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It's easy to see the principles of enthalpy in action with a simple experiment. To make sure that the reaction in your experiment will take place without any foreign contamination, clean and sterilize the container that you plan to use. Scientists use special closed containers called calorimeters to measure enthalpy, but you can achieve reasonable results with any small glass jar or flask. Regardless of the container you use, fill it with clean, room-temperature tap water. You'll also want to conduct the reaction somewhere indoors with a cool temperature. For this experiment, you'll want a fairly small container. We'll be testing the enthalpy-altering effects of Alka-Seltzer on water, so the less water used, the more obvious the temperature change will be. Grab a thermometer and set it in the container so that the temperature-reading end sits below the water level. Take a temperature reading of the water for our purposes, the temperature of the water will represent T1, the initial temperature of the reaction. Let's say that we measure the temperature of the water and find that it's exactly 10 degrees C. In a few steps, we'll use this sample temperature reading to demonstrate the principals of enthalpy. When you're ready to start the experiment, drop a single Alka-Seltzer tablet into the water. You should notice it immediately start to bubble and fizz. As the tablet dissolves in the water, it breaks down into the chemicals bicarbonate (HCO3-) and citric acid (which reacts in the form of hydrogen ions, H+). These chemicals react to form water and carbon dioxide gas in the reaction 3HCO3 + 3H+ 3H2O + 3CO2. Monitor the reaction as it proceeds the Alka-Seltzer tablet should gradually dissolve. As soon as the tablet finishes its reaction (or seems to have slowed to a crawl), measure the temperature again. The water should be slightly colder than before. If it's warmer, the experiment may have been affected by an outside force (like, for instance, if the room you're in is especially warm). For our example experiment, let's say that the temperature of the water is 8 degrees C after the tablet has finished fizzing. In an ideal experiment, when you add the Alka-Seltzer tablet to the water, it forms water and carbon dioxide gas (the latter of which can be observed as fizzing bubbles) and causes the temperature of the water to drop. From this information, we would expect the reaction to be endothermic that is, one that absorbs energy from the surrounding environment. The dissolved liquid reactants need extra energy to make the jump to the gaseous product, so it takes energy in the form of heat from its surroundings (in this case, water). This makes the water's temperature fall. In our example experiment, the temperature of the water fell two degrees after adding the Alka-Seltzer. This is consistent with the sort of mildly endothermic reaction we'd expect.
Grab a clean container and fill it with water. Insert a thermometer into the container. Add one Alka-Seltzer tablet to the container. Measure the temperature when the reaction finishes. Estimate the enthalpy of the reaction.