Tutor profile: Payam Z.
Subject: Organic Chemistry
What of the following is true about Ethers: A) Ethers have sp2 Oxygens B) Ethers can be used as solvents with Grignards C) Can form peroxides and hydroperoxides if exposed to air for a few months D) Have higher boiling point than alcohols becasue they cannot hydrogen bond with themselves E) Two of the above
The correct answer is E. In an ether, sp3 hydrid orbitals of exygen form sigma bonds to both carbons. The C-0-C bond angle is close to 109.5 degrees Although polar, they have weak dipole-dipole interactions thus lowering the boiling point which makes ethers closer to the boiling point of those of hydrocarbons with similar molecular weight.
A 5.0 L flask containing .40 ATM of N2 (g) and .75 ATM of 02(g) at 100 degrees celcius has .10 moles of Ne(g) added. What is the total pressure of the mixture?
First let us get the Ne (g) pressure using the trusty PV = NRT equation Plugging in our known information we get PV = NRT P(5.0) = (.10)(.0821)(373) Pne = .61 ATM Now we use Daltons Law of Partial Pressures equation Ptotal = P(N2) + P(O2) + P(Ne) Ptotal = .40 + .75 + .61 Ptotal = 1.76 ATM If needed I would love to explain the theoretical principles behind PV = NRT and Daltons Law of partial pressures or we can just keep doing example problems until you master all the different ways professors can ask you these questions. I have tons of college-level example problems we can do together or I can work through old exams you have from your school or we can do your homework problems. A lot of chemistry is best taught by doing lots of practice problems and unfortunelty professors spend way too much time on the theory during lecture which leaves their students confused and prone to making easy mistakes come exam time!
Subject: Basic Chemistry
Arrange the following in order of correct boiling point: H2S, H2O, H2Se, H2Te A) H2Te > H2Se > H2S > H2O B) H2Te > H2S > H2Se > H2O C) H2O > H2S > H2Se > H2Te D) H2O > H2Te > H2Se > H2S E) H2Se > H2S > H2O > H2Te
The correct answer is D! Why? Lets first understanding briefly the concept of "boiling point" Basically, we need heat to break apart the attractive forces the molecule has to itself. The weaker those attractive forces are the less heat will be required, meaning the lower the boiling point will be. Molecules with really strong attractive forces will need much more heat and therefore have much higher boiling points. So for boiling point, in general when you increase the molecular weight you increase the boiling point. Now, this is because we have something known as dispersion forces which you can basically think of "attractions" that the molecule has to itself. Technically what is happening is that there are all these temporary "dipole attractions" that result from the electrons in two adjacent atoms. But what you should remember is that unlike other forces like hydrogen bonding, everything has dispersion forces and these forces drastically increase as the molecular size of the molecule you are dealing with increases. Hence the larger the molecular weight is, the stronger the bonds of attraction between the molecule itself requiring more heat to break those bonds, which is why we observe that larger molecular weight of molecules results in higher boiling point than molecules with smaller molecular weight since they have weaker dispersion forces making it easier for heat to break the bonds of attractions. Knowing this we can easily rank boiling point from highest to lowest with H2Te > H2Se > H2S based solely on the molecular weight. Now there is an exception. Although water has the smallest mass, it actually has the highest boiling point. This is because of the hydrogen bonding in the water molecule. Hydrogen bonding is a very special type of attractive force and it is extremely powerful in comparison to other "intermolecular dipole-dipole forces" like dispersion forces. In most of these cases, hydrogen bonding will trump and be more important of a factor in ranking boiling point than dispersion forces. Follow up questions and topics related to this question I would love to elaborate on include: The difference between intermolecular and intramolecular forces? What is a dipole-dipole Force? What are Ion-Dipole Forces? What are Ion-induced Forces? After explaining those topics I will then come up with harder example questions to put our new gained understanding to the test!
needs and Payam will reply soon.