Chapter 15Chapter 15

Chemical ReactionsChemical Reactions

Section 1: Forming New Section 1: Forming New SubstancesSubstances

Chemical reaction Chemical reaction – the process by which – the process by which one or more substances undergo change one or more substances undergo change to produce one or more new substancesto produce one or more new substances The new substances have different chemical The new substances have different chemical

and physical properties compared to the and physical properties compared to the original substancesoriginal substances

New substances are formed in a chemical New substances are formed in a chemical reaction because chemical bonds in the reaction because chemical bonds in the starting substances break, atoms starting substances break, atoms rearrange, and new bonds form to make rearrange, and new bonds form to make new substancesnew substances

Clues to Chemical ReactionsClues to Chemical Reactions

There are several clues that indicate There are several clues that indicate when chemical reaction might be when chemical reaction might be occurringoccurring Clues include:Clues include:

Gas Formation Gas Formation

Solid Formation – the solid formed in a solution Solid Formation – the solid formed in a solution is called a is called a precipitateprecipitate

Color Change Color Change

Energy ChangeEnergy Change

Chemical FormulasChemical Formulas

Chemical Formula Chemical Formula – a shorthand – a shorthand notation for a compound or a diatomic notation for a compound or a diatomic element using chemical symbols and element using chemical symbols and numbersnumbers A chemical formula indicates how many of A chemical formula indicates how many of

each kind of atom is present in a moleculeeach kind of atom is present in a molecule Subscript Subscript – a number written below and – a number written below and

to the right of a chemical symbol in a to the right of a chemical symbol in a formulaformula

Writing Formulas for covalent Writing Formulas for covalent CompoundsCompounds

Covalent compounds are usually composed of Covalent compounds are usually composed of two nonmetalstwo nonmetals

The names of covalent compounds use prefixes The names of covalent compounds use prefixes to tell you how many atoms each element are in to tell you how many atoms each element are in the formulathe formula

Writing Formulas for Ionic Writing Formulas for Ionic CompoundsCompounds

If the name of the compound contains the If the name of the compound contains the name of a metal and nonmetal, the name of a metal and nonmetal, the compound is probably ioniccompound is probably ionic

To write the formula for an ionic To write the formula for an ionic compound, you must make sure the compound, you must make sure the compounds overall charge is zerocompounds overall charge is zero The formula must have subscripts that cause The formula must have subscripts that cause

the charges of the ions to cancel each other the charges of the ions to cancel each other outout

Remember the charge of many ions can be Remember the charge of many ions can be determined by looking at the periodic tabledetermined by looking at the periodic table

Chemical EquationsChemical Equations

Chemical Equation Chemical Equation – a shorthand description – a shorthand description of a chemical reaction using formulas and of a chemical reaction using formulas and symbolssymbols i.e. i.e. 2 H2 H22 + O + O22 → 2 H→ 2 H22OO

Reactants Reactants – the starting materials in a chemical – the starting materials in a chemical reaction (to the left of the arrow)reaction (to the left of the arrow)

Products Products – the substances formed from a chemical – the substances formed from a chemical reaction (to the right of the arrow)reaction (to the right of the arrow) The symbol or formula for each substance in the reaction The symbol or formula for each substance in the reaction

must be written correctlymust be written correctly

Chapter 15 – Quiz 1 Chapter 15 – Quiz 1

1) _________ is the process by which one or more 1) _________ is the process by which one or more substances undergo change to produce one or substances undergo change to produce one or more new substances more new substances

2) Name two of the four clues that a chemical 2) Name two of the four clues that a chemical reaction has occurredreaction has occurred

3) _________ is a shorthand notation for a 3) _________ is a shorthand notation for a compound or molecule using chemical symbols compound or molecule using chemical symbols and numbersand numbers

4) _________ is the number written behind a 4) _________ is the number written behind a symbolsymbol

5) __________ a shorthand description for a 5) __________ a shorthand description for a chemical reaction using chemical formulas and chemical reaction using chemical formulas and symbolssymbols

Section 2: Types of Section 2: Types of Chemical ReactionsChemical Reactions

Most reactions can be grouped into Most reactions can be grouped into one of four categories: synthesis, one of four categories: synthesis, decomposition, single replacement, decomposition, single replacement, and double replacementand double replacement

Synthesis ReactionsSynthesis Reactions

Synthesis Reaction Synthesis Reaction - a reaction in - a reaction in which two or more substances which two or more substances combine to form a single compoundcombine to form a single compound i.e. 2 Hi.e. 2 H22 + O + O22 —> 2 H —> 2 H22OO

Decomposition ReactionsDecomposition Reactions

Decomposition ReactionDecomposition Reaction – a reaction – a reaction in which a single compound breaks in which a single compound breaks down to form two or more simpler down to form two or more simpler substancessubstances i.e. i.e. 2H2H22O O → 2H→ 2H2 2 + O+ O22

Single Replacement Single Replacement ReactionsReactions

Single-replacement reaction Single-replacement reaction – a – a reaction in which an element takes reaction in which an element takes the place of another element that is the place of another element that is part of a compoundpart of a compound i.e. i.e. Zn + 2HCl Zn + 2HCl → ZnCl→ ZnCl22 + H + H22

Double Replacement Double Replacement ReactionsReactions

Double – replacement reaction Double – replacement reaction – a – a reaction in which ions in two reaction in which ions in two compounds switch placescompounds switch places i.e. i.e. NaCl + AgNONaCl + AgNO3 3 → NaNO→ NaNO33 + AgCl + AgCl

Section 3: Energy and Rates of Section 3: Energy and Rates of Chemical ReactionsChemical Reactions

All chemical reactions involve All chemical reactions involve chemical energy, the energy stored chemical energy, the energy stored in chemical bonds.in chemical bonds.

Energy is released or absorbed in the Energy is released or absorbed in the overall reaction depending on how overall reaction depending on how the chemical energy of the reactants the chemical energy of the reactants compares with the chemical energy compares with the chemical energy of the productsof the products

Exothermic vs. Endothermic Exothermic vs. Endothermic ReactionsReactions

If the chemical energy of the reactants is If the chemical energy of the reactants is greater than the chemical energy of the greater than the chemical energy of the products, energy is released during the products, energy is released during the reactionreaction Exothermic Exothermic – a chemical reaction were energy is – a chemical reaction were energy is

releasedreleased If the chemical energy of the reactants is If the chemical energy of the reactants is

less than the chemical energy of the less than the chemical energy of the products, the difference in energy is products, the difference in energy is absorbed during the reactionabsorbed during the reaction Endothermic Endothermic – a chemical reaction in which – a chemical reaction in which

energy is absorbedenergy is absorbed

Energy is Conserved – ItEnergy is Conserved – It’’s a s a Law!Law!

The law of conservation of energy The law of conservation of energy – states – states that energy can neither be created or that energy can neither be created or destroyed.destroyed. i.e. the energy released in exothermic reactions i.e. the energy released in exothermic reactions

was originally stored in the bonds of the was originally stored in the bonds of the reactantsreactants

Also the energy absorbed in endothermic Also the energy absorbed in endothermic reactions is stored in the bonds of the productsreactions is stored in the bonds of the products

Activation energy Activation energy – the minimum amount – the minimum amount of energy needed for substances to reactof energy needed for substances to react

Factors Affecting Rates of Factors Affecting Rates of ReactionsReactions

1) 1) Temperature Temperature An increase in temperature will increase the An increase in temperature will increase the

rate of reaction as the reactant particles move rate of reaction as the reactant particles move faster. A decrease will slow the reaction downfaster. A decrease will slow the reaction down

2) 2) ConcentratioConcentrationn Increasing the concentration of the reactants Increasing the concentration of the reactants

increases the reaction rate as there is a increases the reaction rate as there is a greater chance for the molecules to collide. greater chance for the molecules to collide. Decrease concentration and the reaction slows Decrease concentration and the reaction slows downdown

Factors Affecting Rates of Factors Affecting Rates of Reactions (conReactions (con’’t)t)

33) Surface Area) Surface Area Increasing surface area increases the rate of Increasing surface area increases the rate of

reaction as more reactant particles are reaction as more reactant particles are exposed to each other. Decrease surface area exposed to each other. Decrease surface area and the reaction slows downand the reaction slows down

4) 4) Catalysts and InhibitorsCatalysts and Inhibitors Catalysts Catalysts – a substance that speeds up a – a substance that speeds up a

reaction without being permanently changedreaction without being permanently changed Inhibitor Inhibitor – a substance that slows down or – a substance that slows down or

stops a chemical reactionstops a chemical reaction