stoichiometry ap chemistry in the thermite reaction, a mixture of powdered aluminum and powdered...

StoichiometryAP Chemistry

In the thermite reaction, amixture of powdered aluminum

and powdered iron(III) oxidereact to yield iron and aluminum

oxide. The reaction burns hotenough to be useful in under-

water welding.

2 Al + Fe2O3 2 Fe + Al2O3 + energy

In a reaction:

atoms are rearranged

AND mass

energy

charge

are conserved

Balancing Chemical Equations

=law of

conservationof mass

same # of atomsof each type on each

side of equation

Hint: Start with most complicated substances firstand leave simplest substances for last.

Chemical Equations

2

Solid lithum reacts w/oxygento form solid lithium oxide.

O2(g) Li(s) Li2O(s)+

Li+ O2–

+

4

Aqueous aluminum sulfate reacts w/aqueous bariumchloride to form a white precipitate of barium sulfate.The other compound remains in solution.

Al3+ SO42– Ba2+ Cl–

Al2(SO4)3(s)+ BaCl2 BaSO4 AlCl3(aq) (aq) (aq)+33 2

Methane gas (CH4) reacts withoxygen to form carbon dioxidegas and water vapor.

CH4(g) + O2(g) CO2(g) H2O(g)+2 2

Furnaces burn primarily methane.

CaSi2 + SbI3 Si + Sb + CaI2

Al + CH3OH Al(CH3O)3 + H2

CaC2(s) + H2O(l) C2H2(g) + CaO(s)

3 2 26 3

3262 3

2 C2H2(g) + O2(g) CO2(g) + H2O(l) 5 4 22

C3H8 + O2 CO2 + H2O 5 3 4

Complete combustion of a hydrocarbon,or of a compound containing C, H, and O

(e.g., methanol, CH3OH) yields CO2 and H2O.

C4H10 + O2 CO2 + H2O 13 4 52 8 10

Another pattern of reactivity:

e.g.,

alkalimetal

metalhydroxide

hydrogengas

water + +

2 Rb(s) + 2 H2O(l) 2 RbOH(aq) + H2(g)

Two (of the several) Types of Reactions

combination (synthesis): simpler substances combine to form more complex substances

sodium + chlorine gas sodium chloride

A + B AB AB + C ABC A + B + C ABC

Na Cl2+ NaCl2 2

-- form:

decomposition: complex substances are broken down into simpler ones

lithium chlorate lithium chloride + oxygen

water hydrogen gas + oxygen gas

Li+ ClO3– Li+ Cl–

LiClO3 LiCl + O22 2 3

H2O2 H2 + O22

-- form:

AB A + B ABC AB + C ABC A + B + C

formula weight: the mass of all of the atoms in a chemical formula (unit is amu)

-- If the substance is a molecular substance (e.g., C3H8), then the term molecular weight is also used.

molar mass: the mass of one mole of a substance (unit is usually grams)

-- recall that 1 mole of any substance

6.02 x 1023 particlesof that substance

=

(NH4)3PO4

H: 12 (1.0 g) = 12.0 g

N: 3 (14.0 g) = 42.0 g

m.m. =149.0 g P: 1 (31.0 g) = 31.0 g

NH4+ PO4

3–

O: 4 (16.0 g) = 64.0 g

Find the molar mass and formula weight ofammonium phosphate.

f.w. = 149.0 amu

percentage composition: the mass % of each element in a compound

% of element =g element

molar mass of compoundx 100

Find the percentage of oxygen, by mass, incalcium nitrate.

Ca(NO3)2

% O = = 58.5% O6(16.0) 2(14.0) 40.1

6(16.0)

equation:

Empirical Formula and Molecular Formula

Compound Molecular Formula

EmpiricalFormula

glucose C6H12O6

propane C3H8

butane C4H10

naphthalene C10H8

sucrose C12H22O11

octane C8H18

lowest-termsformula

shows the true number andtype of atoms in a m’cule

CH2O

C3H8

C2H5

C5H4

C12H22O11

C4H9

“What’s your flavorof ice cream?”

Finding an Empirical Formula from Experimental Data

1. Find # of g of each element.

2. Convert each g to mol.

3. Divide each “# of mol” by the smallest “# of mol.”4. Use ratio to find formula.

A ruthenium/sulfur compound is 67.7% Ru.Find its empirical formula.

RuS1.5 Ru2S3

Ru g 67.7

Ru g 101.1Ru mol 1

Ru mol 0.670 0.670 1

S g 32.3

S g 32.1S mol 1

S mol 1.006 0.670 1.5

A sample of a compound contains 4.63 g lead, 1.25 gnitrogen, and 2.87 g oxygen. Name the compound.

PbN4O8

Pb(NO2)4

Pb? 4 NO2–

lead(IV) nitrite

?

Pb g 4.63

Pb g 207.2Pb mol 1

Pb mol 0.0223 0.0223 1

N g 1.25

N g 14.0N mol 1

N mol 0.0893 0.0223 4

O g 2.87

O g 16.0O mol 1

O mol 0.1794 0.0223 8

?