Plan, Ppt10(PS4)

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Plan, Ppt10(PS4)

• Finish EF (Tro, 3.9)
• From masses
• From combustion analysis data
• Chemical Reaction (Tro, 3.10)
• Process (takes time!) original subs ? new subs
• Fixed mass ratio of substances react and form
• Whole-number ratio of FUs react and form
• Chemical Equation (Balanced) later
• Coefficients represent ratio of FUs when reaction
  occurs
• Various problem types (stoichiometry) later
  (Tro, 4.2)

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Empirical Formula (EF) Quick Review

• Need to find the subscripts
• gt lowest whole-number ratio of atoms
• gt lowest whole-number ratio of MOLES (of
• atoms)

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Determining the Empirical Formula of a Compound

• From mass data (of each element A, B, C, ) of a
  sample
• masses (of A, B, C, ) ? moles (of A, B, C,
  atoms) ? ratio of moles ? reduced
  (whole-number) ratio of moles ? formula
  (subscripts)
• From mass data (of each element A, B, C, )
• Get mass data, and do as above!
• Assume 100 g of compound for convenience
• 20.4 of 100 g 20.4 g! (etc.)

• ? NOTE If molar mass is also given, it may be
  easiest to use that mass (rather than 100 g).
  You will then get the molecular formula first
  (which can then be reduced, if needed)

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From Green Handout Sheet (PS4 5 Practice)

• What is the empirical formula of tetraethyllead,
  the lead in gasoline?
• The composition is 29.71 C, 64.06 Pb, 6.23
  H (These percentages are by mass)

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PS Sign-Posting

• The concepts and skills related toproblems 14,
  18, and 19 on PS4 have been covered now in this
  PowerPoint. Give those problems a try now!

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From Combustion Analysis Data

• Often cannot easily get the elements out
• So, react with oxygen
• All C atoms end up in CO2 molecules!
• All H atoms end up in H2O molecules!
• Figure out C in sample from mass of CO2
• Figure out H in sample from mass of H2O
• Figure out a 3rd atom type (O, N, S, ) by mass
  difference (as in Exp 4get Cl by difference)

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Combustion Analysis Figure 3.10
e.g., excess O2, sulfur oxides, nitrogen oxides,
halogen-oxides , etc.
e.g., Mg(ClO4)2
e.g., NaOH
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From Green Handout Sheet (PS4 5 Practice)

• A hydrocarbon of unknown formula CxHy was
  submitted to combustion analysis with the
  following results. What is the empirical formula
  of the hydrocarbon?

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From Green Handout Sheet (PS4 5 Practice)

• Coniine, a toxic substance isolated form poison
  hemlock, contains only carbon, hydrogen, and
  nitrogen. Combustion analysis of a 5.024 g
  sample yields 13.90 g of CO2 and 6.048 g of H2O.
  What is the empirical formula of coniine?

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PS Sign-Posting

• The concepts and skills related toproblem 17 on
  PS4 have been covered now in this PowerPoint.
  Give those problems a try now!
• end of PS4 material

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Discussion of Meaning of a Balanced Chemical
Equation

• Reaction vs. equation (different!)
• Most chemistry texts tend to discuss this
  initially but then use the words interchangeably
  later on(!) Be careful! ?

• Chemical Reaction
• Initial substances (reactants) turn into new
  substances (products)
• Takes time (a process)
• Involves actual amounts
• Can be expressed in a variety of ways grams,
  moles, L, etc.

• Chemical Equation
• Represents (a type of) chemical reaction (but not
  literally)
• Symbols (not reality), on paper
• Formulas, coefficients, an arrow
• Does not involve actual amounts! Only RATIO of
  combination and formation

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Discussion of Meaning of a Balanced Chemical
Equation (Example)
Chemical Reaction 4 g of methane react with 16
grams of oxygen to produce 11 grams of carbon
dioxide and 9 grams of water, all as gases.
Chemical Equation CH4(g) 2 O2(g) ? CO2(g) 2
H2O(g)

• Coefficients represent a ratio only, and they
  represent the ratio of formula units, most
  fundamentally.
• Here, a 1 2 1 2 ratio of molecules of
  CH4, O2, CO2, and H2O but only as reaction occurs.

The reaction occurs over some period of time,
during which the number of molecules of methane
and oxygen steadily decrease as the number of
molecules of carbon dioxide and water increase.
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Discussion of Meaning of a Balanced Chemical
Equation

• Analogy (hypothetical) Working at Lawson
  products
• 25 boxes packed up ? 15 in pocket

• How much money is in my pocket after the
  reaction occurs?

• The equation alone cant answer that!

• How much reaction occurred? (not indicated by
  equation!)

• How much was in your pocket before the reaction?