CHEMISTRY COMMUNITY

Amanda Mei 1B

No, a catalyst doesn't affect the enthalpy of a reaction because enthalpy is a state function - it does not depend on the path taken from reactants to products. It just depends on the compositions of reactants and products. A catalyst provides a new pathway for the reaction with a lower activation e...

Amanda Mei 1B

Does anyone know how to scan from an at home wireless printer? As some people have said, you shouldn't need a printer after you print out the final (if that's how they have us do it). You can then use an app such as "Scannable" that allows you to scan pictures together as a PDF then upload.

Amanda Mei 1B

We'll probably have to download and print the final, do our work by hand, and then scan and upload the completed final. If your computer or wifi crashes, I think you should still be able to do it by phone.

Amanda Mei 1B

CNourian2H wrote:Do you know what time tomorrow he is going to send the test out?

No, Lavelle never said what time he would update us, but it has to be sometime today (although it's getting late).

Amanda Mei 1B

Here's one for the first half of the quarter. viewtopic.php?f=160&t=62044&p=236759&hilit=review+sessions&sid=a8cd0b5608d8cc5fa29e9b0c55df7945#p236759

Amanda Mei 1B

Maybe we would have to identify which substance is a catalyst, given steps of a reaction. Or perhaps conceptual questions about the effect of a catalyst?

Amanda Mei 1B

Expansion causes an increase in work because more work is required to push against an external pressure, whether it be the atmosphere or another thing exerting the pressure.

Amanda Mei 1B

Email your TA. I think they've been scanning the tests for people.

Amanda Mei 1B

Lavelle - If you see this, thank you for your efforts in making this smooth for all of us. Sorry you haven't gotten enough sleep :( It's been a fun ride with you!

Amanda Mei 1B

I believe the final will be posted at 11:30 am on Sunday, the scheduled time. I think it'll either be posted on Lavelle's website and he'll email the link to us. It should be 3 (maybe 4?) hours long. Lavelle hasn't announced how we will download/turn it in yet.

Amanda Mei 1B

I've also been finding it hard to sit down and study, but I know it's important to get into a good habit since next quarter will be like this. I think finding a quiet spot would be super helpful in being able to just sit down and grind for a bit. Definitely setting a routine time to get up and study...

Amanda Mei 1B

Absorption is the process by which a fluid is dissolved by a liquid or a solid, and it involves the entire absorbent. Adsorption is the process by which parts of a substance (it could be gas, liquid or dissolved solid) adhere to a surface of the adsorbent. Lavelle said that adsorption is when reacta...

Amanda Mei 1B

Remember that decreasing the volume of a system causes pressure to increase, since the same number of moles in a smaller space will collide more frequently. If you the pressure of the whole system increases due to a decrease in volume, the system will want to negate that change and restore equilibri...

Amanda Mei 1B

Are there also errors in the back of the textbook?

Amanda Mei 1B

Yes, n is the number of moles of electrons transferred in the redox reaction. After writing the two half-reactions, make sure they have an equal number of electrons being transferred, so that they cancel out when you add the half-reactions back together. Multiply the reactants and products in a reac...

Amanda Mei 1B

A negative E naught means that the reverse reaction is spontaneous.

Amanda Mei 1B

I believe we don't need to know how to derive any equations in this class, just what they mean and how to use them.

Amanda Mei 1B

Ready for some good vibes - thank you Dr. Lavelle!

Amanda Mei 1B

Should it be lnK or lnQ when it reaches equilibrium or the two have the same value. When a reaction reaches equilibrium, the reaction quotient Q (which you solve for the same way as K, but Q is for any stage of the reaction) is equal to the equilibrium constant K. So, lnK and lnQ have the same value.

Amanda Mei 1B

The Van't Hoff Equation shows how the equilibrium constant K depends on temperature. It can be used to calculate K at a different temperature if ΔH° is known, assuming ΔS°is constant.

Amanda Mei 1B

n is the number of moles of electrons transferred in the balanced redox reaction. Write the half reactions and balance them by adding electrons.

For example, Pb²⁺ + Ni → Ni²⁺ + Pb can be written as two half reactions:
Ni → Ni²⁺ + 2e^-
Pb²⁺ + 2e^- → Pb

2 moles of electrons are transferred.

Amanda Mei 1B

The single lines separating the two half-reactions separate the oxidation reaction (anode) and reduction reaction (cathode). You will see the same element with written once and then again with a more positive charge as part of the oxidation half-reaction, and another element that is becoming more ne...

Amanda Mei 1B

The Van't Hoff Equation shows how the equilibrium constant K depends on temperature. It can be used to calculate K at a different temperature if ΔH° is known, assuming ΔS°is constant.

Amanda Mei 1B

Redox reactions are also known as oxidation-reduction reactions because both oxidation and reduction occur, in two half-reactions. Electrons are transferred between species, changing the species' oxidation numbers.

Amanda Mei 1B

To add on, K (equilibrium constant) is easy to measure as opposed to Gibbs Free Energy.

Amanda Mei 1B

Think of heat as either a reactant being absorbed by the reaction (endothermic) or product being given off the reaction (exothermic). If temperature is increased, the reaction will shift away from the side that the heat is on. If temperature is decreased, heat will shift to that side.

Amanda Mei 1B

When a system pushes against an external force (pressure), it loses energy and thus work is negative. Usually this causes volume to expand.

Amanda Mei 1B

Lavelle outlines the grading scheme in the syllabus: Weekly Homework (5 pts per week): 50 pts Chemistry Community (5 pts per week): 50 pts Test 1: 50 pts Midterm: 120 pts Test 2: 50 pts Final: 180 pts Total: 500 pts 50% or higher is required to pass with a C- or higher. He said that the class doesn'...

Amanda Mei 1B

A combustion reaction involves a hydrocarbon (compounds that contains only carbon and hydrogen) burning in (reacting with) diatomic oxygen to produce carbon dioxide and water. You would balance the O 2 last, and you might need to use a fractional coefficient before multiplying all the coefficients b...

Amanda Mei 1B

I believe you also use ratios for R to get everything to have the same units. You can use the ratios of R because they would cancel out (like multiplying by a fraction that equals 1).

Amanda Mei 1B

I got 490 J as well. The solution manual says that's the answer, so the book must be incorrect.

Amanda Mei 1B

Temperature doesn't change during a phase change. Heat added to a substance either raises the temperature or causes a phase change, but not both simultaneously. If a solid is heated at its melting point, its temperature remains the same until it has completely melted because melting requires energy....

Amanda Mei 1B

The system must do the work of expansion by pushing against P, the external pressure. The system loses energy due to this work, so the negative sign is put to account for that.

Amanda Mei 1B

A closed system is physically closed but not thermodynamically closed, meaning that although matter cannot be exchanged between the system and its surroundings, energy can. An example would be a sealed beaker of water, which doesn't insulate, or a pan with a lid on top of it on the stove. An isolate...

Amanda Mei 1B

Aside from the fact that bond enthalpies of non-diatomic molecules are averages from the bonds of many different molecules, their values may vary depending on which chart you use. Since there's no universally agreed upon standard for which molecules are used to determine a bond, those making the cha...

Amanda Mei 1B

For all molecules that are not diatomic, their bond enthalpies are averages from the bonds in many different molecules. For example many molecules can have a C-H bond, and the bond enthalpies of each one is different.

Amanda Mei 1B

A closed system is physically closed but not thermodynamically closed, meaning that although matter cannot be exchanged between the system and its surroundings, energy can. An example would be a sealed beaker of water, which doesn't insulate, or a pan with a lid on top of it on the stove. An isolate...

Amanda Mei 1B

Hess's Law states that enthalpy is a state function, so its value is determined by its current state, not the path taken to obtain that state. Thus, the total enthalpy change of a multi-step reaction can be calculated by adding the enthalpy change at each step of the reaction.

Amanda Mei 1B

If temperature is held constant and there's a decrease in volume, the same amount of molecules will be moving in a smaller space and will collide more often, leading to an increase in pressure. If volume increases, molecules will have more space and collide less frequently, decreasing pressure.

Amanda Mei 1B

Elements in their standard state have a standard enthalpy of formation of 0 because they are are not formed, they just exist. They don't require any energy to get to their most stable form if they're already in that form.

Amanda Mei 1B

Lavelle was explaining how getting burned by steam is more severe than getting burned by boiling water. This is because when steam (water in gas form) hits your skin, it undergoes a phase change to become a liquid. This phase change as the steam hits your skin and becomes liquid releases energy, cau...

Amanda Mei 1B

Hess's Law states that enthalpy is a state function, so its value is determined by its current state, not the path taken to obtain that state. Thus, the total enthalpy change of a multi-step reaction can be calculated by adding the enthalpy change at each step of the reaction.

Amanda Mei 1B

You would divide what you found to be x by the initial concentration, which is given, then multiple by 100 to get the percentage of the initial concentration that has been deprotonated.

Amanda Mei 1B

Yes, we're being tested on chemical equilibrium and acids and bases. I'd review lecture notes and examples and do problems in the textbook.

Amanda Mei 1B

Ruby Richter 2L wrote:If you do the quadratic equation and get a positive and negative number would you just discredit the negative number? Why?

Because you can't have a negative concentration

Amanda Mei 1B

gabbymaraziti wrote:Can we just use K and not specify Kc or Kp? Or would we be marked down?

I don't think you'll be marked down for using K.

Amanda Mei 1B

Divide the x value that you find by the initial concentration to check if it is less than 5% of the initial concentration (less than 5% is deprotonated). This checks if it is small enough to disregard if being subtracted from the initial concentration, making your calculations much easier.

Amanda Mei 1B

K is the equilibrium constant, while Q is the reaction quotient. Though they are calculated the same way, Q can be calculated at any time during the reaction, while K represents when the reaction is at equilibrium. If Q<K, then [reactants] > [products] and the forward reaction is favored. The opposi...

Amanda Mei 1B

There are a couple ways to tell if a reaction is endothermic or exothermic. One way is to see if heat is a reactant of product. If heat is released as a product of the reaction, the reaction is exothermic. If heat is listed on the side of the reactants, it is being absorbed and the reaction is endot...

Amanda Mei 1B

You can think of heat like a reactant or product, depending on whether the reaction is endothermic or exothermic, and apply the same rules by Le Chatelier's Principle as you would do with other reactants and products to figure out which way the reaction will shift.

Amanda Mei 1B

K=1 tells you that there is about the same amount of products as there are reactants, so the energies of products and reactants are equal. An equilibrium reaction is dynamic and the forward and reverse reactions are constantly occurring, so it's rare for the two to have the same amount at the same t...

Amanda Mei 1B

Le Chatelier's Principle can predict the effect of a change in conditions on chemical equilibria. If there is a change in conditions, the equilibrium will shift to counteract the effect of the constraint.

Amanda Mei 1B

Definitely the topics on Outline 1, but I don't know if we'll get to Outline 2 by the time of the test.

Amanda Mei 1B

I think this is the best way to do it since many problems ask you how much product will be formed anyway, so you're essentially answering that question while you determine the limiting reactant. Just convert moles of each reactant to moles of the product you'll be looking for.

Amanda Mei 1B

Delta x represents the uncertainty in position. You can use Heisenberg's Uncertainty Principle (uncertainty of position)(uncertainty of momentum) is greater than or equal to (Planck's Constant /4 pi).

Amanda Mei 1B

The pH scale is based on the self-ionization of water, where two water molecules combine to form hydronium and hydroxide ions. pH is a measurement of how acidic a solution is. A higher concentration of the hydronium ion means the solution is acidic and will have a lower pH (below 7), while a higher ...

Amanda Mei 1B

Amanda Mei 1B

After drawing the Lewis structure, you would count the number of valence electrons of one of the atoms, then subtract the number of bonds that atom makes, then subtract the number of lone electrons on that atom. So valence electrons - (# bonds + # lone electrons).

Amanda Mei 1B

I'm not sure how he's going to curve or how much, but I think he may curve at the end after final grades are determined.

Amanda Mei 1B

Acetic acid only ionizes partly in water (there aren't as many separate acetate and hydrogen ions as whole acetic acid molecules). Acetic acid partially dissociates and releases only some of its hydrogen atoms into the solution and is less capable of losing and donating protons = definition of a wea...

Amanda Mei 1B

Will the final have more questions about the content we learned after the midterm or will it be cover the entire quarter evenly? I don't think Lavelle mentioned this, but it's worth asking your TA. I'm guessing there will probably be a good chunk of questions on content we learned after the midterm...

Amanda Mei 1B · Re: Final Exam It's on Sunday, December 8, from 11:30 am to 2:30 pm.

It's on Sunday, December 8, from 11:30 am to 2:30 pm.

Amanda Mei 1B

Lavelle said in an email that Wednesday is the last day we'll learn new material.

Amanda Mei 1B

Weak Acids:
Acetic acid (CH₃COOH), carbonic acid (H₂CO₃), hydrofluoric acid (HF), hydrocyanic acid (HCN), phosphoric acid (H₃PO₄)

Weak Bases:
Ammonia (NH₃), methylamine (CH₃NH₂), diethylamine ((CH₃CH₂)₂NH)

Amanda Mei 1B

It doesn't matter what type of bond exists there - it counts as one charge cloud.

Amanda Mei 1B

They should be added weekly but sometimes take a while to be added to your grade

Amanda Mei 1B

Trigonal bipyramidal is a type of electron arrangement or geometry, while T-shape is a type of molecular geometry. Any time there are 5 electron groups, the molecule has a trigonal bipyramidal electron arrangement. Lone pairs of electrons are taken into account for molecular geometry. Yes, the rest ...

Amanda Mei 1B

Both are correct but Lavelle uses "bent".

Amanda Mei 1B

Paramagnetic compounds have at least one unpaired (paramagnetic) electron while diamagnetic compounds have all paired electrons. Paramagnetic compounds are attracted to magnetic fields, while diamagnetic compounds are repelled.

Amanda Mei 1B

Bond order is the number of chemical bonds between atoms. To find bond order, count the number of bonds in a molecule and divide that by the number of bonding regions. For example, NO3- has 4 bonds, comprised of two single bonds and one double bond. You would divide 4 bonds by 3 bonding regions/bond...

Amanda Mei 1B

Why is the molecule OCS is considered polar,shouldn't their dipoles cancel out making the molecule non-polar? Oxygen is more electronegative than sulfur, which means the C-S bond will have a stronger partial negative charge on the sulfur and the C-O bond will have a weaker partial negative charge o...

Amanda Mei 1B

I have a follow-up question: How can we tell that dipole moment vectors cancel when they are at angles to one another (e.g. in a molecule with trigonal planar geometry)? Do we have to break up the vectors into their individual components? If all the bonds are between the same 2 atoms, they cancel o...

Amanda Mei 1B

If a molecular had 2 bonding groups and 1 lone pair, or 2 bonding groups and 2 lone pairs, the molecular shape would be bent.

Amanda Mei 1B

When there's a lone pair, it makes the molecular polar because the molecule isn't symmetrical and electrons won't be uniformly distributed.

Amanda Mei 1B

You don't need to know bond order in order to find bond angle or bond length. But to find each one, it'd be helpful to draw the Lewis structure.

Amanda Mei 1B

To find bond order, count the number of bonds in a molecule and divide that by the number of bonding regions. For example, NO 3 - has 4 bonds, comprised of two single bonds and one double bond. You would divide 4 bonds by 3 bonding regions/bond groups to get a bond order of 1.33. For H 2 , you'd div...

Amanda Mei 1B

Sigma bonds form between two atoms along the x-axis, and pi bonds form from p-orbital overlap and connect atoms along the y-axis. Sigma bonds are single bonds, pi bonds are the bonds that form after that.

Amanda Mei 1B

Linear, and the bond angle would be 180 degrees.

Amanda Mei 1B

In class he mentioned that we can just draw a normal Lewis Structure as long as we name the shape I think. Do we have to worry about where we arrange the atoms around the central atom (example: drawing a tetrahedral shape vs square planar) or can we just draw it normally and name the shape after? L...

Amanda Mei 1B

After drawing a Lewis structure, you'd draw a dipole arrow pointing towards the atom with higher electronegativity. This is because this atom attract electrons and gains a partial positive charge. The other atom involved in the bond, the one with the lower electronegativity, would have a partial pos...

Amanda Mei 1B

Electronegativity refers to how strongly an atom attracts bonding electrons to itself. Higher electronegativity means an atom is more likely to attract electrons. EN increases from left to right because more protons means there is a stronger attracted between the positively charged nucleus and negat...

Amanda Mei 1B

The most stable Lewis structure for an ion is the one that minimizes formal charges (minimize having separate charges). Also, the negative formal charge should be on the most electronegative atom.

Amanda Mei 1B

Yes, hydrogen-bonding is the strongest intermolecular force. It's between the hydrogen atom of one molecule and an atom of another molecule with high electronegativity.

Amanda Mei 1B

N, O, and F are the most electronegative elements (upper righthand corner of the periodic table). Hydrogen will only form hydrogen bonds with them because the electronegative atoms attract the electron cloud around hydrogen and leave hydrogen with a positive charge.

Amanda Mei 1B

If you're given an electronegativity table, you can take the absolute value of the difference between the electronegativity values of each atom in the bond. A difference of >1.7 means the bond is ionic, 0.4-1.7 means the bond is polar covalent, and <0.4 means the bond is nonpolar covalent.

Amanda Mei 1B

You don't need to write P_x, P_y, or P_z.

Amanda Mei 1B

Yes, that's all that changes!

Amanda Mei 1B

I don't think so.

Amanda Mei 1B

I think we will.

Amanda Mei 1B

The positively charged nucleus is further from these negatively charged electrons, so its pull is not as strong.

Amanda Mei 1B

There is no ideal resonance structure. The sulfate ion could be drawn with the double bond between the central S atom and any of the various O atoms. The formal charge doesn't change because you're not changing the elements that have that specific bond. The double bond is still between S and O.

Amanda Mei 1B

It's usually the atom that creates the most bonds (has fewest valence electrons).

Amanda Mei 1B

The principle quantum number (n) determines energy level and size of an orbital an electron is in (shell) The angular momentum number (l) describes the shape of the orbital, and it refers to the number of subshells The magnetic quantum number (m l ) labels the different energy levels/orbitals of a s...

Amanda Mei 1B

Hund's Rule says that every orbital in a sublevel must be occupied by one electron before any orbital is occupied by two electrons, and all of these lone electrons have the same spin.

Amanda Mei 1B

I don't believe that sig figs are too important in this class. My TA told us that sig figs are not too big of a deal, as long as you do not round to whole numbers or to only 1 decimal place when the answer should clearly have 2 or 3 decimal places. As for scientific notation, I believe that it is m...

Amanda Mei 1B

Dr. Lavelle mentioned two exceptions in class to the Aufbau principle: Chromium and Copper. Could someone please explain what those exceptions are and why they are there? These exceptions are that these elements do not follow the normal filling order for typical electron configurations. This occurs...

Amanda Mei 1B

It means density, so electron density is the probability of finding an electron in that location. In an area with higher electron density, you're more likely to find an electron there.

Amanda Mei 1B

I read the textbook for sections we cover in class as a way to supplement lectures and clear up anything I don't completely understand during lecture. Aside from watching his audio-visual focus modules, Khan Academy can be a great resource.

Amanda Mei 1B

Since p, momentum, is the mass of an object times its velocity, the unit for mass (kg) times the unit for velocity (m/s) will yield (kg)(m)/(s)

Amanda Mei 1B

The speed of light in a vaccuum is 3.00x10^8 m/s. It is slower in mediums like air, water, and glass. I think that for this class we'll just use this value.

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