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Topic: Predicting spontaneous reactions  (Read 15616 times)

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777888

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Predicting spontaneous reactions
« on: October 20, 2004, 07:31:58 PM »
I don't know how to do some of the questions relating to the spontaneous reactions.

i)Is H2 capable of reducing Ag+? (I say YES, is it right?)

ii)Is Fe(2+) capable of reducing VO(2+)? (How can Fe(2+) be a reducing agent?) ???

iii)Is Fe(2+) capable of reducing Cr(2+) to Cr? (How can Fe(2+) be a reducing agent?)

iv)Is Fe(2+) capable of reducing Cr(3+) to Cr(2+)? (How can Fe(2+) be a reducing agent?)

v)Is it possible for a reagent to redce I2 to I- but not to reduce Cu(2+) to Cu? What RANGE of standard reduction potentials could such a reagent have?
-I say YES, but what is the range? ???

vi)Crystals I2 are added to a solution of NaCl Is this a spontaneous or non-spontaneous reaction?(I don't know if ions/atoms in NaCl are reducing or oxidizing agents. And there's two things in NaCl, how can I know if this is a spontaneous rxn or not?)

Can someone teach me please? THANKS :)
« Last Edit: October 20, 2004, 07:46:40 PM by 777888 »

Tetrahedrite

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Re:Predicting spontaneous reactions
« Reply #1 on: October 21, 2004, 02:06:56 AM »
First of all Fe2+ can act as both an oxidising and reducing agent eg

Fe2+ + 2e- -> Fe(s)  Here it is taking electrons so is an oxidising agent

Fe2+ -> Fe3+ + e- Here it is giving electrons so is a reducing agent

1)the reduction potential for Ag+ is higher than for H+ and it will therefore be reduced.

2)the reduction potention for VO2+ is lower than for Fe2+ therefore no reaction will occur

3) ditto for Cr2+ to Cr

4)ditto for Cr3+ to Cr2+

5) Yes- but s.e.p must fall between 0.54 and 0.34

6)Na+ has an extremely low reduction potential and will therefore not be reduced. Cl2 however has a higher reduction potential than I2 and will therefore be reduced to Cl-

Tetrahedrite

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Re:Predicting spontaneous reactions
« Reply #2 on: October 21, 2004, 02:26:58 AM »
Apologies, the last statement is wrong, higher reduction potential means it will stay as Cl-

777888

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Re:Predicting spontaneous reactions
« Reply #3 on: October 21, 2004, 10:53:08 AM »
When predict, do we just looke at the position of the Oxidizing agent and Reduing agent? (ie, Top or bottom) Because the direction of increasing strength of OA and RA are in opposite directions, does that matter?

And for question 6, which one will be reduced? Na+?

Thanks :)
« Last Edit: October 21, 2004, 10:55:03 AM by 777888 »

777888

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Re:Predicting spontaneous reactions
« Reply #4 on: October 21, 2004, 01:34:10 PM »
For #3 and 4,

iii)Is Fe(2+) capable of reducing Cr(2+) to Cr? (How can Fe(2+) be a reducing agent?)

iv)Is Fe(2+) capable of reducing Cr(3+) to Cr(2+)? (How can Fe(2+) be a reducing agent?)

Are the answers "No"?

Tetrahedrite

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Re:Predicting spontaneous reactions
« Reply #5 on: October 21, 2004, 07:15:19 PM »
yes

Tetrahedrite

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Re:Predicting spontaneous reactions
« Reply #6 on: October 21, 2004, 07:18:52 PM »
Sorry again, the4 above is ambiguous, the answers are no.
Once again Na+ will not be reduced because it has an extemely low reduction potential, much lower than that for I2

777888

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Re:Predicting spontaneous reactions
« Reply #7 on: October 23, 2004, 02:35:36 PM »
How about this one?

Predict if this reaction is spontaneous or non-spontaneous. WHY?

"A lead wire is placed in a solution of CuCl2"

(There are 4 things in this reaction, Pb, Cu(2+), Cl2(-), H2O, which 2 should I compare with?? OA with RA?)

Thanks!

Offline Donaldson Tan

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Re:Predicting spontaneous reactions
« Reply #8 on: October 23, 2004, 04:03:09 PM »
I would quote the reactivity series of metal, and say that since lead is more reactive than copper, it will displace the copper(II) ions in solution. The reaction is spontaneous
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

777888

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Re:Predicting spontaneous reactions
« Reply #9 on: October 23, 2004, 05:05:17 PM »
The trouble is that we will only be given the redox table instead of the activity series.. So is that a way that I can using the redox table to predict?

Thanks!

777888

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Re:Predicting spontaneous reactions
« Reply #10 on: October 23, 2004, 05:30:39 PM »
I have a homework question that I don't understand...

"O2(g) is bubbled into an aqueous slotuion of FeI2 containing excess HCl(aq). Predict ALL spontaneous reactions, in the order in whcih they will occur."

How can there be more than one spontaneous reactions? (It says ALL) According to my text, only the strongest oxidizing agent and the strongest reducing agent will react. How can the reactions have different order?


Offline Donaldson Tan

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Re:Predicting spontaneous reactions
« Reply #11 on: October 23, 2004, 06:46:54 PM »
The trouble is that we will only be given the redox table instead of the activity series.. So is that a way that I can using the redox table to predict?

Look up for the redox values for Pb->Pb2+ and Cu2+->Cu..

"O2(g) is bubbled into an aqueous slotuion of FeI2 containing excess HCl(aq). Predict ALL spontaneous reactions, in the order in whcih they will occur."

oxygen comes in contact all the ions in your solution. it's obvious in this scenario that you should evaluate oxygen's ability as an oxidising agent to oxidise Fe2+, I-, Cl-, so please look up in your table for the relevant redox values in your data table

As long Ecell > 0, the reaction is spontaneous. You question ask for all possible spontaneous reactions, not what reactions actually occur. The spontaneous reaction with the highest Ecell is most likely to occur, thus its order is 1.
« Last Edit: October 23, 2004, 06:50:02 PM by geodome »
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

777888

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Re:Predicting spontaneous reactions
« Reply #12 on: October 23, 2004, 10:42:28 PM »
THANK YOU!

Question: Gold is added to an hydrochloric acid solution. Predict the redox reaction and determine if it's spontaneous.

Steps:
Species initally present: Au   H+   Cl-    H2O

By choosing the strongest reducing/oxidizing agents, and add the half-reactions together, I got the equation:

4H(+) +2H2O->2H2+O2+4H(+)  
2H2O->2H2+O2     Subtract  4H(+) from both sides.

Problem: Would this be spontaneous?? How can I know this by the equation? (There's only 1 reactant so I don't know how to determine which is oxidizing agent and which is reducing agent...)
« Last Edit: October 23, 2004, 10:50:47 PM by 777888 »

777888

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Re:Predicting spontaneous reactions
« Reply #13 on: October 24, 2004, 08:42:03 PM »
THANK YOU!

Question: Gold is added to an hydrochloric acid solution. Predict the redox reaction and determine if it's spontaneous.

Steps:
Species initally present: Au   H+   Cl-    H2O

By choosing the strongest reducing/oxidizing agents, and add the half-reactions together, I got the equation:

4H(+) +2H2O->2H2+O2+4H(+)  
2H2O->2H2+O2     Subtract  4H(+) from both sides.

Problem: Would this be spontaneous?? How can I know this by the equation? (There's only 1 reactant so I don't know how to determine which is oxidizing agent and which is reducing agent...)
Can someone help me? Which half reactions may I compare with(in the redox table)?

777888

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Re:Predicting spontaneous reactions
« Reply #14 on: October 25, 2004, 11:13:14 PM »
I need *delete me* The teacher is always absent so we have to study ourselves. And it's quite hard because chemistry is not an easy subject... :upset:

Can somebody explain why this is a spontaneous or non-spontaneous reacion?

4H(+) +2H2O->2H2+O2+4H(+)  
2H2O->2H2+O2    Subtract  4H(+) from both sides.

Thanks^^


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