ENTHALPY OF HYDRATION OF SODIUM ACETATE

Goal: To determine the enthalpy of this process:

NaOOCCH₃ + 3 H₂O à NaOOCCH₃ ^. 3H₂O

We will determine this value by measuring the enthalpies of solution for anhydrous sodium acetate and for sodium acetate trihydrate. Application of Hess’s Law will give us the DH for the hydration reaction.

NaOOCCH₃(s) à Na⁺ (aq) + CH₃COO^- (aq) ΔH sol’n for the anhydrous

Na⁺ (aq) + CH₃COO^- (aq) + 3 H₂O à NaOOCCH₃ • 3H₂O(s) - ΔH sol’n for the hydrate

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NaOOCCH₃(s) + 3 H₂O à NaOOCCH₃(s) • 3H₂O ΔH of hydration

[Where did the water come from in the last equation? It is the solvent.]

Background: You’ll remember from the “Chemical Reactions” lab that copper sulfate pentahydrate lost its water when heated. (Bunsen burner, blue crystals, turned white … coming back to you?) The water molecules were weakly coordinated to the metal ion; heating broke those bonds and the water was lost. If the loss of water were endothermic (needed heat supplied) in that case, then the reverse process would be exothermic.

It would be difficult to supply just enough heat to dehydrate a hydrate, and it would be difficult to supply just enough water to hydrate an anhydrous compound. In this laboratory we’ll use the less toxic sodium acetate, and measure the heat of solution of its hydrated and anhydrous forms. The heat involved in the two processes will be different – in fact one will be exothermic and the other endothermic as it turns out. Hess’s law allows us to subtract one enthalpy from the other to obtain the enthalpy of hydration as outlined above.

Experimental: Obtain two large dry test tubes, a looped stirrer, and a thermometer. Record room temperature.

Measure out 10.0 mL of deionized water in a graduated cylinder. Weigh out precisely about 1.20 to 1.25 grams of anhydrous sodium acetate. Be sure to close the lid of the jar afterwards – we want to keep it anhydrous. Working quickly, place the dry powder into the test tube. Add the 10.0 mL of water, and begin to stir gently. Measure the solution’s temperature as you stir, and record the highest temperature that the solution gets to.

Measure another 10.0 mL of water. Weigh out precisely about 1.96 to 2.04 grams of sodium acetate trihydrate. Place the solid in the second test tube. Add the 10.0 mL of water, and begin to stir gently. Measure the solution’s temperature as you stir, and record the lowest temperature that the solution gets to.

Calculations: We need to calculate the heat released or absorbed by each reaction. This will be the specific heat of the solution (which we’ll assume is the same for water, 4.18 Joules/degree-gram.) times the grams of solution (10g water plus the mass of the solute used) times the temperature change T_initial - T_final , or Room Temp minus the solution’s final temperature.)

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