Friday, May 12, 2006
Acid-Base analysis + HW
To help those of you who have somehow forgotten how to take a test, here is what you should do:
When you see the common test phrase, "justify your answer", you must seriously and in EXPLICIT DETAIL state your reasons for choosing your answer. That requires you to explain, step by step, WHY AND HOW your answer must be true.
Merely restating the data in the question will NEVER suffice (even on the REGENTS!).
Many questions on the test explicitly stated that you must show the equation and formula used in your calculation. You must write the general formula FIRST and then plug-in the numbers/data correctly.
NOT ONE PERSON correctly explained acidic/basic/neutral salts...the appropriate phrase is (for NaCl), " NaCl is a salt that can be formed from the neutralization of HCl, a strong acid, with NaOH, a strong base thus forming a neutral salt.
Even though some people know what an Arrhenius base is (forms OH- in solution) and a Bronsted base is (H+ acceptor), MOST of you chose a SALT! NaBr which cannot accept H+ (not even the dissociated Br- can because HBr is a STRONG ACID and would IMMEDIATELY release the H+ back to the water). For the record, all NEGATIVELY charged polyatomic ions (except NO3-, ClO4-, and HSO4-) are Bronsted bases because they can easily accept H+. Make sure that you review and CORRECT your test. Do NOT let failure go uncorrected!
...for the hw this weekend, a question from last year follows:
Just want to post a reply to one of the questions about the worksheets (speaking of which, the Table "L" referred to on the worksheets is really Table "N" in our reference Tables):
How do you determine whether or not a reaction will occur in water? I think that the worksheets are using different reference tables because all of my calculations for voltage have been off, but only because they are using different numbers.
Answer:
The reaction in water means the reaction in "aqueous solution". That means that the salt listed is dissolved into its aqueous ions. Then, you just look at which type of single replacement reaction is occurring: metal replaces metal cation or non-metal replaces non-metal anion.
Example:
KI (aq) + Cl2 (g) --> I2 (s) + KCl (aq)
is really
K+ (aq) + I- (aq) + Cl2 (g) --> K+ (aq) + Cl- (aq) + I2 (s)
when you connect the REACTANTS I- and Cl2 on Table N, the line goes downhill which means that the reaction is spontaneous...i.e. chlorine is a more active (attracts electrons better) non-metal than iodine. Note that the K+ (aq) is just a spectator ion because it cannot be oxidized ( K2+ would be waaay less stable than K+).
The worksheet book uses mostly the same electrode potentials (some differ by 0.01 V) so if you are only off by a couple of hundredths, that is fine.
When you see the common test phrase, "justify your answer", you must seriously and in EXPLICIT DETAIL state your reasons for choosing your answer. That requires you to explain, step by step, WHY AND HOW your answer must be true.
Merely restating the data in the question will NEVER suffice (even on the REGENTS!).
Many questions on the test explicitly stated that you must show the equation and formula used in your calculation. You must write the general formula FIRST and then plug-in the numbers/data correctly.
NOT ONE PERSON correctly explained acidic/basic/neutral salts...the appropriate phrase is (for NaCl), " NaCl is a salt that can be formed from the neutralization of HCl, a strong acid, with NaOH, a strong base thus forming a neutral salt.
Even though some people know what an Arrhenius base is (forms OH- in solution) and a Bronsted base is (H+ acceptor), MOST of you chose a SALT! NaBr which cannot accept H+ (not even the dissociated Br- can because HBr is a STRONG ACID and would IMMEDIATELY release the H+ back to the water). For the record, all NEGATIVELY charged polyatomic ions (except NO3-, ClO4-, and HSO4-) are Bronsted bases because they can easily accept H+. Make sure that you review and CORRECT your test. Do NOT let failure go uncorrected!
...for the hw this weekend, a question from last year follows:
Just want to post a reply to one of the questions about the worksheets (speaking of which, the Table "L" referred to on the worksheets is really Table "N" in our reference Tables):
How do you determine whether or not a reaction will occur in water? I think that the worksheets are using different reference tables because all of my calculations for voltage have been off, but only because they are using different numbers.
Answer:
The reaction in water means the reaction in "aqueous solution". That means that the salt listed is dissolved into its aqueous ions. Then, you just look at which type of single replacement reaction is occurring: metal replaces metal cation or non-metal replaces non-metal anion.
Example:
KI (aq) + Cl2 (g) --> I2 (s) + KCl (aq)
is really
K+ (aq) + I- (aq) + Cl2 (g) --> K+ (aq) + Cl- (aq) + I2 (s)
when you connect the REACTANTS I- and Cl2 on Table N, the line goes downhill which means that the reaction is spontaneous...i.e. chlorine is a more active (attracts electrons better) non-metal than iodine. Note that the K+ (aq) is just a spectator ion because it cannot be oxidized ( K2+ would be waaay less stable than K+).
The worksheet book uses mostly the same electrode potentials (some differ by 0.01 V) so if you are only off by a couple of hundredths, that is fine.
# posted by C @ 9:33 AM 
