Click on slide to start movie; here to stop.
Mix equal volumes of A and B by pouring
back and forth into beakers three times. The
reaction starts on the lst pour and ENDs when
the solution turns black….therefore the name
CLOCK REACTION…it tells you how long
the reaction takes from start to finish!
Click on slide to start movie; here to stop.
What would happen
if we did the same
reaction but didn’t
use any starch. Starch
turns iodine black.
1. What about iodine…what does it look like?
What if you dissolve some in water?
2. Where can you get starch?
3. What happens when iodine comes in contact
with starch?
Know what do you expect to
happen IF the two solutions A
and B in the clock reaction don’t
have any starch in them?
1. Will a reaction still happen?
2. Will the result be black?
3. What will the result look like?
Click on slide to start movie; border to stop
What would
happen to the time
for the reaction if
we diluted one
of the reactants?
What would
happen to the time
for the reaction if
we heated one or
both solutions?
Solutions for the iodine clock reaction:
A. 4.3 g KIO3 per liter
B. 0.2 g Na2S2O3, 4 g soluble starch and 5 ml
concentrated sulfuric acid.
You can slow the reaction down by diluting A and
speed it up by adding more sulfuric acid to B.
You can use cornstarch, put 1 tsp in 200 ml water
and heat to boil in microwave….let cool and add
to B. You can use a big pinch of A, small pinch of B
throw in a little sulfuric acid in B and some starch.
Then adjust to suit you….
The Reaction
Mix
no
1
click
2
3
Any HSO4left?
4
yes
1. IO3- + 3HS03- I- +3SO4-2 + 3H+
- + 6H++IO35
I
3 I2 (aq)
+ 3H20
2.
3. I2 +HSO3- + H2O
2I- + SO4-2 + 3 H+
4. I2 + starch == blue solution
I2 solution appears yellowish without starch.