[cuongt]

im having trouble understanding this question.

chlorine is used to treat water to make it safe for drinking. the reaction of chlorine with water is:

Cl2+H2O <-------> HCl + HOCl

State and explain the effect on the safety of water for drinking if the pH of the water is decreased.

I said that, if the pH of the water is decreased it would be more acidic and less safer to drink. teacher said something about the equation shifting to the left or something, i dont understand at all.
the question was worth 3 marks and i got 0 out of the 3.   

[xiankai]

this sounds like Le Chatelier's Principle;

so if u decrease the pH (more H+ ions), then the equilibrium will shift to the right; because more HCl is being produced, so the system shifts to the left to balance it.

[mike]

Cl₂ + H₂O <-------> HCl + HOCl

Yes this is an equilibrium question. At equilibrium the forward and back reaction will stay the same unless you change the concentration of the reactants or products. The reaction will then "shift" to counteract that change, for example if you add more chlorine to the above reaction the left side of the equation is unbalanced and the reaction works to decrease the amount of chlorine. The way it does this is by reacting the newly added chloorine and converting it to products (in other words "shifting the reaction to the product side, or the right side).

So you can see that the same thing will happen if you change the concentration of HCl. By decreasing the pH you are in effect increasing the hydrogen ion (H⁺) concentration, or making the solution more acidic. So you in essence have more HCl (or product) on the right hand side of the equation than you need. In order to decrease this newly aquired excess of acid the reaction will work in reverse to turn it into reactants (hence the reaction is driven back to the reactant side, or the left hand side) this is why the teacher would have said "the equilibrium has been shifted to the left".

In summary, in an equilibrium reaction, if you add something to the left side the reaction will be shifted to the right side. Conversely, if you add something to the right side it will shift to the left side.

[Jamison]

I guess it depends what kind of science teacher you have, but my first go would be to ask the teacher! They know it better than us all. hmmmhmmh

[Borek]

I guess it depends what kind of science teacher you have, but my first go would be to ask the teacher! They know it better than us all. hmmmhmmh

1. There are teachers at chemicalforums.
2. Teachers can be wrong too.

But asking them usually not hurts.

[mike]

2. Teachers can be wrong too.

We can??

 

[Borek]

This was asked by teacher:

http://www.chemicalforums.com/index.php?topic=8324

[Donaldson Tan]

HOCl is the active ingredient that actually disinfects water. The concentration of HOCl must be beyond a critical value for it to be able to disinfect water effectively.

If the pH of water is lower, then the backward reaction is favoured, thus the equilibrium shifts to the left. This means more Cl₂ is needed to push the equilibrium to the right to achieve the minimum level of HOCl concentration to effectively disinfect the water.

[Borek]

Interesting. I have always thought Cl₂ from hydrolyzis is active disinfectant.

Do you have any reference?

[Donaldson Tan]

HOCl is a far stronger oxidising agent than OCl^- and HOCl is the major chloro- component in chlorinated water, not chlorine. Chlorination is the preferred method of water disinfection because chlorine stays in the treated water long enough to prevent enroute re-infections while the water is transported to the consumers.

[Unknown discoverer]

well....it is an entersting question..

chlorine added to water for destroying bacteria and any biological growth...it is added in small amount the range will be from 0.3 ppm min. to  1.0 ppm max  ...so the formation of these chemical compound are minor  and effect to health is law..but if chlorine added in high concentration it will make the water unsafe to drink and makes stomic problems..water must have a PH=7.0 +/- 0.2 to be good for body metabolism...

thanks .

[cuongt]

thanks a lot guys for all the help
i appreciate it