Wednesday, December 06, 2006
Wednesday Test-day
AP: Things looked good after a quick glance at the tests from today...I'll be grading them over the extended weekend.
Honors: You are welcome; yes, today's test was a gift in comparison to the tests from the past three years. If you had timing trouble, once again, you are not answering what is asked for. You have to constantly remind yourself that science writing does NOT require any fluff or extraneous information (unfortunately, filling out an essay with irrelevant prolix information tends to work well in grammar school); a lot of time is wasted writing true but irrelevant facts for which you will never earn a point.
You already knew exactly what was on the test. You just had to answer exactly and only what was asked for. I won't be writing this reminder again.
Today's performance (the degree to which directions were followed) will influence the quantity and style of questions on the next test.
Regents: we discussed polyatomic ions; the common ones are found on TABLE E in your review book. We then practiced writing formulas. Then, we came to the important practical point of relating ionic bonding in salts to their physical property of high melting points. Since ionic bonds are relatively strong, full electrostatic attractions between oppositely charged ions AND each ion in a salt crystal lattice is COMPLETELY SURROUNDED by , on average, SIX OR EIGHT oppositely charged ions (NO MATTER WHAT THE EMPIRICAL FORMULA OF THE SALT IS !!!) , there are MULTIPLE ionic bonds per each ion in any salt sample. Therefore, the ions are "frozen" in place next to each other in a solid salt sample. Relatively large amounts of energy are required to even BEGIN to get the ions to overcome their mutual attractions such that they can at least move over /"mosh with" each other while still remaining in contact. The large energy requirement is exhibited as a relatively HIGH melting point.
Then, another BIG point came up: Coulomb's Law is the reason for different degrees of attraction between differently charged ions that are at different average distances from each other in a lattice. From our last unit, Coulomb's Law is also THE REASON that high Zeff nuclei attract valence electrons stronger than do low Zeff nuclei, so I expected you to know this already.
Bottom line: the higher the charge on the oppositely charged ions, the greater the strength of the ionic bond. Coulomb's Law states that the higher the product of the magnitudes/sizes of the oppositely charged particles, the stronger the force of attraction. Coulomb also states that, the CLOSER that the oppositely charged particles are to each other, the STRONGER the electrostatic force of attraction. This law/observation GOVERNS/RULES all of chemistry!
So, the higher the charges of the cations and anions AND the smaller they are (fewer OPEL's) the stronger the ionic bonds.
For example, NaCl has positive and negative one charged ions, respectively. AlN has positive and negative THREE charged ions! Therefore, the ionic bonds among the Al 3+ and the N 3- ions are much stronger than the ionic bonds among the Na + and Cl - ions. That is why the melting point of aluminum nitride is MUCH HIGHER than the melting point of sodium chloride.
The distances among the ions are not that different (same #'s of OPEL'S) in this case so that is not a significant factor towards the different melting points.
End of story.
Tomorrow there will be a BRIEF quiz. I HIGHLY recommend that you know the symbols of the first 36 elements by now so that you do not have to slow down and look them up. I also recommend that you know all of the halogens and the Group I and Group 2 metal symbols by heart. Know compound formulas, charges of ions, and Lewis dot structures for ATOMS. If the class does not come in prepared, we will have a quiz EVERY DAY FOR THE REST OF THE QUARTER.
Honors: You are welcome; yes, today's test was a gift in comparison to the tests from the past three years. If you had timing trouble, once again, you are not answering what is asked for. You have to constantly remind yourself that science writing does NOT require any fluff or extraneous information (unfortunately, filling out an essay with irrelevant prolix information tends to work well in grammar school); a lot of time is wasted writing true but irrelevant facts for which you will never earn a point.
You already knew exactly what was on the test. You just had to answer exactly and only what was asked for. I won't be writing this reminder again.
Today's performance (the degree to which directions were followed) will influence the quantity and style of questions on the next test.
Regents: we discussed polyatomic ions; the common ones are found on TABLE E in your review book. We then practiced writing formulas. Then, we came to the important practical point of relating ionic bonding in salts to their physical property of high melting points. Since ionic bonds are relatively strong, full electrostatic attractions between oppositely charged ions AND each ion in a salt crystal lattice is COMPLETELY SURROUNDED by , on average, SIX OR EIGHT oppositely charged ions (NO MATTER WHAT THE EMPIRICAL FORMULA OF THE SALT IS !!!) , there are MULTIPLE ionic bonds per each ion in any salt sample. Therefore, the ions are "frozen" in place next to each other in a solid salt sample. Relatively large amounts of energy are required to even BEGIN to get the ions to overcome their mutual attractions such that they can at least move over /"mosh with" each other while still remaining in contact. The large energy requirement is exhibited as a relatively HIGH melting point.
Then, another BIG point came up: Coulomb's Law is the reason for different degrees of attraction between differently charged ions that are at different average distances from each other in a lattice. From our last unit, Coulomb's Law is also THE REASON that high Zeff nuclei attract valence electrons stronger than do low Zeff nuclei, so I expected you to know this already.
Bottom line: the higher the charge on the oppositely charged ions, the greater the strength of the ionic bond. Coulomb's Law states that the higher the product of the magnitudes/sizes of the oppositely charged particles, the stronger the force of attraction. Coulomb also states that, the CLOSER that the oppositely charged particles are to each other, the STRONGER the electrostatic force of attraction. This law/observation GOVERNS/RULES all of chemistry!
So, the higher the charges of the cations and anions AND the smaller they are (fewer OPEL's) the stronger the ionic bonds.
For example, NaCl has positive and negative one charged ions, respectively. AlN has positive and negative THREE charged ions! Therefore, the ionic bonds among the Al 3+ and the N 3- ions are much stronger than the ionic bonds among the Na + and Cl - ions. That is why the melting point of aluminum nitride is MUCH HIGHER than the melting point of sodium chloride.
The distances among the ions are not that different (same #'s of OPEL'S) in this case so that is not a significant factor towards the different melting points.
End of story.
Tomorrow there will be a BRIEF quiz. I HIGHLY recommend that you know the symbols of the first 36 elements by now so that you do not have to slow down and look them up. I also recommend that you know all of the halogens and the Group I and Group 2 metal symbols by heart. Know compound formulas, charges of ions, and Lewis dot structures for ATOMS. If the class does not come in prepared, we will have a quiz EVERY DAY FOR THE REST OF THE QUARTER.
# posted by C @ 12:26 PM 
