Endothermic Reaction (Anna Mkrtchyan)
In this investigation, students will work in groups of 3 to explore what is a chemical reaction and what happens to the reaction rate when you change parameters in the reaction. The materials used in this lesson allow students to safely conduct their investigations, as well as allow for safe and convenient disposal of materials. All materials are available in supermarkets or educational suppliers.
Research Question and Hypothesis
Some chemical reactions absorb energy and are called endothermic reactions. What happens to the reaction rate when you change parameters (ratio of reactants) in the reaction.
Standards
High School Chemistry: Chemical Thermodynamics
7. Energy is exchanged or transformed in all chemical reactions and physical
changes of matter. As a basis for understanding this concept:
a. Students know how to describe temperature and heat flow in terms of the
motion of molecules (or atoms).
b. Students know chemical processes can either release (exothermic) or
absorb (endothermic) thermal energy.
9-12.II.B.1 - Chemical Reactions
Independent variable
The independent variable is time. It will be measured in seconds.
Dependent variables
The dependent variable is Temperature. It will be measured in 0C.
Series
Study the temperature of reaction (dependent variable) as a function of time (independent variable) at various parameters (ratio of reactants) in the reaction (series).
Constants and Controls
Constant-the amount of cabbage juice, the type of chemicals, the way chemicals are mixed in the bag
Control-1:1 Ratio of sodium hydrogen carbonate and citric acid
Materials
Endothermic reaction
citric acid (C6H8O7)
sodium hydrogen carbonate (NaHCO3)
purple cabbage juice
Citric acid is a white crystalline powder and is used as a flavoring and preservative in food and beverages, especially soft drinks. Citric acid can be added to ice cream as an emulsifying agent to keep fats from separating, to caramel to prevent crystallization, or to recipes to replace fresh lemon juice. In the home, it can be used to soften water, which makes it useful in soaps and laundry detergents. It is an active ingredient in some bathroom and kitchen cleaning solutions to remove hard water stains from glass without scrubbing.
Sodium hydrogen carbonate (sodium bicarbonate) has long been known and widely used. It is referred to as baking soda and used in cooking as a leavening agent. Many laboratories keep a bottle within easy reach since it is amphoteric, so it can react with acids or bases. Mixed with water, sodium bicarbonate can be used as an antacid to treat acid indigestion and heartburn. It has also been known to be used in treating scalding, preventing blistering and scarring. Toothpastes containing sodium bicarbonate has shown to have better whitening properties and plaque removal than toothpastes without it. Baking soda is also effective as a odor remover, clothes softener and used in fire extinguishers.
How to make a cabbage juice: Shred a handful of red cabbage and boil for 5 minutes. Let it cool, and store.
Procedures
Get in groups of 3.
Material for each group:
1 gallon sized zip lock bag
a graduated cylinder
thermometer
Put about 4 spoonfuls of the each chemicals in the bag.
Measure out 200 mL of the red cabbage juice into the graduated cylinder.
Get a stop watch (computer, iPhone etc)
Put the juice into the gallon sized zip lock bag.
Take the initial temperature (there should be 1 decimal place to your reading). t (time) initial = 0 seconds
take the readings every 15 seconds.
person #1 - write down the temperatures every 15 seconds
person #2 - hold the bag and try to mix/squish and stir
person #3 - read the reading on the thermometer
Stop when the temperature doesn't change after 3 readings
Everything can be thrown away in the trash. Very safe.
Repeat the investigations using ratio of chemicals to 2:1 and 3:1
The bag gets very cold.
There is a color change.
The bag inflated with carbon dioxide gas.
Sample data and graphs
Analysis & Conclusions
Ask students to complete a summary in their notebooks of what type of reaction they believe this investigation was, endothermic or exothermic. Students should use evidence from the investigations to justify their answers. Students should point out based on the theoretical background that the difference between final and initial temperature is enough to determine the type of reaction. Endothermic reactions absorb heat energy from the environment. Reaction rates can be increased if the concentration of reactants is raised. An increase in concentration produces more collisions. The chances of an effective collision goes up with the increase in concentration. Only experimental observation reveals the link between concentration and reaction rates.
Photos