Temperature Changes Upon Dissolution of Salts 1. Prepare a coffee-cup calorimete

Question

Temperature Changes Upon Dissolution of Salts

1. Prepare a coffee-cup calorimeter by placing one Styrofoam coffee cup inside another.

2. Use a graduated cylinder to measure 50 mL of cold tap water, and pour it into the coffee cup. Record the temperature of the water.

3. Weigh out ~5 grams of calcium carbonate or ammonium nitrate. Record the weight of the salt. Add the salt to the water, and stir to dissolve – use a stirring rod, not the thermometer to stir! Observe the temperature changes that occur while the salt dissolves. Record the highest or lowest temperature observed, as the final temperature.

Reactions:

calcium chloride

ammonium nitrate

magnesium carbonate

sodium chloride

4.            For each reaction, identify the solute and the solvent.

5.            What types of bonds or intermolecular forces need to be disrupted between solute molecules?

6.            What types of bonds or intermolecular forces need to be disrupted between the solvent molecules?

7.            What types bonds or intermolecular forces form between the solute and the solvent?

8.            Review the classification of each reaction as endo- or exothermic and the sign of DH. Given that DH = (solute-solute + solvent-solvent attractive forces)reactants – (solute- solvent attractive forces)products, which attractive forces were stronger for each salt, those of the reactants or those of the products?

Explanation / Answer

Coffee cup calorimeter experiment

4. For each reaction, the added salt CaCl2 or NH4NO3 or MgCO3 or NaCl is the solute and water is the solvent in the system.

5. In all the ioorganic solutes added, electrostatic force of attraction exists which needs to be broken down to dissolve the ions in solvent.

6. In solvent hydrogen bond needs to be disrupted between the water molecules to make up the solution in all the cases.

7. In all the cases, ions are formed from the soutes which are attracted towards the water moelcules via a ion-dipole type of interaction.

8. CaCl2 in water : exothermic, dH = -ve

NH4NO3 in water : endothermic, dH = +ve

MgCO3 in water : exothermic, dH = -ve

NaCl in water : endothermic, dH = +ve

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