chap03-4_bch1049_07MCWC_1d

chap03-4_bch1049_07MCWC_1d - Basic Stoichiometry:...

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1 Stoichiometry Basic Stoichiometry: Calculations with Chemical Formulas and Equations (Chapter 3) What’s Ahead: 1. 1. Chemical equations and reaction types Chemical equations and reaction types 2. 2. Formula weights, Avogadro Formula weights, Avogadro ’s number and the s number and the Mole 3. 3. Empirical formulas from analyses Empirical formulas from analyses 4. 4. Quantitative information from balanced equations Quantitative information from balanced equations Anatomy of a Chemical Equation Reactants appear on left side and products on right side of equation. The states of the reactants and products are written in parentheses to the right of each compound. Coefficients are inserted to balance the equation. CH 4 ( g ) + 2 O 2 ( g ) CO 2( g ) + 2 H 2 O ( g )
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2 Stoichiometry Reaction Types: Combination Reactions • Example: 2 Mg ( s ) + O 2 ( g ) ⎯⎯→ 2 MgO ( s ) • Two or more substances react to form one product. Stoichiometry Decomposition Reactions • Examples: CaCO 3 ( s ) ⎯⎯→ CaO ( s ) + CO 2 ( g ) 2 KClO 3 ( s ) ⎯⎯→ 2 KCl ( s ) + 3 O 2 ( g ) • One substance breaks down into two or more substances. • Reaction on left is used to inflate air bags in cars. • When properly ignited, sodium azide NaN 3 (s) decomposes rapidly, forming N 2 gas which expands the air bag.
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3 Combustion Reactions • Examples: CH 4 ( g ) + 2 O 2 ( g ) ⎯⎯→ CO 2 ( g ) + 2 H 2 O ( g ) C 3 H 8 ( g ) + 5 O 2 ( g ) ⎯⎯→ 3 CO 2 ( g ) + 4 H 2 O ( g ) (above) • Rapid reactions that produce a flame • Most often involve hydrocarbons reacting with oxygen in air. Stoichiometry Formula Weight (FW) • Sum of atomic weights for atoms in a chemical formula. • So, formula weight of calcium chloride, CaCl 2 is: Ca: 1(40.1 amu) + Cl: 2(35.5 amu) 111.1 amu Molecular Weight (MW) • Sum of atomic weights of atoms in a molecule. • For the ethane molecule, C 2 H 6 , molecular weight is: C: 2(12.0 amu) + H: 6(1.0 amu) 30.0 amu
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4 Stoichiometry Percent Composition One can find percentage of the mass of a compound due to each element by using this equation: % element = (number of atoms)(atomic weight) (FW of the compound) x 100 So % of carbon in ethane, C 2 H 6 is: %C = (2)(12.0 amu) (30.0 amu) 24.0 amu 30.0 amu = x 100 = 80.0% x 100 Stoichiometry Avogadro’s Number and the Mole • A mole is the amount of matter that contains as many atoms/molecules/species as the number of atoms in exactly 12 g of isotopically pure 12 C. • Experimentally determined as 6.02 x 10 23 , Avogradro’s number ( N ) . • Therefore a mole of 12 C has a mass of 12 g.
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5 Stoichiometry Molar Mass • By definition, this is the mass of 1 mol of a substance (unit: g/mol) – The molar mass of an element is the mass number for the element in periodic table. – The formula weight (in amu) will be the
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This note was uploaded on 01/11/2011 for the course BCH 1049 taught by Professor Michaelcwchan during the Spring '06 term at City University of Hong Kong.

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chap03-4_bch1049_07MCWC_1d - Basic Stoichiometry:...

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