Ch. 7 Notes Day 4 4/25/16

Objectives SWBAT identify chemical reactions as endothermic or exothermic based on: Graphs Narratives of Experiment Lab Data

Vocabulary Chemical Energy – the energy stored in the chemical bonds of a substance. Exothermic Reaction – a chemical reaction that releases energy into its surroundings. Endothermic Reaction – a chemical reaction that absorbs energy from its surroundings.

Forming bonds is EXOTHERMIC!! (A + B  AB) Exothermic Reactions Energy is released as NEW bonds form in products. Products have a LOWER energy than reactants Light, heat, sound, energy can be observed as reaction occurs. Energy Reactants Products

Endothermic Reactions Breaking bonds is ENDOTHERMIC!! (AB  A + B) Endothermic Reactions Energy is absorbed as reactants break apart. Products have a HIGHER energy than reactants Temperature drop is most common indicator Could see solid to liquid phase change. Energy Reactants Products

Endo or Exothermic Reaction? Use the information and/or descriptions provided to determine if the reaction described is endothermic or exothermic. Chemical A, which is a solid, is mixed with chemical B, also a solid. As the reaction proceeds, Chemical A turns into a liquid, and frost can be seen on the sides of the beaker. Is this endo or exo? Explain why. Endothermic A solid became a liquid Frost formed on the sides of the beaker, indicating that it cooled off.

Bright light was observed at Time = 0:45 Sodium metal is mixed with liquid water. As the reaction proceeds, a student records the following temperatures, and notes the following observations. Bright light was observed at Time = 0:45 Loud sound of gas rushing out of beaker from 0:45 to 3:00 minutes Smoke Alarm was set off at 3:15. Time (minutes) Temperature (˚F) 0:00 65 0:30 66 1:00 70 1:30 75 2:00 90 2:30 140 3:00 3:30 137 4:00 135 Exothermic Temperature rose very quickly, indicating heat was released Light and sound were observed Smoke alarm was activated

Ammonium chloride crystals are mixed with liquid water Ammonium chloride crystals are mixed with liquid water. As the reaction proceeds, the crystals disappear, it only looks like clear water is left. The crystals have completely dissolved in the water. The student calculated the chemical energy of the equation over time, and is shown on the graph below. Energy (kJ/mol) Products Reactants Endothermic The crystals were dissolved, meaning bonds were broken, which is endothermic. The energy of the products is higher than the reactants.