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JoVE Lab Manual
Lab: Chemistry

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Education
Enthalpy of Reaction
 

Learning Objectives

At the end of this lab, students should know...

What is the first law of thermodynamics?

The first law of thermodynamics is the law of conservation of energy, which states that the total energy of an isolated system must remain constant. Although energy can be transferred from one form to another, it can never be created or destroyed.

What is the difference between a state function and a path function?

A path function depends on the transition that a system undergoes from the initial to the final state, whereas a state function is independent of the path taken. A state function is only dependent on the initial and final state. Enthalpy is an example of a state function.

What is enthalpy?

Enthalpy is a thermodynamic property used to describe the heat lost or gained in a system. Reaction enthalpies can be used to determine if a reaction is endothermic or exothermic.

What is Hess's Law?

Hess’s law states that the enthalpy of an overall reaction is equal to the enthalpy of reaction for each of the individual reaction steps.

How is a calorimeter used to measure heat?

Reactants are placed inside of the insulated calorimeter reaction chamber, which prevents energy loss to the environment. The reactants are mixed, and as the reaction occurs, changes in temperature are recorded as ΔT. The thermodynamic heat is calculated using the specific heat capacity, the mass of the substance, and the change in temperature.

List of Materials

  • Magnesium ribbon
    3 g
  • Magnesium oxide
    1 g
  • 150-mL glass beaker
    5
  • 600-mL glass beaker
    5
  • 50-mL glass graduated cylinder
    5
  • 70-mm watch glass
    5
  • Digital thermometer (range includes 60 ºC)
    5
  • Magnetic stir bar
    5
  • 125-mL plastic wash bottle
    5
  • 12-oz polystyrene cups
    10
  • Cardboard lid with a hole punched in the center
    5
  • Data acquisition device
    5
  • Temperature probe (acid-tolerant)
    5
  • Flash drive
    5
  • Non-heating stir plate
    5
  • Hot plate
    5
  • Lab stand
    5
  • Thermometer clamp
    5
  • Stainless steel forceps
    5
  • Scissors
    5
  • 12.1 M hydrochloric acid
    350 mL
  • 2-L volumetric flask with stopper
    1
  • 500-mL glass graduated cylinder
    1
  • 1-L glass graduated cylinder
    1
  • Glass funnel
    1
  • 2-L glass bottle with an acid-resistant cap
    1
  • Square of plastic paraffin film
    1
  • Magnetic wand
    1
  • Airtight plastic container or bag
    2
  • Sheet of emery paper, any mesh
    Dependent on lab size
  • Analytical balance (mimimum 1)
    Dependent on lab size
  • Weighing boats
    Dependent on lab size
  • Deionized water
    Dependent on lab size
  • Disposable plastic pipettes
    Dependent on lab size
  • Laboratory tape
    Dependent on lab size
  • Laboratory wipes
    Dependent on lab size
  • Lab spatula
    Dependent on lab size

Lab Prep

Source: Smaa Koraym at Johns Hopkins University, MD, USA

  1. Preparation of 2.0 M Hydrochloric Acid

    Here, we show the laboratory preparation for 10 students working in pairs, with some excess. Please adjust quantities as needed.

    • To set up for this lab experiment, wear the appropriate personal protective equipment, including a lab coat, chemical splash goggles, and gloves. Hydrochloric acid is extremely corrosive. So, all hydrochloric acid solutions must be used carefully in a fume hood.
    • Prepare 2 L of 2 M HCl, first, carefully measure 330 mL of 12.1 M HCl with a 500-mL graduated cylinder. Close the bottle of concentrated HCl when finished.
    • Place a funnel in the mouth of a 2-L volumetric flask, and carefully pour the HCl into the flask. Then, slowly add 1 L of deionized water to the flask.
    • Place the funnel on a clean surface and seal the top with plastic paraffin film. Carefully invert the flask several times until the mixture appears homogeneous.
    • Then, remove the film and fill the volumetric flask to the line with deionized water.
    • Stir the solution with a large magnetic stir bar until thoroughly mixed, which usually takes 20 - 30 s. Then, turn off the stir plate, remove the flask, and retrieve the stir bar.
    • Label a 2-L glass bottle with ‘2 M HCl’. Then, place the funnel in the mouth of the labeled bottle and carefully transfer the 2 M HCl solution from the flask to the bottle. Cap the bottle and store it in the hood reserved for the instructor.
    • Lastly, put away the concentrated HCl and clean your glassware according to your institution's procedures.
  2. Preparation of the Laboratory
    • Ensure there is a properly labeled waste container for aqueous magnesium waste, per your institution's procedures.
    • Next, set up each fume hood. Refer to the table of materials for a list of equipment. Place a 125-mL plastic wash bottle filled with deionized water in each hood.
    • Cut an approximately 2-inch x 2-inch piece of emery paper for each group.
    • Set out the following lab equipment and glassware at each lab station (we suggest that students work in pairs):
       1    150-mL beaker
       1    600-mL beaker
       1    50-mL graduated cylinder
       1    70-mm watch glass
       1    Digital thermometer
       1    Stir bar
       1    Forceps
       1    Piece of emery paper
       1    Pair of scissors
       2    Polystyrene cups
       1    Cardboard lid with a hole in the center
    • Set out boxes of baking soda, packages of pH paper for neutralizing acid waste, and plastic pipettes for adjusting volume measurements. Ensure that there are paper towels by each lab sink.
    • Prepare the temperature data recording devices. Confirm that the devices are working and charged before distributing them to the fume hoods.
    • Bring a container of magnesium oxide, a package of magnesium ribbon, and a pair of scissors for cutting magnesium to the balance area.
    • Confirm that there is a waste container and a sufficient supply of weighing boats, clean spatulas, and laboratory wipes at the balances.
    • Label a clean, airtight box or bag with ‘magnesium metal pieces’. Cut a piece of magnesium ribbon with a mass of 0.45 – 0.55 g for each student group and place the pieces in the container. Note: Calculate the needed length in advance to help you reliably cut pieces of the appropriate mass.
    • Save any pieces that are too small in a separate container. Afterward, dispose of the weighing boat and put away the scissors.
    • Store the sealed containers of magnesium ribbons in the instructor's hood. Place a stack of weighing boats next to the magnesium ribbons.

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