Additivity of Heats of Reaction: Hess's Law

Recommended for grades 9–12.

Figure from experiment 24 from Science with TI-Nspire Technology

Introduction

In this experiment, you will use a Styrofoam-cup calorimeter to measure the heat released by three reactions. One of the reactions is the same as the combination of the other two reactions. Therefore, according to Hess’s law, the heat of reaction of the one reaction should be equal to the sum of the heats of reaction for the other two. This concept is sometimes referred to as the additivity of heats of reaction. The primary objective of this experiment is to confirm this law. The reactions we will use in this experiment are:

(1) Solid sodium hydroxide dissolves in water to form an aqueous solution of ions.

${\text{NaOH(s)}} \to {\text{N}}{{\text{a}}^{\text{+}}}{\text{(aq) + O}}{{\text{H}}^{\text{-}}}{\text{(aq) }}\Delta {{\text{H}}_{\text{1}}}{\text{ = ?}}$

(2) Solid sodium hydroxide reacts with aqueous hydrochloric acid to form water and an aqueous solution of sodium chloride.

${\text{NaOH(s) + }}{{\text{H}}^{\text{+}}}{\text{(aq) + C}}{{\text{l}}^{\text{-}}}{\text{(aq)}} \to {{\text{H}}_{\text{2}}}{\text{O(1) + N}}{{\text{a}}^{\text{+}}}{\text{(aq) + C}}{{\text{l}}^{\text{-}}}{\text{(aq) }}\Delta {{\text{H}}_{\text{2}}}{\text{ = ?}}$

(3) Solutions of aqueous sodium hydroxide and hydrochloric acid react to form water and aqueous sodium chloride.

${\text{N}}{{\text{a}}^{\text{+}}}{\text{(aq) + O}}{{\text{H}}^{\text{-}}}{\text{(aq) + }}{{\text{H}}^{\text{+}}}{\text{(aq) + C}}{{\text{l}}^{\text{-}}}{\text{(aq)}} \to {{\text{H}}_{\text{2}}}{\text{O(1) + N}}{{\text{a}}^{\text{+}}}{\text{(aq) + C}}{{\text{l}}^{\text{-}}}{\text{(aq) }}\Delta {{\text{H}}_{\text{3}}}{\text{ = ?}}$

Objectives

In this experiment, you will

Combine equations for two reactions to obtain the equation for a third reaction.
Use a calorimeter to measure the temperature change in each of three reactions.
Calculate the heat of reaction, ΔH, for the three reactions.
Use the results to confirm Hess's Law.

Sensors and Equipment

This experiment requires each of the following Vernier sensors and equipment (unless otherwise noted):

Stainless Steel Temperature Probe

Additional Requirements

You may also need an interface and software for data collection. What do I need for data collection?

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List of materials and equipment

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