Useful Information: DU = q + w; q = (S.H.)(m)(DT); 1 cal = 4.184 J
1. DH is called the __________________ change of the reaction or process. Reactions or processes for which DH values are positive are termed _____________thermic.
2. H is a “state function.” What is the essential characteristic of a state function?
3. A reaction takes place in a coffee cup calorimeter that holds 65.0 g of water (Specific Heat = 4.184 J/g°C). The temperature in the calorimeter was observed to rise from 25.4 °C to 28.7 °C.
a. Calculate the heat flow, qcal, for the calorimeter.
b. What is the heat flow, qrxn, for the reaction that took place?
c. What information would we need in order to transform qrxn into DH for the reaction?
4. What is the change in internal energy, DU, of a system which absorbs 476 J of heat and also does work equivalent to 640 J?
5. The thermite reaction is represented as: Fe2O3 + 2 Al à 2 Fe + Al2O3 DH = - 851.5 kJ
Calculate the heat involved in reaction 10.0 g of Al with sufficient Fe2O3.
6. The combustion of acetylene (C2H2, F.W. = 26.04) is used for welding metals:
2 C2H2(g) + 5 O2(g) à 2 H2O(g) + 4 CO2(g) DH = ??
If 100.0 g of C2H2 burns with sufficient oxygen, 4821 kJ of heat is produced. Use this information to calculate the value of DH that should be written in the equation above to make it a thermochemical equation.