Lecture 21 Combustion. Chemical Energy. History of Studies of Burning Reactions Involving Heat Chemical Energy Activation Energy Chapter 11.1  11.5

Combustion The process of burning is called combustion. Ancient view (Aristotle): flammable materials consist of “earth” and “fire”. The phlogiston hypothesis (Becher & Stahl, XVII century): Phlogiston is the substance that supposedly escapes during combustion. Metals heated in air turn into a soft powder. It was assumed that a metal is a compound of the powder and phlogiston.

Problems of Phlogiston Wood burns leaving lighter ashes, but a heated metal leaves a heavier powder (negative phlogiston mass?) Experiments conducted by advocates of the phlogiston theory were qualitative.qualitative The advent of careful measurements helped to disprove phlogiston.

Works of Lavoisier Antoine Lavoisier (1743  1794) has brought chemistry to a new, quantitative, level.Lavoisier Experiment with floating tin Piece of tin floats on a wooden block in water  Heating with sunlight  Tin partly changes into a white powder  The water level rises (4/5 of the air is left in the jar)

Oxygen Oxygen was found by Joseph Priestley in England (1774). OxygenOxygen combines with substances in a process called oxidation and forms oxides. Rapid oxidation accompanied by heating and light is the process of combustion. Slow oxidation includes rusting of metals and digesting food. Oxides of different elements may be solids (metals), liquid (water), or gases (carbon dioxide).

Chemical Reactions Involving Heat Chemical reactions that give off energy are called exothermic (exo  outside), while those involving absorption of energy are called endothermic (endo  inside). C + O 2  CO 2 + energy 9 kJ/g (of CO 2 ) 2H 2 + O 2  2H 2 O + energy 13.6 kJ/g (of H 2 O) N 2 + O 2 + 3kJ/g (of NO)  2NO If a reaction is exothermic, then the reversed one is endothermic. Dissociation of most salts is endothermic, while neutralization is exothermic.

Chemical Energy The heat, involved in a reaction, measures stability of reacting substances. If the decomposition of a substance requires to supply a lot of energy, the substance is stable. If the decomposition is weakly exo- or endothermic, the substance is normally unstable. CO 2, H 2 O, NaCl are examples of stable compounds. NO is unstable and liberates energy when decomposed.

Interpretation of Chemical Energy Changes Example: Na + Cl (ionic bond)ionic bond New position of the electron (from the Na atom near the Cl atom) has less potential energy (PE) with respect to the atomic nuclei because it is more firmly connected to the Cl atom. Example: H + H (covalent bond) A decrease of PE is due to the attraction of each electron to two nuclei instead of one. Chemical energy is electron potential energy

Activation Energy In order to begin many exothermic reactions, some energy has to be supplied (activation energy).activation Example: 2H 2 + O 2  2H 2 O First, covalent bonds between atoms in the hydrogen and oxygen molecules have to be broken. This takes energy. Then energy already liberated in the reaction keeps it going. Some reactions may begin spontaneously. A molecule with enough energy to react is called an activated molecule.

Summary Combustion is a rapid oxidation. Chemical energy is electron potential energy. Many reactions require activation.