This is a cold one. Concentrated acetic acid is called glacial acetic acid because it freezes when it's left out on a cool day. Set this up per the instructions on page 16 of your manual. Remember to record the temperature to the correct number of significant figures. As you're cooling the acid in the ice bath, record the time and temperature, every 15 seconds, until you get at least 3-5 points at about the same temperature (within a few tenths of a degree).
The graph is worth 5 points. Read the graphing guide to see what criteria I'll be looking for. This particular graph will consist of two relatively straight lines: one for the cooling of the acid, slanting down (indicating cooling as energy is lost from the acid); intersecting with a horizontal line drawn through the points for the frozen acid (all at about the same temperature). The temperature is constant during freezing, even though heat is still being removed by the acid. Instead of changing temperature as it loses heat, the acid is now changing state as it loses heat. The intersection of these lines is at the point where the acid freezes. You can determine the rate of freezing by diving the temperature difference by the number of minutes it took to reach this intersection on the graph. If you were recording your data in seconds, then do the conversion to minutes, keeping the proper number of significant figures.
Exothermic processes are those in which energy (heat) is transferred from the system to the surroundings. In an endothermic process, energy goes the other way, from the surroundings into the system. In this case, the "system" is the acid. Is heat being lost or gained by the acid?
The setup is similar: instead of an ice bath, you'll immerse the test tube in a hot water bath. IMPORTANT CHANGE: We will be using the hot plates, rather than the Bunsen burners in this part of the lab. Since water boils at 100 degrees C, which is higher than the boiling points of each of the unknowns, you can use the hot water to bring the unknown liquid to a boil.
The glass of the test tube is often too smooth to allow the formation of gas bubbles, so we use boiling stones in the bottom of the test tube to give the bubbles some crevices in which to get started.
Don't put the thermometer in the liquid and don't let it touch the sides of the tube. We want to measure the temperature of the gas that forms just above the liquid. At the boiling point, the liquid at the surface is hot enough to escape into the gas phase. The gas will condense back to a liquid as it hits the sides of the tube. You'll see it running down the inner walls of the tube. The point of condensation will rise until it starts to come out of the top, but don't let this happen. We don't want the unknown substance to come into contact with the hot plate, because it might ignite and cause a fire.
I probably won't tell you which unknown you have, so pick the one that has a boiling point closest to your experimental data. Use the formula on page 10 of your Manual for the percentage error. Finally, please answer the last question in Part B (page 21) even if your boiling point is exactly the same as the true boiling point. The idea is to think about potential sources of error in the experiment. (Don't even mention "human error." Your answer should address the design of the experiment.)
In this part, you will use a crude calorimeter to measure the change in temperature of water as a solute is added, first using potassium nitrate, then using calcium chloride. Be sure to record your mass and temperature data to the correct number of significant figures.
In this part, think of the chemical reaction (dissolving process) as the "system" and the water as the surroundings. Which way is the heat going? Was it released from the system to the surroundings (exothermic) or was it absorbed into the system from the surroundings (endothermic)? Don't be fooled by the last question: it's a simple conversion, and doesn't involve specific heat.
As always, you must write your own report and do your own calculations for each lab report, even if you're working in a group or with a partner. Sometimes, you will advised to work in groups of 2-6 students (the number will vary depending on the activity), and data will be shared among group members. If you've been instructed to work individually (no partners or groups), then copying of data is not allowed. Copying calculations and answers to questions is NEVER allowed. You must do your own work. Discussing it is okay, but if you copy without thinking, you're cheating. Try at least to reword an answer, or set up a problem in your own way, than to copy directly from someone else.