Thermochemical Stoichiometry
Problems
Thermochemical Stoichiometry
 In a balanced thermochemical equation…
 You can make conversion factors involving energy, as if they
were part of the equation.
 Example:
2P + 5Cl2  2PCl5
∆Ho = -886 kJ
 ∆Ho = standard enthalpy of reaction
 2 mol P = -886 kJ
 5 mol Cl2 = -886 kJ
 2 mol PCl5 = -886 kJ
 You can use these as unit conversions in stoichiometric
calcualtions.
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Problem Solving Strategies
 MAKE YOUR ROAD MAPS.
 In Honors Chemistry, you can convert from moles into…
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Moles of another substance
Concentration
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Molarity, molality, % mass
Numbers of particles
Mass
Liters of gas at STP
P, V, T, NOT at STP
Enthalpy of reaction
molar ratio
equations
Avogadro’s number
molar mass
molar volume (22.4)
ideal gas law
standard enthalpy (kJ/mol)
 You can be asked to convert between all sorts of different
units! Practice!!!! (do HW and review!)
Problem 1
 When a 4.25g sample of solid NH4NO3 dissolves in
60.0g of water the temperature drops from 21.0°C to
16.9°C. Calculate the ∆H for this process. The
specific heat of water (solution) is 4.18 J/g • °C.
 In these problems, assume the specific heat of the
solution is the same as water.
 Also assume no volume change from the addition of
solute.
Problem 2
 When 50.0mL of 1.0 M HCl and 50.0mL of 1.0M
NaOH are mixed in a calorimeter, the temperature of
the mixture increases from 23.0 oC to 29.8 oC.
Neglecting the effects of the calorimeter and
assuming the solution to have the same density and
specific heat of water (4.184 J/gK) calculate the
ΔHrxn in kJ/mole.