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LIMITING REACTANT LIMITING REACTANT • The limiting reactant is used up first • The limiting reactant determines the amount of product • Need balanced equation to proceed

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Page 1: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

LIMITING REACTANTLIMITING REACTANT

• The limiting reactant is used up first

• The limiting reactant determines the

amount of product

• Need balanced equation to proceed

Page 2: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

Limiting ReagentsLimiting Reagents

2H2 + O2 2H2O

10 H2 and 7 O2 10 H2O and 2 O2

reaction stops when one of the reactants is depleted

What if you had only 220.0 g Pb(NO3)2?How much PbI2 would precipitate?

Page 3: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

PERCENT YIELDPERCENT YIELD

PercentYield =

actual yieldtheoretical yield

x 100%

Calculation is just one more stepbeyond a standard stoichiometrycalculation

Page 4: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

PERCENT YIELDPERCENT YIELD

EXAMPLESilicon carbide (SiC) is made from sand (silicon dioxide, SiO2) and carbon at high T. CO is alsoformed. If 100.0 kg of sand are reacted and 55.0 kg SiC are formed, what is the percent yield?

SiO2(s) + 3 C(s) → SiC(s) + 2 CO(g)

Convert kg SiO2 to moles:

100.0 kg x 103 g/kg x 1 mol60.09 g

= 1664 mol SiO2

moles SiO2 = moles SiC

Convert moles SiC to kg:

1664 mol x 40.10 g/mol x 10-3 = 66.73 kg SiC

% yield = 55.0 kg66.73 kg

x 100% = 82.4%

Page 5: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

TITRATIONTITRATION

• Goal: find conc. of unknown• React solution of known conc.

(std. soln.) with soln. of unknown conc.• Find equivalence point (vol.)

soln. of unknown conc.

add std. soln.

Find volume at equivalence point

(need way to signal the equiv. pointsuch as indicator)

Page 6: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

ACIDSACIDS--BASEBASENEUTRALIZATIONNEUTRALIZATION

ACID + BASE → SALT + WATER

HNO3 + KOH → KNO3 + H2O

H+(aq) + NO3–(aq) + K+(aq) + OH–(aq) →

H2O(l) + K+(aq) + NO3–(aq)

completeionic

equation

H+(aq) + OH–(aq) → H2O(l) net ionicequation

eliminate spectator ions

Page 7: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

Strong Acids and BasesStrong Acids and BasesTable 4.2

Strong Acids

Hydrochloric, HClHydrobromic, HBrHydroiodic, HIChloric, HClO3

Perchloric, HClO4

Nitric, HNO3

Sulfuric, HSO4

Strong Bases

Group 1A metal hydroxides(LiOH, NaOH, KOH, RbOH, CsOH)

Heavy Group IIA metal hydroxides(Ca(OH)2, Sr(OH)2, Ba(OH)2)

Page 8: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

TITRATION EXAMPLETITRATION EXAMPLEA flask contains an unknown amount of HCl.This solution is titrated with 0.101 M NaOH.It takes 23.35 mL of NaOH to complete thereaction. How many grams of HCl were there?

HCl + NaOH → NaCl + H2O

(1) Find moles of NaOH used

(23.35 x 10–3 L NaOH)(0.101 mol/L) = 2.36 x 10–3 mol

(2) Find moles of HCl……same as NaOH

(3) Find g of HCl

(2.36 x 10–3 mol)(36.5 g/mol) = 86.1 x 10–3 g

86.1 mg HCl

If we knew the volume of HCl soln., we couldcalc. the M of the HCl soln.

Page 9: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

TITRATING HTITRATING H22SOSO44 with with NaOHNaOH

2 H+(aq) + 2 OH– (aq) → 2 H2O(l) net ionicequation

H2SO4(aq) + 2 NaOH(aq) Na2SO4(aq) + 2 H2O(l)

Begin with 20 mL of 0.1 M H2SO4(aq), titrate with0.1 M NaOH(aq)

As NaOH(aq) is added, H+(aq) ions are used to make H2O(l). When enough NaOH is added, all of the H+ ions are consumed and the indicator turns pink.

What volume of 0.1 M NaOH is required?What is final volume of solution?What is final conc. Of H+(aq) and OH–(aq)?What is final conc. of Na+(aq) and SO4

2–(aq)

Page 10: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

STOICHIOMETRYSTOICHIOMETRY

When chemical reactions involve gases,the balanced equation provides the numberof moles of reactants and products.

The ideal gas equation provides the linkbetween number of moles and P, V, Tof gases.

How much gas is produced or consumedby a chemical reaction?

find n and then V or P

Page 11: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

Air BagAir Bag2 NaN3(s) → 2 Na + 3 N2(g)

sodium azide gas in air bag

How many L of N2 at 735 mm Hg and26 °C are produced from 125 g NaN3?

Moles NaN3125 g

65.01 g/mol= 1.92 mol

Moles N2 (1.92 mol NaN3)(3/2) = 2.88 mol N2

V = (2.88)(0.0821)(299)

0.967= 73.1 L

29 NaN3 + 4 Fe2O3 + NaNO3 →15 Na2O + 8 Fe + 44 N2

2/3 = 0.67 29/44 = 0.66

(40 msec)

Page 12: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

IDEAL GAS LAW EXAMPLEIDEAL GAS LAW EXAMPLECalculate pressure change in cylinder of a car’s engine when 0.250 g of octane [C8H18(l)] is burned with a stoichiometric amount of O2assuming complete combustion.

Cylinder volume = 0.100 L

Initial Temp = 80 oC; final Temp = 700 oC

Page 13: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

ENERGYENERGY

KINETICENERGY

POTENTIALENERGY

• mechanical(moving mass ½ mv2)

[ joule = kg m2/s2]

• mechanical(mass in a placewhere force can act)

• chemical (bonds)

• nuclear(binding energy)

• electrical(moving charge)

• light (photons)

• sound (moleculesmoving uniformly)

• heat (moleculesmoving randomly)

Page 14: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

ENERGY CONVERSIONENERGY CONVERSION

Energy can be converted from oneform to another

When it is converted, the total energyremains constant

Law of Conservation of EnergyFirst Law of Thermodynamics

All energy lost by a system underobservation is gained by the surroundings

During energy conversion, some heatis always produced

Page 15: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

The energy of the universe is constant.

ΔEuniverse = ΔEsystem + ΔEsurroundings = 0

Energy is neither created nor destroyed,

only converted from one form to another.

ΔEsystem = q + w

q is heat gained or lost by the system

w is work done by or on the system

First Law of Thermodynamics First Law of Thermodynamics Conservation of EnergyConservation of Energy

Page 16: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

ΔE = Efinal – Einitial

ΔE is a state function

System Surroundingsenergy ΔE is –

Surroundings Systemenergy ΔE is +

ΔE = q + wwork (work done to system +)

heat (heat added to system +)

State function: a function whose value doesnot depend on pathway used to get topresent state.

ENERGY CHANGESENERGY CHANGES

Page 17: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

ENTHALPYENTHALPYWhen changes occur at constant pressure

ΔE = qp + wexpansion

this is negligible

ΔH = qp

ΔH is the enthalpy changeΔH is the quantity of thermal energy

transferred into a system at constantpressure

Like energy, enthalpy is a state function

Page 18: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

ENTHALPY OF REACTIONENTHALPY OF REACTION

ΔH = H(products) – H(reactants)

Endothermic ΔH > 0Exothermic ΔH < 0

2 H2(g)+ O2(g) → 2 H2O(g) + heat

ΔH = – 483.6 kJ

Characteristics of Enthalpy(1) Enthalpy is an extensive property(2) ΔH for a reaction is equal in magnitude but

opposite in sign to ΔH for reverse reaction(3) ΔH for a reaction depends on states of

reactants and products (gas, liquid)

Page 19: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

2 H2 + O2 → 2 H2O ΔH = −483.6 kJ

Is this reaction exothermic or endothermic? ________

How much heat is given off per mole of O2? _____

How much heat is given off per mole of H2? _____

What is ΔH for 2 H2O → O2 + 2 H2 ? _______

How many kJ of heat are needed to convert9.0 g of H2O into H2 and O2 ?

2 H2O → O2 + 2 H2

THERMOCHEMICAL THERMOCHEMICAL EQUATIONSEQUATIONS

A balanced chemical equation that alsoincludes the enthalpy change.ΔHo delta H standard

standard P (1 bar) & T (usually 25 °C)

Page 20: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

2 HI(g) products

H2(g) + I2(g) reactants

HΔH = 53 kJ

ENDOTHERMIC REACTION

CH4(g) and 2 O2(g) reactants

CO2(g) + 2 H2O(l) products

HΔH = –890 kJ

EXOTHERMIC REACTION

Page 21: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

EXOTHERMIC REACTIONEXOTHERMIC REACTIONDEMONSTRATIONDEMONSTRATION

SO32−(aq) + OCl−(aq) →

SO42−(aq) + Cl−(aq) + heat

sodium sulfite and bleach

Laundry bleach is 5% NaOCl

This is a redox reaction

S4+ → S6+ + 2 e−

Cl1+ + 2 e− → Cl−

Heat is evolvedExothermicΔH is negative

Page 22: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

FLAMING COTTONFLAMING COTTON

Na2O2(s) + 2H2O(l) → 2Na+ + 2OH– + H2O2(aq)

sodiumperoxide

2H2O2(aq) → 2 H2O + O2

reaction produces heathave high conc. of O2cotton has low ignition temperature

….so…..

flame

Page 23: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

ENDOTHERMIC REACTIONENDOTHERMIC REACTIONDEMONSTRATIONDEMONSTRATION

Ba(OH)2•8H2O(s) + 2 NH4(SCN)(s) →Ba(SCN)2(aq) + 2 NH3(g) + 10 H2O(l)

mix two solids

highly endothermic reaction

increase in entropy is driving force

go from two ordered crystals to ionsand gases in solution

Page 24: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

HEAT CAPACITYHEAT CAPACITY

Experimental measurement of heat flow

q = C m ΔT

temp changemass

specific heat

For H2O: 4.184 Jg °C

heat gained or lost

Page 25: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

QUANTITATIVEQUANTITATIVECALORIMETRYCALORIMETRY

EXAMPLEA calorimeter with 200 g H2O is used for areaction. If T rises from 25.0 °C to 33.0 °C,how much energy is being released?

heat capacity of H2O(l) = 4.184 J/°C• g

4.184 J°C g H2O

(200 g H2O)(8.0 °C) = 6694.4 J

6.7 kJ

q = C m ΔT

or

Page 26: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

HESSHESS’’S LAWS LAW

ΔH for a sum of steps is the sameas for the overall process

Reason: ΔH is a state function

Analogy to location and distance

Page 27: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

HESSHESS’’S LAW EXAMPLES LAW EXAMPLE

N2(g) + 2 O2(g) → 2 NO2(g) ΔH = ?

Two steps

N2(g) + O2(g) → 2 NO(g) ΔH = 180 kJ2 NO(g) + O2(g) → 2 NO2(g) ΔH = – 112 kJ

N2(g) + 2 O2(g) → 2 NO2 (g) ΔH = 68 kJ

sum of ΔH values gives ΔH of net reaction

SMOG REACTION

Page 28: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

ΔΔH OF FORMATIONH OF FORMATION

Hess’s law applied to combination reactions

Elementscombine

Compound ΔHfheat of

formation

When all substances in standardstates (T, P, state) then

ΔHfo standard heat of formation

ΔHfo for stable form of element = 0

Page 29: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

ener

gy

reactants

products

-(ΔHFo)R

(ΔHFo)P

ΔHRXNo = (Δ HF)P – (Δ HF)R

elements

ΔΔHHFFoo to calculate to calculate ΔΔHHRXNRXN

oo

C6H12O6 + 6 O2

C(s) + H2(g) + O2(g)

6 CO2 + 6 H2O

ΔHRXNo

Page 30: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

ΔΔHHrxnrxnoo

ΔHrxno = Σn ΔHf

o (products) – Σm ΔHfo (reactants)

an application of Hess’s law

EXAMPLE: What is ΔHrxno for the reaction ?

C2H5OH(l) + 3 O2(g) → 2 CO2(g) + 3 H2O(l)ΔHf

o

valueskJ/mol

–277.7 0 2(–393.5) 3(–285.8)

ΔHrxno = 2(–393.5) – 3(285.8) – (–277.7)= – 1366.7 kJ exothermic

ΔHfo for ethanol

2 C (graphite) + 3 H2(g) + ½ O2(g) → C2H5OH(l)

reactants allin standardstates 1 bar25 °C

ΔHfo = – 277.7 kJ/mol

produce 1 mol of C2H5OH

Page 31: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

ΔΔHHrxnrxnoo

ΔHrxno = Σn ΔHf

o (products) – Σm ΔHfo (reactants)

an application of Hess’s law

EXAMPLE: What is ΔHrxno for the reaction ?

C6H6(l) + 15/2 O2(g) → 6 CO2(g) + 3 H2O(l)ΔHf

o

valueskJ/mol

49.0 0 6(–393.5) 3(–285.8)

ΔHrxno = 6(–393.5) + 3(–285.8) – (49.0)= – 3267.4 kJ exothermic

ΔHfo for benzene

6 C (graphite) + 3 H2(g) → C6H6(l)

reactants allin standardstates 1 bar25 °C

ΔHfo = 49.0 kJ/mol

produce 1 mol of C6H6(l)

Page 32: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

ΔΔHHrxnrxnoo ExampleExample

Calculate ΔHRXNo for

2 C(s) + H2(g) C2H2(g)

From the following information

C2H2(g) + 5/2 O2(g) 2 CO2(g) + H2O(l)ΔHRXN

o = -1299.6 kJ

C(s) + O2(g) CO2(g)ΔHRXN

o = -393.5 kJ

H2(g) + ½ O2(g) H2O(l)ΔHRXN

o = -285.8 kJ

Page 33: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

WHERE DOES THE WHERE DOES THE ENERGY COME FROM?ENERGY COME FROM?

Bond breaking and formation

BOND ENTHALPYenergy to break one mole of bonds

Cl2(g) → 2 Cl(g) ΔHo = 243 kJ

Bond breaking is always endothermic

Bond formation always exothermic

ΔH for a reaction depends on thebreaking and forming of bonds -- thenumbers of bonds and their strengths

(more in Chap. 8)

Page 34: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

COVALENT BONDSCOVALENT BONDSLENGTH, ENERGYLENGTH, ENERGY

BOND LENGTH • •

bond length

Larger atoms have longer bondsMultiple bonds are shorter:

C–O 143 pm but C=O 122 pm

BOND ENERGY

As electron density (and number of bonds)between 2 atoms increases, the bond getsshorter and stronger.

Bond length & bond energy closely related

Page 35: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

COVALENT BONDSCOVALENT BONDSLENGTH, ENERGYLENGTH, ENERGY

Covalent bonds are strongOverall stability of a molecule is related

to the covalent bonds it containsStrength of bond = energy needed to break it

BOND ENERGY

Value depends on identity of atoms involvedand number of shared electron pairs

Value is always positive -- it takes energyto break a covalent bond

C

H

HH

H

C 4 H.... + .

ΔH = 1660 kJ/mol

D(C–H) = 1660/4 = 415 kJ/mol

Page 36: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

COVALENT BONDCOVALENT BONDENERGIESENERGIES

C–C 356 kJ/molC=C 598 kJ/molC≡C 813 kJ/mol

moreelectronsshared

shorterbond length

C–H 416 kJ/molC–Cl 327 kJ/molC–Br 285 kJ/mol

shorterbondlength

Table 8.4

Page 37: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

BOND ENERGIESBOND ENERGIESAND AND ΔΔHH

The ΔH for a reaction can be estimatedfrom bond energies

ΔHrxn = ΔH(bonds broken) – ΔH(bonds formed)

EXAMPLE

Estimate ΔHrxn for

C C

HH

=R R′

C C

HH

R R′

+ Br2

BrBr

Page 38: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

BOND ENERGIESBOND ENERGIESAND AND ΔΔHH

C C

HH

=R R′ C C

HH

R R′+ Br2

BrBr

ΔHrxn = 1[C=C] + 1[Br–Br] – {1[C–C] + 2[C–Br]} break break form form

598 193 356 276x 2

791taken up

908given off

ΔHrxn = 791 – 908 = – 117 kJ/mol

exothermic reaction

Page 39: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

COMBUSION OF METHANECOMBUSION OF METHANE

CH4 + 2 O2 → CO2 + 2 H2O

C – H 413 kJ/molO = O 495 kJ/mol

C = O 799 kJ/molO – H 463 kJ/mol

CONSUMECONSUME RELEASERELEASE

4 x 413 = 16522 x 495 = 990

2642

2 x 799 = 15984 x 463 = 1852

3450

– 808 kJ/mol

expt value = – 802 kJ/mol

Page 40: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

The thermite reaction below is used for welding.What is the ΔHrxn

o for the reaction involving1 mole of Al?

2 Al (s) + Fe2O3 (s) → Al2O3 (s) + 2 Fe (s)

ΔHfo of Al2O3 (s) = – 1669.8 kJ/mol

ΔHfo of Fe2O3 (s) = – 822.2 kJ/mol

A. + 847.6 kJB. – 847.6 kJC. +1895 kJD. – 423.8 kJ E. – 2,492 kJ

OLD EXAM QUESTIONOLD EXAM QUESTION

Page 41: LIMITING REACTANT - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/asbury/Lec37-39_web.pdf · LIMITING REACTANT • The limiting reactant is used up first • The limiting

EXAM QUESTIONEXAM QUESTION

What is the ΔHrxn for this reaction?

2 HCl(g) + F2(g) → 2 HF(g) + Cl2(g)

Bond Bond Dissociation EnergyH−Cl 431 kJ/molF−F 155 kJ/molH−F 567 kJ/molCl−Cl 242 kJ/mol