Energy revisitation.
periodic trends in oxides, halide, hydrides
review Ch 5, 7, 21
Nov 4
25. Wed. Nov. 4, 2009
Chem. 111 with Prof. Knapp
MWF 1:25 – 2:15 in ISB 135
Energy revisitation.
periodic trends in oxides, halide, hydrides
review Ch 5, 7, 21
Nov 4
question sort of related to class: does anyone know if we have the class before thanksgiving off?
Debra –
An important thing to remember about an enthalpy is that when you have to find the enthalpy (delta H) of a chemical reaction (lets say H2O(l) + 3 O2 -> HCN + 2 H2O) you always get your answer by the sum of PRODUCTS minus the REACTANTS, the numbers you use to find this sound be given to you in the form of delta H of f (and don’t forget to multiply the coefficients before the compounds or elements!) .
Debra, Enthralpy is the change in heat for reactions. To find the change in heat you take the molar enthrapies of each substance on the product side and add them together (mutliply the substance”s enthrapy by its coffecient, moles) and subtract the reaction side doing the same thing you did to the product side. This should give you the change in heat.
@ Laura Rossi
the ic pgs is basically the names of the scientists and the contribution to science
Can anyone tell me how to assign the oxidation numbers of HClO4 ?
Can anyone explain what to me what enthalpy is, or give me a hint on how to solve those problems?
Sorry, I made a mistake, the v isn’t really a v, it’s a greek letter, and it represents frequency!
Laura Kozma
Brendan:
I’m not sure which equation you’re looking at, but Planck’s equation doesn’t have an “a looking symbol”. Planck’s equation is E=hv, where h is the constant 6.626X 10^-34 J sec/photon, and v is velocity. This equation can be used to calculate energies.
Laura Kozma
Justin:
Take this reaction as an example
2 Ag+(aq) + Cu(s) -> 2 Ag(s) + Cu2+(aq)
In this reaction, Ag+ ions gain electrons from Cu and are reduced to Ag. Ag+ is the oxiding agent. [Ag+(aq) + e- -> Ag(s)] Cu loses electrons to Ag+ and is oxidized to Cu2+. [Cu(s) -> Cu2+(aq) + 2e-]
Here’s a helpful mnemonic I was taught in AP Chem in high school:
LEO the lion says GER
If a reactant Loses Electrons it’s Oxidized
If a reactant Gains Electrons it’s Reduced
Remember that the reactant that is oxidized is the Reducing agent and the reactant that is reduced is the Oxidizing agent.
So, if the product has less electrons than the reactant, the reactant lost electrons, becoming more positive, and oxidized, it is therefore the reducing agent. If the product has more electrons than the reactant, the reactant gained electrons, becoming more negative, and reduced, it is therefore the oxidizing agent.
I hope that helps!
Laura Kozma
Brendan I think you are referring to the symbol lambda which kind of looks like an upper case A. That stands for the wavelength
Can anyone explain to me the difference between oxidizing agent and reducing agent? In a reactant, if the outcome (product) loses more electrons than the reactant, is that reducing agent? and if the outcome (product) gains more electrons than the reactant, is that a oxidizing agent? I am getting these two agents mixed up.
Thank You!
What does the a looking symbol in Plancks equation represent?
@Derek
Oxidation/reduction is all about transfer of electrons. When something is oxidized, it loses electrons, and when something is reduced it gains electrons. The species that is oxidized is the reducing agent, and the species that is reduced is the oxidizing agent. As to your question about size, an atom that is reduced would actually be bigger because it gains electrons, so there would be more electron-electron repulsions. I think redox is more related to charge than oxidation #’s.
Also, Professor, will there be a bomb calorimetry problem on the exam? If so, could you provide some sort of another example problem for that just to clarify what type of problem and lvl of diffclty we’ll see; thanks!
I have forgot since some old OWLs how to figure out the species oxidized/reduced. I know spec.ox.=reducing agent and spec.red.=oxidizing agent.. I though that if something is reduced its ox.# goes down or something like that, but I’m a bit fuzzy; is it also that reduced means to gain an electron so its sized is reduced..but is this related to ox#s?? Thanks.
im confused with the IC chapter? what is important for the test from those pages?
The radius sizes are in this order form smallest to largest: Mg^2+ < Na^+ < Ne < F^- < O^2-. The radius sizes depend on the number of protons, the more protons in an ion, the greater the attractive force is on the electrons, creating a smaller radius.
also, which of these isoelectric ions has the biggest radius?: Mg^2+, Na^+, Ne, F^-, or O^2-?? m a little ocnfused because they all have the same number of electrons…
thanks Ian Doelling
Zachary,
The sign of the enthalpy, or delta H, is going to be negative.
Ian Doelling
Does anybody know what the sign of the enthalpy is when the reactant is more stable than the product in a reaction?
Adam,
The test is Friday in class. Be prepared it should be a tough one.
Joe D’Onofrio
When is the test?
Sorry I made a few grammar errors in that last post i actually meant to say that “It is important to know that up to chromium the configurations are normal and then from elements Cr until Zn are “weird” ….”
Well Hannah in the book on page 310 there are a list of electron configurations. I assume the problem is with the transition metals in the first row (#20-30). It is important to know that up to chromium the configurations are normal then from elements are “weird.” Cr until Zn this is partially because these elements are more willing to give one or more electrons up because they are usually expressed in terms of +3 or +2. The reason for the odd configurations are that it is easier to remove an electron from the lower energy subshell. This is partially explained by the Aufbau Principle.
Can someone explain about filling in the d orbitals?
A few people mentioned enthalpy and i hope i can give a little help:
1.) a few seem to be confused with heat (q) and enthalpy (h). These two values are technically defined differently but, AS LONG AS NO WORK IS DONE (w=0), then enthalpy and heat can be used interchangeably (q=h). We have yet to do any examples where work is done, but this occurs when pressures and volumes are not constant and the expanding or contracting system does work on the surroundings (in the form of pushing or pulling “stuff” around).
2.) As far as getting answers out and not knowing what they mean:
remember that q, h, and E are with respect to the system. This means that a positive q, h, or E means that the system has gained energy, while a negative means the system has lost energy. It’s the same as balancing a checkbook, keeping track of what goes in and out. If you add 10j, take away 5, then add 3 more you have a final total of 8j. the final total means that you added 8 joules of energy (the total is positive so the system has gained this value).
hope that helped!
Can you please go over more on Hess’s Law and whatever was on ch.21.
Are you still dropping one of the questions? And could you please go over what the question with look like for Enthalpy, planks eqn, and n l ml ms.
Christina-
just remember that an exothermic reaction releases heat. So the enthalpy change for such a reaction will be negative!
Topics we need clarification on please
Enthalpy
Calorimetry
phase change
First laws of thermodynamics
Hess’s law
Enthalpy formation
Nicholas,
In one of the demos on Friday, we saw the various concentrations of peroxide react with a catalyst. The second reaction clearly gave off steam (water) and he said that when he put his hand over it, it was warm. That was an exothermic reaction because it released the heat. If you saw the equation written out for that reaction, it would be +430kJ (I just made that number up as an example). So, that was my best attempt to try to make enthalpy relate to real situations. Hope that helped a little.
Professor Knapp,
I hope you see this before class tomorrow.
Could you please go over the idea of enthalpy and what it stands for without equations? I tried re-reading that section in the book but it just switches from heat to enthalpy without a satisfactory explanation. Also I was wondering if you could [lease explain again why the Bohr model only works for Hydrogen?
Thanks,
Melissa Cruz
Professor,
I am also slightly confused when it comes to enthalpy. I can also do the equations, but i don’t really understand how I got the end product.
Professor,
Would you mind touching on Enthalpy and Thermodynamics tomorrow? Aimee’s explanation above helped a little, but I am still slightly confused.
Thanks,
Nicholas Katz,
Enthalpy is the change of heat when a process is carried out under constant pressure (an isobaric process).
So in the equation for enthalpy it factors in the sum of the energy transferred as HEAT between a system and its surroundings and the energy transferred as WORK between a system and its surroundings (U in the equation).
It also takes into account work, which is occurs when the volume changes under constant external pressure.
So enthalpy is the change in heat that occurs during a chemical reaction.
I hope that helped you.
A combustion reaction uses O2 as the oxidizing agent. In other words, air.
Hi Prof. Knapp,
I have written down that a combustion is always a redox reaction. How you can tell if a combustion reaction took place? Or can anyone else tell me?
Thanks, Alissa M.
Prof. Knapp could you please go over Hess’s Law just a little bit. I still feel a bit fuzzy on that topic. Thank you.
-Danielle
Prof. Knapp,
Will the upcoming exam focus a lot on oxidation or more towards the more recent things we have been doing?
Prof. Knapp,
How extensively do we need to know the demonstrations from Friday’s lecture?
Prof. Knapp can you please go over the demos and their significance during class, I think that would benefit everyone. Thanks.- Mike
Can somebody please explain what enthralpy actually is without using equations. I can do the calculations but I dont really understand the concept.