Practical investigations
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I'm currently writing up a practical on the clock reaction between iodide and peroxodisulphate ions.
Five different catalysts were used and I'm a little confused as some of them appear to have acted as inhibitors. The metal ions used were Copper(II), Iron(II), Iron(III), Zinc(II) and Magnesium(II). can you please list the activaton energies of each of these reactions. Thank you in advance for your help.
I'm not surprised that some of your metal ions, such as magnesium, acted as inhibitors. Did they actually inhibit the reaction, that is make it slower, or just not have any effect at all? It will be difficult to find activation energies for them all.
In this redox reaction between iodide and peroxodisulphate the catalyst changes oxidation state, which is why transition metal ions (note Zn is a d-block metal but strictly NOT a transition metal) are useful.
In its higher oxidation state, e.g. Fe3+, the metal ion must be a strong enough oxidising agent to oxidise iodide ions to iodine. In its lower oxidation state, e.g. Fe2+, the metal ion must be a strong enough reducing agent to reduce peroxodisluphate to sulphate ions. This can be explained in more detail using the electrode potentials of the species involved, but I won't go into that now.
Thus the metal ion catalyst acts as an electron carrier between iodide and peroxodisulphate, shuttling between its two oxidation states. The reaction involving the metal ions as a catalyst has a lower activation energy because it involves the collision of ions of opposite charge, rather than two negative ions colliding.
In a previous answer to a similar question the uncatalysed reaction is said to have an activation energy of 53 kJ/mol, whereas that involving iron ions has an activation energy of 35 kJ/mol. I can't find the value for Cu2+ ions, and Zn2+ and Mg2+ ions will not be catalysts here.
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updated: 02 March 2007
