[chloe]

can anyone answer this question for me ?
Consider this prototypical nucleophilic substitution CH3Br + -OH --> CH3OH + -Br
the effect of doubling the vol. of solvent would be to multiplly the reaction by a factor of
A) 1/4  B) 1/2  C) 2  D) 4
the answer is 1/4 but why??

[Yggdrasil]

What's the rate equation for an SN2 reaction?

[chloe]

What's the rate equation for an SN2 reaction?

rate= K[substituent][nucleophile]

[Padfoot]

How will the concentration terms change?

[chloe]

How will the concentration terms change?

what do you mean by the conc. term ? the volume of the solvent is double, so of course the rate of reaction is going to change too. i just don;t know how and why by a 1/4 but not 2.....when you are doubling....

[Yggdrasil]

Concentration is moles/volume.  If you double volume, you half the concentration.  Since the rate depends on two concentrations, and each concentration is halved, the rate is 1/4 the original rate:

[CH3Br]_new = (1/2)[CH3Br]
[OH-]_new = (1/2)[OH-]
Rate = k[CH3Br]_new[OH-]_new=k(1/2)[CH3Br](1/2)[OH-] = (1/4)k[CH3Br][OH-]

[chloe]

Concentration is moles/volume.  If you double volume, you half the concentration.  Since the rate depends on two concentrations, and each concentration is halved, the rate is 1/4 the original rate:

[CH3Br]_new = (1/2)[CH3Br]
[OH-]_new = (1/2)[OH-]
Rate = k[CH3Br]_new[OH-]_new=k(1/2)[CH3Br](1/2)[OH-] = (1/4)k[CH3Br][OH-]

thank you so much Yggdrasil for answering, it really helped .