The net heat change in a chemical reaction is smae weather it takes place in one ste or several steps. This law is known as:
Correct answer: C. Hess's law
- A. First law of thermodynamics
- B. Henery's law
- C. Hess's law
- D. Joule's law
Explanation
Option A: The first law of thermodynamics states that energy can neither be created nor destroyed, only altered in form. Hence incorrect. Option B: Henry's law is a gas law that states that the amount of dissolved gas in a liquid is directly proportional to its partial pressure above the liquid. The proportionality factor is called Henry's law constant. Hence incorrect. Option C: Hess's law states that the overall change in enthalpy for a chemical reaction is equal to the sum of the enthalpy changes for each step. And this is independent of the path taken. Hence it is the correct option. Option D: Joule's law states that the amount of generated heat in a conductor carrying current is proportional to the square of the current flow through the circuit when the electrical resistance and current supply are constant. Hence incorrect.
This question appeared on the ETEA MDCAT 2005 paper, which you can sit online with every answer explained.
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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
The net enthalpy change in a chemical reaction is same whether it is brought about in two or more different ways in one or several steps. It is known as:
The standard enthalpy changes of formation of iron (II) oxide, FeO(s), and aluminum oxide, Al2O3(s) are -266 kJ mol-1 and -1676 kJ mol-1 respectively. What is the enthalpy change under standard conditions for the following reaction? 3FeO(s) + 2Al(s) → 3Fe(s) + Al2O3(s)
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