NOTE: for chemical reactions, see IRC: Chemical Reactions
8.10 C: identify and demonstrate that loss or gain of heat energy occurs during exothermic and endothermic chemical reactions
8.5 E: investigate how evidence of chemical reactions indicate that new substances with different properties are formed
A chemical reaction is a change in energy of a solution or substance. Chemical reactions are the result of the making and breaking of chemical bonds. Chemical bonds themselves are a form of potential energy, and the making and breaking of these bonds represents the conversion of energy from potential to kinetic form. This is a chemical example of the concept that energy can neither be created or destroyed (the Law of Conservation of Mass).
There are two types of energy change in reactions: exothermic reactions and endothermic reactions. An exothermic reaction is a chemical reaction that produces heat and thus releases energy. The term exothermic is composed of the root exo, which is Greek for outside, and thermic, which means heat. Conversely the prefix endo is Greek for inside, and an endothermic reaction is a chemical reaction that draws heat in. This makes endothermic reactions feel cold to the touch.
Energy input and output affect the different types of chemical reactions. Endothermic reactions requires a continuous input of energy to drive the reaction, where as exothermic reactions release energy and are self-propelled. This fundamental difference arises because atoms are driven to be in their lowest energy state. A charged species (ion) is always inevitably driven towards equilibrium. Endothermic reactions require energy because the reaction that occurs moves the molecules that are reacting to a higher energy state from a lower energy state. Exothermic reactions do not require energy to move forward because the molecules in the reaction are moving toward a lower energy state, thus releasing excess energy in the process.
Cold Propane Torch: how the burning of a propane torch can be an endothermic reaction
The Chemistry in the Kitchen. Ingredients in a chemical reaction (reactants) can be found everywhere, even in the kitchen. Sometimes, we even cause endothermic and exothermic reactions without even knowing it. This experiment tests lemon juice, water, sugar, baking soda for reactivity. We are also going to use a balloon to create a closed system, meaning that nothing can come in or out of the system i.e. the balloon serves as a barrier. From my list of ingredients, I am going to predict that the only chemical reaction we will observe will occur when I mix lemon juice and baking soda because I know that when we mix lemon juice and water or lemon juice and sugar i.e. making lemonade does not explode (exothermic reaction) or cool on its own (endothermic reaction). I predict baking soda and lemon juice will react because I know lemon juice is an acid, and I think baking soda is a base.
In order to test my prediction, I set up an experiment mixing lemon juice and water, water and sugar, water and baking soda, lemon juice and sugar, lemon juice and baking soda. I am not testing to see how much of a particular reactant I need, so I am not measuring quantities out, but rather going by the philosophy of using enough until it works. That means I am providing ample amounts of reactants so that if a chemical reaction is going to proceed, it has enough of each reactant to do so, thus I am eliminating a possible source of error (not enough reactant for the reaction to proceed). I am going to measure temperature over an interval of time when I mix my ingredients. This will tell me three things: 1. if there is a drop in temperature, I can infer that a chemical reaction has occurred, 2. whether the reaction is endothermic or exothermic, depending on whether it gives off heat (raising the temperature) or the opposite (drops in temperature), and 3. it gives me an idea as to how fast the reaction occurs (rate of reaction).
Table 1.
| Initial Temp ( °C) |
Temp @ 15 sec ( °C) |
Temp @ 30 sec ( °C) |
Temp @ 45 sec ( °C) |
Temp @ 60 sec ( °C) |
|
| #1 Water and Juice | 22 | 22 | 22 | 22 | 22 |
| #2 Water and Sugar | 22 | 22 | 22 | 22 | 22 |
| #3 Water and Baking Soda | 21 | 19 | 19 | 19 | 19 |
| #4 Juice and Sugar | 22 | 22 | 22 | 22 | 22 |
| #5 Juice and Baking Soda | 21 | 17 | 17 | 17 | 17 |
Water and juice, water and sugar, and juice and sugar did not vary in temperature over the duration of the experiment. Water and baking soda decreased in temperature by 2 °C over an interval of 15 seconds, and remained at a temperature of 19 °C for the rest of the experiment. Juice and baking soda decreased in temperature by 4 °C over an interval of 15 seconds, and remained at a temperature of 17 °C for the rest of the experiment. Based on the results, my hypothesis of the juice and the baking soda reacting is correct. Due to the drop in temperature, I infer that a chemical reaction occurred between lemon juice and baking soda occurred, that this reaction was endothermic because it dropped in temperature, and that the chemical reaction occurred rapidly (over the course of 15 seconds). Inferring from the drop in temperature, I conclude that a chemical reaction occurred when baking soda was added to water. I failed to predict this reaction. Like the reaction between lemon juice and baking soda, the reaction between baking soda and water is endothermic and occurred rapidly (in 15 seconds).
If all of the reactants were at room temperature before mixing ( 22°C), then why did the juice and baking soda and water and baking soda start out 1 °C cooler than the other trials of the experiment?
This question will require critical thought because the students have to realize that chemical reactions can proceed over such short periods of time
that they appear instantaneous to human.
Students need to identify the characteristics of the ingredients that baking soda did react with and characteristics of the ingredients that baking soda
did not react with and compare them in order to brainstorm other possible reactions that baking soda may undergo.
This is a not so good question because the answer is either warmer or cooler, so the question is too narrow.
Simulation or Hands-on Experiment: Energy and Its Many Forms - experiment illustrating reactions using inexpensive ingredients from the kitchen
Simulation: Chemistry in a Bag - experiments that showcase endothermic and exothermic reactions
Simulation: More chemistry in a bag - this is an example of an exothermic reaction, this experiment will also help students to relate other changes in physical properties with reactions besides temperature