O-Level Chemistry

[HappieNewbie123]

Hi I require some help for these 2 redox reaction questions:


Q1)
Equations given:
1) Cl2(g) + H2S(g) –> 2HCl(g) + S(s)
2) SO2(g) + 2H2S(g) –> 2H2O(l) + 3S(s)
3) Cl2(g) + 2H2O(l) + SO2(g) –> 2HCl(aq) + H2SO4(aq)

a) From the equations given, deduce which of the three gases: chlorine, hydrogen sulfide and sulfur dioxide is the strongest
i) Oxidation agent
ii) Reducing agent

b) State how you arrive at your answers.

Q2) Disproportionation is a reaction in which the same element is both oxidized and reduced in a reaction.
Explain why the following reaction is a disproportionation reaction.
Cl2 + 2NaOH –> NaCl + NaOCl + H2O

(Oxidation state of NaOCl is not balanced? Na is +1, O is -2 and Cl is -1?? I find this a bit strange…)
Thank you!

[Dr.033430Daniel]

HappieNewbie123:

Hi I require some help for these 2 redox reaction questions:


Q1)
Equations given:
1) Cl2(g) + H2S(g) --> 2HCl(g) + S(s)
2) SO2(g) + 2H2S(g) --> 2H2O(l) + 3S(s)
3) Cl2(g) + 2H2O(l) + SO2(g) --> 2HCl(aq) + H2SO4(aq)

a) From the equations given, deduce which of the three gases: chlorine, hydrogen sulfide and sulfur dioxide is the strongest
i) Oxidation agent
ii) Reducing agent

b) State how you arrive at your answers.

Q2) Disproportionation is a reaction in which the same element is both oxidized and reduced in a reaction.
Explain why the following reaction is a disproportionation reaction.
Cl2 + 2NaOH --> NaCl + NaOCl + H2O

(Oxidation state of NaOCl is not balanced? Na is +1, O is -2 and Cl is -1?? I find this a bit strange...)
Thank you!

I asked our new Chemistry tutor these questions. Here are the answers:

1(a)
Strongest Oxidising Agent: Cl2
Strongest Reducing Agent: H2S

(b) From the first two equations, we can see that S always oxidised from -2 in H2S to 0 in S by both Cl2 and SO2. Granted, its ease of being oxidised makes it the strongest reducing agent. Then we look at the 3rd equation, where Cl is still reduced from 0 in Cl2 to -1 in HCl, while S is oxidised from +4 in SO2 to +6 in H2SO4. This indicates that SO2 is more easily oxidised than Cl2 (and thus a stronger reducing agent). Hence, the strength of oxidising agents is as such: Cl2 > SO2 > H2S

2) The reaction is a disproportionation reaction because Cl is 0 in Cl2, and it is concomittantly reduced to -1 in NaCl and oxidised to +1 in NaOCl.

*The oxidiation state of Na is always +1, O is generally -2 (except in cases of peroxides and superoxides), and for Cl to fulfill charge neutrality it has to be +1 in NaOCl.

[iamastudent]

Hi,

May I know what is the oxidation state of ions? Example like the equation:
(S2O3)2- + 4Cl2 + 5H2O –> 2(SO4)2- + 8Cl- + 10H+

What is the oxidation state of H in H+? Is it +1 or is it 0?
O in (SO4)2-? Is it -2? S in (SO4)2-? Is it +6? Thank you

[dioprem]

iamastudent:

Hi,

May I know what is the oxidation state of ions? Example like the equation:
(S2O3)2- + 4Cl2 + 5H2O --> 2(SO4)2- + 8Cl- + 10H+

What is the oxidation state of H in H+? Is it +1 or is it 0?
O in (SO4)2-? Is it -2? S in (SO4)2-? Is it +6? Thank you

The oxidation state of simple ions is always the charge of the ion. For example, in the case of H+, its oxidation state would be +1. In the case of Cl-, its oxidation state would be -1.

In the case of polyatomic ions, such as the sulfate ion, the sum of all the individual oxidation states of the atoms present in an ion must add up to the charge of the ion. In the case of (SO4)2-, the oxidation state of the 1 sulfur atom and 4 oxygen atoms must add up to -2. Oxygen generally has an oxidation state of -2. So the total oxidation state of the 4O atoms would be -8. So in order for the sulfate ion to have a charge of 2-, the S atom must have an oxidation state of +6.

Note that oxygen can have other oxidation states. For example, it will have an oxidation state of -1 in peroxide ions (O2)2-. It can also have an oxidation state of -0.5 in the case of superoxides (O2)-

Hope this helps!

[WhatsappTuitionSG]

Providing Whatsapp-based consultations for Singapore students (Secondary and JC)

Students are charged on a weekly basis for service, allowing students to clarify as many questions as they want, as well as answer as offer them flexibility of learning at their own pace.

Subjects available:

Secondary:
Pure and Science Chemistry, Physics, Biology; E-Math; A-Math

JC:
H1/H2 Biology, Chemistry, Math

Charges: Fixed rate of $35 per subject per week

For interested students/parents, feel free to sign up/know more about this service by contacting us at [email protected]

[Mojostar]

Hi all,


I’m weak in electrolysis…any way to improve in this area? Any book to recommend? Thanks.

[AdonciaTang]

Mojostar:

Hi all,


I'm weak in electrolysis...any way to improve in this area? Any book to recommend? Thanks.

Hi Mojostar. Get a tutor to help u. My niece was also failing in her Pure Chem for a year and she bought all sorts of books to read thinking it would help. It does but its so minimal. After she engaged my DS's tutor, her results improved, because now there is someone who can help to guide her along in her work and teach her the concept which she might get wrong.

[Dr.033430Daniel]

Mojostar:

Hi all,


I'm weak in electrolysis...any way to improve in this area? Any book to recommend? Thanks.

Hi Mojostar – The first thing I would recommend is to read a textbook. These three by local authors are all good:

Chemistry Insights
Chemistry Matters
Discover Chemistry

For question practice, try Conceptual Learning Chemistry by Ong Yean Ling (Blue in Color). I selected this book as a supplement to our Chemistry class worksheets because it looked to be the best one out there.

To get a handle on electrolysis, practice drawing the setups. It is a common exam question to ask you to first draw the setup and then name the products. So once you have studied the topic draw the setup several times for practice, so that you can improve your speed at doing the drawing. Then make your own notes listing all the versions of electrolysis you can find, what goes to the anode, what goes to the cathode etc. There is a logic to it that you should understand in terms of the reactivity of the various ions. But it also helps to write the reactions out a few times so that both the logic and the more common examples stick with you.

Make your notes, then test yourself a week later and see if you can remember.

[spicy]

AdonciaTang:

Mojostar:

Hi all,


I'm weak in electrolysis...any way to improve in this area? Any book to recommend? Thanks.

Hi Mojostar. Get a tutor to help u. My niece was also failing in her Pure Chem for a year and she bought all sorts of books to read thinking it would help. It does but its so minimal. After she engaged my DS's tutor, her results improved, because now there is someone who can help to guide her along in her work and teach her the concept which she might get wrong.

Hi
I have PMed you. Would it be possible to share your tutor's contact?
Thanks !

[achiever2803]

spicy:

AdonciaTang:

[quote=\"Mojostar\"]Hi all,


I'm weak in electrolysis...any way to improve in this area? Any book to recommend? Thanks.

Hi Mojostar. Get a tutor to help u. My niece was also failing in her Pure Chem for a year and she bought all sorts of books to read thinking it would help. It does but its so minimal. After she engaged my DS's tutor, her results improved, because now there is someone who can help to guide her along in her work and teach her the concept which she might get wrong.

Hi
I have PMed you. Would it be possible to share your tutor's contact?
Thanks ![/quote]Hi,

wondering if you are still looking for a tutor for chemistry. if you, do drop me a pm and i can call you to tell you the details.!

Cheers