Endothermic Reactions

endothermic reaction homeschool science experiment

This is a great lab that teaches one of the core chemistry principles: endothermic and exothermic reactions. Every chemical reaction (versus a physical reaction) undergoes a molecular change that can be evidenced by the creation of or the removal of heat, production of a gas (fizzing), by a color change, or by the production of odor, light or a precipitate.

In this simple lab that involves common household products vinegar and baking soda, kids can learn: how to use a thermometer, recording quantitative data, learn about a meniscus, and practice making hypotheses.

This is also a basic lab, but the exploration of endo and exothermic reactions don’t need to stop here. Teach the basic concepts then springboard from this lab to explore hot ice, melting ice cubes with salts, and so on. More advanced ideas can be found here, at ThoughtCo.com.

Safety: Goggles are a must, long hair should be tied up, and while the materials are non-staining, lab coats are always a good idea.

Materials
a disposable plastic cup
vinegar
baking soda
250-mL graduated cylinder
immersion thermometer (an immersion thermometer has the degrees marked right on it, versus on a plastic mount behind the thermometer)
tablespoon measure (I keep a set of measuring spoons just for chemistry lab use)

Background

  1. REVIEW or teach the concepts of a chemical change. A chemical reaction occurs when there’s a chemical change in the substance, transforming it from one material to another. For example, a shiny nail can oxidize and become rusty, fresh milk can sour and become lumpy, or wood burns and becomes ash. Remind them that a chemical change is different from a physical one. A physical change occurs when a material is physically altered, but not on the molecular level. If you rip a piece of paper in two, that’s a physical change. But, the paper still remains paper! If you saw a piece of wood you get sawdust, but that’s still wood, just finely shaved.
  2. ASK the kids to tell you some of the evidence of a chemical reaction (production of odor or gas, change in color, change in temperature, the formation of a precipitate and some even produce light).
  3. INTRODUCE the words endothermic and exothermic. Ask them if they know what the words mean.
  4. TELL them that endothermic means a change in temperature, it gets colder. In an endothermic reaction, all the energy is absorbed and energy produces heat. Endothermic – an END to thermic (heat). Exothermic is the opposite. The chemical change produces energy, thus it produces heat.
  5. SHOW the kids how to use an immersion thermometer.
  6. SHOW the kids how to use the graduated cylinder to measure liquids. Explain that the surface tension on the liquid pulls the liquid up the sides of the graduated cyldinder ever so slightly, causing a dip in the middle. This is called the meniscus. When we measure liquids, we always take the measurement from the bottom of the meniscus.
  7. MEASURE (or let the kids do it) 100 mL of vinegar in the graduated cylinder, then pour it carefully into a disposable plastic cup.
  8. RECORD the temperature of the vinegar.
  9. Using a teaspoon, measure 15 mL of baking soda. Level it with a knife or other straight implement.
  10. ASK the kids to give you some observations about the baking soda. Tell them it is called sodium bicarbonate, and it is a weak base.
  11. ASK the kids what their hypothesis is if you were to combine the sodium bicarbonate and vinegar. (It will fizz, producing carbon dioxide gas in an endothermic chemical reaction).
  12. Insert the thermometer into the vinegar and let the temp rise until it stops. Add the sodium bicarbonate and instruct the kids to watch the thermometer and tell you what’s happening. (The temperature should decrease by a few degrees.)

Clean up by rinsing the vinegar solution down the drain with plenty of water. Recycle the plastic cup.

Extensions
Try using more or less sodium bicarbonate. Kids can graph the temperature change based on using more sodium bicarbonate, or less.

More Advanced Info on What’s Happening
Chemical reactions occur when molecular bonds are broken and are scrambled to create new bonds, and new products. It takes massive (relatively massive) amounts of energy to do this. If a reaction is endothermic, more energy is used to break the bonds than is released when new bonds are formed, hence the drop in temperature. When a reaction is exothermic, there is a surplus of energy released when the new bonds are formed, thus an increase in temperature.

 

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