Tuesday, March 31, 2009

Pre-Lab 2: Kinetics

SCH4U

Pre-lab 2: Kinetics (Molar Enthalpy of Solution)

Background:

Physical and chemical changes always involve an exchange of energy. Nearly all of the time, that energy exchange has a component called HEAT. We can measure the amount of heat given off or taken in by a chemical reaction by measuring the temperature change in the surroundings. This is called CALORIMETRY. Today, you will be measuring the heat given off or taken up by the dissolution of sodium hydroxide (NaOH) in water.

For your pre-lab introduction section:

  1. Write out the equation of what is occurring in the system (NaOH dissolving in water).
  2. Describe some real life examples of when enthalpies are important to you.
  3. Predict what you think you will observe based on the procedure below.
  4. Solve the following: If 5 g of NaOH(s) dissolves in 300 mL of water and the temperature rises from 25 to 35 C, what is the molar enthalpy of solution of NaOH in water and what is your % error based on the information above?

Procedure:

0. Wear safety goggles at all times!

  1. Measure 100 mL of water into one Styrofoam cup. Record the exact volume.
  2. Cut the rim off another cup so that it will nestle inside the first when inverted (with no spaces for heat to enter or escape).
  3. Punch a hole in bottom of the second cup and insert a thermometer inside. DO NOT use the thermometer to punch this hole! Place the second cup with thermometer inside the first so that the thermometer is reading the temperature of the water. Record the temperature.
  4. Measure out 3 g of NaOH. Record the exact mass.
  5. Lift off the second cup and add the NaOH to the bottom cup. Immediately replace the top (with thermometer. Record the maximum or minimum temperature reached. Carefully swirl the two cups while measuring.

Analysis questions: (to be completed after the lab)

  1. Calculate the molar enthalpy of solution of NaOH in water.
  2. Calculate your % error.
  3. What assumptions are we making in this experiment?
  4. What sources of error are there in this experiment and how would each effect your result (overshoot or undershoot of the molar enthalpy of solution)?
  5. Write the reaction you have performed using four of the different methods you’ve studied.

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