Clock Reactions Flashcards
Iodine clock reactions
2 successive reactions occur and an endpoint (colour change) is achieved when a limiting reactant runs out, leaving excess of a reactant
Reaction 1
Hydrogen peroxide + iodine ions (from potassium iodide) + H+ ions (acidic conditions)
Makes water and iodine
Reaction 1: important product
Produces iodine
What does iodine produced from reaction 1 do?
React with thiosulfate ions (from sodium thiosulfate)
To start reaction 2
Reaction 2
Thiosulfate ions + iodine
Makes iodide ions and S4O8- ions
Iodide ions will…
Immediately react with hydrogen peroxide and hydrogen ions in reaction 1
So the iodide ions made from reaction 1 will immediately be recycled to reaction 2
When does the colour change occur?
When the sodium thiosulfate ions in reaction 2 run out
Reaction 2 cannot start
So the iodine can remain in solution instead of immediately reacting
Colour change
Iodine remaining in solution can react with starch to change colour to blue black
How can we assure reaction 1 will occur constantly so we can made iodine again from the iodide ions?
Have hydrogen peroxide be in excess
What are we measuring when we do this?
Time taken for colour change to occur
What are we measuring when we do this?
Time taken for colour change to occur
What is initial rate of reaction proportional to?
1/time taken for a colour change
When can we use the initial rate of reaction method?
By finding the rate of reaction when time = 0 on a continuous monitoring method
By using clock reactions
How is Rate of reaction measured?
The time taken for a fixed amount of thiosulfate ions to run out so time taken for iodine to be produced, as a fixed amount of iodine reacts with the thiosulfate
What is kept constant?
A fixed amount of thiosulfate ions
How is this experiment repeated?
Vary the concentration of iodide ions/ acid catalyst/ hydrogen peroxide but keep other reactants constant to measure rate of reaction changes with respect to the specific reactabt
How do we set up this reaction?
Use measuring cylinders to measure our required volumes of starch, iodide ions, thiosulfate ions and water
Then, once ready add the required volume of hydrogen peroxide
