Standard Chemistry - Thermochemistry

Solve the following problems. Use the correct units and show all work. No credit will be given for answers with no work. Do not assume that the amount of space given directly corresponds to the amount of space needed for a given problem.

1. Use bond energies to calculate the heat of the reaction when benzene burns. The Lewis dot structure for benzene is shown below. A table of bond energies is given on the back page.

2. A student places 5.77 grams of wex (a mythical substance often burned for light on other planets) in a test tube and then melts it. When the wex starts to freeze, the test tube (with the wex) is put into 237.6 g of water at 34.7oC. If the final temperature of the water is 51.9oC, what is the heat of fusion of the wex in cal/g?

3. A block of copper (specific heat of copper = 0.156 cal/g oC) is heated to 137oC. It is then placed on another piece of copper at 21oC. If the final temperature is 38oC, what was the mass of the second piece of copper? (All copper has the same specific heat).

4. Calculate the heat of the reaction:

N2H4 + O2 ® N2 + 2 H2O

Given the following:

H2 + O2 ® H2O -286 cal

2 NH3 + O2 ® N2H4 + H2O -143 cal

N2O + 3 H2 ® N2H4 + H2O -317 cal

2 NH3 + 3 N2O ® 4 N2 + 3 H2O -1010 cal

What does the sign ( + or - ) of the answer mean?

5. Give an example, not using water, that shows how something can have less heat than something else but still be hotter.

6. Our longest day (in the Northern hemisphere) is in mid-June. Our hottest day is usually in August. Why the difference?

7. Entropy both increases and decreases when you build a jigsaw puzzle. How is this possible?

8. Which would be more painful, a burn from steam or a burn from boiling water? Why?

(Hint: think about what happens when you get burned)