CHEMISTRY COMMUNITY

Sarah_Stay_1D

Why are reducing and oxidizing agents the opposites of reduction and oxidation? If something is being a oxidized it is a reducing agent because the process of it losing electrons allows/helps the other substance in the cathode to be reduced. This is true the other way around. If something is being ...

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In example 14.10 it says that to find Q you need to divide left side by the right side concentration. Why isn’t it the other way around - like products over reactants and hence right over left? The left side (anode) is actually the product. In a concentration cell the lower concentration is always ...

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For this HW problem I'm a little confused as to why q=0. The problem doesn't give you any information about heat so is that why we are just assuming q=0?

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The rate constant of the reaction between CO2 and OH- in aqueous solution to give the HCO3- ion is 1.5 x 10^10 L*mol-1*s-1 at 25 Celsius. Determine the rate constant at blood temperature (37 degrees C) given that the activation energy for the reaction is 38 kJ/mol. Could someone please explain step...

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2 A --> B + C, given that [A] is 0.153 mol!L"1 and that after 115 s the concentration of B rises to 0.034 mol!L"1. In this we find At by subtracting 0.068 from Ao. but if B was a reactant and it was increasing would we add it instead of subtracting? The reason that they are subtracting in...

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On the homework problems, I noticed that for first order half life problems, the solution's manual can count the number of half-lives and multiply that number by the half-life for the reaction. However, for second order problems, it seems like that same operation cannot be carried out. Instead, we ...

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Jasmine Botello 2F wrote:Is zero order common or uncommon???

I believe zero-order reactions are fairly common, however they are not as common as first-order reactions.

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Also the time units do not always have to be in seconds. Any time unit works, just make sure that you units are consistent throughout the calculation.

Sarah_Stay_1D

What is pseudo-first order reaction and how is that related to the second-order reaction? You can use a pseudo-first order reaction if you have more than one reactant. You would make the concentration of one of you reactants extremely high, and make the concentration of you second reaction low. Sin...

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For this question, the balanced reaction is 3 Au+ (aq) ---> 2 Au (s) + Au3+ (aq) The cell diagram, though, is not written with Au+ as the reactant on both the anode and cathode side of the diagram. Can someone explain why this is? I thought the cell diagram is written based off of the balanced equa...

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For question 14.17, "Write balanced half-reactions for the redox reaction of an acidified solution of potassium permanganate and iron (II) chloride" ... How do you know which half reactions you are supposed to be using? Potassium permanganate is KMnO4 and iron (II) chloride is FeCl2. Sinc...

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Timothy Kim 1B wrote:For the anode, there is no water. Where did the water come from based on the solution manual?

For the half reaction involving Cr2O7^2- you have to add water in order to balance the oxygens in the reaction.

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Can it be considered standard Gibbs free energy if it is given for a temperature other than 25C? The measured standard Gibbs free energy values in our textbook are all for 25 degrees Celsius. If you calculate the Gibbs free energy for a system not at standard temperature, then you do not have the *...

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So I know that physical states (gas>liquid>solid) affect entropy and molecular size affect entropy (larger means more entropy), but which has more of an effect? As in would a heavy liquid or a light gas have more entropy? A gas will always have a higher entropy than liquid because the difference in...

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The book mentioned that a glass electrode is much easier to use than a hydrogen electrode, however, it didn't really provide an explanation as to why this was the case and so I was a little confused. If someone doesn't mind explaining to me, as well as what the difference between the two is, that w...

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Correct. You will not be required to memorize standard reduction potentials. However, you may be asked to calculate that value using one of the formulas you know.

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In lecture on Monday Lavelle gave the following example problem: Oxygen diflouride is a colorless gas the reacts rapidly with water vapor to produce O2, HF, and heat: OF2 (g) + H2O (g) --> O2 (g) +2HF (g) ∆H = -318 kJ What is the change in internal energy for the reaction of 1.00 moles of OF2 gas at...

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How did they get the equation in 89 and how did they get the partial pressures out of 100? Im confused reading the solution manual for this problem. They get the equation from the explanation of the problem. The problem states that compound A decomposed into compounds B and C. Therefore, A would be...

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So for this one, can someone explain why we are treating one (either hot or cold water) as system and the other as surroundings? Because the heat transfer is happening only at those two? Note that the problem states that this reaction is occurring in a thermally insulated vessel. This means that ne...

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The directions are to balance the redox reaction in basic solution. Part a writes: Action of ozone on bromide ions, O3 (aq) + Br - (aq) --> O2 (g) + BrO3 - (aq). Why does the solutions manual write the first half-reaction as: O3 (g) --> O2 (g)? Don't both O3 and O2 have oxidation numbers of 0? The ...

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If SO2F2 adopts a positionally disordered arrangement in its crystal form, what might its residual molar entropy be? Why do we raise the number of microstates to Avogadro's number? How do we know it's talking about one mole of SO2F2? Why isn't it just raised to the power of 1 because it's one molec...

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A cold pack would be a closed system. It can exchange energy with the surroundings in the form of heat, but it can not exchange matter.

Sarah_Stay_1D

Can someone explain why ∆S=q/T=nR ln(V2/V1)? I know that ∆S=klnW2/W1, so does nR equals k then? I cannot recall whether Lavelle went through in lecture how to derive the equation ∆S=nRln(V2/V1), however it is simply another way to calculate the entropy of the system. However this equation allows yo...

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Water gas is an inexpensive, low-grade fuel that can be made from coal. (a) Is the production of water gas exothermic or endothermic? In the reaction given in the problem, the reactants are carbon (s) and water vapor (g) and the products are carbon monoxide (g) and hydrogen (g). Why is the reaction...

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Can anyone explain why a) is work done by the system? Wouldn’t it be on the system since it is absorbing heat? Hence solid to liquid? We know that ΔU is equal to zero in this case because the problem tells us that the reaction is happening at a constant temperature (because it is a phase change). T...

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Why is the net production 2/3 mole of gas? There are 2 moles of reactants and 4 moles of products so how is the change 2/3? Since you have 3 moles of H2 however the question asks you for the energy for the production of 1 mol, you can divide all the coefficients by 3. So you would have 1/3CH4 + 1/3...

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What is exactly is the difference between heat and enthalpy? Enthalpy is the study of heat released or absorbed in chemical reactions and physical changes. Heat is a type of energy which can be transferred from one object to another. Heat transfer is the result of highly energized atoms or molecule...

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Do we need to know the equations for heat capacity in relation to constant pressure and constant volume where Cp=5/2*R and Cv=3/2*R? I don't remember going over these in lecture, but there were included in one of the homework problems so I just wanted to clarify! Thanks so much! When talking to Lav...

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I was just wondering, but are there any patterns or is there an easy way to figure out what the most stable form is for each element? I know that the standard enthalpy of formation of an element in its most stable form is zero, but what if you're not sure what the most stable form is? Thanks so muc...

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For 8.47, why does the solutions manual use the equation (delta)H = U + P(delta)V instead of just U = q+ w? I'm also confused as to why work would be positive instead of negative because I thought that expansion work done ON a system is negative. You are in a sense actually using ΔU= q + w. First y...

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For homework problem #19, you are asked how much heat is needed for a 500.0 g copper kettle with 400.0 g of water inside to raise the temp of the water form 22 degrees C to 100 degrees C. I understand using the q=mC x change in temp, but I'm assuming you have to account for the copper kettle absorb...

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When calculating the change in enthalpy of a rxn by subtracting the standard enthalpy of formation of the reactants from the standard enthalpy of the products, how do you find the enthalpies of formation for individual products/reactants? Is there a way to calculate this or is there a chart somewhe...

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Why can we equate delta H (heat transfer at constant pressure) and delta U (heat transfer at constant volume)? You can not always equate delta H (enthalpy) to delta U (internal energy). The two are only equal when you have constant pressure and constant volume. This is true because at constant pres...

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I attached a picture which I thought might help. In terms of function, the ignition wires start the combustion, and since there is no change in volume all of the energy is released as heat. The heat raises the temperature of the surrounding water and the temperature of the water is measured using th...

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How do we know when the element is in its formation state? In lecture Lavelle said that for common elements such as nitrogen, oxygen, and carbon (elements towards the top right) we would need to memorize their formation state. However, for more unfamiliar elements we would be told their formation s...

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Can someone explain why mercury in a thermometer is an open system (obviously it can transfer mass+energy), but how? I just looked at the solutions manual and it states the mercury in a thermometer is a closed system. The mercury has a fixed amount of matter, but it can still exchange energy with i...

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elliekauffman1I wrote:When naming structures, there are times when you must use different prefixes such as bis. When do we use these instead of the regular ones?

You would use prefixes such as bis and tris for molecules that are bidentate or ploydentate.

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I was wondering how we know if the "x" in the change row of the ICE table should be positive or negative? Will products always be positive and reactants always negative? The reactants are not always negative in an ice table. For example, if you are given values for the products only, you ...

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The answer for the Lewis structure for this question says that there are more resonance structures available, but what would be the best resonance structure? And do different resonance structures affect the structure of the molecule? We usually say the "best" resonance structure is the on...

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Why can units be ignored during equilibrium constant calculations? Can equilibrium constants still be determined for a system with both solutes and gases? The units are not ignored during equilibrium calculations, all the units simply cancel out. Also, yes you can determine the equilibrium constant...

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So, in layman's terms lol, what does bar even mean, and is it significant in any way? bar is just one unit in which you can measure pressure. There are other ways to measure pressure, one example being atmosphere (atm). Just remember that when you are doing calculations involving pressure, all of y...

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In addition to forming sigma and pi types of bonds similar to p-orbitals, d-orbitals may overlap to form delta-bonds. (a) Draw overlap diagrams showing three different ways in which d-orbitals can combine to form bonds. (b) Place the three types of d-d bonds—sigma ,pi , and delta —in order of stron...

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As far as I know there are no cases in which a double bond is composed of two pi bonds. I believe double bonds always have one sigma and one pi bond.

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What type of bond is stronger and what would we use to determine the bond strength? Triple bonds are the strongest, followed by double bonds and then single bonds. When I am trying to determine which molecule has a stronger bond I typically draw a lewis structure. Also note that for molecules with ...

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When we have to find the structure and shape of an ion, where does the extra electron go? Would it have several different shapes like how Lewis dot structures have several resonance structures? I would not think of it as "extra electrons". The additional electrons in an ion are part of th...

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Can anyone explain 3.9 from the homework problems? I am confused on what the question is asking for. Also for the given electron configurations a.) [Ar] 3d7 b.[Ar]3d6 are these possible? Doesn't the 4s orbital need to be filled for to have an electron in 3d? thanks So The confusing part about this ...

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Hi! Can someone explain how they figured out the answer to 3.37? It asks us to identify the element "E" in this Lewis structure, and E is said to be in Period 3. First you just need to count up the total number of electrons in the structure (32 e-). Since there are 3 Cl atoms and we know ...

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If a molecule such as C2H2 has one triple bond between the two Carbons and single bonds with the two Hydrogens. Is the triple bond shorter than the single bonds? I was confused by resonance, but there can only be one possible Lewis structure for C2H2 so there isn't resonance, right? Triple bonds ar...

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Can someone help me with 2.55? It is asking for the valence-shell configuration of a) alkali metals, b) group 15, c) group 5 transition metals, and d) Cu, Ag, Au. I have the solutions manual so I saw the answers but I couldn't figure out how they got them. The type of orbital (s, p, d, f etc.) that...

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I am having a little trouble imagining what exactly happens to an electron when it is excited and moves up an energy level. An electron revolves around the nucleus in somewhat of an unpredictable way so when it is excited where exactly does it go? Does it move farther away from the nucleus? It may ...

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Can someone help me with how to solve this problem? The question reads, "An excited hydrogen atom emits light with a frequency of 1.14 x 1014 Hz to reach the energy level n = 4. In what principle quantum level did the electron begin?" To solve this problem you need to use the formula E=-h...

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Tatiana R Dis 3E wrote:When is it necessary to use this functions again?

The de Broglie equation is used to measure the wavelength of the ejected electron.

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For this class the work function would be given to you. You would only have to solve for the work function if you had all other variables except the work function. Kinetic energy is different from the energy of a photon. The kinetic energy is the excess energy released from the ejected electron. hv ...

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You do not need to know how to derive ay equations. On the test you will be given a formula sheet that will include the de Broglie equation.

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Don't let the unknown compound freak you out! First just write out a balanced reaction - H2A + 2XOH -> 2H2O + X2A. Since you already know the molar mass of both H2A and XOH, you can convert the grams to moles. You should get 0.00714 moles of H2A and .008 moles of 2XOH. Since H2A and 2XOH are in a 1:...

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I had trouble with this problem too. First have to use the grams of P4 (which you will find to be the limiting reactant of the first equation), to find the grams of P4O6 (the product of the first reaction). You use the grams of P4O6, and the grams of excess O2 (from the first reaction) to find the l...

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In a lab setting, chemist typically try to achieve a a yield of 90% of higher. This is primarily because in an actual experiment, there is often more than one reaction, so if the yield of the first reaction is low, the yield will get lower with each consecutive reaction.

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