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Topic: Errors In Exam Key?  (Read 6792 times)

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albert611

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Errors In Exam Key?
« on: May 17, 2005, 10:47:46 PM »
Hi,
I think I've found some errors in a practice exam I'm taking...would somebody check if I'm right?

1. How does an increase in temp affect the forward and reverse reaction rates for an exothermic reaction:
                    Forward                   Reverse
  (A)             increases                increases
  (B)             increases                decreases
  (C)             decreases               increases
  (D)             increases                no change

   If the reaction shifts to favor the reactant, won't the forward decrease and the reverse increase?

2. Which mixture forms a buffer when dissolved in 1.0 L in water?

  (A) NaOH and HBr
  (B) NaCl and HCl
  (C) HNO2 and HCl
  (D) NH3 and HCl

  I had thought that NaCl and HCl would be the buffer since there are HCl and Cl- ions.

  Let me know if I'm wrong or anything. Thanks! Oh, the correct answers are A and C, respectively.

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Re:Errors In Exam Key?
« Reply #1 on: May 18, 2005, 12:51:06 AM »
1. reaction rate always increase with temperature. in fact, the equilibrium constant is the ratio of the forward rate constant to the backward rate constant. increase in temperature increases both the rate constant, so reaction rate increase as well. however, the position of equilibrium will adjust to the reactant side.

2. ammonia is a very weak alkali, so it won't be able to act as an effective buffer. conversely, HNO2 is a much weaker acid to HCl, and thus it can act as a base in the presence of HCl.
« Last Edit: May 18, 2005, 04:18:13 AM by geodome »
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Offline AWK

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Re:Errors In Exam Key?
« Reply #2 on: May 18, 2005, 03:16:22 AM »
Concerning the second problem - question is not bring up correctly.
Buffering properties can show mixtures of a weak acid and its salt , or weak base and its salt (eventually salt that hydrolyses, but  it is something more complex problem). In this case, as an effective buffer mixture can work only the mixture of an ammonia and HCl but on the condition that there is more moles of ammonia then HCl.
In some cases (biochemist use such a buffer mixtures) buffering properties show also mixtures of KCl + HCl or NaCl + HCl in which the concentration of salts are relatively high in reference to  the acid (though their buffering capacities are rather small).
Following above statement, also NaOH and HBr can be treated as the buffer, but on the condition that a small molar excess of HBr exists.

Chemists never do not  treat  a mixture of HNO2 and HCl as an buffer.

May be Borek put some comments here
AWK

Offline Borek

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Re:Errors In Exam Key?
« Reply #3 on: May 18, 2005, 04:42:22 AM »
D seems to be the best answer.

In a way any mixture here can be trated as a buffer, depending on how do you define a buffer (mixture containing acid and conjugate base? solution that keeps constant pH when adding small amounts of acid or base?) and what cocentrations of substances were used - but they were not given, even if the volume is known. That's stupid.
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Offline Borek

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Re:Errors In Exam Key?
« Reply #4 on: May 18, 2005, 04:44:50 AM »
ammonia is a very weak alkali, so it won't be able to act as an effective buffer.

Geo, you are horribly wrong here - ammmonia buffer is used very often when you need pH near 9.25 (pKa for NH4(+)). Ammonia has pKb of 4.75 so it is exactly as strong as base as acetic acid is as acid.
« Last Edit: May 18, 2005, 04:45:54 AM by Borek »
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Offline Donaldson Tan

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Re:Errors In Exam Key?
« Reply #5 on: May 18, 2005, 06:04:59 AM »
ok. i was taught that ammonia is a weak alkali. guess i learn something new today.
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Offline Borek

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Re:Errors In Exam Key?
« Reply #6 on: May 18, 2005, 06:10:50 AM »
ok. i was taught that ammonia is a weak alkali. guess i learn something new today.

Well, it is a weak alkali, just like acetic acid is a weak acid. It doesn't stop them both from being prefect selection for buffer preparation.
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Garneck

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Re:Errors In Exam Key?
« Reply #7 on: May 18, 2005, 07:13:10 AM »
Borek is absolutely correct here.

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