Heat of Precipitation

Heat of precipitation is the energy change when one mole of precipitate is formed from its ion.

  1. Precipitation reaction occurs when two solutions containing cations and anions of insoluble salts added together.
  2. This reaction is known as double decomposition reaction and is used to prepare any insoluble salt.
  3. Heat of precipitation is the energy change when one mole of precipitate is formed from its ion.
  4. For heat of precipitation, it can be exothermic or endothermic reaction.
  5. Example below shows how the heat of precipitation can calculated from the result of an experiment.


Example:
In an experiment to find the heat of precipitation of magnesium carbonate, 25 cm³ of magnesium nitrate 2.0 mol/dm³ is added into 25 cm³ of sodium carbonate solution, 2.0 mol dm³ in a container. The temperature of the solution decreases by 6°C. What is the heat of precipitation of magnesium carbonate? [Specific heat capacity of the solution = 4.2 Jg-1°C-1, density of the solution =1 g/cm³ ]
Answer:
Step 1: Equation of the reaction
Mg(NO3)2  + Na2CO3 → MgCO+ 2NaNO3

Step 2
Number of mole of Mg(NO3)2 
n= MV 1000 = (2.0)(25) 1000 =0.05mol

Number of mole of Na2CO3 solution
n= MV 1000 = (2.0)(25) 1000 =0.05mol
Number of mole of magnesium carbonate formed = 0.05mol

Step 3
Amount of heat energy been released
= mcθ
= (25+25)(4.2)(6)
= 1260J

Step 4
Heat of precipitation of magnesium carbonate

ΔH= Heat Change Number of mole of precipitated formed ΔH= 1260 0.05 =25,200Jmo l 1 =25.2kJmo l 1

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