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CHEMISTRY FORM 2


1. STRUCTURE OF THE ATOM, AND THE PERIODIC TABLE
2. CHEMICAL FAMILIES AND PATTERNS IN PROPERTIES
3. CHEMICAL BONDING AND STRUCTURE
4. SALTS
5. EFFECT OF AN ELECTRIC CURRENT ON SUBSTANCES
6. CARBON AND SOME OF ITS COMPOUNDS
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Salts: Precipitation (Double decomposition)

4.0 Salts


4.4.5 Precipitation (Double decomposition)


This method is used to prepare insoluble salts. Examples are CaCO3, PbSO4, BaSO4, PbI2, AgCl, and other carbonates of metals except of sodium, potassium and ammonium ions. It involves reacting two solutions. Watch the videos below showing the precipitation reaction between calcium nitrate and sodium carbonate solutions to prepare calcium carbonate.


(courtesy Youtube-Preparation of Calcium Carbonate by CHAMELI DEVI INSTITUTE OF PHARMACY INDORE)

(courtesy Youtube-Precipitation Reaction: Calcium Nitrate and Sodium Carbonate by Doc Tang)

Questions 4.4.5(a)

  1. State the observation made.

  2. Explain why nitrates of metals and compounds of sodium or potassium are used in precipitation reactions.

  3. Write a chemical equation for the reaction.

  4. In this method, we need to wash the solid salt produced. Explain this.


Answers to Questions 4.4.5(a)


Salts are compounds of metals and non-metals; so they are ionic (consist of ions). For soluble salts, water helps in separating the ions, setting them free to combine with other ions in the solution.



To prepare insoluble salts we need a solution that contains the negative ions and another solution that has the positive ions required in the insoluble salt to be made. For example, if we want calcium carbonate, then we use a soluble calcium salt (usually calcium nitrate) and a soluble carbonate (usually of sodium, potassium, or ammonium).


Precipitation of calcium carbonate,high school chemistry , Chemistry

Figure 4.4.5(a) Precipitation of calcium carbonate


The reaction may be represented by the equation,


Ca(NO3)2(aq)  +  2Na2CO3(aq)  ⟶   CaCO3(s)  +   2NaNO3(aq)


NB: Another salt (NaNO3) is formed alongside calcium carbonate; but this remains in solution because it is soluble. The two metals have swooped ions; therefore precipitation is also called double decomposition.


How to prepare lead (II) iodide by precipitation


Watch the video showing the precipitation of lead (II) ions with iodide ions.


(courtesy Youtube-How to prepare solid lead iodide in the laboratory by Chem Labs KENYA)

The full equation (formula equation) for the reaction is

Pb(NO3)2(aq)  +   2KI(aq)  ⟶   PbI2(s)  +   2KNO3(aq)


Nitrate ions (NO3-) and potassium ions (K+) are in solution (aq) before and after the reaction. They have not taken part in the reaction, and are therefore called spectator ions. Only lead (II) ions and iodide ions have reacted. Therefore the reaction can also be represented as


Pb2+(aq)  +  2I-(aq)  ⟶   PbI2(s)


This called an ionic equation, because it involves ions.


Precipitation of lead (II) iodide,high school chemistry , Chemistry

Figure 4.4.5(b) Precipitation of lead (II) iodide


How to prepare lead (II) sulphate by precipitation


Questions 4.4.5(b)

  1. Name two salts that you would use to prepare lead (II) sulphate by precipitation.

  2. Describe how you would proceed to prepare lead (II) sulphate.

  3. Write a formula equation for the reaction.

  4. Write an ionic equation for the reaction.

  5. Write ionic equations for precipitation of silver ions by chloride ions.


Answers to Questions 4.4.5(b)


Figure 4.4.5(c) shows how lead and sulphate ions combine during precipitation of lead (II) sulphate.


Precipitation of lead (II) sulphate,high school chemistry , Chemistry

Figure 4.4.5(c) Precipitation of lead (II) sulphate