CHEMISTRY COMMUNITY

Diane Bui 2J

I think that you can use either one because both serve the same purpose.

Diane Bui 2J

It is false because we do not know the value of delta S. Endothermic means that delta H is positive. According to the gibbs free energy equation (delta G = delta H - TdeltaS), an endothermic reaction would be favorable at a high temp if delta S was positive as well. If delta S was negative and delta...

Diane Bui 2J

I recommend book and old test problems! Gives you the best practice without cramming too much into your brain last minute

Diane Bui 2J

I agree with the answer above, and also both salt bridges and porous disks serve the same function: allowing counterions to flow to prevent charge build up. Hence, I think it shouldn't matter which one you use unless they specify.

Diane Bui 2J

Also note that elements with same phases are separated by a comma and elements with different phases are separated by a single line. Furthermore, the salt bridge is indicated by a double line in the middle while a porous disk is written with a single line in the middle.

Diane Bui 2J

Enzymes affect the rate of reaction because they create a different reaction pathway with a lower activation energy, allowing the reaction to speed up.

Diane Bui 2J

The slow step determines the rate of the reaction because the reaction proceeds only as quickly as the slowest step. This concept is similar to the function of a limiting reactant. The reaction can only make as much product as the limiting reactant allows it to, regardless of how much there is of an...

Diane Bui 2J

Q: What did the chemist snack on during lunch?
A: A 'gram' cracker.

Diane Bui 2J

Q: Why did the bear dissolve in water?
A: Because it was polar.

Diane Bui 2J

Q: Why do chemistry students do well when working with ammonia?
A: Because it's pretty basic stuff.

Diane Bui 2J

Q: Did you hear about the man who got cooled to absolute zero?
A: He's 0K now.

Diane Bui 2J

Q: When one physicist asks another, "What's new?" what's the typical response?
A: C over lambda.

Diane Bui 2J

Adding onto this, we also see how activation energy is related to the rate of a reaction through the function of enzymes. Enzymes affect the rate of the reaction by creating a new pathway with a smaller activation energy, and hence, the reaction proceeds faster because less energy is required for mo...

Diane Bui 2J

This problem was also unclear to me, but I estimated the average between the two numbers and used that value to compare which standard potentials were greater.

Diane Bui 2J

The oxidation reaction that occurs at the anode is written to the left of the salt bridge, and the reduction reaction that occurs at the cathode is written to the right of the salt bridge.

Diane Bui 2J

Platinum metal is often used as an inert electrode because platinum is one of the least reactive elements. Platinum has a large ionization energy, so it can act as an "electron shuttle" without participation in redox chemistry.

Diane Bui 2J

I'm sorry if this may seem like a basic question, but I just wanted to make sure if there was a difference between a reaction being energetically favorable and spontaneous?? I've heard a TA say that a reaction is able to be energetically favorable but not spontaneous, so I was a bit confused.

Diane Bui 2J

By definition, bond enthalpy is the energy required to break a bond. Bond breaking is an endothermic reaction while bond forming is an exothermic reaction. Hence, the bond enthalpy values are positive for breaking bonds and negative for forming bonds.

Diane Bui 2J

I've had different answers from different TA's about this, but does anyone know what the signs are for qsys and qsurr for questions like #7 on the test? I thought that since the ice is gaining heat and the water is losing heat, the equation should be -qice = +qtea. I got the right answer doing this ...

Diane Bui 2J

Yup! To add on to the answer above, heavier molecules have more vibrational energy states, which means higher degeneracy, and hence, higher entropy. In this case, lead is heavier than carbon and thus has a higher entropy.
More complex molecules also tend to be heavier and have higher entropies.

Diane Bui 2J

In this question, you would still perform Hess's Law as usual but the enthalpy of the reaction obtained is for the reaction as written. In this case, the enthalpy of reaction is for 3 moles of C (gr) reacted. To find the enthalpy change for 4 moles of C (gr), you would divide by 3 moles and then mul...

Diane Bui 2J

To add on to the answer above, pressure is inversely proportional to volume according to Boyle's Law, so the smaller the pressure, the greater the volume. Since 1 bar is less than 3 bar, the volume is greater, and hence, entropy increases.

Diane Bui 2J

That equation is used to derive a variation of the equation used to find the entropy change that occurs when a system is heated from T1 to T2. S(T) = S(0) + deltaS(heating from 0 to T) = deltaS(heating from 0 to T) -> This is just stating that the entropy of a system at a temperature T is equal to t...

Diane Bui 2J

Just to add on, like Dr. Lavelle mentioned today in class, we should be careful with which method we are using and the signs of our standard reduction potentials.

Diane Bui 2J

There will be equations on the equation sheet, so focusing on how the equations are used and what they represent is more important, but it'd be helpful to know the equations as well.

Diane Bui 2J

It also depends on whether you are referring to the system or surroundings.

Diane Bui 2J

I also wrote that the internal energy of an isolated system remains constant.

Diane Bui 2J

Hi Timothy,
It's kind of like saying you paid $20 for 5 apples and trying to find the price of one apple. Since we are given the heat value for 0.04998 mol CO, we divide by that value to get the heat per mol CO.

Diane Bui 2J

A spontaneous process means that it has the tendency to occur naturally without an outside force acting on it.

Diane Bui 2J

Adding on above, I think it's useful to draw a diagram of the heating curve to understand what happens first. In this case, the ice melts to liquid water first and then the temperature is raised. Therefore, the specific heat capacity of liquid water instead of solid water.

Diane Bui 2J

Just to ask further about this question, what type of system is a calorimeter?

Diane Bui 2J

Hi RenuChepuru1L, The heat of the reaction is equal to the negative heat of the calorimeter because the calorimeter is gaining heat from the reaction. Hence to calculate heat gained by the calorimeter, we use C(delta T). The question asks for the internal energy change for the reaction of 1.00 mol C...

Diane Bui 2J

In this problem, the standard enthalpy of combustion for C2H2, C2H6, and H2 is given. Hence, the combustion equations for these reactions should be determined: Balanced combustion equation for 1 mole of C2H2: C2H2 + 2.5O2 --> 2CO2 + H2O reaction enthalpy: -1300 kJ Balanced combustion equation for 1 ...

Diane Bui 2J

I believe the question you are referring to is Problem 8.43. D is not the right answer choice because during a phase change, there is no change in temperature, yet in graph D, during melting and vaporization, there is a positive slope to the line, indicating that there is a temperature change when t...

Diane Bui 2J

In this problem, enthalpy density is defined as enthalpy released per liter. So, using the standard enthalpy of formations of the reactants and products, you find the enthalpy of the reaction by subtracting the enthalpy of the reactants from the enthalpy of the products. You would then divide that n...

Diane Bui 2J

The two equations refer to the specific heat capacity and molar heat capacity. Because heat capacity is an extensive property and is therefore not useful in calculations, it's common to use the specific heat capacity (Csp) or the molar heat capacity (Cm). Specific heat capacity is obtained by dividi...

Diane Bui 2J

Actually, we want to use intensive properties if possible because they are much more useful. Extensive properties depend on the amount of substance, while intensive properties do not depend on the amount of substance.

Diane Bui 2J

A state function depends only on the final and initial state of the system and disregards the path taken to bring about change to the system. Heat is not a state function because energy transferred as heat is path-dependent. In other words, the value of heat depends on the specific path taken betwee...

Diane Bui 2J

Also to add on, I worked on this question with a TA and we started off with the general interpretation that -q(copper)=q(water) because heat is lost by the metal and gained by the water. We also established that the Tfinal for both copper and water would be the same.

Diane Bui 2J

This is because the negative charge on the conjugate base is stabilized by the electron withdrawing oxygen atoms. The more oxygen atoms there are, the more stable the conjugate base is, and the greater stability of the conjugate base is an indication of the strength of the acid.

Diane Bui 2J

I agree with what people have said above here. Resonance spreads multiple-bond character over a molecule, which results in a lower energy. Resonance indicates a blending of the structures, and it does so by delocalization of electrons, in which a shared electron pair is distributed over all atoms. R...

Diane Bui 2J

You can also essentially use the ICE table in any acid base equilibrium problem to check your work. For example, when given the equilibrium concentrations of a product and asked to find Kc, you can use ICE to show how you found your Kc when you really don't need to. You can just use the molar ratios...

Diane Bui 2J

In the example you gave, NaOH is an ionic compound and dissociates when it is in water because of hydrogen's and oxygen's partial charges that attract the ions in NaOH, causing it to dissociate into two separate ions. The hydrogen atom has a slightly positive charge, and the oxygen atom has a slight...

Diane Bui 2J

I think it's definitely helpful to memorize at least the common ligands and their charges because, like stated above, it is used to find the oxidation number of the transition metal, and it's also beneficial to work through the problem faster. I'm sure we will be seeing these ligands again in Chem 1...

Diane Bui 2J

I think it is only up to 4.7!

Diane Bui 2J

If I'm understanding this correctly, even though different molecules have the same AX2, AX3, AX4, etc structure, it is due to the lone pairs that create different bond angles due to their repulsion.

Diane Bui 2J

Personally, I think it is easier if you convert all units to SI units before you start calculations so that you don't have to worry about converting at the end. You might even forget to convert at the end, so it's always best to make sure all are in SI units before crunching numbers.

Diane Bui 2J

Just to add on, HOCO is a radical because if you count up the total number of valence electrons, it is an odd number of electrons, meaning that there is one lone pair of electrons in the molecule and in this case, it would go on the Carbon. On the midterm, I personally played around with the electro...

Diane Bui 2J

I also find it easy to remember that Flourine is the most electronegative atom, so as you get closer to Flourine, the atom becomes more electronegative.

Diane Bui 2J

Also, if I'm not mistaken, the test did ask for an ion that was isoelectronic so choosing a parent atom like "Ne" would've been incorrect.

Diane Bui 2J

I'm hoping this is right, but from what I would do is to count up the total number of electrons in the molecule, which seems to me that there are 32 electrons. 21 electrons are from Chlorine due to its 7 valence electrons, and there are 3 Chlorines in the molecule. 32-21 leaves 11 electrons left ove...

Diane Bui 2J

Adding on, the elements in the third row (and above) of the periodic table are able to hold more than an octet as a central atom in a Lewis Structure due to their available d (and f) orbitals.

Diane Bui 2J

Dr. Lavelle showed how moving from the structure with single bonds to the structure with double bonds resulted in lower formal charges for 2 of the oxygen atoms, going from -1 to 0. The structure with the lower formal charge indicates more stability and is therefore the best representation of the mo...

Diane Bui 2J

Also, just to add onto everyone else's responses, ionization energy is the minimum energy needed to remove an electron in its gas phase. This is different from the threshold energy that we learned about in the photoelectric effect, which is the energy needed to remove an electron from a metal. Secon...

Diane Bui 2J

Dr. Lavelle also explained how electrons with the same spin in the same subshell will occupy different orbitals due to electron repulsion. The electrons are more stable if they are in different orbitals.

Diane Bui 2J

Would anyone care to further clarify the difference between a shell, subshell, and orbital, please? I am a little confused on which is which. Sometimes I will hear s subshell and sometimes I hear s orbital.

Diane Bui 2J

Based off of what I remembered from lecture and my notes, the photoelectric effect was an experiment that was testing the threshold energy of a metal. Light was shone on a metal surface to measure the energy needed to remove electrons from the metal. What was unexpected was that electrons were not e...

Diane Bui 2J

There is a 1 to 1 interaction between the photon and the electron! So 1 million electrons would be excited.

Diane Bui 2J

Is De Broglie's equation essentially the same as the original wave equation (wavelength times frequency = speed of light)?

Diane Bui 2J

The problem will most likely state which elements are involved in the combustion, but combustion usually occurs when a hydrocarbon reacts in the presence of oxygen to produce carbon dioxide and water. Hence, the elements of C,H, and O are asked in the problem. Combustion/burning/oxidation of organic...

Diane Bui 2J

Don't forget that 0's before the number are not significant and 0's in between and after the numbers are! For example, 0.0042 has only 2 sig figs, 403 has 3 sig figs, and 50.0 has 3 sig figs.

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