Chemical ReactionsChemical Reactions

Observing Chemical Observing Chemical ChangeChange

Physical change—any change that alters the Physical change—any change that alters the form or appearance of a substance but does form or appearance of a substance but does not make the substance into another not make the substance into another substancesubstance

Chemical change—change in matter that Chemical change—change in matter that produces one or more new substancesproduces one or more new substances

The second one is a chemical reactionThe second one is a chemical reaction

Chemical ReactionsChemical Reactions

Reactants—substances that undergo change Reactants—substances that undergo change in a chemical reactionin a chemical reaction

Products—new substances formed by the Products—new substances formed by the reactionreaction

Chemical changes occur when bonds break Chemical changes occur when bonds break and new bonds form. As a result, new and new bonds form. As a result, new substances are producedsubstances are produced

Evidence for Chemical Evidence for Chemical ReactionsReactions

Precipitate—solid that forms from solution Precipitate—solid that forms from solution during a chemical reactionduring a chemical reaction

Color changeColor change

Formation of gas from solid or Formation of gas from solid or

liquid reactantliquid reactant

However, these do not alwaysHowever, these do not always

indicate a chemical change!indicate a chemical change!

Changes in EnergyChanges in Energy

Chemical reactions usually absorb heat or Chemical reactions usually absorb heat or release heatrelease heat

Breaking bonds between atoms requires energyBreaking bonds between atoms requires energy

Making bonds between atoms release energyMaking bonds between atoms release energy

Endothermic reaction—total making and Endothermic reaction—total making and breaking of bonds results in a net absorption of breaking of bonds results in a net absorption of energy (absorb heat)energy (absorb heat)

Exothermic reaction—total making and breaking Exothermic reaction—total making and breaking of bonds results in net release of energy of bonds results in net release of energy (release heat)(release heat)

Energy Energy

Chemical EquationsChemical Equations

Short, easy way to show a chemical reaction, Short, easy way to show a chemical reaction, using symbols instead of wordsusing symbols instead of words

Use chemical formulas and other symbols Use chemical formulas and other symbols instead of words to summarize a reactioninstead of words to summarize a reaction

Using FormulasUsing Formulas

Combination of symbols that represent the Combination of symbols that represent the elements in a compoundelements in a compound

COCO22 is carbon dioxide is carbon dioxide Tells us that there is one carbon atom and two Tells us that there is one carbon atom and two

oxygen atomsoxygen atoms

Some common formulas are:Some common formulas are: Water (HWater (H22O)O)

Methane (CH4)Methane (CH4) Sugar (CSugar (C1212HH2222OO1111))

Structure of Chemical Structure of Chemical ReactionsReactions

Reactant +ReactantReactant +Reactant Product + Product + ProductProduct

Conservation of MatterConservation of Matter

First demonstrated by French chemist in 1774First demonstrated by French chemist in 1774

Antoine Lavoisier showed that matter is Antoine Lavoisier showed that matter is neither created nor destroyed during a neither created nor destroyed during a chemical reactionchemical reaction

The total mass of the reactants must equal The total mass of the reactants must equal the total mass of the productsthe total mass of the products

The number of atoms stays the same no The number of atoms stays the same no matter how they are arranged. So, their total matter how they are arranged. So, their total mass stays the samemass stays the same