Textbook Question
Without referring to tables, predict which of the following has the higher enthalpy in each case: (b) 2 mol of iodine atoms or 1 mol of I2
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Ch.5 - Thermochemistry
Chapter 5, Problem 44b
Consider the following reaction: 2 CH3OH(g) → 2 CH4(g) + O2(g) ΔH = +252.8 kJ (b) Calculate the amount of heat transferred when 24.0 g of CH3OH(g) is decomposed by this reaction at constant pressure.
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Determine the molar mass of CH_3OH by adding the atomic masses of its constituent atoms: C (12.01 g/mol), H (1.01 g/mol), and O (16.00 g/mol).
Calculate the number of moles of CH_3OH in 24.0 g by dividing the mass by the molar mass.
Use the stoichiometry of the reaction to relate the moles of CH_3OH to the enthalpy change (ΔH). The reaction shows that 2 moles of CH_3OH correspond to an enthalpy change of +252.8 kJ.
Calculate the heat transferred for the moles of CH_3OH you have by setting up a proportion based on the stoichiometry: (moles of CH_3OH / 2 moles) = (x kJ / 252.8 kJ).
Solve for x to find the amount of heat transferred when 24.0 g of CH_3OH is decomposed.
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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 quantities of substances involved in a reaction. In this case, knowing the molar ratios from the balanced equation helps us convert grams of CH₃OH to moles, which is essential for calculating heat transfer.
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Stoichiometry Concept
Enthalpy Change (ΔH)
Enthalpy change (ΔH) represents the heat absorbed or released during a chemical reaction at constant pressure. A positive ΔH indicates that the reaction is endothermic, meaning it absorbs heat from the surroundings. In this reaction, ΔH = +252.8 kJ signifies that 252.8 kJ of heat is required to decompose 2 moles of CH₃OH, which is crucial for calculating the heat transferred for a specific mass.
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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. For CH₃OH, the molar mass is approximately 32.04 g/mol, which allows us to calculate how many moles are present in 24.0 g, facilitating the determination of the heat transferred in the reaction.
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Related Practice
Textbook Question
Without referring to tables, predict which of the following has the higher enthalpy in each case: (c) 1 mol I2(g) and 1 mol H2(g) at 25 °C or 2 mol HI(g) at 25 °C
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Open Question
Consider the following reaction: 2 Mg(s) + O₂(g) → 2 MgO(s) ΔH = -1204 kJ (b) Calculate the amount of heat transferred when 3.55 g of Mg(s) reacts at constant pressure. (d) How many kilojoules of heat are absorbed when 40.3 g of MgO(s) is decomposed into Mg(s) and O₂(g) at constant pressure?
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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