CHEMISTRY COMMUNITY

Elizabeth Kaplan 3I

Hi! I'm very confused on whether catalysts are actually consumed in a reaction or not. In lecture and in a post on Chemistry Community, Dr. Lavelle said that catalysts are consumed first in a reaction, and then are produced. However, there was a question in the textbook that said catalysts are never...

Elizabeth Kaplan 3I

The first equation you wrote is the equation for zero order reactions, and the second equation you wrote is the equation for first order reactions. You can also refer to the equation sheet to help. From left to right and top to bottom on the equation sheet, it writes the equations for first order re...

Elizabeth Kaplan 3I

The orders of a reaction (also called the overall order) is calculated by adding up the orders of each reactant. To determine the order of each reactant, you either use the psuedo rate law (in which there is an excess in concentration of all but 1 reactant) or you compare experiments when all but on...

Elizabeth Kaplan 3I

The graphs are determined through many experiments done in the lab. However, once the graph is determined it is relatively simple to determine the order. If it is a zero order reaction, it is plotting [A] vs time and the slope=-k. If it is a first order reaction, it is plotting ln[A] vs time and the...

Elizabeth Kaplan 3I

[A]0 acts as the y-intercept of the graph for each reaction, no matter the order. However, for a zero order reaction, the y-intercept is [A]0, for a first order reaction, the y-intercept is ln[A]0, and for a second order reaction, the y-intercept is 1/[A]0.

Elizabeth Kaplan 3I

The half-life equation for first order reactions is the only one that is independent of concentration ([A]0). This is especially important because it allows us to use the half-life equation to find how long it takes for a reaction to, for example, get to 1/8 of it's initial concentration. This is so...

Elizabeth Kaplan 3I

What range (in volts) does a voltmeter need to have to measure pH in the range of 1 to 14 at 25 °C if the voltage is zero when pH = 7?

Could someone please explain how they determined that it is the product over reactants fraction? Why is it [H+]/[OH-], and not [OH-]/[H+]? Thank you!!

Elizabeth Kaplan 3I

Hi! I am really confused on how to know which atoms to include in the cell diagram. For example, in textbook question 6L.7 part a, the reaction is AgBr(s)--> Ag+(aq)+Br-(aq). The answer key says that the cell diagram is Ag(s)|AgBr(s)|Br-(aq)||Ag+(aq)|Ag(s). How did they know to include Br- on the an...

Elizabeth Kaplan 3I

Hi! In textbook question 6L.7 part a, the reaction is AgBr(s)--> Ag+(aq) + Br-(aq). The answer key had one half reaction Ag+ to Ag(s) which makes sense, but for the other half reaction they had AgBr(s) to Ag(s) + Br-(aq). Why didn't the second half reaction simply have Br to Br like the first one?

Elizabeth Kaplan 3I

Hi! In the textbook question 6L.5 part d, the reaction is Au+(aq)--> Au(s)+Au3+(aq). In the answer key, they had one half reaction from Au+ to Au(s), which makes sense (the reactant to one of the products) but then for the other half reaction they had Au(s) to Au3+. I am confused why Au+ is not the ...

Elizabeth Kaplan 3I

Hi! I am a little confused on how to know whether to leave a product as one molecule, or whether to separate it into its atoms, when there are 2 reactants and 1 product in a redox reaction. For example in 6L.5, the reaction is Cl2(g)+ H2(g)--> HCl(aq) and the answer key separated HCl into H+ and Cl-...

Elizabeth Kaplan 3I

Hi! I am a little confused how to determine the reactants and products for the half-reactions and ultimately the full balanced redox reaction from a cell diagram? I think the cell diagram doesn't put the reactants and products in a certain order so if someone could explain that to me I'd really appr...

Elizabeth Kaplan 3I

Hi, coefficients are not relevant to determining the oxidation number of an atom, but is relevant when determining the total charge of each side of the reaction to ensure it is properly balanced.

Elizabeth Kaplan 3I

The purpose of a salt bridge is to allow both reactions and sides to remain neutral. If anions are continuously moving from one side to the other, eventually electron repulsion will prevent anions from moving. The salt bridge prevents this from happening.

Elizabeth Kaplan 3I

Hi, I would assume that the anode is always on the left, while the cathode is on the right because Dr. Lavelle said that it flows from left to right, as well as the cell diagram is written with the assumption that the left side of salt bridge or porous disk is referring to the anode, while the right...

Elizabeth Kaplan 3I

Yes, the anode is always the oxidation side of the reaction (usually the left), while the cathode is always the reduction side of the reaction (usually the right). These sides are either separated by a porous disk or a salt bridge.

Elizabeth Kaplan 3I

I like to use OIL RIG, which stands for Oxidation Is Losing, Reduction Is Gaining. You can also use LEO, which stands for Loss of Electrons is Oxidation, and then deduce that reduction must be the opposite.
Hope this helps!

Elizabeth Kaplan 3I

I thought that you can only determine if a reaction is spontaneous or not by knowing both deltaH and deltaS (deltaG= deltaH-TdeltaS), however in my notes it says that you can determine if a reaction is spontaneous or not from deltaS total (if deltaS total is positive the reaction is spontaneous, and...

Elizabeth Kaplan 3I

A state function is when the path taken does not matter; in other words, only the final and initial states matter. Free energy is a state function because we only need the values of the free energies of the products and reactants to determine if the reaction is spontaneous (whether the difference be...

Elizabeth Kaplan 3I

An open system is when both matter and energy can be exchanged with the surroundings, a closed system is when only energy (heat) can be exchanged with the surroundings, and an isolated system is when neither matter nor energy can be exchanged with the surroundings. Remember that the change in intern...

Elizabeth Kaplan 3I

Potassium nitrate dissolves readily in water, and its enthalpy of solution is + 34.9 kJ ⋅ mol − 1 + 34.9 kJ ⋅ mol − 1 . (a) Does the enthalpy of solution favor the dissolving process? (b) Is the entropy change of the system likely to be positive or negative when the salt dissolves? (c) Is the entr...

Elizabeth Kaplan 3I

I am very confused on how you can determine the stability of a compound as temperature increases based on delta S (textbook question 4J.15) and how based on delta G (textbook question 4J.13).

If someone could explain this to me I would really appreciate it!

Elizabeth Kaplan 3I

"Under what conditions, if any, does the sign of each of the following quantities provide a criterion for assessing the spontaneity of a reaction? (a) Δ G ° (b) Δ H ° (c) Δ S ° (d) Δ S tot" The answer manual says that for part d, the reaction is spontaneous when delta S (total)>0. How did ...

Elizabeth Kaplan 3I

"Use the data in Table 9.3 or Appendix 2A to calculate the standard reaction entropy for each of the following reactions at 25 degrees Celsius. For each reaction, interpret the sign and magnitude of the reaction entropy. (d) The decomposition of potassium chlorate: 4 KClO 3 (s) → 3 KClO 4 (s) +...

Elizabeth Kaplan 3I

Considering positional disorder, would you expect a crystal of octahedral cis-MX2Y4 to have the same, higher, or lower residual entropy than the corresponding trans isomer? Explain your conclusion. I don't understand how the solutions manual deduced that there are twelve different orientations for t...

Elizabeth Kaplan 3I

I would recommend looking at the units given in the question and the unit you are trying to solve for. For example, if you are trying to calculate work, whose unit is Joules, and you are using the equation delta n x R x T, you know to use 8.314 J/K.mol because you want to get Joules.

Elizabeth Kaplan 3I

I think it is because q and w are dependent on "the path taken", meaning that you cannot simply subtract the final state minus the initial state. However, for internal energy, all you need to know are the final and initial states, making it a state function.

Elizabeth Kaplan 3I

Internal energy equals change in enthalpy at constant pressure and volume. This is because internal energy equals q (heat) at constant volume (since the system is not doing any work), and q equals delta H at constant pressure.

Elizabeth Kaplan 3I

I recommend writing down what is given in the question! If the question provides the external pressure and volume change of the system, then -PV is the best option! However, if the question specifies moles and temperature, then -nRT is the better option.

Elizabeth Kaplan 3I

The internal energy of a system (deltaU) is equal to the work (w) done by or on the system + the heat (q) absorbed or released by a system. So, in a sense, q is a part of the internal energy but you must also account for the work. The only time the internal energy equals the heat is when there is a ...

Elizabeth Kaplan 3I

Hydrochloric acid oxidizes zinc metal in a reaction that produces hydrogen gas and chloride ions. A piece of zinc metal of mass 8.5 g is dropped into an apparatus containing 800.0 mL of 0.500 M HCl(aq). If the initial temperature of the hydrochloric acid solution is 25°C, what is the final temperatu...

Elizabeth Kaplan 3I

Hi! I was wondering if someone could clarify the difference between q=mCdeltaT vs. q=CdeltaT, and when to use each equation. I'm super confused because for some questions we take the mass into account but others we don't.
Thank you!!

Elizabeth Kaplan 3I

In lecture, Dr. Lavelle did an example on calculating the entropy of 4 carbon monoxide molecules. For W, he used N=4, so 2^4=16. However, I am a little confused because he previously said that N is the number of particles. Since there are only 2 particles (carbon and oxygen), why isn't N=2?

Elizabeth Kaplan 3I

Hi! I think that the different types of systems are significant because they reflect what is staying constant. For example, in open systems, both matter and energy can be transferred, so neither are constant, while in closed systems, only energy/heat can be transferred, so the amount of matter is co...

Elizabeth Kaplan 3I

I don't believe isolated systems change the energy of the system (at least from what we learned so far in lecture) because it does not allow matter or energy to be transferred from the surroundings into the system.

Elizabeth Kaplan 3I

Enthalpy is the amount of energy or heat present in a system, while entropy refers to the amount of disorder or chaos in a system. Enthalpy is also denoted by H, while entropy is denoted by S.

Elizabeth Kaplan 3I

Hi! I'm pretty sure that the only difference between the R constants is the units for pressure. So, although we say 1 bar=1atm, you would choose two different values for R because bar and atm are not exactly equal.

Elizabeth Kaplan 3I

Work is not a state function because it depends on the "path taken". Unlike enthalpy, volume, density, temperature, etc., work does not depend only on the final and initial states.

Elizabeth Kaplan 3I

Hess's Law states that enthalpy is a state function, meaning that only initial and final values are taken into account in the calculation. Hence, this allows to add together enthalpies, which we were not able to do for equilibrium constants (K) previously.

Elizabeth Kaplan 3I

Enthalpy is a change in energy due to a change in temperature. When heat is added, or temperature is increased, the amount of energy in the system is changed, which is represented by enthalpy.

Elizabeth Kaplan 3I

I would recommend looking at whether the event requires heat to be done, or ultimately releases heat when the event is complete. For example, in the instance of baking a cake, this event requires heat to be done, so it is an endothermic reaction. If the event releases heat, it is exothermic.

Elizabeth Kaplan 3I

Hi! I think you are just to supposed to determine which bonds are broken in the reactants and which bonds are formed in the products, and then add the enthalpy values of those bonds together (make sure the enthalpy values for the bonds broken are positive because it requires energy, and that the ent...

Elizabeth Kaplan 3I

By being a state function, this means that only the final and initial states of enthalpy matter. Unlike work, which is not a state function, enthalpy only requires the starting and final values for calculations and does not consider "the path taken".

Elizabeth Kaplan 3I

Textbook Question 4D.15 gives the Delta Hc for the reactants and products, and I was wondering if this is the same as Delta Hf. I calculated the answer as if it was Delta Hf (and got the right number) but the answer key says the value should be negative, even though I got positive in the calculation...

Elizabeth Kaplan 3I

Dr. Lavelle mentioned in Friday's lecture that HBr and HCl are diatomic molecules, but I thought diatomic molecules were molecules like Br2, O2, N2. How do we know when a molecule is diatomic then?

thank you!

Elizabeth Kaplan 3I

An ICE chart is used when the question is asking to find the equilibrium concentrations, however the equilibrium constant equation is used when the equilibrium concentrations are already given. Also, K is usually given when using an ICE chart because it is needed to solve for X.

Elizabeth Kaplan 3I

So far, I believe we have only dealt with atm (atmospheres), besides bar and mol. L-1. Both atm and bar are used to measure partial pressure, and the unit conversion is 1:1. We have not dealt with torr yet, however the unit conversion between atm and torr is 1 atm=760 torr.

Elizabeth Kaplan 3I

Hi! I believe that 10^-3 and 10^3 are used to determine whether the products or reactants in a reaction is STRONGLY favored. If K is greater than 10^3, it is safe to assume that the products are strongly favored. Likewise, if K is less than 10^-3, it is safe to assume that the reactants are strongly...

Elizabeth Kaplan 3I

Hi! If water was a gas, then it would be included in the equilibrium constant because it is not in excess like it is in liquid form.

Elizabeth Kaplan 3I

If the enthalpy is positive, meaning that the reaction is endothermic, this means that heat is a reactant. Thus, increasing heat, or increasing the temperature, will cause the products to be favored and the reaction will shift to the right. Hence, K increases because the products (the numerator) inc...

Elizabeth Kaplan 3I

Carbon monoxide detectors are used in homes to detect this toxic gas. Calculate the concentration of CO(g) in mol.m^-3 at 423 K and 0.80 atm assuming only CO(g) is present. (Hint: Use the conversion factor 1L=(1dm)^3) I understand how to do the problem and I got the concentration to be 0.023M CO, ho...

Elizabeth Kaplan 3I

When a reaction favors one side of the reaction, it means it is producing more of that side. So, if a reaction favors the products, it means that it is using up more of the reactants to create more product. Similarly, if a reaction favors the reactants, it means that it is using up more of the produ...

Elizabeth Kaplan 3I

When there is a coefficient in front of the products, you raise either the concentration of that product, or the partial pressure of that product, to the coefficient that is in front.

Elizabeth Kaplan 3I

The acronym ICE stands for Initial molar concentration, Change in molar concentration, and Equilibrium molar concentration. This acronym is used in an ICE table to determine the equilibrium molar concentrations and K for a chemical reaction.

Elizabeth Kaplan 3I

Hi! It is basically saying that the "new" initial concentration of NO is 0.700 M. This means that you are setting up a new ICE table with 0.700 as the initial concentration, and then simply do the same thing as you did for the previous part of the question to determine the "new" ...

Elizabeth Kaplan 3I

Hi! I think the meaning behind solids and liquids being pure substances is that they do not interact with the other aqueous and gaseous solutions in the chemical reaction, and this is why their concentration do not change (or at least do not change enough to be considered).

Elizabeth Kaplan 3I

The reaction N2O4--> --< 2NO2 is allowed to reach equilibrium in a chloroform solution at 25 degrees C. The equilibrium concentrations are 0.377 mol/L N2O4 and 2.05 mol/L NO2. Calculate the equilibrium concentrations of N2O4 and NO2 after the extra 1.00 mol NO2 is added to 1.00L of solution. I would...

Elizabeth Kaplan 3I

The equilibrium constant Kc=0.56 for the reaction PCl3(g) + Cl2(g)--> --< PCl5(g) at 250 degrees C. Upon analysis, 1.50 mol PCl5, 3.00 mol Pcl3, and 0.500 mol Cl2 were found to be present in a reaction vessel of volume 0.500 L at 250 degrees C. (a) Is the reaction at equilibrium? (b) If not, in whic...

Elizabeth Kaplan 3I

State whether reactants or products will be favored by an increase in the total pressure (resulting from compression) on each of the following equilibria. If there is no change, explain why that is so. (d) 2HD(g) + H2(g)--> --< D2(g) For part (d), I thought that the products will be favored because ...

Elizabeth Kaplan 3I

The following plot shows how the partial pressures of reactant and products vary with time for the decomposition of compound A into compounds B and C. All three compounds are gases. Use this plot to do the following: (a) Write a balanced chemical equation for the reaction. (b) Calculate the equilibr...

Elizabeth Kaplan 3I

In an experiment, 0.020 mol NO2 was introduced into a flask of volume 1.00L and the reaction 2NO2(g)--> --< N2O4(g) was allowed to come to equilibrium at 298K. (a) Using information in Table 5G.2, calculate the equilibrium concentrations of the two gases. (b) The volume of the flask is reduced to ha...

Elizabeth Kaplan 3I

What is the difference between Kc/Kp versus just K? In the textbook, they have a table with different values for K and Kc, but I thought they are the same thing. If someone could clarify this that would be amazing!

Elizabeth Kaplan 3I

Is acidic water hard or soft?
In lecture, Lavelle said that acidic water is hard and basic water is soft, however a few UAs as well as Google all said that acidic water is soft and basic water is hard. Can someone please clarify this?

Elizabeth Kaplan 3I

They are essentially describing the same thing, just emphasizing 2 different factors! Lewis acids/bases focus on electrons, while Bronsted acids/bases focus on protons. Lewis acids accept electrons, while Lewis bases donate electrons. Bronsted acids donate protons, while Bronsted bases accept protons.

Elizabeth Kaplan 3I

Electronegativity is helpful in distinguishing between the strength of acids. The stronger the acid, the greater the electronegativity and electron withdrawing because greater electronegativity allows for the delocalization of electrons and hence stabilizes the conjugate base better.

Elizabeth Kaplan 3I

Acids accept electrons and donate protons, while bases donate electrons and accept protons. Acids can be electron deficient, and bases are usually electron rich (meaning they have a lone pair). There is also a list of strong acids and bases that I would recommend to memorize. Acids will always start...

Elizabeth Kaplan 3I

First, there is a list of strong acids and bases that I would recommend to memorize. I would start by determining the strong acids and strong bases, then the weak acids, and then the weak bases. Acids will always start with H or end in COOH so if the molecule is not on the list of strong acids but s...

Elizabeth Kaplan 3I

Water is amphoteric, meaning that it can act as an acid or a base, depending on the pH of what is surrounded by.

Elizabeth Kaplan 3I

Why wouldn't H20 be bidentate since Oxygen has 2 lone pairs?

Elizabeth Kaplan 3I

Which of the following ligands can be polydentate? If the ligand can be polydentate, give the maximum number of places on the ligand that can bind simultaneously to a single metal center:
(a) HN(CH2CH2NH2)
(b) CO3 2-
(c) H2O
(d) oxalate.

How do you determine which ligands can be polydentate?

Elizabeth Kaplan 3I

The number 3 comes from the fact that Carbon has 4 regions of electron density, 1 orbital from the s subshell and 3 orbitals from the p subshell. Thus, the 3 in sp3 represents the 3 orbitals in the p-subshell.

Elizabeth Kaplan 3I

I believe hybrid orbitals are the result of orbitals that are created when the valence electrons of two atoms/molecules bond or are attracted to each other. They are the average of the orbitals of the valence electrons.

Elizabeth Kaplan 3I

It would be sp3d because the hybridization corresponds to the number of regions of electron density, and since the molecule PBr5 has 5 regions of electron density, it must have an sp3d hybridization.

Elizabeth Kaplan 3I

I think sp3 was so common because it means that the central atom has 4 regions of electron density, and tetrahedral is a common arrangement of electron density. The fact that sp3 was the hybridization for the majority of the Sapling homework may just be a coincidence though.

Elizabeth Kaplan 3I

Delocalized pi bonds are when electrons are free to move around in the molecule. Thus, delocalized pi bonds occur when the Lewis structure has resonance and has double or triple bonds. The delocalized electrons in the double or triple bonds result in a delocalized pi bond.

Elizabeth Kaplan 3I

I think molecules have a bent geometry if there are 3 regions of electron density, and 2 are bonding pairs and 1 is a lone pair. NO2 has 2 bonding pairs (2 between N and O) and one lone pair on the Nitrogen.

Elizabeth Kaplan 3I

I don't think that you can assume a molecule is nonpolar if it is symmetrical because for example, in the case of CH2Cl2, the molecule is symmetrical (tetrahedral), but the dipole moments between C and Cl are much greater than the dipole moments between C and H so the dipole moments don't cancel out...

Elizabeth Kaplan 3I

CH2Cl2 is polar because Cl is way more electronegative than H so the dipole moments between C and Cl are much greater than the dipole moments between C and H. This causes the dipole moments to not cancel out, resulting in a polar molecule.

Elizabeth Kaplan 3I

Sigma bonds are stronger than pi bonds because the orbitals overlap in a sigma bond, while in a pi bond, the 2 orbitals line up but do not overlap. The regions of electron density are not along the internuclear axis in a pi bond, which is why it is weaker than a sigma bond.

Elizabeth Kaplan 3I

Sigma bonds allow bound atoms to rotate, thus the resulting sigma bond has electron density with cylindrical symmetry around the internuclear axis. On the other hand, pi bonds are between 2 orbitals, each with one electron, which are overlapped side by side, and thus the 2 regions of electron densit...

Elizabeth Kaplan 3I

Thank you so much!!

Elizabeth Kaplan 3I

I thought Si has only 2 double bonds, one with each O, which would give it a formal charge of 0. Why does it have a lone pair?

Elizabeth Kaplan 3I

Why is SiO2 polar? Don't the dipole moments cancel out?

Elizabeth Kaplan 3I

Why does CHI3 have a higher melting point than CHF3?

Elizabeth Kaplan 3I

Why does NaCl have a higher melting point than HCl?

Elizabeth Kaplan 3I

Thank you so much that really helps! How do you know though that AlCl3 is a covalent, not ionic, bond?

Elizabeth Kaplan 3I

Why is KCl more soluble in water than AlCl3? I thought that since K has a greater polarizing power than Al, it will cause more covalent character between itself and Cl, meaning that it is less soluble in water. Any help would be greatly appreciated!

Elizabeth Kaplan 3I

Hi! I'm a little confused about how to determine on which atom the radical goes for Lewis Structures. For example, when drawing the Lewis structure for ClO, the radical goes on the Cl atom, not the O atom, and I was wondering why this is. If someone could explain this to me that would be great!

Elizabeth Kaplan 3I

Hi! This is because the electronegativity difference needs to be large enough for the hydrogen to become partially positive, and thus attract another N, O, or F atoms from another molecule.
Hope this helps!

Elizabeth Kaplan 3I

Hi! I don't think it is always this way, I am pretty sure this is just a coincidence. However, what is always true is that a double bond is shorter than a single bond. Because the double bond is shorter, it is stronger and requires more energy to break the atoms apart.

Elizabeth Kaplan 3I

Hi! Lewis acids accept electrons in chemical reactions, while Lewis bases donate electrons. Some helpful tips are that metal cations (such as Co3+), molecules with electron deficient central atoms (such as BeCl2), and molecules with polar double bonds (such as SO3) accept electrons. Atoms/molecules ...

Elizabeth Kaplan 3I

Hi! I think I remember one of the UA's saying that technically they are all resonance structures, and in Chem 14C we will treat them as such, but for now, only the most stable structure should be the final Lewis structure.
Hope this helps!

Elizabeth Kaplan 3I

I am very confused on how to determine the central atom in a Lewis structure. I know we were taught that the most electronegative atoms goes in the center, however sometimes this does not happen. For example in part B of 2B.1 of the textbook problems, the molecule is COCl2 and the answer key puts C ...

Elizabeth Kaplan 3I

The interactions had negative energies because they are all favorable and result in a more stable state than when the molecules were separate. If the energies were positive, it would mean that energy is required for the interaction to take place and the molecules are more stable when they are separa...

Elizabeth Kaplan 3I

Electron affinity is a measurement of how likely an atom is to gain another electron. For example, electron affinity is very high for Group 17 elements, such as F and Cl, because they only need one more electron to complete their octet, so they are very likely to gain another electron. On the other ...

Elizabeth Kaplan 3I

Double bonds are shorter than single bonds because double bonds are stronger and have 2 pairs of electrons that pull the positive nucleus closer together and result in a shorter bond length. On the other hand, single bonds only share 1 pair of electrons that do not pull the positive nucleus as close...

Elizabeth Kaplan 3I

Hi! We know that oxygen is delta negative because it has a higher electron affinity and electronegativity than hydrogen, meaning that it will attract, or pull more electrons to itself. This results in it being delta negative since the oxygen region has more electrons than the hydrogen region.

Elizabeth Kaplan 3I

Hi! The -250kJ/mol is the amount of energy that is released when the two ions interact with each other. Since the value is negative, it means that the attraction is favorable and energy is released, not required, for the attraction to take place.

Elizabeth Kaplan 3I

Complete the electron configuration for P.

The correct answer is [Ne] 3s^2 3p^3, but is it still correct to specify 3px^1 3py^1?

Elizabeth Kaplan 3I

How many electrons in an atom could have these sets of quantum numbers?

n=2

n=5, l=3

n=7, l=3, ml=-1

How do you determine this?

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