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General Chemistry ICHEM 201, Fall 2011Bucknell UniversityD. Rovnyak

Chapter 2-3, McMurry and Fay

Sept 6, 2011

Quiz today. After quiz, please turn onclickers.

Please remain quiet while others finish.

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Molecular Compounds

Generally compounds of non-metals

Both elements require prefixes A variety of ratios are allowed and have been preparedin labs - no easy rules as with ionic compounds

No roman numerals here How do we decide on the order (why not O3N2?)

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ex: HBrO

problem 2.25 is excellent practice (answer in backof book)

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Practice

H2Cr2O7 is

a. chromous acidb. chromic acid

c. dichromous acidd. dichromic acid

(hint: focus on suffix of the oxoanion

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Practice

P2O5 is widely called phosphorouspentoxide. This is

a. technically incorrect

b. an informal name accepted by chemistsc. ambiguousd. all of the above

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Practice

tin(ii) phosphate is

a. Sn3(PO4)2b. Sn2(PO4)3

c. SnPO4d. Sn(PO4)2

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Chapter 3starts

Balancing Chemical Equations (a.k.a. reactions)

enforce conservation of mass

count and balance number of atoms

look at the composition of compounds (aka - formula unit)

ex: one MgBr2 has one Mg2+ ion and two Br- ions

cant change identity of compounds given

ex: need another oxygen? dont change NO to NO2

simple equations can be balanced by inspection,

otherwise use a system:

1. write unbalanced equation2. introduce coefficients, solving one atom at a time2a. use fractions if they help but multiply them out to whole

numbers at the end3. divide coefficients to lowest whole numbers4. always CHECK - you will always know if you are right

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Balancing by Inspection

- easy to count atoms in your head?- final equation required no more than 2 coefficients?

-probably ok, but always be 110% certain; again - you shouldalways know if you get these right/wrong

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C3H8 + O2 -----> CO2 + H2O

Using a system

C 3 1

H 8 2

O 2 3

(i) write down the atom counts on each side

(ii) fix one atom at a time, ignoring the rest, often starting with aheavy atom like C or O; to balance Cs, multiply CO2 by3

C3H8 + O2 -----> 3 CO2 + H2O

C 3 3

H 8 2

O 2 7

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(ii) proceed rationally through the remaining atoms; wheneverpossible, select whole number options; O still messy, sofix H next by multiplying H2O by 4

C3H8 + O2 -----> 3 CO2 + 4 H2O

C 3 3

H 8 8

O 2 10

(iv) fix O by multiply O2 by 5

C3H8 + 5 O2 -----> 3 CO2 + 4 H2O

C 3 3

H 8 8

O 10 10

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Photosynthesis

a CO2 + b H2O ------> c C6H12O6 + dO2

hint: in some cases (example: when pure elements are available) think

about addition, not just multiplication

in the fully balanced equation, thecoefficients c and dare

a. 1 and 6b. 2 and 6c. 1 and 10

d. 1 and 3

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The mole

Big picture : molecules and elements react in whole numberratios - we want to have an easy way to keep track of the

number of molecules

But first:atomic mass units: can be summed to get molecular mass

atomic mass of H : 1.00 amuatomic mass of Cl: 35.5 amu

molecular mass* of HCl: 36.5 amu

* molecular mass must also be anisotope weighted average since it isbased on atomic masses, which we

already know are isotope weighted

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The mole

Number of atoms in 12 g of carbon-12

6.0221415 0.0000010 x 1023

http://www.americanscientist.org/issues/pub/2007/2/an-exact-value-for-avogadros-number

review: howmany sigfigs in the

currentvalue ofNA?

aka:Avogadrosconstant,Avogadrosnumber,Avogadroconstant, NA

key facts:

this is a measured number

the NUMBER of things, not their mass

named in honor of Avogadro who realizedgases of equal volumes had equal number ofparticles (he had no knowledge of, or ability todetermine, the mole)

amu will be molar mass with this definition

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amu tells you molar mass

36.5 amu is the mass of a virtual HCl molecule based

on natural abundance weighting

36.5 g HCl contains 6.022 x 1023 molecules of HCl

Q: The atomic mass of N is 14.0 amu and that of oxygen

is 16.0 amu; what mass of N2O4 contains one moleof N2O4 molecules?

a. 30.0 gb. 36.0 gc. 44.0 gd. 92.0 g

In this textwe use NA to

4 significantfigures, avery commonpractice

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practice with atomic andmolecular molar masses

how many molecules of sucrose (C12H22O11) are in37.3 grams of sucrose?

(i) read off amu from periodic table and add up for allatoms in your molecule

12 C atoms * 12.01 amu = 144.12 amu22 H atoms * 1.01 amu = 22.2 amu11 O atoms * 16.00 amu = 176.0 amu

342.3 amu

(ii) therefore the molar mass if 342.3 g / mol

37.3 g *1 mol

342.34 g*6.022x10

23molecules

mol= 6.56 x1022molecules

sucrose

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Molarity is a unit ofconcentration

Molarity =moles solute(mol)

volume solution(L)

1. add known mass of solute

2. fill to line with solvent

Ex: prepare 58.44 g of NaCl ina 100.00 ml vol flask

Molarity isindicated with acapital M andwith squarebrackets. If asolution is 1 Min glucose then:

[glucose] = 1 M

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counting vs mass

1 mol of many different substances including pureelements and complex molecules. All masses are

different, but each has the same number of molecules

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Stoichiometry

in the production of ammonia, hydrogen and nitrogenmolecules react in a ratio of 3:1

3H2 + N2 ----> 2NH3

If I dont want to waste the starting materials, then I want toalways use hydrogen and nitrogen in a 3:1 molecular ratio

Q: How much nitrogen (by mass) will react perfectly with 1.000 kgof hydrogen?

rule: must have balanced equations before doing stoichiometry

hint : whenin doubtconvert tomoles

1000 gH2( )1 molH2

(2 x 1.008) gH2

= 496.0 molH2(i) how many molecules

of H2 do I have?

496.0 molH2( )1 molN23 molH2

=165.3 molN2(ii) need 1/3 times asmany molecules of N2

165.3 molN2( )28.0 gN2

molN2

= 4630 gN2(ii) change moles N2 tomass of N

2

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stoichiometry

H2 (g) + O2 (g) -------> H2O (g)

Q: What mass of hydrogen gas would react exactly with24.0 g of oxygen gas?

a. 3.02 gb. 1.51 g

c. 0.75 gd. 6.04 g