Chemical ReactionsChemical Reactions

PESS 1PESS 1

Observing Chemical Observing Chemical ChangeChange Review:Review:

– Physical properties: a Physical properties: a characteristic that can be characteristic that can be observed without changing observed without changing the substancethe substance

– Chemical properties: a Chemical properties: a characteristic of a characteristic of a substance that describes a substance that describes a substance’s ability to substance’s ability to changechange

– Physical change: a change Physical change: a change that alters the appearance that alters the appearance of a substance, but does of a substance, but does not change the substancenot change the substance

– Chemical change: a Chemical change: a change that occurs when change that occurs when the substance does change the substance does change into a new substance (AKA into a new substance (AKA chemical reaction)chemical reaction)

Observing Chemical Observing Chemical ChangeChange Chemical changes occur Chemical changes occur

when bonds break and new when bonds break and new bonds form the result is a bonds form the result is a new substance is producednew substance is produced

Evidence that a chemical Evidence that a chemical reactions has occurredreactions has occurred– The substance might The substance might

change formchange form– A solid may appear A solid may appear

when two solutions are when two solutions are mixed together (this is mixed together (this is called a precipitate)called a precipitate)

– A gas might be produced A gas might be produced from solids or liquidsfrom solids or liquids

– Be careful because Be careful because these changes might these changes might also occur in a physical also occur in a physical changechange

Observing Chemical Observing Chemical ChangeChange Changes in Changes in

energy might also energy might also occur in a occur in a chemical reactionchemical reaction– Energy can either be Energy can either be

absorbed or releasedabsorbed or released– EndothermicEndothermic: energy : energy

is absorbed & feels is absorbed & feels cool to the touchcool to the touch

– ExothermicExothermic: energy is : energy is released & feels warm released & feels warm to the touchto the touch

Describing Chemical Describing Chemical ReactionsReactions Chemical equations

– A short, easy way to A short, easy way to show a chemical show a chemical reactionreaction

– Symbols and Symbols and numbers are used in numbers are used in placed of wordsplaced of words

– Formulas in Formulas in equationsequations

COCO22, H, H22O, CO, C66HH1212OO66, NaCl, , NaCl, HH22OO22, NaHCO, NaHCO33

These are the formulas These are the formulas for common substancesfor common substances

Describing Chemical Describing Chemical ReactionsReactions

Structure of an EquationStructure of an Equation– ReactantsReactants are the are the

substances before a substances before a chemical reaction chemical reaction takes placetakes place

– ProductsProducts are the are the substances after a substances after a chemical reaction chemical reaction takes placetakes place

– Reactant + Reactant Reactant + Reactant Product + Product Product + Product

– CaCOCaCO33 CaO + CO CaO + CO2 2

Describing Chemical Describing Chemical ReactionsReactions Conservation of MassConservation of Mass

– Review: Law of Conservation Review: Law of Conservation of Mass (LCM) – matter of Mass (LCM) – matter cannot be created or cannot be created or destroyeddestroyed

– So this means that the same So this means that the same number of atoms that enter number of atoms that enter a chemical reaction must be a chemical reaction must be the same after the chemical the same after the chemical reaction occursreaction occurs

– Open systemsOpen systems: matter can : matter can escape into the surroundings escape into the surroundings (e.g. fire place)(e.g. fire place)

– Closed systemsClosed systems: matter : matter cannot escape into the cannot escape into the surroundings (e.g. baking surroundings (e.g. baking bread)bread)

Describing Chemical Describing Chemical ReactionsReactions Balancing equationsBalancing equations

– Due to the LCM in a chemical equation the Due to the LCM in a chemical equation the number of atoms that in the reactants number of atoms that in the reactants must be the same in the productsmust be the same in the products

– First step: write the equationFirst step: write the equation– Second step: identify which atoms and Second step: identify which atoms and

how many are used in the equationhow many are used in the equation– Third step: Use coefficients (NOT Third step: Use coefficients (NOT

subscripts) to even out the reactants and subscripts) to even out the reactants and the productsthe products

PracticePractice

AnswerAnswer

Describing Chemical Describing Chemical ReactionsReactions Classifying Chemical ReactionsClassifying Chemical Reactions

– SynthesisSynthesis: 2 or more substances combine to form a : 2 or more substances combine to form a more complex substancemore complex substance

HH22 + O + O22 2H 2H22OO– DecompositionDecomposition: a more complex substance breaks : a more complex substance breaks

apart to 2 or more simpler substancesapart to 2 or more simpler substances 2H2H22OO22 2H 2H22O + OO + O22

– ReplacementReplacement: an uncombined element replaces an : an uncombined element replaces an combined elementcombined element

2Cu2Cu22O + C O + C 4Cu + CO 4Cu + CO22

– Double replacementDouble replacement: elements in one compound : elements in one compound trade places with elements in another compoundtrade places with elements in another compound

FeS + 2HCl FeS + 2HCl FeCl FeCl22 + H + H22S S

ExamplesExamples

Controlling Chemical Controlling Chemical ReactionsReactions Activation energyActivation energy

– Remember: motion Remember: motion needs a force to get it needs a force to get it to move, the same goes to move, the same goes for chemical reactionfor chemical reaction

– Activation energy Activation energy is the is the “push” that gets “push” that gets chemical reactions chemical reactions goinggoing

– Examples: a spark is Examples: a spark is needed for combustion, needed for combustion, heat is needed to bake, heat is needed to bake, etc.etc.

Controlling Chemical Controlling Chemical ReactionsReactions Rates of chemical reactionsRates of chemical reactions

– Chemical reactions can Chemical reactions can be controlled by be controlled by changing such factors changing such factors as: surface area, as: surface area, temperature, temperature, concentration levels and concentration levels and using substances called using substances called catalysts or inhibitors catalysts or inhibitors

– Surface area: the Surface area: the smaller the surface area smaller the surface area the quicker the chemical the quicker the chemical reaction (that is why we reaction (that is why we chew our food)chew our food)

Controlling Chemical Controlling Chemical ReactionsReactions

TemperatureTemperature: the greater the : the greater the temperature the quicker the temperature the quicker the chemical reaction (that is why chemical reaction (that is why we put food in the we put food in the refrigerator because it slows refrigerator because it slows bacteria growth)bacteria growth)

ConcentrationConcentration: the greater : the greater the concentration the quicker the concentration the quicker the chemical reaction the chemical reaction

CatalystsCatalysts: a substance that : a substance that increase the reaction rate by increase the reaction rate by lowering the activation lowering the activation energy needed to get the energy needed to get the reaction startedreaction started

InhibitorsInhibitors: this is used to : this is used to decrease the reaction rate by decrease the reaction rate by preventing the reactants from preventing the reactants from combining (preservatives are combining (preservatives are examples of inhibitors) examples of inhibitors)