AS Chemistry Redox/Halogens Question

'Write an equation for the reaction that occurs when chlorine is added to cold water. State whether or not the water is oxidised and explain your answer.'

I wrote the equation:
Cl₂ + H₂O -> HCl + HClO
But I'm not sure whether or not the water is oxidised. Do you work out the oxidation state of H in HCl/HClO? If so I got +1 and +3, but then what do you do with those oxidation states? Does hydrogen both stay the same/lose electrons so that water both stays the same and loses electrons ??? Is only the HClO relevant to decide whether or not H₂O is oxidised?

Reply 1

TSR Jessica

19

Sorry you've not had any responses about this. :frown:

Are you sure you’ve posted in the right place? Posting in the specific Study Help forum should help get responses. :redface:

I'm going to quote in Puddles the Monkey now so she can move your thread to the right place if it's needed. :h:

Spoiler

Reply 2

Fanta4TheBanter

8

Original post by Dopes

'Write an equation for the reaction that occurs when chlorine is added to cold water. State whether or not the water is oxidised and explain your answer.'

I wrote the equation:
Cl₂ + H₂O -> HCl + HClO
But I'm not sure whether or not the water is oxidised. Do you work out the oxidation state of H in HCl/HClO? If so I got +1 and +3, but then what do you do with those oxidation states? Does hydrogen both stay the same/lose electrons so that water both stays the same and loses electrons ??? Is only the HClO relevant to decide whether or not H₂O is oxidised?

Hi there!

This question seems to be poorly worded; are you sure you copied it down correctly? In redox reactions, chemicals themselves aren't usually oxidised/reduced, but it's the ions that make up the chemicals that are oxidised/reduced, if that makes sense.

Also, in this case the Hydrogen ions are always in the +1 oxidation state, so it's actually Chlorine which is both reduced (from oxidation state 0 to -1, in HCl) and oxidised (from oxidation state 0 to +1, in HClO).

Since neither of the ions that make up water are oxidised, I guess you can say water is not oxidised.

If you have any more questions, let me know, and I'll be happy to help. :smile:

(edited 9 years ago)

Original post by Dopes

'Write an equation for the reaction that occurs when chlorine is added to cold water. State whether or not the water is oxidised and explain your answer.'

I wrote the equation:
Cl₂ + H₂O -> HCl + HClO
But I'm not sure whether or not the water is oxidised. Do you work out the oxidation state of H in HCl/HClO? If so I got +1 and +3, but then what do you do with those oxidation states? Does hydrogen both stay the same/lose electrons so that water both stays the same and loses electrons ??? Is only the HClO relevant to decide whether or not H₂O is oxidised?

Chlorine disproportionates in water:

Cl₂ + H₂O --> HCl + HOCl

Chlorine changes from oxidation state zero to oxidation states -1 and +1, nothing else changes ...

Reply 4

Dopes

OP

Original post by Fanta4TheBanter

This question seems to be poorly worded; are you sure you copied it down correctly? In redox reactions, chemicals themselves aren't usually oxidised/reduced, but it's the ions that make up the chemicals that are oxidised/reduced, if that makes sense.

They aren't usually oxidised/reduced meaning chemicals can be oxidised or just that it's unusual to describe them as oxidised or reduced instead of their ions?

Original post by Fanta4TheBanter

Also, in this case the Hydrogen ions are always in the +1 oxidation state, so it's actually Chlorine which is both reduced (from oxidation state 0 to -1, in HCl) and oxidised (from oxidation state 0 to +1, in HClO).

How can you tell which ions keep the same oxidation state? Does the Cl change because it reacts as an element and is in compounds in products?

Reply 5

Fanta4TheBanter

8

Original post by Dopes

They aren't usually oxidised/reduced meaning chemicals can be oxidised or just that it's unusual to describe them as oxidised or reduced instead of their ions?

Just unusual.

Reply 6

Fanta4TheBanter

8

Original post by Dopes

How can you tell which ions keep the same oxidation state? Does the Cl change because it reacts as an element and is in compounds in products?