FORM 5 CHEMISTRY Heat of Precipitation
Chapter 3 : Thermochemistry
3.2 : Heat of Reaction (Part 1)
1. The heat of reaction is normally named according to
the types of reactions that occur.
2. Types of heat of reaction:
a) Heat of Precipitation Chemical Equation:
b) Heat of Displacement
c) Heat of Neutralisation
d) Heat of Combustion
3. The heat of reaction can be determined through Definition:
experiments by establishing the temperature change Heat of precipitation is the heat change when 1 mole of
when the reaction occurs. The value of temperature precipitate is formed from their ions in an aqueous
change obtained is used to calculate heat of solution.
reaction.
Operational definition (based on the experiment):
Calculation Based Questions
Heat of Precipitation: when sodium chloride, NaCl
Determine the number of moles solution is added to silver nitrate, AgNO solution to
of the reactants and products formed, produce 1 mole of silver chloride, AgCl precipitate, the
n mole. thermometer reading increases.
Calculation Based Questions
a) Heat of Precipitation : Mole of
precipitate formed for heat of Question 1
Step 1 precipitation
b) Heat of Displacement : Mole of metal 100 cm3 of 1.0 mol dm−3 of lead(II) nitrate, Pb(NO3)2
displaced for heat of displacement solution is added to 100 cm3 of 1.0 mol dm−3 of sodium
c) Heat of Neutralisation : Mole of water sulphate, Na2SO4 solution. The temperature of the
formed for heat of neutralisation mixture rises from 30.0°C to 33.0°C. Calculate the heat
d) Heat of Combustion : Mole of fuel burnt of precipitation of lead(II) sulphate, PbSO4.
for heat of combustion [Specific heat capacity of solution, c = 4.2 J g−1 °C−1;
density of solution = 1 g cm−3]
Calculate the heat change in the reaction:
Calculate the number of moles.
a) Use formula
Step 2
Q = mcθ Step 1 b) Write the ionic equation
c) Compare the stoichiometry of the ionic
equation
Calculate the heat change for 1 mole of
Step 2 Calculate the heat change.
reactant or 1 mole of product formed in
Step 3
proportion. Calculate the heat change for the formation
Step 3
of 1 mole of precipitate.
State the heat of reaction, ∆H with +/- signs
and the correct units: Step 4 Write the heat of reaction, ∆H.
Step 4
∆H = +/-X kJ mol−1
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Question 2
The following equation shows the precipitation of silver
chloride, AgCl.
AgNO3 (aq) + KCl (aq) → AgCl (s) + KNO3 (aq)
∆H = - 65.5 kJ mol−1
If 20 cm3 of 0.5 mol dm−3 of silver nitrate, AgNO3 solution
is added into 20 cm3 of 0.5 mol dm−3 of potassium
chloride, KCl solution, calculate the rise in temperature
of the mixture.
[Specific heat capacity of solution, c = 4.2 J g−1 °C−1;
density of solution = 1 g cm−3]
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electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the authors of KRflix Academy. 2
