Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
hydrochloric acid : HCl
hydrogen peroxide : H2O2
potassium dichromate : K2Cr2O7
Class:
------
elem=Cr
coordination
redox
Summary:
--------
Dichromate reacts with hydrogen peroxide, also when no acid is present.
An initial compound is formed quickly, which decomposes slowly, releasing
heat. Due to heating up, the reaction proceeds faster and faster, until
all of the peroxide has been used up.
Description:
------------
Add some hydrogen peroxide (30% by weight) to a solution of K2Cr2O7: The
liquid becomes dark grey/purple, almost black. This change is immediate.
After a while, the liquid starts bubbling and evolution of a gas can be
observed. Initially the evolution is slow, but its speed gradually increases.
The liquid also becomes warmer during this reaction. At a certain point in
time, the evolution of gas is so fast, that the liquid starts foaming.
Apparently an equilibrium state is reached (production of heat from the
reaction and loss of heat to the environment) and the evolution of the gas
proceeds at a fairly constant rate. After a while, evolution of the gas
ceases fairly quickly and the liquid becomes lighter. If evolution of gas
has ceased completely, then the liquid is brown/orange. The liquid does not
attain its original bright orange color anymore.
Add some hydrochloric acid (appr. 10% by weight): A little amount of gas
is produced again and the liquid becomes much lighter. It becomes orange,
with a little brown hue.
Add some hydrogen peroxide (30%) again: Liquid becomes dark blue, foams
violently and part of it is ejected from the test tube. When the evolution
of gas ceases again, then the remaining part of the liquid has become green.