Enthalpy change can be_________________?
Correct answer: C. both A and B
- A. calculated by Hess law
- B. can be measured by calorimeter
- C. both A and B
- D. none
Explanation
An enthalpy change can be obtained experimentally from heat measurements in a calorimeter or calculated indirectly using Hess's law by combining known thermochemical equations. Students may choose only a or b when they treat calculation and measurement as alternatives rather than two valid methods.
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About Hess's Law
Hess's law states that the enthalpy change of a reaction is independent of the route because enthalpy is a state function. Calculations involve reversing and adding thermochemical equations, using standard enthalpies of formation or combustion, and finding an unknown reaction enthalpy without confusing enthalpy change with reaction rate.
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Combustion of graphite to form CO2 can be done by two ways. Reactions are given as follows: 1.C + O2 -> CO2 ∆H = -393.7 kJ mol-12.C + ½O2 -> CO ∆H = ?3.CO + ½O2 -> CO2 ∆H = -283 kJ mol-1What will be the enthalpy of the formation of CO?
Given that C + O2 gives CO2 has an enthalpy change of minus 393 kJ per mole, and CO + half O2 gives CO2 has an enthalpy change of minus 283 kJ per mole, the enthalpy of formation of carbon monoxide is
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