Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: kensher on September 12, 2016, 02:41:13 AM
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Hello.
I have multiple practice problems which I haven't figured out. They are all within the chapter of redox reactions.
Practice problem 1
In a solution of KI(aq), starch is added. Which gas makes the solution blue-black?
a) H2
b) N2
c) Cl2
d) O2
My comment
The book hasn't explained this at all, so I wonder how I should figure this out. Any learning resources (preferably a video) would be great.
Practice problem 2
To find the content of Iodine in a solution, the solution gets titrated with thiosulfate. The reaction in the titration flask can be written like this:
I2(aq) + 2S2O3-(aq) :rarrow: 2I-(aq) + S4O62-(aq)
c)
If the starch is added from the very beginning of the titration, then it would be easy to add too much S2O3-. Explain why it will affect the calculated concentration of iodine in the solution.
My comment
Again, the book hasn't mentioned anything about indicators, so I don't know how to figure this out. Any learning resources would be great.
Practice problem 3
Of the following equations, which of them is not spontaneous?
(see attachment – will upload later today)
My comment
Normally, you would put the entalphy change, entrophy change and the temperature into the Gibbs function, but there isn't enough data provided for that. So how can one see it just on the equations alone?
Practice problem 4
2C4H10 + 13O2 :rarrow: 8CO2 + 10H2O ΔHc = -2878 kj/mole
d)
Find ΔH for the combustion reaction.
My comment
I thought the ΔHc was the ΔH, just spesific for combustion reactions. I thefore took:
2 moles * -2878 kj/mole = -5756 Kj. This is the correct number, but my book shows the number as a positive (not negative).
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Reaction attached are not balanced or badly balanced. Moreover, there are only two different reactions or some reactions are misprinted.
Textbooks usually do not repeat some informations. Moreover, information of starch indicator for iodine can be often found at very beginning of general or analitical chemistry texbook.
Some informations can be also find in other sources, eg Wikipedia.Though reaction of titration shows correct coefficients there is misprint in charges.
If the starch is added from the very beginning of the titration, then it would be easy to add too much S2O3-.
Starch forms with iodine the relatively stable complex, and you titrate free iodine.
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Explain why it will affect the calculated concentration of iodine in the solution.
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Are you kidding?
This is stoichiometry and answer should be obvious.
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In a solution of KI(aq), starch is added. Which gas makes the solution blue-black?
Starch solution goes blue-black in the presence of which element?
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«Explain why it will affect the calculated concentration of iodine in the solution»
Are you kidding?
This is stoichiometry and answer should be obvious.
I just paraphrased from the book, and didn't ask the questions myself.
My own comments are exclusively under «My comments».
I will review you other answers later today. Thanks for replying.
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In a solution of KI(aq), starch is added. Which gas makes the solution blue-black?
Starch solution goes blue-black in the presence of which element?
I don't know. The book never touched on it.
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In a solution of KI(aq), starch is added. Which gas makes the solution blue-black?
Starch solution goes blue-black in the presence of which element?
I don't know. The book never touched on it.
Re-read carefully, you need this info for this question. It should have been mentioned. Its even used in questions 2. Question 2 is a pretty complicated question for someone new, but it does expect you to know not just the reaction, but why it happens.
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Here are the attachment for practice problem 3
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Three reaction are not balanced. Even with complete thermodynamic data they can be calculated wrongly.
One reaction is known otherwise that need light.
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In the fourth reaction (with CH2O) probably one reagent on the right side is missing (deep sea thermophilic organisms).
But your reaction (correctly balanced) proceeds in the organic matter heterotrophic organisms in natural waters (marshes).
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Hello.
I have multiple practice problems which I haven't figured out. They are all within the chapter of redox reactions.
Practice problem 1
In a solution of KI(aq), starch is added. Which gas makes the solution blue-black?
a) H2
b) N2
c) Cl2
d) O2
My comment
The book hasn't explained this at all, so I wonder how I should figure this out. Any learning resources (preferably a video) would be great.
Check what is required for making starch solution blue - black ...
Then compare the reactivity of I- to undergo oxidation to give I2 with the gases mentioned in the question..Compare their reduction potentials to know which gas can oxidize I- to I2 ..I am sure this will help you in figuring out this problem...
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Hello.
I have multiple practice problems which I haven't figured out. They are all within the chapter of redox reactions.
Practice problem 1
In a solution of KI(aq), starch is added. Which gas makes the solution blue-black?
a) H2
b) N2
c) Cl2
d) O2
My comment
The book hasn't explained this at all, so I wonder how I should figure this out. Any learning resources (preferably a video) would be great.
Check what is required for making starch solution blue - black ...
Then compare the reactivity of I- to undergo oxidation to give I2 with the gases mentioned in the question..Compare their reduction potentials to know which gas can oxidize I- to I2 ..I am sure this will help you in figuring out this problem...
Thanks for your answer! :)
I researched the problem, and what I could find was this:
1) I2 + I- (from KI) :rarrow: I3-
2) I3- slips into the starch molecule, forms a complex and then change the color of the solution to deep blue.
How does this relate to the reduction potential? And no of the gases will make I3-?
To answer you question, I would guess Cl2 since it has the highest reduction potential, and therefore is the best of the gases at stealing electrons?
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How did you get I2 to make I3-?
It is from oxidation of I- from KI ...now any gas which has more reduction potential than I2 can oxidize I- to I2 ...
Yes Cl2 has more reduction potential...
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That makes sense, but in a solution of KI(aq), don't you only have K+(aq) and I-(aq)?
Where does the I2 come from?
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oh my dear you are adding Cl2 which is a good oxidizing agent and converts I- to I2
That is the reason question.. In a solution of KI(aq), starch is added. Which gas makes the solution blue-black?
It means you need to select a gas from the below list which can convert I- to I2 ...that is the reason I asked you to compare the reduction potentials of all gases and we selected Cl2
a) H2
b) N2
c) Cl2
d) O2
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Got it now!
Thank you for you patience and clarifying answers ☺