CHEMISTRY COMMUNITY

Hussain Chharawalla 1G

Temperature and internal energy are directly proportional. In a sense internal energy is a measure of temperature. If temperature is not changing, then internal energy is 0. If you are expanding then you must be inputting heat. If you are compressing then you must be releasing heat.

Hussain Chharawalla 1G

Dear Dr. Lavelle, I came into your chemistry classes with little chemistry knowledge and definitely no appreciation for the subject! But after these two quarters and your step by step explanations, I have learned more with ease than I never thought I would. It took extra hard work and time to succee...

Hussain Chharawalla 1G

As long as you know how electrolysis can have a negative redox potential and why, I don't believe it is any too different from the regular galvanic problems.

Hussain Chharawalla 1G

You can also use the slow step to make the assumption that the product from the previous reaction is in excess and in equilibrium.

Hussain Chharawalla 1G

I also suggest the Organic Chemistry Tutor. Super helpful for me and he does examples very similar to class with long reviews!

Hussain Chharawalla 1G

Thank you for all your words and wisdom! As a current medical student, what similarities do you find between yourself and other medical students admitted in the program? It seems like as premeds, a lot of people follow the same path (volunteering, research, etc). Do you have any advice on how to set...

Hussain Chharawalla 1G

Also remember you can combine these equations for the vant hoff equation

Hussain Chharawalla 1G

I believe instantaneous rate becomes 0 when the reaction reaches equilibrium

Hussain Chharawalla 1G

You can also think of it as n+m+....=order

Hussain Chharawalla 1G

Be careful that if you are using the formula Ecell= Ecathode- Eanode then you would NOT change the sign

Hussain Chharawalla 1G

You need to use it on both ends when both the cathode and anode ends are aqueous solutions and have no way to conduct the electrons without a solid.

Hussain Chharawalla 1G

The emf (E) is the change in cell potential as current i goes to 0 from what I understand.

Hussain Chharawalla 1G

Be sure to only use oxidizing agent and reducing agent for the reactants in your overall redox reaction,

Hussain Chharawalla 1G

You use the second equation when given gibbs free energy standard formation measurements. You use the first equation when given enthalpy and entropy.

Hussain Chharawalla 1G

The salt bridge is just used to balance the charge as electrons flow to one side. For electrolytic cells, the reaction occurs in the nonspontaneous direction. For galvanic cells, the reaction occurs in the spontaneous direction.

Hussain Chharawalla 1G

What would be the half reaction for HClO3?

Hussain Chharawalla 1G

However, as mentioned in lecture, remember that platinum one one side of the cathode or anode does not imply it is being used on both ends.

Hussain Chharawalla 1G

I may be wrong but I believe you are referring to the concept of deltaG=work(max). In the case of a battery, deltaG will always be negative so I believe work will also be negative.

Hussain Chharawalla 1G

In our discussion, we did an example of balancing redox reactions for: MnO4- (aq) + H2SO3 (aq) --> Mn2+ (aq) + HSO4- In section, it was said that MnO4- was reduced and H2SO3 was oxidized. I didn't quite understand this because to me it seemed like Mn2+ means that two electrons were lost meaning oxid...

Hussain Chharawalla 1G

It is also useful to note that temperature can affect the free energy in both ways either low or high temperature.

Hussain Chharawalla 1G

In addition, make sure to correctly identify the reduction and oxidation half reactions in the beginning. Balance the equation with everything except H and O then balance your oxygen with H2Os.

Hussain Chharawalla 1G

In that case, what is the difference between the reaction and the process?

Hussain Chharawalla 1G

Be sure to not confuse this with the concept of isothermal which means U (internal energy change) is 0. But q and w do not have to be 0.

Your entropy or degeneracy would not change because the isolated system is not interracting with the surroundings in any way.

Hussain Chharawalla 1G

For the most part, you will be explicitly told when the process is reversible. If you are not told, you can most likely tell from the context of the problem (pressure is constant, etc), that the process is irreversible.

Hussain Chharawalla 1G

An exception to the expanding concept is when the external pressure is 0. Then, a system can expand without doing any work (w=0)

Hussain Chharawalla 1G

Wmax is the max work done at constant pressure and temperature and means all free energy is used for work. In an isothermal, reversible expansion Wmax= DeltaG

Hussain Chharawalla 1G

You will generally be told how many states the individual atoms can take. If they are fixed, then there is only one state. Degeneracy is the number of possible states to the power of how many atoms. W= (# states)^ (# atoms). If there is only one possible state then W=1 and entropy is equal to S=kbln...

Hussain Chharawalla 1G

A slight correction to the previous definition. I believe that the sum of work and heat given off in a reaction is equal to change in internal energy (U). Enthalpy is related to heat given off in a reaction during a constant pressure. The heat given off in a reaction is generally denoted by q but in...

Hussain Chharawalla 1G

An example of this is heat capacity vs specific heat capacity. Heat capacity depends on the amount of substance but specific heat capacity does not.

Hussain Chharawalla 1G

If a process is reversible then you can think of it as being done in infinetely small increments. Because of this the system remains in equilibrium with its surroundings and work can be calculated using the integral formula. If it is irreversible, the change is rapid and the system cannot be in equi...

Hussain Chharawalla 1G

It is also useful to know that in an isochoric system work is equal to 0. In isothermal delta U is 0.

Hussain Chharawalla 1G

When you make infinetely small changes, your parameters such as pressure and volume are defined. When you make rapid changes, then they can no longer be defined. There is a khanacademy video that explains this topic well.

Hussain Chharawalla 1G

Yes, and the amount of states the system can take increases.

Hussain Chharawalla 1G

An open system will have constant pressure, and a closed system will have constant volume.

Hussain Chharawalla 1G

Yes, and just to add on you must also account for the specific heat of vaporization or the amount of heat to change phase from liquid to vapor.

Hussain Chharawalla 1G

You can find these values in appendix 2 in the beginning of the book. Find the energy released by the oxidation of zinc. Then use the heat capacity of HCl (same as water) and mass q=mcdetaT to solve for final temp.

Hussain Chharawalla 1G

I believe they will be given to us.

Hussain Chharawalla 1G

Because since it is a state function only the endpoints count not the path taken. Another way to think of enthalpy is the heat content at a standard pressure. There is a khan academy video that can help explain a bit more.

Hussain Chharawalla 1G

Yes bond enthalpies (except diatomics) are inaccurate because they are averages since bond enthalpy depends on properties of surrounding atoms

Hussain Chharawalla 1G

Also be careful to check whether you’re given pKa or pKb and whether you want to find OH or H+ concentration

Hussain Chharawalla 1G

To add on depending on the path taken distance can change and therefore work

Hussain Chharawalla 1G

When adding a weak acid, how does the conjugate base affect pH?

Hussain Chharawalla 1G

What this meant is that when you add a very weak acid or a small amount of acid to a neutral solution, sometimes it does not produce enough H3O to affect the pH. This is because there is already 10^7 (H3O) in a solution of neutral water at 25 degrees Celsius.

Hussain Chharawalla 1G

If you use a weak acid or weak base, wouldn't it's conjugate be a strong base/acid? So if the buffer demands equal concentrations of weak acid and base, how would it be created?

Hussain Chharawalla 1G

When you are given Ka or Kb, that is another indicator you will be likely using an ICE table because you do not know how much of the reactant will dissociate.

Hussain Chharawalla 1G

If it helps, another way to think of delta H or enthalpy is the energy stored in bonds. If the products are in a lower energy more stable configuration, the reaction will be exothermic.

Hussain Chharawalla 1G

K is the equilibrium constant. It gives you the ratio between products and reactants. It can represent Kc or Kp

Kc is the equilibrium constant written in terms of concentration. So it is concentration of products over reactants.

Kp is the equilibrium constant in terms of partial pressure.

Hussain Chharawalla 1G

Yes, shifting to the left means the reverse reaction is favored more reactants will be made. Shifting to the right means forward reaction is favored and more products will be made.

Hussain Chharawalla 1G

Aqueous solutions have a defined concentration. For example 10 moles of salt in one liter of aqueous solution is 10M concentration. However, a liquid by itself cannot be measured in concentration. If you had just water, you would not be able to write it in terms of a concentration.

Hussain Chharawalla 1G

I believe that's the point Dr. Lavelle was trying to make with that demonstration. Pressure itself does not change the reaction quotient. Rather, the change in concentration of the reactants is what affects the reaction quotient. When you add an inert gas, you are increasing the pressure in the cont...

Hussain Chharawalla 1G

To add on, if Q<K that means there is either too much reactants or not enough products. So the reaction will continue in the forward direction.

If Q>K, then there is too many products and not enough reactants, so the reverse reaction is favored.

Hussain Chharawalla 1G

The higher the H+ concentration, the higher the H3O+ concentration (they are the same thing H3O means reacted with water), which results in a lower pH. Same applies for OH.

Hussain Chharawalla 1G

Yes, I believe this is because the max electrons Fe can accomodate are 18 electrons with all its orbitals filled.

Hussain Chharawalla 1G

When drawing the lewis structure, make sure each element has a complete octet then count the lowest charge configuration.

Hussain Chharawalla 1G

If a strong acid reacts with a weak base, then pH<7. If weak acid reacts with a strong base then pH>7. Strong acid strong base will be pH7

Hussain Chharawalla 1G

You can notice how the salt would behave in water. This can only tell you neutral, acid, or basic. However, remember that group 2 elements can react in water although we overlooked that in class.

Hussain Chharawalla 1G

If the question is asking for hybridization, then sp2 is fine. I don't believe it was asking for specific level and hybridization.

Hussain Chharawalla 1G

Also remember the respective strength of intermolecular forces. Hydrogen bonds are much stronger than LDF. Ion-dipole then dipole-dipole, etc. etc. Ionic bonds are weak in water.

Hussain Chharawalla 1G

The electron is delocalized among the atoms. Not just one atom will have a double bond, but multiple ones will at different times (hybrid). This makes the molecule more stable.

Hussain Chharawalla 1G

Uncertainty in position is measured by diameter in meters. It is either provided or you must solve for it knowing delta p.

Hussain Chharawalla 1G

Also remember to always look at molar ratios when looking for the limiting reactant. This is a common mistake.

Hussain Chharawalla 1G

To add on to this, Yes because Chlorine has a greater electronegativity, it will be better in delocalizing electrons from the negative oxygen. He also mentioned that Fluorine would be an even stronger acid, because Fluorine is more electronegative and would delocalize electrons even more, giving mor...

Hussain Chharawalla 1G

For NH3: The electron arrangement would be tetrahedral and the shape using lewis structures would be trigonal pyrimadal. Nitrogen will have 4 areas of electron density. The hybridization of nitrogen will therefore be sp3, because sp3 correlates to 4 areas of electron density. In sp3, there is 1 hybr...

Hussain Chharawalla 1G

Formal charge will always be the greatest contributor to determining the bonding configuration in a molecule. Expanded octets may or may not form as a result of that.

Hussain Chharawalla 1G

You can also think of this in terms of the potential energy equation. (PE=q1q2/r). R is the distance between the electron and nucleus. As R gets bigger and bigger, potential energy approaches 0. As an electron goes farther from the nucleus it reaches higher energy levels and approaches 0 or becomes ...

Hussain Chharawalla 1G

Also remember that it was mentioned in each step of a calculation, you may round to the proper amount of sig figs as your final answer. Rounding error from this will not be an issue from what I understand.

Hussain Chharawalla 1G

Only your final answer needs the correct sig fig amount. However, I remember a post that also mentioned for each individual step you only need to round for the final sig fig number. This way, any rounding errors you may have are okay as long as you use the correct number of sig figs as the final ans...

Hussain Chharawalla 1G

Remember that resonance gives partial double bond characteristics to all surrounding atoms. No, it will not affect the naming.

Hussain Chharawalla 1G

A ring structure forms when a chelate (a complex containing a ligand) attaches to a metal to form the structure. I believe the ring formation is due to the shape of the ligand attaching. An example was done in class with NH2-CH2-CH2-NH2

Hussain Chharawalla 1G

To add on to this, there is a biological instinct to avoid bitter foods because they are associated with poisonous substances.

Hussain Chharawalla 1G

I believe we do not need to know the equation. It is useful to know, however, that Planck's constant was formed to explain the data from blackbody radiation.

Hussain Chharawalla 1G

I am confused as to why these bonds occur. Wouldn't you have electron-electron repulsion between adjacent electron clouds?

Hussain Chharawalla 1G

This is correct. Because it is in row 3, it has access to the d orbital.

Hussain Chharawalla 1G

Yes, just like the above post says. Another thing to remember is that this only applies for the trigonal bipyramidal arrangement or seesaw shape

Hussain Chharawalla 1G

Yes, make sure to be able to differentiate between electron arrangement and shape. While those are the only electron density arrangements we need to know, the number of shapes we need to know is indicated above.

Hussain Chharawalla 1G

It would, but I think for the purposes of the class we only need to remember 700-400 nm.

Hussain Chharawalla 1G

The empirical formula of Riboflavin is C17H20N4O6.

I solved it by identifying the conservation of matter in the reaction, and counted the number of atoms on each end of the reaction to form the empirical formula with the remaining atoms.

Hussain Chharawalla 1G

Yes, the oxidation number of a molecule would be the sum of the formal charges.

Hussain Chharawalla 1G

Oxidation number is based on the number of electrons an atom in ground state gains or loses. Depending on an atom's ionization energy and electron affinity, they are either found as cations or anions.

Hussain Chharawalla 1G

Fluidity, hydrogen bonds, and induced dipole dipole interactions were discussed in detail

Hussain Chharawalla 1G

If it helps, I like to think of it as a modified version of the Bronsted Lowry acid and base idea. In the bronsted definition, an acid donates a hydrogen proton and a base accepts a hydrogen proton. Without a proton, you are left with an electron. The Lewis definition allows you to broaden and apply...

Hussain Chharawalla 1G

Another exception that showed up on the midterm was Nitrogen. Because these atoms have a half filled orbital and are stable, they have a higher ionization energy than expected.

Hussain Chharawalla 1G

As stated before, a coordinate covalent bond has one atom supplies the electron pair. To add on, this relates to Lewis acids because they accept electrons and Lewis bases donate electrons.

Hussain Chharawalla 1G

They aren't used by atoms regularly because atoms in period 3 themselves do not have enough electrons to use the d orbitals. Remember until 4s2, the d is higher energy than s orbital.

Hussain Chharawalla 1G

From what I understood, since covalent bonds share electrons the formal charge formula indicates s/2 to represent an equal sharing. However, in reality there are atoms with higher electronegativities in many cases which cause unequal sharing that is a thing the formula does not account for.

Hussain Chharawalla 1G

It is the most stable form for an atom to have their s and p orbitals filled. When their s and p orbitals are filled (octet rule=8 electrons), they reach noble gas configuration

Hussain Chharawalla 1G

Remember that the dots represent unpaired electrons. The octet rule aims to fill the s and p orbitals completely (8 electrons)

Hussain Chharawalla 1G

Just to add on, a hydrogen atom (or any single electron atom), has s,p, and d orbitals that are degenerate. This means they all have equal energy level because there are no electrons interacting with each other.

Hussain Chharawalla 1G

Electron spin is a property that relates to the direction an electron rotates about its axis. It was discovered by shining a beam of silver atoms in a magnetic field. Because the electrons spin in different directions, their magnetic properties change. This caused atoms to split into two beams. http...

Hussain Chharawalla 1G

H is planck's constant. If you are curious as to what it represents, it was the constant that was used to explain the blackbody radiation model because it accounts for energy that is quantized.

Hussain Chharawalla 1G

As you increase the frequency, you increase the energy of the photon. So yes, you must have a minimum frequency to eject an electron. Increasing the frequency further will just make the electron leave at a higher velocity. As you increase the amplitude, you increase the amount of photons. Therefore,...

Hussain Chharawalla 1G

For electrons to be paired, they will have the opposite spin and will be in the same orbital. For parallel electrons, they will have the same spin but be in different orbitals. This is to create the least electron repulsive force.

Hope that helps,
Hussain

Hussain Chharawalla 1G

Hello everyone, I had a conceptual question about the orbitals. From what I understand, orbitals are mathematical models that show where an electron is most likely to be found. However, I am confused about a few things. 1. A s orbital is a sphere that encloses around the nucleus. According to the de...

Hussain Chharawalla 1G

I also don't understand this. Intuitively, for every one mole of Na2CO3, shouldn't there be 2 moles of Na? And why is the multiplication done in the last step? 2.11g Na2Co3= 0.0199 moles Na2Co3 (1 mole Na2Co3)*(2 mol Na/1 mole Na2Co3)= 0.0398M Na Solution (0.0398)(x)=(2.15*10^3)(0.25) x= 3.4 mL Can ...

Hussain Chharawalla 1G

Just curious. Does anyone know if polarity affects bond strength? If so, how?

Hussain Chharawalla 1G

To add on, think of it intuitively as salt and water. If you have a 10 grams of salt in 10 mL of water and then double the water amount 90 times you would eventually have 10 grams of salt now dispersed in a huge body of water (10*(2)^90) mL. So if you were to take a small sample of this new diluted ...

Hussain Chharawalla 1G

This is just another note to add. One common misconception I had is that 1 kg/m^3 is equal to 1 g/cm^3. This is not true, however, because you have to take into account the powers. So like Dr. Lavelle said, make sure to perform conversions beforehand.

Hussain Chharawalla 1G

Yes, you are looking for the empirical formula because of the key word "ratio" of atoms. And just to clarify, converting between percentages to moles does NOT mean divide by a hundred. It means we are taking a hypothetical sample of 100 grams and taking respective percentages of that. Once...

Hussain Chharawalla 1G

Just to add on, a dilution is more evident when you have already been given a solution. Try to pay close attention to the last statement where it will usually clarify.

Hussain Chharawalla 1G

Remember, an aqueous solution is formed when an ionic compound such as NaCl dissociates. It is represented by (aq) because it's technically a solid but in water dissociates

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