Question

Thanks for the help :)

1. Describe what happens when two substances at different temperatures come into contact. Describe how the law of conservation of energy applies to this system.

2. Explain how the calorimeter works, and how you can calculate the heat given off or absorbed by the reaction being studied.

3. A calorimeter contains 500 g of water at 25°C. You place a hand warmer containing 100 g of liquid sodium acetate (NaAC) inside the calorimeter. When the sodium acetate finishes crystallizing, the temperature of the water inside the calorimeter is 32.2°C. The specific heat of water is 4.18 J/g-°C. What is the enthalpy of fusion (ΔHf) of the sodium acetate? Show your work.

Thanks!

Answer 1

1. When two substances at different temperatures come into contact, the substance that is at higher temperature will transfer energy, to the substance at a lower temperature such that their temperature become balanced.

By the law of conservation of energy the heat released by the substance initially at a higher temperature, is the same amount of heat gained by the substance originally at a lower temperature.

2. A calorimeter contains reactants and a substance to determine the heat absorbed, or the heat capacity of materials. This is done using the following equation, Q = mcΔT, where Q = 'heat absorbed / released.'

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3. We can calculate the energy released by the sodium acetate knowing that all the heat released is absorbed by the water (Part # 1).

$\mathrm{Q =\:m * c * change\:in\:temperature} ,$

$\mathrm{Q =\:500 * 4.18(specific\:heat\:of\:water) * \mathrm{32.2C -25C} } ,$

$\mathrm{Q} = 500 * 4.18 * (32.3 - 25) = 15257 \mathrm{J} = 15.257 \mathrm{kJ}$

Now the enthalpy of fusion is expressed in kJ/mol, so we have to first calculate the moles of sodium acetate.

moles = mass / molar mass

Remember that the molar mass of sodium acetate is 82.0343 g/mol. Therefore the moles will be 100 / 82.0343 = 1.219 mol. Respectively the enthalpy should be 15.257/1.219 = 12.516 kJ/mol.