Rates of reaction - kinetics (A2)
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I'm doing an iodine clock reaction as my A2 coursework piece to study the rate of reaction between hydrogen peroxide and iodide ions. Now, according to the method, potassium chloride is used as one of the reactants, "so that the ionic strength of the various solutions may be kept at a fairly constant level."
What does this mean? Could someone explain it in simple terms.
280708
Corrie writes ....
I doubt you would need to get involved with ionic strength in an A-level investigation, but here is a brief explanation of what it is about.
An ion in solution, although 'free', does not exist in isolation from the other ions present in the solution. In particular, a (+) ion would tend to to have more (-) ions in its vicinity, and vice versa. A given ion therefore experiences an 'ionic atmosphere', which will depend on the concentration of other ions in the solution, and to some extent their nature, i.e. singly - or doubly-charged. Ionic strength is a measure of the intensity of the electrical field experienced by an ion due to other ions in the solution, and is defined by a mathematical equation involving the concentrations of all the ions present and their charges.
When, in doing rate studies for instance, the concentration of one of the reactant ions is varied to obtain information about order and rate constants, the 'ionic atmosphere' changes. So the effect on rate observed may be partially due to the change in this 'atmosphere' - an ion will react at a different rate if it is surrounded by lots of other ions, or only a few. A footballer may score more easily if he/she is surrounded by only a few members of the opposing team than if surrounded by lots of them!
So, to eliminate the effect changing a concentration may have on the 'ionic atmosphere', additional, non-reacting ions may be added to maintain the same 'ionic atmospere' (ionic strength) in the solution for the reacting ions. In your reaction K+ and Cl- ions from added KCl fulfil this purpose.
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updated: 28 July 2008
