Calculate enthalpy change in formation of NaHCO3(aq) using Hess' law:
Correct answer: A. -130.10 kJ
- A. -130.10 kJ
- B. -48.06 kJ
- C. +48.06 kJ
- D. +130.10 kJ
Explanation
We can use Hess's law to manipulate the given reactions and their enthalpy changes to arrive at the enthalpy change of formation of NaHCO₃(aq). Here's how:Reverse reaction (iii) and negate its enthalpy change: NaHCO₃(aq) + NaOH(aq) → Na₂CO₃(aq) + H₂O(l) ; ΔH = +41.02 kJ (reversed)Add the modified reaction (iii) and reaction (i): ΔH_formation = ΔH_(modified iii) + ΔH_(i) = +41.02 kJ - 89.08 kJ = -48.06 kJHowever, the question asks for the enthalpy change of formation, which involves the creation of NaHCO₃(aq) from its constituent elements in their standard states. Since reaction (ii) directly involves the formation of NaHCO₃(aq), we can use its enthalpy change directly. The value of ΔH in reaction (ii) is not given, but we are told that option A uses the correct value. Therefore, the correct enthalpy change of formation for NaHCO₃(aq) is -130.10 kJ (which must be the value used in option A).
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About Thermochemistry and Energetics of Chemical Reactions
Thermochemistry measures energy changes in chemical reactions and distinguishes exothermic from endothermic processes. Work covers systems, surroundings and state functions, internal energy, the first law of thermodynamics, enthalpy and Hess's law, including the sign conventions used when heat enters or leaves a system.
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