a) Formula for calculating molar enthalpy ( H substance).
   
m * c * t
H =
   
n
In calculations of molar enthalpy, using the technique of calorimetry the experimenter has to determine the following:
1) mass of the substance (m) in the surroundings (calorimeter).
2) specific heat capacity of the substance (c) surrounding the system.
3) temperature change in the surroundings (t).
4) moles of substance (n) in the system (reaction that occurs).
Calculation of molar enthalpy
Example: Calculate the molar enthalpy of the reaction if when 0.25 moles of a substance is reacted the heat released produces a 4.5 oC temperature increase in 325 ml of water in a coffee cup calorimeter.
1) Write down the information given.
a) moles of substance = 0.25 moles
b) mass of substance in surroundings = 325 ml = 325 g
c) temperature change = 4.5 C
d) specific heat capacity of water is 4.19 J/g oC
2) Determine the information required.
Molar enthalpy
3) Analysis the problem and make a plan to solve for molar enthalpy.
m * c * t
H substance =
n
4) Apply formula and solve problem.
325g * 4.19 J/goC * 4.5 C
H substance =
0.25 mol.
H substance = 24,511.5 J/mol
5) State answer.
The molar enthalpy of the substance is 24.5 kJ/ mol
 
 
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Calorimetry
To determine the values above , chemists use a technique called calorimetry.
1) They first surround a reacting system with a material whose mass, and specific heat capacity is known. The most common surroundings used is insulated cup and water. ( Chemists often determine the Heat Capacity for these surroundings, to make their calculation easier. )
2) They insulate the system and surrounding so that energy transfer only occurs between them. The insulated surroundings and system is termed a calorimeter.
3) They measure the mass of substance that is to be reacted and perform the reaction, in the calorimeter.
4) Finally they measure the temperature change in the surroundings.
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Activity : Calorimetry Calculations
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