Thermochemistry Lab-Experiment 3: THERMOCHEMISTRY- Heats of Reaction

Chemistry 1A- Experiment 3: THERMOCHEMISTRY- Heats of Reaction.

Thermochemistry deals with the energy changes which accompany chemical reactions. The heat evolved or absorbed in a process at constant pressure, qp , is called the change in enthalpy, H, (or the heat of reaction). A reaction in which heat is lost by the reactants and given off to the surroundings is said to be exothermic, and H is negative. A reaction in which heat is absorbed by the reactants is said to be endothermic, and H is positive.

The general term, heat of reaction, may be classified into more specific categories: 1) the heat of formation, Hf, is the amount of heat associated with the formation of 1 mole of a substance directly from the elements in their standard states, 2) the heat of combustion, Hc, is the amount of heat evolved when 1 mole of a substance is burned with excess oxygen, 3) the heats of fusion, vaporization and sublimation are the heats associated with phase changes, 4) the heat of solution is the heat involved when 1 mole of a substance is dissolved in water, and 5) the heat of neutralization is the heat evolved when an acid and a base react to neutralize each other and form 1 mole of water. In this experiment, you will measure the heat of neutralization of HCl(aq) with NaOH(aq), the heat of solution of NaOH(s), and the heat evolved when HCl(aq) is reacted with NaOH(s).

Heat measurements are performed by carrying out the reaction in a calorimeter, in which the heat of reaction can be determined by measuring the temperature change of the solution, T. The heat released is calculated by taking the product of the specific heat of the solution (Cp, in cal/g °C or J/g °C), the mass of the solution (in grams), and the temperature change (in °C): qp = CpgT .

In cases of appreciable temperature difference between the calorimeter and the surroundings, or when the heat is released over a period of time, it may be advisable to take a series of temperature versus time readings and to extrapolate a graph of these data back to the time of mixing (see figures 1 and 2 below). This way, the correct value for T for a reaction can be obtained.

Experimental Procedure

Complete Calorimetry Simulation Demonstration

Access the calorimetry simulation online. Go to the Experiment tab and “Run Demonstration.” Work through the demonstration to familiarize yourself with the program.

The Heat of Neutralization of HCl(aq) and NaOH(aq)

Press “Run Experiment.” In the first container, add 50.0 mL of 1.00 M HCl solution. In the calorimeter, add 50.0 mL of 1.00 M NaOH solution. Record initial temperatures and amounts in your table on the next pages. Check “Show graph view.” Then, run the experiment. Record the final temperature of the new solution and sketch the shape of the graph in the allocated space.

Assume the density of the new NaCl solution is 1.02 g/mL and its heat capacity is 4.18 J/g°C.

The Heat of Solution for NaOH(s)

Reset the experiment. In the first container, add 2.00 g of solid NaOH. In the second container, add 100. g of liquid water. Record initial temperatures and amounts in your table on the next pages. Check “Show graph view.” Then, run the experiment. Record the final temperature of the new solution and sketch the shape of the graph in the allocated space.

Remember, the mass of the solution is the mass of the solvent and the solute. Assume its heat capacity is 4.18 J/g°C.

The Heat of Reaction for HCl(aq) and NaOH(s)

Reset the experiment. In the first container, add 2.00 g of solid NaOH. In the second container, add 100.0 mL of 0.500 M HCl solution. Record initial temperatures and amounts in your table on the next pages. Check “Show graph view.” Then, run the experiment. Record the final temperature of the new solution and sketch the shape of the graph in the allocated space.

Assume the density of the new NaCl solution is 1.02 g/mL and its heat capacity is 4.18 J/g°C.

Demonstrate, by adding together the chemical equations, that the H for part 4 should be the sum of the H’s you calculated in parts 2 and 3. How closely does your calculated H for part 4 compare to this sum? Find the percent difference/error.

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1. Given the following thermochemical equation:

N2 (g) + O2 (g) 2 NO (g) H = + 43.20 kcal

N2 (g) + 3 H2 (g) 2 NH3 (g) H = – 22.10 kcal

2 H2 (g) + O2 (g) 2 H2O (l) H = – 115.60 kcal

Calculate the H for the following thermochemical equation:

4 NH3 (g) + 5 O2 (g) 4 NO (g) + 6 H2O (l)

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