Chemistry

1. A system absorbs 1.57E2 kJ of heat and the surroundings do 1.096E2 kJ of work on the system. What is the change in internal energy (in kJ) (ΔU) of the system?
2. The air in an inflated balloon (the system) absorbs 125 J of heat from its proximity to a fire. The balloon expands and does 85 kJ of work. What is the change in internal energy (in kJ) (\DeltaU) for the system? Enter only the numerical value.
3. How much heat is liberated (in kJ) from 250 g of silver when it cools from 86 °C to 26 °C? The heat capacity of silver is 0.235 Jg^{-1} °C^{-1}. Note, "heat liberated" implies that the change in heat is negative. Enter a positive number.
4. When 0.1523 g of liquid pentane (C_5H_{12}) combusts in a bomb calorimeter, the temperature rises from 23.7^{\circ}C to 29.8 ^{\circ}C. What is \DeltaU_{rxn} for the reaction in kJ/mol pentane? The heat capacity of the bomb calorimeter is 5.23 kJ/^{\circ}C.
5. When 0.1625 g of magnesium is burned in a bomb calorimeter that has a heat capacity of 3.03 kJ/^{\circ}C, the temperature increases by 1.252^{\circ}C. How much heat (kJ/mol) is liberated during the burning of the magnesium?

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  1. wondering if this is not a test in heat energy, but a test of handling significant digits....or it could be that your instuctor does not care about precision. For example (they all offer similar analysis), question 4:
    you have a multiplication of a four sig digit 0.1523 then a two digit (29.8-23.7 )=6.1, then a three digit 5.23. And the proper answer is only accurate to two significant digits. Wondering what the machine will count as correct...
    good luck.

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    bobpursley

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