Question

 

Activity 3: Design a Proposal for a Hot Pack and a Cold Pack Based on the data and graphs for calcium chloride and ammonium chloride, determine which compound to use and what quantity of each compound will be needed to make a chemical hot pack and cold pack. Both packs should be calculated based on using 100 g of water. The hot pack should reach 60°C and the cold pack should go down to 3.0°C from a room temperature of 25°C.

Create a spreadsheet and graph for CaCl2 and NH4Cl. Plot the mass on the x-axis and change in temperature on the y-axis for both graphs. The slope will be the change in temperature per gram of salt dissolved. The starting temperature for each pack is 25°C.

Hot Pack: Compound needed to achieve 60°C

Grams needed per 100 g of water:

Cold Pack: Compound needed to achieve 3.0°C

Grams needed per 100 g of water:

Answer 1

The experiment is using a coffee cup calorimeter to measure the heat of solution of a chemical salt using 3 different masses. It is recommeneded to use ammonium chloride for the cold packs and calcium chloride for the hot packs.

Calorimetry is the science of measuring heat. Many chemical and physical transformations involve energy transfer in the form of heat. The magnitude and direction of heat may be determined using a calorimeter. A calorimeter is an apparatus that is insulated and prevents heat from flowing in or out of the system. Since the experiment is run under constant pressure (atmospheric), the change in water temperature that is measured is due to the enthalpy of reaction (heat of reaction). The heat of reaction may be calculated using the values measured for change in the water temperature.

The heat transfer, or change in enthalpy in a reaction (q_rxn), is related to the mass of the solution (m), the specific heat capacity of the solution (c), and the temperature change (ΔΤ = Τ_final — Τ_initial).

q_rxn= – (m × c × ΔΤ)

The specific heat capacity of a substance is the amount of energy required to raise 1 g of the substance 1° C. The specific heat capacity of water is 4.186 J / (° C × g). In experiments conducted in aqueous solution, the specific heat capacity of water is generally used.

Many instant hot and cold packs function by dissolving a salt into water. As the salt disassociates, heat is either released in an exothermic reaction or absorbed in an endothermic reaction.

Procedure to design :

Weigh out about 10 grams of CaCl2. Be sure to record the exact mass. Construct a calorimeter by nesting two Styrofoam cups, one inside the other. Add 50 mL of water to the calorimeter. Allow the water to stand for five minutes to reach a stable temperature. Place a small piece of card board to cover the cup. Make a small hole at the center of the card board and insert the thermometer through the hole.Make sure the thermometer bulb is under water. Measure the temperature of water. This is the initial temperature (Ti). 3. Holding the calorimeter steady, add all of the CaCl2 to water, place the cover, and stir rapidly with a thermometer. Be careful with the bulb of the thermometer while stirring. 4. After mixing, time - temperature data should be recorded. One partner should record the temperature while other reads the time and keeps the record. For five minutes, right from the start of mixing, take temperature at intervals of every 30 seconds. The highest temperature reached is the final temperature(Tf) of water. 6. Print the temperature versus time plot using the graph paper provided in the lab book. After recording your data, wash contents of the cup down the sink with lots of water. Repeat steps 1 - 7 using approximately 10 grams of ammonium chloride. The minimum temperature reached in this case is the final temperature (Tf).. . .

Plot graph of temp compared to time. Draw best fits and based on the line determine temp at zero and then calculate average temperature.

Determine temperature change and calculate heat absorbed for each mass of Cacl2.