Describing Chemical Reactions

Describing a Chemical Reaction  Chemical Reaction (rxn) – the process by which one or more substances change to produce one or more different substances.  New / Different chemical and physical properties.  Chemical Change vs. a Physical Change

Chemical Change  In a chemical change, the original substances are known as the reactants  The substances created are known as the ________________. Reactants  Products

Evidence of a Chemical Change  Changes in energy:  Release of energy as heat  Release of energy as light  Production of sound  Reduction or increase of temperature  Absorption or release of electrical energy

Evidence of a Chemical Change  Formation of new substances:  Formation of gas  Formation of a precipitate (an insoluble solid)  Changes in color  Changes in odor

Rxns and Energy Changes  Reactions that release energy (gets hot) is known as an __________________ rxn.  In these reactions, energy is a product (given off).  Consider the combustion of methane: methane + oxygen  carbon dioxide + water + energy

Rxns and Energy Changes  Reactions that absorb energy (feels cold) is known as an __________________ rxn.  In these reactions, energy is a reactant (used up, from ___________ & feels cold).  Consider the decomposition of dinitrogen tetraoxide: dinitrogen tetraoxide + energy  nitrogen dioxide

Reactants MUST Come Together  You cannot hit a ball without touching it.  Reactions use the same concept  The reactants must come together and interact in order to react with one another  What happens with you:  Increase the concentration of the reactants?  Crush up the reactants?  Increase the temperature / pressure?

Chemical Equations  Symbols represent elements and formulas represent compounds  Chemical equations represent a chemical reaction, the make-up of what happens  Chemical Equation – a representation of a chemical reaction that uses symbols to show the relationship between the reactants and the products.

Word Equations  Word equations do not tell us any numerical values or what elements / compounds are interacting. Propane + Oxygen  Carbon Dioxide + Water  Write out the chemical equation from this word equation:

Equations Tell Us A Lot SymbolMeaning (s), ( ℓ ), (g) Substance in the solid, liquid, and gaseous state. (aq)Substance in aqueous solution (dissolved in water).  “produces” or “yields,” indicating result or reaction Reversible Rxn in which products can reform into reactants; final result is a mixture of products and reactants Δ heat  or  Reactants are heated; temperature is not specified Pd  Name or chemical formula of a catalyst, added to speed a reaction.

Chemical Reactions  Classwork: Page 211, #9, 11, 12  Tomorrow: “Balancing Chemical Reactions” and Classifying them.