CHEMISTRY COMMUNITY

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In Test 2, we were asked what species was being oxidized/reduced and for those, we looked at the molecule being reduced or oxidized.

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There are two really helpful resources on Lavelle's website where it tells you what to do for acidic and basic solutions. From that, you can see what differs basic rxns from acidic rxns.

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N stands for the number of electrons transferred. So you would have to separate your redox rxns into the reduction and oxidation rxn in order to see how many electrons are being transferred after you balanced both reactions.

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To determine which rxn is your anode or cathode, remember that your highest Ecell would be your cathode and your lowest one would be your anode. Then when determining Gibbs free energy, you would use -nFE in order to figure it out.

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what of the elements such as platinum that seem to be attached to a cell diagram? Pt(s) is used as a conductor when there is no conductive solid participating in the redox reaction. It is added to the cell diagram but does not participate. Platinum is just used as a conductor when there is no condu...

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There is a step-by-step PDF on Lavelle's website that is super clear and concise when balancing rxns for basic solutions. I have attached it below:

https://lavelle.chem.ucla.edu/wp-conten ... itions.pdf

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We should be given the information in the question whether it is basic or acidic, whether it is explicitly stated or they give us the pH.

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My TA said we would be able to pick it up from the graduate mailbox in Young Hall.

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You are assuming that acetic acid dissolved into CHCH3COO- and H+ due to the pH of 6.

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Yes, the cathode is always (I think) the highest Epotential and the lowest one is the anode.

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If you are referring to the mass of the anode being increased, then the Ecell would not be affected. The mass does not affect the Ecell. As stated before, I believe this was a question on Test 2.

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Yes it does!

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How do you determine n? I find it pretty confusing at times.

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Q: What kind of dogs do chemists have?

A: Laboratory Retrievers.

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You can look at the graphs provided or you can look at the units used in a specific problem if they do not specifically tell you which order it is. That is why it is important to know and memorize the units for the orders!

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The differential rate law is rxn rate as a function of reactant concentration. The integrated rate law tells us how much reactant concentration changes over time, so we add the variable time to it.

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You can just take the derivative to make it easier.

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Test 2 includes the last page on Thermodynamics and what we have learned so far in Electrochemistry!

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The test will be during Week 9 whenever you have your discussion!

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It should be given right next to the molecule, based on the context of the problem.

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The single line represents molecules in different phases, whereas a comma separates the molecules but represents the same phase.

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As stated above, you would change the sign to positive or negative depending on the direction of the reaction from the standard reduction potential. That is why it is also very important to know whether a half reaction is an oxidation or reduction.

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A salt bridge maintains electrical neutrality within the internal circuit, which prevents the cell from rapidly running its reaction to equilibrium. When there is no net change anymore, the cell can no longer do work.

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Why did we use platinum on Friday in Lavelle's lecture? In what cases would we use it?

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How do you combine half reactions together?

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As mentioned before, oxidation is losing electrons while reduction is gaining electrons. You can either memorize "the LEO goes GER" or OIL RIG.

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The answer to this might be obvious but how does this connect to batteries? At a basic level, batteries are storage places for chemical reactions to occur. These chemical reactions can be used to generate electrical energy for our usage. In addition, Professor Lavelle mentioned on Friday that when ...

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I believe we are expected to memorize the most common ones and it also good practice!

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I understand that splitting equations means separating the reduction equation from the oxidation equation but what is the purpose?

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An acronym that I find very useful is as mentioned above, OIL RIG. Oxidation is losing electrons, reduction is gaining electrons.

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For gases, the change in enthalpy is defined by that equation. This is because when gas is produced work is done to clear the other gases so the enthalpy is lower than the internal energy. This is explained in detail on pg 274 in the textbook (4D.2) Thank you so much for explaining! Do you know why...

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The acronym is PD, TV HUGS.

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If you mean 4A, we change the sign of enthalpies when we reverse the equation. For example, we reversed equation 2 because we wanted Vodka on the product side since we have it on the product side in the overall recipe. We then multiplied it by two since we want 2 Vodka on the product side to match t...

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It is very helpful to draw the graph. We see that in order to melt ice, we need to go through the plateau on the heating curve that turns a solid into liquid. So we have to include that in our calculations .

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We know that internal energy is q+w. We know from the question that q is 5.50kJ. Now we need to figure out the work done. Thus, we would be using the equation w = -PΔV to figure that out. Then when work is calculated, we add 5.50kJ + work done and that will be the change in internal energy.

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For 4D7, why are we using the equation DeltaH= DeltaU + DeltaNRT? I do not quite understand why we are including DeltaNRT.

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For 4C7, I know we calculated the enthalpy of vaporization for part b by turning 22.45g of ethanol into moles (which gives you 0.48), and then dividing 21.2kJ of energy by 0.48. This, in turn gives you 44.16 for the enthalpy of vaporization but I do not understand the question conceptually. Why are ...

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For Part B on 4D3, I was a little confused on why we would use 1.40g of carbon monoxide to calculate the internal energy (converting that into moles) instead of 1.00 mol CO(g) which was what was given on part b. I might be totally reading the question wrong, but can someone explain it?

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A chemical rxn is reversible when it can convert back to the reactants. For example, combustion is irreversible since its entirety was "consumed." For reversible rxns, the reactants are never fully consumed or dissociated/ionized.

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As mentioned above, the molecules of water are held together by hydrogen bonding; breaking those bonds require a great amount of energy.

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You will still be able to pick up your Final this week!

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We will most likely be using all three; it definitely depends on what the question is asking. However, keep in mind that Method 2 is the least accurate as mentioned in lecture.

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Enthalpy is a state function because it only depends on initial and final values in the rxn; thus, the path doesn't matter.

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Similarly to the former statement, Hess' law allows us to combine equations to generate new rxns whose enthalpy change can be calculated, since it is a state function.

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K is a constant used when the rxn is at equilibrium. Q can be used at any time in the rxn to determine the shift of a rxn, either left or right.

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As formerly mentioned, pressure and volume are inversely proportional. When volume decreases, pressure increases. When volume increases, pressure decreases.

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The reaction would shift towards the side with fewer moles!

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We test it out by dividing equilibrium concentration/initial concentration x 100. if the value is less than 5%, the approximation is valid.

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The reaction will shift to the left because the reverse reaction is endothermic. Increasing heat/temperature will cause the rxn to favor the reactants.

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Le Chatelier's Principle applies to changes in temperature, volume or concentration.

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Equilibrium constants have no units; this is because it is a ratio of similar quantities.

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If the rxn shifts left, that means that a forward reaction is favored, thus producing more reactants. If the rxn shifts right, that means that a reverse reaction is favored, thus producing more products.

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They are both units of pressure; I think it depends on the question and what units that specific question wants you to use.

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I do not think we will be expected to memorize them as it will most likely be provided in a formula sheet but it is good to understand them!

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As explained by Dr. Lavelle, a reaction tends to favor the side that has less substance to reach equilibrium. If we take away product, the reaction will favor the products in order to balance the products and reactants or reach equilibrium.

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The P stands for pressure
V is volume
n is the number of moles
R is the ideal gas constant
and T is temperature.

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Test 1 will probably consist of Outline 1 and Outline 2!

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I am definitely "(dien)" after that chemistry final!!!!

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In a regular covalent bond, the atoms involved in the bonding share a pair of electrons. However, in a coordinate covalent bond, two electrons are contributed by a single atom.

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Sigma bonds are stronger than pi bonds because sigma bonds head-on overlap while pi bonds overlap side by side. When there is one bond, that forms a sigma bond. When there are two bonds, there is one sigma bond and pi bond. When there are three bonds, there are 2pi bonds and one sigma bond.

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The standard unit for speed of light is m / s. The units are also given in the constants sheet provided to us during finals/midterms.

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When you tell a chemistry joke and there are no reactions :(

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What is the trend for electronegativity and ionization energy?

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I understand the coordinate covalent bonds are covalent bonds in which both electrons come from the same atom, but what does the necessarily mean? I do not quite understand what it is stating.

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What about hemoglobin and myoglobin do we need to know for the final?

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Can we assume that melting and boiling points will always be higher for hydrogen bonds than any other intermolecular force?

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Can resonance structures determine the strength of an acid? Or is there no correlation between both?

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I know atomic radius and ionic radius both follow the same periodic trend but what is the difference between them?

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What are some key concepts to know about coordinate compounds for the final?

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A Lewis acid is an electron-pair acceptor while a Lewis base is an electron-pair donor.

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Yes H2O is an amphoteric compound so it depends fully on what it is reacting with.

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I think for class, we need to know mainly about Cisplatin, which binds to DNA to kill cancer cells.

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Since long bonds are farther away from the nuclei, it is easier to remove H+ so thus long bonds are strong acids. Conversely, short bonds are harder to break so they are weak acids.

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Lavelle went over neutralization and relative acidity. He also stated that an acidic solution's H3O+ is greater than OH- and the opposite occurs for basic solutions.

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Strong bases are completely ionized in water while weak bases are partially/incompletely ionized in water.

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A Lewis acid is an electron acceptor while a Bronsted acid is a proton donor.

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Strong acids are completely ionized in water or solution while strong bases are completely ionized in solution. Weak acids and bases are partially dissociated in water/solution.

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The one amphoteric compound we have gone over in lecture is H2O since it can act as a base and acid. I think it is sufficient enough to know that H2O is the main amphoteric compound.

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You always draw the Lewis structure first to determine the hybrid orbitals.

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O2 would be considered nonpolar since the bonds are evenly distributed and cancel out, thus there is no net dipole. We can also say that because they have the same electronegativity, there is never an electronegativity difference between them, therefore it is nonpolar.

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The higher the electronegativity, the stronger the bond is and the shorter the bond length is. Since the electronegativity is higher, then the closer a bond becomes to being ionic rather than covalent.

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A single bond is a sigma bond, while a pi bond occurs with double and triple bonds. For double bonds, there is one pi bond and sigma bond. For a triple bond, there is two sigma bonds and one pi bond.

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If an H bonds with a Fluorine, Nitrogen, or Oxygen, it is considered a hydrogen bond.

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A coordinate covalent bond is when one atom donates both electrons in a covalent bond.

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A bent shape can have up to one or two lone pairs around the central atom.

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Yes, lone pairs are considered regions of electron density because they alter the molecular structure significantly. For example, a lone pair on a central atom can turn a trigonal planar into a trigonal pyramidal.

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The bond length between atoms with two or more bonds are shorter than those with single bonds. The more electrons, the stronger the pull is and the shorter the bond length is.

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We can tell when a VSEPR model has distortion because usually the central atom will have a lone pair or more. When the central atom has a lone pair, it pushes the bond electrons closer to one another, thereby making the bond angle smaller.

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For a molecule to be polar, it must possess a non-zero value of dipole moment. In the molecule CCl4, the bond polarity is canceled by the symmetric tetrahedral shape causing the dipole moment to be zero.

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The bond length is determined experimentally, so we do not determine the bond lengths ourselves.

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Double bonds are shorter because the pull/attraction to the nucleus is greater than single bonds.

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What exactly are the characteristics of distorted electrons?

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Electronegativity goes down the periodic table because as the electrons are farther away from the nucleus, there is decreased attraction thereby lessening the pull.

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What is exactly the significance of coordinate covalent bonds and how does it help us understand bond structure in general?

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Elements past the third period can have expanded valence shells due to the presence of a d-orbital.

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Usually, covalent bonds are between two nonmetals and involves the sharing of electron pairs between atoms. There might be some exceptions to it, as the individual above stated but mainly, covalent bonds are between two nonmetals.

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How exactly would I represent anions and cations in Lewis structures? Would these be implied by removing an electron from the Lewis structure if it is cation and adding an electron if it is an anion?

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It is stated that d-orbitals can carry more than 8e-. The examples that Dr. Lavelle gave in lecture were the elements, Chlorine, Phosphorus and Sulfur.

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Is the purpose of resonance structures to help us draw the best/most accurate Lewis Structure? I am still slightly confused on resonance and what the purpose of it is.

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Can someone explain whether the octet rule is more important than obtaining a 0 formal charge or vice versa?

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