CHEMISTRY COMMUNITY

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First use the first order half life equation to find K. Using K, you can calculate the ratio of the final/initial concentrations. that ratio can be used to find what fraction of the 150mg have been used.

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the rate law tells you the overall rate of the reaction in terms of its reactants, this will include K. the rate order depends on the exponent on each reactant, which indicates the number of molecules of that substance that needs to collide. Molecularity refers to the number of species in an element...

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Hi all,

I am looking at the midterm, question 4. How do I determine if a substance will or will not remain protonated by comparing PH between that substance and its surroundings?

Thanks in advance

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Does the rate determining step always have the largest activation energy? How do I determine the rate determining step by looking at a reaction profile?

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In lecture Dr. Lavelle said that raising the temperature increases rate of reaction in both the forward and reverse reaction. However, I previously thought that raising the temperature increases the value of K, which as I understand is the ratio of the Ks of the forward and reverse reactions. Could ...

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a pseudo rate law is obtained under lab conditions. Consider a reaction with three reactants, A, B, C, each with n=1. The order would be considered 3. However, under lab conditions the concentrations of A and B can be made very high, and would be considered constant. Thus, the rate depends only on t...

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changing the concentration of the reactants changes E cell. Increasing the concentration of the products shifts the equilibrium to favor the products, thus E cell increases.

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if a reaction is endothermic, and the temperature is raised, the rate constant of the forward reaction also is raised. Likewise, if a reaction is exothermic, does that mean with an increase in temperature k' will increase more than k of the forward reaction?

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"elementary" is used in many practice problems to describe reactions. What exactly does it mean when a reaction is "elementary?"

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Im looking at chapter 7 problem 7A.17 given a table of concentrations and rates for multiple reactions, the question asks to find the order of each reactant. I know from class to find the order you compare two separate reactions when the concentration of one reactant changes while the other concentr...

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If second order reactions take place when two molecules collide in the slow step, how do first and zero order reactions take place? In other words, could someone explain conceptually what takes place in a zero or first order reaction?

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Hi all. I want to clarify my understanding of the NERST equation. When using the natural log of Q, at 298K, RT/F = 0.025693. However, at the same temperature but when using log of Q, RT/F = 0.05926. Could someone confirm/correct my understanding?

Thanks!

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The standard cell potentials are very helpful when the cell is spontaneous. The greater the magnitude of charge the stronger the reducer/oxidizer.

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look at the states of the products - if there is a solid as a product that could be considered a precipitate.

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In terms of electrolytic cells, it is important to keep in mind that it is not spontaneous, thus the voltage of the reduction reactions aren't super important because regardless of voltage, the cell will need additional energy to move the process.

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Hi,

How do I identify what substance is the strongest reducing/oxidizing agent based off of their E?

Thanks in advance.

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Suppose I am balancing a redox reaction, and need to multiply one of the half reactions by some coefficient. Would the E for that half reaction also be multiplied by the same coefficient?

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I haven't come across any questions that require you to balance using OH and H unless the problem specifically mentions an acidic/basic solution.

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The Nernst equation can help you calculate concentration, from which you can use to find PH. Khan academy has a super helpful video on the Nerst Equation, just search on youtube!

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How far in chapter 6 have we covered in lecture so far?

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Hi all, I'm looking at question 6L.5.B. I don't understand why Pt is used as an inert conductor on BOTH sides. The answer key has Pt as an inert conductor even when there is solid iodine in the reaction. Could someone explain to me why Pt is used as in inert conductor despite there being a solid pre...

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in addition^ I understand how Chlorine was able to be oxidized, but I don't understand how to write out a reduction equation for chlorine in this case

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Hi all, looking at question 6k.5.d, chlorine gas is both the oxidizing and reducing agent. Could someone help me understand how this happens?

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Hi all,

Im having trouble understanding what it means when a redox reaction takes place in an acidic or basic solution. Could someone clarify how to balance and conceptually what takes place when a redox reaction takes place in an acidic or basic solution? Thanks!

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Today in lecture Dr.Lavelle said that Le Chatelier's principle is used to explain the change in Ecell when the concentration of a product/reactant is changed. Could someone further explain/clarify exactly how changing the concentration of a product/reactant influences Ecell?

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If E cell is negative, does that indicate that the reaction is not favorable? Can there every be a negative value for E cell?

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I did not catch in lecture today what Dr. Lavelle was explaining about the relationship with Wmax and deltaG, are they always equal?

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Hello,
I thought that when temperature increased, so did K. However I'm looking at 5J.13 and the K given for a higher temperature is smaller than the K given for a lower temperature. Could someone clarify the relationship between temperature and K?

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Hi, i'm looking at question 4 on the Pizza Rolls review sheet. When do I know to change the sign of the enthalpies given?

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I know the equation Q=mc(deltaT), where m=mass of substance. However I have come across a very similar equation, Q=nc(deltaT), where n=moles of substance. How do I know if I should use the mass or the moles of a substance when finding Q? One idea I had was to use moles when calculating at a constant...

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How do I know when to convert to kelvin/celsius when solving problems? Is it safe to always use kelvin?

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Hey.
Could someone explain where the equation for work, W=nR(deltaT) comes from? This is for calculating work at constant pressure.

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Could someone explain when solving for q: when we include moles, when we include mass, and when we don't include any value for mass/moles?

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Could someone clarify exactly what U, internal energy, is? Beyond the equation U=q+W, why is U significant?

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when considering Work, the energy from work of expansion, why is it important that the system is at equilibrium?

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In Wednesday's lecture, (the 29th) an equation put W, work, in an integral. Could someone explain what this equation means and why is is helpful? Thanks in advance.

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I highly recommend watching the Organic Chemistry tutor on youtube. I find his videos super helpful. He goes very slowly, but does many practice problems as well as conceptual explanations as he solves them. I also recommend khan academy.

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Could someone explain why the standard enthalpy of formation of an element in its most stable form is zero?

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Hi. In lecture, it was stated that enthalpy was a state property, meaning its value is "determined by its current state." Could someone expand what
determined by its current state" means?

Thanks!

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is oxidizing/burning/breaking down the same thing as combustion?

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Is the concentration of a conjugate acid always equal to the concentration of hydroxide ions? Similarly is the concentration of the conjugate base of a reaction equal to the concentration of hydronium ions?

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could someone explain why, at the same concentration, a weak acid would have a higher PH than a strong acid?

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K will change in response to change in temperature depending if it is endothermic or exothermic. If the reaction is endothermic, thinking of heat as a reactant, and the reaction will favor formation of the products to reduce stress on the overall reaction. Likewise, if the reaction is exothermic, he...

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To avoid calculation errors, yes, it is best to convert to mols. If you don't you need to be careful with your units throughout the entire problem. Since Molarity is in Mol/L, it is easiest to convert to mols.

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It can be calculated by (conjugate acid)/(initial base) * 100%.
for an acid, it would be (conjugate base)/(initial acid) * 100%

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If a problem doesn't specifically state if it is endothermic or exothermic, how do we determine which it is based off the stoichiometric equation?

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In lecture Lavelle said that Kw is a constant, 1.0 x 10^-14. I'm looking at a homework problem, and it gives kw as 2.1 x 10^-14, could someone clarify the meaning of the value of Kw? The homework question im referring to is 6A.21

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In class Dr. Lavelle said that when pressure increases, equilibrium will favor the side with less mols of gas. He said however, that reasoning is not sufficient as an explanation. Could someone explain what would be an appropriate explanation for why an equilibrium would favor the side with less mol...

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Also having trouble with this problem. I follow your work until you plug into the Kc formula. Could you explain how you solved for x? Could you also explain why you omitted the x variable in the denominator with 0.06?

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The value of K helps us determine how favored the reaction, forward or backwards, is. A very small K indicates high concentration of reactions, while a very large K indicates there is a high concentration of products.

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In lecture Dr.Lavelle used the word "favored" when describing if equilibrium "sits" to the left or to the right. Could someone clarify what it means for equilibrium to "favor" a side? Thanks!

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I'm confused when to use partial pressure or when to use concentration for K, when a problem doesn't specifically ask for one or the either. If the products and reactants are in the gas phase, does that mean I should always use partial pressure? Thanks in advance!

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What factors influence rate of reaction for a reaction reaching equilibrium? For example, does higher concentration influence rate of reaction to equilibrium?

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What exactly happens to products and reactants at equilibrium? I know there is still movement, but why doesn't a reaction stop when it reaches equilibrium?

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I am confused about the order of naming in the compound [Co(H2NCH2CH2NH)2(CN)(Cl)]Cl. the correct name is, chlorocyanobis(ethylenediamine)cobalt(III)chloride, however, I don't understand the thinking behind the positioning of the names of the two Cl ions. If Cl is an anion, why does it lead in the n...

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Im confused when to use, or not to use equations involving momentum(p). Furthermore, how do I know when mass plays a factor in calculations in relationship to wavelength, frequency, etc. Similarly, when is it not right to use the equation C=(wavelength)(frequency)?

Thanks in advance!

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Explain why hypobromite is a stronger base than morphine. I my thought process was first that since morphone has more oxygen atoms, and that Nitrogen is more electronegative than Bromine, thus morphone would be stronger. Could someone explain why hypobromite is actually a stronger base?

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From reading in the textbook I am confused on the difference between acidic and basic oxides. Furthermore, are all oxides (acidic or basic) amphoteric? The section of textbook I am referring to is 6A.3. Thanks in advance!

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Hybridization is when orbitals fuse together forming a new orbital- with new properties that influence shape etc. This allows for bonding, and is more energetically favorable.

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the concentrations of hydronium and hydroxide are characteristics of acids and bases. Since acids donate a proton (H ion), that proton reacts with water, forming hydronium. Thus, when an acid reacts with water, there is a higher concentration of hydronium. Similar for bases, except with hydroxide. B...

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First identify the type of reaction. A strong acid completely dissociates in water, thus the result will be hydronium + anion. Similarly, a strong base also completely dissociates in water, but the result will be hydroxide instead of hydronium.

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Some quick tips: acids are named based on their anion, the ion that is produced when dissolved in water. For strong bases, since most contain the compound OH, a polyatomic ion, strong bases follow the rules for naming ionic compounds. Weak bases can also follow these same rules for ionic compounds, ...

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You will need to identify first if the base is strong or weak. From that identification you can then conclude if there was complete dissociation.

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Does a lone pair have the same repulsion effect on single bonded electrons as double bonded electrons?

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The text book reads that the lowest energy level structure of a molecule is achieved when the lone pairs are on the equatorial plane. Could someone explain the difference between axial and equatorial planes and how to identify them when placing lone pairs of electrons on an atom?

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I know that lone pairs push bonded atoms away, so atoms atoms with lone pairs have smaller angles. Is there a set number of degrees that a lone pair causes an angle to decrease? Or do i just need to memorize angles.

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I think we additionally will need to know how the intermolecular forces affect properties of the molecules.

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What is the purpose of electron geometry? If we use molecular geometry to name the shape of a compound, why do we look at electron geometry?

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If a molecule is polar, this is an uneven charge distribution. Meaning, the molecule does not have an even distribution of charges. A quick trick is to look at the actual structure of the molecule, looking for symmetry. Symmetric molecules tend to be non-polar. However, the most sure way is to look ...

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The shape of a molecule does influence strength of attraction. For example, different shapes allow for some molecules to "fit" together better than others. Thus shape can influence the strength of attraction between different molecules.

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Dispersion forces for different elements impact their boiling/melting points. Although elements within the same group have similar qualities, they are not identical. For example, down a group the atomic radii increases. Larger atoms are more polarizable, thus have stronger forces.

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I would either write out all resonance structures, or, if all resonances structures contribute equally to the hybrid, draw the solid and double lines in one structure.

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l can be any value that satisfies n-1. Thus, if n=12, l could have any value between 0 and 11.

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What is the difference between subshells and orbitals? Many practice questions asked to determine the number of orbitals/subshells from a given set of quantum numbers, for example, "how many orbitals can have the given quantum numbers: n=2, l=1." I do know that "number of orbitals&quo...

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I believe it is just a property. compounds that donate a proton (H+) are considered an acid, while compounds that accept a proton, are considered basic.
Hope this helped!

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I'm confused what the differences are between polarizability and delocalized electrons. This is in context of the chemical bonds unit.

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Today in a review session the TA said 18, but I have not encountered in problems that use electrons past 10.

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I have come across many problems where the reactants were named without the chemical equation. I would suggest you learn common compounds/naming compounds and be able to write out the reactions. However, I think we are given tables on exams with equations.

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The Balmer series has electrons rest at energy level2 and is for visible light, while the Lyman series has electrons rest at energy level1 and is for UV light

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The intensity of light is proportional to the square of its amplitude

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That is the Rydberg formula, it is used to find the wavelength(for frequency) of light emitted when an electron changes energy levels.

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In the past there have been different standards set for the atomic scale, such as oxygen because its many bonding characteristics. However Oxygen has many isotopes that that vary in weight etc, so carbon 12 was chosen which as similar to the previous scale.

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When electrons go down in energy levels, they release energy that can be mapped. For this reason it is helpful measure these wavelengths to identify an atom.

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I don't understand in dilution, when a question states that, for example, when a 10mL solution is diluted with 20mL, is the final volume 30mL or 20mL? Would the "final" volume that would be used in the equation be the total volume of the final solution, or the amount added?

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Some animals do see on a different light spectrum. Just as humans see some colors differently than some animals, spiders and insects have the ability to see ultraviolet light.

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I wouldn't group photons with electrons or protons. Although there are some commonalities, their movement is independent of mass and do not carry a charge.

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When calculating molar mass do you include the mass of the hydrate? For example, CuSO4 • 5H2O, would the molar mass be just the CuSO4, or would it include the 5 moles of H2O?

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One trick I learned was to calculate the amount in moles of each reactant. Then, divide by that reactant's coefficient that is given in the balanced equation. The reactant with the lower ratio is the limiting reactant.

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A student asked Professor Lavelle this same question in lecture on Friday. He said he uses all the decimal places, and waits to round until the final answer.

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You would first find the molarity of the new solution using the equation M1V1=M2V2, then using the equation Molarity = (Moles)/(Volume) find the moles in 100mL.

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