CK-12-Chemistry Intermediate

http://www.ck12.org Chapter 17. Thermochemistry

Known

• mass of SO 2 = 58.0 g


• molar mass of SO 2 = 64.07 g/mol


• ∆H =−198 kJ for the reaction of 2 mol SO 2

Unknown

• ∆H =? kJ

The calculation requires two steps. First, the mass of SO 2 is converted to moles. Then, mol SO 2 is multiplied by the
conversion factor (−198 kJ/2 mol SO 2 ).

Step 2: Solve.

∆H= 58 .0 g SO 2 ×

1 mol SO 2

64 .07 g SO 2

×

−198 kJ

2 mol SO 2

=− 89 .6 kJ

Step 3: Think about your result.

The mass of sulfur dioxide is slightly less than 1 mol. Since 198 kJ is released for every 2 mol of SO 2 that reacts,
the heat released when about 1 mol reacts is one half of 198. The 89.6 kJ is slightly less than half of 198. The sign
of∆H is negative because the reaction is exothermic.

Practice Problem

3. Given the reaction below for the decomposition of mercury(II) oxide:
   (a) What is∆H when 0.750 mol of HgO fully reacts?
   (b) A certain reaction produces 28.4 g of Hg. How much heat was absorbed in this reaction?
   (c) What volume of oxygen gas at STP is produced in a reaction that absorbs 432 kJ of heat?

Lesson Summary

• Enthalpy is the heat content of a system. When a chemical reaction or physical process occurs at constant
  pressure, the heat absorbed or released by the system is equal to the enthalpy change of the system.


• A calorimeter is an insulated device used in the laboratory to measure the enthalpy change during a reaction.


• Thermochemical equations show the heat that is either absorbed or released during a reaction. The enthalpy
  change (∆H) for a reaction can be used as a conversion factor in solving problems.

Lesson Review Questions

Reviewing Concepts

1. What experimental condition is required for the heat change in a reaction to be numerically equal to the
   enthalpy change (∆H)?