Define a mechanism use the material below or starters for 10. Also use the the iodination of propanone as context.Note some of the video footage from MEA_EU_01_vid maybe useable here too or th data collected.

In nucleophilic substitutions of R-Br to give R-OH, there are two possible mechanisms:
Either [A]: the R-Br bond first breaks giving R+, followed by attack of
     OH- ions to give the product R-OH.
Or [B]: as the R-Br bond breaks, a new bond simultaneously forms
      with the attacking species OH-.
The two mechanisms have different rate equations. Links to organic chemistry

This comparison could be shown using molecular models to emphasise the difference.
[A] The breaking of the covalent bond from R (an alkyl group) to the halogen (bromine) is the slow step here. The second part of the reaction is between two ions, a carbocation R+ and the anion OH-, and is faster as there are most reactions between ions when compared with cleavage of covalent bonds. Changing the concentration of the anion will not affect the rate since it is only involved in the fast step. 

This could make a good anaimation??

The slowest step is rate-determining. 
[B] Here both species, the original R-Br molecule and the anion OH-, are together involved. Changing the concentration of either will necessarily affect the rate.