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Untitled - Kelly Walsh High School

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Thermochemistry 105<br />

Quick Tip<br />

Some problems use the total heat capacity of the calorimeter (calorimeter +<br />

water). This means that the summation has already been done for you, so that<br />

you do not need to do the summation a second time.<br />

We have already noted that the process was exothermic (the value is negative)<br />

and should be reported as 28.50606 kJ (unrounded). The sign comes from<br />

convention (the definition of exothermic) and not from the calculation.<br />

To get the final answer, we must divide the energy released by the moles of glucose.<br />

We find the moles of glucose from the mass given in the problem and the<br />

molar mass determined from the atomic weights of the elements.<br />

(1.5886 g C6H12O6 )a 1 mol C6H12O6 b<br />

180.16 g C6H12O6 8.81772 103 mol C6H12O6 (unrounded)<br />

Combining the enthalpy change with the moles gives us the final answer:<br />

H reaction <br />

28.50606 kJ<br />

8.81772 10 3 mol<br />

3233.8153 3234 kJ/mol<br />

Calorimetry problems, such as this one, and most other thermochemistry problems,<br />

will require a systematic step-by-step approach as seen in this example.<br />

In this chapter, you learned about thermochemistry, the heat changes accompanying<br />

chemical reactions. You learned about calorimetry, the technique used to<br />

measure these heat changes, enthalpies, and the types of heat capacities that we<br />

can use in thermochemistry calculations. Finally, you learned about Hess’s law<br />

and how we can use it to calculate the enthalpy change for a specific reaction.<br />

1. True/False. Temperature is an extensive property.<br />

2. True/False. A nutritional Calorie is the same as a “normal” calorie.<br />

3. True/False. One way to determine the heat of reaction is to use a calorimeter.<br />

4. True/False. An exothermic process has a negative enthalpy.<br />

5. True/False. Water cooling from 40C to 25C is an exothermic process.<br />

6. Write the definition of a joule in terms of SI base units.<br />

7. What are the units of specific heat capacity?

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