Hess’s Law

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Hess’s Law

Hess’s Law

Purpose:

To calculate the neutralization energy in the reaction of magnesium oxide with hydrochloric acid and the reaction of magnesium metal with hydrochloric acid using Hess Law.

Introduction:

Germain Henri Hess was a 1900th century chemist who formulated Hess’s Law, which marked the beginning of thermochemistry. Hess’s law allows us to calculate the  of a reaction by adding up the  of other reactions with the same substances or elements in them. Hess’s law can be used in calorimetry when trying to find the overall energy released or absorbed in a reaction.[pic 1][pic 2]

        Materials:

  • Table-top balance
  • 50mL graduated cylinder
  • Styrofoam cup and lid (calorimeter)
  • Thermometer
  • 1.0M hydrochloric acid
  • magnesium metal
  • magnesium oxide

        Procedures: Part 1- Reaction of Magnesium Oxide with Hydrochloric Acid

  1. Weigh the calorimeter (without the lid).
  2. Measure 30.0mL OF 1.0M hydrochloric acid using a graduated cylinder, and place it in the calorimeter.
  3. Weigh the calorimeter with the hydrochloric acid.
  4. Take the temperature of the hydrochloric acid solution using a thermometer.
  5. Weigh out approximately 1.00g of magnesium oxide.
  6. Add the magnesium oxide to the calorimeter.
  7. Place on the lid and insert the thermometer. Swirl the cup gently in order to mix the magnesium oxide with the hydrochloric acid. Take the highest temperature reached.

        Procedures: Part 2- Reaction of Magnesium metal with Hydrochloric Acid

  1. Weigh the calorimeter (without the lid).
  2. Measure 40.0mL of 1.0M hydrochloric acid using a graduated cylinder and place it in a calorimeter.
  3. Weigh the calorimeter with the hydrochloric acid.
  4. Take the temperature of the hydrochloric acid solution using a thermometer.
  5. Weigh out approximately 0.10g of magnesium metal.
  6. Add the magnesium metal to the calorimeter.
  7. Place on the lid and insert the thermometer. Swirl the cup gently in order to mix the magnesium metal with the hydrochloric acid. Take the highest temperature reached.

        Data: Part 1

Weight of calorimeter

10.14g

Weight of calorimeter and HCI

39.33g

Temperature of HCI before reaction

25.3oC

Highest temperature reached during reaction

39.1oC

        Data: Part 2

Weight of calorimeter

10.40g

Weight of calorimeter and HCI

49.64g

Temperature of HCI before reaction

25.1oC

Highest temperature reached during reaction

36.7oC