SCH4U1
Molar Enthalpy of a Chemical Change
Remember that molar enthalpy is a convenient way of describing energy changes
involved in a variety of physical and chemical changes involving 1 mole of a particular
reactant or product. Some examples of molar enthalpy are shown in the table below.
Table 1 Some Molar Enthalpies of Reaction (Hx)
Type of Molar Enthalpy
Example of Change
solution (Hsoln)
solid  aqueous
combustion (Hcomb)
 carbon dioxide + water
vaporization (Hvap)
liquid  gas
condensation (Hcond)
gas  liquid
freezing (Hfr)
liquid  solid
neutralization (Hneut)
acid + base  a salt + water
formation (Hf)
X + Y  XY
Pre-lab:
Part A:
For each of the following experiments in (a) and (b):
(i)
Write a balanced chemical equation (with the state of matter)
(ii)
State the type of molar enthalpy
(iii)
Use the data in the table to calculate the corresponding molar enthalpies
(a) Table 2 Observations for Calorimetry Investigation
Quantity
Mass of water
Initial temperature of calorimeter
Final temperature of calorimeter
Mass of acetone, C3H6O (l), burned
Measurement
100.00 g
20.0 ºC
25.0 ºC
0.10 g
(b) Table 3 Observations for Calorimetry Investigation
Quantity
Mass of calcium carbonate, CaCO3 (s)
Mass of polystyrene cup
Mass of polystyrene cup and
1.00 mol/L hydrochloric acid, HCl (aq)
Initial temperature of calorimeter
Final temperature of calorimeter
Measurement
4.20 g
3.00 g
173.20 g
29.0 ºC
31.0 ºC
Part B:
Enthalpy of neutralization can be expressed per mole of either base or acid consumed.
If the base is the limiting reagent, the moles of base or the corresponding moles of acid
required to completely consume the base should be used to determine the molar
enthalpy change. On the otherhand, if the acid is the limiting reagent, the moles of acid
or the corresponding moles of base required to completely consume the acid should be
used to determine the molar enthalpy change.
Read Investigation 5.2.1 on pages 348 –349 in your textbook.
Given that 30.0 mL sample of 1.00 mol/L sulfuric acid is added to 50.0 mL of 1.00 mol/L
sodium hydroxide solution in polystyrene calorimeter,
(a) determine the limiting reagent
(b) using the limiting reagent, calculate the corresponding amount of moles of
sulfuric acid or sodium hydroxide
Part C:
Calculate how to make 100.0 mL of 1.00 M solution of sodium hydroxide (i.e. determine
the mass of NaOH to dissolve in 100.0 mL of water)