Ignition of ammonium dichromate results in copious sparks, green solid, and smoke

Decomposition of Ammonium Dichromate

Purpose To observe a decomposition

reaction and its products

Demonstration

The reactant was stable in the open air

The reaction required heat to start it

The reaction generated much heat and only become self-sustaining above 225 degrees Celsius

The chromium(VI)oxide product weighed only 9 grams but had a high volume

Concepts

1. Decomposition Reactions

2. Redox Reactions

3. Heats of Reaction

4. Chemical Changes

1. Decomposition Reactions A single substance gives more than a

single substance upon reaction

ABC A + B + C

(NH4)2Cr2O7(s) N2(g) + 4H20(g) + Cr2O3(s)

May be exothermic (give off heat)

or endothermic (require heat)

This demonstration is exothermic!

2. Redox Reactions In a redox reaction, one substance loses electrons

while another substance gains electrons The substance that loses electrons is said to be oxidized and/or

the reducing agent The substance that gains electrons is said to be reduced and/or

the oxidizing agent

In this demonstration ammonium dichromate is both oxidizing agent and reducing agent

(NH4)2Cr2O7(s) N2(g) + 4H20(g) + Cr2O3(s)N: Changes from 3- to 0. OxidizedCr: Changes from 6+ to 3+ ReducedO and H: No change

3. Heats of Reaction The amount of heat liberated per mole of

substance Heat of reaction for ammonium dichromate

is 315 kJ/mol where kJ stands for ‘kilojoules’

This demonstration uses 15g Heat liberated in this demonstration:

grams/molecular weight= moles moles X heat of reaction= heat

Therefore: 15/252.10 X 315 =19 kJ = 4541cal

4. Chemical Changes

Chemical Changes include any changes in which a new substance is produced

The ammonium dichromate underwent a chemical change in this demonstration and formed three different substance

Conclusions Inorganic compounds can be

decomposed into simpler substances

The water and nitrogen from this reaction were released into the atmosphere

The reaction was a redox reaction

in which nitrogen is oxidized and chromium is reduced

Comments Applications of ammonium

dichromate lithography porcelain finishes pigment manufacture recording materials

Applications of chromic oxide abrasives pigment in printing banknotes

All chromium compounds are dangerous to health