Untitled - Kelly Walsh High School

Thermochemistry 105
Quick Tip
Some problems use the total heat capacity of the calorimeter (calorimeter +
water). This means that the summation has already been done for you, so that
you do not need to do the summation a second time.
We have already noted that the process was exothermic (the value is negative)
and should be reported as 28.50606 kJ (unrounded). The sign comes from
convention (the definition of exothermic) and not from the calculation.
To get the final answer, we must divide the energy released by the moles of glucose.
We find the moles of glucose from the mass given in the problem and the
molar mass determined from the atomic weights of the elements.
(1.5886 g C6H12O6 )a 1 mol C6H12O6 b
180.16 g C6H12O6 8.81772 103 mol C6H12O6 (unrounded)
Combining the enthalpy change with the moles gives us the final answer:
H reaction
28.50606 kJ
8.81772 10 3 mol
3233.8153 3234 kJ/mol
Calorimetry problems, such as this one, and most other thermochemistry problems,
will require a systematic step-by-step approach as seen in this example.
In this chapter, you learned about thermochemistry, the heat changes accompanying
chemical reactions. You learned about calorimetry, the technique used to
measure these heat changes, enthalpies, and the types of heat capacities that we
can use in thermochemistry calculations. Finally, you learned about Hess’s law
and how we can use it to calculate the enthalpy change for a specific reaction.
1. True/False. Temperature is an extensive property.
2. True/False. A nutritional Calorie is the same as a “normal” calorie.
3. True/False. One way to determine the heat of reaction is to use a calorimeter.
4. True/False. An exothermic process has a negative enthalpy.
5. True/False. Water cooling from 40C to 25C is an exothermic process.
6. Write the definition of a joule in terms of SI base units.
7. What are the units of specific heat capacity?