Endothermic and Exothermic Reactions

Endothermic and Exothermic Reactions

Description

A small amount of water soluble salt is placed in a test tube. A small amount of water at room temperature is added. A change in temperature is sensed by a finger. When the temperature changes, one of the terms (exothermic or endothermic) is applied to describe the reaction.

Set

Chemical changes are usually accompanied by a change in heat. If heat is absorbed in a reaction, the reaction is said to be endothermic. The products are higher in heat content than the reactants. A reaction in which heat is given off is exothermic. In this case, the products are lower in heat than the reactants. The amount the temperature increases or decreases in a given reaction can be measured by placing a thermometer in the reaction vessel.

Precautions

Use nontoxic chemicals. Do not ingest chemicals.

Procedure

For each salt tested, place sample in tube, place thermometer in tube, note temperature, add water, and note temperature again.
Repeat the procedure for each salt tested.

Handout Makeup

Name ___________________________ Class ________

Teacher__________________________

DoChem 093 Endothermic and Exothermic Reactions

Watch the Movie. Use the 'Salt Observations' to answer the questions.

Salt Observations

NaCl	salt dissolves, no temperature change
CaCl₂	salt dissolves, temperature increases
NH₄Cl	salt dissolves, temperature decreases

Write net ionic equations to show dissolving process. Use your observations as a basis for adding an energy term to the appropriate side of each equation. Describe the reaction using the terms exothermic or endothermic where appropriate.
Instant chemical cold packs and heat packs are available for treating athletic injuries or even for warming hands or feet in winter. Explain how instant chemical cold packs and heat packs work.

Teachers Guide

Purpose

To demonstrate endothermic and exothermic reactions.

Materials

(for 10 students working in pairs)

15 13- x 100-mm test tube
10 g each NaCl, NH₄Cl, and CaCl₂
5 wash bottle containing tap water
15 stopper or sufficient plastic wrap to cover test tubes
5 grease pencil or labels for test tubes
5 test tube racks
5 thermometer (optional)

Lab Hints

CaCl₂ is deliquescent and needs to be covered with a cork or plastic wrap. You may want to discuss this with your students. If test tubes are to be left for any length of time, it is best to cover all of them with plastic wrap.
Students should be able to determine if heat is released or absorbed just by touching the test tubes.
Label all test tubes.

Time

Teacher preparation: 10 minutes

Class time: 25-30 minutes (depends on number of salts)

Hazards

Chemical toxicity is possible.

Precautions

Use nontoxic chemicals. Do not ingest chemicals.

Disposal

The chemicals suggested for this experiment may be disposed of safely at the sink.

Sample Data

Salt Observations

NaCl	salt dissolves, no temperature change
CaCl₂	salt dissolves, temperature increases
NH₄Cl	salt dissolves, temperature decreases

Closure?

Closure Question:

Write net ionic equations to show dissolving process. Use your observations as a basis for adding an energy term to the appropriate side of each equation. Describe the reaction using the terms exothermic or endothermic where appropriate.

Answers to Closure Question:

NaCl(s) --> Na⁺(aq) + Cl^-(aq)
CaCl₂(s) --> Ca²⁺(aq) + 2Cl^-(aq) + energy (exothermic)
energy + NH₄Cl(s) --> NH₄⁺(aq) + Cl^-(aq) (endothermic)

Applications

Most students, especially the athletes in class, should be familiar with instant cold packs and heat packs. If you have one available, demonstrate it and have students read the label and check the ingredients. Ask them to explain, in their own words, how cold packs and heat packs work.

Makeup Answers

See closure answers.
Cold chemical packs mix two chemicals which undergo an endothermic reaction. Hot packs mix two chemicals which undergo an exothermic reaction.

Key Words

endothermic
exothermic
solubility
deliquescent
reactants
net ionic equations
heat of reaction