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A2 level question regarding rate equation of ester hydrolysis

My question comes from the AQA june 2011 unit 4 chemistry paper (q5,e,i)

"Hydrolysis of [ester] in an excess of water was found to follow first order kinetics.
Write a rate equation for this hydrolysis reaction using [ester] to represent the ester."

My initial answer was rate = k[ester][h2o]

but the actual answer is rate = k[ester]

Why is water not included in this rate equation? My only ideas could be that the fact that it is in excess causes it to not affect the rate, or perhaps that it is a catalyst so doesn't appear. Can anyone help me understand why?

Thank you
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Borek
4
If it is in excess its concentration doesn't change (is constant) so you won't find any relationship between reaction speed and water concentration.
Original post by mintsponge
My question comes from the AQA june 2011 unit 4 chemistry paper (q5,e,i)

"Hydrolysis of [ester] in an excess of water was found to follow first order kinetics.
Write a rate equation for this hydrolysis reaction using [ester] to represent the ester."

My initial answer was rate = k[ester][h2o]

but the actual answer is rate = k[ester]

Why is water not included in this rate equation? My only ideas could be that the fact that it is in excess causes it to not affect the rate, or perhaps that it is a catalyst so doesn't appear. Can anyone help me understand why?

Thank you


Well they have told you the order with respect to the ester is one and haven't mentioned anything about the order with respect to water .That implies the concentration of the ester is the only thing affecting rate thus r=k[ester]^1
The catalyst in this reaction is a strong acid catalyst .


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