066 – Endothermic & exothermic reactions

Chemical reactions and physical changes often involve energy transfers, evidenced by changes in temperature. These energy exchanges can be categorized as either endothermic, where energy is absorbed from the surroundings, or exothermic, where energy is released. Understanding these processes is crucial for applications ranging from industrial chemistry to biological systems.

This experiment examines two scenarios: the dissolution of sodium hydroxide (NaOH) in water and the reaction between citric acid (C6H8O7) and sodium bicarbonate (NaHCO3). By measuring temperature changes, students will classify each process as endothermic or exothermic and calculate the associated energy changes. This hands-on activity enhances comprehension of energy transfer in chemical processes and provides practical experience in data collection and analysis.

Educational Goals

  • Understanding energy transfer: Students will explore the concepts of endothermic and exothermic reactions by observing temperature changes during chemical and physical processes.
  • Developing laboratory skills: Students will gain proficiency in using calorimeters, digital thermometers, and other laboratory equipment to measure and analyze energy changes.
  • Applying theoretical knowledge: By applying formulas for energy calculations (e.g., ), students will connect theoretical principles to experimental data.
  • Enhancing analytical thinking: Students will interpret their observations to classify reactions and deduce the underlying energy dynamics.
  • Promoting collaboration: Students will work in teams to conduct experiments, record data, and analyze results, fostering teamwork and communication skills.
  • Encouraging critical evaluation: By comparing their results with hypotheses, students will critically evaluate the accuracy and implications of their findings.

By completing this experiment, students will deepen their understanding of energy transfer in chemical processes and enhance their practical and analytical skills.

Protocol

Experiment 1 : Water + sodium hydroxide

  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. Using the weighing boat, weigh approximately 4 g (approximately 2 mL) of sodium hydroxide powder.
  6. Pour the contents of the weighing boat into the calorimeter.
  7. Attach the calorimeter lid to the calorimeter.
  8. Activate the green button of the stirrer on the calorimeter lid.
  9. Insert the digital thermometer into the calorimeter lid.
  10. The temperature of the mixture will appear in the results table.
  11. Stop the agitator by pressing the red button.
  12. Remove the thermometer from the calorimeter lid.
  13. Remove the lid of the calorimeter and empty its contents into the recovery bin.
  14. Rinse the calorimeter with distilled water and empty its contents into the recovery tank.

Experiment 2 : Citric acid + sodium bicarbonate

  1. Measure 50 mL of citric acid 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. Using the weighing boat, weigh approximately 4.5 g (approximately 2 mL) of sodium bicarbonate.
  6. Pour the contents of the weighing boat into the calorimeter.
  7. Attach the calorimeter lid to the calorimeter.
  8. Activate the green button of the stirrer on the lid of the calorimeter.
  9. Insert the digital thermometer into the calorimeter lid.
  10. The temperature of the mixture will appear in the results table.
  11. Stop the agitator by pressing the red button.
  12. Remove the thermometer from the calorimeter lid.
  13. Remove the calorimeter lid and empty its contents into the recovery bin.
  14. Rinse the calorimeter with distilled water and empty its contents into the recovery tank.

Anticipated Outcomes

NaOH(s) + H2O

  • The dissociation of NaOH(s) into Na+ and OH– (strong base) releases 44.5kJ of energy per moles of NaOH
  • The reaction takes 1 to 2 seconds to arrive at completion.
  • For 100mL of water and 4g of NaOH(s), the increase in temperature will be approximately 10.6 ℃.

NaHCO3(s) + citric acid (solution)

  • The reaction between NaHCO3(s) and citric acid (weak acid) will release CO2(g) and absorb 20kJ of energy per mole of NaHCO3.
  • For 50mL of water and 4.5g of NaHCO3(s), the decrease in temperature will be approximately 4.7 ℃.
  • When citric acid (a weak, triprotic acid) mixes with baking soda (sodium bicarbonate, a base), they neutralize each other and make carbon dioxide gas, water, and a salt called sodium citrate. The CO₂ bubbles cause the fizzing you see—this is the same idea behind bath bombs and some “volcano” demos.
  • The process is endothermic, meaning it absorbs heat from the surroundings, so the mixture often feels cold. Because citric acid can donate three H⁺ ions, it can react with up to 3 moles of NaHCO₃ per mole of acid.

Summary of Assignment by Grade Range

Grades 6-8

Focus: Introduction to energy changes and qualitative observations.

  • Students will observe temperature changes and identify reactions as exothermic or endothermic.
  • Emphasis will be placed on understanding the relationship between temperature and energy transfer.

Expected Outcomes:

  • Recognition of energy transfer during chemical and physical changes.
  • Development of basic observational and recording skills.
  • Introduction to the role of energy in chemical processes.

Grades 9-10

Focus: Intermediate exploration of energy calculations and reaction classification.

  • Students will measure temperature changes, calculate energy values, and classify reactions using theoretical concepts.
  • They will analyze the role of chemical bonding in energy changes.

Expected Outcomes:

  • Improved ability to connect experimental data to theoretical principles.
  • Deeper understanding of energy transfer and reaction dynamics.
  • Enhanced laboratory techniques and analytical skills.

Grades 11-12

Focus: Advanced analysis and critical evaluation of energy dynamics.

  • Students will conduct detailed energy calculations, evaluate their results, and prepare comprehensive lab reports.
  • They will explore the broader implications of energy changes in chemical and physical processes.

Expected Outcomes:

  • Mastery of experimental techniques and energy calculations.
  • Proficiency in scientific writing and critical analysis.
  • In-depth understanding of energy dynamics in chemical processes and their applications.

This structured approach ensures that students at all levels can engage meaningfully with the experiment, building their knowledge and skills progressively.

Laboratory essentials

Instruments

  • Calorimeter
  • Electronic balance
  • Analog & numeric thermometers
  • Spatulas
  • Graduated cylinder 100mL

Products

  • NaOH(s)
  • NaHCO3(s)
  • Citric acid 1M
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