# Additivity of Heat of Reaction

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Computer Additivity of Heats of Reaction: Hess’s Law 18 (1) Solid sodium hydroxide dissolves in water to form an aqueous solution of ions. (2) Solid sodium hydroxide reacts with aqueous hydrochloric acid to form water and an aqueous solution of sodium chloride. NaOH(s) + H+(aq) ) + Cl–(aq)  → H2O(l) + Na+(aq) + Cl–(aq) ∆H2 = ?  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, ∆H, for the three reactions. Use the results to confirm Hess’s law. Ev al Figure 1 ua tio Na+(aq) + OH–(aq) + H+(aq) ) + Cl–(aq)  → H2O(l) + Na+(aq) + Cl–(aq) ∆H3 = ?  n (3) Solutions of aqueous sodium hydroxide and hydrochloric acid react to form water and aqueous sodium chloride. co NaOH(s)  → Na+(aq) + OH–(aq) ∆H1 = ?  Chemistry with Vernier py 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: 18 - 1 Computer 18 You will use a Styrofoam cup in a beaker as a calorimeter, as shown in Figure 1. For purposes of this experiment, you may assume that the heat loss to the calorimeter and the surrounding air is negligible. Even if heat is lost to either of these, it is a fairly constant factor in each part of the experiment, and has little effect on the final results.