CHEMISTRY COMMUNITY

David Effio 1H

Hey All,

Does anyone know where we are assigned to go for the final? I know the rooms are either Moore 100 or CS76. My last name starts with E but I don't know where I am assigned, and I can't find any announcement that states where we are assigned.

David Effio 1H

The cathode is always the site of reduction, hence electrons flow towards it, and the anode is always the site of oxidation, hence electrons flow away from it. The only time that they flow the opposite way is when an electrical current causes the non-favorable process to occur, driving electrons the...

David Effio 1H

For galvanic cells, yes only a positive voltage or 0 is possible. Only electrolytic cells can have negative voltages.

The cathode should ideally have a positive E˚ and the anode should have a negative E˚, which should result in an E˚cell equal to a positive number or 0.

David Effio 1H

I believe that everyone gets different numbers, but I'll try to explain based on my numbers. In order to get the correct answer, you need to choose the species with the strongest reducing power and strongest oxidation power. In my case, Sn had the highest reducing power and an E of -0.14 V. Pb2+ had...

David Effio 1H

I believe that if no problems for that section are assigned in the textbook for it, we don't really need to know it. Seeing that Lavelle did not really address corrosion in his lectures, so far, we may not need to know it.

David Effio 1H

No, if you look at the equation, the half-life only takes into account the rate constant, which is dependent on temp and activation energy only.

David Effio 1H

Looking at the equation, the instantaneous rate depicts the rate of either reactants or products at a very specific point in time. It is the derivative, and therefore the tangent line to the graph of the reaction. The instantaneous rate of a reactant and product can vary. However, the unique rate is...

David Effio 1H

Yes, the order matters, as on the outside are the material of the cathode and anode, while on the inside, connected by the salt bridge represented by a || are the ions of the two materials, which will be balancing the charges to assure that the reaction doesn't all occur at once and the battery reac...

David Effio 1H

So for this problem, there is a "hidden" component where water will appear in the reaction, creating an electric current that is then passed through a platinum cathode and anode. The two half-reactions would look like: 2H2O (l) --> O2(g) + 4H+ (aq) + 4e- anode (oxidation) O2 (g) + 2 H2O (l...

David Effio 1H

6N goes over the Nernst equation, which Lavelle announced won't be in Test 2. My best guess is that 6M is the cutoff for this week's test.

David Effio 1H

To find the value of n, the number of electrons, you should try to find what the half reactions will look like. Make sure the charges, reactants and products are all balanced first. You will then be able to plug in n for the formula.

David Effio 1H

This is because changes in the volume of the system affect a gas more than a liquid or solid. Most of the time we will be working with gases since they are capable of doing work by expansion. However, when adding energy to a liquid or solid, they will probably not expand much, if at all (assuming th...

David Effio 1H

Dr. Lavelle mentioned in class that having the correct sig figs will account for 2 points out of a 120 pt test. Honestly, do not worry too much but definitely try to get the hang of using them correctly, as in future classes they may not be as lenient.

David Effio 1H

Honestly surprised that Kelvin and Celsius would fight considering that an increase of 1C is 1K. I would at least expect a temporary alliance to defeat the evil Fahrenheit.

David Effio 1H

When work is done by a system, work is negative. This is because work is essentially in energy units (J or kJ), and work is how a system loses energy. Therefore, if a system does work, the system is losing that energy it had. When work is done on a system, that energy released is absorbed by the sys...

David Effio 1H

So the Trans would have 3 orientations because it can either be on the y-axis in the configuration shown on the image or across from each other on the "x-plane". That is, a molecule on the left that sticks out and the one on the right inside the page, or the molecule on the right that stic...

David Effio 1H

Enthalpy change of combustion and enthalpy change of the reaction are not the same thing. To solve the change in enthalpy of the reaction, you have to use the enthalpy change of combustion. In this case this would be: DeltaH rxn = DeltaH of C2H6 - (DeltaH of C2H2 + 2 * DeltaH of H2) = -1560 - (-1300...

David Effio 1H

Let me try to set up a situation for you: Entropy is essentially a measure of disorder. With that logic: Say we transfer 1000 J of energy to a system. This system is currently sitting at 283.15 K (10 C). Here the entropy would be 3.53 J/K, a high entropy. Then let's say we add 1000J of energy to a s...

David Effio 1H

I would assume this is correct. Since the problem does not really mention any temperature change so trying to calculate for change in temperature using the heat capacity would get you 0.

David Effio 1H

You are correct. Following the announcement we should only be responsible for anything from day 1 up until today, 2/6/19, which covered entropy. I am almost certain that Dr. Lavelle will go over some problems on Friday, so anything NEW we learn after entropy will most likely be reserved for Test 2. ...

David Effio 1H

In terms of entropy, if the DeltaS value is positive, the reaction is spontaneous, while if the DeltaS value is negative, the reaction is non-spontaneous. This is a good way to remember this. Another way is when measuring the change in volume, if V1 is greater than V2, then the reaction is non-spont...

David Effio 1H

An integral is a very complicated thing to explain (especially in writing), but here is a basic breakdown of why it is useful: In calculus, an integral allows us to determine the area under a graph using two reference points (the number at the bottom of the S shape, and the number at the top). This ...

David Effio 1H

Mathematically, constant pressure deltaH would be represented as: deltaU = qP + w w = -P * deltaV, under constant pressure. Since deltaH = deltaU + (P * deltaV) deltaH = (qP - P * deltaV) + (P * deltaV) deltaH = qP Similarly, the heat content at constant volume, qV, will be the same as deltaH, becau...

David Effio 1H

As far as I know, the coolant in a refrigeration coil does not absorb or release heat or matter. It simply changes its internal temperature to do all the necessary cooling via changes in the pressure within the coil. I found this link with the inner workings of a refrigerator which I hope does a bet...

David Effio 1H

Calculating the pH and pOH from their concentrations is the easiest way to tell if an acid or base is strong or weak. pH of around 1 or lower is usually a strong acid, while 13 or higher is a strong base. Memorizing the list of the most common strong acids is a great plan too. It's also worth noting...

David Effio 1H

I believe if units are given in a problem, use the units given. In any case that you must convert from concentration to Ppressure using the ideal gas law, I would stick to using atmospheres, since Lavelle since to prefer using that.

David Effio 1H

I'm a little confused as to what it means that enthalpy is a state function. I understand that we can add and subtract them freely, but I do not understand how that is related to it being a state function.

David Effio 1H

Lavelle mentioned superacids, which have negative pH. This may be the case, but a better explanation is that a pH of 0 is equal to a 1 M concentration of H3O+ ions (10^0 = 1), so anything with molarity higher than 1 will be calculated to have a negative pH. A pH of 0.176, goes with an H3O+ concentra...

David Effio 1H

So one dead giveaway of a wake acid or base is the presence of acidity or basicity constant (Ka or Kb respectively). Only a weak acid/base would have one of these. A chemical reaction involving a weak acid/base would have a double arrow, as some of the reactants would form back from the product thro...

David Effio 1H

This is an extremely rare case where Q=1, meaning there are exactly as many products as there are reactants. If no K value is given in the problem, try checking the charts in section 5G, which give the equilibrium constants at various temperatures. I don't think the problem would ask you to guess th...

David Effio 1H

Technically, this is correct, because the H3O+ concentration is much higher than what it would normally be. This would probably mean this is a very strong acid (or a superacid) that can dissociate completely. Therefore, the OH- concentration is almost nothing. It's possible that the answer on the ba...

David Effio 1H

I believe you would use 0.22 as the Ppressure of HCl because it was present at the start. As it is the only reactant present, the pressure present in the container has to come from the HCl, as there is nothing else within the container. (Side note: You would assume nothing else is present in the con...

David Effio 1H

It usually depends on what operation you are performing (addition/subtraction or multiplication/division.) I'd say for pH, if the given numbers are molar concentrations or liters of solution, use the least possible numbers after the decimal. I know for 6B.5 on edition 7, which asks to calculate pH/p...

David Effio 1H

I want to say that this could have been an error in the answer, as the problem does not allude to unit conversion, so you should assume that you have to use the given numbers. With the given Ppressure in kPa, Q would be less than K, meaning NH3 would be formed and the forward reaction is favored, bu...

David Effio 1H

Yes, any molecule in gas or aqueous form should be included in the equilibrium constant.

David Effio 1H

Personally, I balanced the equation to get in the habit in case Lavelle gives us an unbalanced equation and doesn't ask us to balance it before finding an equilibrium constant. I heard from other students that the chemistry textbooks normally have some mistakes, so I want to say this is probably one...

David Effio 1H

So I noticed that on part B of problem 5G.3 on page 408, the equation has 10 Nitrogen on one side, and 14 on the other. I noticed the rest of the equation is balanced, and that even the answer on the back shows the partial pressure to a power of 7. Is this a mistake with the textbook?

David Effio 1H

First start with the formula for K: K = P product / P reactant In this case, the product is 2X, and the reactant is X 2 , which you can get from the balanced equation X 2 <=> 2X Now when taking into account the initial pressure given, 0.10 bar, this pressure accounts for the 11 original X 2 molecule...

CHEMISTRY COMMUNITY

Search found 38 matches

Test Location 14B W19

Re: Flow of electrons

Re: Positive Voltage

Re: Test 2 #6 part c

Re: Corrosion

Re: 1/2 life

Re: Difference between instantaneous and unique rate

Re: Cell Diagrams

Re: 6L.3 part d

Re: Topic 6N

Re: Problem 6L.1

Re: Whether PV is significant

Re: Sig Figs

Re: temperature joke - very funny

Re: Work done vs work on system

Re: Question on 7th edition 4G. 5

Re: practice midterm question

Re: Entropy as Temperature Increases

Re: 4A3 7th Edition

Re: Chem 14B Midterm

Re: Determining Spontaneity of a Reaction

Re: Calculus in Chem

Re: Enthalpy & Constant Pressure and Volume

Re: Problem 8.1 (c) 6th edition

Re: OH and H3O

Re: units for partial pressure of gases

State Function?

Re: Help on Question 6B.9 (7th edition)

Re: difficulties recognizing weak acids and bases

Re: 5I.7 7th edition

Re: 7th Edition Question 6B.9

Re: 7th Edition HW Problem 5I.29

Re: 12.29

Re: Question about self-test 5I.2A

Re: H2O

Re: 7th Edition - 5G.3 Mistake?

7th Edition - 5G.3 Mistake?

Re: Number 5G.5 question