Chemistry question help please Ka

Hi, please could I have some help on question 5a of this paper. I’m really confused about Ka, I thought water will be part of the expression because it is reacting in it and in the paper I did before, water was included but now it isn’t?
Paper: https://pmt.physicsandmathstutor.com/download/Chemistry/A-level/Past-Papers/CAIE/Paper-4/QP/November%202016%20(v2)%20QP.pdf
Thanks!

Hi, please could I have some help on question 5a of this paper. I’m really confused about Ka, I thought water will be part of the expression because it is reacting in it and in the paper I did before, water was included but now it isn’t?
Paper: https://pmt.physicsandmathstutor.com/download/Chemistry/A-level/Past-Papers/CAIE/Paper-4/QP/November%202016%20(v2)%20QP.pdf
Thanks!

As with most equilibrium constants, [H2O] can be disregarded because water in this case is a (virtually) pure liquid. At A level and equivalent, the explanation for why this is the case is simply that [H2O] changes very little during the reaction and so can be disregarded. In reality, it’s because equilibrium constants are based on a quantity related to concentrations called the activity. In aqueous solutions, the activity of water is (very close to) 1, since it’s an almost pure liquid but the activities of all other species are well approximated by their concentrations.
If an equilibrium constant does include the concentration of water, it’s usually because the water is part of a mixture of liquids and so is not pure and so its concentration does change significantly. This is best exemplified with esterification reactions where a carboxylic acid and alcohol react. This is never the case with Ka problems, though since the water molecules always far outnumber the acid molecules.

Thank you, I just have one question. So if water is reacting, then it would be included in the kc expression but this is the never the case for a Ka expression?

More often than not, [H2O] does not show up in Kc expressions. The only time it shows up would be in an instance where its concentration is likely to change very significantly (e.g you mix 1.00 g of water and 2.00 g of ethyl ethanoate and leave it for a while). The biggest telltale sign is if there are multiple species in the equation whose state symbols are (l) (e.g H2O (l) + HCOOCH3 (l) <=> HCOOH (l) + CH3OH (l) or similar).

In question 7d of this paper https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FChemistry%2FA-level%2FPast-Papers%2FCAIE%2FPaper-4%2FQP%2FNovember%202016%20(v1)%20QP.pdf it was included. Is it because it was reacting and the conc has changed significatnly?

In general, I would expect not - trichlorocyanuric acid isn't likely to be added to any amount of water in large quantities and so the water molecules should vastly outnumber the molecules of anything else in there. Even I as a second year chem undergrad would have got that question wrong in an exam simply because it isn't intuitive to include water in this particular Kc expression.
However, based on the stoichiometric ratio (e.g for each mole of trichlorocyanuric acid that reacts, three moles of water are also consumed) and that there is no indication of the amounts of reagents used, it is possible that [H2O] does change significantly. My suspicion is that this is the line of reasoning they wanted you to use, but it's well worth checking newer papers (along with the mark schemes and examiner's reports) to see if anything similar comes up and what answers they expect.