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Ch.5 - Thermochemistry
Brown - Chemistry: The Central Science 15th Edition
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Tutti i libri di testoBrown 15th EditionCh.5 - ThermochemistryProblema 46c
Capitolo 5, Problema 46c

At one time, a common means of forming small quantities of oxygen gas in the laboratory was to heat KClO3: 2 KClO3(s) → 2 KCl(s) + 3 O2(g) ΔH = -89.4 kJ c. Now consider the reverse reaction, in which KClO3 is formed from KCl and O2. What is Δ𝐻 for the formation of 19.1 g KClO3 from KCl and O2?

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Identify the given reaction: 2 KClO_3(s) \(\rightarrow\) 2 KCl(s) + 3 O_2(g) with \(\Delta\) H = -89.4 \(\text{ kJ}\).
Recognize that the reverse reaction is being considered: 2 KCl(s) + 3 O_2(g) \(\rightarrow\) 2 KClO_3(s).
For the reverse reaction, the enthalpy change (\(\Delta\) H) will be the opposite sign of the forward reaction, so \(\Delta\) H = +89.4 \(\text{ kJ}\) for the formation of 2 moles of KClO_3.
Calculate the molar mass of KClO_3: K (39.10 g/mol) + Cl (35.45 g/mol) + 3 \(\times\) O (16.00 g/mol) = 122.55 g/mol.
Determine the \(\Delta\) H for the formation of 19.1 g of KClO_3 by using the ratio: \(\Delta\) H = \(\left\)(\(\frac{19.1 \text{ g}\)}{2 \(\times\) 122.55 \(\text{ g/mol}\)}\(\right\)) \(\times\) 89.4 \(\text{ kJ}\).

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Thermochemistry

Thermochemistry is the study of the heat energy associated with chemical reactions and changes of state. It involves understanding how energy is absorbed or released during reactions, which is quantified as enthalpy change (ΔH). In this context, the ΔH value indicates whether a reaction is exothermic (releases heat) or endothermic (absorbs heat), which is crucial for calculating the energy changes in the formation of KClO3.
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Stoichiometry

Stoichiometry is the branch of chemistry that deals with the quantitative relationships between the reactants and products in a chemical reaction. It allows chemists to calculate the amounts of substances consumed and produced in a reaction based on balanced chemical equations. In this question, stoichiometry is essential for determining how much KClO3 can be formed from a given mass of KCl and O2, using the coefficients from the balanced equation.
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Reverse Reactions and Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the same, regardless of the number of steps taken to complete the reaction. This principle is particularly useful when considering reverse reactions, as the ΔH for the reverse process is equal in magnitude but opposite in sign to that of the forward reaction. In this case, calculating the ΔH for the formation of KClO3 from KCl and O2 requires applying Hess's Law to the given reaction.
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