Deadlines

Teaching Events‎ > ‎

Endothermic and Exothermic Reactions (Tamara Desso)

Title: Endothermic and Exothermic Reactions

Principle(s) Investigated: Chemical reactions, conservation of energy, endothermic and exothermic reactions

Standards :  

8.5.c Chemical reactions are processes in which atoms are rearranged into different combinations of molecules. As a basis for understanding this concept: Students know chemical reactions usually liberate heat or absorb heat.

8.9.e Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other three strands, students should develop their own questions and perform investigations. Students will: Construct appropriate graphs from data and develop quantitative statements about the relationships between variables.

Materials:

Baking Soda

Vinegar

Thermometer

Plastic baggie

Index card

Steel wool

Timer


Procedure

Open DATA SHEET

Reaction 1: Vinegar + Baking Soda

You may want to warn your students that this trial will go very quickly. It may be best to do a quick run through once before actually performing the experiment so they understand how quickly they will have to take measurements.

Each member of the group will have a specific job for each reaction.

**CHOOSE A DIFFERENT JOB FOR EACH REACTION. **

Please write the names of your group members who will:

1.       Time Reader- tells group when to take a measurement

2.       Observer- watches as the reaction takes place and takes notes on any color, odor, texture, etc. changes

3.       Thermometer Reader- reads off temperatures every time the time reader says so

4.       Temperature Recorder- writes down the temperature said by thermometer reader on the data table

*If you only have 3 people in your group, do NOT include an observer. Instead record your observations as a group after the 30 seconds is over.

1.       Observer: tear off the back sheet of this packet that says “Observation Boxes” and keep this close at hand. Time Reader: place 20ml vinegar in the plastic baggie. Put the thermometer in the plastic baggie. Thermometer Reader: hold the thermometer and plastic baggie so they do not fall over. Read the temperature and Temperature Recorder: will record it in the chart under "Time 0".

2.       Thermometer Reader: Measure 2 scoop of baking soda and place this on the paper square.

We will do the next part together. Wait for the teacher to put the stop watch up on the screen.

3.       Thermometer Reader: ONLY WHEN TEACHER SAYS START: dump all the baking soda in the plastic baggie. Gently swirl/mush the plastic baggie to mix reactants. Time reader: call out the time every 3 seconds. When each 3 seconds is called, Temperature Recorder should record the temperature in the chart. Observer: quietly write down observations in the observation box for reaction 1.

Reaction 2: Steel Wool + Vinegar + Oxygen

Choose a different job from the one that you chose for reaction one. Write your name in the proper slot in the group members chart.

1.       Observer: take the “Observation Boxes” and keep this close at hand. Time Reader: place steel wool in beaker and add 40ml vinegar to the beaker so that the steel wool is completely covered. We will let this sit for about 2 minutes. Observer: record observations of the steel wool in the vinegar solution. Thermometer reader: during the wait time, place the thermometer through the hole in the index card. 

We will do the next part together. Wait for the teacher to put the stop watch up on the screen.

2.    After about two minutes have passed AND THE TEACHER TELLS YOU TO, Temperature Recorder: pour the vinegar out of the beaker into an empty beaker. Thermometer reader:put thermometer into the steel wool.  Read the temperature and Temperature Recorder: will record it in the chart under "Time 0".

3.        Time reader: call out the time every 30 seconds. When each 30 seconds is called, Temperature Recorder should record the temperature in the chart. Observer: quietly write down observations in the observation box for reaction 1. We will observe this reaction for a total of 5 minutes.



Student prior knowledge: Students will have just finished ionic and covalent bonding as well as a small unit on graphing and data analysis. They should understand that ionic and covalent bonds hold compounds and molecules together, respectively. An introduction will be given on chemical changes and chemical equations, including reactants and products. Additionally, energy conservation will be discussed and students should be able to understand that chemical energy can be converted to thermal energy. They will be taught about endothermic and exothermic reactions before taking part in the lab activity.

Explanation: Baking soda and vinegar will react vigorously following the equation:
NaHCO3 + CH3COOH => CH3COO-Na+ + H2O + CO2
This reaction is endothermic, meaning that the breaking and forming of bonds overall absorbs more energy than it releases. Students will see a decrease in the temperature of the system as the reaction procedes. The reaction takes place VERY quickly so it may be a good idea to have students practice each of their respective jobs once through before actually adding the baking soda to the vinegar.

Steel wool has a protective coating that is easily degraded by vinegar. Allow the steel wool to soak in vinegar for 1-2 minutes and then pour out the vinegar. The exposed and acidified iron in the steel wool (steel wool is an alloy of carbon and iron) will react with oxygen in the air to form rust: 
4Fe + 3O2 => 2Fe2O3

This reaction takes quite a bit longer than the baking soda and vinegar so you may wish to do this first so that students are more familiar with their roles in the lab group before attempting the vinegar and baking soda experiment. This reaction releases more energy than it absorbs during the breaking and forming of bonds and therefore is an exothermic reaction. Students will notice a rise in temperature of about 2-3 degrees Celsius over a period of about 5 minutes.

Questions & Answers: 

1. Describe another process in which energy is converted from one form to another. Types of energy include: electrical, mechanical, chemical, and thermal.

There are many processes in which energy is converted from one form to another. In cars, chemical energy is converted to mechanical energy when the fuel is burned and the tires rotate. A windmill turns mechanical energy into electrical energy that can power our computers, phones, and microwaves. Your body converts chemical energy into mechanical energy after you have had your lunch and are going out to play soccer.

2. What process is occurring to cause these reactions to release or absorb thermal energy?

Chemical reactions involves the breaking and forming of chemical bonds. Certain reactions require an overall input of thermal energy where others release net thermal energy. In either case, there is a transfer between chemical and thermal energy.

3. Why is it important to let the steel wool soak in vinegar prior to exposing it to the oxygen in the air?

The vinegar removes the protective coating on steel wool. Steel wool is an alloy that contains iron. The iron is not exposed to oxygen when the protective coating is present. After vinegar removes the coating it acidifies the iron which makes it much easier for the iron to react with oxygen in the air.

Applications to Everyday Life: 

1. Converting chemical to mechanical energy to move cars: The chemical energy stored in natural gas undergoes a combustion reaction with oxygen which is an exothermic reaction. The energy is converted to mechanical energy to move the car from one place to another.

2. Instant cool/ hot packs: Cold packs contain chemicals that react when a small pouch that is separating them is broken. The reaction is endothermic which makes it feel cold. The reverse is true of hot packs. When the small pouch is broken and the chemicals come in contact with one another they react exothermically, producing heat.

3. Burning a candle: Candles are made of paraffin which reacts with oxygen in the air when enough initial energy is provided (lighting with a match). This reaction is exothermic, giving off heat energy to the surroundings.

Photographs: Please see attached photos.

Videos: 

The following videos show how to perform the steel wool and vinegar as well as the baking soda and vinegar experiments. 

Č
ć
Tamara Desso,
Nov 17, 2011, 1:51 PM
ą
Tamara Desso,
Dec 12, 2011, 4:30 PM
ĉ
Tamara Desso,
Nov 17, 2011, 1:50 PM
Comments