# Additivity Of Heats Of Reaction: Hess’s Law Essay

1149 WordsNov 15, 20115 Pages
Experiment 7 Additivity of Heats of Reaction: Hess’s Law Introduction This report discusses an experiment that combines equations for two reactions to obtain the equation for a third reaction, uses a calorimeter to measure the temperature change in each of three reactions, calculates the heat of reaction (∆H) for three reactions, and uses the results to confirm Hess’s law. This experiment will use a Styrofoam-cup calorimeter to measure the heat released by three reactions. These measurements of heat released will be used to confirm Hess’s law. Hess’s law states that the heat of reaction of the one reaction should be equal to the sum of the heats of reaction for the other two. In the case of this experiment one of the reactions performed will have the same amount of heat released as the other two reactions combined. There are three reactions that take place in this experiment. They are as follows: · NaOH(s) à Na+(aq) + OH-(aq) · NaOH(s) + H+(aq) + Cl-(aq) à H2O(l) + Na+(aq) + Cl-(aq) · Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq) à H2O(l) + Na+(aq) + Cl-(aq) It is important to be aware of these reactions in this experiment for one reason; knowing the reactions will allow one to combine equations for two reactions to obtain the equation for the third reaction. There are also several math related concepts that one should be aware of in this experiment. First, when determining the mass of 100 mL solution, it is necessary to assume that the density of all the solutions is 1.00 g/mL. Secondly, to calculate the heat released by each reaction the following equation will be used: · Q = Cp * m * ∆t (Cp = 4.18 J/goC). Finally, it is necessary to know how to find ∆H. This can be found with the following formula: · ∆H = -q. Additivity of Heats of Reaction: Hess’s Law relates to the concepts being studied in several ways. This experiment mainly relates to