Rates of reaction - kinetics (A2)
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What is the proposed mechanism for the reaction between hydrogen peroxide and iodide ions in acid conditions?
Igloo writes
A simplified version of the mechanism is:
Step 1: H2O2 + I- -> H2O + IO- (SLOW)
Step 2: H+ + IO- -> HIO (FAST)
Step 3: H+ + I- + HIO -> I2 + H2O (FAST)
Since the rate determining step is Step 1 and this involves 1 molecule of hydrogen peroxide colliding with 1 iodide ion, you can see why this reaction is first order with respect to each of these species, and second order overall. Since the hydrogen ions do not become involved in this “slow” step, the reaction is zero order with respect to H+.
Rate = k[H2O2]1[I-]1[H+]0 = k[H2O2][I-]
The overall reaction is usually written as:
H2O2 + 2H+ + 2I- -> 2H2O + I2
You may like to know that this reaction is sometimes referred to as the Harcourt-Essen reaction. If you want to take this further you may well be able to find more information on the internet. For example try using “Harcourt Essen Reaction” as a search phrase in www.google.co.uk or www.google.com
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updated: 09 November 2004
