# Reaction rates MCQ's Help!

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#1
I'm struggling on how to answer these:

1) https://imgur.com/0foFP1P
2) https://imgur.com/N6dU1Xe

For 1) I know it is true that benzenesulphonic acid plays a part in affecting the rate of the reaction even if it not a reactant itself but a catalyst instead so the rate equation will look something like this...

rate = k [CH3COCH3]^a [HOCH2CH2OH]^b [H+] ^1, where a and b are the orders with reactants propanone and ethane-1,2-diol respectively

I know definitely that the order with respect to the catalyst is first order because it mentions when the concentration of acid doubles then rate also doubles too which shows a directly proportional relationship.

I don't really know if there is enough information to work out what the overall order of the reaction is if we have 2 unknown orders a and b which I don't know. Is there a a way to calculate it or not?

I think the answer could be option c as there is a catalyst being used i.e. benzenesulphonic acid

With option D I don't know if I can find the rate constant units since I don't know the values of order a and b but only the order for the [H+]

For 2) I'm not sure how to work out the concentration of P and it's Ph

Do I need to use the formula Ph = - log [H+] for this?

Any help will be very grateful! Thanks!
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1 year ago
#2
(Original post by Yatayyat)
I'm struggling on how to answer these:

1) https://imgur.com/0foFP1P
2) https://imgur.com/N6dU1Xe

For 1) I know it is true that benzenesulphonic acid plays a part in affecting the rate of the reaction even if it not a reactant itself but a catalyst instead so the rate equation will look something like this...

rate = k [CH3COCH3]^a [HOCH2CH2OH]^b [H+] ^1, where a and b are the orders with reactants propanone and ethane-1,2-diol respectively

I know definitely that the order with respect to the catalyst is first order because it mentions when the concentration of acid doubles then rate also doubles too which shows a directly proportional relationship.

I don't really know if there is enough information to work out what the overall order of the reaction is if we have 2 unknown orders a and b which I don't know. Is there a a way to calculate it or not?

I think the answer could be option c as there is a catalyst being used i.e. benzenesulphonic acid

With option D I don't know if I can find the rate constant units since I don't know the values of order a and b but only the order for the [H+]

For 2) I'm not sure how to work out the concentration of P and it's Ph

Do I need to use the formula Ph = - log [H+] for this?

Any help will be very grateful! Thanks!
1. Orders wrt to CH3COCH3 and HOCH2CH2OH cannot be determined
we can only deduce that the answer is C

2. Yes. Rate = k[P]^2 x 10^-pH
Last edited by username3249896; 1 year ago
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#3
(Original post by BobbJo)
1. Orders wrt to CH3COCH3 and HOCH2CH2OH cannot be determined
we can only deduce that the answer is C

2. Yes. Rate = k[P]^2 x 10^-pH
Okay so for Q 1 it is C but how did you know 'Rate = k [P]^2 x 10^-pH'

How did you exactly come to that statement? And with what numbers are we plugging in to find [P] and pH?

EDIT: Nevermind, I see that you just had to rearrange for [H+] in pH = - log[H+] where [H+] turns out to be equal to 10^-pH, then substitute that into the given rate equation which was rate = k [P]^2 [H+]

But how do we know what the rate constant is to be to find out which is the slowest reaction?
Last edited by Yatayyat; 1 year ago
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#4
(Original post by BobbJo)
1. Orders wrt to CH3COCH3 and HOCH2CH2OH cannot be determined
we can only deduce that the answer is C

2. Yes. Rate = k[P]^2 x 10^-pH
Okay so for 2. I realised that the rate constant doesn't really matter here if we are trying to find what the slowest reaction could be since multiplying by the same constant for every answer shouldn't affect which rate is the biggest or smallest value.

I have performed the calculations for ever single option but neglected the rate constant and this doesn't affect which rate is the slowest.

Turned out to be that option C has the smallest value i.e. is the slowest rate. While option A, B and D all gave the same rate as each other.

So is the answer C then?
0
1 year ago
#5
(Original post by Yatayyat)
Okay so for 2. I realised that the rate constant doesn't really matter here if we are trying to find what the slowest reaction could be since multiplying by the same constant for every answer shouldn't affect which rate is the biggest or smallest value.

I have performed the calculations for ever single option but neglected the rate constant and this doesn't affect which rate is the slowest.

Turned out to be that option C has the smallest value i.e. is the slowest rate. While option A, B and D all gave the same rate as each other.

So is the answer C then?
it says same conditions & same temperature so rate constant is constant
hence
A rate = 0.01k
B rate = 0.01k
C rate = 0.009k
D rate = 0.01k
hence C
so yes
0
#6
(Original post by BobbJo)
it says same conditions & same temperature so rate constant is constant
hence
A rate = 0.01k
B rate = 0.01k
C rate = 0.009k
D rate = 0.01k
hence C
so yes
Thank you!
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