Frequently Asked Questions s39

Frequently Asked Questions

Question = I think these are mistakes: for the april 2000 provincial q's on chapter 2 (7 10 12) mostly 10 and 12 why are the answers that?

Answer = There was a mistake. Jan 2000 answers were typed again in error. The correct answers are for april 2000 Unit 2 MC’s as follows: 7. D 8. C 9. B 10. B 11. B 12. D 13. C

Question = Which year was the provincial you gave to us to practice on ?

Answer = April 2002

Question = Define activated complex and intermediates? Also the difference, plz

Answer = Activated complexes are: high point on reaction profile, unstable, last only for a few moments, high energy. Intermediates are: A dip on reaction profile (higher than final product though); they are really just a product that will keep on reacting; stable, can last for a while as long you prevent it from reacting, low energy (just not as low as final product) See page 29 in Hebden. HOOBr and HOBr are two intermediates. There are three activated complex that exist at the top of each hill. For example there is one between HBr + O2 (reactants) and HOOBr (first intermediate). Since it live for just a millisecond, the structure is not known and is not drawn in.

Question = Can you explains to me the concept of molecular solution when dissolved in water? And also the understanding of LEAST SOLUBLE? salt is soluble?

Answer = A molecular compound is a covalent compound. It is not an ionic compound (salt). So when water attacks a covalent compound, there are no ions to tear off. A soluble molecular compound will just mix with the water. Identify a molecular compound as organic, lots of carbon. (I.e.: CH3OH, CH3CH2OH)

To compare solubilities, first check the “Solubility of Common Compounds in Water” table. If you still have to decide between two or more compounds as which is least soluble and they BOTH show “Low solubility”, check next the Ksp table. The lowest Ksp value will be the least soluble. For example, both Fe(OH)3 and CaCO3 are low solubility. -39 But Fe(OH)3 is less soluble than CaCO3 cause its ksp value of 2.6 x 10 is smaller.

Question = Solve this problem: What is the maximum[Br-] which can exist in a solution of 0.025M Pb(NO3)2?

Answer = This is a common question for the provincia!

First, “can exist” means without forming a precipitate.

Second, decide what ppt will form. It will be PbBr2 cause Br- will not react with NO3- and Pb(NO3)2 is soluble.

Third, write the dissociation for PbBr2

2+ PbBr2  Pb + 2Br-

Ksp = [Pb2+][Br-]2

Fourth, fill in the info known

6.6 x 10-6 = [0.025M][Br-]2

2+ note: [Pb(NO3)2] = [Pb ] from stoichiometry.

Fifth, solve for [Br-]

[Br-] = 0.016 M