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Ch.5 - Thermochemistry
Chapter 5, Problem 46b

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 For this reaction, calculate H for the formation of (b) 10.4 g of KCl.

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1
Identify the molar mass of KCl by adding the atomic masses of K (potassium) and Cl (chlorine).
Calculate the number of moles of KCl in 10.4 g using the formula: \( \text{moles of KCl} = \frac{\text{mass of KCl}}{\text{molar mass of KCl}} \).
Use the stoichiometry of the balanced chemical equation to determine the relationship between moles of KCl and the enthalpy change (\( \Delta H \)).
Calculate the enthalpy change for the formation of the calculated moles of KCl using the relationship: \( \Delta H = \text{moles of KCl} \times \frac{-89.4 \text{ kJ}}{2 \text{ moles of KCl}} \).
Express the final enthalpy change in kJ for the formation of 10.4 g of KCl.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions based on the balanced equation. It allows us to determine the relationships between the amounts of substances involved in a reaction, such as how many grams of a product can be formed from a given mass of reactants. In this case, stoichiometry will help us relate the mass of KCl produced to the amount of KClO3 reacted.
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Enthalpy Change (ΔH)

Enthalpy change (ΔH) is a measure of the heat energy absorbed or released during a chemical reaction at constant pressure. In the given reaction, ΔH = -89.4 kJ indicates that the reaction is exothermic, meaning it releases energy. Understanding ΔH is crucial for calculating the energy associated with the formation of products, such as KCl, from the reactants.
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Molar Mass

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). It is essential for converting between the mass of a substance and the number of moles, which is necessary for stoichiometric calculations. To find the amount of KCl formed, we need to know its molar mass to convert the given mass (10.4 g) into moles for further calculations.
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Related Practice
Textbook Question

Consider the following reaction: 2 CH3OH(g) → 2 CH4(g) + O2(g) ΔH = +252.8 kJ (d) How many kilojoules of heat are released when 38.5 g of CH4(g) reacts completely with O2(g) to form CH3OH(g) at constant pressure?

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Textbook Question

When solutions containing silver ions and chloride ions are mixed, silver chloride precipitates Ag+(aq) + Cl-(aq) → AgCl(s) H = -65.5 kJ (a) Calculate H for the production of 0.450 mol of AgCl by this reaction. (b) Calculate H for the production of 9.00 g of AgCl. (c) Calculate H when 9.25⨉10-4 mol of AgCl dissolves in water.

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Textbook Question

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 For this reaction, calculate H for the formation of (a) 1.36 mol of O2

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Textbook Question

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) The decomposition of KClO3 proceeds spontaneously when it is heated. Do you think that the reverse reaction, the formation of KClO3 from KCl and O2, is likely to be feasible under ordinary conditions? Explain your answer.

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Textbook Question

Consider the combustion of liquid methanol, CH3OH(l): CH3OH(l) + 3/2 O2(g) → CO2(g) + 2 H2O(l) ΔH = -726.5 kJ (a) What is the enthalpy change for the reverse reaction?

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Textbook Question

Consider the combustion of liquid methanol, CH3OH(l): CH3OH(l) + 3/2 O2(g) → CO2(g) + 2 H2O(l) ΔH = -726.5 kJ (b) Balance the forward reaction with whole-number coefficients. What is ΔH for the reaction represented by this equation?

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