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Additivity of Heats of Reaction: Hess's Law

Figure from experiment 18 from Chemistry with Vernier


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{  =  ?}}


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 features the following Vernier sensors and equipment.

Option 1

Option 2

Additional Requirements

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

Standards Correlations

See all standards correlations for Chemistry with Vernier »

Chemistry with Vernier

See other experiments from the lab book.

1Endothermic and Exothermic Reactions
2Freezing and Melting of Water
3Another Look at Freezing Temperature
4Heat of Fusion of Ice
5Find the Relationship: An Exercise in Graphing Analysis
6Boyle's Law: Pressure-Volume Relationship in Gases
7Pressure-Temperature Relationship in Gases
8Fractional Distillation
9Evaporation and Intermolecular Attractions
10Vapor Pressure of Liquids
11Determining the Concentration of a Solution: Beer's Law
12Effect of Temperature on Solubility of a Salt
13Properties of Solutions: Electrolytes and Non-Electrolytes
14Conductivity of Solutions: The Effect of Concentration
15Using Freezing Point Depression to Find Molecular Weight
16Energy Content of Foods
17Energy Content of Fuels
18Additivity of Heats of Reaction: Hess's Law
19Heat of Combustion: Magnesium
20Chemical Equilibrium: Finding a Constant, Kc
21Household Acids and Bases
22Acid Rain
23Titration Curves of Strong and Weak Acids and Bases
24Acid-Base Titration
25Titration of a Diprotic Acid: Identifying an Unknown
26Using Conductivity to Find an Equivalence Point
27Acid Dissociation Constant, Ka
28Establishing a Table of Reduction Potentials: Micro-Voltaic Cells
29Lead Storage Batteries
30Rate Law Determination of the Crystal Violet Reaction
31Time-Release Vitamin C Tablets
32The Buffer in Lemonade
33Determining the Free Chlorine Content of Swimming Pool Water
34Determining the Quantity of Iron in a Vitamin Tablet
35Determining the Phosphoric Acid Content in Soft Drinks
36Microscale Acid-Base Titration

Experiment 18 from Chemistry with Vernier Lab Book

<em>Chemistry with Vernier</em> book cover

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Dev Reference: VST0087

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