Decomposition of Ammonium Dichromate

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

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 (NH 4 ) 2 Cr 2 O 7 (s) N 2 (g) + 4H 2 0(g) + Cr 2 O 3 (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 (NH 4 ) 2 Cr 2 O 7 (s) N: Cr: O and H: N 2 (g) + 4H 2 0(g) + Cr 2 O 3 (s) Changes from 3- to 0. Oxidized Changes from 6+ to 3+ Reduced 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