Chapter 16 Chemical Reactions That Involve Heat

The study of the changes in heat in chemical reactions. Thermochemistry

Chemical Reactions Involve ENERGY Changes in ENERGY result from bonds being broken and new bonds being formed.

Requires ENERGY Breaking Bonds

Releases ENERGY Bond Formation

Both absorption and release of energy occurs. In a chemical reaction We detect the net result. Measure the temperature of the surroundings.

System: Reactants and Products System & Surroundings Surroundings: Solvent, container, atmosphere above the reaction, etc.

Types of Reactions

RELEASE HEAT! Exothermic Reactions C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O (g) kJ Combustion reactions are exothermic! HEAT is listed as a product in the reaction! HEAT ENERGY

Exothermic Reactions Energy needed to break bonds must be LESS THAN the energy released when new bonds are formed. Surroundings will have a higher temperature after the reaction!

NaOH (s) Na + (aq) + OH - (aq) Beginning Temp of Surroundings: 25.4 ° C Ending Temp of Surroundings: 29.5 ° C Change in Temp of Surroundings: +4.1 ° C +4.1 ° C Means heat was GIVEN OFF EXOTHERMIC

NaOH (s) Na + (aq) + OH - (aq) +4.1 ° C Calculate HEAT per MOLE if you have 5.0 grams of NaOH to start. 5.0 g NaOH 40 g NaOH 1 mol NaOH mol NaOH = +4.1 ° C mol NaOH = + 33 ° C per mole NaOH Means HEAT is given off.

ABSORB HEAT! Endothermic Reactions C (s) + H 2 O (g) kJ CO (g) + H 2 (g) HEAT is listed as a reactant in the reaction! HEAT ENERGY

Endothermic Reactions The energy needed to break bonds is GREATER THAN the energy released when new bonds are formed. Surroundings will have a lower temperature after the reaction!

Exo & Endo Demos

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