[f1987]

Studying for my Physical chemistry exam at uni and having a few problems with the following questions, any help appreciated. Thanks.

1. The influence of the ionic strength on the rate constant of a reaction in aqueous solution is called the kinetic salt effect.  State giving reasoning whether the reaction rate of the folloing reaction will increase or decrease if you increase the ionic strength of the solution;
H202(aq) + 2H+(aq) +2Br-(aq) -> 2H20 + Br2(aq)

2. Make a sketch and write down the cell reaction and Nernst equation for the chlorine electrode (first bit I can do fine) Calculate by how much the potential of this electrode changes if the concentration of the chloride ions is increased by a factor of 2.

[oceane]

1.  The kinetic salt effect is about seeing if the transition state is more or less charged than the reactants.

Adding a salt increases the ionic strength of the solution & so stabilises charged species.  If your reactants are more charged, they'll be more stabilised (lower in Gibbs energy) than your transition state & so the activation energy is larger. If the transition state is more charged, then it's more stabilised compared to the reactants, so activation energy is lower.

Lower activation energy equals a faster rate.

have a look at the mechanism of the reaction to see which is the rate determining step.  Is charge being built up or reduced in the transition state (this could take the form of the same charge but spread over a number of centres)

If you want this in quantitative form, you need to use the Debye-Huckel expression for activity and put that into the rate equation.

2. Nernst equation: Ecell = E0 - (RT/nF) ln a

where a = activity of species, alternatively can use concentration as an approximation.