Endothermic and Exothermic Reactions (Tayler Hickland)

Title: Exploring Endothermic and Exothermic Reactions

Principle(s) Investigated:

• Exothermic reactions
• Endothermic reactions
• Changes in states of matter (liquid to solid, solid to liquid)
• Solubility
• Supersaturation
• Supercooling
• Heat of crystallization
• 1^st law of thermodynamics

Standards :

Science Content Standards for California Public Schools

• Structure of Matter
  • 3. Each of the more than 100 elements of matter has distinct properties and a distinct atomic structure. All forms of matter are composed of one or more of the elements. As a basis for understanding this concept:
    • b. Students know that compounds are formed by combining two or more different elements and that compounds have properties that are different from their constituent elements.
    • c. Students know atoms and molecules form solids by building up repeating patterns, such as the crystal structure of NaCl or long-chain polymers.
      • d. Students know the states of matter (solid, liquid, gas) depend on molecular motion.
      • e. Students know that in solids the atoms are closely locked in position and can only vibrate; in liquids the atoms and molecules are more loosely connected and can collide with and move past one another; and in gases the atoms and molecules are free to move independently, colliding frequently.

• Reactions
  • 5.Chemical reactions are processes in which atoms are rearranged into different combinations of molecules. As a basis for understanding this concept:
    • c. Students know chemical reactions usually liberate heat or absorb heat.

Materials:

Procedure:

Student prior knowledge: What prior concepts do students need to understand this activity?

Students are already able to:

• recall that liquids can be cooled below their freezing point and remain liquid (based on "Project Deep Freeze" lab they did with supercooled water)
• categorize substances as solids, liquids or gases based on physical appearance and properties
• describe the motion and arrangement of particles in solids, liquids and gases
• define the terms endothermic reaction and exothermic reaction
• define the terms solute, solvent and solution
• identify reactants and products in a chemical equation
• identify whether something is an element, mixture or compound

Explanation:

Exothermic Reaction

Supersaturated: containing an amount of a substance greater than that required for saturation as a result of having been cooled from a higher temperature to a temperature below that at which saturation occurs (definition from www.Merriam-Webster.com) .

This reaction is an example of a supersaturated solution. The solution can be made by two different processes. One is by using sodium acetate trihydrate and simply heating it over a flame. The sodium acetate will essentially dissolve in its own water. The other method is to take sodium acetate anhydrate and dissolve it in boiling water (about 2ml of water for every 3g of sodium acetate). Both create a supersaturated solution of sodium acetate and water. This supersaturated solution then cools and becomes unstable, since it contains more solute than it can normally hold. When it is disturbed, and a nucleation point is presented, the solute will fall out of the solution and rapidly crystallize. The heat that we feel is the heat of crystallization.

Below are a few explanations for this reaction from various sources:

"All liquids have a freezing point- the temperature at which they turn from a liquid into a solid. Water’s freezing point is 0^◦C (32^◦F). The freezing point of sodium acetate is much higher, 58^◦C (136.4^◦F). However, by creating the liquid solution and cooling it slowly, we are able to supercool the sodium acetate- to bring it down to a temperature below its freezing point without it freezing. In a super cooled state, the liquid will not begin to crystalize and turn into a solid without being given a starting point to crystalize around. By pouring the liquid onto another surface, or by introducing a seed crystal, we are disturbing the super cooled state and providing a starting site for crystallization. Since the solution is at a much cooler temperature than its freezing point, it must heat up in order to freeze" (taken from http://berkshiremuseum.org/wp-content/uploads/2012/10/111712-Kitchen-Kaboom.pdf).

"At temperatures above 58^◦C, NaOAc•3H2O begins to lose its water of hydration and begins to dissolve in that water. The solution, once completely dissolved, can be cooled to give a solution which is supersaturated in the anhydrous and trihydrate form of sodium acetate. In the presence of a seed crystal, this solution will “freeze” but in doing so must reach its freezing point which is 58^◦C and thus the crystallizing solution warms up. The heat of solution of NaOAc•3H2O is 19.7 kJ/mol, therefore the dissolution process is endothermic and the crystallization is exothermic" (taken from http://ncsu.edu/project/chemistrydemos/Thermochem/SatNaOAc.pdf).

Another good explanation can be found here: http://www.chem-toddler.com/solutions-and-solubility/supersaturated-thiosulphate.html. It describes the process with sodium thiosulfate, which is a very similar to the "hot ice" from sodium acetate.

Endothermic Reactions:

Ammonium nitrate and water

Ammonium nitrate and water are usually what is used in instant cold packs. When the ammonium nitrate crystals dissolve in the water, the following reaction occurs: NH₄NO₃ + heat --> NH₄⁺ + NO₃^-. This reaction absorbs heat from the surroundings and feels very cold. It is an endothermic reaction.

Barium hydroxide and ammonium nitrate

When you mix barium hydroxide (octahydrate) and ammonium nitrate, both of which are dry solids, you get a very cold liquid. This is due to an endothermic reaction that is occurring between the two compounds (the reaction is as follows: Ba(OH)₂^.8H₂O(s) + 2NH₄NO₃(s) → Ba(NO₃)₂(aq) + 2NH₃(aq) + 10H₂O(l)). Students have not really learned about atoms and the period table or about chemical reactions at this point, so this is mostly just a demo of endothermic reactions and a introduction to chemical reactions. The teacher can point out the ammonia in the products, and have students gently waft the beaker to detect the odor of the ammonia.

Questions & Answers: Give three thought-provoking questions and provide detailed answers.

• Q: Why did the liquid sodium acetate solution turn into a solid when it came in contact with a solid crystal?
  • A: The liquid sodium acetate solution was supersaturated and supercooled. This means that there was more solute dissolved than the solution could normally hold, and it was liquid at room temperature even though its freezing point is 58^◦C. The sodium acetate should have been solid at that point, but there was no "template" for the crystals to form around. The solid crystal acted as this template, which caused the sodium acetate in the solution to crystallize and come out of the solution.
• Q: Why does the Sodium Acetate get hot when it crystallizes? Where did this "energy" come from?
  • A: The Sodium Acetate got hot when it crystallized because energy was being released. When it was in liquid form, the particles were free to move around. When it began to solidify, the particles slowed down and became locked in place to form the solid crystals. The energy that was previously in the motion of the particles had to go somewhere when the slowed down, so it went out out into the surroundings and we felt it as heat.
• Q: Consider the processes of cellular respiration and photosynthesis that you learned about last year in Life Science. Which of these processes is endothermic, and which is exothermic? Why? (Hint: The equation for cellular respiration is C₆H₁₂O₆ + O₂ → CO₂ + H₂O + Energy, and the equation for photosynthesis is CO₂ + H₂O + Energy → C₆H₁₂O₆ + O₂)
  • A: Cellular respiration is exothermic because energy is being released from the reaction as a product, and photosynthesis is endothermic because energy is going into the reaction as a reactant.

Applications to Everyday Life:

Photographs: Include a photograph of you or students performing the experiment/demonstration, and a close-up, easy to interpret photograph of the activity --these can be included later.

Videos: Include links to videos posted on the web that relate to your activity. These can be videos you have made or ones others have made.

Additional Resources:

http://www.wikihow.com/Make-Hot-Ice

*Sodium acetate can be purchased as either sodium acetate trihydrate or sodium acetate anhydrate. Sodium acetate trihydrate can be melted down on its own, and does not need any additional water. Sodium acetate anhydrate, however, needs to be mixed with water before it can be used for hot ice. Mix 2ml of boiling/very hot water for every 3g of sodium acetate anhydrate. It is best to do this in a beaker that is in a boiling water bath or over a hot plate.