Clock Reactions

Most chemical reactions proceed at a comfortable rate, gradually changing from on set of materials into other materials. Chemists refer to these as reactants (starting species) and products (final species.)

Usually we see a reaction mixture slowly changing from one form to another.

For example, if we put some copper metal into dilute nitric acid

• the solution starts colorless
• we start to see bubbles form on the copper
• the solution becomes faintly blue
• the gas above the solution is faintly brown
• as reaction proceeds the colors become more intense
• the copper metal slowly disappears and the bubbles slow down and stop
• the blue (Cu⁺² in solution) becomes more intense

Titrating an acid is rather different

• we start with a solution of the acid (colorless)
• we add an acid-base indicator (phenolphtalein)
  • in an acid solution this is colorless
• as we add base (sodium hydroxide) a reaction occurs
  • the acid is slowly used up
  • but, until it's all consumed, the solution stays acidic
  • solution remains colorless
• finally we react the point where we've used up all the acid
  • now the added base makes the solution basic or alkaline
  • in a basic solution, phenolphthalein in red
• at this point, one or two drops of base causes the color to change abruptly.

Clock reaction #1

We mix together several species

• Iodide ion I-
• Hydrogen Peroxide, H₂O₂
• solution also contains starch
  • Iodide and H₂O₂ react to produce I₂
  • Normally the I₂ reacts with starch and the solution turns dark blue
• but we also add a little thiosulfate ion
  • this reacts with I₂ and quickly removes it
  • thus, the solution remains colorless
• eventually, we run our of thiosulfate ion
  • now, the iodine collects and reacts with the starch
  • the color turns dark blue in less than a second
  • but it may take 15, 30, 45 seconds or more before the change occurs

showmanship:

• use several flasks, vary the amount of thiosulfate
• the color changes occur in as equence, one flask after another

Clock reaction #2-- the Oscillating Clock

• Several solutions are mixed
  • hydrogen peroxide, H₂O₂
  • potassium iodate (KIO₃)
  • starch
  • a catalyst (Manganese Ion)
  • a catalyst speeds up a reaction
  • an acid, sulfuric
  • and malonic acid
• Each solution is colorless and the mixture starts colorless
  • in a short time (15-30 seconds) a golden color forms
  • this is I2 produced from H₂O₂ and IO₃- ion
    • the starch stays colorless because that reaction needs both I- (Iodide) and Iodine (I2)
• Slowly, the I₂ begins to react with malonic acid
  • one of the products is I- ion
  • after about 45 seconds we have both I- and I2 and the solution turns blue
  • But the reaction continues to use up the Iodine
  • also, H₂O₂ begins to use up the I-
    • So, no I2 and no I- so solution turns colorless
• But we still have most of the initial reactants
  • so reaction starts over again
    • We'll see this color cycle repeat every 30-45 seconds
    • We often see 30-50 cycles before the solutions run out of ingredient