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LAB in Exothermic Endothermic. Last updated October 17, 2016.

Lab

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Submitted by
Mo Wong
Cypress Springs High School
Cypress, Texas

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Summary

In this lab, students determine whether mixing two chemicals is endothermic or exothermic. One is a physical change, one is a chemical change.

Grade Level

High school

Objectives

By the end of this lesson, students should be able to

identify reactions as either endothermic or exothermic.
write balanced chemical equations including heat energy.

Chemistry Topics

This lesson supports students’ understanding of

Endothermic/exothermic reactions
Chemical changes

Time

Teacher Preparation: 10–15 minutes

Lesson: 40 minutes

Materials

For each group:

NH₄Cl powder
Water bottle
Thermometer
Balance
Test tubes
Spatula
Weigh boat
10-mL graduated cylinder
Zinc metal
HCl

Safety

Always wear safety goggles when handling chemicals in the lab.
Students should wash their hands thoroughly before leaving the lab.
When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
Students should be reminded to handle acid carefully. If any gets on their skin, they should alert you and immediately flush the area with water.

Teacher Notes

This is a quick, simple lab that allows students to witness endothermic and exothermic processes; one from a physical change, one from a chemical change.

For the Student

Lesson

Background

In chemical and physical changes, energy can be transferred to or from the surroundings. For example, when a fire burns, it transfers heat energy to the surroundings. Objects near a fire become warmer and the temperature rises. In this experiment, you will make observations and evaluate whether heat energy is released or absorbed.

Purpose

To determine whether a process is exothermic or endothermic.

Safety

Always wear safety goggles when working with chemicals in a laboratory setting.
Handle acid carefully. If any gets on your skin, alert your teacher and immediately flush the area with water.

Materials

NH₄Cl
Water
Thermometer
Balance
Test tubes (2)
Weigh boat
10-mL graduated cylinder
Zinc
<1.0-M HCl

Procedure

Part I

Measure 5.0 mL of distilled water. Pour the water into a test tube.
Find the temperature and record in your data table.
Measure 1.0 g of NH₄Cl. Place your sample into the test tube with the water.
Measure the final temperature and complete your data table.

Part II

Measure 5.0 mL of hydrochloric acid. Pour the acid into a test tube.
Find the temperature and record in your data table.
Add a small piece of zinc to the hydrochloric acid. Gently stir until all zinc has reacted.
Measure the final temperature and record it in your data table.

Data

*Part I*
Initial temperature, T₁	°C
Final temperature, T₂	°C
Change in temperature, DT	°C

*Part II*
Initial temperature, T₁	°C
Final temperature, T₂	°C
Change in temperature, DT	°C

Analysis

Did you observe a physical or chemical change when you added NH₄Cl to the distilled water? Describe the evidence to support your answer.
Did you observe a physical or chemical change when you added zinc metal to hydrochloric acid? Describe the evidence to support your answer.
When you added NH₄Cl to distilled water, that was an (endothermic / exothermic) process and energy was (absorbed / released) by the system. Explain.
When you added zinc metal to hydrochloric acid, that was an (endothermic / exothermic) process and energy was (absorbed / released) by the system. Explain.
What you observed in part I is NH₄Cl_(s) + H₂O_(l) → NH₄⁺_(aq) + Cl^-_(aq) ΔH = 20 kJ/mol
Rewrite this equation with the heat energy on the reactant or product side.
What you observed in part II is Zn_(s) + 2 HCl_(l) → ZnCl_2(aq) + H_2(g)
Is its ΔH_rxn positive or negative? Explain.
Draw the general potential energy diagram for each part.