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# chapter3 - 3 Stoichiometry Calculations with Chemical...

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1 3 Stoichiometry: Calculations with Chemical Formulas and Equations Stoichiometry: the area of study that examines the quantities of substances consumed and produced in chemical reactions. 3.1 Chemical Equations: Chemical reactions are represented in a concise way by chemical equations. Coefficients: the numbers in front of elements or compounds represent the relative numbers of molecules. Example: 2H 2 + O 2 2H 2 O react with produce 2 moles 1 mole 2 mole of hydrogen of oxygen of water ---Reactants/Reagents------ --products— Balancing Equations: Rule: a chemical Equation must have an equal number of atoms of each elements on each side of the arrow and in charge, if charged species appear in the equation. (Dalton’s atomic theory) *Note: (1) subscript and coefficient have different meanings. (2) When balancing an chemical equation, only place a coefficient in front of a formula and NEVER change subscripts. (Placing a coefficient in front of a formula changes only the amount of the substance and not its identity); (2) Don’t write a symbol for an atom when an element occurs as a diatomic molecule. Eg. H 2 , N 2 , O 2 , F 2 , Cl 2 , Br 2 and I 2 under normal conditions.

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2 The process of balancing an equation: (1) Write an unbalanced equation (2) Determine the coefficients to balance the equation. By convention, coefficients in an equation are usually the smallest possible integers. Example: Natural gas (main component is methane CH 4 ) burn is air to produce carbon dioxide and water. Unbalanced equation: CH 4 +O 2 CO 2 +H 2 O 1 CH4 contains 1C and 4H 1C needs 1O 2 and produces 1CO 2 ; 4H needs 1O 2 and produces 2H 2 O. So totally 2 O 2 are required to completely reacts with 1CH 4 . Balanced Equation: CH 4 + 2O 2 CO 2 + 2H 2 O 1C 1C 4H 4H 4O 4O
3 Indicating the states (s-solid, l-liquid, g-gas, aq-aqueous (water) solution) of reactants and products: CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O (g) Example: Balance the following equations: (a) Fe (s) + O 2 (g) Fe 2 O 3 (s) (b) C 2 H 4 (g) + O 2 (g) CO 2 (g) + H 2 O(g) (c) Al (s) + HCl (aq) AlCl 3 (aq) + H 2 (g) (d) H + (aq) + CO 3 2- (aq) H 2 O (l) + CO 2 (g) . 3.2 Some Simple Patterns of Chemical Reactivity 1. Combination Reactions Combination reaction: two or more substances react to form one product. E.g.: 2H 2 (g) + O 2 (g) 2H 2 O (l) Example: Writing balanced equation for the following combination reaction: lithium metal and fluorine gas react. .

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5 2. Decomposition reaction: one substance undergoes a reaction to produce two or more other substances. E.g.: CaCO 3 CaO + CO 2 Example: Write balanced equation for the following decomposition reaction: solid barium carbonate is heated. . →

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6 3. Combustion in air: Combustion reactions: rapid reactions that produce a flame. Most of the combination reactions involve O 2 as a reactant. E.g.: Fuel + Oxygen Heat + Water + Carbon dioxide Products of complete combustion reactions with O 2 : C CO 2 (g); H H 2 O (l or g depending on the conditions of the reaction) The chemical equation for stoichiometric burning of
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