Chapter08_Fall11

# Chapter08_Fall11 - 8 Stoichiometry Ratios of Combination...

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10/27/2011 1 Stoichiometry: Ratios of Combination 8 8.1 Chemical Equations 8.2 Combustion Analysis 8.3 Calculations with Balanced Chemical Equations 8.4 Limiting Reactants 8.5 Periodic Trends in Reactivity of the Main Group Elements Chemical Equations A chemical equation uses chemical symbols to denote what occurs in a chemical reaction. Ammonia and hydrogen chloride react to produce ammonium chloride. NH 3 + HCl NH 4 Cl Chemical formulas indicate elements or compounds that are reacting or being formed Labels in parentheses indicate the physical state (s) solid (l) liquid (g) gas (aq) aqueous (dissolved in water) NH 3 ( g ) + HCl ( ) NH 4 Cl ( s ) SO 3 ( ) + H 2 O ( l ) H 2 SO 4 ( aq ) reactants products Chemical equations must be balanced so that the law of conservation of mass is obeyed. Balancing Chemical Equations Balancing is achieved by writing stoichiometric coefficients to the left of the chemical formulas. Balancing Chemical Equations 1. Change the coefficients of compounds before changing the coefficients of elements. 2. Treat polyatomic ions that appear on both sides of the equation as units. 3. Count atoms and/or polyatomic ions carefully, and track their numbers each time you change a coefficient. Example Write the balanced chemical equation that represents the combustion of propane. Step 1: Write the unbalanced equation: C 3 H 8 ( ) + O 2 ( ) CO 2 ( ) + H 2 O( ) Step 2: Leaving O 2 until the end, balance each of the atoms: C 3 H 8 ( ) + 5 O 2 ( ) 3 CO 2 ( ) + 4 H 2 O( ) Step 3: Double check to make sure there are equal numbers of each type on atom on both sides of the equation.

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10/27/2011 2 Examples O 2 (g) + 2 H 2 2 H 2 O (g) O 2 (g) + H 2 (g) H 2 O (g) C 8 H 18 (l) + O 2 (g) CO 2 (g) + H 2 O (g) SiO 2 (s) + C (s) SiC (s) + CO (g) + 3 C (s) SiC (s) + 2 CO (g) H | C | O C 8 H 18 + O 2 CO 2 + H 2 O 18 8 2 2 1 3 C 8 H 18 + O 2 8 CO 2 + H 2 O 18 8 2 2 8 17 C 8 H 18 + O 2 8 CO 2 + 9 H 2 O 18 8 2 18 8 25 C 8 H 18 + 25/2 O 2 8 CO 2 + 9 H 2 O 18 8 25 18 8 25 2 C 8 H 18 + 25 O 2 16 CO 2 + 18 H 2 O 36 16 50 36 16 50 Examples Al (s) + H 2 SO 4 (aq) Al 2 (SO 4 ) 3 + H 2 O 2 (g) + 2 H 2 (g) 2 H 2 O (g) SiO 2 (s) + 3 C (s) SiC (s) + 2 CO (g) 2 C 8 H 18 (l) + 25 O 2 (g) 16 CO 2 (g) + 18 H 2 O (g) 2 Al (s) + 3 H 2 SO 4 (aq) Al 2 (SO 4 ) 3 (aq) + 3 H 2 (g) Al (s) + HCl (aq) AlCl 3 (aq) + H 2 (g) 2 Al (s) + 6 HCl (aq) 2 AlCl 3 (aq) + 3 H 2 (g) Patterns of Chemical Reactivity Three of the most commonly encountered reaction types are combination, decomposition, and combustion. Combination – two or more reactants combine to form a single product NH 3 ( g ) + HCl( ) → NH 4 Cl( s ) Decomposition – two or more products form from a single reactant CaCO ± ? Δ → CaO ? + CO 2 ° Combustion – a substance burns in the presence of oxygen. Combustion of a compound that contains C and H (or C, H, and O) produces carbon dioxide gas and water. CH ² O( l ) + O ² ( ) → CO ² ( ) + H ² O( ) Combustion Analysis The experimental determination of an empirical formula is carried out by combustion analysis.
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Chapter08_Fall11 - 8 Stoichiometry Ratios of Combination...

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