# Chem A-level multiple choice question- help neededWatch

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#1
Attachment 736796

how is it A? The energy level diagram is for an exothermic reaction and the activation energy is B, and we know that catalysts lower the activation energy so B would be altered, right?

and this one i have no clue

help would be greatly appreciated, thanks!
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1 year ago
#2
(Original post by Mark jam)
Attachment 736796

how is it A? The energy level diagram is for an exothermic reaction and the activation energy is B, and we know that catalysts lower the activation energy so B would be altered, right?

and this one i have no clue

help would be greatly appreciated, thanks!
Q14 should be B so I think they made a mistake.

Q18 - Think about how the rate depends on the concentration of H2 and NO. You can write an expression for the rate as rate=k[H2][NO]^2 where k is a constant. So if you triple the H2 concentration the rate will also triple as the reaction is 1st order wrt H2. If you triple the NO concentration the rate will increase by a factor of 3^2=9 as the reaction is 2nd order wrt NO. Overall, the rate increases by a factor of 9*3 = 27 which is D.
0
1 year ago
#3
it's B, i think its just a mistake
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#4
(Original post by Anonymouspsych)
Q14 should be B so I think they made a mistake.

Q18 - Think about how the rate depends on the concentration of H2 and NO. You can write an expression for the rate as rate=k[H2][NO]^2 where k is a constant. So if you triple the H2 concentration the rate will also triple as the reaction is 1st order wrt H2. If you triple the NO concentration the rate will increase by a factor of 3^2=9 as the reaction is 2nd order wrt NO. Overall, the rate increases by a factor of 9*3 = 27 which is D.
in the equilibrium constant, isnt h2 to the power of 2 as well?

Also, what does it mean by " first order" and "second order"
Thanks!
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1 year ago
#5
(Original post by Mark jam)
in the equilibrium constant, isnt h2 to the power of 2 as well?

Also, what does it mean by " first order" and "second order"
Thanks!
The question has nothing to do with the equilibrium constant. It is to do with rate equations.

Basically the if the rate of reaction is first order with respect to a reactant then rate is proportional to the concentration of that reactant.
If the rate of reaction is 2nd order with respect to a reactant, then the rate is proportional to the square of the concentration of that reacant.

To generalise if the rate of reaction is nth order wrt a reactant, then the rate is proportional to [R]^n where R is that reactant.
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