Acid and bases

bonkysleuth SG

Copper(II) carbonate is the main ingredient used to produce algaecide. An algaecide is a chemical treatment for swimming pools and fish ponds that kills algae.

Suggest with a plausible reason whether the algaecide will be able to work effectively to remove algae if the fish pond is found to have a pH of 4.

I have some doubts that need some fixing over here. Firstly, I realise many questions ask about whether carbonates can be used in acidic medium to get rid of something (let's say calcium carbonate. If CaCO3 is added to acid rain, does it increase the pH and make it less acidic?) So the general question is, are carbonates used to increase alkalinity of substances?

I trawled the net for more information regarding this and sad to say, the results yielded are mostly copper sulphate is usually used for algae control. So may I know whether copper(II) carbonate works here? I think it should be yes since carbonate reacts with acid to give water, CO2 and a salt. So, ultimately, the acidity is decreased to become more alkaline.

Please do assist!

UltimaOnline SG

Carbonates react with acids as follows :

2H+ + CO3 2- --> CO2 + H2O.

Hence, the copper(II) carbonate in the algaecide would be reacted away (in the form of CO2) and hence in acidic pH (eg. pH 4) would not be able to remain in carbonate form to carry out its function as a algaecide.

The Qn stated that copper(II) carbonate is an ingredient. It doesn't require you to explain why it is helpful as an ingredient, in terms of how it kills algae. It only asks you to consider what reaction the carbonate might undergo at pH 4.

>>> If CaCO3 is added to acid rain, does it increase the pH and make it less acidic?) So the general question is, are carbonates used to increase alkalinity of substances? <<<

Again, as I've explained above, carbonates increase pH (only up to neutral pH, if sufficient carbonate is present) by reacting with, and hence removing, H+ ions from acids. That's all. It doesn't cause pH to go above neutral pH (which is pH 7 for room temperature).

>>> So, ultimately, the acidity is decreased to become more alkaline. <<<

Rephrase it to say it becomes less acidic, rather than more alkaline. It cannot make the pH alkaline (above neutral pH), only less acidic (up to neutral pH).