Chem 2AP Hess's Law Lab.docx - Finding the Heat of...

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Finding the Heat of Formation using Hess’s Law and Other Methods Wu, A., Wu, K., Falat, A. Lab Section C, 10/27/2016 Introduction: The heat of reaction for a highly dangerous or explosive reaction be very difficult to calculate. This is one of many reactions that is too difficult to replicate in order to solve for its heat of reaction. Germain Henri Hess states that the heat of reaction is the same whether it occurs in one step or in a series of steps because the heat of reaction is a state function, meaning that the value is independent of its path; (Anne Marie Helmenstine, 2015). A difficult heat of a reaction can be broken into several smaller reactions in order to solve for the overall heat of reaction. For example, H 2(g) → 2H (g) ΔH = 436 kJ Br 2 → 2Br (g) ΔH = 193.9 kJ 2H (g) + 2Br (g) → 2HBr (g) ΔH = -2(365.7 kJ) H 2(g) + Br 2(g) → 2HBr (g) ΔH rxn overall = -101.5 kJ T o solve for the overall heat of reaction, simply balance the individual step equations, look up standard enthalpies for each step, flip/change coefficients of equations by multiplication in order to cancel out terms on opposite sides (changing the equation around requires a multiplication of -1 of that individual reaction’s heat of reaction), and sum up the individual steps. This is all because heat of reactions are constant and depend on the moles present in a reaction. Therefore, any large reaction’s heat of reaction can be solved for as long as the individual reactions’ heat of reactions are known. If the heat of reactions for HCl + NaOH
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