Determination of the Enthalpy for Decomposition of Hydrogen Peroxide

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Determination of the Enthalpy for Decomposition of Hydrogen Peroxide Objective: To construct a coffee cup calorimeter, measure its calorimeter constant, and determine the enthalpy of decomposition and formation of hydrogen peroxide. Background: This experiment is a classic thermodynamics lab. In it, we attempt to measure the enthalpy (H) of a chemical reaction. The main obstacle is that this is a quantity that cannot be measured directly. It instead is observed as heat from one substance is transferred to another while being monitored with a thermometer. A temperature change in a known substance (such as water) can be related to a certain amount of heat (q).

q   mass  T f  Ti   Specific heat  Our calorimeter is going to be a set of Styrofoam coffee cups. These cups are well known insulators, and they should keep most heat inside themselves. They are not perfect, however, and they will absorb some of the heat from the process, so we need to determine the calorimeter constant. For any process, the first law of thermodynamics must be followed.

E  0  qsystem  qsurroundings However, if the calorimeter itself is going to absorb some energy, it must be accounted for as well. E  0  qsystem  qsurroundings  qcalorimeter In the first portion of our experiment, a known mass of water with a known temperature will be used to determine how much energy is being absorbed by the calorimeter. Once this is known, the change in enthalpy from the decomposition of hydrogen peroxide can be determined. 2H2O2(aq)  2H2O(l) + O2(g) A solution of hydrogen peroxide will be added to the calorimeter and its temperature will be monitored for a short time. A small amount of iron nitrate will be added to catalyze the reaction and produce a measureable temperature change. This temperature change will be plotted over time. Using this plot, the mass of solution, its specific heat, the calorimeter constant, and the temperature change, we will be able to determine the enthalpy for the reaction.

Procedure Week One – Determination of the calorimeter constant Objective – To measure the amount of energy absorbed by the calorimeter while observing a change in temperature.

1. Using a top loading balance, determine the mass of Styrofoam cup and a stir bar. 2. Place about 100mL of warm deionized water in the cup. Monitor its temperature for 3-4 minutes. Record the temperature every minute. 3. Using a top loading balance, determine the mass of a second Styrofoam cup and a stir bar. 4. Place about 100mL of room temperature water in the cup. Monitor its temperature for 34 minutes. Record the temperature every minute. 5. When the temperature of the warm water is about 20C above the room temperature water, quickly pour it into the room temperature water. Continue monitoring the temperature and recording it every minute. 6. After several minutes, when the temperature is no longer changing quickly, record the mass of the combined water. 7. Repeat this procedure at least one more time. Week One – Preparation of a standard potassium permanganate solution Objective: To prepare a potassium permanganate solution with a known concentration. This will be used in a second week to standardize a hydrogen peroxide solution.

1. Obtain enough potassium permanganate to make 250.0mL of a 0.02M solution. NOTE: Use the analytical balance for this operation. It is NOT necessary to get exactly the amount you calculated. It is only important to be close to this amount, and to record the amount you took accurately. 2. Dissolve this in a small amount of water before diluting it to a final volume of 250.0mL in a volumetric flask. 3. Transfer this to a clean dry bottle, and store it for use next week.

Week Two – Determination of the Concentration of hydrogen peroxide. Objective: Determine the concentration of hydrogen peroxide and use this result to know how many moles are being decomposed.

1. Pipet 2.00mL of hydrogen peroxide into a small Erlenmeyer flask....
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