enthalpy change...can anyone help me?

2007-04-10 12:19 am

this is the questions
100 cm cube of water were placed in a plastic beaker The temperature was 18.7 C. 16.6 g of solid potassium iodide were added and the temperature fell to 13.4 C.
(take the c of water be 4.18 J/g/C)

a.
In this experiment it is necessary to make the potassium iodide dissolve as quickly as possible. Explain why. How is this brought about?

更新1:

b. Calculate the amount of heat absorbed by the solutioin. (assume that the density of the solution is the same as the density of water, which is 1.00 g/cm cube) c. Calculate the number of moles of potassium iodide weighed out. d. Calculate △H for dissolving potassium iodide in water.

更新2:

20 marks will be given to the best solution!!

回答 (1)

Uncle Michael

2007-04-10 1:49 am

✔ 最佳答案

a.
When dissolving KI in water, energy is absorbed.
In this experiment, it is assumed that energy is absorbed from the solution only, and the amount of energy loss of the solution can thus be calculated using expression :
Energy change = m•c•ΔT

The dissolving of KI must be as quick as possible. Otherwise, energy would be gained from the surroundings to make a smaller magnitude in the change of temperature (ΔT). This would cause an error and make the experiment value of ΔH become less endothermic.

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b.
Mass of the solution, m = 100 g

Energy absorbed
= m•c•ΔT
= 100 x 4.18 x (18.7 - 13.4)
= 2220 J
= 2.22 kJ

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c.
Molar mass of KI = 39.01 + 126.9 = 165.91 g mol-1
Mass of KI used = 16.6 g

No. of moles of KI used
= mass / (molar mass)
= 16.6 / 165.91
= 0.1 mol

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d.
ΔH for dissolving KI in water (endothermic)
= +2.22 / 0.1
= +22.2 kJ mol-1

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