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Determining the Heat of Reaction

Description of Activity

Title: Determining the Heat of Reaction

Overview: Working in groups of 2-4, students will experimentally classify solubility as either endothermic or exothermic buy measuring temperature increases or decreases in a micro scale calorimeter. Using the calorimeter, students will then explore reactions with acids and metals, determine the ÆH for the reaction, and rank those results.

This activity should require about three 50 minute lab periods.

Materials: Skill Development Activity:

plastic sauce cups with lids (the kind used in fast food restaurants)
thermometers
rubber bands

Ammonium Chloride
Ammonium Nitrate
Ammonium Sulfate
Copper (II) sulfate
Calcium chloride
Sodium hydroxide
Zinc sulfate

Inquiry Lab Activity:

2.0 M Hydrochloric Acid
2.0 M Sulfuric Acid
Zinc (very small pieces)
Magnesium (ribbon)

Background

The amount of energy either given off or absorbed by a reaction (ÆH) can be measured with a calorimeter. Calorimeters allow investigators to use the law of conservation of energy to describe the energy flow in a chemical reaction. The energy of the reaction is equal to but opposite in sign to the energy gained or lost by the calorimeter itself. As the reaction progresses, the temperature of the reacting mixture and the immediate environment (a calorimeter) either increases or decreases. By evaluating the energy transfer in the calorimeter, both a qualitative and a quantitative evaluation of the reaction is possible.

A reaction that absorbs energy from the environment is called an endothermic reaction. On the macroscopic level, the temperature of the substances and the environment decreases. Endothermic reactions are symbolized by a positive ÆH.

A reaction that releases energy to the environment is called an exothermic reaction. On the macroscopic level, the temperature of the substances and the calorimeter increase. The products in such a reaction are at a lower potential energy than were the reactants. Exothermic reactions are symbolized by a negative ÆH.

In order to effectively measure the heat transfer between the reaction being studied and the calorimeter, precautions must be taken to retain the heat energy within a measurable environment. A calorimeter is used to isolate the reaction from the surroundings.

In the coffee-cup calorimeter used in this laboratory exercise the following relationships exist:

ÆHreaction = - ÆHcalorimeter
ÆHcalorimeter = m C ÆT

where m is the mass of the water, C is the specific heat of water, and ÆT is the temperature change of the water. The value for C is accepted to be 4.18 J/g¥¡C.

Teaching and Learning Strategies

Prerequisite instruction:

Students must be able to calculate the ÆH of a reaction before completing the guided inquiry activity.

A simple and economical calorimeter can be made by using two plastic sauce containers, a lid, and a small rubber band. Wrap the rubber band around one of the cups. Place the cup with the rubber band inside of the other cup (it should form a tight seal). Punch a hole in the lid for the thermometer.

The Handbook of Chemistry and Physics should be available as a reference.

Safe Operating Procedures:

Safety glasses and aprons should be worn at all times while in the chemistry laboratory.

Some of the salts used in the heat of solution problem are toxic. Special care should be taken with the acids used in the inquiry procedure.

Dispose of chemicals appropriately. See Flinn Chemistry Catalog for disposal procedures.

Skills Development Lab

Procedures:

1. Construct a calorimeter as demonstrated by your instructor.
2. Add 15.0 mL of water to the calorimeter.
3. Record the temperature of the water as your initial temperature
4. Add 0.5 grams one of the salts to the calorimeter. Record the temperature change every fifteen seconds until there is no further change in temperature.
5. Repeat this procedure with each of the different salts.

Assessment of Skills Development Lab

1. Which substance caused the greatest temperature change per gram of solute.
2. Which substance caused the greatest temperature change per mole of solute.
3. Describe any patterns that you notice.
4. Classify each of the heats of solution as either endothermic or exothermic.

Invitation to Learn

Design and carry out an experiment to rank (by quantitative analysis) each of the following acid/metal reactions from the greatest ÆH to the smallest ÆH.

2.0 M Hydrochloric Acid and 2.0 M Sulfuric Acid with both Zinc and Magnesium

Assessment of Learning:

Suggested Evaluation Tools

The lab report should include the calculation of ÆH for each of the reactions that the students tested.

1. Compare your values of ÆH with those currently accepted. You may find those values either in your textbook or in the Handbook of Chemistry and Physics.
2. What factors might account for the differences between your calculated values and the accepted values?

Alternative Evaluation Tools

Alternatives to the written report may include the following:

A contest which rewards the least experimental error or the most reproducible results may add interest to the activity.

1. In an exothermic reaction:

A. ÆH is negative.
B. The products have less potential energy than the reactants.
C. Heat is given off to the environment
D. All of the above
E. None of the above

2. Which of the following is NOT involved in the calculation of heat absorbed as a substance reacts?

A. Mass of the sample.
B. Specific heat.
C. Change in temperature.
D. Density of the sample.

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This document was submitted for posting to the Internet by the State Science Specialist. Any questions concerning content should be directed to that individual.

Updated September 25 1997 by Michelle Dumas