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Additivity of Heats of Reaction: Hess’s Law

Experiment #18 from Chemistry with Vernier

Education Level
High School


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 sensors and equipment. Additional equipment may be required.


Teaching to an educational standard? This experiment supports the standards below.

International Baccalaureate (IB) 2025/Chemistry
Reactivity 1.2.4—An application of Hess’s law uses enthalpy of formation data or enthalpy of combustion data to calculate the enthalpy change of a reaction.

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This experiment is #18 of Chemistry with Vernier. The experiment in the book includes student instructions as well as instructor information for set up, helpful hints, and sample graphs and data.

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