Experiment 18 Additivity of Heats of Reaction Hess\'s Law In this experiment, yo

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Experiment 18 Additivity of Heats of Reaction Hess's Law 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. NaOH(s)- Na-(aq) + OH (aq) }l "? (2) Solid sodium hydroxide reacts with aqueous hydrochloric acid to form water and an aqueous NaOH(s) + H.(aq) ) + Cl (aq)- H20(1) + Na+(aq) + Cl (aq) AH2-7 solution of sodium chloride. (3) Solutions of aqueous sodium hydroxide and hydrochloric acid react to form water and aqueous sodium chloride. Na (aq1+ OH (aq) + H.(aq) )-O-(aq)- H2O(l) + Na'(aq) + Cl (aq) Alb = ? 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, Al, for the three reactions. .Use the results to confirm Hess's law. Figure 1 Chemistry with Comouters 18-1

Explanation / Answer

Q6.

Q = heat of solution

Q = m*C*(Tf-Ti)

m = mass of solution = 100 g of solution approx

C = 4.184 J/gC , assume it is water's

dT = Tf-Ti = 4.6°C for trial 1

then

Q = 100*4.184*4.6

Q = 1924.64 J

Q7

heat abosrbed by solution = heat released by reaction

HRxn = -Q = -1924.64 J

Q8

mol of NaOH = mass/MW = 2.038/40 = 0.05095 mol of NaOH

Q9

HRxn = H/n = -1924.64/0.05095

HRxn = - 37775.07 J/mol

HRxn = -37.07 kJ/mol

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