Heat and Physical Change
Lab: Heat & Physical Changes
There are three "mini-experiments" within this lab, and all of them deal with heat and physical changes.
During the first part of the lab, you will construct a cooling curve by monitoring the temperature of lauric acid as it cools. If you look at the diagram to the right, you will see what a typical cooling curve looks like. The temperature of the substance drops while it is in the liquid phase steadily until a certain point. Then, it seems like the temperature gets "stuck" for a period of time. Finally, the substance begins to drop in temperature again once it is in the solid phase. The part of the curve where temperature levels out is the freezing point (and also the melting point) of the substance. Make sure you are recording the temperature every 15 seconds and measuring to the nearest 0.1 of a degree.
The next two parts of the lab require the use of a calorimeter - in the lab we will use coffee cups or a small thermos. Calorimeters are devices that are used to monitor what is happening to the energy of a system by measuring the temperature change of water as the reaction/experiment proceeds. Inside of the calorimeter is a known mass of water. We use water because we know it's specific heat (4.18 J/g•oC) and water will not boil away easily. As a physical change occurs inside the calorimeter, the water will either increase or decrease in temperature. If the temperature of the water increases, we know an exothermic change occurred and energy was given off from the system to the surroundings (water). If the temperature of the water decreases, we know an endothermic change has occurred and energy was absorbed by the system from the surroundings. We assume that the calorimeter is a perfect insulator and that any temperature change in the water is a direct result of energy either being gained or lost by the system.
Dissolving is regarded as a physical change by most scientists. In this lab you will dissolve calcium chloride and ammonium chloride in the calorimeter. Monitor the temperature change that occurs and record the highest (or lowest) reading. To find the heat of solution, or the energy required/released by the process of dissolving the solid, use the formula q = mcDT and plug in the information for water, since you have measured its temperature change, and solve for q. Since you need to find the heat of solution (DH) and not simply the amount of energy gained/lost by the water, change the sign of q. For example, if the water temperature increased when the solid was added, that means the dissolving process was exothermic and DH should be negative. Your final answer should be in kJ/mol. Convert the mass of the solid in grams into moles by using the molar mass and convert J into kJ.
Finally, you will identify an unknown metal using specific heat. You will heat your metal using boiling water so that the metal is at 100 °C. Then, you will place your metal into the calorimeter and measure the temperature change of the water. The energy gained by the water must be equal to the energy lost by the metal. Therefore, the equation q = mcDT for the water and the metal must be equal to one another. The final temperature of the water and the metal will be the same. Watch the video on the right to see the procedure we will use in the lab.
Safety and Disposal Instructions for Lab #10
Goggles must be worn at all times.
The lauric acid will be reused in this lab - do not discard!
NH4Cl (aq) and CaCl2 (aq)can be disposed of down the drain.
The flat part of the curve shows us what the freezing/melting point is of the substance.
This is very similar to the procedure we will use to find the specific heat of an unknown metal.