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A.The thermo chemical standard state of a substance is its most stable state under one atmosphere pressure and at some specific temperature (298 K if not specified)
B.A superscript zero, such as ?H0, indicates a specified temperature of 0?C.
C.For a pure substance in the liquid or solid phase, the standard state is the pure liquid or solid.
D.For a pure gas, the standard state is the gas at a pressure of one atmosphere.
E. For a substance in solution, the standard state refers to one-molar concentration.

17. Fuel values of hydrocarbons increase as the H/C atomic ratio increases. Which of the following compounds has the highest fuel value?
A.C2H6
B.C2H4
C.C2H2
D.CH4
E.C6H6

18. Calculate ?H° (in kJ) for reaction 3 is __________ kJ.

A.196
B.-196
C.1087
D.-1384
E.-543

19. When we put 100-g piece of metal at 100°C into 100 g of water at 20°C, the final temperature is 60°C.

20. With reference to enthalpy changes, the term standard conditions means __________.
A.a only
B.b only
C.c only
D.a and c
E.a and b