Ch.5 - Thermochemistry

Problem 55b

Chapter 5, Problem 55b

When a 5.10-g sample of solid sodium hydroxide dissolves in 100.0 g of water in a coffee-cup calorimeter (Figure 5.18), the temperature rises from 20.5 to 33.2 °C. b. Using your result from part (a), calculate ΔH (in kJ/mol NaOH) for the solution process. Assume that the specific heat of the solution is the same as that of pure water.

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Identify the mass of the solution, which is the sum of the mass of NaOH and the mass of water.

Calculate the heat absorbed by the solution using the formula: q = m * c * ΔT, where m is the mass of the solution, c is the specific heat capacity of water (4.18 J/g°C), and ΔT is the change in temperature.

Convert the heat absorbed (q) from joules to kilojoules by dividing by 1000.

Determine the number of moles of NaOH by dividing the mass of NaOH by its molar mass (approximately 40.00 g/mol).

Calculate ΔH in kJ/mol by dividing the heat absorbed (in kJ) by the number of moles of NaOH.

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Key Concepts

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

Enthalpy Change (ΔH)

Enthalpy change (ΔH) is the heat content change of a system at constant pressure, often associated with chemical reactions or physical processes. It is expressed in kJ/mol and indicates whether a process is exothermic (releases heat, ΔH < 0) or endothermic (absorbs heat, ΔH > 0). In this context, calculating ΔH for the dissolution of sodium hydroxide involves determining the heat absorbed by the water and relating it to the amount of solute dissolved.

Specific Heat Capacity

Specific heat capacity is the amount of heat required to raise the temperature of one gram of a substance by one degree Celsius. For this problem, it is assumed that the specific heat of the solution is the same as that of water, which is 4.18 J/g°C. This assumption simplifies calculations, allowing us to use the temperature change of the water to determine the heat absorbed during the dissolution of sodium hydroxide.

Calorimetry

Calorimetry is the science of measuring the heat of chemical reactions or physical changes. In this scenario, a coffee-cup calorimeter is used to measure the temperature change of the water when sodium hydroxide dissolves. By applying the principles of calorimetry, we can calculate the total heat absorbed by the water, which is then used to find the enthalpy change per mole of NaOH dissolved.

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Related Practice

Textbook Question

Consider the data about gold metal in Exercise 5.24(b). b. Suppose that the same amount of heat is added to two 10.0-g blocks of metal, both initially at the same temperature. One block is gold metal, and the other is iron metal. Which block will have the greater rise in temperature after addition of the heat?

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Textbook Question

Consider the data about gold metal in Exercise 5.24(b). c. What is the molar heat capacity of Au(s)?

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Textbook Question

When a 5.10-g sample of solid sodium hydroxide dissolves in 100.0 g of water in a coffee-cup calorimeter (Figure 5.18), the temperature rises from 20.5 to 33.2 °C. a. Calculate the quantity of heat (in kJ) released in the reaction.

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Textbook Question

(b) Is this process endothermic or exothermic?

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Textbook Question

A 2.200-g sample of quinone (C₆H₄O₂) is burned in a bomb calorimeter whose total heat capacity is 7.854 kJ/°C. The temperature of the calorimeter increases from 23.44 to 30.57 °C. (a) What is the heat of combustion per gram of quinone?

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Textbook Question

A 2.200-g sample of quinone (C₆H₄O₂) is burned in a bomb calorimeter whose total heat capacity is 7.854 kJ/°C. The temperature of the calorimeter increases from 23.44 to 30.57 °C. b. What is the heat of combustion per mole of quinone?

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