expansion is greater since it...

Anika Chakrabarti 1A

For A, use the equation w = -P(delta V), which finds the work done during an irreversible expansion. Then, convert your answer to joules using the conversion of 101.325 J/ 1 L.atm. For part B, since it is a reversible expansion, use the equation w = -nRT ln(V2/V1). This was the formula that came fro...

Anika Chakrabarti 1A

I think there was a homework problem where we needed to use those ratios, so I'm assuming we should know it.

Anika Chakrabarti 1A

The ideal gas law is PV = nRT, and work is equal to -P(change in V). So, if you know the number of moles and the temperature of a reaction, you can solve for work using the idea that work = -nRT. I think there was an example of this in Friday's lecture.

Anika Chakrabarti 1A

Why is using bond enthalpies to calculate H the least accurate method?

Anika Chakrabarti 1A

In the heating curve graph that Lavelle showed in class, the amount of energy needed for melting is much lower than the energy required for vaporization. This means that heat is absorbed in order to go from a liquid to a vapor, so the vapor would have a higher enthalpy.

Anika Chakrabarti 1A

When H2O is a solvent it is in so much excess that its concentration doesn't change.

Anika Chakrabarti 1A

Some examples of state functions are energy, pressure, and temperature.

Anika Chakrabarti 1A

For a proton transfer between water molecules, the reactants would be 2 H2O molecules and the products would be H3O+ and OH-. Then when you write Ka, you exclude H2O and its just Ka = [H3O+] [OH-], which equals 10^-14 (Kw).

Anika Chakrabarti 1A

When the pressure is increased by decreasing volume, the reaction will move in whichever direction will reduce the pressure. This means that it will move towards the side with fewer moles (so fewer molecules), which causes the pressure to decrease.

Anika Chakrabarti 1A

The homework problem 5I.25 is a good example of this type of situation. In this problem, there are initial concentrations of both reactants and products present, and you end up getting a quadratic equation where x is two positive values.

Anika Chakrabarti 1A

An endothermic reaction is one that requires heat to form products. For example, the reaction N2 -> 2N is endothermic because it requires energy to break the N-N bonds and form product. When an endothermic reaction is heated, its equilibrium constant increases, since more heat is absorbed by the rea...

Anika Chakrabarti 1A

For ICE tables, you can simplify the calculation if K is less than 10^-3.

Anika Chakrabarti 1A

I think K and Kc mean the same thing, but Kc is just more specific because it tells us that we're dealing with concentrations.

Anika Chakrabarti 1A

When solving for x in an ICE table, how do we know when to use the quadratic equation and when to assume that x is very small and simplify the expression.

Anika Chakrabarti 1A

What are partial pressures? I understand that we use the partial pressures of R and P to calculate Kp, but what do the values actually represent?

Anika Chakrabarti 1A

You'll get the same answer in the end, but I would choose the ratio that makes solving for x the simplest.

Anika Chakrabarti 1A

For ICE tables, only include the reactants and products that you would use to calculate K.

Anika Chakrabarti 1A

K is a fixed ratio and will therefore always be the same number (for each reaction at a certain temperature), but Q is the P/R ratio calculated at any point in the reaction. Q can be used to figure out whether a reaction is at equilibrium, and if not, which direction the reaction is moving.

Anika Chakrabarti 1A

Is there a way to tell whether a coordination compound will be tetrahedral or square planar?

Anika Chakrabarti 1A

I'm pretty sure we aren't going to do actual calculations with Ka until 14B. What does Kw represent, though? I don't remember learning that

Anika Chakrabarti 1A

The unit is J, but in the E=hv equation you are finding the energy of a photon, so technically this value means the amount of energy per photon. I think you can just write J in your answer though.

Anika Chakrabarti 1A

Chelates form when a ligand can bind to a metal atom at more than one site, creating a ring of atoms.

Anika Chakrabarti 1A

I think it's important to know the charges and names of the different ligands to help with naming coordination compounds.

Anika Chakrabarti 1A

When an acid is dissolved in water, it loses a proton (hydrogen ion) and transfers it to water, forming a hydronium ion (H30+). I think H30+ is more specific and it is what you would use when writing a chemical equation.

Anika Chakrabarti 1A

For Lewis acids and bases, it would be helpful to draw the structures to see whether the molecule has a lone pair that it can donate.

Anika Chakrabarti 1A

I've never heard of ferricyanide, but I think it's safer to just follow the naming system we learned in class

Anika Chakrabarti 1A

In the textbook, it says that the ligands ethylenediamine (en) and oxalato are bidentate, diethylenetriamine (dien) is tridentate, and (edta) is hexadentate. I'm not sure if these will always be polydentate though, or if it is only in certain cases.

Anika Chakrabarti 1A

Based on the homework, I'm guessing it will mostly just be naming.

Anika Chakrabarti 1A

I had trouble with part c at first, but H2SeO4 will also have hydrogen bonding. This is because its lewis structure has the four oxygens bonded to the central Se atom, and the two H atoms are bonded to two of the hydrogens. Any time an H atom is bonded to an O, F or N atom, there will be hydrogen bo...

Anika Chakrabarti 1A

A molecule that has three regions of electron density will have a bent shape if one of the regions is a lone pair and the other two are bonding pairs. If all three regions are bonded atoms, then the shape is trigonal planar. In this problem, the molecule has a bent shape because Cl has a lone pair a...

Anika Chakrabarti 1A

There are also dipole-induced dipole forces, which occur between a polar and a nonpolar molecule.

Anika Chakrabarti 1A

If the intermolecular forces are between two of the same molecule, each molecule must have a hydrogen atom attached to an F, O, or N atom.

Anika Chakrabarti 1A

Lone pairs are more likely to be found in equatorial positions because they experience less repulsion.

Anika Chakrabarti 1A

Is it possible for a molecule to have the potential to form both dipole-induced dipole and dipole-dipole bonds?

Anika Chakrabarti 1A

How do we predict the bond angles of a molecule?

Anika Chakrabarti 1A

Does a greater repulsion strength between bonding pairs result in a larger bond angle? How would we determine which bonding pairs have a greater repulsion when looking at a molecule with multiple bonds?

Anika Chakrabarti 1A

Why is the London Dispersion Force found in every bond? London dispersion forces happen in all atoms because they all have the potential to form instantaneous dipole moments, which is when the electron density is not evenly distributed around the nucleus and is more concentrated in one area. This c...

Anika Chakrabarti 1A

There is not a dipole moment for CH bonds because the electronegativity difference between C and H is very small, so the bond is considered nonpolar.

Anika Chakrabarti 1A

What causes certain interactions to form liquids or solids?

Anika Chakrabarti 1A

Why are the attractive forces of hydrogen bonds always -20 kJ/mol?

Anika Chakrabarti 1A

Is there a difference between dispersion and london forces?

Anika Chakrabarti 1A

What makes some molecules such as salts more soluble in water?

Anika Chakrabarti 1A

Triple bonds have the largest number of shared electrons and the greatest attraction between atoms, so they are harder to break and shorter in length. Single bonds are the largest in length, so they are the weakest.

Anika Chakrabarti 1A

What is the difference between "highly polarizable" and "high polarizing power"? They both result in an ionic bond with a greater covalent character.

Anika Chakrabarti 1A

Does polarizing power follow any trends on the periodic table?

Anika Chakrabarti 1A

At the end of the lecture on Friday Lavelle talked about the interactions that give rise to attractive forces (ion-ion, ion-dipole, etc.) but why are the kJ/mol always negative?

Anika Chakrabarti 1A

What causes an induced dipole?

Anika Chakrabarti 1A

Ionization energy is the minimum energy required to remove an electron, electron affinity is an atom's likelihood of gaining an electron, and electronegativity is how tightly an atom holds onto their electrons. All three trends increase as you go up and to the right on the periodic table.

Anika Chakrabarti 1A

The valence of an atom means the number of covalent bonds it has, and valence electrons are the electrons in the outer shell.

Anika Chakrabarti 1A

I think this happens because the double bond can occur at three different locations (remember the three different structure diagrams for NO3 that he showed in class), so this bond length of 1.24 A is kind of like an average or combination of the different structures.

Anika Chakrabarti 1A

I had a hard time with this problem too. For part A, we can use the De Broglie equation (wavelength = h/p) since we know the mass and velocity of the electron. For B, they give the frequency required to remove one electron, so you can use that in the equation E = h x frequency. This gives the energy...

Anika Chakrabarti 1A

What does the effective nuclear charge describe about an atom? Do certain elements tend to have a higher or lower effective nuclear charge?

Anika Chakrabarti 1A

This means that electrons always fill up the lowest energy orbitals first before occupying the higher orbitals. So when we write electron configurations, we start with 1s, then 2s, etc.

Anika Chakrabarti 1A

How are we supposed to know what the spin state of an electron is? For all of the homework problems I've done so far they have only been asking about the first three quantum numbers, but I don't understand how we would know what the spin state is unless it is given.

Anika Chakrabarti 1A

Why do certain elements (like Cr and Cu) not follow the same electron configuration pattern as most elements?

Anika Chakrabarti 1A

How do we know when to include x, y, and z when writing electron configurations? For example, nitrogen is 1s^2, 2s^2, 2px^1, 2py^1, 2pz^1

Anika Chakrabarti 1A

I'm having a hard time visualizing the electron configurations that we learned about on Friday. What does it mean when two electrons are paired? How is this different than two electrons being parallel?

Anika Chakrabarti 1A

I don't think that increasing the frequency would increase the number of electrons ejected, since the number of electrons ejected still depends on the number of photons. But, if the energy of light is already greater than the minimum energy needed to eject an electron, then increasing the frequency ...

Anika Chakrabarti 1A

With the photon model, energy is proportional to frequency, whereas with the wave model, energy is proportional to intensity. What is the difference between frequency and intensity?

Anika Chakrabarti 1A

The De Broglie equation (wavelength = h/p) can be used to find the wavelength of any moving particle with a momentum. The equation E(n) = -[hR/n^2] can be used to find the energy of an electron at a certain energy level (n), and then you can calculate the change in energy if you know the initial and...

Anika Chakrabarti 1A

I think for most of the problems we'll be doing this quarter we can just assume that C = the speed of light in a vacuum.

Anika Chakrabarti 1A

I'm having a hard time conceptualizing what a photon is. Lavelle described them as "packets" of energy, but they also travel in waves, and each photon interacts with one electron?

Anika Chakrabarti 1A

In the lecture last week Lavelle explained that to find the kinetic energy of an ejected electron, you subtract the work function from the energy of a photon (E=hv). What does the work function represent? Is it something that we need to know how to find or will it always be a given number in problems?

Anika Chakrabarti 1A

In problem M9, it asks to find the net ionic equation for when copper (II) nitrate reacts with sodium hydroxide to produce a precipitate of light blue copper (II) hydroxide. I understand what a net ionic equation is, but I wasn't able to figure out how to get the answer shown in the solutions book. ...

Anika Chakrabarti 1A

Once you convert each of the reactants to moles, you can just choose one of the reactants and use the mole ratio from the balanced equation to find the amount of moles that are required of the other reactant. If the number of moles calculated of the second reactant is less than the amount given in t...

Anika Chakrabarti 1A

I feel like I understand the concept of molarity and dilutions, but problem G25 threw me off because the solution is diluted by doubling the volume 90 times. How would we go about solving this problem and finding the number of molecules in the final solution?

Anika Chakrabarti 1A

What does it mean when compounds have water molecules added to them? For example, in G17 A it asks for the mass of CuSO4 needed for a dilution, but in part B it asks for the mass of CuSO4*5H2O. What is the difference between these two compounds? Also, if we're finding the molar mass of CuSO4*5H2O, d...

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