10.1 - Reversible Changes

• non-reversible changes

  only go in one direction

  e.g.: baking bread, burning fuel (combustion)
• reversible changes

  a process that can be made to go in either direction by changing the conditions

  e.g.: haemoglobin binding to oxygen, hydrated cobalt (II) chloride to anhydrous cobalt (II) chloride
• showing reversible change using cobalt (II) chloride

  heat pink hydrous cobalt (II) chloride to form blue anhydrous cobalt (II) chloride

  add water to reverse
• test for water using cobalt (II) chloride

  blue cobalt chloride paper turns pink when exposed to water
• ammonium chloride

  NH3 + HCl -> NH4 (white smoke)

  reverse by heating but do it in a fume cupboard because HCl is toxic
• observations when heating ammonium chloride

  NH3 diffuses across cotton wool faster turning red litmus paper blue then HCl turns litmus paper red

  a white smoke of NH4Cl appears above the tube as gases meet and cool
• reversible reaction between iron and steam

  3Fe(s) + 4H2O(g) -> Fe3O4(s) + 4H2(g)

  forward reaction happens when concentration of steam is high and hydrogen is swept away, keeping its concentration low

  backward reaction happens when concentration of hydrogen is high and steam is swept away, keeping its concentration low