System vs Surroundings A couple definitions first: The system is whatever we are studying—a particular reaction or change. The surroundings are everything else.

Chemical Reactions and Energy Chemical changes (chemical reactions) always involve a change in energy. If energy is absorbed by the system, the reaction is said to be endothermic. If energy is released from the system, the reaction is said to be exothermic.

Endothermic Reactions… do not often occur spontaneously because energy must be supplied in order for the reaction to take place. usually feel cold. Heat goes in—into the new bonds that form as the reaction takes place. (endothermic = heat goes in). Most common example: coldpacks used to reduce swelling after an injury.

Exothermic Reactions… can occur spontaneously. often feel warm or even hot because… energy is released from the system to the surroundings in the form of heat. The heat exits –exothermic– as the reaction proceeds. Examples: a fire, handwarmers

Chemical Reactions and Energy Suppose you combine two solutions in a test tube and they react. How would you determine if the reaction is endothermic or exothermic? Let’s find out…

Wear your goggles!! 1) Place a scoopful of potassium nitrate in a test tube. Set it in the test tube rack. 2) Measure ~25 mL water into a beaker. Pour into the test tube. 3) Stopper the test tube, and shake until dissolved. 4) Repeat the process with 2-3 scoops of calcium chloride. 5) Which is exothermic? Which is endothermic?

Chemical Reactions and Energy Which felt cold? The potassium nitrate solution Which felt hot? The calcium chloride solution An endothermic reaction feels cold to the touch, whereas an exothermic reaction feels warm or hot. Why?

Because… Exothermic reactions feel hot because the system releases heat and the surroundings heat up. Endothermic reactions feel cold because the system pulls energy from the surroundings as it absorbs energy.