Enthalpy of H₂O₂ Decomposition Reaction

10 points

Purpose: To determine the chemical enthalpy (heat) generated or absorbed in the decomposition reaction of a 2.6% solution of hydrogen peroxide.

Pre-Lab Questions:

1. You can assume that H₂O₂ has the same density as water (1g/ml). If you use 40 ml of H₂O₂ in this experiment, how many grams are you using? (this is EASY!)

2. A 3% solution of H₂O₂ purchased from the drug store is roughly 2.6% H₂O₂ and 97.4% water (by mass). How many grams of your sample is actually H₂O₂?

3. Convert these grams of H₂O₂ into moles of H₂O₂.

4. Write a balanced equation for the decomposition reaction of H₂O₂ breaking down into H₂O and O₂ .

5. Use the following information about the DH_f values for H₂O and H₂O₂ to determine the DH_rxn for the balanced equation you have in question #4.

DH_f value for H₂O: -286 kJ/mole DH_f value for H₂O₂: -191 kJ/mole

6. Is DH_rxn for this reaction positive or negative? Is it exo or endothermic?

7. Divide your answer to #5 in half because in the balanced equation you have 2 moles of H₂O₂ and you are going to want to solve the equation for one mole.

8. Multiply your answer to #7 by the number of moles of H₂O₂ you have in your sample to determine the predicted amount of heat generated/absorbed by this reaction. Record this on your data table as Predicted value for q _H2O2.

9. What is your predicted value for q_H2O? (remember H₂O₂ gives off the heat, while the water in the sample will receive the heat)

Procedure:

1. Put about ½ of a scoopful of MnO₂ into the calorimeter. The amount doesn’t matter too much. The MnO₂ is used only as a catalyst and doesn’t affect the reaction results.

2. Measure out EXACTLY 40 mL of 3% hydrogen peroxide solution. Take its temperature and record this as initial temperature of the H₂O₂

3. QUICKLY add the 40 mL of hydrogen peroxide to the calorimeter and put on the lid.

4. Record the HIGHEST TEMPERATURE REACHED. This is final temperature of the H₂O₂ .

5. When you are done, dump the solution in a garbage bag lined trash can – use the one in front of the class.

Post Lab Calculations/Questions:

1. Calculate the Actual Value of q_H2O of this experiment (remember q will come out in Joules, you will need to convert to KJ in order to compare it to your answer #9 in the pre-lab.)

q_H2O = mass_H2O x Specific Heat_H2O x DT_H2O (Specific heat of H₂O = 4.18 J/g^oC)

Record this answer as Actual value of q_H2O

(Hint, for the mass of water, use 40 grams. We can assume that most of that 40 grams is water (97.4% of it is anyway) and so the mass of water affected is 40 grams. If you want to be more accurate, use 97.4% of 40 grams – which is 39.0 grams).

2. Why is your answer to Post Lab Calculation #1 a positive number but Pre-Lab Calculation #8 is a negative number?

3. Find the % error = ½Experimental value – Theoretical Value½ x 100

Theoretical value

Data Table:

Predicted value for q _{H2O2 _________________kJ} Predicted value for q _{H2O _________________kJ}

initial temperature of the H₂O_{2 _ _________}^o_C final temperature of the H₂O_{2 ____________}^o_C

Actual value of q_H2O ________kJ

What is due for this lab:

1. Pre-lab questions (show work where necessary)

2. Data Table

3. Post-Lab questions (show work where necessary)