2 Heat of Precipitation

Thermochemistry
Heat of precipitation

- precipitate is unsoluble salt
- precipitate must be prepared through double bond decomposition or precipitation method Do you still remember what is meant by double bond decomposition?
[please refer to salts notes]

General equation double bond decomposition/precipitation;

Ionic equation for precipitation reaction.

Salt
Solubility in water
Li+, Na+, K+, NH4+
All salt dissolve in water
Nitrate, NO3-
All nitrate salt dissolve in water
Chloride
All chloride salt dissolve dissolve in water except;
PbCl2 - lead(II) chloride (dissolve in hot water)
AgCl - argentums/silver chloride
HgCl - hydroargentum chloride, mercury chloride
Sulphate
All sulphate salt dissolve in water except;
PbSO4 , BaSO4 , CaSO4
Carbonate
All carbonate salt not dissolve in water except;
Li2CO3 Na2CO3 , K2CO3 , (NH4)2CO3
Oxide
All oxide not dissolve in water except;
Na2O , K2O , CaO
Hydroxide
All hydroxide not dissolve in water except;
NaOH, KOH, Ca(OH)2 , Ba(OH)2

Formula to determine the heat change;

Heat released/absorbed, H = mcӨ [unit = J or kJ]

Symbol
Description
Unit m mass of solution
1cm3 = 1 g c specific heat capacity of solution
4.2 J g-1 oC-1
Ө
temperature change oC

To determine precipitation heat of silver chloride, AgCl

In this experiment you must have the following data;

Data tabulation
Initial temperature of sodium chloride NaCl /oC x oC
Initial temperature of silver nitrate, AgNO­3 /oC y oC
Average initial temperature for both solution

Highest temperature for the solution z oC
Temperature change z – (x + y) oC = Ө oC 2

Chemical equation for the reaction;

AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq)

Ionic equation for the reaction;

Ag+ (aq) + Cl- (aq)  AgCl (s)

Calculation of heat of precipitation for AgCl;
1. Calculate the number of mole of precipitate formed

No. of mol NaCl =