Conservation of Massflemingapchem.weebly.com/uploads/2/4/6/5/24658308/ap_chem_we… · Law of...

ConservationofMass

Law of Conservation of Mass •  Law of Conservation of Mass =

mass in a closed system can neither be created nor destroyed

Law of Conservation of Mass • What does this mean for us?

– In a balanced chemical equation, the number and kinds of atoms on each side of the equation should be equal.

Law of Conservation of Mass

Balancing Equations • A balanced equation has the same

number of each element on both sides of the equation

Balancing Equations • We use coefficients to indicate

multiples of molecules or compounds

Balancing Equations • Coefficients are multiples of the

entire formula • How does this affect each atom?

Rules 1.  Write out correct formulas (if not

already written) Ex/ Li + H3PO4 à H2 + Li3PO4

Rules 2. Count the number of atoms of each type on both sides

–  If possible, keep polyatomic ions together

Ex/ Li + H3PO4 à H2 + Li3PO4

Rules 3. Balance the elements one at a time by adding coefficients in front.

à Generally, balance H, O, & monatomic atoms last

Ex/ Li + H3PO4 à H2 + Li3PO4

Rules 4. Double-check – always Ex/ Li + H3PO4 à H2 + Li3PO4

Rules Never’s: •  Never add or change subscripts on

formulas •  Never put a coefficient in the middle of a

formula

Try These: •  AgNO3+Cu→Cu(NO3)2+Ag

• C3H8 + O2 à H2O + CO2

Balanced Chemical Equations • When baking cookies, a recipe is

usually used, telling the exact amount of each ingredient. – If you need more, you can double or triple the amount

Stoichiometry • Greekfor“measuringelements”•  Stoichiometry=thecalculationofquantitiesinchemicalreactions

•  Allstoichiometriccalculationsmustbeginwithabalancedchemicalequation

Balanced Chemical Equations • Waystointerpretabalancedchemicalequation:– Moles– Particles(atoms,molecules,formulaunits)

– Mass– Volume

2H2 + O2 à 2H2O •  For every two moles of hydrogen and

one mole of oxygen, 2 moles of water form

2 mol H2: 1 mol O2: 2 mol H2O

Particles •  Relating the number of molecules 2H2 + O2 → 2H2O (2 x 6.022 x 1023 molec H2) + (1 x 6.022 x 1023 molec L O2) → (2 x 6.022 x 1023

molec H2O)

Volume •  If we are at STP... 2H2 + O2 → 2H2O (2 x 22.4 L H2) + (1 x 22.4 L O2) → (2 x 22.4 L H2O)

Mass 2H2+O2à2H2O

•  Becauseofthelawofconservationofmass,themassofthereactantsmustequalthemassoftheproducts.

Conversions

Steps to Calculate Stoichiometric Problems

1.  Correctly balance the equation. 2.  Convert the given amount into

moles. 3.  Set up mole ratios. 4.  Use mole ratios to calculate moles

of desired chemical. 5.  Convert moles back into final unit.

Example: 2CO+O2à2CO2

HowmanylitersofcarbonmonoxideatSTPareneededtoreactwith4.80gofoxygengastoproducecarbondioxide?

Example:Solidlithiumhydroxideisusedinspacevehiclestoremoveexhaledcarbondioxidefromthelivingenvironmentbyformingsolidlithiumcarbonateandliquidwater.Whatmassofgaseouscarbondioxidecanbeabsorbedby1.00kgoflithiumhydroxide?Steps:1.  Writebalancedequation2.  Planunitsteps3.  Setupconversionfactors

Example:Solidlithiumhydroxideisusedinspacevehiclestoremoveexhaledcarbondioxidefromthelivingenvironmentbyformingsolidlithiumcarbonateandliquidwater.Whatmassofgaseouscarbondioxidecanbeabsorbedby1.00kgoflithiumhydroxide?Steps:1.  Writebalancedequation

LiOH(s)+CO2(g)àLi2CO3(s)+H2O(l)2LiOH(s)+CO2(g)àLi2CO3(s)+H2O(l)

2.  Planunitsteps3.  Setupconversionfactors

Example:Solidlithiumhydroxideisusedinspacevehiclestoremoveexhaledcarbondioxidefromthelivingenvironmentbyformingsolidlithiumcarbonateandliquidwater.Whatmassofgaseouscarbondioxidecanbeabsorbedby1.00kgoflithiumhydroxide?Steps:1.  2LiOH(s)+CO2(g)àLi2CO3(s)+H2O(l)2.  Planunitsteps

kgLiOHàgLiOHàmolLiOHàmolCO2àgCO22.  Setupconversionfactors

Example:Solidlithiumhydroxideisusedinspacevehiclestoremoveexhaledcarbondioxidefromthelivingenvironmentbyformingsolidlithiumcarbonateandliquidwater.Whatmassofgaseouscarbondioxidecanbeabsorbedby1.00kgoflithiumhydroxide?Steps:1.  2LiOH(s)+CO2(g)àLi2CO3(s)+H2O(l)2.  kgLiOHàgLiOHàmolLiOHàmolCO2àgCO23.  Setupconversionfactors

LimitingReactants

StoichiometricMixture•  Atypeofmixturethatcontainsrelativeamountsofreactantsthatmatchthenumbersinthebalancedequation

StoichiometricMixture•  Whenhydrogenisthelimitingreactant

LimitingReactant•  Whatisalimitingreactant?•  Alimitingreactantlimitstheamountofproductthatcanform– Whenitistotallyconsumed,thereactioniscompleteàthereactioncannotcontinuewithoutit

LimitingReactant•  Note:Somemixturescanbestoichiometric•  Thismeansthatallreactantsrunoutatthesametime•  Requiresdeterminingwhichreactantislimiting

LimitingReactant•  Howdowedeterminethelimitingreactant?

– Method1• Comparethemolesofthereactantstoseewhichwillbeconsumedfirst

– Method2• Comparetheamountofproducteachwouldproduce,astheLRwillalwaysproducelessproduct

Example•  Nitrogengascanbepreparedbypassinggaseousammoniaoversolidcopper(II)oxideathightemperatures.Otherproductsofthereactionaresolidcopperandwatervapora.  Ifasamplecontaining18.1gofNH3isreactedwith

90.4gofCuO,whichisthelimitingreactant?b.  HowmanygramsofN2willbeformed?

90.4gofCuO,whichisthelimitingreactant?First,let’swriteourbalancedequation.

NH3(g)+CuO(s)àN2(g)+Cu(s)+H2O(g)2NH3(g)+3CuO(s)àN2(g)+3Cu(s)+3H2O(g)

90.4gofCuO,whichisthelimitingreactant?Next,let’sconvertbothofourreactantstomoles

2NH3(g)+3CuO(s)àN2(g)+3Cu(s)+3H2O(g)

90.4gofCuO,whichisthelimitingreactant?Now,comparethemolesofreactantsusingmoleratiostoseehowmuchoftheotherreactanteachrequires.

90.4gofCuO,whichisthelimitingreactant?First,wewriteourbalancedequationagain.

90.4gofCuO,whichisthelimitingreactant?Then,again,weconvertbothofourreactantstomoles

90.4gofCuO,whichisthelimitingreactant?Next,let’susestoichiometrytocomparetheamountofN2thatwouldbeproduced

90.4gofCuO,whichisthelimitingreactant?b.  HowmanygramsofN2willbeformed?

•  Usethelimitingreactantasyourstartingquantityforyourcalculation

•  molCuOàmolN2àgN2

LimitingReactant•  Ifweweretoobservethisreactioninalab,wouldwegetexactlythatamountofN2?

•  TheoreticalYield:– AmountofproductformedwiththeLRisentirelyconsumed–  Inotherwords,thevaluewecalculatefromourbalancedequation

•  PercentYield:–  Theactualyieldofourproductthatweobtain.

TryThis:•  Methanol(CH3OH),alsocalledmethylalcohol,canbemanufacturesbycombininggaseouscarbonmonoxideandhydrogen.Suppose68.5kgCO(g)isreactedwith8.60kgH2(g).

a.  Whatisthelimitingreactant?b.  HowmuchCH3OHwillbeproduced?c.  If3.57x104gofCH3OHisactuallyproduced,whatisthe

percentyieldofmethanol?SHOWALLSTEPSANDUNITSCLEARLY

SampleQuestion•  Inthereaction2A+3B→C,4.0molesofAreactwith4.0molesofB– Whichofthefollowingchoicesbestanswersthequestion,“whichreactantislimiting?”a.  Neitherislimitingbecauseequalamounts(4.0mol)ofeach

reactantareusedb.  Aislimitingbecause2issmallerthan3(thecoefficientsin

thebalancedequation)c.  Aislimitingbecause2molisavailablebut4.0molisneededd.  Bislimitingbecause3islargerthan2(thecoefficientsinthe

balancedequation)e.  Bislimitingbecause4.0molisavailablebut6.0molis

needed

SampleQuestion•  Limitingreactantinareaction:

a.  Hasthesmallestcoefficientinabalancedequationb.  Isthereactantforwhichyouhavethefewestnumber

ofmolesc.  Hasthelowestratioof[molesavailable/coefficientin

thebalancedequation]d.  Hasthelowestratioof[coefficientinthebalanced

equation/molesavailable]e.  Noneofthese

SampleQuestion•  Considerthefollowingbalancedequation:A+5B→3C+4D– Whichofthefollowingchoicesbestanswersthequestion,“whenequalmassesofAandBarereacted,whichislimiting?”a.  IfthemolarmassofAisgreaterthanthemolarmassofB,

thenAmustbelimitingb.  IfthemolarmassofAislessthanthemolarmassofB,then

Amustbelimitingc.  IfthemolarmassofAisgreaterthanthemolarmassofB,

thenBmustbelimitingd.  IfthemolarmassofAislessthanthemolarmassofB,then

Bmustbelimiting

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