1. Ch. 7 Notes Day 4
4/25/16

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

3. 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.

4. 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

5. 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

6. 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.

7. 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

8. 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.