CHEMICAL REACTIONS The Process of Chemical Change – or – “What the heck have I just made?”

CHEMICAL REACTIONSCHEMICAL REACTIONSThe Process of Chemical Change

– or –“What the heck have I just made?”

CHEMICAL CHANGESCHEMICAL CHANGESIn a chemical change a new

substance is formedSigns of a chemical change

◦Color change◦Change in odor◦Release of a gas◦Precipitate – solid formed suddenly

in a liquid solution

CHEMICAL CHANGESCHEMICAL CHANGESEnergy changes

◦Temperature change◦Flame or explosion◦Luminescence or fluorescence

Chemical equations – a way of describing a reaction

Reactants and products◦reactant 1 + reactant 2 products

CHEMICAL EQUATIONSCHEMICAL EQUATIONSWord equationsacetic acid + sodium bicarbonate

sodium acetate + carbon dioxide

+ waterChemical equationsHC2H3O2 + NaHCO3 NaC2H3O2 + CO2 + H2O

CHEMICAL EQUATIONSCHEMICAL EQUATIONSChange to a chemical equation:Calcium + fluorine gas calcium

fluorideCa + F2 CaF2

Chemical EquationsChemical EquationsSymbols for physical state

(s) = solid(l) = liquid(g) = gas

(aq) = aqueous (dissolved in water)HC2H3O2(aq)+ NaHCO3(s)NaC2H3O2(aq)+ CO2(g)

+H2O(l)

Energy in Chemical Energy in Chemical ReactionsReactionsEnergy in chemical reactions

◦Exothermic reactions – heat is released

◦Reaction vessel feels warmEnergy appears on the right side

of the equation as a productC2H5OH + 3O2 2CO2 + 3H2O + energy

ENERGYENERGYEndothermic reactions

◦Take in energy from the surroundings

◦Reaction container gets cold◦Energy appears on the left side of

the equation as a reactantenergy+Ba(OH)2(s)+2NH4NO3(s)Ba(NO3)2(aq)+2H2O(l)

+2NH3(g)

BALANCING EQUATIONSBALANCING EQUATIONSLaw of conservation of matter

and energy◦Matter and energy cannot be created

or destroyed – only interconverted ◦All the atoms in the reactants show

up in the products

Balancing EquationsBalancing EquationsCoefficients – used to indicate

numbers of molecules or formula units

5NaCl (five formula units of sodium chloride)

2H2O (two molecules of water)

Balancing EquationsBalancing EquationsCounting atoms

5NaCl means five sodium atoms and five

chlorine atoms

2H2Omeans four (2x2) hydrogen atoms and

two oxygen atoms

Balancing EquationsBalancing Equations

3Ca(OH)2

means three calcium atoms, six (3x2) oxygen atoms and six hydrogen atoms Multiply the subscript times the coefficient for every atom in the

parentheses


4Ba(NO3)2

4 barium atoms, 8 (4x2) nitrogen atoms, and 24 (4x3x2) oxygen atoms

2Na2CO3 . 10H2O

4 (2x2) sodium atoms, 2 carbon atoms, 26 (2x3 + 10x2) oxygen atoms, and

40 (2x10x2) hydrogen atoms

Balancing EquationsBalancing EquationsThere must be the same number of

atoms on each side of the equation

H2 + O2 H2O22H 2O 4H 2O2H 2O 2H 1O

24H 2O 4H 2O


NaI + BaCl2 NaCl + BaI2(s)21Na 1I 1Ba 2Cl 1Na 1Cl 1Ba 2I1Na 1I 1Ba 2Cl 2Na 2Cl 1Ba 2I2Na 2I 1Ba 2Cl 2Na 2Cl 1Ba 2I

2

•Polyatomic ions can be counted as a group if they appear on both sides

AgNO3 + Na2SO4 Ag2SO4(s)+ NaNO3221Ag 1NO3 2Na 1SO4 2Ag 1SO4 1Na 1NO32Ag 2NO3 2Na 1SO4 2Ag 1SO4 1Na 1NO32Ag 2NO3 2Na 1SO4 2Ag 1SO4 2Na 2NO3

Types of Chemical Types of Chemical ReactionsReactions

Synthesis reaction

A + B CC + O2 CO2

2Na + Cl2 2NaClNa + Cl2 NaCl


Decomposition

C A + BH2O H2 + O2

KClO3 KCl + O22KClO3 2KCl + 3O2

(NH4)2Cr2O7N2+4H2O+Cr2O3

22


Single displacement

A + BC AC + BMetals displace metals,

nonmetals displace nonmetals

Fe + CuSO4Cu + FeSO4

Cl2+2NaBr(aq)Br2+2NaCl(aq)Cl2+NaBr(aq)Br2+NaCl(aq)


Double displacement

AB + CD AD + CBMetals and nonmetals switch partners

NaCl+AgNO3NaNO3+AgCl(s)

2KI + Pb(NO3)2PbI2 + 2KNO3 KI+Pb(NO3)2PbI2(s)+KNO3


Combustion Element or compound reacting

with oxygen

2Mg + O2 2MgO

2H2 + O2 2H2O

Combustion ReactionsCombustion ReactionsCarbon containing compounds

give CO2 and water

CH4 + O2 CO2 + H2OCH4 + 2O2CO2+ 2H2O

C2H5OH+O2CO2+H2O C2H5OH+3O22CO2+3H2O

EquilibriumEquilibriumDynamic equilibrium occurs when

the forward and reverse reactions happen at the same rate.

N2 + 3H2 2NH3

At equilibrium, ammonia is being made and decomposed at the same speed.

EquilibriumEquilibrium

At equilibrium, the reaction seems to stop.

In order to achieve equilibrium, reactions must be reversible, or able to occur in both directions.

LeChatelier’s PrincipleLeChatelier’s PrinciplePosition of equilibrium – how

much product and reactant is present at equilibrium

LeChatelier’s principle: If an equilibrium system is stressed, the system will shift to relieve the stress.

LeChatelier’s PrincipleLeChatelier’s PrincipleStresses – adding or removing

reactants, products or energy.

N2+3H22NH3+energy6H2O + CoCl4-2 Co[H2O]6

+2 + 4Cl- + energy

blue pink Add Cl-

Remove Cl-

•Add energy•Remove energy

KineticsKineticsKinetics: reaction rates, or how

fast does a reaction goActivation energy is the energy

required to start a reactionReaction rates are measured in

units of (product)/(unit time)

KineticsKineticsFactors affecting reaction rates

◦Temperature◦Concentration/surface area

Limiting reagent – the reactant that is completely used up in a reaction

◦Catalyst – a substance that speeds a reaction without being used up

KineticsKineticsInhibitor – a substance that slows

a reaction.

CHEMICAL REACTIONS The Process of Chemical Change – or – “What the heck have I just made?”

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