Chemical Reactions & Chemical Reactions & EquationsEquations

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Evidence of Chemical ReactionsEvidence of Chemical Reactions• Release of a gasRelease of a gas

Example: bubbles formed when magnesium and Example: bubbles formed when magnesium and hydrochloric acid were mixed hydrochloric acid were mixed

• Color changeColor change Example: color went from clear to yellowish orange Example: color went from clear to yellowish orange

when potassium iodide was added to hydrogen when potassium iodide was added to hydrogen peroxideperoxide

• Formation of a precipitateFormation of a precipitate Example: cloudiness occurred when COExample: cloudiness occurred when CO22 gas passed gas passed

through limewaterthrough limewater• Change in temperature, light, sound, smellChange in temperature, light, sound, smell

Example: temperature increased when hydrochloric Example: temperature increased when hydrochloric acid and sodium carbonate were mixedacid and sodium carbonate were mixed

Endothermic or Exothermic?Endothermic or Exothermic?

CaOCaO (s) + H(s) + H22O(l) O(l) Ca(OH) Ca(OH)22(aq) + 82 kJ(aq) + 82 kJ

Since energy is a product, this reaction is Since energy is a product, this reaction is exothermic.exothermic.This reaction will generate enough heat to fry an egg!This reaction will generate enough heat to fry an egg!

NHNH44NONO33(s) + H(s) + H22O(l) + energyO(l) + energy NH NH44NONO33(aq)(aq)

Since energy is a reactant, this reaction is Since energy is a reactant, this reaction is endothermic.endothermic.This is the cooling reaction of a cold pack.This is the cooling reaction of a cold pack.

Chemical EquationsChemical Equations

ReactantsReactantssubstances substances before the before the changechange

CC33HH88 OO22++

Propane and oxygenPropane and oxygen reactreact to to produceproduce carbon dioxide, carbon dioxide, water, and energy. water, and energy. This reaction is not yet balanced.This reaction is not yet balanced.

++

ProductsProductssubstances substances after the after the changechange

COCO22 HH22OO energyenergy++

Symbols in EquationsSymbols in Equations

++ ““plus” or “and” indicates multiple reactants or productsplus” or “and” indicates multiple reactants or products

““yields” or “produces” separates reactants & products, arrow points in the direction of yields” or “produces” separates reactants & products, arrow points in the direction of the reactionthe reaction

(s)(s) ““solid”solid”

(l)(l)

(g)(g)

indicate phase of a reactant or productindicate phase of a reactant or product““liquid”liquid”

““gas”gas”

(aq)(aq) ““aqueous” dissolved in water aqueous” dissolved in water (not the same as liquid (l))(not the same as liquid (l))

indicates reaction is reversibleindicates reaction is reversible

N.R.N.R. ““no reaction”no reaction”

over the arrow, indicates heating is necessaryover the arrow, indicates heating is necessary

Balancing EquationsBalancing Equations• When balanced, an equation accounts for the conservation When balanced, an equation accounts for the conservation

of:of: massmass chargecharge energyenergy

DO NOT CHANGE SUBSCRIPTS!DO NOT CHANGE SUBSCRIPTS!

• Balance the numbers of atoms of each element on Balance the numbers of atoms of each element on both sides of the equation by determining the both sides of the equation by determining the coefficients of the compounds. coefficients of the compounds.

An Example: Forming WaterAn Example: Forming Water

H2 (g) + O2 (g) H2O (l)

On left: 2 H and 2 OOn left: 2 H and 2 O On right: 2 H and 1 O On right: 2 H and 1 OTherefore, not balancedTherefore, not balanced

We could use 1/2 OWe could use 1/2 O22 on the left but that is not on the left but that is not

done, we always want whole number coefficients.done, we always want whole number coefficients.

Instead we double the coefficients of HInstead we double the coefficients of H22 and H and H22OO

2H2 (g) + O2 (g) 2H2O (l)

There are now 4 H’s and 2 O’s on each side.There are now 4 H’s and 2 O’s on each side.

Rules for Balancing EquationsRules for Balancing Equations

1.1. Write the formulas for all reactants and productsWrite the formulas for all reactants and products2.2. Don’t change formulas (subscripts), only coefficients.Don’t change formulas (subscripts), only coefficients.3.3. Balance elements that only occur once on each side of the Balance elements that only occur once on each side of the

equation firstequation first4.4. To determine the number of atoms of an element, multiply To determine the number of atoms of an element, multiply

the coefficient by the subscript for the element.the coefficient by the subscript for the element.

Tips: Tips: for reactions involving oxygen, balance it lastfor reactions involving oxygen, balance it lastpolyatomic ions can be balanced as a unitpolyatomic ions can be balanced as a unit

6H6H22OOcoefficientcoefficient subscriptsubscript

Balancing ExampleBalancing ExampleWrite the equation for burning ethylene (CWrite the equation for burning ethylene (C22HH44) to ) to

produce carbon dioxide and water.produce carbon dioxide and water.

CC22HH4 4 + O+ O22 CO CO22 + H + H22OO

C and H both appear once on each side of the equation whereas C and H both appear once on each side of the equation whereas O appears in O appears in bothboth compounds on the right hand side. compounds on the right hand side.

Multiply COMultiply CO22 by 2 to balance C by 2 to balance C

Multiply HMultiply H22O by 2 to balance HO by 2 to balance H

CC22HH4 4 + O+ O22 22COCO22 + + 22HH22OO

There are now 6 O’s on the right so multiply OThere are now 6 O’s on the right so multiply O22 by 3 to finish. by 3 to finish.

CC22HH4 4 + + 33OO22 2CO 2CO22 + 2H + 2H22OO

Balancing Practice #1Balancing Practice #1

K + FK + F22 KF KFUnbalanced equation:Unbalanced equation:

Balance F:Balance F: K + K + FF22 22KKFF

Balance K:Balance K: 2K2K + F + F22 2K2KFF

Balancing Practice #2Balancing Practice #2

Ca + HCa + H22O O Ca(OH) Ca(OH)22 + H + H22 Unbalanced:Unbalanced:

Notice Ca is fine:Notice Ca is fine: CaCa + H + H22O O CaCa(OH)(OH)22 + H + H22

Balance O:Balance O: Ca + Ca + 22HH22OO Ca( Ca(OOH)H)22 + H + H22

Hydrogen is already taken care of so we’re done.Hydrogen is already taken care of so we’re done.

Balancing Practice #3Balancing Practice #3

NHNH44Cl + Ca(OH)Cl + Ca(OH)22 NH NH33 + H + H22O + CaClO + CaCl22

22NHNH44ClCl + Ca(OH) + Ca(OH)22 NH NH33 + H + H22O + CaO + CaClCl22Cl:Cl:

2N2NHH44Cl + Ca(OH)Cl + Ca(OH)22 22NNHH33 + H + H22O + CaClO + CaCl22N:N:

2NH2NH44Cl + Ca(Cl + Ca(OOH)H)22 2NH 2NH33 + + 22HH22OO + CaCl + CaCl22O:O:

H:H: 22NNHH44Cl + Ca(OCl + Ca(OHH))22 22NNHH33 + + 2H2H22O + CaClO + CaCl22

10 H on each side, so we’re done!10 H on each side, so we’re done!

5 Types of Chemical Reactions5 Types of Chemical Reactions

• CombustionCombustion• SynthesisSynthesis• DecompositionDecomposition• Single ReplacementSingle Replacement• Double ReplacementDouble Replacement

Some reactions fall into multiple categories.Some reactions fall into multiple categories.

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Combustion ReactionsCombustion ReactionsA substance reacts with oxygen to create A substance reacts with oxygen to create products containing oxygen. Heat may be products containing oxygen. Heat may be required to initiate. Exothermic.required to initiate. Exothermic.

2Mg(s)2Mg(s) + O+ O22(g) 2MgO(s)(g) 2MgO(s)

MetalsMetals produce metal oxides (like rust produce metal oxides (like rust and tarnishes).and tarnishes). QuickTime™ and a

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hydrocarbonhydrocarbon + + OO2 2 (g)(g) COCO2 2 (g)(g) + + HH22O(g)O(g)

Hydrocarbons (C, H, O)Hydrocarbons (C, H, O) - heat homes, cook - heat homes, cook food, and power transportation. food, and power transportation. Always get Always get COCO22(g) and H(g) and H22O(g)O(g). .

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Practice CombustionPractice CombustionWrite the balanced equation for the Write the balanced equation for the combustion of heptane gas (Ccombustion of heptane gas (C77HH1616))

Write out the reactants and products:Write out the reactants and products:

Balance the equation (ignore heat):Balance the equation (ignore heat):

CC77HH1616(g) + O(g) + O22(g)(g) COCO22(g) + H(g) + H22O(g) + heatO(g) + heat

CC77HH1616(g) + 11 O(g) + 11 O22(g)(g) 7 CO7 CO22(g) + 8 H(g) + 8 H22O(g)O(g)

Synthesis ReactionsSynthesis Reactions• The combining of two or more substances to The combining of two or more substances to

form a more complex substance.form a more complex substance.

2Mg2Mg + O+ O22 2MgO 2MgO magnesium oxidemagnesium oxide

2Na2Na + Cl+ Cl22 2NaCl 2NaCl sodium chloridesodium chloride

Examples:Examples:

general formgeneral formA + B ABA + B AB

Note: A & B can be elements or compoundsNote: A & B can be elements or compounds

Decomposition ReactionsDecomposition Reactions• Compound breaks down into two or more Compound breaks down into two or more

simpler substances (elements or compounds).simpler substances (elements or compounds).

general formgeneral formAB A + BAB A + B

NHNH44NONO33(s)(s) NN22O(g) + 2HO(g) + 2H22O(g)O(g)

Examples:Examples:

2H2H22O(l)O(l) 2H2H22(g) + O(g) + O22(g)(g)electricityelectricity

Single Replacement ReactionsSingle Replacement ReactionsOne One elementelement takes the place of another takes the place of another elementelement. . Metals replace metals. Non-metals replace non-Metals replace metals. Non-metals replace non-metals. metals. Not reversibleNot reversible..

general formsgeneral formsA + BC BA + CA + BC BA + C

ClCl22(aq) + 2KBr(aq) 2KCl(aq) + Br(aq) + 2KBr(aq) 2KCl(aq) + Br22(aq)(aq)Example:Example:

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A + BC AC + BA + BC AC + B

Double Replacement ReactionsDouble Replacement ReactionsWhen solutions of two soluble ionic compounds are When solutions of two soluble ionic compounds are mixed, an insoluble product (precipitate) forms.mixed, an insoluble product (precipitate) forms.

general formgeneral formAB + XY AY + XBAB + XY AY + XB

Pb(NOPb(NO33))22(aq) + 2KI(aq) PbI(aq) + 2KI(aq) PbI22(s)(s) + 2KNO + 2KNO33(aq) (aq) Example:Example:

Remember “inny outy” ruleRemember “inny outy” rule

More on Double ReplacementMore on Double Replacement• When 2 ionic solutions are mixed there are When 2 ionic solutions are mixed there are

several events that could happen:several events that could happen: Nothing, everything is solubleNothing, everything is soluble A precipitate forms (possibly two)A precipitate forms (possibly two) A gas is formed A gas is formed

(like FeS(s) + H(like FeS(s) + H22SOSO44(aq) --> FeSO(aq) --> FeSO44(aq) + H(aq) + H22S(g)S(g) A covalent molecule forms (like HA covalent molecule forms (like H++ + OH + OH-- --> H --> H22O)O)

• Ions that remain in solution are called Ions that remain in solution are called spectator spectator ionsions because they don’t participate in the because they don’t participate in the reaction.reaction.

Practice - Identifying Reaction TypesPractice - Identifying Reaction Types

single replacementsingle replacement

decompositiondecomposition

double replacementdouble replacement

Ca(s) + CuSO4(aq) CaSO4(aq) + Cu(s) Ca(s) + CuSO4(aq) CaSO4(aq) + Cu(s)

2AlCl2AlCl33(l) 2Al(l) + 3Cl(l) 2Al(l) + 3Cl22(g) (g)

ZnSOZnSO44(aq) + SrCl(aq) + SrCl22(aq) ZnCl(aq) ZnCl22(aq) + SrSO(aq) + SrSO44(s)(s)

4Na(s) + O4Na(s) + O22(g) 2Na(g) 2Na22OO (s)(s)synthesis, combustionsynthesis, combustion