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Types of reactions

We will look at three types of reactions that occur in aqueous solutions. These are precipitation reactions, acid-base reactions and redox reactions. Precipitation and acid-base reactions are sometimes called ion exchange reactions. Redox reactions are electron transfer reactions. It is important to remember the difference between these two types of reactions. In ion exchange reactions ions are exchanged, in electron transfer reactions electrons are transferred. These terms will be explained further in the following sections.

Ion exchange reactions can be represented by:

AB(aq) + CD(aq) AD + CB
Either AD or CB may be a solid or a gas. When a solid forms this is known as a precipitation reaction. If a gas is formed then this may be called a gas forming reaction. Acid-base reactions are a special class of ion exchange reactions and we will look at them seperately.

The formation of a precipitate or a gas helps to make the reaction happen. We say that the reaction is driven by the formation of a precipitate or a gas. All chemical reactions will only take place if there is something to make them happen. For some reactions this happens easily and for others it is harder to make the reaction occur.

Ion exchange reaction
A type of reaction where the positive ions exchange their respective negative ions due to a driving force.

Interesting fact

Ion exchange reactions are used in ion exchange chromatography. Ion exchange chromatography is used to purify water and as a means of softening water. Often when chemists talk about ion exchange, they mean ion exchange chromatography.

Precipitation reactions

Sometimes, ions in solution may react with each other to form a new substance that is insoluble . This is called a precipitate .

Precipitate
A precipitate is the solid that forms in a solution during a chemical reaction.

Demonstration : the reaction of ions in solution

Apparatus and materials:


4 test tubes; copper(II) chloride solution; sodium carbonate solution; sodium sulphate solution

Method:


  1. Prepare 2 test tubes with approximately 5 ml of dilute Cu(II) chloride solution in each
  2. Prepare 1 test tube with 5 ml sodium carbonate solution
  3. Prepare 1 test tube with 5 ml sodium sulphate solution
  4. Carefully pour the sodium carbonate solution into one of the test tubes containing copper(II) chloride and observe what happens
  5. Carefully pour the sodium sulphate solution into the second test tube containing copper(II) chloride and observe what happens

Results:


  1. A light blue precipitate forms when sodium carbonate reacts with copper(II) chloride
  2. No precipitate forms when sodium sulphate reacts with copper(II) chloride

It is important to understand what happened in the previous demonstration. We will look at what happens in each reaction, step by step.

  1. Reaction 1: Sodium carbonate reacts with copper(II) chloride.
    When these compounds react, a number of ions are present in solution: Cu 2 + , Cl - , Na + and CO 3 2 - . Because there are lots of ions in solution, they will collide with each other and may recombine in different ways. The product that forms may be insoluble, in whichcase a precipitate will form, or the product will be soluble, in which case the ions will go back into solution. Let's see how the ions in this example could have combined with each other:
    Cu 2 + + CO 3 2 - CuCO 3
    Cu 2 + + 2 Cl - CuCl 2
    Na + + Cl - NaCl
    2 Na + + CO 3 2 - Na 2 CO 3
    You can automatically exclude the reactions where sodium carbonate and copper(II) chloride are the products because these were the initial reactants. You also know that sodium chloride ( NaCl ) is soluble in water, so the remaining product (copper carbonate) must be the one that is insoluble. It is also possible to look up which salts are soluble and which are insoluble. If you do this, you will find that most carbonates are insoluble, therefore the precipitate that forms in this reaction must be CuCO 3 . The reaction that has taken place between the ions in solution is as follows:
    2 Na + + CO 3 2 - + Cu 2 + + 2 Cl - CuCO 3 + 2 Na + + 2 Cl -
  2. Reaction 2: Sodium sulphate reacts with copper(II) chloride.
    The ions that are present in solution are Cu 2 + , Cl - , Na + and SO 4 2 - . The ions collide with each other and may recombine in different ways. The possible combinations of the ions are as follows:
    Cu 2 + + SO 4 2 - CuSO 4
    Cu 2 + + 2 Cl - CuCl 2
    Na + + Cl - NaCl
    Na + + SO 4 2 - Na 2 SO 4
    If we look up which of these salts are soluble and which are insoluble, we see that most chlorides and most sulphates are soluble. This is why no precipitate forms in this second reaction. Even when the ions recombine, they immediately separate and go back into solution. The reaction that has taken place between the ions in solution is as follows:
    2 Na + + SO 4 2 - + Cu 2 + + 2 Cl - 2 Na + + SO 4 2 - + Cu 2 + + 2 Cl -

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Source:  OpenStax, Siyavula textbooks: grade 10 physical science [caps]. OpenStax CNX. Sep 30, 2011 Download for free at http://cnx.org/content/col11305/1.7
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