Hess's law states that the enthalpy change of a reaction
Correct answer: B. is the same whether the reaction occurs in one step or several
- A. depends on the number of steps taken
- B. is the same whether the reaction occurs in one step or several
- C. is always negative
- D. depends on the catalyst used
Explanation
Because enthalpy is a state function, the total change depends only on the initial and final states, so a reaction that cannot be measured directly can be calculated through a series of steps that can. This is how the enthalpy of formation of carbon monoxide is obtained, since burning carbon always gives some carbon dioxide as well. Hess's law is essentially the first law applied to chemical change.
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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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More Hess's Law questions
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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