Chemical Reactions

Observing Chemical Change Physical change—any change that alters the form or appearance of a substance but does not make the substance into another substance Physical change—any change that alters the form or appearance of a substance but does not make the substance into another substance Chemical change—change in matter that produces one or more new substances Chemical change—change in matter that produces one or more new substances The second one is a chemical reaction The second one is a chemical reaction

Chemical Reactions Reactants—substances that undergo change in a chemical reaction Reactants—substances that undergo change in a chemical reaction Products—new substances formed by the reaction Products—new substances formed by the reaction Chemical changes occur when bonds break and new bonds form. As a result, new substances are produced Chemical changes occur when bonds break and new bonds form. As a result, new substances are produced

Evidence for Chemical Reactions Precipitate—solid that forms from solution during a chemical reaction Precipitate—solid that forms from solution during a chemical reaction Color change Color change Formation of gas from solid or Formation of gas from solid or liquid reactant However, these do not always However, these do not always indicate a chemical change!

Changes in Energy Chemical reactions usually absorb heat or release heat Chemical reactions usually absorb heat or release heat Breaking bonds between atoms requires energy Breaking bonds between atoms requires energy Making bonds between atoms release energy Making bonds between atoms release energy Endothermic reaction—total making and breaking of bonds results in a net absorption of energy (absorb heat) Endothermic reaction—total making and breaking of bonds results in a net absorption of energy (absorb heat) Exothermic reaction—total making and breaking of bonds results in net release of energy (release heat) Exothermic reaction—total making and breaking of bonds results in net release of energy (release heat)

Energy

Chemical Equations Short, easy way to show a chemical reaction, using symbols instead of words Short, easy way to show a chemical reaction, using symbols instead of words Use chemical formulas and other symbols instead of words to summarize a reaction Use chemical formulas and other symbols instead of words to summarize a reaction

Using Formulas Combination of symbols that represent the elements in a compound Combination of symbols that represent the elements in a compound CO 2 is carbon dioxide CO 2 is carbon dioxide Tells us that there is one carbon atom and two oxygen atoms Tells us that there is one carbon atom and two oxygen atoms Some common formulas are: Water (H 2 O) Water (H 2 O) Methane (CH4) Methane (CH4) Sugar (C 12 H 22 O 11 ) Sugar (C 12 H 22 O 11 )

Structure of Chemical Reactions Reactant +ReactantProduct + Product Reactant +ReactantProduct + Product

Conservation of Matter First demonstrated by French chemist in 1774 First demonstrated by French chemist in 1774 Antoine Lavoisier showed that matter is neither created nor destroyed during a chemical reaction Antoine Lavoisier showed that matter is neither created nor destroyed during a chemical reaction The total mass of the reactants must equal the total mass of the products The total mass of the reactants must equal the total mass of the products The number of atoms stays the same no matter how they are arranged. So, their total mass stays the same The number of atoms stays the same no matter how they are arranged. So, their total mass stays the same