065 – Hess’s Law

This laboratory session is designed as a comprehensive exploration of chemical reactions and thermal exchanges through four distinct experiments, each aimed at understanding different aspects of thermochemistry and chemical kinetics.

Educational Goals

  • Volume and temperature measurement techniques: Students will refine their skills in using graduated cylinders for volume measurements and thermometers for temperature observations, enhancing their precision and accuracy in experimental chemistry.
  • Observation of chemical reactions: Participants will gain insights into the nature of chemical reactions, specifically how the mixing of different substances can lead to thermal changes, illustrating the principles of thermochemistry.
  • Exploration of reaction variations: By altering components such as solvents or reactants, students will explore how experimental conditions affect reaction outcomes, fostering a deeper understanding of chemical kinetics.
  • Thermochemistry and kinetics concepts: This laboratory aims to provide a practical understanding of thermochemistry and chemical kinetics, emphasizing the thermal effects of chemical reactions and the factors influencing reaction rates.

Through these experiences, students will not only become familiar with standard experimental procedures in chemistry but also gain practical experience in manipulating laboratory equipment and interpreting experimental data.

This hands-on approach to learning allows students to apply theoretical knowledge of chemistry to real-world scenarios, reinforcing their comprehension of fundamental principles within the discipline. The laboratory session highlights the importance of precise measurement and control in chemical experimentation, offering valuable lessons in the thermal behavior of chemical reactions and the impact of varying experimental conditions.

Protocol

Experiment 1: Water + Alcohol

  1. Measure 100 mL of distilled water using the graduated cylinder.
  2. Pour the contents of the graduated cylinder into the calorimeter.
  3. Immerse the tip of the digital thermometer in the liquid to take its temperature.
  4. The initial temperature of the water will appear in the results table.
  5. Measure 100 mL of ethanol using the graduated cylinder.
  6. Pour the contents of the graduated cylinder into the calorimeter.
  7. Attach the lid to the calorimeter.
  8. Activate the green button on the stirrer on the calorimeter lid and let it stir for a few seconds.
  9. Insert the digital thermometer into the lid of the calorimeter.
  10. The temperature of the mixture will appear in the results table.

Remove the thermometer from the lid of the calorimeter.

  1. Remove the lid of the calorimeter and empty the contents of the calorimeter into the recovery tray.
  2. Rinse the calorimeter with distilled water and empty its contents into the recovery bin.
  3. Rinse the graduated cylinder with distilled water and empty its contents into the recovery tray.

Experiment 2: Water + NH4Cl

  1. Measure 100 mL of distilled water using the graduated cylinder.
  2. Pour the contents of the graduated cylinder into the calorimeter.
  3. Immerse the tip of the digital thermometer in the liquid in order to take its temperature.
  4. The initial temperature of the water will appear in the results table.
  5. Weigh approximately 9.2g (6mL) of powdered ammonium chloride using the weighing pan.
  6. Pour the contents of the weighing bucket into the calorimeter.
  7. Attach the lid to the calorimeter.
  8. Activate the green button on the stirrer on the calorimeter lid and let it stir for a few seconds.
  9. Insert the digital thermometer into the lid of the calorimeter.
  10. The temperature of the mixture will appear in the results table.
  11. Remove the thermometer from the lid of the calorimeter.
  12. Remove the lid from the calorimeter and empty the contents of the calorimeter into the recovery tray.
  13. Rinse the calorimeter with distilled water and empty its contents into the recovery bin.
  14. Rinse the graduated cylinder with distilled water and empty its contents into the recovery tray.

Experiment 3: Water + CaCO3

  1. Measure 100 mL of distilled water using the graduated cylinder.
  2. Pour the contents of the graduated cylinder into the calorimeter.
  3. Immerse the tip of the digital thermometer into the liquid in order to take its temperature.
  4. The initial temperature of the water will appear in the results table.
  5. Weigh approximately 9.5g (3.5mL) of powdered calcium carbonate using the weighing pan.
  6. Pour the contents of the weighing bucket into the calorimeter.
  7. Attach the lid to the calorimeter.
  8. Activate the green button on the stirrer on the lid of the calorimeter and let it stir for a few seconds.
  9. Insert the digital thermometer into the lid of the calorimeter.
  10. The temperature of the mixture will appear in the results table.
  11. Remove the thermometer from the lid of the calorimeter.
  12. Remove the lid from the calorimeter and pour the liquid content into the black recovery tray and transfer the solids using the spatulas.
  13. Rinse the calorimeter with distilled water and empty its contents into the recovery tray.
  14. Rinse the graduated cylinder with distilled water and empty its contents into the recovery tray.

Experiment 4: HCl + CaCO3

  1. Measure 100 mL of hydrochloric acid (HCl) 2 M using the graduated cylinder.
  2. Pour the contents of the graduated cylinder into the calorimeter.
  3. Immerse the tip of the digital thermometer in the liquid to take its temperature.
  4. The initial temperature of the water will appear in the results table.
  5. Weigh approximately 9.5g (3.5mL) of powdered calcium carbonate using the weighing boat.
  6. Pour the contents of the weighing boat into the calorimeter.
  7. Attach the lid to the calorimeter.
  8. Insert the digital thermometer into the calorimeter lid.
  9. Start the stopwatch.
  10. Press the green stirrer button on the calorimeter lid and let it stir for a few seconds.
  11. Observe the reaction occurring in the temperature vs. time graph.
  12. When the reaction is complete (the temperature will have reached a plateau); stop the stirrer by pressing the red button and remove the calorimeter lid.
  13. Remove the thermometer from the calorimeter lid.
  14. Remove the lid of the calorimeter and empty the contents of the calorimeter into the recovery tank.
  15. Rinse the calorimeter with distilled water and empty its contents into the recovery tank.
  16. Rinse the graduated cylinder with distilled water and empty its contents into the recovery tank.

Note: The reaction speed is accelerated 10x.

Experiment 5: HCl + NaOH

  1. Measure 100 mL of 1M NaOH using the graduated cylinder.
  2. Pour the contents of the graduated cylinder into the calorimeter.
  3. Immerse the tip of the digital thermometer in the liquid to take its temperature.
  4. The initial temperature of the water will appear in the results table.
  5. Measure 100mL of 1M HCl using the graduated cylinder.
  6. Pour the contents of the graduated cylinder into the calorimeter.
  7. Attach the lid onto the calorimeter.
  8. Press the green agitator button on the calorimeter lid and let it stir for a few seconds.
  9. Insert the digital thermometer into the calorimeter lid.
  10. The temperature of the mixture will appear in the results table.
  11. Remove the thermometer from the calorimeter lid.
  12. Remove the lid of the calorimeter and empty the contents of the calorimeter into the recovery tank.
  13. Rinse the calorimeter with distilled water and empty its contents into the recovery tank.
  14. Rinse the graduated cylinder with distilled water and empty its contents into the recovery tank.

Anticipated Outcomes

Experiment 1: Water + Ethanol

Adding 100mL of ethanol to 100mL of water is an exothermic reaction, which should raise the water temperature by about 8°C. There is a release of 1.537 kJ/mol per mole of ethanol. When ethanol (C2H5OH) is mixed with water (H2O), the two liquids form a solution. This process involves the breaking and forming of intermolecular forces. Initially, the hydrogen bonds between water molecules and the van der Waals forces between ethanol molecules are broken. New hydrogen bonds form between the water and ethanol molecules. The formation of these new intermolecular forces releases energy, resulting in an exothermic reaction that increases the temperature of the solution. The specific heat capacities of the substances and the total energy released during the formation of the new bonds contribute to the observed temperature change.

Experiment 2: Water + NH4Cl

This is an endothermic reaction, where the reaction consumes 14.8 kJ/mol per mole of NH4Cl. The temperature should drop by about 5 to 8 degrees.

Experiment 3: Water + CaCO3

Since CaCO3 is insoluble in water, no significant temperature change is expected, indicating no reaction.

Experiment 4: HCl + CaCO3

This should result in a reaction where CaCO3 reacts with HCl to produce calcium chloride, water, and carbon dioxide, leading to a temperature increase of about 19.5 C over 35 seconds (speed accelerated 10 times). Each mole of CaCO3 should react with 0.5 mole of HCl to produce 1600 kJ. This experiment involves an acid-base reaction where hydrochloric acid (HCl) reacts with calcium carbonate (CaCO3) to form calcium chloride (CaCl2), water (H2O), and carbon dioxide (CO2).

Experiment 5: HCl + NaOH

An exothermic neutralization reaction is expected. The temperature should rise, ideally by about 6.4°C, indicating the release of energy. The reaction produces 54 kJ/mol per mole of HCl. This experiment features a neutralization reaction, a type of exothermic reaction where an acid (HCl) and a base (NaOH) react to form water (H2O) and a salt (NaCl). During the reaction, the hydrogen ions (H+) from the acid react with the hydroxide ions (OH−) from the base to form water.

In each experiment, the observed temperature changes are indicators of the energy dynamics involved in the chemical processes, reflecting the exothermic or endothermic nature of the reactions.

Summary of Assignment by Grade Range

1.1.1 Summary of Assignment by Grade Range

Grades 3-5 (Ages 8-10)

  • Focus: Basic introduction to chemical reactions, temperature changes, and measurement techniques.
  • Activities: Observing simple thermal changes during chemical reactions, using thermometers and graduated cylinders, basic safety instructions.

Grades 6-8 (Ages 11-13)

  • Focus: Intermediate understanding of thermochemistry, chemical kinetics, and measurement accuracy.
  • Activities: Conducting reactions, measuring volumes and temperatures, observing how different reactants and solvents affect reaction outcomes, following detailed safety protocols.

Grades 9-12 (Ages 14-18)

  • Focus: Advanced understanding of thermochemistry, Hess’s Law, and experimental precision.
  • Activities: Accurately measuring volumes and temperatures, conducting detailed experiments to explore the thermal effects of chemical reactions, analyzing how changes in reactants and solvents influence reaction rates, detailed recording and interpretation of results, adhering to advanced safety protocols, reinforcing concepts of chemical kinetics and thermodynamics.

Laboratory essentials

Instruments

  • Beaker (500ml & 1000ml)
  • Calorimeter
  • Electronic Scale
  • Graduated Cylinders (70ml & 250ml)
  • Spatulas
  • Thermometers
  • Timer
  • Tweezers

Products

  • NH4Cl (powder)
  • CaCO3 (powder)
  • Ethanol (liquid)
  • HCl 1M (solution).
  • HCl 2M (solution).
  • NaOH 1M (solution)
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