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Posted by Ryan On April - 7 - 2009 0 Comment

A double replacement reaction is a type of aqueous reaction where two ionic compounds are dissolved in water allowing the aqueous ions from one compound to collide and react with the aqueous ions from the other compound.

These reactions are beautiful, but they are toxic and silver can be expensive. I show these reactions to my intro chemistry class as a demonstration. This minimizes waste, expense, and the handling of these chemicals by my students with limited laboratory experience. I do let my advanced chemistry students do this in a predicting chemical reaction laboratory activity.

In this video, solutions of potassium chromate and silver nitrate are mixed, producing a red silver chromate precipitate. In the second reaction, potassium iodide and lead (II) nitrate are mixed, producing a yellow lead iodide precipitate. To predict these reactions, it is recommended that you use your solubility rules. Remember that all nitrates and alkali metals (such as potassium) are soluble. The nitrate and alkali metals are considered spectator ions in the reaction.

Double replacement reactions:

Reaction 1: Potassium Chromate and Silver Nitrate

  • Complete Molecular Reaction: K2CrO4(aq) + 2AgNO3(aq) ⇌ Ag2CrO4(s) + 2KNO3(aq)
  • Net Ionic Reaction: CrO42+(aq) + 2Ag+(aq) ⇌ Ag2CrO4(s)

Reaction 2: Potassium Iodide and Lead (II) Nitrate

  • Complete Molecular Reaction: 2KI(aq) + Pb(NO3)2(aq) ⇌ PbI2(s) + 2KNO3(aq)
  • Net Ionic Reaction: 2I-(aq) + Pb2+(aq) ⇌ PbI2(s)

Note:  There are a number of teachers and websites  that will use a single reaction arrow to describe a double replacement reaction, but please remember that you will revisit this concept again as Ksp in the equilibrium chapter.

Categories: Chemical Reactions, Videos

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